High Daytime Temperature Stress Effects on the Physiology of Modern ...

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tion (Hoagland and Arnon, 1 0). After emergence, each pot was thinned to a single plant. The study included two factors, cultivar and temperature, arranged in a ...
High Daytime Temperature Stress Effects on the Physiology of Modern Versus Obsolete Cotton Cultivars Robert S. Brown and Derrick M. Oosterhuis1 RESEARCH PROBLEM The U.S. cotton industry has faced very difficult times in recent years due to an increased problem with year-to-year yield variability. It is speculated that this increased yield variability is due to modern cultivars being more sensitive to adverse environmental conditions such as high temperatures during the critical first five weeks of flowering and boll development (Oosterhuis, 1990). However, information is lacking on the differential physiological responses of modern versus obsolete cultivars under high temperature stress conditions to explain why this variability exists. The goal of this study was to quantify the effect that elevated daytime temperatures had on physiological responses of modern versus obsolete cotton cultivars. BACKGROUND INFORMATION Even though cotton is a tropical plant and is accustomed to growing in hot climates, it does not necessarily grow best in excessively high temperatures. The ideal temperature range for cotton is reported to be from 20 to 30EC, and once temperatures reach about 35EC, growth begins to decrease (Reddy et al., 1991). Burke et al. (1988) reported that the temperature range of cotton for optimal metabolic activity, also known as the thermal kinetic window, is 23-32EC with the optimum for photosynthesis at 28EC. Temperatures exceeding 32EC are associated with a decrease in photosynthesis and carbohydrate production (Perry and Krieg, 1981). Unfortunately, in the mid-South, average daily maximum temperature in August and some years in July are well above this optimum temperature for photosynthesis. Furthermore, it is speculated that our modern, commercially available cultivars are more sensitive to high temperature stress conditions and, therefore, will have reduced physiological functioning necessary for Graduate assistant and distinguished professor, respectively, Crop, Soil, and Environmental Sciences Department, Fayetteville. 1

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optimal growth, development, and yield in cotton. The objective of this study was to evaluate physiological responses of modern versus obsolete cotton cultivars to high daytime temperature stress. RESEARCH DESCRIPTION A growth chamber study was designed at the Altheimer Laboratory in northwest Arkansas at Fayetteville in 2004 to evaluate the effect that high daytime temperature stress had on physiological responses of modern and obsolete cultivars. Three seeds per pot were planted into 2-L pots filled with Sunshine Mix (Sungrow Horticulture, Sanbena, Canada) and watered as needed with a half-strength Hoagland’s nutrient solution (Hoagland and Arnon, 1950). After emergence, each pot was thinned to a single plant. The study included two factors, cultivar and temperature, arranged in a completely randomized design with four replications. Temperature treatments included maximum daytime temperatures of 30, 34, and 38EC, while nighttime temperatures were held constant at 20EC. The second factor was cultivar and consisted of two modern and two obsolete cultivars. The modern cultivars selected were ‘SureGrow 747’ and ‘Stoneville 474’ and the obsolete cultivars were ‘Stoneville 213’ and ‘Deltapine 16’. Plants were initially grown in two large walk-in growth chambers (Conviron Environments Inc., Pembina, N.D.) until two weeks after first flower (FF2). At this time, half of the plants in each growth chamber were placed with half the plants from the other growth chamber to limit the possibility that treatment differences were the result of different growing conditions between the two growth chambers. Also, one-third of the plants in each chamber were planted three days after the start of the experiment so that final physiological measurements were conducted on plants of the same age. Beginning at FF2, two separate growth chambers were programmed for 30 and 34EC daytime temperatures and a photoperiod of 14 h. Three days after plants were exposed to these temperatures, a series of physiological measurements were taken to characterize physiological responses of contrasting genotypes under high-temperature stress. Measurements included leaf photosynthesis, chlorophyll fluorescence, membrane leakage, total soluble protein, and chlorophyll. Finally, the remaining plants were placed in a growth chamber programmed for 38EC, and measurements taken three days after temperature imposition. RESULTS AND DISCUSSION Leaf Photosynthesis and Chlorophyll Fluorescence There were no significant differences in leaf photosynthesis between modern and obsolete cotton cultivars at lower ambient temperatures of 30 and 34EC, however, at an elevated temperature of 38EC the obsolete cultivars showed a significant (p