This item is only available as the following downloads:
1 DIURNAL RESPONSE OF SHOOT GROWTH TO TREE WATER ST RESS By EMILY ELAINE MASSEY A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2011
2 2011 Emily Elaine Massey
3 To my p arents, Rhonda and Terry Massey
4 ACKNOWLEDGMENTS The efforts of numerous people went into the formation, implementation, and completion of this thesis, and to them I owe many thanks. First, I would like to thank Dr. Richard C. Beeson, Jr. for reading this manuscript and programming my irrigation time cl ocks. His expertise was greatly appreciated throughout the editing of this thesis. I would also like to extend my gratitude to my committee members Drs. Michael E. Kane and Timothy A. Martin for their support and manuscript review. I learned much about water relation techniques, plant physiology, and balance from all of them. Second, I would like to extend thanks to Christopher Fooshee and Brian Pearson for their technical support, guidance, and statistical expertise. A special thanks also goes out to E dward Tillman, Cody Abston, Mark Jackson, and Steve Toomoth for their aid in the implementation and setup of my research. I would like to thank all staff and professors at the MREC and Dr. William MacKay for their kind words and inspiration Finally, I would like to acknowledge my family and friends for their support and encouragement with a special thanks to my parents, Ma rc Malone, Sarah Carls, and Tim and Becky Moss. I am especially thankful to Brandon Burmeister for all of his support and review of this manuscript as well
5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 7 LIST OF FIGURES ................................ ................................ ................................ .......... 9 LIST OF ABBREVIATIONS ................................ ................................ ........................... 10 ABSTRACT ................................ ................................ ................................ ................... 12 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 14 Water P otential ................................ ................................ ................................ ....... 15 Water Balance in Plants ................................ ................................ .......................... 17 Water Movement from Soil to Roots ................................ ................................ 17 Water Movement in X ylem ................................ ................................ ............... 18 Water Movement in Leaves ................................ ................................ .............. 19 Plant Gas Exchange ................................ ................................ ............................... 19 Transpiration ................................ ................................ ................................ .... 19 Stomatal Conductance ................................ ................................ ..................... 19 Photosynthesis ................................ ................................ ................................ 21 P lant Growth ................................ ................................ ................................ ........... 21 Plant Growth: Cell Wall Expansion and Turgor Pressure ................................ 22 Plant Growth: Turgor Pressure ................................ ................................ ......... 23 Plant Growth: Elongation Rates ................................ ................................ ....... 24 Irrigation Effects on Plant Growth ................................ ................................ ........... 25 Water Stress ................................ ................................ ................................ ........... 25 Water Stress Effects on Plant Growth ................................ .............................. 26 Maintenance of Turgor Pressure: Osmotic Adjustment ................................ .... 27 Water Stress Effects on Stomatal Aperture ................................ ...................... 28 Water Stress Effects on Photosynthesis ................................ .......................... 29 Quantification of Water Status, Gas Exchange, and Plant Growth ......................... 30 Pressure Chambers ................................ ................................ ......................... 30 Pressure Volume Curves ................................ ................................ ................. 30 Porometers and Infrared Gas Analyzers ................................ .......................... 33 Plants of Study ................................ ................................ ................................ ........ 33 Quercus virginiana ................................ ................................ ........................... 33 Ulmus parvifolia ................................ ................................ ................................ 34 2 MATERIALS AND METHODS ................................ ................................ ................ 38 Season One ................................ ................................ ................................ ............ 38
6 Shoot Growth Measurements ................................ ................................ ........... 40 Xylem Water Potential Measu rements ................................ ............................. 40 Pressure Volume Curves ................................ ................................ ................. 41 Relation of Turgor Pressure to Shoot Growth ................................ ................... 42 Leaf Gas Exchange Measurements ................................ ................................ 43 Season Two ................................ ................................ ................................ ............ 43 Shoot Growth Measurements ................................ ................................ ........... 45 Xylem Water Potential Measurements ................................ ............................. 45 Pressure Volume Curves ................................ ................................ ................. 45 Relation of Turgor Pressure to Shoot Growth ................................ ................... 46 Statistical Analysis ................................ ................................ ................................ .. 46 3 RESULTS ................................ ................................ ................................ ............... 55 Season One ................................ ................................ ................................ ............ 55 Shoot Elongation Rate ................................ ................................ ..................... 55 Relation of Turgor Pressure to Shoot Growth ................................ ................... 56 Pressure Volume Curves ................................ ................................ ................. 56 Leaf Gas Exchange ................................ ................................ .......................... 57 Season Two ................................ ................................ ................................ ............ 58 Shoot Elongation Rate ................................ ................................ ..................... 58 Relation of Turgor Pressure to Shoot Growth ................................ ................... 59 Pressure Volume Curves ................................ ................................ ................. 61 4 DISCUSSION ................................ ................................ ................................ ......... 71 Seasons One and Two ................................ ................................ ........................... 71 Leaf Gas Exchange ................................ ................................ .......................... 71 P ressure Volume Curves ................................ ................................ ................. 73 Shoot Elongation Rate ................................ ................................ ..................... 77 Relation of Turgor Pressure to Shoot Growth ................................ ................... 79 LIST OF R EFERENCES ................................ ................................ ............................... 84 BIOGRAPHICAL SKETCH ................................ ................................ ............................ 92
7 LIST OF TABLES Table page 2 1 Tree fertilization schedule across S easons O ne and T wo. ................................ 48 3 1 Shoot elongation rates measured across a five day period for irrigation regimes across species. ................................ ................................ ..................... 62 3 2 Chinese elm shoot elongation rates measured across a five day period for irrig ation regimes. ................................ ................................ ............................... 62 3 3 Interaction between species, irrigation regime, and time of growth for turgor pressure normalized ( P Norm ) growth rate across species. ................................ 62 3 4 Interaction of species and shoot age for relative symplastic water content at incipient plasmolysis ( SWC IP ) across species. ................................ ................... 63 3 5 Interaction of irrigation regime and shoot age for relative symplastic water content at incipient plasmolysis ( SWC IP ) across species. ................................ ... 63 3 6 Interaction of species and shoot age for osmotic potential at incipient IP ) across species. ................................ ................................ .... 63 3 7 Interaction of irrigation regime and shoot age for osmotic potential at incipient IP ) across species. ................................ ................................ .... 64 3 8 Interaction between species, irrigation regime, and shoot age for maximum turgor pressure (P max ) across species. ................................ ............................... 64 3 9 Interaction of irrigation regime and shoot age for bulk modulus of elasticity of Chinese elm during across irrigation regimes ................................ .................... 65 3 10 Interaction of species and shoot age for maximum modulus of elasticity (E max ) across species. ................................ ................................ ......................... 65 3 11 Interaction of species, irrigation regime, and time of growth for shoot elongation across species. ................................ ................................ ................. 66 3 12 Interaction of irrigation regime and time of growth for shoot elongation in live w measurements. ................................ ............. 66 3 13 Interaction of irrigation regime and time of growth for shoot elongation in Chinese elm. ................................ ................................ ................................ ....... 67 3 14 Interaction between species and time of growth for turgor pressure normalized ( P Norm ) growth rate across species. ................................ ............... 67
8 3 15 Interaction of irrigation regime and time of growth for turgor pressure normalized ( P Norm ) growth rate in Chinese elm ................................ ............... 68 3 16 Interaction of species and shoot age for osmotic potential at incipient IP ) during Season Two across species. ................................ .... 68 3 17 Interaction of species and shoot age for maximum turgor pressure (P max ) during Season Two across species. ................................ ................................ ... 68
9 LIST OF FIGURES Figure page 1 1 Photographs of Quercus virginiana taken in April 2011 at the Mid Florida Research and Educati on Center in Apopka, Florida ................................ .......... 36 1 2 Photographs of Ulmus parvifolia taken in April 2011 at the Mid Florida Research and Education Center in Apopka, Florid a ................................ .......... 37 2 1 Open side gutter connected sawto oth style greenhouse located at the Mid Florida Research and Education Center in Ap opka, Florida .............................. 48 2 2 Series of frequ ently irr igated trees (FI) ................................ ............................... 49 2 3 Series of lysimeter trees (LDI or LDI 2 Daily or LLI) ................................ .............. 50 2 4 Shoot age distinction of study trees for Ulmus parvifolia ................................ ... 51 2 5 Shoot age distinction of study trees for Quercus virginiana ............................... 51 2 6 Laboratory pressure volume (P V) curve station located at the Mid Florida Research and Education Center in Apopka, Fl orida ................................ ........... 52 2 7 x ) and subsequently estimated turgor p ) of frequently irrigated (FI) live oak in Season Two ................................ .............. 53 2 8 x ) and subsequently estimated p ) of frequently irrigated (FI) Chinese elm in Season Two. ................................ ..... 54 3 1 x ) and subsequently p ) of live oak in Season T wo ................................ ................................ ................... 69 3 2 x p ) of Chinese elm in Season Two. ................................ ................................ .......... 70
10 LIST OF ABBREVIATION S 1/r s S tomatal conductance w D ensity of water g G ravitational potential l L eaf water potential m M atric potential p P ressure or turgor potential P Norm T urgor pressure normalized s O smotic potential T Total water potential w W ater potential x X ylem water potential z E lectrical potential IP O smotic potential at incipient plasmolysis A P hotosynthesis ABA A bscisic acid E T ranspiration E o B ulk modulus of elasticity E max M aximum modulus of elasticity EST Eastern Standard Time ET E vapotranspiration rate ET A A ctual evapotranspiration rate Exp. 1 Experiment 1 Exp. 2 Experiment 2 F
11 FI F requently irrigated g G ravity g s S tomatal conductance h H eight hr Hour IRGA I nfrared gas analyzer LDI O nce daily well irrigated lysimeter LDI 2 Daily T wice daily well irrigated lysimeter LLI W ater limited lysimeter irrigation LSD Least Significant Difference mm M illimeters MPa M egapascals Max night M aximum night Min night M inimum night P max M aximum turgor pressure P n P hotosynthesis P V P ressure v olume r c C uticular resistance r i I ntercellular resistance r p S tomatal pore resistance SWC IP R elative symplastic water content at incipient plasmolysis z j C harge
12 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science DIURNAL RESPONSE OF SHOOT GROWTH TO TREE WATER STRESS By Emily Elaine Massey August 2011 Chair: Richard C. Beeson, Jr. Co chair: Michael E. Kane Major: Horti cultural Science Few studies have measured angiosperm shoot elongation, and fewer still have related elongation rates of trees to turgor pressure. The objective of this thesis was to quantify effects of turgor pressure on shoot elongation for two common Florida landscape trees, Quercus virginiana Mill. (live oak) and Ulmus parvifolia e Jacq. (Chinese elm). Methodologies were developed over a variety of irrigation regimes to measure and quantify turgor pressure influenced growth rates. Data were collected over two seasons. In Season One, h igher nighttime turgor pressures were expected to result in higher nighttime shoot elongat ion rates. Shoots of well watered trees elongated at rates 13 times higher than those of mildly water stressed trees. Elongation rates between day and night were s imilar. Turgor pressure normalized growth rates were greater d uring the day than at night. In Season Two, measured shoot elongation rates were similar between day and night under well watered conditions for both species. Timing of irrigation events segregated shoot elongation rates. For Chinese elm, regimes with night irrigation events produce d greater growth rates than dawn only, with rates similar between night and day within regimes.
13 For live oak, segregation occurred based on proximity to an irrigation event, with greater elongation occurring during the day or night period immediately afte r an event Turgor normalization accounted for these differences in growth rates in live oak. However turgor normalized rates for Chinese elm were greater during day than at night within an irrigation regime, and did not correspond with turgor pressures. Results suggest a turgor pressure threshold in Chinese elm above which increases in shoot elongation rate do not occur, and that this threshold is low. Thus growth for some species is directly related to turgor, and this can account for differences in d ay and night shoot elongation. Differences between species in regards to turgor pressure normalized growth rates may indicate cell wall loosening mechanisms also differ in live oak and Chinese elm. Further experiments should focus on validating this result Results from this study are expected to influence irrigation scheduling for live oak and Chinese elm and add to knowledge of turgor pressures role in shoot elongation of these two species.
14 CHAPTER 1 INTRODUCTION Plant growth is affected by internal physiological and chemical processes, which in plant water status, and plant water status impacts turgor pressure in cells (Kramer and Boyer, 1995) Maintenance of turgor pressure is required for plant growth (Anderson and Brodbeck, 1988) with decreases in growth measured when substrate water availability is reduced (Ripullone et al., 2009) Plant cell expansion is driven by two processes, which consist of the passive uptake of water and stress relaxation through biochemical inputs. These two process es interact in a continuous cycle of turgor driven elastic extensions and biochemical stress relaxation until growth ceases (Taiz, 1994) Mechanisms involving cell wall expansi on are currently under review with increasing knowledge being found as to the key players involved in cell wall loosening However, current measurements of cell wall extendibility are often hard to quantify in vitro given current method ology and the sensi tivity of short lived cell wall loosening proces ses within cell wall spaces (Cosgrove, 2011) While much is known about plant water relations and the phy siology of plant growth, few studies have quantified diurnal plant growth rates. Fewer still have related measured growth rates to turgor pressure. The research presented here examined relationships between turgor pressure and shoot elongation of expandi ng shoots for two commonl y grown landscape trees in the s outheastern United States ( Quercus virginiana and Ulmus parvifolia ). Results of this study will guide both nursery producers and landscape managers in the scheduling of irrigation for optimum or red uced shoot growth.
15 Water Potential Chemical potential is a measure of free energy available for work in a system, quantified in joules. I t is governed by the two laws of thermodynamics and calculated from differences between a substance and the same substance in a standard state (Taiz and Zeiger, 2006) C hemical potential is applicable to the movement of water in a plant when water is compared to its stand w ) within a plant, and is the measure of free energy of water divided by water volume (Jm 3 ) (Raven et al., 2005) Movement of water in plants is affected by gravity, osmosis, mechanical pressure, and/or matrix effects like surface tension Total water potential measurements are made using psychrometers or pressure chambers because Jm 3 is equivalent to pressure units such as pascals (Nobel, 2009) Typically in moist soils, di urnal water potential patterns in plants are U shaped with w close to zero at dusk and dawn and a maximum, negative midd ay level. For example, Koch et al. (1994) w measurements between 2 and 3 MPa for five rain forest trees, and Cline and Campbell (1976) found similar results for maple ( 2) and pine ( 2.5 MPa) species. T s ), pressure potential p m g z ) w s p m g z (Raven et al., 2005) Osmotic potential is the potential of water molecules to mo ve across a semipermeable membrane. Its magnitude depends on solute concentration because solutes lower the chemical potential of water. Free movement of water generally occurs from hypotonic solutions ( i.e., solutions with more water than solutes) to hy pertonic
16 solutions ( i.e., solutions with more solutes than water) or from areas of high to low water potential (Lambers et al., 2008) Osmotic potential can be measured directly using osmometers (Nobel, 2009) p ) is pressu re exerted by water against cell walls. Pressure w s gradients within plants cells. Turgidity is sustained in plants when ions, sugars, and other solutes accumulate in cell vacuoles. Solute w causing an influx of water into cells and an increase in rigidity (Lambers et al., 2008) Turgor pressure maintenance is important for many plant processes ( i.e., stomatal aperture, plant growth, and negative values or tension help pull water through xylem vessels) and i s measured using pressure probes or calculated from pressure volume curves (P V curves) (Nobel, 2009) Matric potential is a measure of how strongly water is held in soil pores and within cell walls and spaces. It is particularly large for dry surfaces and small surface areas ( i.e., cell wall spaces and soil pores) where adhesion ( i.e ., the attraction of water to the surfaces of other objects) is strong When adhesive forces are strong, the number of potential (Lambers et al., 2008) Matric potential is measured using tensiometers and pressure plates (Kramer and Boyer 1995) Gravitatio nal potential is generally only w ), acceleration due to gravity (g), and plant height (h ) with a vertical distance of 10 m equaling a change of w (Taiz and Zeiger, 2006) Electric potential is the amount of work required to move an electrical charge, and it is consideration when ions are present in a solution. Electrical potential is a product of charge (z j
17 constant (F), and electric potential difference across the membrane at equilibrium (Nobel, 2009) Generally, matric and gravitational potentials are ignored from water potential equations for predicting water movement in short plants, given their negligible impact w s p z becomes important only when ions are present. Water Balance in Plants In addition to internal processes, the environment external to plants has a major impact on plant water balance. The plant water cycle begins with water movement through soil and ends with water vapor exiting plant stomata into the atmosphere (Lambers et al., 2008) This cycle is driven by transpiration and is explained by the cohesion tension theory. In this theory tension, along with cohesive ( attraction of water to other water molecules) and adhesive properties of water, creates a pull on water from the soil into plant roots and throughout the plant (Taiz and Zeiger, 2006) Water Movement from Soil to R oots The plant water cycle begins in the soil. Water movement in soils depends on soil type and structure because components of soil determine pore spaces sizes and channels between individual particles of soil. Movement of water in soils is dictated by s and hydrostatic pressure, with the former being normally negligible due to the low solute concentrations in soils (Kramer and Boyer, 1995) As water is depleted from soils, adhesion increases between water and soil particles and water becomes increasing harder to remove from soil. Water transport into roots occurs via passive transport due to negative hydrostatic pressure generated by tra nspiration and water potential gradients across tissues. Root hairs increase surface area available for water absorption. Movement of water through
18 root tissue occurs through two pathways: apoplastic ( i.e., movement through intercellular spaces) or sympl astic ( i.e., movement through spaces within cellular membranes via plasmasdesmata and aquaporines) (Nobel, 2009) Plasmodesmata and aquaporins are more efficient than osmosis through lipid bilayers (Taiz and Zeiger, 2006) Plasmodesmata are microscopic memb rane lined channels connecting neighboring cells (Raven et al., 2005) Aquaporins are water selective channels consisting of proteins embedded in plasma membranes which facilitate active transport of water into cells (Taiz and Zeiger, 2006) Water Movement in Xylem Once water transverses the root cortex, it travels through an extensive number of hollow, dead tubes called tracheids and/or vessel elements. Flow of water through this path encounters little resistanc e and is driven by negative hydrostatic pressure gradients generated by leaf transpiration (Kramer and Boyer, 1995) However, large generated tensions in xylem may cause weakened cell walls to collapse or lead to increases in air bubble formation or embolisms. Embolism s lead to cavitation ( i.e., breaks in water continuity transport through xylem tissue). Many mechanism have evolved in plants to cope with these situations ( i.e., constant formation of new xylem in a secondary growth and development of pit membranes in ve ssel elements) (Lambers et al., 2008) Capillary rise also contributes to water flow through xylem tissue. Xylem narrows as water flows up a tree through the wood, branch junctions, and leaf venation. This narrowing of the xylem cou properties allows for water to flow against gravity (Taiz and Zeiger, 2006)
19 Water Movement in Leaves Water exits xylem elements in leaves and moves into mesophyll cells and intercellular air spaces using a process similar to water movement in soils. Water w gradients generated by relative water vapor pressure differences between leaf substomatal cavities (~100% relative humidity) and bulk air, completing transpiration (Lambers et al., 2008) Plant Gas Exchange Transpiration Transpiration is affected by conditions external to ( i.e., relative humidity and temperature) and within leaves. Boundary layer resistance also affects transpiration. Boundary layer resistance is related to length of water vapor diffusion due to a layer of unstirred air adjacent to the leaf surface. Resistance generated from boundary layers is proportional to layer thickness. It is dependent on wind speed; canopy thickness; and leaf size, shape, and surface. Resistances within plant leaves are cuticular (r c ) and stomatal (r s ) ( i.e., sum of intercellular space (r i ) and stomatal pore (r p ) resistances) (Kramer and Boyer, 1995) Stomatal Conductance Stomatal conductance (g s ) is a measure of flux rate of water vapor inside a leaf normalized by the water vapor pressure difference between the two end points Stomatal conductance, which is equivalent to the inverse of stomatal resistance, is a function of stomatal density and stomatal p in plant guard cells (Kramer and Boyer, 1995) Generally stomata open at dawn when photo receptors in guard cells are activated by
20 blue light. This activation triggers an increase in potassium concentration and water flows into guard cells increasing turgidity and stomatal aperture (Taiz and Zeiger, 2006) As water vapor escapes thro ugh open stomata, carbon dioxide diffuses into plant leaves. Patterns for g s follow an inverted U shape with leve ls at dawn and dusk at zero and maximum levels peaking around midday (Wullschleger et al., 2000) Water loss usually peaks around midday in response to maximum vapor pressur e gradients, triggering stomatal closure in isohydric tree species (Tardieu and Simonn eau, 1998) Carbon dioxide assimilation begins to slow as stomatal aperture decreases (Larcher, 1980) In anisohydric tree species, stomatal aperture remains wide and closure is not w rates approach a maximum negative value (Tardieu and Simonneau, 1998) Isohyrdric and anisohydric are terms used to describe the extent of tissue hydration under c hanging environmental conditions. Typically, isohydric plants have great control over transpiration rates, and anioshydric plants are less sensitivity to transpiration rates (Franks et al., 2007) This value varies by species. By dusk, most, if not all stomata are clo sed in response to declining light levels. Guard cells close and carbon assimilation ceases (Raven et al., 2005) Stomatal conductance not only varies diurnally and in response to environmental factors, but also among species. Stomatal cond uctance of 140 316 mmol m 2 s 1 were measured in Eucalyptus pauciflora over a three month period (Korner and Cochrane, 1985) Gallego et al (1994) reported g s of around 150 200 mmol m 2 s 1 for Quercus pyrenaica Andrade et al (1998) found similar rates to those of Q. pyrenaica in Ficus insipida Luehea seemannii and Spondias mombin (178 211 mmol m 2 s 1 ); with lower rates in Anacardium excelsum and Cecropia longipes (35 125 mmol m 2 s 1 ). Stomatal
21 conductance varied depen ding on propagation method for Ulmus glabra (~51 98 mmol m 2 s 1 for micropropagted and grafted species respectively ) (Durkovic et al., 2010) Photosynthesis In plants, a balance between minimizing water loss by transpiration and sustaining carbon dioxide diffusion for assimilation is controll ed by stomatal aperture Assimilation of carbon dioxide, in part, determines photosynthetic (A) rates in autotrophic organisms. energy in plant chloroplasts (Raven et al., 2005) Carbohydrates produced during A are important for biomass accumulation and growth in plants (Taiz and Zeiger, 2006) Diurnal photosynthesis (mol m 2 s 1 ) pat terns resemble those of g s with A being close to zer o at dawn and dusk and maximum A occurring just prior to midday. Maximum net photosynthetic rates were measured at ~22 mol m 2 s 1 for Picea abies and ~16 mol m 2 s 1 for Quercus pubescens respectively (Haldimann et al., 2008; Maier Maercker, 1998) Photosynthesis for three year old grafted and micropropagated Ulmus glabra were ~33 and 23 mol m 2 s 1 respectively (Durkovic et al., 2010) Plant Growth Plant growth is defined as irreversible enlargement of cells (Hsiao et al., 1976) Growth consists of cell division and cell elongation. Cell division occurs when a parent cell divides into two or more daughter cells. Mitotic processes result in production of cells. Mitosis consists of five distinct phases: interphase, prophase, metaphase, anaphase, and telophase. Cytokinesis is the final step to this process (Raven et al., 2005) During cell elongation, plant cells can enlarge from 10 to 10,000 fold in volume from meristematic to mature cells depending on their function. Fa ctors influencing cell expansion are cell type, age, auxin and gibberellin concentrations, and environmental
22 conditions such as light levels and water availability (Taiz and Zeiger, 2006) Two processes must take place for plant elongation to occu r. First, water must be taken up by the elongating cells. Second, cell wall area must increase. Plant Growth: Cell Wall Expansion and Turgor Pressure Restriction of growth is typically caused by plant cell walls Cell walls of plants consist of cellulo se microfibrils embedded in a matrix of pectin, hemicellulose, and proteins (Taiz, 1994) However cell walls of growing cells are typically less ridged than those of non growin g cells. Stress relaxation is the key to cell wall expansion. First, turgor pressure inside cells increases as water flows into growing plant cells from adjacent cells due to water potential gradients. This increase in pressure creates a strain or tensi on to build up in cell walls This strain is relieved through stress relaxation via biochemical input (Taiz, 1994) Wall loosening enzymes are thought to be triggered by low pH in the cell walls, which is promoted in these areas by the presence of auxin induced protein extrusion These enzymes cleave components of the cell wall matrix and allow cell wall extension (Hager et al., 1971; Rayle and Cleland, 1970) Initially, hydrolases ( i.e. cellulases, hemicellulases and pectinases) were thought to be the enzymes responsible for cleaving of bonds within the cell wall, but now transglycosylases such as xyloglucan endotransglycosylase are thought to be the primary enzymes involved Transglycosylases are thought to be key players in elongation due to their ability to lay down cellulose in a biased orientation as bonds are systematic weakening between cell wall polymers of load bearing walls (Bonner and Varner, 1976; Carpita and Gibeaut, 1993; Fry, 1989; Fry et al., 1992; Smith and Fry, 1991) Later studies focused on one particular protein termed expanins which appear ed to induce cell wall extension Like
23 hydrolases and transglycolases expansins are activated by low p H However unlike hydrolases, expansins extend cell walls without causing time dependent weakening (Cosgrove, 1989) Expansins are thought to disrupt hydrogen bonds between cellulose fibers, causing slippage to occur between cellulose microfibrils and the polysa cc haride matrix As this slippage occurs, polysaccharides shift their positions in response to the orientation of cellulose microfibrils and this creates the formati on of new hydrogen bonds to form decreasing stress in cell walls (McQueen Mason and Cosgrove, 1994; Taiz, 1994) Various research has outlined w ays to measure cell wall mechanics of growing plant cells (Burgert, 2006; Cosgrove, 2011; Geitmann, 2006) Extensibility is measured by extending a clamped specimen in various ways while recording cell wall behavior. These processes are typically car ried out in a extensometer (Cosgrove, 1993; Geitmann and Ortega, 2009) However, despite the efforts of these studies, cell wall extensibility cannot be fully measured in vitro due the fact that cell wall extensibility is a multifaceted process consisting of cell wall mechanics, which can be inferred from extendibility, and wall loosening processes. The latter is hardest to measure because cell wall loosening processes are influenced by short lived conditions within cell walls, which may be further dependent on plant species and plant environment (Cosgrove, 2011) Plant Growth: Turgor Pressure Studies indicate that turgor must be maintained for growth to occur. Anderson and Brodbeck (1988) found in newly expanding flushes in pe cans that leaf elongation rate was exponentially related to turgor pressure with maximum leaf elongation occurring at higher, overnight turgor pressures. Furthermore, Zwack et al. (1998) found when three Freeman maple cultivars were exposed to drought conditions that relative shoot
24 extension growth decreased in the morning and increased during the afternoon. B ulk turgor pressure seems to play less of a role in elongation in many non woody species. Leaf elongation of maize and sorghum was l ower during higher nighttime turgor pressures. This slower elongation rate was attributed to temperature ( < 12 C) and not p because early in the morning they saw a noticeable increase in leaf p (Acevedo et al., 1979) Leaf elongation rates in rice were also greater during the daytime, despite a lower bulk turgor during t he daytime, when nighttime temperatures fell below 27 o C When nighttime temperatures were above this temperature threshold, nighttime elongation was greater (Cutler et al., 1980) Plant Growth: Elongation Rates Few studies have reported short term diurnal shoot growth rates (mm h r 1 ), and of those conducted there appears to be great variability in grow th rates among species. Borchert (197 6) measured pin oak sapling growth rates of 0.7917 mm h r 1 Reich et al. (1980) measured stem height growth rates from 0.0164 to 0.0424 mm h r 1 in blackjack oak seedlings. Field grown Zea mays leaf elongation rates fluctuated diurnally (0.4 to 3.6 mm h r 1 ) wi th minimal rates occurring from 0000 0500 hr (10 12 C) and maximal elongation at 1200 h r (32.5 C). Rates of growth can vary throughout a growing season depending on the plant species. Kramer (1943) reported that growth of young Fraxinus americana Juglans nigra Quercus alba and Q. borealis in North Carolina occurred in recurrent waves (eposidial), whereas, some tropical evergree n tree species continuously produce new growth throughout the growing season (Koriba, 1958) In addition to variable growth rates, much debate exists over daily diurnal patterns of shoot elongation. Studies of spruce, pine, and pecan indicate that shoot and leaf elongation occurred mostly at night (Anderson and Brodbeck, 1988; Fielding, 1955;
25 Mork, 1941) while other studies of woody species, maize, and sorghum indicated that a majority of shoot growth occurs during the day (Acevedo et al., 1979; Danilov, 1954) Irrigation Effects on Pla nt Growth Many studies have sought to maximize growth and marketability of plants by optimizing irrigation regimes. Beeson (1992) showed Rhodo dendron sp. and Elaeagnus pungens cyclically irrigated to 100% container capacity produced significantly greater growth than plants overhead irrigated once per day. Cyclic irrigation also increased rates of height growth and trunk diameter in Magnolia gra ndiflora compared to plants irrigated once per day (Beeson and Keller, 2003) Amounts of water supplied during irrigation also affect plant growth. Doubling irrigation volumes significantly increased growth of Acer rubrum and Lagerstroemia indica compared to plants overhead irrigated and plants micro irrigated using 50% of the o verhead irrigation volume (Beeson and Haydu, 1995) These results suggest direct spray stake irrigation and cy clical irrigation increased substrate water availability for plants throughout the day, maintaining a less negative plant water potential and therefore likely higher turgor pressures. This in turn led to increased plant growth rates. However these exper iments examined irrigation regimes implemented during the day. Irrigation at night has been shown to offset some of the reduction in growth due to decreased substrate water availability and may lead to increased water efficiency (Yacoubi et al., 2010) and growth in plants due to higher nightly turgor pressures (Anderson and Brodbeck, 1988) Water Stress Water stress occurs when plants are unable to absorb adequate amounts of water due to environmental or physiological constraints. Irrigation is often supplied to plants
26 to mitigate water stress or put an end to it. Water stress can be a result of drought, increased salinity of soils, and under irrigation. Wilting occurs when plants lose water pressure of turgidity and it is common sign of water stress. When plants receive inadequate amounts of water, problems may arise such as hormonal, biochemical, or physiological changes; the production of fewer or smaller leaves; lower crop yields; a slowing or decrease growth in shoot growth; or death (Simpson, 1981) Water Stress Effects on Plant Growth The process most sensitive to water stress is cell growth, consisting of cell division and cell enlargement, of which the latter is the most affected in plants (Hsiao and Acevedo, 1974) Enlargement occurs through uptake of water. When plants experience water stress, water uptake may slow and growth may be retarded or stop in response to lowe r water potentials and a loss of turgor pressure (Loc khardt, 1965) However, decreases in growth are often dependent on stress severity and plant species. Zwack et al. (1998) reported that leaf elongation was lower for drought stressed Freeman maple cultivars (19 to 87 cm for drought and control trees, respectively). While other studies suggest water stress may vary among species and growth patterns. Lotan and Zahner (1963) found late summer drought had no significant effect on Pinus resinosa height during the current season of growth. Whereas, Zahner (1962) found late summer drought significantly reduced the number of growth flushes and height of Pinus taeda Kozlowski (1982) suggested this is s pecies mediated where plants with one fixed growth cycle exhibit affects in the subsequent growth season, and plants with rec urrent flushes tend to exhibit e ffects in the same growth season.
27 Several species of pines and oaks exhibit mechanisms to countera ct a loss in turgor (Danilov, 1954; Kozlowski, 1964) For instance, Bahari et al. (1985) found that under drought stress, several oak species displayed restricted leaf conductance and s at full turgor. Growth of leav es and shoots are particularly susceptible to cessation for many species during periods of water deficit; however, roots are more variable with either slowing or slight increase in growth occurring (Raper and Kramer, 1983; Xu et al., 2006) Maintenance of Turgor Pressure: Osmotic Adjustment When water loss exceeds water supply and cell water volume decreases, loss of cellular turgor may result and plant processes other than growth may be affected, such as reduction in stomatal aperture and metabolic processes. Osmotic adjustment can maintain turgor pressure in plant cells. Osmotic adjustment is characteri zed by an increase in the concentration of solutes such as potassium, amino acids or amino acid derivatives, and/or sugars in cell vacuoles (Kramer and Boyer, 1995) Increased w within the cells. The resulting water potential gradient between the cells and adjacent xylem causes an influx of water into the cells and allows for turgor to be maintained (Lambers et al., 2008) Osmotic adjustment initially occurs from starch and other components of the cell (Kramer and Boyer, 1995) with later solutes a ccumulated through photosynthesis (Wang and Stutte, 1992) Many studies have shown that osmotic adjustment may be a response to water stress in plants. Species of oak, pine, and rapeseed were able to maintain turgor longer through the accumulation of higher concentrations of solutes in the roots and shoots of water stressed species (Osonubi and Davies, 1978; Koppenaal, et al ., 19 91; and Meier,
28 et al. 1991; Norouzi, et al., 2008). Osmotic adjustments in leaves and the roots of three s in leaves of these species by 0.25 to 0.60 MPa (Parker and Pallardy, 1988) Some s shifts of oaks are not due to water stress, but due to seasonality or phenological responses with plants constantly undergoing changes to maintain biochemical and physiological balances in order to more efficiently operate (Abrams, 1990) Kwon and Pallardy (1989) looked at chronic stress on three oak species. Their data indicate that solute accumulation for some species of oaks may be limited under pe rsistent water deficit. Water Stress Effects on Stomatal Aperture Another process affected by water stress in plants is stomatal aperture. Typically this process is not as sensitive as processes involved in plant growth (Hsiao and Acevedo, 1974) Effects of water deficit on gas exchange and subsequent effects on w have been well documented for non woody and woody species. Generally pla nts close stomata to minimize water loss during stress and maintain an adequate water status. Stomatal aperture is mediated by availability of moisture in soils and vapor pressure of the air. Closure begins to take place prior to water stress in leaves. In drying soils, it can be initiated by root signaling via translocation of abscisic acid (ABA) from plant roots to leaves. If a suitable water status can be maintained, preservation of turgor may persist and wilt may be avoided (Lambers et al., 2008) Abscisic acid also is implicated in other plant processes and is not solely synthesized in the roots of plants. x ), measured at the leaf level also decrease (Mazzoleni and Dickmann, 1988) Under a prolonged water l ) and g s for young oak seedlings declined
29 sharply (Fort et al., 1997) l was similarly found in three Fagaceae species during the peak drought period (Abrams et al., 1990) Water stress may lead to full or partial stomatal closure with the severity mediated by species and environmental conditions. Water Stress Effe cts on Photosynthesis While stomatal closure minimizes water loss, a decrease in carbon assimilation and subsequent photosynthetic rates may also result. Studies have also shown that when stomates close, or when turgor in plants is low, carbon dioxide in the sub stomatal cavity decreases (Omae et al., 2007) Decreases in ca rbon dioxide mean plants will have a reduced photosynthesis rate. During a water reduction experiment, Gu et al. (2007) demonstrated a strong relationship in the reduction of g s and P n rates for four birch genotypes. Decreased evapotranspiration rates (ET), g s and P n rates were also measured in four drought stressed oleander clones (Niu et al., 2008) Reduced carbon fixation will limit biomass accumulation in most cases and plants may all ocate less carbon and assimilates towards growth. Water deficits lead to a slowing of shoot elongation and often an inhibition or slight increase in root elongation through a variety of mechanisms (Raper and Kramer, 1983) Other studies indicate that a majority of fixed carbon is allocated to root elongation over elongation of shoots. A decrease in water availability reduced shoot dry weight and increased root dry weight for three grass seedlings (Xu et al., 2006) Decreases of 15 20% in precipitation reduced shoot elongation of plants in a Mediterranean macchia due to increased hydraulic resistance in the so il plant continuum (Ripullone et al., 2009)
30 Quantification of Water Status, Gas Exch ange, and Plant Growth Pressure Chambers Pressure chambers are devic es used to measure x ) in plants. Leaves or shoots are typically severed from plants and placed inside an airtight pressure chamber with petioles or stems exposed o utside through a seal. Compressed gas, typically nitrogen, is forced into a chamber until water appears at the cut end of the exposed tissue. Pressure amounts inside devices are indicated on a gauge. This device works because water in plant xylem is und er tension. When samples are collected, water within xylem recedes deeper in the sample. As pressure is increased in a chamber, water is forced from the sample to the cut end if the stem. The corresponding pressure is equal and opposite to tension withi n the sample at this time (Nobel, 2009) Pressure Volume Curves Water parameters in plants can be assessed using pressure volume (P V) curves. Pressure p s to cha x (Nobel, 2009) Curves are constructed using pressu re chambers and plant structures such as leaves, shoots, or roots (Dreyer et al., 1990; Husken et al., 1978; Kandiko et al., 1980) Tissue removed from plants is fully hydrated shortly after removal, and periodic measurements of weight and balancing pressure readings are taken as the ti ssue is dehydrated past the turgor loss point. Dehydration can be caused by evaporation or by incrementally expressing xylem sap by subjecting the tissue to increased pressure s Reciprocals of balancing pressure are calculated and plotted against a measu re of the tissue water content or volume (Abrams, 1988b) Pressure volume curves are often
31 used to estimate relationships b w components and water tissue contents (Tyree and Jarvis, 1982) Some of these components include effects of turgor pressure on cell size and elasticity (Steudle et al., 1977) Other parameters can also be ascertained like symplastic water content. This is a measureme nt that quantifies amounts of water in the symplast, the space within the plasma membrane of the living cells (Nobel, 2009) Values of 0.81, 0.73 to 0.84, 0.69 to 0.86, 0.77 and 0.82 have been measured in shoots of U. parvifolia leaves of Juglans nigra leaves and shoots of Q. alba expanding leaves (Kandiko et al., 1980; Parker and Pallardy, 1985; Parker and Pallardy, 1987; Ranney et al., 1991; Ranney et al., 1990) A related parameter is the percentag e of apoplastic water, which is a measurement that quantifies the percentage of water within the apoplast or outside plasma membranes and in the intercellular cell spaces (Raven et al., 2005) Relative symplastic water content represents amounts of water within intracellular for expanding and mature tissue (Ranney et al., 1991) 0.78 to 0.83 in leaves and shoots of Q. alba (Parker and Pallardy, 1987) and 0.81 in U. parvifolia respectively (Ranney et al. 1990) W hereas relative symplastic water content at incipient plasmolysis (SWC IP ) is a measure of water volume within intracellular spaces as plasma membranes begin to withdraw from cell walls and turgor is lost inside the cell. Low relative symplastic water volu me at incipient plasmolysis suggests cell walls are more flexible, meaning cells of this tissue could lose more water before plasma membranes retreat from cell walls, thus allowing for this immature or actively growing tissue to
32 maintain turgor for a longe r period of time (Nobel, 2009) Values ranging from 0.17 to 0.22 have been measured in leaves and shoots of Q. alba (Parker and Pallardy, 1987) and values of 0.141, 0.208, 0.186, 0.238, and 0.156 for Q. robur Q. petraea Q. pubescens mature Q. ilex shoots, and in immature Q. ilex shoots, respectively (Dreyer et al., 1990) Corcuera et al. (2002) also calculated higher values of 0.92, 0.85, and 0.73 SWC IP for mature shoots of Q. petr aea Q. pyrenaica and Q. cerris respectively. Osmotic potential at incipient plasmolysis IP ) is also determined from P V curves. This is a measure of solute concentrations in cells (Nobel, 2009) Osmotic adjustment of water stressed plants is a way to ma intain turgor at lower water potentials (Kramer and Boyer, 1995) Measurements of 1.37 and 1.84 MPa have been observed (Ranney et al., 1991) Similar values were also measured in leaves and shoots of Q. alba seedlings ( 1.62 to 1.77 MPa) (Parker and Pallardy, 1987) leaves of J. nigra seedlings ( 1.42 to 1.58 MPa) (Parker and Pallardy, 1985) and in terminal shoots of U. parv ifolia ( 1.30 MPa) (Ranney et al., 1990) Maximum turgor pressure (P max ) within cells is collected from P V curves as wel l. Beeson and Gilman (1992) quantified turgor pressures of 2.5 MPa for Quercus virginiana Bulk modulus of elasticity (E o ) estimates cell wall rigid ity in plant samples. Low bulk modulus of elasticity indicates flexibility of cell walls, which aids in the ability of immature tissue being able to elongate and grow. Ranney et al. (1991) reported E o values of 7.92 MPa for expanding leaves and 10.23 MPa cherry. Values ranging from 3.61 to 8.73 MPa have been reported for leaves and shoots of Q. alba (Parker and Pallardy, 1987) and 9.15 MPa for shoots of U. parvi folia
33 (Ranney et al. 1990) Maximum modulus of elasticity (E max ) measurement is similar to the bulk modulus elasticity a nd is a measure of the rigidity of the plant tissue (Nobel, 2009) Parker and Pallardy (1985) reported values of 2.45 to 11.71 MPa for leaves of J. nigra Porometers and Inf rared Gas Analyzers Stomat al conductance is quantified using a porometer. Portable steady state porometer systems compute g s by measuring the flux in water vapor through leaf stomatal pores. Photosynthesis is measured using portable gas exchange systems such an infrared gas analy zer. Infrared gas analyzers (IRGA) measure concentrations of specific gases such as carbon dioxide and water vapor in air sample. Machines determine concentrations or concentration differences by measuring amounts of absorption occurring as an infrared l aser is emitted through samples. Systems typically have two IRGA cells ( i.e., a r eference and a measuring cell ). A beam of infrared light is emitted at one end of the device. As it passes through, gases in the chambers absorb the radiation and the remaining unabsorbed laser quantity is measured. Internal calculations determine the differenc e between the emitted radiation and the amount of radiation not absorbed (Burough, 1980) Plants of Study Quercus virginiana Quercus virginiana Mill., live oak or evergreen oak is a member of the Fagaceae, beech and oak family (S mith, 1977) Quercus is Latin for oak and virginiana refers to Virginia, where this species is native (Watkins et al. 2005) Q. virginiana is widely distributed along warmer c o astal regions of North America with a native range restricted to the Atlantic coast from southeastern Virginia to Florida and along the Gu lf
34 Coast from Florida to Texas Q virginiana is found in diverse habitats, but usually grows in well drained sandy coastal areas (Cullina, 2002) It is found throughout Florida (zones 7 10), and is plentiful in hammocks, scrubland, and lake margins (Cullina, 2002; Taylor, 1988; Watkins et al., 2005) Known for its low slung, horizontal b ranches festooned with Spanish moss and other bromeliads, Q. virginiana has evergreen leaves that persist throughout the year with flushes occurring from March to September in Florida and (Brockman, 2001; Taylor, 1988) (Figure 1 1 ). Height is generally 40 80 feet, but a variety of different forms ( i.e ., shrubby or dwarfed to large and spreading) can persist depending on location (Brockman, 2001; Cullina, 2002) Q. virginiana is mainly propagated from seed (Watkins et al., 2005) It is considered to b e a fast growing species when young and under optimal growing conditions (Maccubbin and Tasker, 1997) This spe cies is highly drought tolerant and has potential to root deeply, with the latter being an important trait for trees native to United States regions prone to hurricanes (Watkins et al., 2005) In the 1800s and 1900s, the durable wood of Q. virginiana was used in ship building (Maccubbin and Tasker, 1997) Present ly Q. virginiana is a popular ornamental landscape trees in Florida residential areas and along roadsides because of the drought tolerant nature and shade produced from the low, wide crown (Maccubbin and Tasker, 1997; Taylor, 1988; Watkins et al., 2005) Ulmus parvifolia Ulmus parvifolia Jacq., Chinese elm or lacebark elm, is a member of Ulmacea, elm family (Smith, 1977) Ulmus is Latin for elm and parvifolia refers t o the small leaves (Watkins et al., 2005) U. parvifolia is native to the O rient (Brockman, 2001; Watkins et al ., 2005) U. parvifolia is distributed throughout the southern and central United States
35 and along Atlantic, Gu lf, and Pacific coasts. It is planted throughout Florida (zones 6 9), but grows best in northern and central Florida on well drained s oils (Taylor, 1988; Watkins et al., 2005) This species is known for having intricately patte rned, exfoliating bark and a weeping growth habit with semi evergreen leaves that typically flush from mid March to August in Florida (Brockman, 2001; Watkins et al., 2005) (Figure 1 2 ). Height is generally 20 60 feet (Brockman, 2001) U. parvifolia is propagated by seed and from cuttings (Watkins et al., 2005) e c ultivars were released in the late 1990s. Cultivars were cloned from a tree located on the University of Georgia campus in Athens (More and White, 2002) Chinese elm is considered to be a fast growing species when young and under optimal growing conditions and can grow u p to 3 feet per year (Taylor, 1988) (Figure 1 2 ). U. parvifolia has a high drought tolerance and appears to h ave a relatively shallow root system (Watkins et al., 2005) e cultivar is a common landscape tree in Florida It is planted in buffer strips around parking lots and in h ighway medians due to this species tolerance of drought (Watkins et al., 2005) In the future Chinese elm might be used to combat Dutch elm disease due to resistance and given that it will hybridize with American elm species (More and White, 2002)
36 Figure 1 1. Photographs of Q uercus virginiana taken in April 2011 at the Mid Florida Research and Education Center in Apopk a, Florida. A) Mature tree. B) Mature leaves. Photos are courtesy of Emily Massey
37 Figure 1 2. Photographs of Ulmus parvifolia taken in April 2011 at the Mid Florida Research and Educatio n Center in Apopka, Florida A) Mature tree B) Colorful trunk bark Photos are courtesy of Emily Massey. B B
38 CHAPTER 2 MATERIALS AND METHOD S Season One In May 2010 2.5 year old seedlings of Quercus virginiana Mill. (live oak) and Ulmus parvifolia e Jacq (Chinese elm) were obtained from Cherry Lake Tree Farm in Groveland, Florida. Trees were transplanted from #3 air based root pruning containers to #7 black polyethylene containers (Nursery Supply, Co., Inc., Spartanburg, SC) using a substrate mixture of 60:40:10 (pine bark:Nupeat:coarse sand; Florida Potting Soil, Apopka, FL) with 2.9 kg of dolomite limestone and 0.86 kg of micronutrients per m 3 (Micro Max, Scotts Miracle Gro Co. Marysville, OH). The through a 12.5 mm screen. Trees were grown in full sun at the M id Florida Research and Education Center in Apopka, FL and micro irrigated 3 times per day with two individual spray stakes per container (blue nursery pot stakes, 0.66 Lpm at 133 kPa, Maxijet Inc., Dundee, FL). In January 2011, remaining trees were trans planted into #10 black polyethylene containers (Nursery Supply, Co., Inc.) using the same substrate blend (Florida Potting Soil). Trees were fertilized throughout both seasons of data collection (Table 2 1). Data were collected during periods of shoot el ongation. In the summer of 2010, data collection began in early June. Six trees of each species were housed in an open side gutter connected sawtooth style greenhouse (18.5 x 24.6m) aligned east x west (Figure 2 1). Trees were randomly arranged in two r ows aligned north x south with three control trees and three lysimeter trees of each species within each row. Control
39 trees were placed in wire basket stands above ground (Figure 2 2). Lysimeter trees were individually placed in suspension weighing lysim eters to monitor actual evapotranspiration (ET A ) and for individual irrigation control (Figure 2 3). A lysimeter consisted of a S shaped aluminum mini load cell (SSM 100, Interface Force, Inc., Scottsdale, AZ) suspended from a tripod support structure. A tree was suspended from a load cell by three chains that extended to a basket within a dry well placed in the ground such that the top of a container was about ground level (Beeson, 2011) Lysimeter data was collected and irrigation of lysimeters controlled with a data logger (CR10X, Campbell Scientific, Inc., Logan, UT) attached to a multiplexer (AM 16 32, Campbe ll Scientific, Inc., Logan, UT) and a control port module (SDM CD16AC, Campbell Scientific, Inc., Logan, UT). Mass of each tree was recorded every half hr At midnight ET A of each tree was calculated by subtracting mass at midnight from that recorded a t 0500 hr EST (Eastern Standard Time). Irrigation volume was 125% ET A which was applied in three sub volume s ( midnight, 0100, and 0200 hr EST ) Irrigation of control trees was managed by a Sterling time clock (Superior Control Co., Inc., Valencia, CA) with one solenoid valve controlling irrigation for both species. Irrigation was applied at 1030, 1500, 2100 hr EST daily using two blue pot stakes (Maxijet Inc., Dundee, FL) per tree. Irrigation was applied for 5 minutes each cycle to insure thorough re wetting of substrate. These trees will hereafter be referred to as frequently irrigated (FI). After data was collected under well watered conditions, a drought irrigation regime was imposed on lysimeter trees such that trees were slowly water stressed ( ~ 25 July 2010 and 30 July 2010 for live oak and Chinese elm, respectively) Drought stress was
40 initiated by applying nightly irrigation equivalent to 90% of d aily ET A in three sub volumes as described previously. When daily ET A had declined to about 90% of the well irrigated rate, nightly irrigation was increased to 105% of daily ET A to maintain the water stress level. Hereafter, well watered lysimeter trees will be referred to as once daily well irrigation lysimeter (LDI) and water stressed lysimeter trees will be referred to as water limited lysimeter irrigation (LLI). Shoot Growth Measurements Six elongating branches were selected on each tree. Branches were selected throughout a canopy. Lengths (millimeters) of elongating branches were measured at dawn (~0530 hr EST) and dusk (~1930 hr EST) using a CD caliper (Mitutoyo Corp., Japan). Elongation was recorded for five days and growth rates (mm/hr 1 ) were calculated for day and nighttime measurements Initial data was collected o n FI and LDI trees. After water stress had been imposed and stabilized, shoot elongation measurements were collected for an additional five days at dusk and dawn for the same six branches on LLI trees. Xylem Water Potential Measurements x ) was measured during four collection s of shoot elongation measurements. Measurements were recorded every two hr s from pre dawn (~0500 hr EST) to post dusk (~2100 hr EST) weather permitting, and collected from FI and LDI trees only. Measurements were concluded ~1900 hr EST when evening weather was cloudy or rainy. Measurements were made on individual, mature stems Soilmoisture Equipme nt Corp., Santa Barbara, CA). Shoots were selected throughout a canopy. Shoots to be measured were covered with plastic wrap and aluminum foil one
41 hr w x (Begg and Turner, 1970) Covered stems were removed from a branch with sharp shears (FELCO 2 Green Felco SA Corp., Les Geneveys sur Coffrane Switzerland) and enclosed in the pressure chamber within 45 seconds. Pressure was increased at a rate of 25 kPa s 1 using compressed nitrogen until water was just visible at the cut end. Pressure Volume Curves Three to four mature individual shoot s (~1 1 cm long) were collected for constructi on of pressure volume (P V) shortly after bud break for each tree. Bud break occurred in recurrent flushes from around March thr ough September for both species. P ressure volume curves were constructed just prior to shoot elongation measurements for well watered trees. Mature tissue was tissue with fully expanded leaves and a stem that appeared to be lignified throughout ( Figure 2 4 and Figure 2 5). Morning samples were taken throughout the canopy. Ends of stems w ere re cut under water and placed in a 200mL beaker so that freshly cut ends were submerged in water. Plastic bags were secured over the tops of beakers and tissues were placed at ~3 o C for >24 hr s to re saturate tissues and minimize transpiration (Ranney et al., 1990; Turner, 1988) Water poten tial was measured as described above (Figure 2 6) Volume of water loss was determined prior to pressurization by weighing each sample using an analytical balance (Mettler AE100, Mettler Toledo Inc., Columbus, OH). Samples transpired freely on laborato w measurements. Pressure volume curves were constructed using Microsoft Excel 2010 (Microsoft Corp., Redmond, WA) and calculations were made as described previously (Meier et al., 1992; Tyree and Hammel, 1972; Urban et al., 199 3)
42 Similar technique was followed for immature tissue with P V curves constructed in the same manner. Immature growth was collected when new shoots had reached ~10 cm. Immature tissue had fully expanded leaves and a fairly flexible stem throughout (Figure 2 4 and 5). Lignified tissue (~2 cm) was needed on the proximal end of the stem for proper insertion into the pressure chamber. Relation of Turgor Pressure to Shoot Growth p x measurement for each t ree based on mean values from constructed P V curves for that tree (Fig. 2 6) p were plotted against corresponding balance pressures for each tree with mature and immature growth data analyzed separately. Equations were calculated using line ar regression for each tree and age of tissue. p x measurements and corresponding reg p x p resulted. These values were recorded as zero All p p values calculated were not altered. p were integrated over time (MPa hr) for each tree. Measurement collection never extended past 2030 EST. Instead midnight measu rements were based on x of the follow ing morning prior to sunrise Integrated values were divided into respective growth measurements to calculate pressure normalized growth rates (mm MPa hr 1 ) (Anselmi et al. 2004) Turgor pressure normalized ( P Norm ) growth rates were calculated for measurements recorded on trees under well watered conditions only. All calculations were made and the graphs were constructed using Microsoft Excel 2010 (Microsoft Corp., Redmond, WA).
43 Leaf Gas Exchange Measurements Transpiration rate (E; mmol m 2 s 1 ), and net photosynthesis (A; mol m 2 s 1 ) were measured using a n ADC LC4 LC i portable gas exchange system (BioScient ific Ltd., Mountain View, CA). Calibration of the syste m occurred two years prior. Measurements were only collected from FI and LDI trees. Measurements were made at 0900, 1130, 1415, 1630, and 1900 hr EST on sunny days. Data collection for trees were on the 7 July 2010 (LDI elms), 24 June 2010 (FI elms), 7 July 2010 (LDI oaks), and 26 July 2010 (FI oaks). Samples were collected from the entire canopy. Leaves were fully expanded, mature, and located in full sun. These measurements were integrated over time to estimate a daily transpiration and net photosyn thesis for each tree (Myers, 1988) Stomatal conductance (g s ; mmol m 2 s 1 ) 2 s 1 ) were measured using a Li 1600 portable steady state porometer (L i Cor Inc., Lincoln, NE). Initial measurements were collected from FI and LDI trees with subsequent measurements collected for LLI after trees were placed under water stress (~25 July 2010 and 30 July 2010 for live oak and Chinese elm, respec tively) Measurements were made between ~1200 and ~1300 EST on sunny days. Data collection for trees were on the 8 July 2010 (LDI oaks), 26 July (FI oaks and LLI oaks), 31 July 2010 (LDI elms), and 2 August 2010 (FI elms and LLI elms). Leaves were fully expanded, mature, and located in full sun throughout the tree canopy. Season Two In 2011 new trees in #10 containers were randomly arranged in the same experimental system described above. FI trees were subjected to the same pro tocol described in Seas on One. L ysimeter trees were initially irrigated using a different well
44 watered regime from that of 2010. Under this new regime, ~50% of ET A occurring before solar noon (1300 hr EST) was applied individually for each lysimeter tree. At midnight (EST), d aily ET A was calculated for each tree as the difference in mass between 0600 hr and midnight EST plus the volume of the irrigation applied at solar noon. Night irrigation was calculated by multiplying daily ET A by 1.3, then dividing by 2, with half applie d at midnight and the other half applied at 0015 hr (Exp. 1 ). Hereafter, well watered lysimeter trees from Season 2 will be referred to as twice daily well irrigation lysimeter (LDI 2 Daily ). After data collection under the well watered regime, two additi onal irrigation regimes were imposed on lysimeter trees (Exp. 2) to manipulate turgor pressure over a 24 hr period In the minimum night regime (Min night), daily ET A was calculated 0.5 hr after sunset (1945 hr EST) by subtracting the maximum mass recorde d at 2030 hr EST the night before, by that measured at 1945 hr EST. Irrigation volume was 130% daily ET A with half applied at 1945 hr EST and the other half at 2000 hr EST. Maximum mass was determined after 30 minutes of drainage at 2030 hr EST. The Mi n night regime was implemented to maximize turgor pressure over the night and minimize turgor pressure over the day. The second additional regime was a maximum night regime (Max night). Here daily ET A was calculated at sunrise (0630 hr EST) by subtracting the maximum mass recorded at 0715 hr EST the morning before, by that measured at 0630 hr EST. Irrigation volume was 130% daily ET A with half applied at 0630 hr EST and the other half at 0645 hr EST. Maximum mass was determined after 30 minutes of drainage. The Max night regime was implemented to maximize turgor pre ssure over the day and minimize turgor pressure over the night. There was a one
45 day interval between one regime to the next. Overall protocol for Season Two was similar t o Season One, except gas exchange measurements which were not collected during Season Two due to equipment malfunction Shoot Growth Measurements Usually five elongating branches were selected on each tree for growth measurements for all irrigation regi mes. For some trees, a maximum of three elongating branches were available. Branches were selected throughout a canopy and measurements were collected by sunrise (~0625 hr EST) and by sunset (~2000 hr EST) for three days in concurrence with diurnal water potential measurements. Shoot elongation measurements began one day after changing irrigation regimes. Xylem Water Potential Measurements x were measured during collection of shoot elongation measurements as described during Season One. Measure ments were made for three days for each irrigation regime (FI, LDI 2 Daily Max night, and Min night). Measurements were taken approximately every two hr s from pre dawn (~0600 hr EST) to post dusk (~2045 hr EST) weather permitting. Measurements were made on individual, mature shoot s cm in length) using a pressure chamber. Pressure Volume Curves Three to four immature shoot s (~11 cm long) were collected for construction of P V curves after bud break for all trees as described in Season One. Pressure volume curve measurements were made on three immature shoots for LDI 2 Daily and FI trees w me asurements. Single replication curves were constructed for mature tissue for each LDI 2 Daily tree post implementation of irrigation regimes. Curves were constructed following the same protocol as outlined in data
46 collection during Season One. Pressure v olume curves were constructed shortly after budbreak for all trees. Curves were constructed prior to shoot elongation measurements for well watered trees. Relation of Turgor Pressure to Shoot Growth Turgor pressure was related to growth using the same pr otocol described in w measurements were collected for three days for each regime. Given the nature of the two irrigation p and w were constructed to determine if trends we re apparent as expected. General trends were compared to trees under a well watered condition (LDI 2 Daily ) and similar trends were seen for FI trees (Figure 2 7 and Figure 2 8). Statistical Analysis Experimental design consisted of a complete randomized design with trees serving as replicates (N=3 per treatment) and measurements within individual trees serving as subreps In Season One, growth elongation rate and pressure normalized growth rates were analyzed as a 2 x 2 x 2 factorial, with two species, t wo irrigation regimes (FI vs LDI), and two time intervals (day vs night). Relative symplastic water content at incipient plasmolysis, osmotic potential at incipient plasmolysis, maximum turgor pressure, bulk modulus of elasticity, and maximum modulus of e lasticity were also analyzed as a 2 x 2 x 2 factorial, with two species, two irrigation regimes (FI vs LDI), and two growth ages (mature growth vs immature growth). Stomatal conductance (g s ), light level, and leaf temperature measurements were analyzed as repeated measurements using a split plot design with tree water status ( FI and LDI vs LLI) as the main plot and with two species and two irrigation regimes (well watered vs water
47 stressed) Integrated measurements of photosynthesis (A) and transpiration ( E) were analyzed as a 2 x 2 factorial, with two species and two irrigation regimes (FI vs LDI). In Season 2, growth elongation rates and pressure normalized growth rates under well watered conditions were analyzed as a 2 x 2 factorial (FI and LDI, day and night growth rates) as repeated measures using a split plot design for Exp. 1. When comparing among irrigation regimes of lysimeter trees, data was analyzed independently by species (Exp. 2). Within a species, data was analyzed as a repeated measurement s using a split plot design, with three irrigation regimes (control, Max night, and Min night), and two time intervals (day vs night), replicated three times. Data was subjected to an analysis of variance and means separation between variables was obtai Difference (LSD). All data was analyzed using SAS (9.2 SAS Instit ute Inc., Cary, NC).
48 Table 2 1. Tree fertilization schedule across S easons O ne and T wo. Date (day, month, year) Type Amount (g) 20 May 2010 16 04 08 z 9.5475 14 June 2010 16 04 08 9.5475 9 August 2010 16 04 08 7.5565 14 September 2010 16 04 08 7.5565 12 October 2010 14 14 14 y 6.9401 2 February 2011 16 04 08 7.5565 22 March 2011 16 04 08 7.5565 4 April 2011 16 04 08 7.5565 22 April 2011 16 04 08 7.5565 16 May 2011 16 04 08 7.5565 z (N:P:K) quick release fertilizer (Pro Source One, Win F ield Solutions Inc., Elwood, IN). y (N:P:K) control release fertilizer (Osmocote Scotts Miracle Gro Co., Marysville, OH). Figure 2 1. Open side gutter connected sawtooth style greenhouse located at the Mid Florida Research and Education Center in Apopka, Florida. Study trees were housed inside throughout Seasons One and Two of data collection. Photo is courtesy of Emily Massey.
49 Figure 2 2. Series of frequently irrigated trees (FI). A) Quercus virginiana B) Ulmus parvifolia Photos are courtesy of Emily Massey.
50 Figure 2 3 Series of lysimeter trees (LDI or LDI 2 Daily or LLI) A) Quercus virginiana B) Ulmus parvifolia Photos are courtesy of Emily Massey.
51 Figure 2 4. Shoot age distinction of study trees for Ulmus parvifolia A) Mature shoot. B) Immature shoot. Photos are courtesy of Emily Massey. Figure 2 5. Shoot age distinction of study trees for Quercus virginiana A) Mature shoot. B) Immature shoot. Photos are courtesy of Emily Massey.
52 Figure 2 6. Laboratory pressure volume (P V) curve station located at the Mid Florida Research an d Education Center in Apopka, Florida Station includes an orange tank of compressed nitrogen, a pressure chamber (Plant Moisture Status Console, Soilmoisture Equipment Corp., Santa Barbara, CA), and an analytical balance (Mettler AE100, Mettle r Toledo Inc., Columbus, OH). Photo is courtesy of Emily Massey.
53 Figure 2 7. Diurnal water potential ( x ) and subsequently estimated turgor pressure ( p ) of frequently irrigated (FI) live oak in Season Two. Values were averaged over replicates (N=3) and three days of measurement (N=3).
54 Figu re 2 8. x ) and subsequently estimated turgor pressure p ) of frequently irrigated (FI) Chinese elm in Season Two. Values were averaged over replicates (N=3) and three days of measurement (N=3).
55 CHAPTER 3 RESULTS Season One Shoot Elongation Rate Growth rates measured for a s ingle day were w measurements. Measurements were recorded for frequently irrigated (FI) and once daily well irrigated lysimeter (LDI) trees. Shoot elongation rates were also analyzed over a two five day period s Five day me asurements were recorded for FI and LDI, then water limited lysimeter (LLI) trees. Across the single day, shoots of Chinese elm (0.296 mm hr 1 ) elongated at a greater rate than live oak shoots (0.208 mm hr 1 ; P =0.043). Elongation rate was greater for FI trees (0.308 mm hr 1 ) than LDI or LLI trees (0.213 mm hr 1 ; P =0.016). Day and night elongation rates were similar within irrigation regimes. Differences among irrigation regimes were further evident when measurements were compared across a five day period (Table 3 1). Shoots of FI trees el ongated at greater rates than those of LDI and LLI, as seen in the single day comparison. Shoot elongation of LLI trees was slowest ( P =0.002; Table 3 1). Differences between species were not significant. To gain greater insight, species were further anal yzed separately. For live oak there were no differences among irrigation regimes. However, shoot elongation of Chinese elm was highly dependent on irrigation regime ( P =0.036; Table 3 2). Elongation rate was 13 times greater for LDI trees than LLI trees, and 21 times greater for FI trees. Day and night elongation rates were not significantly different ( P >0.05) within either species.
56 Relation of Turgor Pressure to Shoot Growth Turgor pressure normalized ( P Norm ) growth rates were calculated for each samp p and shoot elongation. Calculations were made for shoots under FI and LDI conditions only. When species were analyzed together, there was a three way interaction between species, irrigation regim e and time of growth ( P 0.0001; Table 3 3 ). Overall daytime rates were higher than nighttime rates, with more variability associated with daytime rates. Daytime rates for live oak were similar across irrigation regimes, while Chinese elm daytime irrigati on rates varied. While not significantly different, nighttime rates for Chinese elm varied 10 fold and LDI for live oak were double that of FI trees. Pressure Volume Curves Pressure volume curves were constructed from mature and immature shoots collected from well watered trees (FI and LDI). R elative symplastic water content at incipient plasmolysis (SWC IP ) had a pair of two way interactions between species and shoot age ( P =0.030) and between irrigation regime and shoot age ( P = 0.007). In the first inter action, mature shoots from both species and those of immature live oak had higher SWC IP than immature Chinese elm shoots (Table 3 4). In the second interaction, shoots from LDI trees and mature tissue from FI trees had greater SWC IP than immature shoots f rom FI trees (Table 3 5). IP ) also had two two way interactions between species and shoot age ( P =0.001) and irrigation regime and shoot age ( P live oak shoots had IP than immature live oak shoots (Table 3 6). In the second
57 interaction, shoots from FI trees and mature shoots of LDI trees were more negative when compared to immature tissue from LDI trees (Table 3 7). There was a three way interaction between species, irrigation regime, and shoot age for maximum turgor pressure (P max ; P = 0.004 ). Here mature shoots had a higher P max than immature shoots (Table 3 8). Mature shoots P max were double those and less variable than imma ture shoots. Mature shoots of Chinese elm had similar P max values. Immature FI Chinese elm P max values were similar to immature FI live oak. Mature shoots (10.866 MPa) had a greater bulk modulus of elasticity (E o ) compared to immature shoots (3.270 MPa; P Shoots from LDI trees (7.325 MPa) also had a larger E o than shoots from FI trees (6.463 MPa; P =0.0001 ). For live oak, mature shoot E o (11.308 MPa) was greater than that of immature shoots (3.612 MP a; P ). Bulk modulus of elasticity for LDI trees (8.116 MPa) was also greater than FI trees (7.143 MPa; P =0.015 ). For Chinese elm there was a two way interaction between irrigation regime and shoot age ( P =0.041 ; Table 3 9). As in the oaks, mature shoot E o was greater than that of immature sho ots; however E o of mature shoots from LDI trees was also greater than mature shoots from FI trees. A two way interaction also occurred between species and shoot age for maximum modulus of elasticity (E max ) ( P =0.007 ; Table 3 10). Mature shoots had a larger value for E max when compared to immature shoots, with mature live oak shoots being larger than mature Chinese elm shoots (Table 3 10). Leaf Gas Exchange Stomatal conductance (g s ) was measured for all irrigation regimes. Well watered trees (FI and LDI) had a significantly higher mean g s (225 mmol m 2 s 1 ) than those under a water deficit (150 mmol m 2 s 1 ; LLI) ( P = 0.001) However, net photosynthesis
58 and transpiration rates were not significantly different. Light levels within the greenhouse were not different. Season Two Shoot Elongation Rate In Experiment 1 (Exp. 1), shoot elongation rates were measured over a period of five continuous days. To ensure a well watered condition, initial growth measurements were recorded on frequently irrigated (FI) and twice daily well irrigated lysimeter ( LDI 2 Daily ) trees for a five day period. There was a three way interaction between species, irrigation regime, and time of growth for shoot elongation rates collected over a five day period ( P =0.032; Table 3 11). Shoots of FI Chinese elm elongated at the hig hest rate. Rates of shoot elongation in LDI 2 Daily live oak trees also exhibited high rates. Elongation rates for remaining trees were similar (Table 3 11). Notable for each species and irrigation regime combination, shoot elongation rates were the same between day and night except for the LDI 2 Daily Chinese elm. w measurements occurred across three different irrigation regimes. Regimes consisted of trees which were LDI 2 Daily minim um night irrigation (Min night), and Maximum night irrigation (Max night). Min night trees were irrigated shortly after sunset. Max night trees were irrigated shortly after dawn. Irrigation volumes were calculated by measuring actual evapotranspiration r ates (ET A ) daily. Species were analyzed separately. There was a two way interaction between irrigation regime and time of growth for live oak trees ( P =0.0068). Shoot elongation was highest during Min night at night and for LDI 2 Daily trees at night. Sho ot elongation was lowest for Max night irrigation at night and for Min night irrigation during the day (Table 3 12). A lthough shoot elongation rate
59 of M ax night during the day was 25% greater than that of Max night at night, elongation rates were similar. The same two way interaction also occurred in Chinese elm trees ( P =0.044). Shoot elongation was the highest for LDI 2 Daily trees during the night and shoot elongation was the lowest for trees irrigated under the Max night regime during the night (Table 3 13). Relation of Turgor Pressure to Shoot Growth Turgor pressure normalized ( P Norm ) growth rates were calculated using the elongation rates described above and followed the protocol outlined in Season One. New P V curves were generated for immature growth prior to any alteration in irrigation w measurements. A single P V curve was made for all lysimeter trees after the last irrigation regime (Max night) was implemented. When analyzed across species there was a two way interaction between species and time of growth ( P =0.042). Turgor pressure n ormalized growth rates were higher in Chinese elm than in live oak trees, with daytime Chinese elm rates being three and four times greater than live oak daytime and nighttime rates, respectively. Daytime rates were also greater than nighttime rates for b oth species with daytime ( P Norm ) growth rates double to those of nighttime rates in Chinese elm. Daytime live oak rates were higher, but not the same extent as Chinese elm rates (Table 3 14). To better understand the relationship between turgor and sh oot elongation, species were analyzed separately across irrigation regimes. Variation was found between treatments for live oak ( P =0.010). Twice daily well irrigated lysimeter (LDI 2 Daily ; 0.039 mm MPa hr 1 ) trees had greater P Norm growth rates when co mpared to those of the Max night irrigation (0.023 mm MPa hr 1 ) regime. The Min night (0.031 mm MPa hr 1 ) regime produced intermediate P Norm growth rates that were similar to the
60 other two A two way interaction between irrigation and time of growth oc curred in Chinese elm ( P =0.003). For Chinese elm, LDI 2 Daily and Min night trees had the greatest ( P Norm ) growth rates during the daytime, with rates three to four times greater than the lowest values. Rates for nighttime Max night and Min night were t he lowest (Table 3 15). x p ) used to P Norm ) growth rates were further investigated. x ) and calculated diurnal turgor pre p ) values were graphed (Figure 3 1 and Figure 3 2). Graphed patterns followed similar trends x was generally constant over night from sunset to sunrise (Figure 3 1 and 3 x decli ned after sunrise to reach a minimum after midday (~1400 to 1600 Eastern Standard Time; EST), later than normally reported for more northern latitudes (Abrams and Knapp, 1987) Thereafter, x increased relatively quickly to achieve a minimum at sunset. This general trend was independent of irrigation regime. However, there were characteristics of each irrigation regime tha p LDI 2 Daily and Min night irrigation had relatively p x values at sunrise and sunset. Max night irrigation p x values than those of the other two treatments at sunset. A small peak was visible ~0700 for Max night irrigation for both the water potential and turgor pressure graph. This peak was not seen for the other two irrigation regimes. Around midday, turgor pressure for both s pecies was zero with the duration of this loss of turgor longer for Chinese elm. Max night and LDI 2 Daily x values were visibly less negative than those of Min night irrigation. While data were
61 not analyzed statistically, Max night values for both sunri se and sunset were more x p ) than those of Min night and LDI 2 Daily and values for the Min night and LDI 2 Daily appeared to be similar. Pressure Volume Curves Pressure volume curves were constructed following protocol as outlined i n Season One, with an exception in the number of samples collected. Measurements were made on three immature shoots for LDI 2 Daily and FI trees, with only one mature shoot measured for each LDI 2 Daily tree. Mature shoots (0.632) had a higher SWC IP than im mature shoots (0.400; P T here was a two IP (Table 3 16; P IP than mature Chinese elm shoots. A two way interaction betwe en species and shoot age also was measured for maximum turgor pressure (P max ) ( P= 0.048). Larger P max values were measured in mature shoots over immature shoots (Table 3 17). Mature shoots (10.473 MPa) had a greater bulk modulus of elasticity (E o ) when compared against immature shoots (3.641 MPa; P between species, with a higher E o for live oak shoots (4.975 MPa) when compared to Chinese elm (4.260 MPa; P =0.0007). Live oak shoots (2.218 MPa) had a larger value for ma ximum modulus of elasticity (E max ) when compared to shoots of Chinese elm (1.817 MPa; P =0.0022). Maximum modulus of elasticity was also larger for mature shoots (3.453 MPa) when compared to immature shoots (1.778 MPa; P ied, with E max for LDI 2 Daily trees (2.407 MPa) higher than FI trees (1.498 MPa; P
62 Table 3 1. Shoot elongation rates measured across a five day period for frequently irrigated (FI), once daily well irrigation lysimeter (LDI), and once daily limi ted irrigation (LLI) regimes across species Irrigation regime Shoot elongation (mm hr 1 ) FI z 0.330 a y LDI 0.259 b LLI 0.027 c z Each mean represents N=270, 311, and 259 for FI, LDI, and LLI respectively. y Means with same letter were not Table 3 2. Chinese elm shoot elongation rates measured across a five day period for frequently irrigated (FI), once daily well irrigation lysimeter (LDI), and once daily limited irrigat ion (LLI) regimes. Irrigation regime Shoot elongation (mm hr 1 ) FI z 0.409 a y LDI 0.262 b LLI 0.019 c z Each mean represents N=162, 162, and 99 for FI, LDI, and LLI respectively. y not Table 3 3. Interaction between species, irrigation regime, and time of growth for turgor pressure normalized ( P Norm ) growth rate over a single day for frequently irrigated (FI) and once daily well irrigation lysim eter (LDI) across species Species*irrigation regime*time of growth P Norm growth rate (mm MPa hr 1 ) Chinese elm*FI*day z 0.073 a y live oak*LDI*day 0.046 b live oak*FI*day 0.032 bc Chinese elm*LDI*day 0.020 c Chinese elm*FI*night 0.002 d live oak* LDI*night 0.001 d live oak*FI*night 0.0004 d Chinese elm*LDI*night 0.0002 d z Each mean represents N=18, 18, 12, 18, 18, 18, 12, and 18 for the combinations above in descending order. y
63 Table 3 4. Interaction of species and shoot age for relative symplastic water content at incipient plasmolysis ( SWC IP ) during Season One across frequently irrigated (FI), and once daily well irrigation regimes (LDI) across species Species*shoot age SWC IP live oak*mature z 0.634 a y Chinese elm*mature 0.592 a live oak*immature 0.578 a Chinese elm*immature 0.394 b z Each mean represents N=19, 20, 17, and 19 for the combinations above in descending order. y Table 3 5. Interaction of irrigation regime and shoot age for relative symplastic water content at incipient plasmolysis ( SWC IP ) during Season One across frequently irrigated (FI) and once daily well irrigation lysimeter (LDI) across species Irrigation regime*shoot age SWC IP LDI*mature z 0.616 a y LDI*immature 0.599 a FI*mature 0.577 a FI*immature 0.439 b z Each mean represents N=18, 18, 16, and 23 for the combinations above in descending order. y sign Table 3 6. Interaction of species and shoot age for osmotic potential at incipient IP ) during Season One across frequently irrigated (FI) and once daily well irrigation lysimeter (LDI) across species Species*shoot age IP (MPa) live oak* mature z 2.594 a y Chinese elm*immature 2.494 a Chinese elm*mature 2.431 a live oak*immature 1.754 b z Each mean represents N=18, 23, 18, and 16 for the combinations above in descending order. y same letter were not
64 Table 3 7. Interaction of irrigation regime and shoot age for osmotic potential at incipient IP ) during Season One across frequently irrigated (FI) and once daily well irrigation lysi meter (LDI) across species Irrigation regime*shoot age IP (MPa) FI*immature z 2.781 a y LDI*mature 2.583 ab FI*mature 2.434 b LDI*immature 1.630 c z Each mean represents N=19, 19, 17, and 20 for the combinations above in descending order. y Table 3 8. Interaction between species, irrigation regime, and shoot age for maximum turgor pressure (P max ) during Season One across frequently irrigated (FI) and once daily well irrigation lysimeter (LDI) across species Species*irrigation regime*shoot age P max (MPa) live oak*FI*mature z 4.731 a y Chinese elm*LDI*mature 4.104 b live oak*LDI*mature 4.086 b Chinese elm*FI*mature 3.800 b Chinese elm*FI*immature 1.936 c live oak*FI*immature 1.588 cd Chinese elm*LDI*immature 1.367 d live oak*LDI*immature 1.31 d z Each mean represents N=8, 9, 10, 9, 12, 7, 11, and 9 for the combinations above in descending order. y
65 Table 3 9. Interaction of irrigatio n regime and shoot age for bulk modulus of elasticity of Chinese elm during Season One across frequently irrigated (FI) and once daily well irrigation lysimeter (LDI). Irrigation regime*shoot age E o (MPa) LDI*mature z 11.28 a y FI*mature 9.782 b FI*immature 3.124 c LDI*immature 2.930 c z Each mean represents N=18, 18, 12, and 11 for the combinations above in descending order. y Table 3 10. Interaction of species and shoot age for maximum modulus of elasticity (E max ) during Season One across frequently irrigated (FI) and once daily well irrigation lysimeter (LDI) across species Species*shoot age E max (MPa) live oak*mature z 2. 198 a y Chinese elm*mature 1.610 b live oak*immature 1.144 c Chinese elm*immature 1.144 c z Each mean represents N=18, 18, 16, and 23 for the combinations above in descending order. y were not
66 Table 3 11. Interaction of species, irrigation regime, and time of growth for shoot elongation over a period of five days during Experiment 1 (Exp. 1) for Season Two. Measurements were collected from fre quently irrigated (FI) and twice daily well irrigation lysimeter (LDI 2 Daily ) across species Species*irrigation regime*time of growth Shoot elongation (mm hr 1 ) Chinese elm*FI*day z 0.352 a y Chinese elm*FI*night 0.339 a live oak* LDI 2 Daily *night 0.236 b live oak* LDI 2 Daily *day 0.193 bc live oak*FI*day 0.176 c Chinese elm* LDI 2 Daily *day 0.163 c live oak*FI*night 0.148 c Chinese elm* LDI 2 Daily *night 0.108 d z Each mean represents N=75, 60, 60, 75, 75, 75, 60, and 60 for the combinations above in descending order. y Table 3 12. Interaction of irrigation regime and time of growth for shoot elongation in live w measurements during Experiment 2 (Exp. 2) for Season Two. Irrigation regime*time of growth Shoot elongation (mm hr 1 ) LDI 2 Daily z *ni ght y 0.252 a x Min night w *night 0.218 ab Max night v *day 0.195 ab LDI 2 Daily *day 0.169 b Max night*night 0.155 bc Min night*day 0.091 c z Twice daily well irrigation lysimeter (LDI 2 Daily ). y Each mean represents N=43, 27, 27, 43, 27, and 27 for the combinations above in descending order. x w Minimum night irrigation regime (Min night) v Maximum night irrigation regime (Max night)
67 Table 3 13. Interaction of irrigation regime and time of growth for shoot elongation in w measurements during Experiment 2 (Exp. 2) for Season Two. Irrigation regime*time of growth Shoot elongation (mm hr 1 ) LDI 2 Daily z *night y 0.320 a x LDI 2 Daily *day 0.279 ab Min night w *night 0.231 bc Min night*day 0.183 cd Max night v *day 0.140 de Max night*night 0.076 e z Twice daily well irrigation lysimeter (LDI 2 Daily ). y Each mean represents N=43, 27, 27, 43, 27, and 27 for the combinations above in descending order. x w Minimum night irrigation regime (Min nigh t) v Maximum night irrigation regime (Max night) Table 3 14. Interaction between species and time of growth for turgor pressure normalized ( P Norm ) w measurements during Experiment 2 (Exp. 2) for Season Two acros s frequently irrigated (FI) and twice daily well irrigation lysimeter (LDI 2 Daily ) across species Species*time of growth P Norm growth rate (mm MPa hr 1 ) Chinese elm*day z 0.107 a y Chinese elm*night 0.055 b live oak*day 0.0364 c live oak*night 0.0267 d z Each mean represents N=143, 143, 140, and 140 for the combinations above in descending order. y
68 Table 3 15. Interaction of irrigation regime and time of growth for turgor pressure normalized ( P Norm ) growth rate in Chinese elm during Experiment 2 (Exp. 2) for Season Two for twice daily lysimeter irrigation regimes (LDI 2 Daily ), Minimum night regime (Min night), and Maximum ni ght regime (Max night) Each mean represents N=36. Irrigation regime*time of growth P Norm growth rate (mm MPa hr 1 ) LDI 2 Daily *day 0.154 a z Min night*day 0.081 b LDI 2 Daily *night 0.065 c Max night*day 0.061 c Max night*night 0.039 d Min night*night 0.037 d z Table 3 16. Interaction of species and shoot age for osmotic potential at incipient IP ) during Season Two across species Species*shoot age IP (MPa) Chinese elm*immature z 2.495 a y live oak*immature 2.287 a live oak*mature 1.808 a Chinese elm*mature 1.179 b z Each mean represents N=18, 18, 3, and 3 for the combinations above in descending order. y Table 3 17. Interaction of species and shoot age fo r maximum turgor pressure (P max ) during Season Two across species Species*shoot age P max (MPa) Chinese elm*mature z 4.14 a y live oak*mature 3.57 a live oak*immature 2.19 b Chinese elm*immature 2.13 b z Each mean represents N=18, 18, 3, and 3 for the combinations above in descending order. y
69 Figure 3 x ) and subsequently estimated turgor pressure p ) of live oak in Season Two. Values were averaged over replicates (N=3) and three days of measurement (N=3). Treatments include twice daily well watered lysimeter irrigation (LDI 2 Daily ), minimum night irrigation (Min night) and maximum night irrigation (Max night).
70 Figure 3 2 x ) and subsequently estimated turgor pressure p ) of Chinese elm in Season Two. Values were averaged over replicates (N=3) and three days of measurement (N=3). Treatments include twice daily well watered lysimeter irrigation (LDI 2 Daily ), minimum night irrigation (Min night) and maximum night irrigation (Max night).
71 CHAPTER 4 DISCUSSION Seasons One and Two Measurements were collected for containerized Quercus virginiana and Ulmus parvifolia seedlings over two growing seasons. In 2010, data was collected from June thr o u gh September, and in 2011 data was collected from March thr o u gh June. All measurements were collected on actively growing trees, which were subjected to monitored irrigation regimes. Frequently irrigated (FI) trees were irrigated three times a day to conta iner capacity. In Season One, lysimeter trees were irrigated once daily (LDI) to ensure a well irrigated condition. These trees were later irrigated at 90% ET A in order to impose mild water stress (water limited lysimeter irrigation, LLI). In Season Two FI trees were watered following the same protocol outlined in Season One. Protocol was altered for well watered lysimeter trees such that lysimeter trees were irrigated twice daily (twice daily well irrigated lysimeter, LDI 2 Daily ). T wo additional regimes were included in which irrigation was applied once daily. For trees irrigated under the minimum night regime ( Min night), irrigation was applied shortly after sunset. Post w and therefore minimum water stres s throughout the night. For trees irrigated under the maximum night regime (Max night), irrigation was applied shortly after dawn. Dawn irrigation resulted in sub w during the night and therefore an elevated level of water stress. Leaf Gas Exch ange Stomatal conductance (g s ) was collected within periods of shoot elongation during Season One for all trees around midday only. Higher mean g s was measured in well
72 watered trees of both species (225 mmol m 2 s 1 ; FI and LDI) when compared to those un der a water deficit (150 mmol m 2 s 1 ; LI). Overall, g s for well watered trees agreed with those of previously studied species. Gallego et al (1994) reported g s around 150 200 mmol m 2 s 1 for Quercus pyrenaica Lower g s for water stressed trees were also consistent with previously cited literature. Plants close stomata to minimize water loss during stress and maintain water status. Stomatal aperture is mediated, in part, by availability of moisture in soils (Lambers et al., 2008) Decreased g s rates were measured in four drought stressed oleander clones (Niu et al., 2008) Gu et al. (2007) also demonstrated a strong relationship in the reduction of g s and P n rates in four birch genotypes under water deficit. No differences were measured in light levels of the greenhouse No differences were found for mean photosynthetic or transpiration rates between species or irrigation regimes. Mean photosynthetic rates of 22 and 21 mol m 2 s 1 for live oak and Chinese elm, respectively were similar to other woody s pecies. Maximum photosynthetic rates were measured at ~22 mol m 2 s 1 for Picea abies and ~16 mol m 2 s 1 for Quercus pubescens respectively (Haldimann et al., 2008; Maier Maercker, 1998) While rates for three year old grafted and micropropagated Ulmus glabra were ~33 and 23 mol m 2 s 1 respectively (Durkovic et al., 2010) These differences in g s yet similarities in leaf gas exchange, indicates that differences in growth rates and pressure volume (P V) curve parameters were more than likely due to changes in irrigation regime and not changes in photosynthetic rates. Furthermore, these results support that w ell watered trees (FI and LDI) we re
73 phy siologically similar despite the differing amounts and number of irrigation cycles applied to them. Pressure Volume Curves Overall, mature shoots in both seasons had a higher relative symplastic water volume at incipient plasmolysis ( SWC IP ) than immature s hoots, with mature shoot values ranging from (0.634 0.577; Table 3 4 and Table 3 5). In Season Two, there were no interactions with shoot age (0.632 and 0.400 for mature and immature shoots). However in Season One, effects of shoot age interacted with sp ecies and irrigation regime separately. Immature shoots of live oak (0.578) and LDI (0.599) trees had values similar to mature shoots (Table 3 4 and Table 3 5). High values measured in immature shoots of live oak indicate they are less plastic than immat ure shoots of Chinese elm. Limited interactions with shoot age in Season Two may have been the result of imbalance in shoot sam ple numbers due to only one shoot replication having been analyzed per LDI 2 Daily tree with no samples collected from FI trees Alternatively, three immature shoot replications were collected for FI and LDI 2 Daily trees. T issue collected was based on visual estimation of maturity and flexibility of tissue. High flexibility of shoots was attributed to little or no lignin in this tissue. Identification of mature and immature growth was easier for Chinese elm opposed to live oak. Immature shoots of Chinese elm were highly flexible with red coloration in the leaves (Figure 2 4 and Figure 2 5). While new flushes of live oak appear ed more flexible than mature tissue, young live oak tissues were distinguished by pale green leaves and red brown stems in contrast to dark green leaves and light brown stems in mature shoots (Figure 2 4 and Figure 2 5).
74 High SWC IP is typically found in ma ture shoots; whereas lower values are often associated with immature, actively growing tissue due to the highly flexible cell walls (Nobel, 2009) As in this study, higher values were measured in mature shoots (0.238) when compared to immature shoots (0.156) from Q. ilex (Dreyer et al., 1990) Corcuera et al. (2002) also calculated similar SWC IP values measured in study trees in mature shoots Q. frainetto and Q. cerris (0.73 and 0.72). IP ) were variable between Seasons One and Two. There was a two way interaction between species IP in both seasons (Table 3 6 and Table 3 17). In Season One, mature shoots and immature Chinese elm shoots ( 2.594 to 2.431 MPa) had a m ore IP than immature live oak shots ( 1.754 MPa; Table 3 6). In contrast the opposite was found in Season Two, where live oak shoots and immature Chinese elm shoots ( 2.495 to IP than mature Chinese elm shoots ( 1.179 MPa; Table 3 17) In Season One there was also a two way interaction between irrigation regime and shoot age (Table 3 7). Here shoots from FI trees ( 2.781 to 2.434 MPa) and mature shoots of LDI trees ( 2.583 MPa) were more negative when compare d to immature shoots from LDI trees ( 1.630 MPa; Table 3 7). This interaction was absent in Season Two. Osmotic potential at incipient plasmolysis is variable and can depend on environmental conditions such as temperature and wind speed. Plants will ofte n increase solute concentrations within cell s p in plant cells during periods of stress. Osmotic adjustment is the accumulation of solutes in cell vacuoles (Kramer and Boyer, 1995) Many studies have show n that osmotic adjustment is a response to water
75 str ess in plants (Osonubi and Davies, 1978; Koppenaal, et al., 1991; and Meier, et al., 1991). Despite f requent irrigation cycling for FI trees in Season One and FI and LDI 2 Daily in Season Two, immature sho ots of Chinese elm had the most negative IP both seasons. In addition, visible wilt was noticed for some immature shoots of Chinese elm during Season Two. Immature growth is known for having poor control of guard cells and stomatal aperture, leaving trees vulnerable to increased transpiration (Kirkham, 2005) It is poss ible that slight to mild water stress due to poor stomatal control may have triggered some osmotic adjustment in Chinese elm which could account for more negative IP Values measured for IP in this study were similar to previously cited literature. M easurements of 1.37 and 1.84, 1.30, 1.62 to 1.77, and 0.99 to cherry, mature terminal shoots of U. parvifolia mature leaves and shoots of Q. alba seedlings, and Q. ellipsoi dalis respectively (Abrams, 1988a; Parker and Pallardy, 1985; Parker and Pallardy, 1987; Ranney et al., 1991) In Season One there was a three way interaction between species, irrigation regime, and shoot age for P max (Table 3 8). Similar p atterns occurred across seasons in regards to shoot age, with larger P max values measured for mature shoots over immature shoo ts (Table 3 8 and Tab le 3 18). In Seasons One and Two, P max calculated for mature shoots were double those calculated in immature shoots, with little variation between mature shoot means in Season One (Table 3 8 and Table 3 18) There was no difference i n P max between irrigation regimes for Season Two. p can cause changes in cellular content and volume within cells (Kirkham, 2005) Although turgor
76 pressure is one of the major forces in plant growth, few studies c ould be located in which turgor was reported. One such study by Beeson (1994) calculated values of 2.5 MPa in of Quercus virginiana While conditions of that study were e xtreme and not comparable to thi s study, it indicates live oak is a highly drought tolerant species. It also p of that magnitude. This insight coupled with the higher SWC IP in Sea son One supports less plasticity of live oak shoots when compared to shoots of Chinese elm. Mature shoots (10.366 to 10.473 MPa) had a greater bulk modulus of elasticity (E o ) compared against immature shoots (3.270 to 3.641 MPa) for both Seasons (Table 3 9). In Season Two, live oak (4.975 MPa) had slightly higher E o than Chinese elm (4.260 MPa), and LDI 2 Daily trees (2.407) had a greater E o than FI trees (1.498 MPa). Low E o estimates indicates flexibility of cell walls, which aids in the ability of imma ture tissue being able to elongate and grow (Nobel, 2009) Values reported in this study are consistent with previously cited literature. Ranney et al. (1991) reported lower values of E o cherry. Values of 8.73 MPa have been reported for leaves and shoots of Q. alba (Parker and Pallardy, 1987) and 9.15 MPa for shoots of U. parvifolia (Ranney et al., 1990) Similar trends were found between seasons for values of maximum modulus of elasticity (E max ). Across seasons E max was larger for mature shoo ts compared to immature shoots, and E max was greater for live oak than Chinese elm (Table 3 10). However unlike in Season One, E max varied among irrigation regimes. Maximum modulus of elasticity for LDI trees (2.41 MPa) was higher than FI trees (1.50 MP a).
77 Maximum modulus of elasticity measurement is a measure of the rigidity of the plant tissue (Nobel, 2009) Parker and Pallardy (1985) reported similar values (2.45 MPa) for leaves of J. nigra Shoot E longation Rate Few studies have reported short term, diurnal shoot growth rates and of those conducted there appears to be great variability in growth rates among species. Borchert (1976) measured pin oak sapling growth rates of 0.7917 mm h 1 Reich et al. (1980) measured elongation of stem height rates from 0.0164 to 0.0424 mm h 1 in blackjack oak seedlings. Elongation rates of Chinese elm (0.296 mm hr 1 ) and Live oak (0.208 mm hr 1 ) when averaged over FI and LDI irrigation regimes were both lower when compared to pin oak saplings, bu t higher when compared to stem height of backjack oak seedlings. Across both Seasons frequently irrigated (FI) Chinese elm elongated at higher rates than other trees. Similarly, LDI and FI irrigation regimes of live oak trees produced comparable growth rates each season This suggests shoot elongation of live oak was less dependent on irrigation re gime than Chinese elm. LDI in Season One was irrigated only once per day. Greater sensitivity of Chinese elm sho ot growth being more negative w than that of live oak is found in Season Two Exp. 2. Chine se elm elongation rates were greatest under conditions imposed by the LDI 2 Daily with nighttime elongation rates u nder Max night irriga tion being the lowest (Table 3 13). In both seasons overall growth rates were similar between day and night within irrigation regime and species with the exception of nighttime LDI in Chinese elm. (Table 3 11). Higher rates of elongation were often associated with Chinese elm trees when under non water limiting irriga tion regimes This coupled with low measured values of
78 SWC IP ,P ma x, E o E max in both seasons, and in contrast to p in live oak indicates t hat shoots of Chinese elm are highly plastic R eported values of high p within plant shoo ts of live oak may be indicative of high lignification or another less plastic s ubstance This suggests shoots are less plastic, which was true of species in this study. Given this information and the lower SWC IP in Chinese elm, it is hypothesized that C hinese elm shoots may be more plastic than live oak and that this higher plasticity led to greater shoot elongation under well watered conditions. Turgor pressure must be maintained for plant growth to occur, and turgor pressure is mediated, in part, by plant and substrate water status (Anderson and Brodbeck, 1988; Kramer and Boyer, 1995; Ripullone et al., 2009) When plants experience water stress, water uptake may slow and growth may cease in response to lower water potentials and a loss of turgor pressure (Lockhardt, 1965) Zwack et al. (1998) reported that leaf elongation was lower for drought stressed Freeman maple cultivars (19 to 87 cm for drought and control trees, respectively). While trees here were not water stressed to the point of termination of growth rates, pat terns for this data were comparable with highest shoot elongation for tr ees irrigated to excess and lowest for trees experiencing a water deficit ( i.e ., FI> LDI>LLI). S hoot el ongation in Chinese elm appears to be sensitive to irrigation regime; whereas live oak shoot elongation is not. Beeson and Haydu (1995) reported similar growth in live oak regardless of increased volume of irrigation or increase in cycle above a certain minimum level However, multiple irrigation regimes applied in sub volumes throughout the day have been found to optimize growth rates for some species. Beeson (1992) showed Rhododendron sp. and Elaeagnus pungens cyclically irrigated
79 to 100% container capacity produced significantly greater growth than plants over head irrigated once per day. Cyclic irrigation also increased rates of height growth and trunk diameter in Magnolia grandiflora compared to plants irrigated once per day (Beeson and Keller, 2003) Given the large difference of Chinese elm growth rates between FI and LLI Chinese elm may require increased frequency and volume of water application during irrigation to optimize growth rates as well. Relation of Turgor Pressure to Shoot Growth For Season One and Season Two Exp. 1 P Norm elongation rates were greater durin g the daytime and independent of species within well watered irrigation regimes (FI, LDI, LDI 2 Daily ). These rates were always highest in Chinese elm irrigated in excess of container capacity at mid morning and mid afternoon, and shortly after sunset (FI) in Season One. Greater elongation rates of elm were also found in Season Two (Exp. 1) independent of irrigation regime. Elm shoot elongation both during the day and at night was greater than at either time of day for live oak. This is likely because the re was little different between the FI and LDI 2 Daily regimes. Both applied irrigation during the mid afternoon and re saturated containers at midnight. The main different was the quantity applied in the middle of the day, excess verse ~50% of ET A Whe n irrigation was restricted to only midnight (LDI) in Season One, both the FI and LDI regimes applied to live oak produced greater elongation rates than that of elms. This suggests easily availability water was more critical to shoot growth of Chinese elm than for live oak. Easily available water is considered that held in a container substrates at matric potentia ls less than 5 kPa (Beardsell et al., 1979; Bunt, 1983) Chinese elm appears to have a more plastic nature when compared to live oak, as indicated by lower measured SWC IP and lower bulk and maximum moduli of elasticit y. Lower values of SWC IP
80 suggest these shoots may have been more susceptible to water loss and may be more susceptible to water loss than other plant material measured. Furthermore, higher osmotic potential at incipient plasmoysis was found both s easons and observation of wilt was seen in Season Two for immature shoot s of Chinese elm. Both of these indicate that immature flus hes of Chinese elm may have osmotically adjusted in order to maintain turgor pressure at lower water potential. Wilting was never apparent in immature growth of live oak and higher osmotic potentials at incipient plasmolysis was only apparent in immature shoots of live oak for one season. In Season Two, three irrigation regimes were appli ed in an attempt to manipulate P both overall and during the night time. As discussed above, the LDI 2 Daily regime was considered optimum for promoting maximum shoot elongation ( Beeson and Haydu, 1995). The Min n w and therefore P dur ing the nighttime by re saturating the sub strate shortly after stomata closur e, a s indicated by Figures 3 1 and 3 x by sunset and sunrise the next morning. The Max night was hypothesized to be the opposite of the Mi n night. By re saturating the container substrate only at dawn, trees would come to equilibrium when stomata closed at sunset with substrate available water approximately half depleted by ET A during the day. w should have caused P throughout the night until the container was re saturated at dawn. These hypothesized effects were a chieved as seen in Figs. 3 1 (oak) and 3 2 (elm) However, statistical analysis was not performed for any values graphed, and without these analy ses only possible trends may be reported Similar trends were observed for Min night and LDI 2 Daily irrigation A t midnight and dawn v alues appeared
81 to be similar, with p resulted from low er w Whereas for the Max night irrigation, p also resulted from lower w but values were higher at dawn than those of midnight for both parameters. Turgor pressure was lost in both species around midday with w for Min night around 1.6 to 1.9 MPa for live oak and Chinese elm respectively Values for Max night and LDI 2 Daily were l ess negative around 1.25 to 1.5 MPa for live oak and Chinese elm, respectively Water potential and turgor pressure values change d little between sunset to dawn. S pecies responses appeared to be s imilar with the exceptio n of longer duration s of minimum values close to zero for p for Chinese elm p P Norm elongation rates for Chinese elm were greater during the day than at night (Table 3 14). However for live oak, when separated by irrigation regime, there were no differences in P Norm elongation rates between day and night. Thus shoot elongation in live oak was directly related to turgor pressures. The less negative the diurnal water potential due to p Norm growth rate (Fig. 3 1). However for Chinese elm, P Norm daytime growth p was zero or less for many hr s during the mid afternoon (Fig. 3 2). These differences are proposed due to p above which there was no measurable increa se in shoot p was higher in shoots during the night than during day. Dividing similar shoot elongation rates (mm hr 1 p calculated at night, results P Norm elongation rates (mm MPa hr 1 ) at night even th ough actual elongation rate is similar. Thus it is hypothesized that thresholds for maximum growth of Chinese
82 p calculated here at night. This is probably due to increased plasticity of immature Chinese elm shoots as indicated by lower SWC IP and E o It is also possible that Chinese elm and live oak undergo different processes during cell wall expansion as dictated by the composition of their respective cell walls. Cosgrove (2011) suggested that differences in plasticity may be dependent on characteristics of the cell wall such as the number and bundling of cellulose microfibrils in wall cross sectional areas; orientation of th e microfibrils relative to the direction of extension; and the density, hydration, and cross linkage of the matrix. This coupled with differences in immature cell wall plasticity may attribute to differences in shoot elongation rates and the role of turgo r pressure in growth of these two species. Further pursue of shoot elongation rates under mild water stress may tease out more accurate values for these threshold p for maximum shoot elongation in Chinese elm. Expected outcomes of this study were two fold. The first being possible changes to irrigation scheduling for both the species. The second is new physiological knowledge in regards to growth rates, turgor pressure, and possibly cell wall loosening mechanisms. First, r es ults fr om this study indicate these species likely require different irrigation schedules. Shoot elongation of Chinese elm trees was highly dependent on irrigation volume and frequen cy, indicati ng that increased irrigation will result in greater s hoot elo ngation and more marketable plants for tree nurseries. Live oak had little increase in shoot elongation rate between frequently irrigated (3 times daily) and twice daily irrigated (LDI 2 Daily ) r egimes. This indicates that increased frequency and volume d id not result in subsequent increase s in shoot elongation, which indicate that increased
83 irrigation is superfluous. Therefore, increased irrigation will only result in increased water usage and higher, unnecessary costs for nurseries. Furtherm ore, this research increases the understanding of shoot elongation and how it relates to turgor pressure First, q uantification of straight shoot elongation measurements was important given that few studies have presente d this information. Second turgor pressure thresholds were indicated implying that high nighttime turgor pressures ma y not be the only forces promoting shoot elongation for these two species Differences found here are indicative of more plastic tissue in immature s hoots of Chinese elm shoots com pared to live oak and/ or differences in cell wall loosening processes. Further study would need to confirm either of these two possibilities. However, cell wall extensibility may be hard to measure based on current methods of quantification for cell wall mechanics and cell wall loosening.
84 LIST OF REFERENCES Abrams, M.D. 1988a. Comparative water relations of three successional hardwood species in Wisconsin. Tree Physiol. 4:263 273. Abrams, M.D. 1988b. Sources of variation in osmotic potentials with special reference to North American tree species. For. Sci. 34:1030 1046. Abrams, M.D. 1990. Adaptations and response to drought in Quercus species of North America. Tree Physiol. 7:227 238. Abrams, M.D.and A.K. Knapp. 1987. Sea sonal water relations of three gallary forest hardwood species in northeastern Kansas. For. Sci. 32:687 696. Abrams, M.D., J.C. Schultz, and K.W. Kleiner. 1990. Ecophysical responses in mesic versus xeric hardwood species to an early season drought in cent ral Pennsylvania. For. Sci. 36:970 981. Acevedo, E., E. Fereres, T.C. Hsiao, and D.W. Henderson. 1979. Diurnal Growth Trends, Water Potential, and Osmotic Adjustment of Maize and Sorghum Leaves in the Field. Plant Physiol. 64:476 480. Anderson, P.C.and B.V Brodbeck. 1988. Net CO2 assimilation plant water relations characteristics of pecan growth flushes. J. Am. Soc. Hort. Sci. 133:444 450. Andrade, J.L., F.C. Meinzer, G. Goldstein, F.C. Holbrook, J.C. Cavelier, P. Jackson, and K. Silvera. 1998. Regulation of water flux through trunks, branches, and leaves in trees of a lowland tropical forest. Oecologia 115:463 471. Anselmi, S., M. Chiesi, M. Giannini, F. Manes, and F. Maselli. 2004. Estimation of Mediterranean forest transpiration and photosynthesis throug h the use of an ecosystem simulation model driven by remotely sensed data. Global Ecol. Biogeogr. 13:371 380. Bahari, Z.A., S.G. Pallardy, and W.C. Parker. 1985. Photosynthesis, water relations, and drought adaptations in six woody species of oak hickory f orests in central Missouri. For. Sci. 31:557 569. Beardsell, D.V., D.G. Nichols, and D.L. Jones. 1979. Physical properties of nursery potting mixtures Scientia. Hortic. 11:1 8. Beeson, R.C. 1992. Restricting overhead irrigation to dawn limits growth in con tainer grown woody ornamentals. HortScience 27:996 999.
85 Beeson, R.C. 1994. Water relations of field grown Quercus virginiana Mill. from pre harvest through containerization and 1 year into a landscape. J. Amer. Hortic. Sci. 119:169 174. Beeson, R.C. 2011. Weighing lysimeter systems for quantifying water use and studies of controlled water stress for crops grown in low bulk density substrates. Agr. Water Manage. 98:967 976. Beeson, R.C.and E.F. Gilman. 1992. Water stress and osmotic adjustment during post di gging acclimation of Quercus virginiana produced in fabric containers. J. Amer. Hortic. Sci. 10:208 214. Beeson, R.C.and J. Haydu. 1995. Cyclic microirrigation in container grown landscape plants improves plant growth and water conservation. J. Environ. Ho rt. 13:6 11. Beeson, R.C.and K. Keller. 2003. Effect of cyclic irrigation on growth of magnolias produced using five in ground systems. J. Environ. Hort. 21:148 152. Begg, J.E.and N.C. Turner. 1970. Water potential gradients in field tobacco. Plant Physiol 46:343 346. Bonner, J.and J.E. Varner. 1976. Plant biochemistry. Academic Press, New York. Borchert, R. 1976. Differences in shoot growth patterns between juvenile and adult trees and their interpretation based on systems analysis of trees. Acta. Hort. 5 6:123 130. Brockman, C.F. 2001. Trees of North America: A Guide to Field Identification. Golden Guides from St. Martin's Press, New York. Bunt, A.C. 1983. Physical properties of mixtures of peat and minerals of different particles sizes and bulk density fo r potting substrates. Acta. Hort. 150:143 153. Burgert, I. 2006. Exploring the micromechnaical design of plant cell walls. Am. J. Bot. 93:1391 1401. Burough, I. 1980. Gas analyzer and gas analyzing. In: U.S.P. Office (Ed., Google Patents,July 19, 2010. An dros Inc., Berkeley, CA, United States of America, pp. 1 7. Carpita, N.C.and D.M. Gibeaut. 1993. Structural models of primary cell walls in flowering plants: Consistency of molecular structure with the physical properties of the walls during growth. Plant J. 3:1 30.
86 Cline, R.C.and G.S. Campbell. 1976. Seasonal and diurnal water relations of selected forest species. Ecology 57:367 373. Corcuera, L., J.J. Camarero, and E. Gil Pelegrin. 2002. Functional groups in Quercus species dervied from the analysis of p ressure volume curves. Trees 16:465 472. Cosgrove, D.I. 1989. Characterization of long term extension of isolated cell walls from growing cucumber hypocotyls. Planta 177:121 130. Cosgrove, D.I. 1993. Wall extensibility: It's nature, measurement, and relationship to plant cell growth. New Phytol. 124:1 23. Cosgrove, D.I. 2011. Measuring in vitro extensibility of growing plant cell walls. vol. 715. Humana Press, New York. Cullina, W. 2002. Native trees, shrubs, and vines: a guide to using, growing, and propagating North American woody plants. Houghton Mifflin Company, Boston. Cutler, J.M., P.L. Steponkus, M.J. Wach, and K.W. Shahan. 1980. Dynamic Aspects and Enhancement of Leaf Elongation in Rice. Plant Physiol. 66:147 152. Danilov, M.D. 1954. Diurnal pe riodicity in the growth and shoots of some tree and shrub species. Doklady Akad. Nauk. S.S.S.R. 96:205 208. Dreyer, E., F. Bousquet, and M. Ducrey. 1990. Use of pressure volume curves in water relation analysis on woody shoots: Influence of rehydration and comparison of four European oak species. Ann. For. Sci. 47:285 297. Durkovic, J., I. Canova, T. Priwitzer, M. Biroscikova, P. Kapral, and M. Saniga. 2010. Field assessment of photosynthetic characteristics in micropropagated and grafted wych elm ( Ulmus gl abra Huds.) trees. Plant Cell Tiss. Org. 101:221 228. Fielding, J.M. 1955. The seaonsal and daily elongation of the shoots of Monterey pine and the daily elongation of the roots. Forestry & Timber Bur. Australia 75. Fort, C., M.L. Fauveau, F. Muller, P. La bel, A. Granier, and E. Dreyer. 1997. Stomatal conductance, growth and root signaling in young oak seedlings to partial soil drying. Tree Physiol. 17:281 289. Franks, P.J., P.L. Drake, and R.H. Froend. 2007. Anisohydric but isohydrodynamic: Seasonally cons tant plant water potential gradient explained by a stomatal control mechanism incorporating variable plant hydraulic conductivity. Plant, Cell Environ. 30:19 30.
87 Fry, S.C. 1989. The structure and function of xyloglucan. J. Exp. Bot. 40:1 11. Fry, S.C., R.C Smith, K.F. Renwick, D.J. Martin, S.K. Hodge, and K.J. Matthews. 1992. Xyloglucan endotransglycosylase, a new wall loosening enzyme activity from plants. Biochem. J. 282:821 828. Gallego, H.A., M. Rico, G. Moreno, and I. Santa Regina. 1994. Leaf potentia l and stomatal conductance in Quercus pyrenaica Willd. forests: Vertical gradients and responses to environmental factors. Tree Physiol. 14:1039 1047. Geitmann, A. 2006. Experimental approaches used to quantify physical parameters at cellular and subcellul ar levels. Am. J. Bot. 93:1380 1390. Geitmann, A.and J.K.E. Ortega. 2009. Meachnics and modeling of plant cell growth. Trends Plant Sci. 14:467 478. Gu, M., C.R. Rom, J.A. Robbins, and D.M. Oosterhuis. 2007. Effect of water deficit on gas exchange, osmotic solutes, leaf abscission, and growth in four birch genotypes ( Betula L.) under a controlled environment. HortScience 42:1383 1391. Hager, A., H. Menzle, and A. Krauss. 1971. Experiments and hypothesis of the primary effect of auxins on expansion growth. P lanta 100:47 75. Haldimann, P., A. Galle, and U. Feller. 2008. Impact of an exceptionally hot dry summer on photosynthetic traits in oak ( Quercus pubescens ) leaves. Tree Physiol. 28:785 795. Hsiao, T.C.and E. Acevedo. 1974. Plant responses to water deficit s, water use efficiency, and drought resistance. Agr. Meteorol. 14:59 84. Hsiao, T.C., E. Acevedo, E. Fereres, and D.W. Henderson. 1976. Water stress, growth, and osmotic adjustment. Philosophical Transactions of the Royal Society of London B, Biological S ciences 273:479 500. Husken, D., E. Steudle, and U. Zimmermann. 1978. Pressure probe technique for measuring water relations of cells in higher plants. Plant Physiol. 61:158 163. Kandiko, R.A., R. Timmis, and J. Worrall. 1980. Pressure volume curves of sho ots and roots of normal and drought conditioned western hemlock seedlings. Can. J. For. Res. 10:10 16. Kirkham, M.B. 2005. Principles of soil and pane water relations. Elsevier Academic Press, Boston.
88 Koch, G.W., J.S. Amthor, and M.L. Goulden. 1994. Diurna l patterns of leaf photosynthesis, conductance, and water potential at the top of a lowland rain forest canopy in Cameroon: Measurements from the Radeau des Cimes Tree Physiol. 14:347 360. Koriba, K. 1958. On the periodicity of tree growth in the tropics, with reference to the mode of branching, the leaf fall, and the formation of resting buds. Gard. Bull. 17:11 81. Korner, C.and P.M. Cochrane. 1985. Stomatal responses and water relations of Eucalyptus pauciflora in summer along an elevational gradient. Oe cologia 66:443 455. Kozlowski, T.T. 1964. Shoot growth in woody plants. Bot. Rev. 30:335 392. Kozlowski, T.T. 1982. Water supply and tree growth. Commonw. Forest. Bur. 43:57 95. Kramer, P.J. 1943. Amount and duration of growth of various species of tree seedlings. Plant Physiol. 18:239 251. Kramer, P.J.and J.S. Boyer. 1995. Water relations of plants and soils. Academic Press, San Diego. Kwon, K.W.and S.G. Pallardy. 1989. Temporal changes in tissue water relations of seedlings of Quercus acutissima Q. alb a and Q. stellata subjected to chronic water stress. Can. J. For. Res. 19:622 626. Lambers, H., F.S. Chapin, and T.L. Pons. 2008. Plant physiological ecology. Springer, New York. Larcher, W. 1980. Physiological plant ecology. Springer Verlag, Berlin. Lock hardt, J.A. 1965. Plant biochemistry. Academic Press Inc, New York. Lotan, J.E.and R. Zahner. 1963. Shoot and needle responses of 20 year old red pine to current soil moisture regimes. For. Sci. 9:497 506. Maccubbin, T.and G.B. Tasker. 1997. Florida garden er's guide. Cool Springs Press, Franklin, Tennessee. Maier Maercker, U. 1998. Dynamics of change in stomatal response and water status of Picea abies during a pesistant drought period: A contribution to the traditional view of plant water relations. Tree P hysiol. 18:211 222. Mazzoleni, S.and D.I. Dickmann. 1988. Differential physiological and morphological responses of two hybrid Populus clones to water stress. Tree Physiol. 4:61 70.
89 McQueen Mason, S.and D.I. Cosgrove. 1994. Disruption of hydrogen bonding b etween plant cell wall polymers by proteins that induce wall extension. Proc. Natl. Acad. Sci. USA 91:6574 6578. Meier, C.E., R.J. Newton, J.D. Puryear, and S. Sen. 1992. Physiological responses of Loblolly Pine ( Pinus taeda L.) seedlings to drought stress : Osmotic adjustment and tissue elasticity J. Plant Physiol. 140:754 760. More, D.and J. White. 2002. The illustrated encyclopedia of trees. Timber Press, Portland. Mork, E. 1941. Om sambandet mellom temperature og. Nekst. Medd. Det Norske Skogforsoksvesen 8:1 89. Myers, B.J. 1988. Water stress integral a link between short term stress and long term growth. Tree Physiol. 4:315 323. Niu, G., D.S. Rodriguez, and W. Mackay. 2008. Growth and physiological responses to drought stress in four oleander clones. J. Amer. Hortic. Sci. 133:188 196. Nobel, P.S. 2009. Physicochemical and environmental plant physiology. Academic Press, Boston. Omae, H., A. Kumar, K. Kashiwaba, and M. Shono. 2007. Assessing drought tolerance of snap bean ( Phaseolus vulgaris ) from genotypic differences in leaf water differences in leaf water relations, shoot growth and photosynthetic parameters. Plant Prod. Sci. 10:28 35. Parker, W.C.and S.G. Pallardy. 1985. Genotypic variation in tissue water relations of leaves and roots of black walnut ( J uglans nigra ) seedlings. Physiol. Plant. 64:105 110. Parker, W.C.and S.G. Pallardy. 1987. The influence of re saturation method and tissue type on pressure volume analysis of Quercus alba L. seedlings. J. Exp. Bot. 38:535 549. Parker, W.C.and S.G. Pallardy 1988. Leaf and root osmotic adjustment in drought stressed Quercus alba Q. macrocarpa and Q. stellata seedlings. Can. J. For. Res. 18:1 5. Ranney, T.G., N.L. Bassuk, and T.H. Whitlow. 1991. Turgor maintennance in leaves and roots of 'Colt' cherry trees ( Prunus avium x pseudocerasus ) in response to water stress. J. Hortic. Sci. 66:381 387. Ranney, T.G., T.H. Whitlow, and N.L. Bassuk. 1990. Response of five temperate deciduous tree species to water stress. Tree Physiol. 6:439 448.
90 Raper, C.D.and P.J. Kram er. 1983. Crop reactions to water and temperature stresses in humid, temperate climates: Westview special studies in agriculture/aquatic science and policy. Westview Press, Boulder, CO. Raven, P.H., R.F. Evert, and S.E. Eichhorn. 2005. Biology of plants. W .H. Freeman and Company, New York. Rayle, D.and R.E. Cleland. 1970. Enhancement of wall loosening and elongation by acid solutions. Plant Physiol. 46:250 253. Reich, P.B., R.O. Teskey, P.S. Johnson, and T.M. Hinckley. 1980. Periodic root and shoot growth i n oak. For. Sci. 26:590 598. Ripullone, F., M. Borghetti, S. Raddi, E. Vicinelli, R. Baraldi, M.R. Guerrieri, A. Nole, and F. Magnani. 2009. Physiological and structural changes in response to altered precipitation regimes in Mediterranean macchia ecosyste m. Trees Struct. Funct. 23:823 834. Simpson, G.M. 1981. Water stress on plants. Praeger, New York. Smith, J.P. 1977. Vascular plant families. Mad River Press Inc., Eureka, CA. Smith, R.C.and S.C. Fry. 1991. Endotransglycosylase of xyloglucans in plant cell suspension cultures. Biochem. J. 279:529 535. Steudle, E., U. Zimmermann, and U. Luttge. 1977. Effect of turgor pressure and cell size on the wall elasticity of plant cells. Plant Physiol. 59:285 289. Taiz, L. 1994. Expansins: Proteins that promote cell w all loosening in plants. Proc. Natl. Acad. Sci. USA 91:7387 7389. Taiz, L.and E. Zeiger. 2006. Plant physiology. Sinauer Associates, Inc., Publishers, Sunderland, MA. Tardieu, F.and T. Simonneau. 1998. Variability among species of stomatal control under fl uctuating soil water status and evaporative demand: Modelling isohydric and anisohydric behaviors J. Exp. Bot. 49:419 432. Taylor, N. 1988. Taylor's guide to trees. Houghton Mifflin Company, Boston. Turner, N.C. 1988. Measurement of plant water status by t he pressure chamber technique. Irrigation Sci. 9:289 308. Tyree, M.T.and H.T. Hammel. 1972. The measurement of the turgor pressure and the water relations of plants by the pressure bomb technique. J. Exp. Bot. 23:267 282.
91 Tyree, M.T.and P.G. Jarvis. 1982. Water in tissues and cells. In: O.L. Lange, P.S. Nobel, C.B. Osmond, and H. Ziegler (Eds.), Encyclopedia of plant physiology (New Series) Springer Verlag, Berlin, pp. 35 77. Urban, L., A. Jaffrin, and A. Chraibi. 1993. Analysis of pressure volume curves o f leaves of Rosa hybrida cv. Sonia. J. Exp. Bot. 44:605 613. Wang, Z.C.and G.W. Stutte. 1992. The role of carbohydrates in active osmotic adjustment in apple under water stress. J. Am. Soc. Hort. Sci. 117:816 823. Watkins, J.V., T.J. Sheehan, and R.J. Blac k. 2005. Florida landscape plants: Native and exotic. University of Florida Press, Gainesville. Wullschleger, S.D., K.B. Wilson, and P.J. Hanson. 2000. Environmental control of whole plant transpiration, canopy conductance, and estimates of the decoupling coefficient for large red maple trees. Agr. Meteorol. 104:157 168. Xu, B., F. Li, L. Shan, Y. Ma, N. Ichizen, and J. Huang. 2006. Gas exchange, biomass partition and water relationships of three grass seedlings under water stress. Weed Biol. Manag. 6:79 88 Yacoubi, S., K. Zayani, N. Zapata, A. Zairi, A. Slatni, R. Salvador, and E. Playan. 2010. Day and night time sprinkler irrigated tomato: Irrigation performance and crop yield. Biosyst. Eng. 107:25 35. Zahner, R. 1962. Terminal growth and wood formation b y juvenile loblolly pine under two soil moisture regimes. For. Sci. 8:345 352. Zwack, J.A., W.R. Graves, and A.M. Townsend. 1998. Lead water relations and plant develpoment of three Freeman maple cultivars subjected to drought. J. Amer. Hortic. Sci. 123:37 1 375.
92 BIOGRAPHICAL SKETCH Emily Elaine Massey was born in J acksonville, Illinois. In 2008, she received her B.S. from Illinois College in Jacksonville, Illinois with a major in b iology/ c hemistry and a minor in e nvironmental s cience. While attending Illin ois College, she studied symbiotic and asymbioitc orchid propagation at the Orchid Recovery Program under Dr. Lawrence Zettler Her work centered on propagation and restoration of threatened and rare species from Illinois and south Florida. In 2009, she continued her ecological based work at the University of Florida in Gainesville, Florida. She received her M.S. from the C ollege of Agriculture and Life Science in 2011. Her work with two popular Florida landscape trees has added to the s cientific knowledge of shoot elongation and the effects of turgor pressure on shoot elongation rates In addition, data collected from this study will be helpful in regulating irrigation cycles for live oak and Chinese elm In her free time, she enjoys reading, watching movies, and spending time with her three cats. She is an avid fan of Bob Dylan and playing board games.