Drought Stress in Field-Grown Gossypium hirsutum Limits Net Photosynthesis By Decreasing Stomatal Conductance, Increasing Photorespiration, and Increasing the Proportion of Gross Photosynthesis Lost to Dark Respiration.

Renewed interest in methods to improve irrigation efficiency and drought tolerance of agronomic crops has yielded positive results; however, a clear understanding of the physiological mechanisms that interact to decrease net carbon fixation and drive yield loss has not been attained.   To elucidate the effect of drought stress on the underlying mechanisms contributing to diminished source strength, three cultivars of Gossypium hirsutum were grown in the field under contrasting irrigation regimes during the 2012 and 2013 growing season near Camilla, Georgia, USA. Physiological measurements were conducted on three sample dates during each growing season (providing a broad range of plant water status) and included, predawn and midday leaf water potential (Ψ_PD and Ψ_MD), gross and net photosynthesis, dark respiration, photorespiration, and chlorophyll a fluorescence. End-of-season lint yield was also determined. Ψ_PD ranged from -0.31 to -0.95 MPa, and Ψ_MD ranged from -1.02 to -2.67 MPa, depending upon irrigation regime and sample date.  G. hirsutum responded to water deficit by decreasing stomatal conductance, increasing photorespiration, and increasing the ratio of dark respiration to gross photosynthesis, thereby limiting P_N and decreasing lint yield (lint yield declines observed during the 2012 growing season only).Conversely, even extreme water deficit, causing a 54% decline in P_N, did not negatively affect actual quantum yield, maximum quantum yield, or photosynthetic electron transport. It is concluded that P_N is primarily limited in drought-stressed G. hirsutum by decreased stomatal conductance, along with increases in respiratory and photorespiratory carbon losses, not inhibition or down-regulation of electron transport through photosystem II. It is further concluded that Ψ_PD is a reliable indicator of drought stress and the need for irrigation in field-grown cotton.