Defining drought sensitivity and assessing alternative irrigation practices for Gossypium hirsutum grown in the southeastern United States
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Drought represents a major threat to cotton production worldwide. In the current dissertation, three experiments were conducted under field conditions to assess 1) the underlying limitations to net photosynthesis under drought, 2) the potential for plant-based irrigation scheduling to improve water productivity in Georgia cotton production, and 3) the impact of leaf development on drought and heat tolerance. For experiment 1, predawn water potential (ΨPD) ranged from -0.31 to -0.95 MPa, and midday water potential (ΨMD) ranged from -1.02 to -2.67 MPa for the 2012 and 2013 growing seasons combined for irrigated and dryland cotton. Cotton responded to water deficit by decreasing stomatal conductance, increasing photorespiration, and increasing the ratio of dark respiration to gross photosynthesis, thereby limiting PN and decreasing lint yield. Even extreme water deficit did not negatively affect primary photochemistry. For experiment 2, G. hirsutum plants were grown under fully-irrigated, dryland, and three predawn water potential (ΨPD) thresholds (-0.5, -0.7, -0.9 MPa). ΨPD was an effective means of determining the need for irrigation in cotton, and in the current study, yield and water productivity were maximized at a season-long average ΨPD threshold of -0.5 MPa. Canopy temperature-derived crop water stress index (CWSI) exhibited a non-linear relationship with ΨPD between -0.4 and -0.7 MPa (r2 = 0.81) and with lint yield (r2 = 0.81). For experiment 3, increased photosystem II thermotolerance was observed for young leaves early in the growing season. PN in young leaves was not negatively impacted by leaf temperatures as high as 37°C or by extreme drought. For example, in young leaves, as TLeaf increased from 31 to 37°C, no decline in PN was observed whereas, PN in more mature leaves declined by 66% over the same temperature range when high Tleaf was drought induced. The substantial differences in heat and drought tolerance between two different stages of leaf development may provide opportunities to improve drought and heat tolerance by regulating pre-existing genes within the same genotype.