STOMATAL RESPONSE TO WATER VAPOR PRESSURE DEFICIT: AN UNSOLVED ISSUE

Abstract

Leaf-to-air water vapor pressure deficit (VPD) is an important environmental factor that can affect stomatal functioning in higher plants. The objective of this review is to provide an overview on the stomatal response to VPD in higher plants and the possible mechanisms that have been proposed to explain such response. There are conflicting results about whether stomata respond to VPD or not. Soil water stress and leaf position are factors that may affect the stomatal response to VPD and can help to explain these conflicting results. When stomata do respond to VPD, the mechanism causing such response is not well understood, and two contrasting hypotheses have been proposed. The feedforward hypothesis states that stomatal conductance (Gs) decreases directly as VPD increases, with abscisic acid (ABA) in the leaves probably triggering the response. The feedback hypothesis states that Gs decreases as VPD increases because of an increase in transpiration (E) that lowers the leaf water potential. These two mechanisms have been the subject of vigorous debates as there are published results that support both hypotheses. The results of this review show that stomatal response to VPD is still a controversial issue and constitutes a good research rationale for the current generation of plant physiologists.