The direct response of stomata to temperature (DRST, the response with the leaf-to-air vapor gradient, Δ w, held constant) is poorly studied due to the difficulty of keeping Δ w 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 Δ w, this mechanism predicts a stronger positive DRST at higher Δ w than at lower Δ w. We tested this prediction by measuring the DRST at two different values of Δ w, 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 Δ w was far stronger than predicted from theory, suggesting a role for other mechanisms.

Plants differ widely in how soil drying affects stomatal conductance ( g s) and leaf water potential ( ψ leaf), and in the underlying physiological controls. Efforts to breed crops for drought resilience would benefit from a better understanding of these mechanisms and their diversity. We grew 12 diverse genotypes of common bean ( Phaseolus vulgaris L.) and four of tepary bean ( P. acutifolius; a highly drought resilient species) in the field under irrigation and terminal drought, and quantified responses of g s and ψ leaf, and their controls (soil water potential [ ψ soil], evaporative demand [Δ w] and plant hydraulic conductance [ K]). We hypothesized that (i) common beans would be more ”isohydric” (i.e., exhibit strong stomatal closure in drought, minimizing ψ leaf decline) than tepary beans, and that genotypes with larger ψ leaf decline (more ”anisohydric”) would exhibit (ii) smaller increases in Δ w, due to less suppression of evaporative cooling by stomatal closure and hence less canopy warming, but (iii) larger K declines due to ψ leaf decline. Contrary to our hypotheses, we found that half of the common bean genotypes were similarly anisohydric to most tepary beans; that isohydric genotypes experienced less canopy warming and hence smaller increases in Δ w in drought, and similar declines in K; and that stomatal closure was similar in isohydric and anisohydric genotypes. g s and ψ leaf were virtually insensitive to drought in one tepary genotype (G40068). Our results highlight the potential importance of non-stomatal mechanisms for leaf cooling, and the variability in drought resilience traits among closely related crop legumes.

Changes in leaf temperature are known to drive stomatal responses, because the leaf-to-air water vapor gradient (Δ w) increases with temperature if ambient vapor pressure is held constant, and stomata respond to changes in Δ w. However, the direct response of stomata to temperature (DRST; the response when Δ w is held constant by adjusting ambient humidity) has been examined far less extensively. Though the meager available data suggest the response is usually positive, results differ widely and defy broad generalization. As a result, little is known about the DRST. This review discusses the current state of knowledge about the DRST, including numerous hypothesized biophysical mechanisms, potential implications of the response for plant adaptation, and possible impacts of the DRST on plant-atmosphere carbon and water exchange in a changing climate.