1 MANAGEMENT OF CENTER PIVOT IRRIGATION ON FLORIDA POTATO: IMPACT ON PLANT PHYSIOLOGY AND YIELD COMPONENTS By SETH ANDREW BYRD A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF T HE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2012
2 2012 Seth Andrew Byrd
3 To Mom and Dad
4 ACKNOWLEDGMENTS I would like to thank: Dr. Diane Rowland for her guidance and for giving me this opportunity; Dr. Jerry B ennett who was an excellent resource in this experience and throughout my time as a graduate student ; Dr. Lincoln Zotarelli for being a great source of information and always taking time to answer my never ending questions; Dr. David Wright and Dr. Ashok Alva for providing me with advice and examples to assist me with the writing process; Dr. George Hochmuth for his support and knowledge. I would also like to thank John Nordgaard, Clay Pederson, and everybody at Black Gold Farms Inc., especially Gregg Halv erson for the research opportunity, as well as Scott Prospect, Jodie Matthews and the rest of the staff of the Live Oak farm for their help with the management of the project. Thank you to everybody that assisted with the field and lab work associated wit h this project, I would like to extend a sincere thanks to my family my Mom and Dad for their unrelenting support and encouragement, my brother for his support and service, and my extended f amily for all of their Ashley for the inspiration.
5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 7 LIST OF FIGURES ................................ ................................ ................................ .......... 9 LIST OF ABBREVIATIONS ................................ ................................ ........................... 10 ABSTRACT ................................ ................................ ................................ ................... 11 CHAPTER 1 LITERATURE REVIEW ................................ ................................ .......................... 13 Effects of Water Stress in Potato P roduction ................................ .......................... 13 Management Considerations for Reduced Irrigation ................................ ............... 18 2 THE POTENTIAL FOR POTATO TO ACCLIMATE TO REDUCED IRRIGATION: EFFECTS ON PHYSIOLOGICAL PROCESSES, YIELD, AND QUALITY ................................ ................................ ................................ ................ 24 Introduction ................................ ................................ ................................ ............. 24 Materials and Methods ................................ ................................ ............................ 28 Irrigation Treatments ................................ ................................ ........................ 29 Physiological Measurements ................................ ................................ ............ 30 Plant Nutrient and Soil Measurements ................................ ............................. 32 Yield and Grade ................................ ................................ ............................... 33 Statistical Analysis ................................ ................................ ............................ 34 Results ................................ ................................ ................................ .................... 34 Crop Management ................................ ................................ ............................ 34 Physiological Measurements ................................ ................................ ............ 35 Plant and Soil Nutrients ................................ ................................ .................... 37 Yield and Quality ................................ ................................ .............................. 38 Discussion ................................ ................................ ................................ .............. 38 3 THE RELATIONSHIP BETWEEN SAP FLOW AND SOIL MOISTURE UNDER REDUCED IRRIGATION IN POTATO PRODUCTION ................................ ........... 64 Introduction ................................ ................................ ................................ ............. 64 Materials and Methods ................................ ................................ ............................ 66 Results and Discussion ................................ ................................ ........................... 70 4 CONCLUSIONS ................................ ................................ ................................ ..... 82
6 LIST OF REFERENCES ................................ ................................ ............................... 85 BIOGRAPHICAL SKETCH ................................ ................................ ............................ 92
7 LIST OF TABLES Table page 2 1 Measurement and collecti on dates with DAP for 2011 ................................ ....... 46 2 2 Measurement and col lection dates with DAP for 2012 ................................ ....... 47 2 3 ANOVA results for physiologica l me asurements in 2011 and 2012 .................... 48 2 4 Effect of partial irrigation schedule on the means of 2011 physiological measurements, including fluorescence (Fv/Fm,), SPAD, RWC and s ) ............ 48 2 5 Effect of partial irrigation schedule on the means of 2012 physiological measurements, including fluorescence (Fv/Fm), SPAD, RWC and s ) ............. 50 2 6 ANOVA results for leaf area index (LAI ) measurements in 2011 and 2012 ........ 52 2 7 ANOVA results for soil nutrients measured by fertility samples in 2011 and 2012 ................................ ................................ ................................ ................... 55 2 8 2011 Soil fertility nutrient results for full and partial irrigation treatments, at both de pths and seven sampling periods ................................ ........................... 56 2 9 2012 Soil fertil ity nutrient results for full and partial irrigation treatments, at both de pths and seven sampling periods ................................ ........................... 57 2 10 ANOVA results for soil nutrients measured by PRS in 2011 and 2012. Measurements include PRS probes results for NO 3 P, K, and Ca ...... 58 2 11 2011 PRS probe results (measured in micro grams/10cm 2 /14 days) for both full and partial irrigation tre atments measured at both depths .................... 58 2 12 2012 PRS probe results (measured in micro grams/10cm 2 /14 days) for both full and partial irrigation tre atments measured at both depths .................... 59 2 13 ANOVA results for plant nutrient measurements in 2011 and 2012. Measurements include petiole nutrient samples for NO3, P, K, and Ca ............. 59 2 14 Plant n utrient results for both full and partial irrigation treatments in 2011 and 2012. Measurements include petiole results of NO 3 P, K, and Ca .................... 60 2 15 ANOVA results for 2011 and 2012 yields. M easurements include tubers per plant (#/plant), yield (kg/hectare), specific gra vity (Spc. Grav.) and net value .... 60 2 16 Results of yield, number of marketable tubers per plant, specific gravi ty, and net value for b oth treatments in 2011 and 2012 ................................ ................. 61
8 3 1 P values from ANOVA results for measurements in 2011 and 2012. Measurements include PMV ................................ ................................ ............... 76 3 2 Average daily Percent Max Value (PMV) from soil moisture capacitance probes in 2011 and 2012 across all c ollection dates and depths ........................ 76 3 3 R 2 values of linear and quadratic regressions of total daily water use (TDWU) and soil moisture as percent of maxim um value (PMV) in 2011 and 2012 ......... 80
9 LIST OF FIGURES Figure page 2 1 Effect of partial irrigation schedule on the means of stomatal conductance measurements for the top and bottom surfaces of the leaf, and total stomatal conductance (sum of top and bottom conductance) from 9 April 2011 21 April 2011, and 9 M ay 2011 ................................ ................................ ................ 49 2 2 Effect of partial irrigation schedule on the means of stomatal conductance measurements for the top and bottom surfaces of the leaf, and total stomatal conductance (sum of top and bottom conductance) from 26 April 2012 (A), 8 May 2012 (B) ................................ ................................ ................................ ...... 50 2 3 Effect of partial irrigation schedule on the results of the 2011 FV/Fm stress bioassay. Dates include 9 April AM and PM, 21 April AM and PM, and 9 May AM and PM ................................ ................................ ................................ ......... 51 2 4 Effect of partial irrigation schedule on the results of the 2012 Fv/Fm stress bioassay. Dates include 26 April AM and PM, and 8 May AM a nd PM .............. 52 2 5 LAI values from plots in the full and partial irrigat ion treatments in 2011 and 2012 ................................ ................................ ................................ ................... 53 2 6 Tuber dry weighs from part it ioning samples in 2011 and 2012 .......................... 54 2 7 Marketable yield results (in kg/ha) from taken from full and partial irri gation treatments 2011 and 2012 ................................ ................................ .................. 61 2 8 Number of marketable tubers per plant average taken from full and partial irrigat ion treatments in 2011 and 2012 ................................ ............................... 62 2 9 Tuber specific gravity taken from both full and partial irrigat ion treatments in 2011 and 2012 ................................ ................................ ................................ .... 63 3 1 Seasonal sap flow rates (g/h/cm 2 ) from full and partial irrigat ion treatments in 2011 and 2012 ................................ ................................ ................................ .... 77 3 2 2011 sap flow (grams/hour; not calibrated to leaf area) with PMV at 10 cm and 30 cm in the full an d partial irrigation treatments ................................ ......... 78 3 3 2012 sap flow (grams /hour; not calibrated to leaf area) with PMV at 10 cm and 30 cm in the full an d partial irrigation treatments ................................ ......... 79 3 4 Linear and quadratic regressions of 2011 TDWU with PMV at 10 and 30 cm dep th in the full an d partial irrigation treatments ................................ ................. 81
10 LIST OF ABBREVIATIONS DAP Days after planting G S S tomatal conductance measurement representing gas exchange rates on either the top or bottom of the leaf, or for the whol e leaf l Leaf water potential PMV Percent of maximum value, method of relating un calibrated soil moisture readings to the maximum value recorded for each sensor which is set as the saturated reading. Each value logged is then converted into a percent o f the maximum value. RWC Relative water content s Stem water potential TDWU Total daily water use, the sum of sap flow readings over 24 hours
11 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 MANAGEMENT OF CENTER PIVOT IRRIGATION ON FLORIDA POTATO: IMPACT ON PLANT PHYSIOLOGY AND YIELD COMPONENTS By Seth Andrew Byrd December 2012 Chair: Diane Rowland Major: Agronomy Proper irrigation scheduling can lead to higher returns and more sustainable production practices. This is especially true for potato during the tuber bulking stage when the largest irrigation applications are needed to meet greater crop demand. In an attempt to reduce irrigation input with a minimal reduction in yield, we evaluated a novel deficit irrigation treatment utilizing mild water stress in a commercial potato production field in Florida with the FL 1867 cultivar. The irrigation treatments in this pro ject consisted of: the normal irrigation schedule (FULL); and an irrigation skip, or a dry pass, followed by typical irrigation for two passes of the center pivot (PARTIAL). The partial irrigation treatments were designed to be initiated after primary tub er initiation was complete. To monitor the effect of the partial irrigation schedule on the crop, plant physiological measurements and soil and plant nutrient samples were taken throughout the growing season, and yield and quality measurements were quanti fied. Plant water use was determined through the use of sap flow sensors, and soil moisture was logged continuously during the season with the use of capacitance probes. In both years, the irrigation treatment had no effect on plant physiological process es, and only a slight
12 impact on certain plant and soil nutrients. The yield with the partial irrigation treatment was 25% lower than the yield of the full irrigation. This significant reduction in yield in 2011 was most likely due to the partial irrigati on treatment occurring too early in the season, and led to a delay of the initiation of the partial irrigation treatment in 2012. This delayed resulted in no significant difference in yield in 2012. A relationship was found between total daily water use (as measured with sap flow) and soil moisture showing the efficacy of using soil moisture as a tool for efficient irrigation scheduling. This study shows the potential for a reduced irrigation schedule as a viable option for Florida potato growers and as a sustainable management option for potato production.
13 CHAPTER 1 LITERATURE RE VIEW Effects of Water Stress in Potato Production The potato is a profitable crop grown across northern Florida on nearly 14,730 hectares with a value of approximately $144.8 mi llion (USDA, NASS, 201 2 b ) Compared to peanut which is the main crop in the north Florida area and is planted on over 4.5 times as many hect ares in Florida potato production is approximately $13 million more valuable (USDA, NASS, 201 2 b ) to the state E ffective i rrigation management is vital for economically sustainable potato production, but represents a significant expense for potato growers. In Florida, most potato production is primarily on sandy soils even though maximal crop yields are typically o n loam soils (Rowell and Coolong, 2011). Potatoes require 40 to 80 cm of water per growing season, which usually requires supplemental irrigation, but the amount of water applied depends on location and climate (Haverkort, 1982; Scherer et al., 1999). Ac cording to the FAO (2008) the potato produces substantially more calories of dietary energy than corn, wheat, and maize ; more protein per land area than wheat or maize ; and more dietary calcium per land area than wheat or rice. The potato has four general growth stages : vegetative, tuber initiation, tuber bulking, and maturation (Scherer et al. 1999). Tuber initiation and bulking are the period s in which the plant requires the most water and nutrients to increase tuber size ; therefore, w ater stress can h ave the biggest impact on potential yield during this period (Steyn et al. 2007 ; Scherer et al. 1999). Water stress during tuber bulking has the potential to impact grade and yield as a result of reductions in: tuber number and size ( Hassanpanah 2010 ; Schaflei tner et al., 2007); above ground biomass (Steyn et al.
14 2007 ; Jefferies, 1993; Jefferies and MacKerron, 1987 ) ; tuber dry matter (Darwish et al., 2006; Jefferies and MacKerron, 1989 ; Jefferies, 1993; Jefferies and MacKerron, 1987 ) ; and specific gra vity ( Shock et al. 2006 ; Ojala et al., 1990; Miller and Martin, 1987b ). The effect of water stress on tuber dry matter can be compounded in Florida as this trait is also reduced by high temperatures (Haverkort and Verhagen, 2008 ) In addition, w ater stre ss during the tuber bulking stage can affect the physiological ch aracteristics of the plant, causing decreased photosynthesis and transpiration (Gordon et al., 1999; Ojala et al., 1990; van Loon 1981), decreased stomatal c onductance (Schafleitner et al. 2007), and decreased leaf area index (Steyn et al. 2007). It can be concluded that significant water stress in potato will most likely lead to a reduction in yield and profit. Water stress is most frequent in areas that have low and erratic rainfall patt erns, or soils with low water holding capacity, thus making irrigation essential in these production environments (Ojala et al., 1990) There are many potato production areas in North Florida with these characteristics, making i rrigation vital for a succe ssful potato c rop One step to ensure a high yield is proper irrigation scheduling to avoid significant levels of water stress in the crop Irrigation recommendations for optimal potato yields are typically at level s that maintain available soil moisture in the rooting zone between 6 0% and 7 0% depending on soil type, location, and other climatic varia bles (Aegerter et al., 2008; Tomasiewicz et al., 2003; Scherer et al., 1999). Past research has found a negative impact on yields when available soil moist ure drops below 65% to 50% (Costa et al ., 1997 ; Ojala et at., 1990; van Loon, 1981 ). Deficit irrigation (DI) strategies, which are imposed by applying sub optimal levels of water below crop use, is one method proposed to conserve water in potato
15 producti on (Shock and Fiebert, 2002; Alva et al., 2002), even though most research clearly shows that the yield of potato is reduced with dec reased irrigation amounts (Alva et al., 2012 ; Badr et al., 2012; Onder et al., 2005; Alva et al., 2002; Shock and Feibert, 2002; Shock, 1995; Miller and Martin, 1987b). However, in a study using surface and subsurface drip irrigation, Onder et al. (2005 ) found an increase in tuber yield in one year with a treatment that applied 66% of the amount of water that the control (100 %) received, and found no significant difference in another year of the study between the yield and size of tubers in the 66% and control surface drip treatments. In another drip irrigation study, Yuan et al. (2003) found an increase in marketable tuber y ield as the amount of water applied increased, however no significant difference was found between the treatments that applied 0.75, 1.00, and 1.25 times pan evaporation. Another method to reduce the amount of water applied is to reduce irrigation frequen cy However, t his technique can result in a decline of total yield and number of tubers ( Hassanpanah, 2010 ; Miller and Martin, 1987a), as well as the number of tubers per plant and average tuber weight per plant (Badr et al., 2012). For example, when red ucing the frequency of irrigation applications, Hassanpanah (2010) found a decrease in tuber yield with both mild and severe water stress, but the significance of the reduction varied across the six cultivars in the study. A specific type of deficit irri gation, regulated deficit irrigation (RDI) in which reduced irrigation application is timed to certain developmental stages that are more tolerant of mild drought stress may have potential to conserve water in potato production. There are developmental st ages in potato that can withstand at least some level of reduced water applicati on better than other stages. For example, Lynch et al.
16 (1995) found that water stress initiated in the early and midseason growth periods has a more detrimental effect on yiel d than stress initiated in the late season, and that early and midseason stress resulted in a decrease of tubers per plant, with early season stress having the biggest effect. But the success of RDI is mixed at best. For example, i n a study by Miller and Martin (1987b) using sprinkler irrigation, a reduction in the amount of water applied over the last two months of the season, or a 10 day dry period during either tuber initiation or bulking was found to decrease yield by reducing tuber size. It was also found that U.S. No.1 tuber yield was significantly decreased in all reduced irrigation treatments compared to full irrigation in two out of three varieties (Miller and Martin, 1987b). In a study using reduced drip irrigation at different growth stages, F abeiro et al. (2001) found that a reduction in the amount of water applied during the bulking and ripening (late tuber bulking) stages reduced tuber dry matter and tuber yield compared to a season long full ET irrigation schedule. Reduced irrigation amou nts can affect crop quality as well by increasing the number of small tubers or by affecting specific gravity compared to irrigation that meets full crop demand (Wolfe et al., 1983). Specific gravity is a measure that plays an important role in the market yield, especially in potatoes used in processing There is a near linear relationship between specific gravity and tuber dry matter content (Wilson and Lindsay, 1969), and higher specific gravity leads to increased chip yield and reduced oil consumption during the frying process for processing potatoes (Lulai and Orr, 1979). Reducing the f requency of irrigation typically decreases specific gravity (Lynch et al., 1995; Miller and Martin 1987b) ; while r educing the overall amount of water applied to the cr op had little effect (Miller and Martin 1987b). Wolfe et al. (198 3 )
17 found an increase in specific gravity in tubers exposed to severe water stress, imposed by applying 33% of the water th at was applied to the fully irrigated treatment While it is obviou s a number of experiments have been conducted on the effects of water stress and reduced irrigation on potatoes, there are still some options that nee d to be explored. Many of the prior studies show that mild reductions in irrigation, either through tradi tional DI or RDI, have the potential to preserve yield and quality while conserving water (Onder et al., 2005 ; Yuan et al., 2003 ; Miller and Martin, 1987b). R esearch to investigat e the effects of very conservative water reductions is rare so little is know considering the mixed results of the few studies that have utilized mild deficits, it is clea r that more research is needed to identify the particular levels and timing s of mildly reduced irrig ation that will be effective in commercial potato production for preserving economic returns. The studies included above show that the yield of potato is reduced when water stress, imposed either by drastic irrigation reduction or imposition of drought c onditions, occurs during either tuber initiation or tuber bulking. However, the tuber initiation stage lasts approximately 1.5 to three weeks (depending on variety) while the tuber bulking stage lasts two to three months (Scherer et al., 1999) Therefore, if a reduced irrigation strategy was to be implemented, timing it for the tuber bulking stage, as opposed to the tuber initiation stage, gives much more time and inherent flexibility to efficiently decrease water use. There are other missing pieces of in formation a s well, including the fact that m ost of the irrigation studies have been conducted in the northern half of the United States mostly centered in the potato pr oducing region of the northwest and
18 excluding most of the southeastern U.S. climate and regional environments. Further, international research has taken place in countries with climates that greatly differ from the southeastern U.S., or are focused on utilizing drip irrigation, an irrigation system not traditionally used for U.S. potato pro duction. Many of the past studies have also focused on moderate to severe stress and in most cases sustaining that stress throughout the season. It does not appear that much work has been focused on developing water management systems that utilize applic ations of mild stress targeted to particular growth stages under overhead sprinkler irrigation systems, the primary irrigation method in U.S. potato production. Management Considerations for Reduced Irrigation One primary consideration in a management sche me that targets mild water deficits is the maintenance of appropriately mild stress level but this requires regular monitoring of soil moisture levels in order to trigger water application correctly. There are several different methods employed for deter mining soil moisture or soil water content, including: the feel and appearance method, which involves physically handling the soil to feel for moisture content and visually inspecting the color (Tomasiewicz et al. 2003 ; Scherer et al. 1 999 ) ; neutron prob e measurement s (Tomasiewicz et al., 2003; Lynch et al. 1995) ; resistance blocks (Tomasiewicz et al., 2003); soil water budgeting or the checkbook method (Tomasiewicz et al ., 2003); time domain reflectomet ry or TDR (Gordon et al., 1999; Costa et al. 1997) ; gravimetric water content (Costa et al. 1997); and soil water tension through the use of tensiometers or granular matrix sensors (Shock et al. 2006). T he most popular method for growers is the feel method which is used on over 80% of U.S. irrigated cr op acreage ( S c herer et al. 1999) Capacitance probes have also been used as alternatives to the feel method and were evaluated
19 specifically for Florida crop production (Fares and Alva, 2000 a ). The benefit of using capacitance probes is that they offer a c ontinuous, nondestructive, and accurate method of soil moisture monitoring in comparison to the feel method and are able to provide soil moisture values that can be utilized for making irrigation decisions (Fares and Alva 2000 b ). While soil moisture monitoring is important for successful irrigation management, plant physiological performance can show stress levels and water application needs as well. The soil moisture level or the alternative calculation of plant available water in the soil can be r elated to plant uptake and use through the measurement of sap flow, which measures water movement through the plant (Starr et al., 2008). This technique has been used successfully in potato in several studies. In a study of three potato cultivars in Nova Scotia, Canada in which plants were either irrigated, rain fed, or sheltered to impose different levels of soil water depletion and water stress, Gordon et al. (1999) found that sap flow in the drought stressed crop was at most, 30% of the sap flow in th e irrigated crop Patterns of daily sap flow differed among well watered and stressed crops as well. F low rates reached their highest levels before noon and gradually decreased through the afternoon in the stressed crop; while the flow in irrigated plant s peaked in mid afternoon and then rapidly declined (Gordon et al., 1999) Another study on potato sap flow, relating soil moisture at different depths to plant transp iration, showed that sap flow rates could successfully measure the time periods when wat er flow was occurring in the plant and the primary soil depth where water uptake was predominant ( Starr et al. 2008).
20 Other physiological factors that have been used to determine and measure water stress in potatoes include: leaf area index, stomatal cond uctance, chlorophyll fluorescence, chlorophyll content, water potential, and relative water content Leaf area index (LAI) has been found to be reduced in plants in deficit irrigation treatments or in water stressed conditions compared to fully irrigated or well watered plants ( Liu et al., 2006; Gordon et al., 1999; Jefferies and MacKerron, 1989; Wolfe et al. 1983 ). Another common response to water stress is stomatal closure (Chaves et al. 2003) which is commonly seen in water stressed potato plants (Ha ssanpanah, 2010; Iwama 2008 ; Costa et al. 1997 ; Jefferies, 1993; van Loon, 1981); but the differences in stomatal response between drought stressed and w ell watere d plants can vary depending on variety (Schafleitner et al. 2007) and stress level Overa ll, potatoes close their stomata at relatively low soil moisture deficits with transpiration and photosynthesis declining rapidly under soil moisture stress, in comparison to other crops ( Ojala et al., 1990). Chlorophyll fluorescence can also be used as a predictor of water (Chaves et al. 2003) and nitrogen (Goffart et al. 2008) stress Chlorophyll fluorescence shows the relative efficiency of photosystem II and typically increases under water stress (Chaves, 2003; and Schreiber, 1986). A study by Bur ke (2007) utilizing a stress assay monitoring levels of chlorophyll fluorescence, illustrate d the relationship between chlorophyll fluorescence and irrigation levels or water stress. In this assay, Fv/Fm was measured on samples of cotton leaves grown in either irrigated or dryland conditions which were then exposed to elevated temperatures under dark conditions (Burke, 2007). Fv/Fm is a measure of the efficiency of photosystem II to convert light energy into photosynthetic yield, namely O 2 ( Sethar et al. 2002 ; Maxwell and Johnson, 2000). The study showed
21 that the rate of decline of Fv/Fm in dry land plants is slower than that of irrigated plants, due to the reduction in new growth and source sink relationships in plants undergoing water stress (Burke, 2 007). This method has also been successfully used to screen for drought tolerance in stay green and senescent sorghum plants (Burke et al., 2010). SPAD chlorophyll measurements can be used as an indicator of chlorophyll and nitrogen levels in the leaf to assess the condition of the foliage ( Goffart et al. 2008 ; Vos and Bom, 1993 ). Two direct measures of water status and stress are water potential ( including either stem s or leaf l ) and relative water content (RWC). Liu et al ( 2006) found that an irrigation treatment that applied 70% of full irrigation significantly reduced leaf water potential in several of the measurements; while Liu et al. (2005) found that withholding water from potato plants during tuber initiation and tuber bulking stages decreases the l compared to well watered plants. Other studies have found that both leaf RWC and l declined when potato plants w ere exposed to droug ht ( Moorby et al., 1975) and that drying cycles result ed in decreased RWC in comparison to well watered plants (Liu et al., 2005; Wilcox and Ashley, 1982; Epstein and Grant, 1973). Stem water potential ( s ) is another method of quantif ying plant water st atus, and is more sensitive to small changes in environmental conditions tha n leaf water potential (McCutchan and Schakel, 1992). Past work by Lampinen et al., (1995) has shown that there is a strong linear relationship between s and water applications, making it a good indicator of plant water status in response to irrigation. One important factor related to yield and that can be directly affected by water application is nutrient availability and uptake/use in the plant. Therefore, studies examining alt ered irrigation application in potato production should include
22 measurements of nutrient availability in the soil as well as nutrient concentrations in the plant To monitor plant available nutrient levels in the soil that are susceptible to leaching by i rrigation, ion membranes or probes have been used in studies to measure the supply of nutrients such as nitrate (Jowkin and Schoenau 1998). Ion exchange resins have proved be accurate in field studies (Qian and Schoenau, 2001; Schoenau et al., 1993), wit h the best results occurring with increased duration of probe burial (two weeks or more) (Sharifi et al., 2009). Nutrient uptake in the plant can be assessed by p etiole sampling which has been found to be correlated with yield (Scherer et al. 1999); spe cifically, postive r elationships between potato yield and petiole nitrate concentrations have been found (Meyer and Marcum, 1998 ). In past studies nitrogen levels were higher in the petioles of potato plants under deficit or reduced irrigation (Alva et a l., 2012 ; Carter and Bosma 1974). These above discussed crop physiological characteristics are important for inclusion in a study of irrigation applications aimed at applying mild water deficits in potato production in an effort to conserve water while ma intaining yield and quality. This is because these physiological responses c an help determine the mechanisms behind crop tolerance or susce ptibility to mild water deficits and may provide insight into how to manipulate these processes through irrigation a mount and timing. To determine the efficacy of using a reduced irrigation schedule for North Florida potato growers under center pivot irrigation a study was cond ucted implementing mild deficit irrigation timed to tuber bulking. The goal of this study w as the development of a reduced irrigation strategy for potato production that results in the conservation of water while maintaining yield and qualit y. The research aimed to quantify and evaluate plant
23 physiological and yield response s to a mild water de ficit timed to the tuber bulking stage I t was also important to determine the relationship of soil moisture to crop water use. To accomplish this specific objective the relationship between potato sap flow and soil moisture was determined
24 CHAPTER 2 T HE POTENTIAL FOR POTATO TO ACCLIMATE TO REDUCED IRRIGATION: EFFECTS ON PHYSIOLOGICAL PROCESSES, YIELD, AND QUALITY Introduction In terms of spring potato, Florida ranks first in the United States in area harvested and pla nted, and second in production (US DA NASS, 2012 a ). Potato is also a major crop within the state of Florida comprising nearly 10% of the $1.65 billion vegetable industry in the state (http://www.florida agriculture.com/fass_activities.htm). However, potato production requires large amount s of irrigation to not only meet the water requirement of the crop, but also to regulate high soil temperatures that can harm the tubers. I n Florida this translates into millions of liters of water per growing season used for potato production on an avera ge size field to meet the crop requirement of 40 80 cm of water per growing season (Haverkort, 1982; Scherer et al., 1999) and to combat high soil temperatures One of the primary irrigation technologies for Florida potato production is seepage irrigati on which has been the primary irrigation system for potato production for over a century in the eastern and southern regions of Florida. However, there is significant potato hectarage in Florida irrigated with overhead irrigation, usually delivered throu gh center pivot irrigation systems. There are more reduced irrigation options for growers using pivot irrigation as surface applied irrigation can be controlled, compared to growers using seepage irrigation which relies on raising the water table itself to meet crop water demand. This is because l arge amounts of water are required to raise the water table to the depth of potato roots, while pivots can apply a set amount of water based on environmental conditions or plant requirements. Total cost for p i vot irrigation is approximately $121/acre during a growing season (J. Norgaard, Black Gold, Inc., personal communication), but this value is dependent on
25 year, location, rainfall, and type of pivot (electric or diesel, etc.). Florida potato g rowers tend to use frequent irrigation applications to overcome the lack of rainfall and high temperatures that often occur durin g the potato growing season in n orth Florida. However, this practice can be inefficient and economically unsustainable if the crop or soil water status is not closely and accurately monitored. Water stress can have a negative impact on the physiological characteristics of the potato plant including decreases in stomatal conductance (Hassanpanah, 2010), leaf area index (Steyn et al., 2007), re lative water content and stem water potential (Liu et al., 2005). Potatoes are considered to be sensitive to water stress and past work has proven the negative effect water stress, either through drought or reduced irrigation, has on the yield and qua lity of potatoes. Past work has shown that the number and size of tubers are reduced when potatoes undergo water stress (Badr et al., 2012; Hassanpanah, 2010; Schafleitner et al., 2007). Quality factors that are impacted by water stress include specific gravity (Shock et al., 2006; Ojala et al., 1990), and malformations and external defects; however, the extent of the impact is somewhat cultivar dependent (Miller and Martin, 1987b). Water stress can make potatoes susceptible to physiological disorders s uch as growth cracks (Hiller and Thornton, 2008) and hollow heart (Christ, 1998). Despite the sensitivity of potato to water stress, there has been some past success in either increasing yield (Onder et al., 2005) or finding no significant differences bet ween fully and partially irrigated potato es by using care fully timed mild water stress during certain developmental periods of the crop (Yuan et al., 2003; Fabeiro et al., 200 1 ; Miller and Martin, 1987b). Onder et al. (2005) found that applying
26 66% of ful l irrigation (based on the difference between field capacity and soil water content) using surface drip irrigation had no significant effect on yield in one year and significantly increased yields in another yield. The frequency of irrigation applications was every nine to ten days (Onder et al., 2005) which is much less than the approximately daily applications used in Florida production. Yuan et al. (2003) found no significant difference in marketable tuber yield when 75% of irrigation (based on pan ev aporation) was applied compared to 100% and 125% irrigation treatments. Fabeiro et al. (2001) also found no significant difference in yield when a deficit irrigation treatment (based on a modified ET value) that applied 84% of the control was timed to the tuber bulking stage. Miller and Martin (1987b) tested three reduced irrigation treatments, consisting of reducing application amounts for the last eight to ten weeks of the season by 82 and 79% of the control (based on ET) in years one and two, respectiv ely, and by withholding water for approximately ten days during tuber initiation and bulking. Overall, there was no significant effect on the number of tubers produced under reduced irrigation, and U.S. No. 1 tuber yield was not significantly reduced for one of the three cultivars in the study (Miller and Martin, 1987b). Another cultivar only showed a significant reduction in U.S. No. 1 yield under the treatment that withheld irrigation during tuber bulking (Miller and Martin, 1987b). In another study, n o difference in yield was found by Waddell et al. (1999) between irrigated potatoes with 40% available water compared to 70%. The studies by Onder et al. (2005), Yuan et al. (2003), Fabeiro et al. (2001 ), and Miller and Martin (1987b) that found no signifi cant decrease in yield under reduced irrigation were conducted in Turkey, Japan Spain and Washington state U.S.
27 respectively, while no attempt with similar results has been made in a climate similar to Florida. Further, with the exception of Miller an d Martin (1987b) and Waddell et al. (1999) the studies mentioned above used drip irrigation to deliver water while the concept of mild reductions in water application timed to certain developmental stages ha s not been tested in Florida utilizing center pi vot irrigation From the past studies it can be hypothesized that if a reduced irrigation strategy was to be implemented, the tuber bulking stage would be a more potentially successful target than the tuber initiation period. Tuber initiation has proven to be the most sensitive to water stress, and the longer length of the tuber bulking stage allows more flexibility for the implementation of a reduced irrigation strategy. Irrigation scheduling used in n orth Florida for chipping potatoes must take into acc ount not only the low occurrence of precipitation during the growing season, but also the high temperatures. The effect of water stress on tuber dry matter can be compounded in Florida as this trait is also reduced by high temperatures (Haverkort and Verha gen, 2008 ). A typical irrigation strategy for center pivot irrigation in the n orth Florida area is to apply 0.76 cm to 1.27 cm of water at each irrigation pass from a center pivot beginning at flowering. Depending on the amount of water being applied, th e size of the field, and the type of pivot, an irrigation pass will occur every 2 4 to 3 2 hours. This schedule is maintained by running the pivot constantly, unless a significant precipitation event occurs, from the beginning of tuber initiation until one to two days before harvest. While this schedule may result in upwards of two to three times the water lost to evapotranspiration (ET) being replaced, these application rates
28 water needs, but also are employed to mitigate the high temperatures present during the growing season. Therefore, to further investigate the development of an irrigation strategy that utilizes mild water stress timed to tuber bulking, as well as provides some degree of temperature regulation, a study was init iated on a commercial potato production field in n orth Florida. This study aimed at imposing a mild stress through a dynamic irrigation schedul e that could be easily adopted by growers, and used for testing the economic and environmental sustainability of this new scheme for the n orth Florida region. Materials and Methods The research was conducted in the spring of 2011 and 2012 in potato fields under the operation of Black Gold Potato Inc The commercial fields were located in 82 elev. 21.1 meters in 2011; and 30 of the fields in this region was not the same for both years d ue to traditional rotation practices The soil in this area is an Alpine fine sand (Thermic, coated Lamellic Quartzipsammets) which is classified as being excessively drained, having moderately rapid permeability, and very low available water holding capacity (USDA and NRCS 2006). Soil from the fields in both year s contained 95 to 96% sand and field capacity was at 5.1% water by volume while permanent wilting point was at 1.4% water by volume. The potato cultivar Frito Lay 1867 (FL 1867) a widely grown cultivar processed for chip products in the U.S., was planted in both years Potatoes were planted on 18 and 16 February in 2011 and 2012, respectively. For both yea rs the inter row spacing was 86 cm while the intra row spacing between tuber seed pieces was 25 cm.
29 Irrigation Treatments M ost Florida potato produc tion fields grown under overhead irrigation receive about 1 cm of wat er applied to the crop on a 24 32 hour basis In the production fields utilized in this study, this was accomplished with a single pass of a Valley (Valmont Irrigation, Valley, NE) cente r pivot irrigation system. The center pivot system in 2011 took 30 hour s to apply 1 cm over the entire field; while in 2012 it took approximately 26 hours Once this irrigation regime was started (roughly 40 45 days after planting), the irrigation syst em was run non stop for a two month period up to harvest unless a rainfall event in excess of 1. 3 cm was received. The irrigation treatment s imposed in this project consist ed of : 1) the normal irrigation schedule as just described (FULL); and 2) an irriga tion skip, or a dry pass, followed by typical irrigation for two passes of the system (PARTIAL). The partial irrigation treatments were designed to be initiated after primary tuber initiation was complete; treatments began on 8 April (49 days after planti ng DAP) in 2011 and 16 April (60 DAP) in 2012. In order to conform to design and equipment limitations presented by working in a commercia l production field, irrigation plots were laid out with two sectors of approximately 5 degrees of the pivot circle; one sector served as the full treatment which represented the typical irrigation application regime used for the commercial field, and one sector as the partial water application treatment composed of reduced irriga tion during the bulking stage. In both years the location of the study occu r r ed under one pivot span, and was located between the fifth and sixth tower from the pivot point Each treatment section took up an area of approximately 41 by 30.5 meters. Within each sector, four sampling and measur ement subplots of approximately 8 by 8 meters were arranged randomly across each sector and contained a capacitance soil moisture sensor, two sets of PRS probes Within
30 each plot, sampling and physiological measurements were taken at regular intervals du ring the season. An area of approximately 12 meters between the treatment sections was used to stop the pivot and start the pump to apply water to the full section; no measurements were taken and no sensors were installed in this area. Fertilizer applica tion totals for 2011 were 474 76 476 (kg of N P K per hectare) and 2012 totals were 350 63 372 25 (kg of N P K Ca per hectare). The irrigation treatment was scheduled so that it did not disturb or alter fertilization applications in either year. Physiolog ical Measurements Physiological measurements we r e taken three times during the tuber initiation and bulking growth stage s on 9 April, 21 April, and 9 May (50, 62, and 80 DAP) in 2011, and in 2012 on 26 April, 8 May, and 10 May (70, 80, and 82 DAP) The th ree measurement dates in 2011 corresponded to early, mid, and late tuber bulking, while in 2012 measurement dates corresponded with mid and late tuber bulking. At each date, measurements were taken at two time periods: during morning (0900 1200 hours) a nd afternoon (1500 1800 hours) periods. Due to inclement weather in 2012, the morning (AM) measurements for the second time period were taken on 8 May and the afternoon (PM) measurements on 10 May. At each period, physiological measurements were perfor med after one irrigation event that followed a skip in the partial irrigation treatment was applied to all plots At the initiation of each measurement period, dark adapted chlorophyll fluorescence (Fv/Fm) was measured on the first fully expanded leaf of four plants per subplot using the OS 1 fluorometer (OptiSciences Inc., Hudson, NH). Leaves were dark adapted for at least 30 minutes prior to each measurement. After measurement, a section of the leaf was excised ( avoiding the mid vein ) and placed in a s mall tube of distilled wa ter on ice for further laboratory analysis. S tomatal conductance
31 (g s ) was then measu red on the first fully expanded leaf of another set of four plants within each subplot using a Decagon SC 1 Porometer (Decago n Devices, Inc. Pullm an, WA) The leaf was then removed and chlorophyll status was estimated using a SPAD chlorophyll meter (Konica Minolta Sensing, Inc., Ramsey, NJ) and the leaf samples were then placed in a plastic bag on ice for subsequent determination of relative water content (RWC) in the laboratory Stem water potential was measured in the field using a PMS Model 610 Pressure Chamber (PMS Instrument Company, Albany, OR.) on four plants in each subplot between the hours of 1200 and 1700 Foil bags were placed on leaves for at least 30 minutes prior to measurement to allow leaf water potential to equilibrate with the stem similar to the procedure used by Williams and Araujo (2002). In the laboratory, the leaf that had been measured for stomatal conductance in the fiel d was analyzed for RWC. The leaf was weighed for fresh weight, floated on distilled water under a light tray for three hours to maintain physiological activity, and subsequently removed and weighed to g et the turgid weight. Next, t he leaf was placed in a n oven at 6 0 C fo r a minimum of 72 hours, removed and weighed for the determination of dry weight. These measurements were used to determine RWC using the formula: RWC = [(FW DW) / (TW DW)] x 100 Where FW is fresh weight, DW is dry weight, and TW is turgid weight. In the laboratory, the leaf sections that were collected in the field after measurement of Fv/Fm were removed from the distilled water and mounted on moist filter paper following the protocol of the Stress Test Bioassy (Burke 2007) and Stay Green Bioassay (Burke et al. 2010) Briefly, filter paper (Whatman Papers, England,
32 UK) containing the leaf sections was wrapped in thin plastic wrap (The Glad Products Company, Oakland, CA) and kept moist in a glass tray during measurement Once l eaf sections were prepared and mounted on the filter paper, an initial Fv/Fm reading was taken prior to incubation at 40 C Measurements of Fv/Fm were repeated at 30, 60, and 90 minutes (this reading was omitted on 9 April 2011) with continued incubati on at 40 C in between readings (Burke 2007 ; Burke et al. 2010). Additional measurements of plant growth and nutrient status throughout the season included leaf area index (LAI), biomass partitioning, and petiole analyses (Tables 2 1 and 2 2). Leaf area index was measured once a week for six consecutive weeks in 2011 and five consecutive weeks in 2012 during the growing season using the LAI 2200 Plant Canopy Analyzer (LI COR Environmental, Lincoln, NE) One reading was taken from each subplot for a tota l of four measurements each in the full and partial irrigation treatment each week Biomass partitioning samples were taken twice during the 2011 growin g season and three times in 2012 For these measurements, four plants were dug with approximately 22 c m of the tap root and all tubers intact from both t he full and partial irrigation treatments in 2011 and 2012, respectively. Plants were placed into plastic bags and maintained on ice until seperation into stems, leaves, and tubers. The plant samples wer e then dried at 60 C for a minimum of 72 hours and weighed. Plant Nutrient and Soil Measurements Soil samples were taken periodically during the season for fertility analysis in both years (Tables 2 1 and 2 2). At each sampling period, soil samples were taken at two depths in each subplot, one in the rooting zone (0 25 cm ), an d one below t he rooting zone (45.7 6 1 cm; a depth at which the nutrients are likely no long er
33 accessible to the roots ) The fertility samples were analyzed for NO 3 P, K, and Ca, ( Ag v ise Laboratories, Northwood, ND) To assess the availability of nutrients to the crop itself a set of Plant Root Simulator (PRS ) probes (Western Ag Innovations, Saskatoon, Canada) were installed four times during the season in both years for two w eek durations (Tables 2 1 and 2 2). The probes simulate root nutrient uptake (Schoenau et al., 1993) and thus characterized soil nutrient availability from emergence until near the end of tuber bulking. The probes were installed at two depths in each subp lot, one set in the rooting zone at 25 cm and one below the rooting zone at 61 cm. Each set of probes consist ed of one anion and one cation probe Each time a set of probes was removed after a two week burial a new set of probes was installed in the sam e location immediately after removal of the old set, with the exception of the first removal After the first removal, there were five (in 2011) and six (in 2012) days until the second installation to avoid installing the probes within two days of a ferti lizer application which would likely quickly saturate the membranes Each day a set of probes was installed or rem oved, soil samples were taken for fertility analysis. After removal, the probes were shipped to Western Ag Labs (Saskatoon, Canada) for ana lysis of NO 3 P, K, and Ca. P etiole samples were taken (one per subplot) once a week for five consecutive weeks that coincided with the initiation of flowering (Tables 2 1 and 2 2). The first fully expanded leaf was collected, and thirty petioles were sa mpled in each subplot. These samples were analyzed for NO 3 P, K, and Ca by Agivse Laboratories (Northwood, ND). Yield and Grade Yield was determined at six undisturbed locations in the full and partial sections in 2011 and 2012 by digging and collecting the potatoes in a 4 .7 m length of row, no
34 later than 5 days before t he field was commercially harvested. In 2011 yields were measured on 7 June (109 DAP) while in 2012 yields were measured on 23 May (97 DAP). The tubers were collected and fresh weight d etermined. Tubers were sized into marketable (> 4.76 cm) and undersized (< 4.76 cm) categories The number and weight was recorded for the marketable tubers and specific gravity of a n 11 to 14 kg subsample of tubers was determined from a sample of each o f the six samples from the full and partial irrigation sections by using the formula ( C. Pederson Black Gold Inc., personal communication): Specific gravity = (weight in air ) divided by [(weight in air) (weight in water)] Net value was based solely on the difference in irrigation costs determined by subtracting the irrigation cost for each treatment ( based on a percentage in terms of difference in cm app lied ) from the gross value determined by the yield ($0.298/kg). The net value did not take other co sts (fertilizer, chemicals, fuel, etc.) into consideration. Statistical Analysis ANOVA were run on each of the physiological characteristics, soil measurements, and yield and quality (JMP Pro 9 software, SAS Institute Inc., Cary, NC) Factors in each m odel included: treatment (trt), date, time of day (TOD), depth, and the two way interactions of trt x date, trt x TOD, and trt x depth where applicable. When multiple comparis on test, unless otherwise noted. Results Crop Management Potatoes were planted on 18 February and harvested on 10 June (110 DAP) in 2011 while in 2012 potatoes were planted on 16 February and harvested on 23 May (97
35 DAP) in 2012. The 2011 season was long er compared to 2012 due to market demand and processor orders for the FL 1867 variety in these commercial fields. In 2011 the irrigation treatment was initiated on 7 April (48 DAP) which resulted in 14 irrigation skips and a difference of 13.5 cm of wate r applied between the full and partial irrigation treatment s. In 2012 the irrigation treatment was initiated on 16 April (60 DAP) and resulted in nine irrigatio n skips and a difference of 9.3 cm of water applied between the full and partial irrigation tre atments Based on the number of tubers quantified by partitioning collections prior and subsequent to initiation of partial irrigation in 2011 the first skip in 2011 likely occurred just prior to the end of primary tuber initiation; therefore, the irriga tion treatment in 2012 was delayed to avoid applying reduced irrigation during the latter part of tuber initiation. Rainfall totals during the growing season (19 February to 10 June in 2011; 16 February to 25 May in 2012) were 20. 5 and 7.9 cm for 2011 and 2012, respectively. In 2011, 57.4 and 43.9 cm of irrigation were applied to the full and partial irrigation treatments respectively; while in 2012 these totals were 58.3 and 49.1 cm for the full and partial plots, respectively. Physiological Measureme nts Overall, partial irrigation had no effect on the physiological characteristics measured in this study for either 2011 or 2012 (Table 2 3). However, across traits, there were differences among dates, time of day (TOD) in both years, and treatmen t by da te interaction in 2012. Date had an effect on Fv/Fm measured in the field in both years although there was no consistent pattern of change. H owever, if data are averaged over all dates in Fv/Fm was statistically lower in the afternoon time period in comp arison with the morning (Table 2 4 and 2 5 ). For SPAD measurement date had no pattern in 2011, while in 2012 SPAD decreased at the later sampling date; time of day
36 had a significant impact in 2011 with increased SPAD values in the afternoon. A s would be expected RWC was higher in the morning measurements compared to the afternoon. In 2012, TOD significantly affected RWC measurements with three out of four of the measurements being lower in the PM sample In 2012 s was higher at the later sampling date. For all measurements in both 2011 (Fig. 2 1) and 2012 (Fig. 2 2 ) g s top, g s bottom, and g s total decreased from the first measurement to the last measurement date and within a date, decreased from the morning mea surement to the late afternoon measurement This decrease from the morning to the late afternoon is most likely due to the increased vapor pressure deficit at the late afternoon Due to early senescence in 2012, the g s values measured on 8 May were much lower than those shown in all other measurements. The fluorescence bioassay showed a similar decline in readings for both full and partial treatments once subjected to the heat treatment however, there were no differences in the rate of decline between irrigation treatment s (Figures 2 3 and 2 4). Date had an effect on LAI in both 2011 and 2012 (Table 2 6), showing a parabolic trend of LAI over the season in 2011 but no uniform pattern in 2012 (Fig 2 5). For both years, there was no effect of the reduc ed irrigation treatment on LAI The partial irrigation treatment did not have an effect on the t uber dry weights taken at the two partitioning samples in 2011 or the three samples taken in 2012. Date showed an effect on the weights in 2012 (p val ue <0.00 01), while in 2011 dates were not different This may be evidence that the partial irrigation treatment began prior to the end of tuber initiation; i.e. in 2011 the lack of increase in tuber weight between the two dates measured indicate that tubers had n ot begun bulking (Fig. 2 6).
37 Plant and Soil Nutrients T he partial irrigation treatment had only a slight effect on soil and plant nutrients, while the effect of soil depth on soil nutrients measured by soil fertility samples (Table 2 7 ) was more pronoun ced. In 2011 only soil P was affected by irrigation treatment In six out of the seven samples across time P was lower in the pa rtial irrigation treatment (Table 2 8 ). NO 3 measured by fertility samples was affected by date, but there was no consistent trend during the season. Depth had an effect on all fertility nutrient measurements in 2011, as values generally decrease d at deeper depths In 2012 NO 3 K, and Ca showed a date effect however no clear pattern was observed Depth again had an effect i n 2012 with values in the shallower depth being generally higher (Table 2 9 ). Depth showed an effect on the PRS results (Table 2 10), with values typically declining with depth. For the nutrients measured by the PRS probes in 2011, the partial irrigati on treatment showed a significant effect on K, yet no pattern was observed (Table 2 1 1 ). NO 3 and P were affected by date, but again no clear pattern was observed For the PRS values in 2012, only NO 3 was impacted by date and depth, with date not resulti ng in a pattern while NO 3 values were higher in the shallower depth measured across the season (Table 2 1 2 ). The irrigation treatment showed an impact on the Ca measured by petiole samples in 2011 (Table 2 13) with the partial irrigation treatment havin g higher Ca values than the full irrigation treatment in four out of five measurements (Table 2 1 4 ). Date had an effect on all petiole nutrient results measured in 2011, with P levels declining throughout the season while there was no trend for NO 3 K and Ca. Date had an effect on petiole nutrients in 2012 with levels of NO 3 P, and K decreasing with time, while Ca measured in the full irrigation increased with time, but had no trend in the partial treatment.
38 Yield and Quality Total y ield and the number of marketable tubers per plant were impacted by the reduced irrigation treatment in 2011 (Table 2 1 5 ). The yield of the partial irrigation treatment was 34,877 kg/hectare compared to the f ull irrigation treatment yield of 46,590 kg/hectare ( Table 2 16; Fi g. 2 7) This likely was most attributable to a reduction of one marketable tuber per plant in the partial treatment (Fig. 2 8). However, irrigation treatment had no effect on specific gravity (Fig. 2 9). In contrast the yield, number of marketable tub ers per plant, as well as specific gravity was not significantly affected by irrigation in 2012. The yield in 2012 of the partial irrigation treatment was 39,984 kg/hectare while the yield in the full irrigation treatment was 41,342 kg/hectare. In 2011 th e net value in the partial irrigation treatment was $10,150/ha, significantly less compared to the full irrigation treatment net value of $13,567/ha for a difference of $ 3,415.55 per hectare between the two treatments. In 2012 however, the re was no effect of the irrigation treatment on the net value as the partial irrigation treatment had a value of $11,651/ha while the full treatment value was $12,005/ha for a difference of $ 354 per hectare in comparison to the full irrigation treatment Yield was lower across both treatments in 2012 than in 2011 most likely due to the shortened season in 2012 that reduced the amount of time the plants were in the tuber bulking stage. Discussion The primary aim of this study was to test an irrigation scheduling s trategy t hat utilized irrigation skips to apply a mild wat er stress during tuber bulking. From the physiological measurements in both years, it is apparent that plant processes were generally not affected by the reduced irrigation schedule. This evidence of susta ined
39 normal plant physiological performance shows the potential for the implementation of a reduced irrigation schedule in Florida potato production. While scarce, there are studies illustrating that some reductions in irrigation amounts in potato can be i mplemented without reducing physiological performance. In an experiment screening for drought tolerant potato varieties Schafleitner et al. (2007) imposed a drought for 41 days on 16 potato genotypes during tuber bulking and found no significant differen ce s in Fv/Fm measurements between the droughted and irrigated plants. The results of the stress bioassay in the present study revealed similar overall patter n s as those found by Burke (2007) and Burke et al. (2010) : a decline in Fv/Fm under heat treatment However, the lack of difference in the pattern of Fv/Fm decline under heat between the two irrigation treatments lends support there was little to no stress in the partial irrigation treatment. In past studies, stomatal opening, measured e ither by con ductance or resistan ce, has exhibited responses to water stress often reducing gas exchange compared to well watered plants (Epstein and Grant, 1973; Jeffries, 1993; Schafleitner et al., 2007) However, t he lack of a significant difference in stomatal c onductance with partial irrigation in both years of the present study in FL 1867 which is a cultivar considered to be quite drought susceptible indicates minimal water stress was imposed throughout the season under the partial irrigation schedule The g eneral trend in stomatal conductance within a season after imposition of partial irrigation treatments also revealed a potential acclimation response of stomat a to the reduced irrigation schedule. At early tuber b ulking in 2011, there was a numerical redu ction in stomatal conductance in the partial compared to the full irrigat ion treatment; while at late tuber bulking, this
40 trend in stomatal conductance was reversed. This shows that the plants experiencing mild reductions in water application may have the ability to acclimate to a reduced irrigation schedule and, over time, equilibrate back to optimal gas exchange rates. Typically, potato shows a reduction in LAI in response to water stress (Jeffries and MacKerron, 1989; Shahnazari et al., 2007; Steyn et al., 2007). However in the present study there was no significant effect of the reduced i rrigation treatment on LAI in either year. An important difference between the current study and previous studies ( Jeffries and MacKerron, 1989; Shahnazari e t al., 2 007; Steyn et al., 2007 ) that examin ed decreases in LAI under water reductions is that th o se previous studies typically utilize d fairly severe water reductions and applications of water stress for extended periods of time. For example, Shahnazari et al. ( 2007) found reduced LAI under 50 and 70% of full irrigation (soil water maintained near field capacity) from periods encompassing tuber initiation, to maturity; while Jefferies and MacKerron (1989) found reductions in LAI in plants that received no water a pplication after emergence. These studies represent drought conditions that were much more severe than those in the current study and help explain why there were no significant reductions in LAI under the partial irrigation treatment. Potato RWC and water potential (typically leaf water potential) are also physiological characteristics often strongly impacted by water reductions and are usually indicative of the magnitude of applied water stress (Wilcox and Ashley, 1982; Moorby et al., 1975; Epstein a nd Grant, 1973). However previous studies primarily utilized more severe and prolonged periods of drought than those imposed in the current study. For example, Liu et al. (2005) reported a significant decline in RWC and
41 l in plants that were exposed to nine to 14 day drought periods during tuber initiation and tuber b ulking. The lack of significant differen ces in these traits due to the partial irrigation treatment in the current stud y gives further evidence that plant water status was not detrimenta lly affected under the partial irrigation schedule Water reduction has the potential to detrimentally impact nutrient availability to the crop. However in the current study, the reductions in irrigation had no effect on soil nutrients measured either by soil samples or PRS probes in both years, except for soil P in 2011. The difference in means between the P m easured in the full irrigation treatment to that of the partial irrigated treatment was 17 ppm (130 ppm vs. 112 ppm), likely indicating a very small to no biological impact on P availability. This is supported by the results of the petiole samples that showed no difference between irrigation treatments in P uptake by the plants. The only petiole measurement affected by reduced irrigation was pe tiole Ca in 201 1 when the value in the partial irrigation treatment was higher than in the full irrigation treatment This actually could be beneficial because low levels of calcium in the tuber have been linked with the developmen t of the physiological d isorder internal heat necrosis ( Davies 1998; Sterrett and Henninger 1991 ) ; a disorder that is a common problem in Florida chip potato production. The lack of impact on soil nutrients and plant nutrient availability is an important finding in the current s tudy because other research has documented water reduction can detrimentally impact these characteristics (Johnson et al., 2005; Meyer and Marcum, 1998). The lack of impact on physiological functi oning of the potato crop by the reductions in irrigation in the current study help explain the relatively small effects on
42 yield and quality in both years with the exception of the yield loss in 2011 under partial irrigation In 2011, the irrigation treatment was initiated just prior to the end of primary tub er initiation and this likely led to the significant yield loss and the reduction of one marketable tuber per plant that occurred during that year. As shown in the results of studies by Steyn et al. (2007) and Lynch et al. (1995), water stress during the tuber initiation stage has the most severe impact on tuber yield In 2012, the irrigation treatment was delayed to ensure that tuber initiation would not be impacted, and likely resulting from this delay, there was no difference in yield between full and partial irrigation treatments. Further, the average number of tubers per plant in 2012 was also not a ffected by the partial irrigation treatment Yield losses in prior studies illustrate how sensitive potato is to moderate and severe levels of water stre ss timed parti cularly to tuber initiation. Jefferies and MacKerron (1989) found a yield loss of 56% and 63% in consecutive years comparing irrigated potatoes to potatoes grown in drought conditions. Other studies that applied deficit irrigation have show n yield losses at 60 (Darwish et al., 2006) and 80% ET (Badr et al., 2012; Costa et al., 1997). Steyn et al. (2007) stressed potatoes at different growth stages and found yield losses with stress during tuber init iation and tuber bulking; while Lynch et a l. (1995) imposed water stress on potatoes during early and midseason and observed a decline in the number of tubers per plant and in yield Howe ver, what is different among tho se studies in comparison to the current study is that past w ork has been compl eted on small scale research plots that are not necessarily representative of a field under commercial production. The current study was located in a field under commercial production and the typical irrigation schedule used across Florida was compared to a reduced irrigation schedule. The
43 strategy used for imposing the reduced irrigation in this study appears to be unique. Due to the setting, reducing application amounts or incorporating prolonged dry periods as used in prior studies could not be implem ented in the current study, as interruption to management activities needed to be minimized. The result was an irrigation schedule that reduced water applications by shutting off irrigation at intermittent times during the season If this schedu le was implemented on an entire field, saving fuel or electricity by shutting down the pivot would provide further cost saving potential for the grower. Moderate losses in yield for the partial irrigation treatment could possibly be offset by the increase d costs associated with pumping greater water amounts in the full irrigation treatments. The reduced irrigation schedule implemented in 2011 resulted in a savings of $ 69 per hectare for the grower. Factoring the yield los s measured in that year ( 11,713 k g/hectare) along with the irrigation savings in the partial treatments, there was still a large loss in value for the grower at $ 3,415.55 per hecta re, a 25% reduction in profit in comparison to the full irrigation treatment The reduced irrigation schedul e in 2012 resulted in a savings in irrigation costs of $ 50 per hecta re Factoring in the small reduction in yield that year, this corresponded to a pro fit reduction ( $ 354 per hectare ), which was not significantly different from the full irrigation treatme nt. The reduction in yield by 25% in 2011, even with a 13. 5 cm reduction in water supplied is not an economically acceptable scenario for commercial potato producers in Florida. The yield loss observed in 2011 can be most likely attributed to the timing of the initiation of the partial irrigation treatment. The amount of tubers set by the plants was impacted by reducing irrigation during the tuber initiation phase that led to a reduction in the number
44 of marketable tubers on each plant and resulted in a yield loss compared to the full irrigation treatment. In contrast, by delaying the initiation of reduced irrigation treatments and decreasing the severity of water reduction by 16% (9. 3 cm), the 2012 partial irrigation treatment resulted in no significant yield or value loss in comparison to the full irrigation and represents a possible water conservation strategy for Florida potato growers. There was also no reduction in tuber quality under the reduced irrigation schedule. Specific gravity was not reduc ed in the partial irrigation treatment showing that neither fresh yields n or processing quality was impacted in either year. This is particularly important in chipping potatoes as even years when the yields are high a low specific gravity can result in p requ irements of processors. The results of the current study confirm the findings of past studies that potato is a crop sensitive to water stress T he effect of water stress will significantly differ depending o n what growth stage is impacted When the partial irrigation treatment was initiated during the late tuber initiation stage in 2011 yield was significantly reduced even though the measured physiological processes were not affected. When the partial irrig ation treatment was delayed and confined to the tuber bulking stage alone in 2012, no differences were found in the physiological processes or yield between the full and partial irrigation treatments. This research shows that potato yield is more sensitiv e to water stress during the tuber initiation sta ge as opposed to tuber bulking. However, because of the inconsistent effects on yield in the two years, more research is required to more precisely tune the timing and amount of water need ed to implement a reduction in irrigation that will result in water savings but not reduce yields, as observed in 2012.
45 For a reduced irrigation schedule to be accepted as a common practice among potato producers in n orth Florida, there must be a way to relate the schedulin g of irrigation (or irrigation skips) to easily measureable environmental conditions such as soil moisture, as the physiological characteristics measured in this study would not be convenient or realistic for growers to conduct over a season on multiple fi elds. Vegetative measurements, such as NDVI sensors that could be installed on pivots or canopy temperature sensors located in the field could be possible means of giving growers easy to use information that might be used to make irrigation decisions. It can be determined from this study that a reduced irrigation strategy shows promise as a future technique for irrigation management of chipping potatoes in n orth Florida. With minimal impact on physiological processes in both years of this study due to re duced irrigation, there appears to be options for water conservation strategies that utilize mild reductions in water application timed to the tuber bulking stage.
46 Table 2 1. Measurement and collection dates with DAP for 201 1. Measurement Collection Numb er (DAP) 1 (DAP) 2 (DAP) 3 (DAP) 4 (DAP) 5 (DAP) 6 (DAP) 7 (DAP) Soil Fertility 3 March (14) 10 March (20) 24 March (34) 29 March (39) 12 April (53) 26 April (67) 10 May (81) Probes 10 24 March (20 34) 29 March 12 April (39 53) 12 26 April (53 67) 26 April 10 May (67 81) Petioles 29 March (39) 5 April (46) 12 April (53) 19 April (60) 26 April (67) LAI 7 April (48) 14 April (55) 21 April (62) 28 April (69) 4 May (75) 12 May (83) Partitioning 9 April (50) 21 April (62)
47 Table 2 2 Measurement and collection dates with DAP for 2012 Measurement Collection Number (DAP) 1 (DAP) 2 (DAP) 3 (DAP) 4 (DAP) 5 (DAP) 6 (DAP) 7 (DAP) Soil Fertility 1 March (14) 8 March (21) 22 March (35) 28 March (41) 12 April (56) 27 April (71) 10 May (85 ) Probes 8 22 March (21 35) 28 March 12 April (41 56) 12 27 April (56 71) 27 April 11 May (71 85) Petioles 3 April (47) 10 April (54) 17 April (61) 24 April (68) 1 May (75) LAI 10 April (54) 17 April (61) 24 April (68) 1 May (75) 8 May (8 2) Partitioning 14 April (58) 1 May (75) 16 May (90)
48 Table 2 3. ANOVA results for physiological measurements in 2011 and 2012 1 Trait Factors df Fv/Fm gs top gs bottom gs total SPAD RWC s 2011 Trt. 1 0.5170 0.6627 0.1160 0.2201 0.1726 0.3509 0.0925 Date 2 <.0001 <.0001 <.0001 <.0001 <.0001 0.0036 0.0929 TOD 1 <.0001 <.0001 0.0463 <.0001 <.0001 <.0001 N/A Trt. x date 2 0.2530 0.9971 0.1702 0.4501 0.0801 0.4494 0.1658 Trt. x TOD 1 0. 3512 0.7482 0.4891 0.5005 0.9796 0.8138 N/A 2012 Trt. 1 0.2970 0.0972 0.5877 0.7040 0.6996 0.3338 0.2116 Date 1 0.0036 0.0002 <0.0001 <0.0001 <.0001 0.8867 0.0025 TOD 1 <0.0001 <0.0001 <0.0001 <0.0001 0.0690 <0.0001 N/A Trt. x date 1 0.0046 0.1183 0.0 344 0.0111 0.2121 0.8322 0.5387 Trt. x TOD 1 0.9199 0.8285 0.3483 0.5433 0.1314 0.0828 N/A 1 Measurements include photosystem II efficiency measured in the field (Fv/Fm), stomatal conductance (gs) of the top and bottom of the leaf, and the total, chlorop hyll content (SPAD), relative water content, previously listed), time of day (TOD), and two way interactions of treatment by date, and treatment b y time of day. Table 2 4 Effect of partial irrigation schedule on the means of 201 1 physiological measurements, including field fluorescence (Fv/Fm,), SPAD, RWC and s ). Fv/Fm SPAD RWC Date Full Partial Full Partial Full Partial Full Partial 9 April AM 0.81 0.82 44.1 44.6 82.7 84.2 N/A N/A PM 0.69 0.70 46.8 45.5 81.4 80.6 4.2 4.4 21 April AM 0.80 0.80 47.3 45.9 87.7 85.1 N/A N/A PM 0.71 0.66 47.1 48.8 84.0 83.6 4.5 4.5 9 May AM 0.79 0.80 41.3 44.0 85.2 84.9 N/A N/A PM 0.79 0.80 43.2 44.7 83.6 82.7 4.5 5.9
49 Figure 2 1. Effect of partial irrigation schedule on the means of s tomatal conductance measurements for the top and bottom surfaces of the leaf, and total stomatal condu ctance (sum of top and bottom conductance) from 9 April 2011, 21 April 2011, and 9 May 2011.
50 Table 2 5 Effect of partial irrigation schedule on the means of 201 2 physiological measurements, including field fluorescence (Fv/Fm), SPAD, RWC and s ). Fv/Fm SPAD RWC s (bars) Date Full Partial Full Partial Full Partial Full Partial 26 April AM 0.76 0.76 47.4 47.9 92.0 92.6 N/A N/A PM 0.71 0.75 48.4 45.9 88.3 86.0 4.7 5.6 8 May AM 0.77 0.79 41.7 42.1 90.1 90.6 N/A N/A PM 0.77 0.74 40.1 40.6 90.2 88.5 3.3 3.6 Figure 2 2 Effect of partial irrigation schedule on the means of s tomatal conductance measurements for the top and bottom surfaces of the leaf, and total stomatal conductance (sum of top and bott om conductance) from 26 April 2012 (A), 8 May 2012 (B).
51 Figure 2 3 Effect of partial irrigation schedule on the r esults of the 2011 FV/Fm stress bioassay. Dates include 9 April AM and PM, 21 April AM and PM, and 9 May AM a nd PM.
52 Figure 2 4 Effect of partial irrigation schedule on the r esults of the 201 2 Fv/Fm stress bioassay. Dates include 26 April AM and PM, and 8 May AM and PM. Table 2 6 ANOVA results for leaf area index (LAI) measurem ents in 2011 and 2012 1 Trait Factor df LAI* 2011 Trt. 1 0.4135 Date 5 <0.0001 Trt. x date 5 0.2706 2012 Trt. 1 0.9035 Date 4 0.0414 Trt. x date 4 0.2629 1 Factors include irrigation treatment (full and partial irrigation), dates (as previousl y listed), and the two way interaction of treatment by date
53 Figure 2 5. LAI values from plots in the full and partial irrigation treatments in 2011 and 2012.
54 Figure 2 6. Tuber dry weighs from pa rtitioning samples in 2011 and 2012.
55 Table 2 7 ANOVA results for soil nutrients measured by fertility samples in 2011 and 2012. 1 Trait Factor df NO 3 fert. P fert. K fert. Ca fert. 2011 Trt. 1 0.3079 0.0022 0.4100 0.8661 Date 6 <0.0001 0.1702 0.1562 0.1203 Depth 1 <0.0001 <0.0001 <0.0001 <0.0001 Trt. x date 6 0.6910 0.6785 0.7696 0.4968 Trt. x depth 1 0.6040 0.3431 0.2188 0.9800 2012 Trt. 1 0.9133 0.9770 0.7110 0.0946 Date 6 <0.0001 0.3190 <0.0001 0.0159 Depth 1 <0.0001 <0.0001 <0.0001 <0.0001 Trt. x date 6 0.8835 0.2483 0.9144 0.1975 Trt. x depth 1 0.0849 0.0819 0.1831 0.1598 1 Measurements include fertility nutrient samples (fert.) for NO3, P, K, and Ca. Factors include irrigation treatment (full and partial irrigation), dates (as previous ly listed), depth, and two way interactions of treatment by date, and treatment by depth.
56 Table 2 8 2011 Soil fertility nutrient results for full and partial irrigation treatments, at both depths (0 25 cm, 46 61 cm) and seven sampling periods. Soil Fer tility 2011 Collection Number Nutrient (units) Treatmen t Depth (cm) 1 2 3 4 5 6 7 NO 3 (kg/ha ) Full 0 25 34 49 80 66 82 63 38 NO 3 (kg/ha ) Partial 0 25 25 57 91 72 89 58 58 NO 3 (kg/ha ) Full 46 6 1 6 6 16 10 87 66 29 NO 3 (kg/ha ) Partial 46 6 1 6 9 25 21 9 0 55 22 P (ppm) Full 0 25 180 185 178 195 198 170 193 P (ppm) Partial 0 25 138 185 160 165 185 138 165 P (ppm) Full 46 6 1 79 73 94 94 60 90 37 P (ppm) Partial 46 6 1 4 8 64 89 70 58 75 38 K (ppm) Full 0 25 63 79 74 80 50 49 40 K (ppm) Partial 0 25 62 9 4 83 85 49 51 68 K (ppm) Full 46 6 1 27 17 28 35 37 49 44 K (ppm) Partial 46 6 1 41 24 39 35 31 28 26 Ca (ppm) Full 0 25 546 508 530 536 523 518 551 Ca (ppm) Partial 0 25 580 477 555 511 445 500 623 Ca (ppm) Full 46 6 1 292 267 325 292 277 325 329 Ca (p pm) Partial 46 6 1 454 24 1 319 260 278 267 262
57 Table 2 9 2012 Soil fertility nutrient results for full and partial irrigation treatments, at both depths (0 25 cm, 46 61 cm) and seven sampling periods. Soil Fertility 2012 Collection Number Nutrient (unit ) Treatment Depth (cm) 1 2 3 4 5 6 7 NO 3 (kg/ha ) Full 0 25 38 26 43 43 63 35 11 NO 3 (kg/ha ) Partial 0 25 29 25 43 33 63 30 11 NO 3 (kg/ha ) Full 46 6 1 6 7 17 2 29 16 7 NO 3 (kg/ha ) Partial 46 6 1 15 8 12 8 30 16 18 P (ppm) Full 0 25 125 140 155 155 176 16 8 159 P (ppm) Partial 0 25 123 146 160 13 3 153 160 138 P (ppm) Full 46 6 1 25 26 24 32 51 4 46 P (ppm) Partial 46 6 1 72 25 29 27 35 51 48 K (ppm) Full 0 25 68 43 43 44 2 6 21 26 K (ppm) Partial 0 25 51 48 43 34 23 22 27 K (ppm) Full 46 6 1 16 11 13 14 2 3 19 24 K (ppm) Partial 46 6 1 32 12 10 18 25 20 25 Ca (ppm) Full 0 25 295 287 292 297 325 375 345 Ca (ppm) Partial 0 25 281 294 290 321 339 329 382 Ca (ppm) Full 46 6 1 178 184 185 203 195 220 174 Ca (ppm) Partial 46 6 1 240 19 6 187 202 233 199 232
58 T able 2 1 0 A NOVA results for soil nutrient s measured by PRS in 2011 and 2012 Measurements include PRS probes results for NO 3 P, K, and Ca 1 Trait Factor df NO 3 PRS* P PRS* K PRS* Ca PRS* 2011 Trt. 1 0.7196 0.1605 0.0492 0.4418 Date 3 0. 0422 0.0190 0.8303 0.2444 Depth 1 0.0005 <0.0001 <0.0001 0.0003 Trt. x date 3 0.8710 0.4321 0.2375 0.3607 Trt. x depth 1 0.8473 0.4033 0.1225 0.3301 2012 Trt. 1 0.2463 0.8589 0.5079 0.8267 Date 3 <0.0001 0.6403 0.8470 0.0756 Depth 1 <0.0001 0.7960 0 .6313 0.6754 Trt. x date 3 0.0507 0.9505 0.9621 0.9976 1 Factors include irrigation treatment (full and partial irrigation), dates (as previously listed), depth, and two way interactions of treatment by date, and treatment by depth. Table 2 1 1 2011 PRS probe results (measured in micro grams/10cm 2 /14 days) for both full and partial irrigation tre atments measured at both depths. PRS Probes 2011 Collection Number Nutrient Treatment Depth (cm) 1 2 3 4 NO 3 Full 0 25 524 432 300 380 NO 3 Partial 0 25 529 355 347 172 NO 3 Full 46 6 1 185 156 233 201 NO 3 Partial 46 6 1 88 293 200 12 0 P Full 0 25 20 27 25 10 P Partial 0 25 10 19 20 27 P Full 46 6 1 2 4 6 27 P Partial 46 6 1 2 3 3 4 K Full 0 25 603 795 504 165 K Partial 0 25 463 394 484 406 K Full 46 6 1 163 1 3 4 107 426 K Partial 46 61 72 67 170 224 Ca Full 0 25 865 1108 546 649 Ca Partial 0 25 731 649 687 588. Ca Full 46 6 1 511 456 481 579 Ca Partial 46 6 1 313 552. 543 426
59 Table 2 1 2 2012 PRS probe results (measured in micro grams/10cm 2 /14 days) for bo th full and partial irrigation treatment s measured at both depths PRS Probes 2012 Collection Number Nutrient Treatment Depth (cm) 1 2 3 4 NO 3 Full 0 25 385. 1077 894 269 NO 3 Partial 0 25 48 5 1116 429 221 NO 3 Full 46 6 1 29 139 186 138 NO 3 Partial 46 6 1 58 133 127 148 P Full 0 25 16 14 33 14 P Partial 0 25 13 33 14 11 P Full 46 61 2 3 4 2 P Partial 46 61 1 4 3 2 K Full 0 25 210 452 241 216 K Partial 0 25 285 331 286 235 K Full 46 61 30 38 135 95 K Partial 46 61 39 34 50 44 Ca Full 0 25 484 1252 860 520 Ca Partial 0 25 657 1033 778 516 Ca Full 46 61 125 355 640 345 Ca Partial 46 61 193 353 378 479 Table 2 1 3 ANOVA results for plant nutrient measurements in 2011 and 2012 Measurements include petiole nutrient samples for NO3 P, K, and Ca 1 Trait Factor df Petiole NO 3 Petiole P Petiole K Petiole Ca 2011 Trt. 1 0.3836 0.0841 0.2283 0.0484 Date 4 <0.0001 <0.0001 <0.0001 <0.0001 Trt. x date 4 0.3156 0.0317 0.4581 0.1793 2012 Trt. 1 0.2733 0.3685 0.1447 0.8136 Date 4 <0.0001 <0.0001 <0.0001 <0.0001 Trt. x date 4 0.1476 0.0268 0.4151 0.0909 1 Factors include irrigation treatment (full and partial irrigation), dates (as previously listed), depth, and the two way interaction of treatment by date.
60 Table 2 1 4 Plant nu t rient results for both full and partial irrigation treatments in 2011 and 2012. Measurements include petiole results of NO 3 P, K, and Ca Petioles 2011 Collection Number Nutrient (units) Treatment 1 2 3 4 5 2011 NO 3 (ppm) Full 27776 29194 27465 21285 13345 NO 3 (ppm) Partial 26888 26760 26105 21343 15148 P (%) Full 0.6 0. 5 0. 3 0.1 0.1 P (%) Partial 0. 6 0.4 0.3 0.1 0.1 K (%) Full 9. 1 12.9 12.5 9.4 8.0 K (%) Partial 9. 1 11.9 11.2 9.9 7.7 Ca (%) Full 1.0 0.9 0.9 1.2 1.6 Ca (%) Partial 1.1 1.0 1.0 1 .2 1.7 2012 NO 3 (ppm) Full 26433 19416 12995 10621 10153 NO 3 (ppm) Partial 24144 18193 13220 11447 10286 P (%) Full 0.5 0.5 0. 4 0. 4 0.2 P (%) Partial 0. 6 0.5 0.4 0. 4 0. 2 K (%) Full 11.1 7.6 5.5 4.0 2 .0 K (%) Partial 10.6 7.8 5.6 3.4 1 .5 Ca (%) Full 0.6 0.7 1.0 1.1 1.2 Ca (%) Partial 0.7 0.8 1.1 1.1 1.0 Table 2 1 5 ANOVA results for 2011 and 2012 yields Measurements include tubers per plant (#/plant), yield (kg/hectare ) specific gravity (Spc. Grav.) and net value 1 Trait Factor Df #/plant k g/hectare Spc. Grav. Net Value 2011 Trt. 1 0.0420 0.0003 0.1475 0.0005 2012 1 0.7147 0.5834 0.5747 0.6810 1 Treatment is the only factor.
61 Table 2 1 6 Results of yield, number of marketable tubers per plant, specific gravity, and net value for both treatments in 2011 and 2012. Treatment Yield (kg/ha) Marketable tubers per plant Specific Gravity Net Value/ha ($) 2011 Full Irrigation 46,590a 7 a 1.088 a 13,567a Partial Irrigation 34,877b 6 b 1.084 a 10,150b 2012 Full Irrigation 41,342a 5 a 1.094 a 12,00 5a Partial Irrigation 39,984a 5 a 1.095 a 11,651a Figure 2 7 Marketable yield results (in kg/ha) from taken from full and partial irrigation treatments 2011 and 2012 A B A A
62 Figure 2 8 Number of marketable tubers per plant average taken from full and partial irrigation treatments in 2011 and 2012 A B A A
63 Figure 2 9 Tuber specific gravity taken from both full and partial irrigation treatments in 2011 and 2012 A A A A
64 CHAPTER 3 THE RELATIONSHIP BET WEEN SAP FLOW AND SO IL MOISTURE UNDER REDUCED IRRIGATION I N POTATO PRODUCTION Introduction Managing water use efficiently is a challenge faced by the commercial agriculture industry across the U.S., as approximately 80 water is used in agricultural operations (USDA, 2012). Optimizing the efficiency of irrigation can lead to a more economically and environmentally sustainable operation for producers. For crops such as potato in Flori da, this is especially tru e because the crop requires 40 to 80 cm of water per growing season (Haverkort, 1982; Scherer et al., 1999), and the normal growing season in the state often encompasses long periods of minimal rainfall. Combined with these envir onmental limitations, the shallow rooting depth of potato (Ojala, 1990) makes proper irrigation scheduling and management pivotal for producing a high yielding and profitable crop. Many past studies have focused on the response of potato to various amou nts and timing of water stress in an effort to explore more conservative irrigation options. Much of this work has used some type of soil moisture measurement, either alone or in conjunction with ET, to schedule water applications to maintain soil water s tatus at various thresholds for both well watered and water limited treatments (Shahnazari et al., 2007; Onder et al., 2005; Shock et al., 1992; Lynch et al., 1995; Jefferies and MacKerron, 1989). Thus, the common assumption often made in most studies is that soil moisture is indicative of crop water use and can be used to quantify crop stress level s For example, Lynch et al. (1995) u sed a soil moisture tension of 4 0 kPa to initiate irrigation of a well watered treatment and 80 kPa to initiate irrigati on for a water
65 stress treatment. Past researchers have further refined these thresholds and determined that there is a negative impact on tuber yields when soil available water drops below 65% to 50% of water holding capacity ( Costa et al., 1997; Ojala et at., 1990; van Loon, 1981 ). R ecommendations for irrigation scheduling in commercial production of potato are based on measurements of soil water status and typically advocate keeping the available soil water in the rooting zone between 60% and 70% at a m inimum (Aegerter et al., 2008; Tomasiewicz et al., 2003; Scherer et al., 1999). There are many different methods and technologies available to determine soil moisture content ( Shock et al., 2006 ; Tomasiewicz et al., 2003; Scherer et al., 1999;; Gordon et al., 19 99; Costa et al., 1997; Lynch et al., 1995 ). Capacitance probes determine soil water content by measuring the dielectric constant of the soil surrounding the sensor (Fares and Alva, 2000 a ). Capacitance probes have shown promise for irrigation sched uling in other crop s in Florida, such as in bell pepper and tomatoes (Zotarelli et al., 2009). Despite the assumption of soil moisture measurements as accurate surrogates for potato water use and stress, few studies have compared the quantification of soi l moisture with crop water use directly (Starr et al., 2008; Gordon et al., 1999). The m easur ement of sap flow is a direct and continuous quantification of plant transpiration on a fine time scale (usually 15 to 60 minutes) but results can also be summed over days or seasons to determine total crop use The theory and methodology behind the use of sap flow sensors is described by Gordon et al. (1997) and Smith and Allen (1996). The technique has been used successfully in potato (Gordon et al., 1997) and it has been shown that potato plants undergoing water stress have reduced sap flow
66 compared to plants not under wate r stress (Gordon et al., 1999). Directly relating soil moisture with sap flow measurements could test how indicative soil moisture reading s are of crop water status and overall crop transpiration and the feasibility of using soil moisture sensors for aiding in irrigation scheduling for potato. There are two previous studies that have successfully monitored sap flow in relation to soil moist ure measurements (Starr et al., 2008; Gordon et al., 1999) but both studies were aimed primarily at determining the variation between irrigated and drought stressed potatoes, and did not directly exami ne the relationship between the two for irrigation sche duling purposes. The aim of the current study is to impose a mild stress and quantify the effects on sap flow and soil moisture compared to the ful l irrigation treatment By coupling soil moisture and sap flow measurements that are logged simultaneously this study will provide real time comparisons and interactions of sap flow and soil moisture through the soil profile. The specific objectives of the study were to: 1) determine if there is a significant relationship between 24 hour daily sap flow totals to average daily soil moisture; and 2) evaluate how the strength of this relationship is affected by moisture measurements at differing soil depths. Materials and Methods The research was conducted in 2011 and 2012 in potato fields under the operation of Black Gold Potato, Inc The commercial elev. 21 meters elev. 16.5 meters in 2012); however the exact location of the fields in this region was not the same for both years due to traditional rotation practices. The soil in this ar ea is an Alpin fine sand (Thermic, coated Lamellic Quartzipsammets) which
67 is classified as being excessively drained, having moderately rapid permeability, and very low ava ilable water holding capacity (USDA and NRCS 2006). The potato cultivar Frito Lay 1867 (FL 1867), a widely grown cultivar processed for chip products in the U.S., was planted in both years Potatoes were planted on 18 and 16 February in 2011 and 2012 res pectively. For both years the inter row spacing was 86 cm while the intra row spacing between tuber seed pieces was 25 cm. Most Florida potato production fields grown under overhead irrigation receive about 1 cm of water applied to the crop on a 24 32 hou r basis. In the production fields utilized in this study, this was accomplished with a single pass of a Valley (Valmont Irrigation, Valley, NE) center pivot irrigation system. The center pivot system in 2011 took 30 hours to apply 1 cm over the entire fi eld; while in 2012 it took approximately 26 hours. Once this irrigation regime was started (roughly 40 45 days after planting), the irrigation system was run continuously for a two month period up to harvest unless a rainfall event in excess of 1. 3 cm w as received. The irrigation treatments in this proposed project consisted of: 1) the normal irrigati on schedule as just described (full ); and 2) an irrigation skip, or a dry pass, followed by typical irrigation for two passes of the system ( partial ). The partial irrigation treatments were designed to be initiated after primary tuber initiation was complete; treatments began on 8 April (49 days after planting DAP) in 2011 and 16 April (60 DAP) in 2012. In order to conform to design and equipment limitat ions presented by working in a commercial production field, irrigation plots were laid out with two sectors of approximately 5 degrees of the pivot circle; one sector served as the full treatment which represented the typical irrigation application regime used for the commercial field, and one sector as the partial water
68 application treatment composed of reduced irrigation during the bulking stage. Within each sector, four sampling and measurement subplots of approximately 18 m x 24 m were arranged randoml y across each sector Eight Adcon SM1 soil moisture sensors (Adcon Telemetry, Klosterneuburg, Austria) were located in the fi eld ( four in the full irrigation treatment and four in the partial irrigation treatment ) to provide logged seasonal soil moisture levels. Soil moisture sensors were installed on 14 March (24 DAP) and 3 April (47 DAP) in 2011 and 2012, respectively. The sensors measured relative soil moisture at six depths in the soil profile (10, 20, 30, 40, 50, and 60 cm). Black Gold Farms, Inc. utilizes these Adcon sensors to indicate soil moisture status in their production fields. The company utilizes the raw voltage values from the sensors but does not calibrate the sensors for actual volumetric water content ( VWC); instead the company monitor s the rise and fall in relative moisture readings to determine the overall moisture status of the field Therefore, individual sensors may have variable raw values even within the same irrigation treatment. In order to test the applicability of the Adcon data managed in this way, a maximum value was found for each sensor across all six depths measured during the entire season This maximum reading for each sensor was considered to be 100% for that individual sensor, and all other readings from that senso r were expressed as percent of this maximum value. This was done for each of the eight sensors in both years and all relative soil moisture values are presented as the percent of maximum value (PMV). Sap flow collars (Dynamax Inc., Houston, TX) were insta lled 1 April (42 DAP) in 2011 and 21 April (65 DAP) in 2012 on four plants in both the full and partial irrigation
69 treatments in close proximity (approximately 1 2 meters) to the soil moisture sensors. The soil moisture sensors and sap flow collars log ge d a reading every 15 minutes ; s ap flow values were logged in grams of water flow per hour (g/h) The initial stem size accommodated SGA 10 sap flow sensors but these were changed later in the season to SGA 13 sensors to accommodate increased stem diameter When collars were changed in mid season and when they were removed for harvest (2 June and 21 May in 2011 and 2012, respectively), the above ground portion of the plant was collected for leaf area determination. Canopy tissue was stored in a plastic ba g on ice for transport and, once in the lab, the leaves were removed from the plant and were scanned using a Licor model 3100 area meter (LI COR Environmental, Lincoln, NE) which gave leaf area in cm 2 This value was then used to express normalized sap fl ow rate (NSFR) in grams per hour per cm 2 of leaf area (g/h/cm 2 ). To e nsure erroneous data points that represented flow rates exceeding actual values did no t skew the analysis, all overflow values were removed. Overflow values were determined by assessing the threshold maximum va lue for sap flow; in most cases flow did not exceed 200 gram s/hour Therefore, the values exceeding 200 g/h were removed in both years (approximately 0.02% and 0.03% of sap flow values in 2011 and 2012, respectively) To calculat e total daily water use (TDWU in g/cm 2 ), NSFR values were summed over each 24 hour period (midnight to midnight) for the duration of the collar installation period (57 and 29 days in 2011 and 2012, respectively). The average TDWU values were calculated by averaging the TDWU for each individual gauge across the entire season, and then averaging those values by treatment.
70 Data for TDWU were analyzed using both linear and non linear regression ( JMP Pro 9 software SAS Institute Inc., Cary, NC) For each regr ession, TDWU by treatment (full and partial) was regressed with PMV by depth (10, 20, 30, 40, 50, and 60 cm). Differences in average TDWU between treatments were compared using ANOVA Results and Discussion Rainfall totals during the growing season were 20.5 cm for 2011 and 7.9 cm for 2012. In 2011, 57.4 cm and 43.9 cm of irr igation were applied to the full and partial irrigation plots respectively (a 23% reduction in water applied), while in 2012 the full irrigation plots received 58.3 cm of irrigation and the partial irrigation plots received 49.1 cm of irrigation (a 16% reduction in irrigation applied). In 2011, daily average flow rates measured in the full irrigation treatment ranged from 0.000106 g/h/ cm 2 to 0.066564 g/h/cm 2 while in the partial ir rigated treatment daily average flow rates ranged from 0.000110 g/h/ cm 2 to 0.106076 g/h/ cm 2 (Fig. 3 1) Daily average flow rates in 2012 in the full irrigated plants ranged from 0.000165 g/h/cm 2 to 0.054886 g/h/cm 2 while in the partial irrigated plants fl ow rates ranged from 0.000101 g/h/cm 2 0.073312 to g/h/cm 2 These maximum flow rates were similar to the range of maximum flow rates determined by Gordon et al. ( 1999). Average flow rates in both years typically peaked for all plants between 13:15 and 17: 45 hours and flow gradually declined after 19:00 hours to eventual zero flows overnight This pattern of sap flow follows other studies in potato which generally show a midday peak (Gordon et al., 1999). However more specifically Gordon et al. (1999) fo und that sap flow peaked in stressed plants before noon, while in irrigated plants flow peaked in mid afternoon. The irrigated plots in that study had an available water deficit (compared to field capacity) of 16% compared to 81% in the stressed crop (Gor don et al., 1999). This di fferential time
71 of day response between full and partial irrigated plants was not found in the current study, most likely due to the fact that the difference in water deficit was not as severe as in Gordon et al. (1999). Over th e season, flow rates in the current study began to decline at approximately ten days before harvest (89 90 DAP). This decline in late season sap flow observed in this study has not been documented in previous studies on potato. There was no significant impact in either year of the irrigation treatment on average TDWU over the sampling (for 2011 and 2012, respectively: F ratio = 3.3094 and 0.5650; p value = 0.1188 and 0.4807). There was, however, a numerical difference in each year, as the TDWU on averag e was 35% higher in the partial irrigation treatment in 2011; while in 2012 this trend was in the opposite direction, with TDWU in the full irrigation treatment being 16% higher t han in the partial irrigated treatment While the increase in T DWU for the partial irrigation treatment in 2011 is somewhat puzzling, the magnitude of decline in sap flow for the partial irrigation treatment in 2012 agrees with Gordon et al. (1999) who found a 28% reduction in sap flow when 20% of available soil water was deplete d in the chipping variety Monona. The PMV values varied between irrigation treatments and among soil depths, and there was an interaction between treatment and depth in both 2011 and 2012 (Table 3 1 ). This interaction was driven by higher PMV values in the full i rrigated plots than the partial irrigated plots at all depths except 50 cm in 2011, and at all depths except 10 cm and 20 cm in 2012 (Table 3 2 ). These results agree somewhat with the findings of Alva (2008) who found soil moisture to be decrea sed under partial irrigation (70% ET) throughout the soil profile compared to replacing full ET.
72 There was a general visual relationship between daily sap flow rates and PMV in both 2011 and 2012 (Fig ures 3 2 and 3 3) but the shape of this pattern differe d some what between full and partial irrigation treatments and by depth On a daily basis, the peak in sap flow rate in the full irrigation treatment corresponded with a decline in the 1 0 cm PMV; however in the partial irrigation treatment, this daily peak in sap flow rate c orresponded with a similar decline in PMV at deeper depths in the 40 cm (in 2011) and 30 cm (in 2012) This relationship of decreasing PMV with increasing plant uptake is in agreement with Starr et al. (2008) who found declines in soil moisture in the potato hill associated with peak sap flow rates. This may also indicate rooting depth and root water uptake activity because soil moisture will decline as root uptake (in response to transpiration) occurs at a given depth. Because these p atterns are present at the 10 cm depth for the full irrigation and 30 cm depth for the partial irrigation, and this may be indicative of a more active and deeper root system in the partial treatment. There was a direct relationship between TDWU and averag e daily PMV as shown by both linear and p olynomial regressions (Table 3 3 ). In general, sap flow was significantly related to PMV at deeper d epths (> 30 cm) in the partial irrigation treatment compared in the full irrigation treatment (10, 20 cm) (Fig. 3 4) This also indicates a deeper overall rooting depth and water uptake activity in the partial as compared to the full irrigation and indicates that roots likely responded to the reduced irrigation schedule by expandin g deeper into the soil profile than plants under full irrigation. However, the direction of the relationship between TDWU and PMV varied between positive and negative. For 2011 in the full irrigation, the direction of the linear relationship was negative, while the partial irrigation showe d a negative parabolic relationship. The
73 negative relationship between TDWU and PMV in the full irrigation is indicative of a saturated soil profile, so that as additional water is added through irrigation, transpiration is inhibited due to stomatal closu re, leading to declining TDWU. However, for plants in the partial irrigation treatment with a negative parabolic relationship, TDWU increased with PMV up to a threshold PMV value (typically 8 0 PMV), after which TDWU declined with increasing PMV. This is an indication tha t root activity in the partial irrigation treatment was able to respond through increased transpiration to increasing soil water up to a threshold, beyond whi ch transpiration was inhibited. In 2012, the same negative linear relationship w as evident b etween TDWU and PMV in the full irrigation treatment ; while in the part ial irrigation treatment the quadratic relationship was opposite to that in 2011 (i.e. there was a positive parabolic between TDWU and PMV). This indicates that as water w as applied root activity was inhibited in both the full and partial irrigation treatments in 2012. This could be expected based on the increased water application rates in 2012 in the partial irrigation section in comparison to 2011. These trends illustr ate that soil water levels are likely high and saturating the root system, thus inhibiting water uptake when irrigation is applied. However, moderate water restriction in this system is detrimental to yield (as was seen in 2011) likely indicating that the root systems were small and inadequate to withstand large water deficitis Finally the magnitude of the relationship between TDWU and PMV is somewhat smaller (comparing R 2 values) than for other environmental variable s examined in other studies For exam ple, Gordon et al., (1999) obtained R 2 values of 0.54 to 0.78 and 0.58 to 0.81 when relating sap flow to solar radiation and vapor pressure deficit, respectively.
74 In another study, Hingley and Harms (2008) found R 2 values of 0.72 when relating sap flow to temperature and 0.66 when sap flow was related to net radiation. However, radiation and vapor pressure deficit would be expected to have a stronger impact on sap flow as these factors directly drive plant tra nspiration at the leaf level. Overall, there was a direct relationship between soil moisture as measured by capacitance sensor readings and TDWU in the current study showing that soil moisture sensors could be used successfully for irrigation scheduling. The use of calibrated soil moisture sensors w ould assist in relating sap flow rates to accurate measures of soil moisture content and could lead to standard soil moisture thresholds or trigger points being developed to time irrigation applications. Of course, sap flow sensors themselves could be use d to provide irrigation decisions and they have proven to be successful for irrigation scheduling in grapevines (Patakas et al., 2005), and olive trees ( Fernandez et al., 2001). However, triggering irrigation applications based on sap flow alone would als o be a difficult concept for commercial producers to adapt, as the data collection and analysis can be time consuming especially if multiple fields are involved. The results of this study show the direct relationship between sap flow and soil moisture and that there is potential for utilizing these sensors for irrigation scheduling in Florida potato production. The next challenge for Florida potato producers is to manage early season water applications in such a way that root growth is enhanced. If soil moisture sensors were to be used in commercial potato production for irrigation scheduling, the data from the current study show that monitoring the appropriate depths would be critical to achieving efficient irrigation applications. U nder the typical irr igation schedule (full treatment), shallow soil depths tend to best reflect the status and water use of the crop,
75 while under a reduced irrigation schedule (partial treatment) monitoring the soil moisture at deeper depths appear to be critical for efficien t irrigation scheduling. Using soil moisture sensors for irrigation scheduling would be an appropriate avenue to explore for potato production as commercial producers have access to the sensors and most already have them in place in their fields.
76 Tabl e 3 1 P values from ANOVA results for soil moisture average daily Percent Max Value (PMV) measurements in 2011 and 2012. 1 Trait df PMV 2011 PMV 2012 Factors Trt. 1 <.0001 0.0187 Date 5 <.0001 <.0001 Trt. x depth 5 <.0001 <.0001 1 Factors include irrig ation treatment (full and partial irrigation), depth (as previously listed), and the tw o way interaction of treatment by depth. Table 3 2 Average daily Percent Max Value (PMV) from soil moisture capacitance probes in 2011 and 2012 across all collection dates and depths Soil Depth Treatment 10 cm 20 cm 30 cm 40 cm 50 cm 60 cm 2011 Full Irrigation 54.9 e* 52.9 f 64.3d 82.1 a 71.2 c 77.9 b Partial Irrigation 51.7 e 46.4 f 56.6 d 77.7 a 72.4 c 75.1 b 2012 Full Irrigation 31 0 d 26.1 f 31.8e 45.1 a 4 1.6 c 44.0 b Partial Irrigation 36.1 d 26.6 f 24.4 e 43.6 a 40.3 c 40.6 b *Means within an irrigation followed by the same letter are not significantly different
77 Figure 3 1 Seasonal normalized sap flo w rates ( NSFR ) from full and partial irrigation treatments in 2011 and 2012 1 1 The partial irrigation treatment started on 49 DAP in 2011 and 60 DAP in 2012
78 Figure 3 2 2011 sap flow (grams/hour ; not calibrated to leaf area ) and PMV at 10 cm and 30 cm in the full and partial irrigation treatments 1 1 This figure illustrates the relationship observed in 2011 between daily sap flow or transpiration peaks and the corresponding deletion of soil moisture at 10 cm in the full irrigation treatment and the soil moisture depletion at 30 cm in the partial irrigation treatment.
79 Figure 3 3 2012 sap flow (grams/hour; not calibrated to leaf area ) and PMV at 10 cm and 30 cm in the full and partial irrig ation treatments. 1 1 This figure illustrates the relationship observed in 2012 between daily sap flow or transpiration peaks and the corresponding deletion of soil moisture at 10 cm in the full irrigation treatment and the soil moisture depletion at 30 cm in the partial irrigation treatment.
80 Table 3 3 R 2 values of linear and quadratic regressions of total daily water use ( TDWU ) and soil moisture as percent of maximum value ( PMV ) in 2011 and 2012. 1 Full Partial Soil Depths Linear Quadratic Linear Qua dratic 2011 10 cm 0.32 N** 0.38 N** NS NS 20 cm 0.21 N** 0.28 P** NS NS 30 cm NS NS NS 0.18 N** 40 cm 0.22 P** 0.24 N** 0.08 P* 0.25 N** 50 cm NS NS NS 0.17 N** 60 cm NS NS NS 0.14 N* 2012 10 cm 0.36 N** 0.39 P** NS NS 20 cm 0.20 N* 0.28 P* NS NS 30 cm 0.20 N* 0.24 P* 0.19 N* 0.30 P* 40 cm NS NS NS NS 50 cm NS NS NS NS 60 cm 0.20 N* 0.25 P* NS 0.56 P** 1 Regressions done by depth for full and partial irrigation treatments. Direction of regression is indicated as either positive (P) or negativ e (N) with significance at 0.05 (*) and 0.01 (**).
81 Figure 3 4 Linear and quadratic regression s of 2011 TDWU with PMV at 10 and 30 cm depth in th e full and partial irrigation treatment s. 1 1 Regressions of TDWU with soil moisture (at 10 and 30 cm depth s) in the full and partial irrigation treatment s. I ndicates significant regression at p<0.05.
82 CHAPTER 4 CONCLUSIONS Plant physiological processes and nutrient content in measured in the soil and plant showed only minimal effect from the irrigation treatment in 2011, and no effect in 2012. However, yield was significantly decreased in 2011 and not affected at all in 2012. It is apparent from the results of the present study that while stress as measured by physiological p rocesses was not present with the partial irrigation treatment, there was some negative impact as yield was decreased by 25% in plants under partial irrigation. It may be that because the partial irrigation treatment used application skips followed by a r eturn to the full irrigation schedule, that there was not a stress induced as water applications were not halted for prolonged periods of time, and that the amount applied when applications were made were not reduced compared to the full irrigation treatme nt. The water deficit in comparison to the total amount applied in the full irrigation treatment was accrued across a long period of time in the partial irrigation treatment. So there were no drought periods or season long reductions in application amoun ts but intermittent periods of skipped applications. In 2011 there was a 13.5 cm decrease in water applied to the partial irrigation treatment in comparison to the full irrigation treatment over the last 62 days of the season. In 2012 the partial irrigati on treatment took place over the last 37 days of the season. The two reasons for the time period of the treatment occurring over a shorter period of time in 2012 than 2011 are the delay in the initiation of the treatment in 2012 (12 days later than in 201 1) and that the 2012 season ended, due to harvest, 13 days earlier in 2011. The yield across both treatments was reduced in 2012 compared to
83 2011 due to the harvest occurring earlier and thus the tuber bulking stage was shorter in 2012 than in 2011. When looking at the amount of water saved in each season (13.5 cm in 2011; 9.3 cm in 2012) when compared to the full irrigation, 2011 is higher due to the longer period of time the partial irrigation treatment was in use, but the amount of time it days per 1 c m of deficit in each year is approximately the same (4.6 days per 1 cm deficit in 2011; 4.0 days per 1 cm deficit in 2012). So the treatment and skip schedule was relatively uniform for both years meaning that the same partial irrigation strategy was used in both seasons, only the initiation and duration was changed. This leads to the determination that the timing of the initiation of the partial irrigation schedule was the biggest factor in the yield reduction that occurred in 2011. From observation mad e by digging up whole plants to check growth stage status in 2012, it was apparent that the first two irrigation skips as part of the partial irrigation treatment in 2012 most likely occurred during primary tuber initiation, and the third skip may have als o occurred during tuber initiation. The resulting yield loss experienced in the partial irrigation treatment in 2011 seems to be most likely due to the first two to three irrigation skips, and is also linked to a reduction in the number of marketable tube rs per plant. By inadvertently starting the partial irrigation treatment during primary tuber initiation in 2012, it appears that the number of marketable per plant present at the end of the season was impacted leading to the yield reduction. However, in 2012 when plant development was closely monitored and the partial irrigation treatment was initiation once primary tuber initiation had completed and tuber bulking began, there was no effect on the number of marketable tubers per plant or yield. These fi nding agree with work done by Steyn et al. (2007) and Lynch et al. (1995) that yield and tubers per
84 plant (Lynch et al., 1995) are impacted most severely with water stress during tuber initiation, and that tuber bulking is less sensitive to water stress co mpared to tuber initiation. While there was no evidence of stress in the present study, the yield results in 2011 appear to be evidence of the impact of the partial irrigation treatment occurring during the tuber bulking stage. The results observed in 2 012 show the potential for some type of reduced irrigation strategy, whether the partial schedule used in the present study or some other type of deficit irrigation, for implementation for commercial chip potato producers in Florida. One of the keys for s uccessful implementation of this strategy is ensuring that the start of the reduced irrigation does not impact primary tuber initiation. Another pivotal part of the strategy would be using in field sensors to monitor environmental conditions, such as soil moisture or even soil temperature, so that the reduced irrigation strategy does not lead to stress conditions and harm plant function and yield. Through the partial irrigation treatment implemented in 2012, there was a water savings of 927,098 liters per hectare compared to the full irrigation. If this schedule were to be used over an averaged sized field of 54 hectares it would result in a savings of 50,274,638 liters of water over the season. If this type of water savings can be implemented without im pacting the yield or profit for producers as was found in the present study, significant savings in water and energy can be realized in the future for commercial producers.
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92 BIOGRAPHICAL SKETCH Seth Byrd is from Thurmond, North Carolina and received his Bachelor of Science in Agronomy (Soil Science) from N. C. State University in 2007. He worked as an agronomic consultant until enrolling in graduate school at the University of Florida in 2011.