Diurnal Shifts in Stomatal Regulation in Response to Water Availability

Diurnal patterns of stomatal conductance are central to understanding plant responses to different types of drought conditions and the resulting impact on environmental fluxes of water and CO2. That maximum stomatal opening shifts earlier in the day has long been observed, but the underlying physiology remains unclear. We studied the stomatal diurnal trend in Vitis vinifera under both soil and VPD drought while keeping light, temperature and CO2 constant in growth chamber conditions. We also examined the recovery of the diurnal trend when the drought condition was relieved. As drought conditions progressed, stomatal closure occurred earlier in the day compared to well-watered controls. The shift to early stomatal closure was not linked to leaf water potential, suggesting instead that it could reflect heterogeneity in diurnal soil water conductance. After rewatering to increase soil hydraulic conductivity uniformly, the diurnal shift was immediately eliminated. The sensitivity of diurnal stomatal conductance to soil drying was not paralleled by a reduction in net assimilation. Instead, net assimilation decreased with leaf water potential. The complex response of stomatal conductance and net assimilation suggests that differential soil hydraulic conductivity during the day might be an important driver of diurnal stomatal cycles during drought. Understanding the nonlinear nature of stomatal response to drought will improve our ability to integrate stomatal regulation of water and CO2­ fluxes in hydraulic models and guide physiological research.