Temperature alters the response of hydraulic architecture to CO2 in cotton plants (Gossypium hirsutum)

Elevated CO2 and temperature are expected to result in drought stress with increased intensity and frequency, yet our understanding of subsequent plant response is generally limited. The objective of this study was to investigate the impacts of elevated CO2 and temperature on physiological traits affecting drought tolerance of cotton plants (Gossypium hirsutum). We grew cotton plants in the glasshouse under two CO2 treatments (Ca: 420 ppm; Ce: 640 ppm) and two temperature treatments (Ta: 32/24 °C; day/night; Te: 36/28 °C; day/night) with adequate irrigation and fertilization. Plant allometry, leaf gas exchange and a suite of hydraulic characteristics (xylem resistance to drought-induced embolism in the leaf and stem, leaf tolerance to dehydration-induced loss of rehydration capacity, and water transport capacity of stem) were examined. Xylem anatomical traits of the leaf and stem were also examined to elucidate the structural basis for potential physiological adjustments. Ce increased canopy leaf area and decreased leaf level water loss at Ta, and decreased stomatal conductance and transpiration at both temperatures. Moreover, Ce significantly increased stem conductivity at Ta, but xylem tissue was less resistant to drought induced embolism. Te altered the pattern of xylem conductivity and embolism resistance response to CO2, with the stem less hydraulically conductive while the xylem was more tolerant to embolism under Ce. The variation of stem conductivity and embolism resistance of the stem across CO2 and temperature treatments was likely to be partially explained by xylem anatomy. Overall, CO2 and temperature had interactive effects on traits associated with water relations of cotton, such that elevated CO2 compromised drought tolerance under ambient temperature, but these negative impacts were partially mitigated by elevated temperature.