The Stomatal Response to Temperature Is Enhanced by High Evaporative Demand, Consistent With a Partially Hydraulic...

The direct response of stomata to temperature (DRST, the response with the leaf-to-air vapor gradient, Δw, held constant) is poorly studied due to the difficulty of keeping Δw constant while changing leaf temperature. Most published data suggest a positive response, though the mechanisms behind such a response are unknown. We propose that a hydraulic mechanism should contribute to the DRST, wherein temperature decreases the viscosity of water, increasing hydraulic conductance and thereby increasing leaf water potential, which in turn drives stomatal opening. Because the sensitivity of leaf water potential to changes in hydraulic conductance should be proportional to transpiration rate and hence to Δw, this mechanism predicts a stronger positive DRST at higher Δw than at lower Δw. We tested this prediction by measuring the DRST at two different values of Δw, in six diverse angiosperm species. Our results are consistent with the hypothesis that a hydraulic mechanism contributes to the DRST, though the response varies widely across species, and in three of six species the effect of Δw was far stronger than predicted from theory, suggesting a role for other mechanisms in enhancing the effect of Δw on the DRST.