Physiological Tradeoffs of Stomatal Closure Under High Evaporative Gradients in Field Grown Soybean.

Rainfall variability is the main constraint to agriculture, thus it is a matter of priority to understand plant behavior that leads to avoidance of soil water deficit. One focus has screened for crop varieties that decrease stomatal conductance under high vapor pressure deficit (VPD), a proxy for the evaporative gradient. However, the link between lower water use and physiological consequences in field environments is not yet clear. Particularly under soil water deficit, such water conservative crops are at risk of photosynthetic heat damage and photoinhibition from decreased evaporative cooling, high light and stomatal constraints on photosynthesis. A field experiment on soybeans demonstrated that considerable variation in leaf temperature relative to air temperature occurred, leading to the evaporative gradient differing from VPD. Soil water deficit led to lower stomatal conductance, particularly under low evaporative conditions, not just under hot conditions. Non-stomatal photosynthetic limitations were observed due to combined occurrence of stomatal closure and high temperature under high VPD. Although leaves reached temperatures higher than the threshold for a decrease in maximum photochemical efficiency, and displayed non-stomatal photosynthetic limitations, no photoinhibition or damage was observed by nighttime. The results demonstrate that more understanding of physiological strategies for achieving altered water use is needed to avoid tradeoffs and heat stress.