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Temperature and drought are major abiotic limitations to crop productivity worldwide. While abiotic stress physiology research has focused primarily on fully expanded leaves, no studies have investigated photosynthetic tolerance to concurrent drought and high temperature during leaf ontogeny. To address this, Gossypium hirsutum plants were exposed to five irrigation treatments, and two different leaf stages were sampled on three dates during an abnormally dry summer. Early in the growing season, ontogenic PSII heat tolerance differences were observed. Photosystem II was more thermotolerant in young leaves than mature leaves. Later in the growing season, no decline in young leaf net photosynthesis (P N ) was observed as leaf temperature increased from 31 to 37 °C, as average midday leaf water potential (Ψ MD ) declined from −1.25 to −2.03 MPa. In contrast, mature leaf P N declined 66% under the same conditions. Stomatal conductance (g s ) accounted for 84–98% of variability in leaf temperature, and g s was strongly associated with Ψ MD in mature leaves but not in young leaves. We conclude that young leaves are more photosynthetically tolerant to heat and drought than mature leaves. Elucidating the mechanisms causing these ontogenic differences will likely help mitigate the negative impacts of abiotic stress in the future. [ABSTRACT FROM AUTHOR]

Respiratory carbon evolution by leaves under abiotic stress is implicated as a major limitation to crop productivity; however, respiration rates of fully expanded leaves are positively associated with plant growth rates. Given the substantial sensitivity of plant growth to drought, it was hypothesized that predawn respiration rates ( R PD ) would be (1) more sensitive to drought than photosynthetic processes and (2) highly predictive of water-induced yield variability in Gossypium hirsutum . Two studies (at Tifton and Camilla Georgia) addressed these hypotheses. At Tifton, drought was imposed beginning at the onset of flowering (first flower) and continuing for three weeks (peak bloom) followed by a recovery period, and predawn water potential ( Ψ PD ), R PD , net photosynthesis ( A N ) and maximum quantum yield of photosystem II ( F v / F m ) were measured throughout the study period. At Camilla, plants were exposed to five different irrigation regimes throughout the growing season, and average Ψ PD and R PD were determined between first flower and peak bloom for all treatments. For both sites, fiber yield was assessed at crop maturity. The relationships between Ψ PD , R PD and yield were assessed via non-linear regression. It was concluded for field-grown G. hirsutum that (1) R PD is exceptionally sensitive to progressive drought (more so than A N or F v / F m ) and (2) average R PD from first flower to peak bloom is highly predictive of water-induced yield variability. [ABSTRACT FROM AUTHOR]

Much effort has been expended to improve irrigation efficiency and drought tolerance of agronomic crops; however, a clear understanding of the physiological mechanisms that interact to decrease source strength and drive yield loss has not been attained. To elucidate the underlying mechanisms contributing to inhibition of net carbon assimilation under drought stress, 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. [ABSTRACT FROM AUTHOR]