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1 MECHANISMS OF COGONGRASS [ Imperata cylindrica (L.) Beauv.] COMPETIT ION, LOW LIGHT SURVIVAL, AND RHIZOME DORMANCY. By JINGJING WANG A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLM ENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2008
2 2008 Jingjing Wang
3 To my parents.
4 ACKNOWLEDGMENTS I sincerely thank my commite e chair Dr. Jay Ferrell and co c hair Dr. Greg MacDonald for giving me t he opportunity to work on this proje ct. I am grateful for the count less hours they put in to help me understand my work, write paper s analyze data and practice seminar presentation s I also thank my committee membe rs Dr. Lynn So l lenberger and Dr. Curtis Rainbolt for their insightful thoughts and suggestions to this research. Special thanks go to everybody I worked with in thi s weed science group, including Justin Snyder, Bob Querns, Eileen Guest Barton Wilder, Bran don Fast, Bucky Dobrow, Brett Bultimei er, Chris Mudge, Kurt Vollmer, Jim Boyer and Courtney Stokes They were very helpful with my work, and were my first American friends I would also like to thank all the faculty, staff and students of the Ag r onomy dep artment for their help and g uidance throughout the past two and a half years. Lastly I thank my parents, my best friend Sean, and all my other friends in China and the US. Without their continuous love and support, I would not be the person I am today.
5 TA BLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ ........... 4 LIST OF TABLES ................................ ................................ ................................ ...................... 7 LIST OF FIGURES ................................ ................................ ................................ .................... 8 ABSTRACT ................................ ................................ ................................ ............................... 9 1 INTRODUCTION ................................ ................................ ................................ .............. 11 Cogongrass Problem Statement ................................ ................................ .......................... 11 Taxonomy ................................ ................................ ................................ .......................... 13 Morphology and Biology ................................ ................................ ................................ ... 13 Physiology ................................ ................................ ................................ ......................... 16 Rhizome Physiology ................................ ................................ ................................ .......... 17 Habitat ................................ ................................ ................................ ............................... 19 Management ................................ ................................ ................................ ...................... 20 Rationale ................................ ................................ ................................ ............................ 21 2 COGONGRASS BAHIAGRASS COMPETITION as a function of SOIL PH .................... 23 Introduction ................................ ................................ ................................ ........................ 23 Research Objectives and hypothesis ................................ ................................ ................... 26 Objective ................................ ................................ ................................ ..................... 26 Hypothesis ................................ ................................ ................................ .................. 26 Materials and Methods ................................ ................................ ................................ ....... 26 Results ................................ ................................ ................................ ............................... 29 Plant B iomass ................................ ................................ ................................ .............. 29 Replacement Series ................................ ................................ ................................ ..... 30 Relative C rowding C oefficient (RCC) and A ggressivity (A) ................................ ........ 31 Discus sion ................................ ................................ ................................ .......................... 32 3 GROWTH AND PHOTOASSIMILATE PARTITIONING OF COGONGRASS UNDER DIFFERENT LIGHT INTENSITIES ................................ ................................ ... 40 Introduction ................................ ................................ ................................ ........................ 40 Objectives and Hyphothesis ................................ ................................ ............................... 42 Materials and Methods ................................ ................................ ................................ ....... 42 Plant M aterial P reparation ................................ ................................ ........................... 42 Light I ntensity T reatment ................................ ................................ ............................ 43 Data C ollection and M athematical A nalysis of G rowth ................................ ............... 43 Results and Discussion ................................ ................................ ................................ ....... 44 Results from Experiment 1 and 2 ................................ ................................ ................. 44 Overall D iscussion ................................ ................................ ................................ ...... 48
6 4 ABSCISIC ACID CONCENTRATION IN DORMANT COGONGRASS RHIZOMES .... 56 Introduction ................................ ................................ ................................ ........................ 56 Materials and Methods ................................ ................................ ................................ ....... 59 Results and Discussion ................................ ................................ ................................ ....... 60 5 C ONCLUSION ................................ ................................ ................................ .................. 63 APPENDIX: PHYTODE TED ABSCISIC ACID TEST KIT EXPERIMENT PROTOCOL ...... 65 LIST OF REFERENCES ................................ ................................ ................................ .......... 69 BIO GRAPHI CAL SKETCH ................................ ................................ ................................ ..... 78
7 LIST OF TABLES Table page 2 1 Cogongrass and bahiagrass under different soil pH (low and high) levels and density levels as measured by relativ e yield (RY) and relative yield total (RYT). ....................... 38 2 2 C ogongrass and bahiagrass as measured by relative crowding coefficient (RCC) and aggressivity (A). ................................ ................................ ................................ ............ 39 3 1 Experiment 1: G rowth of mature cogongrass plants as affected by light intensity for 12 weeks ................................ ................................ ................................ ........................ 50 3 2 Experiment 1: G rowth of newly established cogon grass plants as affected by light intensity for 12 weeks ................................ ................................ ................................ .... 51 3 3 Experiment 2: G rowth of mature cogongrass plants as affected by light intensity for 12 weeks ................................ ................................ ................................ ........................ 53 3 4 Experiment 2: G rowth of newly established c ogongrass plants as affected by light intensity for 12 weeks ................................ ................................ ................................ .... 54
8 LIST OF FIGURES Figure page 2 1 Average s hoot biomass under different plant densi ty levels and different soil pH levels. ................................ ................................ ................................ ............................ 35 2 2 Shoot biomass per plant under different plant de nsity levels and different soil pH levels. ................................ ................................ ................................ ............................ 36 2 3 Relative yield (RY) and relative yield total (RYT) of cogongrass and bahiagrass shoot biomass ................................ ................................ ................................ ............... 37 3 1 Experiment 1: Relative g rowth rate (Kw) compared with: A ) relative leaf expansion rate (Ka ) B ) relativ e leaf weight growth rate (Kl) C ) leaf area partitioning coefficient (LAP) and D ) leaf weight partitioning coefficient ( LWP). ................................ ............. 52 3 2 Experiment 2: Relative g rowth rate (Kw) compared with: A ) rel ative leaf expansion rate (Ka) B ) relativ e leaf weight growth rate (Kl) C ) leaf area partitioning coefficient (LA P) and D ) leaf weight partitioning coefficient (LWP). ................................ ............. 55 4 1 Experiment 1: ABA con centration (pmol/g DW) in cogongrass rhizome and scale leaves at different sections .. ................................ ................................ ........................... 60 4 2 Experiment 2: ABA con centration (pmol/g DW) in cogongrass rhizome and sca le leaves at different sections ................................ ................................ ............................ 61
9 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 MECHANISMS OF COGONGRASS [ Imperata cylindrica (L.) Beauv.] COMPETITION, LOW LIGHT SURVIVAL, AND RHIZOME DORMANCY By Jingjing Wang Dec ember 2008 Chair: Jason Ferrell Cochair: Gregory MacDonald Major: Agronomy Cogongrass [Imperata cylindrica (L.) Beauv.] has been reported as a serious perennial pest throughout the tropical and subtropical areas of the world and has been ranked the 7th w orst weed worldwide. It is an invasive C4 grass weed, and possesses a very strong and aggressive rhizome system, which is the major reason for its survivability and competitiveness. The low shoot to rhizome ratio greatly contributes to energy conservation and rapid re growth after chemical and mechanical control methods such as burning or cutting. Cogongrass thrives better in soils with low pH (pH about 4.7), low fertility, low organic matter, although it can grow in a wid e range of soils. The goal of this research is to provid e biological information concerning cogongrass growth and competitiveness to facilitate future research on integrated management of cogongrass. The first experiment evaluated the influence of soil pH on the relative competitiveness of cogongrass and bahiagrass. Based on previous research on different pH requirements for optimal growth of cogongrass and bahiagrass (cogongrass about 4.7, bahiagrass about 5.5 to 6.5), we hypoth esized soil pH wo uld influence the competition between these 2 species. O u r results showed that when soil pH increased, bahiagrass competitiveness was increased and cogongrass
10 competitiveness was decreased. Therefore cogongrass invasion into bahiagrass pastures appears to be strongly related to soil pH. The second e xperiment evaluated light intensity effects on cogongrass growth. Mature and young cogongrass plants were grown under different light intensity levels (75 25, 15, 2 and 1% of full sunlight) to measure growth pattern s and changes in rhizome to shoot ratio Mature plants had a completely developed rhizome system, and under low light intensities, the plants maintained a constant rhizome to shoot ratio. At light intensit y below the light compensation point, rhizome to shoot ratio increased dramatically indica ting possible rhizome dormancy. Young plants sacrificed rhizome growth for shoot production with rhizome to shoot ratio decreasing as light intensity decreased. The third experiment evaluated abscisic acid (ABA ) con centration in cogongrass rhizomes and rhi zome scale leaves as a function of apical position The relation ship between ABA con centration and rhizome dormancy was evaluated. Our results showed that ABA likely plays a role in cogongrass rhizome dormancy. The rhizome position that includes rhizome ti ps, the ABA con centration was significantly higher than in axillary nodes, including shoots and scale leaf tissue. Further research is still needed to confirm the actual function of ABA in cogongrass rhizome dormancy and the complex interaction between dif ferent plant growth hormones, such as auxin, also needs to be addressed in the future.
11 CHAPTER 1 INTRODUCTION I nvasive plant species are currently disrupting natural areas b y displacing native flora and fauna through several mechanisms. Non native specie s are often more proliferat e and less sensitive to environmental changes (Marion, 1986) and able to adapt to a wide range of environmental conditions These species are also capable of exploit ing a variety of niches minimi zing the effects of external press ure from the ecosystem (Reichard, 1996; Grace, 1999). Cogongrass Problem Statement Cogongrass ( Imperata cylindrica (L.) Beauv.), named as j apgrass, bladygrass, speargrass, alang alang and lalang alang (Dozier et al., 1998), is a cosmopolitan and disruptiv e grass species found throughout the tropi cal and subtropical areas of the world. It has been found on virtually every continent and is considered a troublesome weed in over 73 countries (MacDonald, 2004). Cogongrass is also regarded as the 7 th most hard t o control weed worldwide ( Holm et al., 1977 ). P e ndleton (194 8 ) first stated that the complete eradication of this noxious weed should be under taken as the hazard of this weedy species far outweigh s its benefits. Cogongrass has infested more than 500 mill ion hectares of land worldwide ( Holm et al., 1977 ; Dozier et al., 1998) including areas in Europe, northern Africa the Middle East, Australia and New Zealand and more than 35 million hectares in Asia (Garrity et al., 199 6 ; Holm et al., 1977). In the Unit ed States, cogongrass is found throughout much of the southern G ulf R egion (Dickens, 1974; Elmore, 1986) including southern Alabama, Georgia, Louisiana, Mississippi and Florida (Shilling et al., 1995). A survey of cogongrass distribution on Florida highway rights of way from 1984 to 1985 showed cogongrass widely distributed from the north central region southward through the central Florida ridge north of Lake Okeechobee (Willard et al., 1990)
12 T he highest distribution frequencies were in counties where cog ongrass was used for forage and soil stabilization during the 1950s (Willard et al., 1990). There were two mechanisms of cogongrass introduction into the United States. Cogongrass was inadvertently introduced from Japan to Alabama as a packing material in 1912 (Dickens, 1974; Tabor, 1949, 1952) and intentional ly introduc ed as a potential forage from the Philippines to Mississippi in 1921 (Hubbard et al., 1944; Dickens and Buc hanan, 197 5 ; Patterson et al., 1983; Tabor, 1949, 1952). F orage trials conducted i n Texas, Mississippi, Alabama and Florida proved that cogongrass was not a desirable forage species, but b y this time cogongrass had spread from the original points of introduction. Points of introduction into Florida were at The University of Florida Expe riment Station in Gainesville, USDA Plant Introduction Station in Brooksville and the Soil Conservation Service Reclamation Area in Withlacoochee (Hall, 1983; Willard, 1988). The quick spread of cogongrass was aided by cattlemen who took the grass f rom the Florida Experiment Station and established it on ranches throughout the state. By the 1950s, it had already covered more than 1000 acres of land in central and northwest Florida (Dickens, 1974). Cogon grass is generally considered to hav e little utility. I t is undesirable to grazing animals because the leaves have serrated margin s that accumulate silicon becom ing sharp and abrasive (Coile and Shilling, 1993). However, s ometimes it can be used for thatch, short term forage production, soil stabilization an d even paper making (Watson and Dallwitz, 1992). An Imperata cylindrica var. r ubra ornamental variety of this grass was developed in the United States called Rubra Red Baron or Japanese Blood Grass which is not aggressive (Greenlee, 1992). Sever a l natural compounds with medic inal value have also been discovered in cogongrass such as imperanene and cyl indrene (Matsunaga et al., 19 9 4 ).
13 Taxonomy Imperata is a genus of the tribe Andropogoneae, subtribe Saccharine. This genus has nine species includ ing I. brasiliensis I. brevifolia I. cheesemanii I. condensate, I. congerta, I. contracta, I. cylindrical, I. minutiflora and I. tenius (Gabel, 1982). Cogongrass is the most variable and important species in this genus and can be identified from other s pecies as having two anthers instead of one (Gabel, 1982; Hitchcock, 1951). Cogongrass was grouped by Hubbard et al., ( 1944 ) and Santiago ( 1980) into five varieties Varieties M ajor and Africana are the most widespread, damaging and variable (Brook, 1989). However, recent studies show all 5 varieties to be integrated; making true distinction s difficult (Clayton and Renvoize, 1982). Morphology and Biology Cogongrass is a stemless, tufted grass species. The above ground portion of the plant is slender with flat leaf blades a rising from rhizomes. Leaf blades are 1 to 2 cm wide and 15 to 120 cm long with prominent, white and slightly off center mid veins and very narrow and sharp tips ( Hubbard et al., 1944) Stomata can be found on both surfaces of the leaf bl ades (Bryson and Carter, 1993). Cogongrass i nflorescence is a 10 to 20 cm long panicle containing an average of 460 individual spikelets (Bryson and Carter, 1993; Holm et al., 1977; Shilling et al., 1997). A single plant can produce as many as 3000 seeds (Sajise, 1972). Seeds are small and attached to a plume of silky white hairs allow ing seeds to travel as long as 15 miles over open country (Hubbard et al., 1944). Larger spikelet clumps and greater wind speed favor larger dispersal d istance s (M cDonald et al., 1996 ) Flowering is promoted through external stress es such as cold, mowing, tillage, or burning (Sa jise, 1972) although some studies suggested flowers produced under such conditions cannot produce viable seed s (Eussen, 1980). In the Philippines, cog ongrass flower s year round (Holm et al., 1977) while flowering usually occur s in the late winter and early spring
14 i n the United States (Shilling et al., 1997; Willard, 1988). C ogongrass has been observed flowering throughout the year (Gaffney, 1996) suppor ting the argument by Dickens and Moore (1974) and Shilling et al. (1997) that cogongrass flowering, at least in southern US, does not depend on photoperiod. Cogongrass seed germination requires light and appears to be photochrome mediated (Sajise, 1972; S oemarwoto, 1959, Dickens and Moore, 1974)). The pH requirement of cogongrass seed germination is usually less than 5.0 (Sajise, 1972) and the optimum temperature for germination was reported to be 30 C (Dickens and Moore, 1974). Most seeds will remain viab le for at least three months if stored under cold and dry condition s G ermination rate s are as high as 95 to 98% immediately after harvest, provid ing adequate spikelet fill occurs (Santiago 1965; Shilling et al, 1997). Seed v iability has been shown to dro p to 50% by 7 months and 0% by 11 months (Shilling et al., 1997). Cogongrass seedlings generally emerge in groups (Shilling et al., 1997) and are surprisingly weak competitors with other grass seedlings (Dozier et al., 1998; Shilling et al., 1997; Willard and Shilling, 1990). Cogongrass seedling s were shown to be less competitive than bahiagrass seedlings and were not able to establish in areas with > 75% coverage of bahiagrass sod. However, cogongrass ramets arising from rhizomes were shown to be more comp etitive than bahiagrass seedlings (Shilling et al., 1997; Willard and Shilling, 1990). Cogongrass seedling growth has been shown to be enhanced primarily by decreasing in ter specific competition (King and Grace, 2000a, 2000b). Cogongrass is a prolific seed producer; however, o nly cross pollination from geographically isolated and heterogenous populations can produce viable seeds McDonald et al. ( 1996), sho w ed that cogongrass population s in Florida o nly produce self incompatib le seeds (McDonald et al., 1996) H owever, viable seed production may be occur ing in Florida recently
15 due to co mingling of distinct populations Seed would explain cogongrass invasion into new sites over large geographic areas and infestations has been found in isolated areas of Alabama and Mississippi, however, rhizome spread through m ovement of contaminated s oil during construction or intentional forage planting has been acknowledged as the major mechanism of spread in the U.S. ( Hubbard et al., 1944; Wilcut et al., 1988 ; Patterson et a l., 198 0, Patterson and M cWhorter, 198 0 ). Cogongrass r hizomes are defined as C strategist that can thrive in established population of other plants (Tominaga, 2003). Rhizomes are long, white and tough with prominent node s and short internodes. The cataphyl ls (scale leaves) are brownish red and serve as a protective sheath around the rhizome (Ayeni, 1985; English, 1998). The sharp tip of the rhizome can grow into other plants roots, bulbs and tubers and cause physical damage as well as infection (Boonitee an d Ritchit, 1984; Eussen and Soerjani, 1975). The sclerenchymous fibers under the epidermis and the sclerotic tissue surrounding the vascular bundles help rhizomes to conserve water and resist breakage, disruption, desiccation and heat from fire (Holm et al ., 1977). Axillary bud dorman cy is maintained through auxin imposed apical dominance (Gaffney, 1996) Rhizomes can pro duce over 40 tons per hectare fresh weight with density as high as 89 m linear length within 1 m 2 of soil surface area (Lee, 1977). Rhizo mes form a dense mat underground and comprise over 60% of the total plant biomass These structures facilitate rapid re growth after defoliation (Sajise, 1976) and can give rise to 350 shoots in 6 weeks and cover a 4 m 2 area in 11 weeks (Eussen, 1980). Rhi zomes are resistant to heat but susceptible to cold temperatures (Wilcut et al., 1988 Hubbard et al., 1944). Rhizomes are usually found within the top 8 to 40 cm of soil depending on soil type but have the ability to grow as deep as 120 cm in the soil ( G affney, 1996).
16 Physiology Cogongrass is a perennial, rhizomatous, C4 plant that is able to tolerate drought, fire, cultivation and short term shade (Terry et al., 1997). T he light compensation point for cogongrass is 32 to 35 mol m 2 s 1 (Gaffney, 1996 ; Ramsey et al., 2003), which is approximately 2% of ambient light intensity in North Central Florida (Gaffney, 1996) Some research suggested shading is beneficial for cogongrass control because it reduce s carbohydrate storage, rhizome production and shoo t weight This results in decreased regeneration vigor increasing susceptibility to herbicide s (MacDicken et al., 1997). Patterson (1980) showed that s hading change d the total biomass, biomass distribution and plant morphology of cogongrass C ompared to pl ants in exposed habitats, cogongrass in shaded habitats possessed decreased total plant biomass, biomass distribution in rhizomes and an increase in biomass distribution in leaves. C ogongrass adaptation s to light intensity level changes were realized by sp ecific leaf area and leaf area ratio changes tolerating a 50% reduction in sunlight. Moosavi Nia and Dore (1979) observed the changes of herbicide movement under increasing shade. They found that glyphosate symptoms appearance w as delayed in the shaded pl ants with increased efficacy L ight is a requirement for cogon g rass rhizome propagation with light stimulat ing rhizome bud growth (Soerjani and Soemarwoto, 1969). Vegetative reproduction by rhizome s is responsible for the local spread of cogongrass and ne w population establishment. A single rhizome system contains huge number s of viable buds each with the potential to give rise to new shoots. R hizome r egeneration capacity increases with increasing age, weight, length, thickness and number of these visible buds. Younger buds have reduced regeneration capacity because of the lack of roots (Terry et al., 1997). Ayeni (1985) developed a model of cogongrass rhizome growth and development. Early rhizome growth starting at the third or fourth leaf stage is verti cal. W hen rhizomes start to
17 develop scale leaves at the fifth leaf stage, rhizome growth become s horizontal. R hizome tips then begin to grow upward and form secondary shoot s and rhizomes. Buds on the convex side of the rhizome usually form new rhizomes but buds on the concave side will either develo p shoots or remain dormant. Rhizomes and shoots develop at the same time on more mature plants but shoots develop first in stressed p lants. Rhizome Physiology Correlative inhibition is a term defining the growt h inhibiting effects of one part of the plant on another (Goebel, 1900; Smith and Rogan, 1980). In the case of apical dominance apex buds exert control on the growth of axillary buds There are four stages of apical dominance development : 1) axillary bud formation, 2) imposition of inhibition, 3) the rel ease of apical dominance and 4) subsequent growth of axillary buds Correlative inhibition is primarily present in the first two stages (Cline, 1997) I n these stages the growing apex is a stronger competit or for water and nutrients with a higher sink activity than axillary buds This activity also influences herbicide efficacy because systemic herbicides move primarily to actively g rowing plant parts, and dormant buds with lower sink acitivit ies will accum ulate a sub lethal herbicide dose. The apex will be killed by the herbicides, releasing axillary buds which will start to grow T herefore the whole plant will escape from herbicide treatment. T here are several proposed mechanisms of apical dominance. T he most widely accepted is the auxin inhibition hypothesis. Auxin produced at apical buds will move b asipetally along the rhizome and directly inhibit lateral bud growth This theory was confirmed by reproducing apical dominance through the application of ex o genous auxin after removal of the apex (Leakey and Chancellor, 1975). Another theory is nutritional differences among plant parts. The apex is a stronger sink than basal parts so nitrogen and carbohydrates accumulate in the apex thus reducing growth of no n sink regions (McIntyre, 1990). McIntyre (1971, 1987) also suggest ed
18 that the rhizome apex could be a stronger competitor for water than axillary buds. The ratio of p lant hormone concentration also appear s to have effects in apical dominance. For example, auxin may cause inhibition of axillary buds in development stage II, but has promotional effects on stage IV. Cytokinin promotes the development of stage I, which is bud formation and will cause release of apical dominance (stage III) when exogenously app lied (Cline, 1997; Tamas, 1987). Collectively t hese data infer apical dominance is the comprehensive effect of several factors including hormone s nutrients, and g rowth factors In terms of rhizome activity, generally lower light intensity will increase a pical dominance because of the reduced amount of carbohydrates available and the competition between nodes for these carbohydrates (McIntyre, 1990). However, research on quackgrass showed that under lower light intensity, shoots will be produced in all rhi zome nodes instead of being restricted to the nodes close to the apical end due to apical dominance (McIntyre, 1970). Some research showed that the percentage of buds producing shoots w as not influenced by light in quackgrass (Chancellor, 1968; Jonson and Buchholtz, 1963), whereas Leakey et al. (1978) found that light would delay or prevent shoot formation which is called light induced inhibition It was suspected to due to the low rate of transpiration result ing to low water potential in the rhizomes unde r low light intensity Parental plants also have effects on axillary bud growth. R esearch on quackgrass rhizomes showed that the viability of buds increased with increasing distance from the parent plant (Dekker and Chandler, 1985; Tardif and Leroux, 1990 ) and it was suggested that the smaller internodes providing less nutrition might be the reason for the low viability of the basal buds (Tardif and Leroux, 1990). Leakey et al. (1975) found that in quackgrass rhizomes the removal of the apex would not full y release axillary buds from dormancy unless the whole rhizome was
19 removed from the parent plant. It was suggested that cytokinins from the roots on the parental plants, as well as auxins, nutrients and other hormonal factors are the reasons for parent pla nt factors. In cogongrass, r emov al of the apex will release the axillary buds from apical dominance with b uds closest to the actively growing apex being less dormant and more likely to sprout (Gaffney, 1996). S cale leaves may also contribute to bud dorman cy One scale leaf is produce d per rhizome node and its major function is as a protective sheath. Robertson et al. (1989) found that removal of scale leaves from quackgrass rhizomes was effective in promoting sprouting of previously dormant buds English ( 1998) found similar results w ith cogongrass. Roberston et al (1989) suggested that the reason might not be due to the factor of scale leaves but the exposure of buds to light and the secondary effects of light This may also be the case in cogongrass as s prouting of cogongrass rhizomes is two to three times greater in light than under dark condition s (Holm et al., 1977). Research also suggests that the ABA produced in scale leaves might contribut e to bud dormancy but low levels of ABA were found in the sc ale leaves (Taylor et al., 1995). Habitat Cogongrass usually thrive s in tropical and subtropical areas where annual rainfall is 75 500 cm (Bryson and Carter, 1993). This species does not infest cultivated areas but is found in disturbed areas such as ro adways, pastures, min e sites forest land, park and other recreational areas It grows under a wide range of soil conditions (Bryson and Carter 1993, Gaffney, 1996) and can tolerate soils with low pH (4.7), poor fertility, and low organic matter (Sajise, 1 980). Ramakrishnan and Saxena (1983) reported its high efficiency in nutrient uptake association s with m ycorrhiza (Brook, 1989) and high utility of phosphorus (Brewer and Cralle, 2003) The competitive nature of cogongrass contributes to its ability to e xclude other species and establish
20 monotypic stands In addition to direct competition, cogongrass interferes other plant growth through allelopathy (Eussen, 1979 ; Casini et al., 1998; Koger and Bryson, 2003). Management Herbicide trials have been conduc ted extensively on cogongrass with h undreds of materials tested, but imazapyr and glyphosate have been shown to be the only consistently effective herbicides T he most effective application dosages are 4.5 kg/ha and 0.8 kg/ha for glyphosate and imazapyr r espectively (Willard et al., 1996). Compared to glyphosate, imazapyr provides a longer period of control due to soil activity, but off target effect s are a concern with its usage (MacDonald et al., 2002). It was shown that imazapyr applied in late summer o r early fall can provide cogongrass control as long as 18 month (Dozier et al., 1998). Herbicide efficacy can be influenced by many factors. Development al stage at time of applica tion influence s efficacy Under greenhouse conditio ns, glyphosate applied t o 8 week old cogongrass provide d better control than at 12 weeks after planting It was hypothesized that the delay in application timing allowed the rhizome system to become more established (Willard, 1988). Seasonal a pplication t iming is another importan t consideration for control A pplication in the fall has consistently shown better herbicide activity relative to spring applications due to basipetal movement of photosynthates in the fall of the year (Johnson et al., 1999 ; Gaffney, 1996; Tanner et al., 1 992). Essentially, fall applications result in great er herbicide loading into the rhizome complex. However low rainfall during that time of the year may caus e low availa ble soil moisture th us reduc ing herbicide movement and activity (D ozier et al., 1998). Cogongrass responds differently to mechanical treatment Shallow tillage only provides short term red uctions in shoot growth, while deep tillage provide s better control by cut ting rhizomes into fragments and by bringing material to the soil surface to de siccate Multiple and deep t illage more than 15 cm over a period of months to 2 years may achieve good results
21 (Shilli ng et al., 1995). Disking has been shown to provide a 27% decrease in rhizome biomass one time and a 66% decrease when performed twice (Wi llard et al., 1997). Mowing alone does not control cogongrass but can reduce rhizome and foliage biomass (Willard and Shilling, 1990; Willard et al., 1996). It has been suggested that one of the most effective cogongrass management practice s is integrating disking with herbi cide applications. Deep disking 2 0 to 30 cm to break apical dominance and promote new leaf growth, followed by herbicide application to regrown shoots, followed by postapplication discing will enhance herbicide incorporation in the soil (Dozier et al., 1998). Integrated management is necessary for cogongrass management since single management method s have often fail ed to provide effective manage ment Incorporating different methods including burning, till ing, mowing, cultural and chemica l control will provide more effective long term control ( G affney, 1996). After control t he niche released by cogongrass must be rapidly replaced by desirable plant species to exclude cogongrass reinvasion The cho ice of a desirable species is dependent on species tolerance to the previous cogongrass management techniques and soil type. Some have suggested bahiagrass ( Paspalum notatum ) and hairy indigo ( Indigofera hirsut a Harv.) might be alternative choices. Rationale Emphasis has been made on the importanc e of an integrated management system to suppress cogongrass. Although field trials integrate different weed control methods physiological and biological information concerning cogongrass will be helpful in refining control strategies T he biggest issues w ith cogongrass management include competitiveness low light tolerance and rhizome dormancy. The se factors aid in the ability of cogongrass to compete and exclud e other species. These studies will focus on the biological characteristics of cogongrass comp etitiven ess, rhizome system development under different environmental
22 conditions and the potential for rhizome dormancy. T he specific objectives of these experiments are: Evaluate the competitiveness of cogongrass and bahiagrass under different soil pH co nditions Evaluate the growth patterns of cogongrass as a function of maturity level under different light intensities Evaluate the level of abscisic acid in cogongrass rhizome s as a function of node position T he hypotheses addressed follow: Cogongrass co mpetitive ness with bahiagrass change s as a function of soil pH Shoot to rhizome ratios in cogongrass will change as a function of light intensity and maturity level Abscisic acid is present in cogongrass rhizomes at levels and locations that indicate a rol e in bud dormancy
23 CHAPTER 2 COGONGRASS BAHIAGRASS COMPETITI ON AS A FUNCTION OF SOIL PH Introduction Cogongrass [ Imperata cylindrica (L.) Beauv. ] is a warm season, rhizomatous, perennial, C4 grass that has become a serious pest throughout the tropical and subtropical areas of the world (Favley, 1981; Holm et al., 1977). It is listed on the Federal Noxious Weed List (USDA Animal and Plant Health Inspection Service 2006) and several states Noxious Weed List s including Florida, Alabama, Mississippi, North C arolina, Vermont and Hawaii ( USDA Natural Resources Conservation Service Plants Profile, 2008 ) F lorida has large tracts of grazing land ranging from improved pastures to rangeland. R angelands here means long term pasture settings that generally receive low er levels of input and thus experienc e a greater degree of species invasion that further reduces forage performance. Cogongrass is currently increasing in importance Bahiagrass ( Paspalum notatum ) is a pasture specie s at risk of infestation by cogongrass (Shilling et al., 1997; Willard and Shilling, 1990). Bahiagrass is the most widely utilized forage in Florida, covering an estimated 2.5 million acres (Chambliss, 1996). Bahiagrass is a warm season perennial species w ith a deep fibrous root system, and can thrive in dry, infertile soils (Beard, 1980; Watson and Burs on, 1985). It requires little to no irrigation, minimal fertilization and is susceptible to relatively few insect or disease pests (Chambliss, 1996) The op timal condition for bahiagrass growth is a soil pH of about 5.5 to 6.5 Seeds are common ly used to establish bahiagrass, but seedlings are weak competitors (Beard, 1980; Watson and Burson, 1985). This slow establish ment rate from seeds and poor growth unde r moderate or heavy shade makes bahiagrass susceptible to competition from aggressive grass species (Busey and Myers, 1979; Watson and Burson, 1985).
24 C ogongrass generally infests areas with low poor fertility and low organic matter (Saji se, 1980; Wilcut et al., 1988). Cogongrass does not tolerate tillage, and is therefore commonly found on roadways, pastures, reclaimed mining areas, forest land, parks and other recreational areas ( Gaffney, 1996). Cogongrass is highly competitive often f orm ing monotypic stands. M echanism s of i nterference can include competition, allelopathy and physical injury ( Eussen and Soerjani, 1975 ; Eussen, 1979 ). These mechanisms retard growth, cause yellowing and die back and otherwise reduce yield and/or growth of othe r crops and desirable plants (Hubbard et al., 1944; Soerjani, 1970). Once cogongrass has invaded an area, establish ment of other desirable grass species is difficult due to the dense rhizome system This extensive rhizome network physically excludes other vegetation and quickly extract s soil moisture and nutrients from shallow soil layers (Boonitee and Ritdhit, 1984 ; Terry et al., 1997 ; Casini et al., 1998; Koger and Bryson, 2003). Cogongrass is capable of reinvading an unoccupied ecological niche aft er effective control methods are implemented. Therefore, the reestabl ishment of desirable forages is critical to prevent re in vasion by cogongrass (Shilling et al ., 1997; Yandoc et al 2004). A ccording to Gaffney (1996), the success of this strategy relie s on the tolerance of the desirable species to the herbicide used for cogongrass and agronomic conditions favorable for growth. He suggested that the use of herbicides followed by the establishment of either bermudagrass (Cynodon dactylon) will effectively suppress cogongrass and prevent reinfestation. Barron et al., ( 2003) reported similar results using bahiagrass and bermudagrass under field conditions According to Shilling (1997), the degree of invasiveness of a given species at a given time can be def ined as the relative ability of that species to displace other species. Competitiveness can be regarded as the determinant of displacement ability. Therefore, the relative
25 competitiveness of bahiagrass and cogongrass can be a crucial factor in cogongrass i nvasion into bahiagrass pastures. Bahiagrass is usually established from seeds with a relatively slow growth rate (Beard, 1980) while cogongrass most commonly spreads from rhizomes. Willard and Shilling (1990) showed that the rate of cogongrass establishm ent is more rapid than that of seedling bah iagrass Additional greenhouse studies by Willard et al., (1990) demonstrated the influence of growth stage on competition between bahiagrass and cogongrass. R esults showed bahiagrass seedlings to be less competit ive than cogongrass emerging from rhizomes, while established bahiagrass i.e. in this case bahiagrass planted 8 weeks before cogongras, showed more c ompetitiveness than cogongrass Furthermore, bahiagrass establishment combined with one mowing maximized c ompetitiveness. Under conditions of no nutrient or water stress, cogongrass effectively competed with seedling bahiagrass but no t with established bahiagrass. This information provides us with the basic theory of evaluating the relative competitiveness bet ween these two species. Under optimal environmental conditions and at certain growth stage s bahiagrass can actually be m ore competitive than cogongrass (Shilling 1997). T herefore, if we improve growth conditions f or bahiagrass, it will likely prevent the establishment of cogongrass. A s stated previously cogongrass has the ability to reinvade into an unoccupied ecological niche Therefore if could be achieved long term cogongrass control may be realized The classic replacement series experimental design (de Wit, 1960) has been widely used in experimental studies of interspecies competition. For a two species design, the experiment consists of a pure stand of each species alone and combinations of mixture s of different ratios while maintaining constant overall density T he biomass each species contributes to the total biomass at each different planting ratio determines competitiveness at a given ratio. Performance
26 of the species in mixture compar ed with that in a pure stand is used to assess relative competitiveness and aggression. E nvironmental conditions such as fertilizer, water status, soil pH etc. can also be manipulated to study the influence of different parameters on competitiveness. In s ummary, i t is noted that optimum soil pH for b ahiagrass is 5.5 to 6.5 while cogongrass soils, low soil pH appears to have a greater negative impact on bahiagrass than o n cogongrass (Shilling, 1997). Soil p H could play a major role in the management of competitiveness between these two species. Research Objectives and hypothesis Objective The objective of this experiment was to study the competition between rhizomatous emerging co gongrass established from rh izomes and seedling bahiagrass as affected by two different levels of soil pH (pH= 6.8 pH=4.5). Hypothesis Our hypothesis is that t he relative competitiven e ss of cogongrass and bahiagrass will be changed by different soil pH levels. Materials and Methods Preliminary density and fertility stud ies performed by Shilling et al., (1997) showed that cogongrass density of 144 plants/m 2 and bahiagrass density of 720 plants/m 2 are optimal plan t densities for both species. To minimize intraspecific competition and m aximize interspecific competition, soil fertility level was achieved based on previous research by Shilling et al. (1997)
27 with 45.4 kg ha 1 N by using 14 14 14 fertilizer 1 These methods will be used in the following experiments. Cogongrass plant s used in this experiment were propagated from 10 cm long rhizome segments collected from a population in Gainesville, FL. Rhizomes were planted in plastic flats and covered with commercial potting mix 2 and were placed in direct sunlight area and watered daily. Aft er two weeks, plants with 2 leaves attached to a single node were selected and transplanted. Bahiagrass seeds were planted in plastic flats similar to cogongrass and approximately 3 weeks after planting, 2 leaf stage seedlings of equal size were select ed a nd transplanted. Both species were transplanted at the same time into 4L volume pots at desired density levels. Plants were then placed in a greenhouse under the following envirionmental conditions: 16 h day: 8 h night photoperiod and 30 0 C day: 20 0 C nigh t temperatures. In this study, t wo pH treatment levels were evaluated pH= 6.8 and pH=4.5. Soil was a Chandler fine sand and was gathered from the University of Florida P lant Science R esearch and E ducation C enter in Citra, FL. For each soil pH level, a repl acement series model for studying competition between the two species was established C ogongrass densi ties were 0, 1, 2, 4, and 8 shoot s/pot and corresponding bahiagras s densit ies were : 40, 20, 10, 1 and 0 plants/pot respectively The resulting proporti ons were : (40:0), (20:1), (10:2), (1:4) and (0:8) for a single pot ; these proportions were based on previous research (Shilling et al., 1997) The experiment al unit was one pot and it was considered as one replication. T he experiment was established as a 2 (pH levels) by 5 (densities) factorial in a completely randomized design with 4 replications. The study was conducted in August 2007 and repeated in October 2007. At 8 w ks after trans planting, 1 Scotts Osmocote 14 14 14 2 Fafrad super fine germination mix. Conrad Fafrad. Inc. P.O.Box 790. Agawam, MA 01001 0790
28 species were harvested and separated. Plant tissues were plac ed in a 75 0 C oven for 3 days and dry weights were determined Competitive indices were based upon shoot biomass and calculated as follows (de Wit, 1960; de Wit and van den Bergh, 1965; McGilchrist and Trenbath, 1971; Rejmanek et al., 1989; Radosevich, 19 88): Relative yield (RY) : the yield of each species in mixture as a percent of its monoculture yield und er the same growing conditions. RY of species A =biomass production of species A at a particular proportion/ biomass production of species A in monocultu re Relative yield total (RYT) : the absolute yield of each species within a given proportion. RYT =RY of species A + RY of species B Relative crowding coefficient (RCC) : a measure of the relative competitiveness of one species over another. RCC of species A with respect to species B = ((dry weight/plant o f species A at a particular proportion )/(dry weight/plant of species B at a particular proportion))/ (( dry weight/plant of species A at monoculture )/(dry weight/plant of species B at monoculture)) RCC is a n index. An RCC of 1.00 indicates equal competiti veness between the two species and RCC increases as a certain species competitiveness increases. Aggressivity (A) : another way to measure the rel ative competitive ability of each species A of species A = (( dry weight/plant of species A at a particular proportion )/(dry weight/plant of species A at monoculture)) (( dry weight/plant of species B at a particular proportion )/(dry weight/plant of species B at monoculture)) A value of 0 denotes equal competitiv e ness while s pecies with positive values are considered to be more competitive. All data were subjected to ANOVA. A r eplacement series curve model under different pH levels was created based on RY and RTY of both cogongrass and bahiagrass.
29 Results Plant B iomass ANOVA results indicated no run by density interaction ; therefore data from both experiments were pooled As expected, total biomass for cogongrass increased as the proportion of cogongrass plants per density level increased. However, t here was n o sig nificant difference (P>0.05) for cogongrass biomass between the two pH levels within each level of plant density (Figure 2 1A ) This trend was also observed f or bahiagrass (Figure 2 1 B ) There was also no significant difference (P>0.05) between bahia grass shoot biomass within each level of plant density between the two different soil pH levels On a per plant basis, cogongrass biomass declined sligh tly at the higher densities (4: 01 and 8:0). There was no significant difference between biomass at each pH level w ithin a given density (Figure 2 2 A ). For bahiagrass (Figure 2 2 B ), single plant shoot biomass also decreased as the proportion of bahiagrass in the mixture increased. T here was no difference in individual plant biomass as a function of soil pH at each density level, except at the lowest bahiagrass: cogongrass proportion (4:01). At this level, bahiagrasss shoot biomass was two times greater under high soil pH condition In terms of total plant biomass per pot and single plant biomass, cogongra ss perform ed better under lo w soil pH, while bahiagrass perform ed better under high soil pH. Our results indicated that cogongrass growth is similar under widely different soil pH values, while low soil pH level negatively impacted bahiagrass biomass prod uction. For single plant biomass, there was no statistical d ifference between cogongrass at different density levels. T he situation for bahiagrass was different. S ingle plant biomass increased as plant density decreased. However, there was an obvious trend that under higher levels of competition (1:20, 2:10), cogongrass
30 performed better under low pH conditions. U nder higher levels of competition (2:10, 4:1), bahiagrass performs better under high soil pH conditions. Replacement Series Under low soil pH (Fig ure 2 3 A ), relative yield curves of cogongrass and bahiagrass crossed at the density level of cogon: bahia=2:10, indicating similar relative yield and thus similar competitiveness at this density. U nder high soil pH conditions (Figure 2 3 B ), relative yie ld curves of cogongrass and bahigrass crossed at the density level between cogon:bahia=2:10 and cogon:bahia=4:1 At pH 6.8 a lower density of bahiagrass (i.e. greater density of cogongrass) achieved the same level of competitiveness as the pH 4.7 soils. T herefore, high soil pH favors bahiagrass competitiveness A similar conclusion can also be drawn by the species contribut ion to total yield. The area below relative yield (RY) line of a certain species shows yield con tribution of that species to total yiel d and the area below relative yield total (RYT) line shows total yield. Under low soil pH, cogongrass and bahiagrass contributed similarly to the total yield w hile bahiagrass contributed more to the total yiel d unde r high soil pH Moreover, the area diffe rence of relative yield of cogongrass/bahiagrass between different soil pH levels indicated the competitiveness changes for both species. Comparing with low soil pH, cogongrass area, i.e., its relative yield was slightly decreased and bahiagrass area, i.e. its relative yield was increased under high soil pH Numeric value s for relative yield (RY) an d relative yield total (RYT) are presented in table 2 1. ANOVA results indicated that for cogongrass RY was only significantly higher at low soil pH at 2:10 de nsities, but not significantly different between two pH levels at 1:20 and 4:01 density level. However the overall trend is that co gongrass RY at all densities is higher at low soil pH. B ahiagrass RY at high pH was significantly higher than that at low pH at 4:01, however, there is also a obvious trend that bahiagrass RY are higher under high soil pH at all densities
31 T he biggest RY difference for both species between high and low soil pH occurred at the cogon: bahia density of 2: 10. For RYT, t here was no significant difference between pH levels (P=0.79 ) RYT of mixed species in each density level was always below 1, indicating antagonistic effects between cogongrass and bahiagrass. Relative C rowding C oefficient (RCC) and Aggressivity (A) RCC is an index indicating the relative competitiveness of one species to another. The relative competitiveness of a species increases as the RCC increases A RCC of 1 indicat es both species are equally competitive (Harper, 1977). ANOVA results showed that cogongrass RCC at low pH was not significantly higher than at high pH however, the overall trend was cogongrass RCC was higher at low soil pH Conversely bahiagrass RCC at low pH was significantly lower than at high pH except for cogongra : bahiagrass=2:10 A t low soil pH, b ahiagrass was more competitive than cogongrass at cogongrass: bahiagrass=1:20 and 4:1 but less competitive than cogongrass at cogongrass: bahiagrass=2:10 (Table 2 2) However, at high soil pH, bahiagrass RCC was significantly higher than cogongrass at almost all density levels except for cogongrass: bahiagrass=1:20 For aggre s si vity (A), t he species with a positive A value is considered to be the more competitive negative value the less competitive, and a A value o f 0 means equal competitiveness betw een species. For cogongrass aggre s sivity (A) value, ANOVA indicated a pH by density interaction (P=0.01). The aggressivity of c ogongrass at low soil pH had positive A values at cogon: bahia =1:20 and 2: 10, but negative value s at cogon: bahia =4:1 while in teracting with bahiagrass. A t low densities and low pH cogongrass showed more aggress ivity than bahiagrass. Under high soil pH, cogongrass A values were all negative at all density levels, indicating bahiagrass was more aggressive than cogongrass
32 RCC and A values did not reflect the exact same trend compared to relative competitiveness of cogongrass and bahiagrass as a function of density and soil pH However, it is clear by both ind ices that bahiagrass competitiveness was greatly enhanced at high soil pH T he largest difference between both RCC and A of bahiagrass and cogongrass occurred at the ratio of cogon: bahia= 2:10 Once again this appears to be the density at which bahiagrass competitiveness against cogongrass is most profoundly affected by soil p H Therefore t here is a significant difference between high and low soil pH on the relative competitiveness of cogongrass and bahiagrass. High soil pH increased bahiagrass competitiveness. Both cogongrass and bahiagrass competitiveness were significant ly different at two different soil pH levels and c ogongrass competitiveness was not greater than bahiagrass at low soil pH. Discussion A deWit replacement model was used to determine the relative competitiveness of the two plant species. Willard and Shilling (1990) mentioned in their research on competition between cogongrass and bahiagrass that s ince compari son s w ere made between two species at different stage s of development, prudence should be exercised when drawing conclusion s based on the replacement mod el However, t he data provide essential background information for further research on competition between cogongrass and bahiagrass. Although high and low soil pH treatment s did not result in significant plant biomass difference s for either species rel ative competitiveness was shown to be significantly different as a function of soil pH. Cogongrass RY, RCC and A were all lower under low soil pH, conversely bahiagrass RY, RCC and A were all significantly greater under high soil pH conditions Our resul ts differ somewhat from p reviou s research which focused on the influence of propagule type for cogongrass and bahiagrass. Shilling et al. (1997) compared the relative
33 competitiveness between cogongrass seedlings and bahiagrass seedlings and cogongrass rame ts and bahiagrass seedlings. These data indicated that bahiagrass seedlings were not as competitive as cogongrass ramets (i.e., cogong rass established from rhizomes but were more competitive than cogongrass seedlings. T herefore propagule type is an impor tant factor in the relative competitiveness between the two species. Research conducted by Willard et al. (1990) reflected similar results as Shilling et al. (1997) where cogongrass maintained advantage over seedling bahiagrass regardless of the species mi xture but was less competitive than est ablished bahiagrass. However, under field conditions, it is more commo n that bahiagrass pastures are established from seeds and cogongr ass is introduced or invades by rhizomes. Fo r established bahi agrass pastures, hu man or natur al disturbance may play an important role in cogongrass invasion. In many areas c ogongrass does not become dominant until disturbance releases it from competition however after invasion, cogongrass can successfully displace the original speci es and establish a monoculture (Will ard et al., 1990; Eussen and Soerjani, 1975). In previous research (Willard et al., 1990 and Shilling et al., 1997), neither study noted the pH of the soil used in the experiments Since previous research is somewhat co ntradictary as to the competition between cogongrass and bahiagrass, it is important to discern whether differing soil conditions would change competitiveness. Our results indicated that at pH value < 5, t he competitiveness between cogongrass and bahiagra ss was simi l ar. At pH > 5 bahiagrass showed greater competitiveness over cogongrass support ing the environmental conditions which favor cogongrass and bahiagrass. O ptimum soil pH for bahiagrass is 5.5 to 6.5 (Chambliss, 1996), while cogongrass has the abi lity to thrive in habitats et al., 1988). The significant difference s between cogongrass and bahiagrass performance at different soil pH indicated soil
34 pH plays an important role in the c ompetitivene ss between the two species Cogongrass ramets have been shown to effectively compet e with bahiagra ss seedlings, however, when soil pH is favorable for bahiagrass, the competitive edge of cogongrass is lost. Moreover our studies demonstrated that cogongras s ramets did not show overwhelmingly greater competitiveness over bahiagrass even at a soil pH that is consid ered to favor cogongrass growth Bahiagrass seedlings are generally regarded as weak competitor s because of s low growth rate; however, our research showed that under certain conditions, bahiagrass competitiveness can be elevated Therefore, proper soil conditions could defer cogongrass invasion into bahiagrass.
35 Figure 2 1 Average s hoot biomass under different plant density levels and d ifferent soil pH levels. D ata represent ed as the mean of 8 replications with standard error A ) C ogongrass shoot biomass per pot/rep B ) B ahiagrass shoot biomass per pot/rep
36 Figure 2 2 Shoot biomass per plant under different plant density levels and different soil pH levels. D ata represent ed as the mean of 8 replications with standard error A ) C ogongrass shoot biomass per plant B ) B ahiagrass shoot biomass per plant
37 Figure 2 3 Relative yield (RY) and relative y ield total (RYT) of cogongrass a nd bahiagrass shoot biomass D ata represent ed as the mean of 8 replications with standard error A ) L ow soil pH 4.7 B) H igh soil pH 6.8
38 Table 2 1 Cogongrass and bahiagrass under different soil pH levels (low and high) and density levels as measured by re lative yield (RY) and relative yield total (RYT) Density RY Cogon RY Bahia RYT (cogon: bahia) low high low high low high 0:40 a 1 1 1 1 1:20 0.19 0. 0 5 A b 0.12 0.02 A 0.66 0.35 A 0.76 0.04 A 0.85 0.04 A 0.88 0.06 A 2:10 0.42 0. 06 A 0.24 0.0 3B 0.39 0. 0 5 A 0.57 0.06 A 0.81 0.06 A 0.81 0.08 A 4:01 0.66 0.05 A 0.59 0.05 A 0.04 0.01 B 0.12 0.03 A 0.70 0.05 A 0.70 0.05 A 8:00 1 1 1 1 a D ata based on aboveground shoot biomass and are represent ed as the mean of 8 replications with standard error. b Mean separation is only shown between different soil pH (low and high) with in each density level for RY cogongrass, RY bahiagrass and RYT respecitively. The two data within the same row between low and high soil pH wi th the same letter means they are not significantly different according to Fisher s Protected LSD test at P 0.05
39 Table 2 2 C ogongrass and bahiagrass as measured by relative crowding coefficient (RCC) an d aggressivity (A) Density RCC Cogon RCC Bahia A cogon b (cogon: bahia) low high low high low high 0:40 0 0 a 0 0 N/A N/A 1 0 1 0 1:2 0 1.23 0.44 A 0.64 0.12 A 1.25 0.22 A 2.55 0.73 A 0.06 0.04 A 0.52 0.17 B 2:10 1.49 0.49 A 0.63 0.20 A 1.10 0.22 B 3.26 0.65 A 0.11 0.03 A 1.31 0.32 B 4:01 0.91 0.17 A 0.53 0.19 A 1.44 0.25 B 4.71 0 .65 A 0.47 0.19 A 3.52 1.34 B 8:00 N/A N/A 0 0 0 0 1 0 1 0 a D ata based on aboveground shoot biomass and are represent ed as the mean of 8 replications with standard error. b Bahiagrass has the same aggressivity value as cogongrass, but the sign is opposite. c Mean separation is only sh own between different soil pH (low and high) with in each density level for RCC cogongrass, RCC bahiagrass and A cogongrass respecitively. The two data within the same row between low and high soil pH with the same letter means they are not significantly di fferent according to Fisher s Protected LSD test at P 0.05
40 CHAPTER 3 GROWTH AND PHOTOASSI MILATE PARTITIONING OF COGONGRASS UNDER DIFFERENT LIGHT INTE NSITIES Introduction Cogongrass [ Imperata cylindric a (L.) Beauv. ] is a tropical, rhizomatous grass native to southeast Asia (Holm et al., 1977). It is a serious weed throughout the warmer regions of the world and mainly infests non cultivated areas (Dickens, 1974; Holm et al., 1977). Cogongrass grows vigorously in a wide variety of environmental conditions (Ho lm et al., 1977) and competitive against other plant species for nutrients and light (Boonitee and Ritdhit, 1984) According to Eussen and Wirjahardja (1973), plants which can survive competition with cogongrass usually have a deeper root system and taller canopy than cogongrass. P revious studies have investigated the impact of shading on cogongrass (Patterso n et al., 1980 ; Patterson 1980 ) These studies focused on the presence of sun and shade cogongrass ecotypes. T he y evaluated cogongrass growth under shade levels which simulate conditions under agro nomic crop canopies. Flint and Patterson ( 1980) collected mature cogongrass plants from shaded and exposed habitats and found that plants from both areas exhibit ed adaptation to shade through an increas e in specific leaf area, leaf wei ght ratio and leaf ar ea ratio. Short term (90 days) shading decreased dry matter, leaf area, growth rate and net assimilation rate in plants although p artitioning of plant biomass increase d in leaves. Plants from shaded and full sun habitats responded similarly to shading in dicating little evidence to support the existence of sun and shaded ecotypes of cogongrass However, considering the low ( 32 35 mol m 2 s 1 ) light compensation point of cogongrass (Ramsey et al. 2003), the light intensity levels of this previous study m ay not have imposed significant stress for cogongrass. Additionally, Flint and Patterson ( 1980) only collected aboveground biomass data while changes in underground biomass were not mentioned.
41 Furthermore only mature plants were examined and they did not address the impacted of low light on cogongrass at establishement. However, MacDickens et al., (1997) stated that Imperata spp. is shade in tolerant, w ith shading quickly reduc ing carbohydrate storage, rhizome production, and shoot dry weight with i ncreas ed susceptibility to competition and herbicides. Their research results indicated that herbaceous cover crops and tree fallows with fast growing species such as Egyptian riverhemp ( Se s bania sesban ) thorn mimosa ( Acacia nilotica ) or lead tree ( Leucaena leu cocephal a ) could provide shade based control of cogongrass. Menz and Grist (1996) also suggested increasing rubber ( Ficus elastica ) planting density to shade Imperata would be a bioeconomic approach to control Imperata spp The success of cogongrass is la rgely due to rhizome regeneration capacity. Rhizomes can reproduce and spread at a rate of 350 shoots in 6 weeks and can cover 4 m 2 in 11 weeks (Eussen, 1980). Additionally, the presence of apical dominance prevents axillary bud formation and maintai ns lar ge rhizome base and carbohydrate storage Decreasing light intensity will likely change the growth pattern and carbon assimilate partitioning of the whole plant, especially the rhizome system. It is plausible that cogongrass will reduce rhizome biomass to support greater leaf biomass thus changing the plant shoot to rhizome ratio under low light intensities. Low l ight conditions are often associated with a shift in biomass allocation pattern s between shoots and roots. Light quantity will influence the phot osynthesis rate, which will determine the amount of substrate available for growth. A model Thornley (1972) developed can be applied directly to light influence s on the shoot to rhizome ratio. This model states that the growth of shoots and rhizomes both d epend on the concentration of labile carbon (C) and nit rogen (N) and the transpor t of C and N between shoots and rhizomes. T ranslocation depends on the difference in C and N concentration between shoot and rhizome. Rhizome growth is
42 determined by the resid ue of assimilate available after shoot growth Therefore, in this model, greater light levels will result in greater available carbon for rhizome growth. Conversely, low light levels will shift carbon allocation to shoots, since the shoot is the do minant s ink, regardless of light regime. Objectives and Hyphothesis T he objective of this experiment was to examine the effect of different light intensities on the growth and carbon allocation of newly established and mature cogongrass. Our hypothesis is that lo wer light intensities will enhance carbon assi milate partitioning to the shoot at the expense of the rhizome changing shoot to rhizome ratio It is also assumed that changes in root: shoot partitioning will differ between newly established and mature cogo ngrass plants Materials and Methods Plant M aterial P reparation : Cogongrass rhizome materials we re collected from a single population in Gainesville, FL. For mature plants, r hizomes were transplanted in 4 L pots filled with commercial potting soil 1 Plants were allowed to grow under natural sunlight conditions for 12 weeks to ensure full rhizome development All leaves were removed prior to shading exposure For newly established plants, 10 cm long rhizome segments that were sprouted in plastic trays Trays were filled with commercial potting mix and placed outside under natural conditions in August 2007 After 3 weeks, uniform shoots were selected and the shoots with their connected rhizhome nodes were detached from the original rhizome segments and were tr ans planted to pots similar to mature plants, and remained outside for an additional 2 weeks 1 Fafrad super fine germination mix. Conrad Fafrad. Inc. P.O.Box 790. Agawam, MA 01001 0790
43 before treatment All plants were fertilized and watered as needed for the duration of the study. Initial density was 4 plants per pot for both mature and newly es tablished plants. Light I ntensity T reatment : Both mature cogongrass plants and newly established plants were placed in a greenhouse (25 o C day/20 o C night) under different light intensities. Respective light treatments were 75% 25%, 15%, 2% and 1% of full sunlight ( 2200 mol m 2 s 1 ) The 75% of full sunlight was not achived by artificial treatment ; instead, it is due to the sunlight reduction from the top of greenhouse. The 2% sunlight treatment reflects the light compensation point ( near 35 mol m 2 s 1 ) for cogongrass (Ramsey et al., 2003) 1% was lower than cogongrass light compensation point and was considered as stress. L ayers of shade cloth placed 1 meter above the plants were used to achieve the desired light levels and actual light intensity was mea sured by quantum s ensor 2 The experiment al design was a 2 (plant age) by 5 (light intensity ) factorial blocked by light regime with 6 replications. T he study was initiated on Sept. 8th, 2007 and repeated on Jan. 20th, 2008. Data C ollection and M athematica l A nalysis of G rowth : At the time of study initiation, 5 pots were harvested from each maturity group and separated into aboveground (shoot) and belowground (rhizome ) biomass. T his served as a time zero fro m calculating growth rate and other parameters fro m the time course of the studies. Plan ts were harvested after 12 weeks of shade exposure to evaluate shoot to rhizome ratios. P lant height was measured just prior to harvesting and plants were separated into shoots and underground biomass Leaf area was me as ured using a automatic leaf area meter 3 A ll plant materials were dried at 70 0 C oven for 3 days and s hoot dry weight (DW) rhizome dry weight (DW) and total plant dry weight (DM) 2 Li Cor LI 1 70 3 Li Cor Model 3100 Area Meter Li Cor Bioscience 4647 Superior Stree t Lincoln Nebraska 68504
44 were measured. From these data p arameters relating to growth analysis wer e calculated. Parameters include d leaf weight ratio (LWR), rhizome weight ratio (RWR) rhizome leaf ratio (RLR), specific leaf area (SLA), leaf area ratio (LAR) leaf area duration (LAD), net assimilating rate (NAR), relative growth rate (RGR Kw ), relativ e leaf area expansion rate (Ka), relative leaf growth rate (Kl), leaf area partition coefficient (LAP), leaf weight partition coefficient (LWP) and dry matter production over the period ( W). All calculations w ere based on Potter and Jones (1977) and Patte rson (1980) as follows: LWR= shoot DW/total DW, g g 1 RWR= rhizome DW/total DW g g 1 RLR=rhizome DW/shoot DW, g g 1 SLA=leaf area/shoot DW, dm 2 g 1 LAR=leaf area/total DW, dm 2 g 1 LAD= (leaf area 1)/ln(leaf area)*time duration, total amount of leaf area pr esent during the interval, dm 2 days Ka=ln(leaf area)/time duration, relative leaf area expansion rate/day Kl=ln(shoot DW)/time duration, relative leaf growth rate/day Kw (RGR)=ln(total DW)/time duration, relative growth rate/day L AP=(Ka*leaf area)/( Kw*tota l DW) LWP=(Kl*shoot DW)/(Kw*total DW) NAR=(Kw*total DW)/leaf area, average DW production per unit leaf area, g dm 2 day 1 W =NAR*LAD Data w ere subjected to analysis of variance to test for treatment difference s. M eans are presented with standard errors a (LSD) Results and Discussion Statistical analysis detected a significant treatment by run interatction; t herefore data for each experiment will be presented separately but will be discus sed together Results from Experiment 1 and 2 In both experiment 1 and 2, f or mature cogongrass plant height, shoot dry matter (DW), rhizome DW total DW and leaf area were all significantly reduced as light intensity decreased
45 ( Table 3 1 and Table 3 3 ). Leaf weight ratio (LWR), was significantly lower at the 1% light intensity compared to the higher light regimes R hizome weight ratio (RWR) and r hizome to leaf ratio (RLR) w ere significantly higher at the 1% light intensity compared to the other li ght inte nsit ies Newly established cogongrass plant height, leaf area, shoot DW, rhizome DW and total DW significantly decreased in both experiments as light intensity decreased to 15% (Table 3 2 and Table 3 4 ). In experiment 1, t he LWR of newly established plants was significantly lower at 75% full sunlight as compared to the lower light intensities. RWR and RLR were conversely significantly higher at 75% full sunlight than other light intensity levels For all 3 ratios, there were no significant difference s between light intensity levels 25%. RLR was the principle differen ce between mature and young cogongrass growth and dry matter production relative to light intensity T his is likely because mature plants possessed significant rhizome biomass before shade treatments while young plants did not Mature plants had similar RLR even at 2% of full sunlight which is close to cogongrass light compensation point T his indicates matu re cogongrass was likely able to maintain an adequate RLR through sacrificing rhizome reserve to maintain shoot growth. At the 1% light intensity the higher RLR suggests cogongrass sacr ificed leaf production to maintain a viable rhizome system T his furth er suggests that m ature c ogongrass could become dormant when adverse conditions such as intense shading arise T herefore, mature cogongrass showed the ability to adjust to different light intensities to either maintain its low rate growth by balancing the ratio between rhizome and shoot production or tend to become dormant. Young plants did not withstand shading as well as mature plants. When the light intensity declined to 25% of full sunlight, the decreased RLR suggests the plants sacrificed rhizome
46 prod uction in favor of shoot production Additionally, young plants, with immature rhizome systems, do not appear to have the ability to sacrifice leaf biomass as a means of conserving rhizome biomass for future growth. LWR represents the investment of plant b iomass into photosynthetic tissue and LAR, the product of SLA and LWR, represents the level of distribution of this tissue as light harvesting structure in the form of leaf area (Patterson, 1980; Patterson et al 1980; Patterson et al., 197 9 ). As previousl y stated, LWR was decreased significantly under shading for mature plants and increased significantly for young plants (P<0.05). As light intensity decreased, m ature cogongrass did not devote more carbon assimilation products into photosynthetic tissue, bu t rather kept a balance between photosynthetic and r hizome tissue. Young cogongrass did not have the same level of reserved rhizome biomass to support the whole plant; therefore the increase in LWR indicates adaptation to low irradiance at the whole plant level. For m ature cogongrass LAR was constant regardless of light intensity variation, indicating the ratio of leaves to total plant weight remained constant. Y oung cogongrass LAR increased significantly as light intensity decreased; therefore the incre ase of leaf area compared to the plant total weight also implied adaption to low irradiance. NAR did not vary with light intensity for m ature cogongrass plants (Table 3 1 and Table3 3 ), but sharply declined in y oung cogongrass plants (Table 3 2 and Table 3 4 ) Mature plants were able to maintain the same rate of dry matter production per unit leaf area regardless of light intensity This is probably due to the mature rhizome system that allows a larger proportion of photosynthate to be invested into shoot growth instead of further rhizome development Conversely, y oung plants were attempting to develop shoots and rhizomes simultaneously
47 Kw, SLA and LAD did not differ between young and mature plants ( Table 3 1, Table 3 2 Table 3 3 and Table 3 4 ) Addition ally, their trend with respect to light intensity was similar. Kw and LAD decreases significantly as light intensity decreased while SLA increased significantly as light intensity increased. Based on previous research by Patterson et al., ( 1979 ), our resul ts indicated that c ogongrass leaves were thinner under shading but the distribution of leaf biomass as leaf area was significantly increased under low light intensity. The parameters most related to leaf partitioning are Ka, Kl, LAP and LWP. LAP indicate s the partitioning of total daily biomass accumulation into new leaf area and similarly, LWP indicates the partitioning of total daily biomass accumulation into new leaf weight. Ka is the actual leaf area expansion rate and Kl is the actual leaf weight gro wth rate. The partitioning of daily biomass accumulation into leaf area/weight will determine the area/weight growth of the next day, which will depend on the partitioning rate to a larger ex tent (Potter and Jones, 1977). T he Ka and Kl of m ature cogongrass both decreased significantly as light intensity decreased but LAP and LWP did not As mentioned before, mature cogongrass assimilation rate remained constant throu ghout all light intensities t herefore, the proportion of carbon assimilates partitioning to leaf area expan sion and leaf weight growth were constant. However, actual leaf weight growth and leaf expansion rate declined Once again, t his indicates that mature cogongrass has the ability to maintain regular growth pattern s or self impose dormancy und er adverse conditions Young cogongrass Ka and Kl also decreased significantly as light intensity decreased (P<0.05) but LAP and LWP i ncreased significantly from 7 5 to 2% full sunlight and only LWP decreased significantly from 2% to 1% full sunlight (P<0. 05). Young cogongrass assimilation
48 rate decreased significantly throughout decreasing light intensities, therefore although the partitioning of assimilate to leaves increased the actual growth was still decreased. The correlation of parameters Ka, Kl, L AP and LWP wi th relative growth rate (Kw) is shown in Figure 3. 1 and Figure 3.2 In both experiments, y oung plants showed correlation between K l and Kw as well as Ka and Kw. It indicates for young plants, i ncreases in growth were associated with increases in both relative leaf area and leaf biomass but poorly with the partitioning coefficients i.e. the actual leaf area and leaf weight increase instead of the partitioning proportion determines plant growth. Mature cogongrass growth correlation with leaf ar ea expansion was less than young plants, but it was still well correlated with the increase in relative leaf biomass It suggests for mature plants, leaf weight increase is more crucial for their growth instead of leaf area expansion. Overall D iscussion P revious research on parameters related with growth rate and leaf partitioning rate showed a decrease with shading (Patterso n, 1980; Patterson et al., 1979; Holly and Ervin, 2007). O ur experiment s with young cogongrasss plants showed similar results to prev ious research. D ecrease s in growth, leaf partitioning and RLR indicates how young cogongrass adapts to decreasing light intensity. However mature cogongrass with a fully established rhizome system has the ability to maintain balance between rhizomes and shoot growth These data suggest that mature plants have the ability to conserve rhizome biomass when placed under intense shade T h ese data do not support our hypothesis tha t lower light intensity will en hance more carbon assimilation partitioning to the shoot increasing the shoot to rhizome ratio. Rather the influence of shading to cogongrass growth and carbon partitioning is a function of maturity level. Our results indicated that cogongrass without a n established rhizome system, shading from neigh boring plants might provide a powerful competitive function and could help to explain why
49 cogongrass does not invade and displace heavily shaded ecosystems Utilization and cultivation of competitive neighboring c rop s could produce a shading canopy reduci ng plant vigor and consequently reduc ing cogongrass spread. Moreover, as shading promotes high relative aboveground biomass allocation, aboveground control practices might be more effective.
50 Table 3 1 Experiment 1: G rowth of mature cogongrass plants as a ffected by light intensity for 12 weeks Light Intensity % of full sunlight Parameter 75 % 25% 15% 2% 1% P lant H eight ( cm ) 86 3 ab 97 4 a 81 6 bc 72 4 cd 64 2 d L eaf A rea(cm 2 ) 1214122 a 85292 b 847108 b 699.162 b 33464 c S hoot DW(g) 9.871.09 a 6.010.63 b 4. 380.54 bc 3.760.51 c 1.610.32 d R hizome DW (g) 41.544.13 a 29.764.07 b 21.971.95 bc 19.242.64 cd 12.542.28 d Total DW(g) 51.45.15 a 35.774.10 b 26.362.4 bc 233.06 cd 14.152.55 d Leaf Weight Ratio (LWR) 0.190.01 a 0.180.02 a 0.170.01 a 0.160.01 a 0.110. 01 b R hizome Weight Ratio (RWR) 0.810.01 b 0.820.02 b 0.830.01 b 0.830.01 b 0.890.01 a R hizome : Leaf Ratio (RLR) 4.270.23 b 5.311.07 b 5.160.41 b 5.260.49 b 8.290.78 a Specific Leaf Area(dm 2 /g)(SLA) 1.240.04 c 1.430.07 bc 1.930.02 a 1.870.30 ab 2.100.06 a Leaf Area Ratio(dm 2 /g)(LAR) 0.240.01 a 0.260.04 a 0.320.01 a 0.310.06 a 0.240.02 a Leaf Area Duration(cm 2 days)(LAD) 1533 4 1332 a 1245 1 104 9b 1125 3 120 8b 940 9 1964 b 5109836 c Relative Leaf Area Expansion Rate/day(Ka) 0.0780.001 a 0.0760.001 ab 0.0750. 001 ab 0.0690.004 bc 0.0630.002 c Relative Growth Rate/day(Kw) 0.0950.001 a 0.0910.001 b 0.0870.001 bc 0.0860.001 c 0.0790.002 d R elative Leaf Growth Rate/day (K l ) 0.0760.001 a 0.0710.001 b 0.0670.001 bc 0.0650.001 d 0.0550.002 d Leaf Area Partition Coef ficient (LAP) 0.1970.009 a 0.2180.041 a 0.2730.019 a 0.2720.061 a 0.1900.024 a Leaf Weight Partition Coefficient (LWP) 0.1540.007 a 0.1430.021 a 0.1280.010 a 0.1260.013 a 0.0800.011 b Net Assimilation Rate (g/dm 2 day) (NAR) 0.400.01 a 0.410.07 a 0.280.0 1 a 0.670.43 a 0.360.03 a a A ll data represents the mean of 6 replications followed by standard error. b For each parameter, values sharing the same letter within a row are not significantly different according to Fisher s Protected LSD test at P 0.05
51 Table 3 2 Experiment 1: G rowth of newly established cogongrass plants as affected by light intensity for 12 weeks Light Intensity % of full sunlight Parameter 75 % 25% 15% 2% 1% P lant H eight ( cm ) 92 1 a 93 3 a 43 5 b 52 3 c 51 1 c L eaf A rea (c m 2 ) 2598158 a 112491 b 36570 c 24142 cd 458 d S hoot DW (g) 19.201.47 a 6.680.53 b 1.710.34 c 1.080.17 c 0.310.07 c R hizome DW (g) 35.494.59 a 5.140.74 b 0.930.28 c 0.710.33 c 0.270.12 c Total DW(g) 54.685.90 a 11.811.21 b 2.650.61 c 1.800.47 c 0.580.18 c Leaf Weight Ratio (LWR) 0.360.02 b 0.580.03 a 0.660.03 a 0.670.06 a 0.580.04 a R hizome Weight Ratio (RWR) 0.640.02 a 0.420.03 b 0.340.03 b 0.330.14 b 0.420.04 b R hizome : Leaf Ratio (RLR) 1.830.15 a 0.750.08 b 0.520.07 b 0.570.17 b 0.790.17 b Specific Leaf Area(dm 2 /g)(SLA) 1.370.07 b 1.690.07 b 2.130.18 a 2.170.11 a 1.570.14 b Leaf Area Ratio(dm 2 /g)(LAR) 0.500.05 c 0.970.04 b 1.430.18 a 1.440.11 a 0. 12 0.09 a Leaf Area Duration(cm 2 days)(LAD) 29706158 1 a 143681021 b 5490897 c 390 7 582 c 1037156 d Rela tive Leaf Area Expansion Rate/day(Ka) 0.0870.001 a 0.0780.001 b 0.0640.002 c 0.0600.003 c 0.0410.002 d Relative Growth Rate/day(Kw) 0.0950.001 a 0.0780.001 b 0.0 68 0.002 c 0.0560.003 c 0.0430.003 d Relative Leaf Growth Rate/day (K l ) 0.0840.001 a 0.0720.0 01 b 0.0560.002 c 0.0510.002 c 0.0360.003 d Leaf Area Partition Coefficient (LAP) 0.4580.050 c 0.9630.043 b 1.5340.236 a 1.5490.16 a 0.8960.110 b Leaf Weight Partition Coefficient (LWP) 0.3160.019 c 0.5320.027 ab 0.6140.034 a 0.6190.059 a 0.4980.044 b Ne t Assimilation Rate (g/dm 2 day) (NAR) 0.2030.023 a 0.0820.005 b 0.0460.005 c 0.0410.005 c 0.0520.011 bc a A ll data represents the mean of 6 replications followed by standard error. b For each parameter, values sharing the same letter within a row are not significan tly different according to Fisher s Protected LSD test at P 0.05
52 A B C D Figure 3. 1 Experiment 1: Relative g rowth rate (Kw) compared with: A ) rel ative leaf expansion rate (Ka) B ) relativ e leaf weight growth rate (Kl) C ) leaf area parti tioning coeff icient (LAP) D ) leaf weight partitioning coefficient (LWP) All data represents means of 6 replications.
53 Table 3 3 Experiment 2: G rowth of mature cogongrass plants as affected by light intensity for 12 weeks Light Intensity % of full sunligh t Parameter 75 % 25% 15% 2% 1% P lant H eight ( cm ) 119 8 ab 132 5 a 119 7 ab 104 5 b 106 2 b L eaf A rea(cm 2 ) 7113 588 a 4385 123 b 2757 170 bc 1522 239 c 1437 239 c S hoot DW(g) 75.98 13.91a 38.33 10.92b 22.62 1.32b 13.06 1.87b 9.58 1.25b R hizome DW (g) 83.49 18.54a 53.32 7.43ab 54.35 1.26ab 38.24 3.60b 33.26 4.51b Total DW(g) 159.47 31.99a 91.66 18.19ab 76.97 0.06ab 51.3 1.83b 42.8 5.10b Leaf Weight Ratio (LWR) 0.48 0.03a 0.39 0. 04ab 0. 29 0.0 2ab 0. 26 0.0 4c 0. 23 0.0 3c R hizome Weight Ratio (RWR) 0. 52 0.0 3c 0. 60 0.0 4bc 0. 71 0.0 2ab 0. 74 0.0 5a 0. 77 0.0 3a R hizome : Leaf Ratio (RLR) 1.09 0.11c 1.61 0.26bc 2.41 0. 19ab 3.10 0. 65a 3.55 0. 52a Specific Leaf Area(dm 2 /g)(SLA) 1. 01 0. 16b 1.43 0.0 2b 1. 22 0.0 1ab 1. 16 0. 02b 1.48 0.0 7a Leaf Area Ratio(dm 2 /g)(LAR) 0. 48 0.0 7a 0. 45 0 .04 ab 0.3 6 0.0 2ab 0.310.06 b 0. 3 40.0 4ab Leaf Area Duration(cm 2 days)(LAD) 72104 5302a 46517 11744b 31309 1692bc 18640 2520c 17715 2531c Relative Leaf Area Expansion Rate/day(Ka) 0.0 9 80.001 a 0.0 92 0.00 3ab 0.0 88 0.001 bc 0.0 81 0.00 1cd 0.0 80 0.002 d Relati ve Growth Rate/day(Kw) 0. 107 0.00 2a 0. 101 0.00 2b 0.0 99 0.001 b 0.0 95 0.001 bc 0.0 93 0.00 1c R elative Leaf Growth Rate/day (K l ) 0.0 99 0.00 2a 0.0 90 0.00 3b 0.0 86 0.001 bc 0.0 79 0.001 cd 0.0 76 0.00 1d Leaf Area Partition Coefficient (LAP) 0. 45 0.0 7a 0. 41 0.0 5ab 0. 32 0.0 2ab 0. 26 0.06 ab 0. 29 0.0 04b Leaf Weight Partition Coefficient(LWP) 0. 45 0.0 2a 0. 36 0.0 4ab 0. 25 0.0 4bc 0. 22 0.0 4c 0.0 19 0.0 2c Net Assimilation Rate (g/dm 2 day)(NAR) 0. 24 0.0 4a 0. 23 0.0 2a 0.280.0 2a 0. 34 0. 06a 0. 28 0.03 a a A ll data represents the mean of 6 replications followed by standard error. b For each parameter, values sharing the same letter in a row are not significantly different according to Fisher s Protected LSD test at P 0.05
54 Table 3 4 Experiment 2: G rowth of newly established c ogongrass plants as affected by light intensity for 12 weeks Light Intensity % of full sunlight Parameter 75 % 25% 15% 2% 1% P lant H eight ( cm ) 124 5 a 115 2 a 94 4 b 72 6 c 59 3 c L eaf A rea(cm 2 ) 1447 39 a 1006 91 b 276 35 c 118 35 c 23 4 c S hoot DW(g) 24.19 1.98a 10.51 0. 95b 2.11 0.34 c 1.0 1 0. 35c 0. 14 0.0 3c R hizome DW (g) 32.33 2.82a 8.25 0. 95b 0.9 6 0. 17c 0. 39 0. 14c 0. 07 0. 03c Total DW(g) 5 6.52 4.63a 1 8.76 1. 89b 3.08 0. 51c 1. 39 0.4 9c 0. 21 0. 03c Leaf Weight Ratio (LWR) 0. 43 0.0 1c 0.5 6 0.0 1bc 0.6 9 0.0 1ab 0. 74 0.06 a 0. 69 0. 12ab R hizome Weight Ratio (RWR) 0. 57 0.0 1a 0.4 4 0.0 1ab 0.3 1 0.0 1bc 0. 26 0. 06c 0. 31 0. 12bc R hizome : Leaf Ratio (RLR) 1. 34 0. 06a 0.7 8 0.0 3b 0. 45 0.0 1bc 0. 41 0.1 2c 0. 61 0. 33bc Spe cific Leaf Area(dm 2 /g)(SLA) 0.59 0.0 4d 0.96 0.0 4c 1.34 0. 06b 1.24 0. 08b 1. 74 0.1 0a Leaf Area Ratio(dm 2 /g)(LAR) 0. 26 0.0 2d 0. 54 0.0 2c 0.92 0. 04b 0.90 0. 07b 1.19 0. 19a Leaf Area Duration(cm 2 days)(LAD) 17824 1 904a 1 3062 1 024b 4388 462c 2134 5 48cd 642 82d Relative Leaf Area Expansion Rate/day(Ka) 0.08 1 0.001 a 0.07 7 0.001 a 0.06 2 0.00 1b 0.0 49 0.00 5c 0.0 35 0.002 d Relative Growth Rate/day(Kw) 0.09 6 0.001 a 0.0 83 0.001 b 0.0 63 0.002 c 0.05 0 0.00 5d 0.0 3 30.00 1e Relative Leaf Growth Rate/day (K l ) 0.08 6 0.001 a 0.07 7 0.001 b 0.05 9 0.002 c 0.0 47 0.00 5d 0.0 29 0.00 2e Leaf Area Partition Coefficient (LAP) 0. 22 0.0 2c 0. 50 0.0 2c 0.91 0. 05b 0.88 0. 08b 1.28 0. 28a Leaf Weight Partition Coefficient(LWP) 0.3 8 0.0 1c 0.5 2 0.0 1bc 0.6 4 0.0 1ab 0.6 9 0.0 7a 0. 62 0.0 14ab Net Assimilatio n Rate (g/dm 2 day)(NAR) 0. 39 0.0 4a 0. 15 0.0 1b 0.0 7 0.0 1c 0.0 6 0.0 1c 0.0 9 0.01 c a A ll data represents the mean of 6 replications followed by standard error. b For each parameter, values sharing the same letter in a row are not significan tly different according to Fisher s Protected LSD test at P 0.05
55 A B C D Figure 3 2 Relative growth rate (Kw) compared with: a) relative leaf expansion rate (Ka) b) relative leaf weight growth rate (Kl) c) leaf area partitioning coefficient (LAP) d) leaf weight partitioning coefficient (LWP) All data represents means of 6 replications from Experiment 2.
56 CHAPTER 4 ABSCISIC ACID CONTEN T IN DORMANT COGONGR ASS RHIZOMES Introduction Cogongrass ( Imperata cylindrica (L.) Beauv.) is one of the most troublesome weedy species in the world. As a persistent invasive species, it possesses several survival strategies including an extensive rhizome system, adaptation to poor soils, drought tolerance, prolific wind disseminated seed, fire adaptation, and high genetic plastic ity (Hubbard et al., 1944; Holm et al., 1977; Brook, 1989; Dozier et al., 1998). Among these strategies, the most important one is the persistent and aggressive rhizome system. Cogongrass can colonize new habitats by seed dispersal, but spread is mostly f rom vegetative rhizome growth (Tominaga, 2003). Growth from a s ingle rhizome can extend 1 m per year and total rhizome growth from one plant can be more than 12 m per year (Tominaga, 2003). R esearch has shown this extensive rhizome network can exceed 40 to ns of fresh weight per hectare (Terry et al., 1970). Rhizomes are mainly found in the top 15 cm of fine soils and top 40 cm of coarse soils (Holm et al., 1977; Gaffney, 1996). Cogongrass has a low shoot to rhizome ratio with rhizomes compris ing over 60% of the plant total biomass (Holm et al., 1977) This extensive rhizome system allows cogongrass to survive control procedures The regeneration capacity of rhizomes has been shown to be positively correlated with increased age, weight, length and thickness (Ayeni, 1985). Rhizomes are highly resistant to breakage and disruption, and drought (Ayeni, 1985; English, 1998). Apical dominance plays an important role in cogongrass rhizome growth. Apical dominance means the apical bud (or tip) of the rhizome produce s auxin which not only promotes cell division on the apical bud, but also diffuses basipetally and inhibits the development of lateral bud growth (Cline, 1994). These buds would otherwise compete with the apical tip for
57 light and nutrients. T he persistent and large rhizome mass with numerous dormant buds provides a mean s of energy conservation and regeneration capacity. Apical dominance contributes to the ability of cogongrass to survive control methods therefore repeat ed treatments are needed for long t erm control Axillary buds are abundant in cogongrass, but often dormant Therefore better unde rstanding of bud dormancy could lead to better manipulation of rhizome growth and the development of more effective control measures. Research done by Gaffney and Shilling (1995) confirmed apical dominance in cogongrass through auxin (indole 3 acetic acid (IAA)) imposed dormancy. The y also showed that Napthalam a chemical that inhibits polar auxin transport can activate axillary buds. Extensive research perfor med on quackgrass ( Agropyron repens (L.)), which is a rhizomatous, perennial grass, also reported IAA induced apical dominance (Chancellor and Leakey, 1972). English (1998) performed research on c ogongrass rhizomes and h er results indicated that differe nt part s of the rhizome performed differently in the same growing stage. Sprouting of apical and central buds was significa ntly higher than the basal buds Therefore, further investigation into the viability and level of the dormancy of basal buds may prov ide useful information. English (1998) also mentioned that apex, scale leaves and root removal will influence axillary bud development. Releasing rhizome dormancy could improve cogongrass control in different ways. Dormant buds are not susceptible to trans located herbicides because they are not active metaboli c sinks As the top growth of plants is killed by herbicides, axillary buds are often viable and able to sprout. Releasing rhizome dormancy will enhance bud sprouting at the expense of carbohydrates re served in rhizomes T his will depl et e the potential for regeneration and increase
58 the concentration of herbicide into axillary bud tissue Moreover it will increase the shoot to rhizome ratio and increase the target for foliar applied herbicides. As bud d R esearch has shown secondary dormancy is closely related to abscisic acid ( ABA ) c oncentration especially in the case of seed dormancy and germination (Bewley, 1997). AB A is also thought to play a role in apical dominance. It has been postulated as a possible auxin induced second messenger that directly repress e s axillary bud outgrowth (Tucker, 1978). Research on quackgrass showed that ABA plays a principle role in bud do rmancy where exogenously applied ABA i nhibit ed sprouting in quackgrass buds (Taylor et al., 1995. Pearce et al., 1995). ABA applied to quackgrass rhizomes with the apex intact stimulate d sprouting in axillary buds but ha d no effect with the apex removed. Research by Cline and Oh (2006) showed that basally applied ABA c ould restore apical dominance in Ipomoea and Solanum suggest ing the interaction among ABA and auxin. Research on purple nutsedge ( Cyperus rotundus L.) and yellow nutsedge ( Cyperus esculentus L.) showed that exogenous ABA inhibited nutsedge tuber sprouting and this mi ght be a natural dormancy mechanism in nutsedge (Jangaard et al., 1971). In contrast, Kojima (1993) found as asparagus ( Asparagus officinalis ) spears lateral buds initiated growth, ABA concentration in lateral buds plus scale leaves associated with the tip region was the highest among all tested tissue In asparagus rhizomes, ABA concentration was also higher in younger regions where buds would readily grow. Therefore, we suspect t hat the ABA co ncentration will be different between dormant and non dormant buds of cogongrass determine if this relation ship between ABA level is positive ly or negative ly correlated to dormancy.
59 In summary, t he objective of this experiment is to determi ne the levels of abscisic acid in cogongrass rhizomes as a function of nodal position We hypothesize that the secondary dormancy at rhizome nodes enforced by apical dominance is related to ABA content and that ABA content differs along the length of the r hizome Materials and Methods Rhizome s from m ature cogongrass plants were collected from a single population in Gainesville, FL. R hizomes were transplanted in 4 L pots containing commercial potting mix 1 Plants were placed under greenhouse conditions with temperatures of 25 o C day /20 o C night. Plants were watered and fertilized as needed. After 6 months of growth, rhizomes extending from the bottom of four pots were randomly selected. These rhizomes were covered with scale leaves that had accumulated larg e amount of anthocyanins. Although extended from the pot and exposed to light conditions, these rhizomes remained dormant and did not form shoots. Two rhizomes > 20 cm length were detached from each pot and quick frozen in liquid nitrogen. Rhizomes were th en segmented into 4 sections each containing 7 nodes from the tip basipet ally. T he actual separatio n between s ections was made between two buds. For each s ection nodal tissue and scale leaves were separated. All tissue was placed into a freeze dryer 2 fo r 3 days to achieve complete dryness and ground with a mortar and pestle u sing liquid nitrogen. After grinding, ABA was extracted from the tissues with 10 ml 100% methanol for 12 hours on a rotary shaker 3 (150 rpm) at 4 C. 1 Fafrad super fine germination mix. Conrad Fafrad. Inc. P.O.Box 790. Agawam, MA 01001 0790 2 Microporcessor control corrosion resistant freeze dryer, Kinetics Dura Dry TM MP, FTS system 3 Gyrotiry rotary shaker, Model No. G 2, New Brunswick Scientific Inc. Edison New Jersey
60 Methanol was evaporated at 0 o C a nd the residue resuspended in 1.0 ml tris buffered saline 4 All above stated ste ps were based on MacDonald (1994 ). Analysis for ABA was performed using an enzyme linked immunoassay specific for ABA while using () cis trans ABA as a standard Detailed step s and solution preparation based on the immunoassay instruction manual can be found in the A ppendix. The experiment was repeated twice and ANOVA was performed to test differences between experiments Means were separated by significant difference (LSD) Results and Discussion Statistical analysis detected a significant interaction (P>0.05) between the two experimental runs therefore data from the two runs are presented separately Figure 4 1 Experiment 1: ABA con centra tion (pmol/g DW) in cogongrass rhizome and scale leaves at different sections All data represents means of 8 replications followed by standard error Section 1 4 represents sections from farther away from to being closer to parental plants. 4 Turbo Vap LV Evaporator, Zymark
61 Figure 4 2 Experiment 2: A BA c oncentration (pmol/g DW) in cogongrass rhizome and sca le leaves at different sections All data represents means of 8 replications followed by standard error Section 1 4 represents sections from farther away from to being closer to pare ntal plants. A bscisic acid content (pmol/g DW) in cogongrass rhizome tissue and scale leaves at different nodal positions are shown in Figure s 4 1 and 4 2 for E xperiment 1 and 2 respecitively Section 1 refers to those nodes nearest to and including the tip, while position 2, 3, and 4 represent segments closest to farth est away from the tip respectively. Although the actual ABA concentration from the two sets of experiments w as significantly different (P<0.05), the relative trend of ABA concentration in the two experiments was very similar. R hizome segments and scale leaves close s t to the rhizome tip had the highest concentration of ABA and the level was significantly higher (P<0.05) than all the other parts of the same rhizome. For nodes and scale leaves in segments 2 3 and 4 the ABA concentration levels were similar and not significantly different (P>0.05). These results indicate a possible relation ship between the level of ABA concentration and cogongrass rhizome dormancy. Buds closest to the apex ar e usually less dormant than basal buds
62 (Taylor et al., 199 5 ). Previous research on quackgrass ( Elytrigia repens ) also showed simi lar results (Taylor et al., 199 5 ; Pearce et al., 1995). They concluded that rhizome tips and a xillary buds closest to the tip h ad the highest level of ABA concentration T he actual ABA level present in rhizomes harvested at different experiment periods varied differently, which is similar to our experiment. However, t he actual value of ABA content in cogongrass rhizomes and quackg rass rhizomes are quite different. Leaky (197 5 ) suggested that the small amount of ABA ( 200 pg/mg DW ) was not sufficient to account for bud inhibition, while Taylor et al. (1995) suggested it might be the balance of several growth hormones interacting to gether. Therefore, further research on cogongrass relative to this point is also necessary. The ABA concentration in scale leaves for cogongrass and quackgrass were also very differ ent. Quackgrass scale leaves ha ve about 20 times less ABA con centration in scale leaves compared to rhizome tissues (Taylor et al., 1995) Our results showed in each section of cogongrass rhizome, the ABA concentration in rhizome tissue and scale leaves were comparable. Previous research by English (1998) showed that cogongrass s cale leaves were an inhibitory factor to bud growth and removal of scale leaves around the axillary buds of the rhizome lead to increased sprouting of the axillary buds. The comparable level of ABA c oncentration in scale leaves sugg est s the possibility of ABA functioning as a dormancy enhancing role of scale leaves.
63 CHAPTER 5 CONCLUSIONS T hese three projects were focused on studying cogongrass growth response s As a strong competitior, cogongrass competitiveness against bahiagrass was evaluated at t wo different soil pH conditions. At soil pH 4.5 cogongrass ramets were more competitive than bahiagrass seedlings. Cogongrass was able to thrive on its own at low pH conditions while bahiagrass was slightly affected by cogongrass. At a soil pH of 6.8, bah iagrass seedlings showed significantly greater competitiv eness against cogo ngrass ramets. T herefore as soil pH shifted from low to high, bahiagrass seedling competitivenss was increased while cogongrass competitiveness was decreased. At optimal soil pH fo r forages bahiagrass seedling s are strong competitor s againt cogongrass. As the soil pH decreases bahiagrass weakens while cogongrass has a strong adaptation ability to thrive in low pH conditions. Therefore cogongrass becomes the more competitive spec ies when the soil pH decreased. Our results indicate that decreases in soil pH often associated with poor soil fertility might be the reason for cogongrass invasion into bahiagrass pastures. Under low light intensities, mature cogongrass plants with a ful ly developed rhizome system and young cogongrass without strong rhizome system responded differently. As light intensity decreased to below the cogongrass light compensation point, mature plants went from full growth to maintenance growth to dormancy. It i ndicates that at low light intensity conditions, mat ure cogongrass plants sacrifice d rhizome tissue for shoot production to maintain a low level of growth. As light intensity decrease s below the light compensation point, cogongrass stopped shoot production but maintained rhizome biomass In mature plants, the rhizome to shoot ratio increased as light intensity decreased. Y oung plants sacrificed rhizome production to produce
64 more shoots to maintain growth rate, with a marked decline in rhizome to shoot rati o as light intensity decreased. Based on previously suggested auxin imposed apical dominance theory, our results showed that abscisic acid might also play a role in cogongrass rhizome dormancy. Nodes nearest to the rhizome tips have significantly higher am ount of abscisic acid content than all other positions along rhizomes. Nodal tissue and associated scale leaves showed comparable amount s of abscisic acid content T he content level decreased in the same trend as compared to positions without rhizome tips. Further research is still needed to confirm the actual function of abscisic acid in cogongrass rhizome dormancy.
65 APP ENDIX PHYTODETEK A BSCISIC ACID TEST KIT EXPERIMENT PROTOCOL Catalog number: PDK 09347/0096 Competitive ELISA, for the quantitative determ ination of Abscisic Acid Procedures Prepare tracer solution: Add 1 m L distilled water to each vial of lyophilized ABA tracer. Wait 5 minutes to allow for complete reconstitution. Add 4 m L of tracer diluent to each ABA tracer vial and mix well to insure pro per ABA tracer dilution. Prepare standards : Weigh 26.43 mg of 2 cis (S) ABA and dissolve in 10.0 m L of a bsolute methanol. If an enantiomeric ABA compound is used, weigh 52. 86 mg of the compound. Add 100 L of this solution to 9.90 m L of Absolute methanol. This makes a stock solution (SS) with a c oncentration L Store this stock solution in an amber bottle, in the dark at 20 C or lower. Following the chart below, prepare standards by di luting the stock solution in TBS buffer (buffer formulation on page 7). New standards should be prepared each time the test is run. NSB=Nonspecific Binding, Bo=100% Binding Cup A BA Solution TBS Buffer Picomoles (ABA/m L ) Dilution A1= NSB 50ul of SS 4.95 m L 1000 1:100 B1 L of A1 1.80 m L 100 1:10 C1 L of B1 2.00 m L 20 1:5 D1 L of C1 2.00 m L 4 1:5 E1 L of D1 2.00 m L 0.8 1:5 F1 L of E1 2.00 m L 0.16 1:5 G1 L of F1 2.00 m L 0.032 1:5 H1=Bo L of TBS The sensitivity is optimum between 0.0064 and 0.16 picomoles ABA/m L
66 Remove desired number of test wells from the pouch and place in test well holder. Seal pouch and r eturn to freezer L of standard or sample extract to each well. Standards and samples should be run in duplicate. L diluted Tracer prepared in Step 1 to each well using a multichannel pipette. Mix by gently tapping plate. Cover test wells with Plate sealer or place in a humid box (airtight plastic box lined with damp paper towel). Incubate test wells in refrigerator at 4 C for 3 hours. Prior to the end of the incubation period prepare Substrate solution: Dissolve 1 Substrate tablet in 5 m L Substrate diluent. One tablet is sufficient to perform 16 test wells. After the 3 hour incubation, remove the test wells from the refrigerator and dump contents of the test wells into the sink. Wash test wells by adding 2 L of the Wash solution to each well with a multichannel pipette. Dump contents of the test wells into the sink. Repeat this step 2 more times. Then, grasping the test well holder upside down, firmly tap on paper towel to shake out remaining drops of Wa sh solution. L of substrate solution to each well using a multichannel pipette. Cover test wells with Plate sealer or place in a humid box. Incubate at 37 C for 60 minutes. Test is not valid unless Bo reads greater than 0.750 O.D. If the value is below this, increase the Substrate incubation time until the desired O.D. is obtained (not to exceed 30 additional minutes). L (1 drop) Stop reagent to each well. Wait 5 minutes. Read color absorbance at 405 nm. Record optical densities. Calculations Calculate the means of the optical densities of duplicate standards or samples. Calculate the % Binding of each standard point or sample by the following: % Binding = (Standard or Sample O.D. NSB O.D.) / (Bo O.D. NSB O.D.) x 100 Note: B = STD or Sample O.D. L of A1 (100 picomoles ABA/m L L Tracer = 0% Binding. L L Tracer = 100% Binding.
67 Plot the % Binding (B/Bo) versus the ABA concentration (picomoles/m L ) and draw the best fit curve on 4 cycle semi log pape r (sigmoid curve). Determine ABA concentration by interpolation of the sample % Binding from the standard curve. Linear standard curves can also be drawn using a Log Logit transformation of the data as follows: Logit (B/Bo) = Ln [(B/Bo)/(100 (B/Bo))].
68 B uffer formulations Stop Reagent Dissolve in 800 m L distilled water: Sodium hydroxide 40.0 g Adjust volume to 1 L. Store at room temperature. Substrate Diluent Dissolve in 800 m L distilled water: Magnesium chloride 0.1 g Sodium azide 0.2 g Diethanolamine 97.0 m L Adjust pH to 9.8 with hydrochloric acid. Adjust final volume to 1 L with distilled water. Store at 4 C. TBS Buffer / Tracer Diluent Dissolve in 800 m L distilled water: Trizma base 3.03 g Sodium chloride 5.84 g Magnesium chloride hexahydrate 0.20 g Sodium azide 0.20 g Adjust pH to 7.5. Adjust volume to 1 L. Store at 4 C. Wash Solution Dissolve in 1000 m L distilled water: Sodium chloride 8.00 g Sodium phosphate, dibasic (anhydrous) 1.15 g Potassium phosphate, mo nobasic (anhydrous) 0.20 g Potassium chloride 0.20 g Tween 20 0.50 g Sodium azide 0.20 g Adjust pH to 7.4
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78 BIO GRAPHI CAL SKETCH Jingjing was born in 1984 in a small city called Suzhou in China. She grew up and went to school there until she went to college in Nanjing China S he received her B achelor of S cience in plant biotechnology from Nanjing Agriculture Univeristy. A fterwards she decided to travel to the US to pursue further study in agriculture She came to University of Florida in August 2006 starting her m aster s research in the Department of Agrono my specializing in weed science Her research focu s ed on biological charac teristics of the invasive weed cogongrass. She is expected to receive a Maste r of S cience in a gronomy in December 2008. Jingjing plans to attend business school at the University of Florida in pursuit of another Master of Science degree in management. Her future plan is to use what she learnt from both degrees and find a position in agr icultural business in China.