Beyond guard cells: Epidermal anatomy determines transient stomatal opening following rapid changes in hydration status

Abstract

The rapid and transient increase of stomatal conductance in response to a sudden water deficit in plants, known as the wrong-way response (WWR), is hypothesized to result from rapid turgor loss of epidermal cells, which pulls the guard cells apart, leading to the opening of the stomatal pore. Variations in WWR amplitude and duration could stem from differences in epidermal anatomy and/or cell density, which influence the localized turgor responses driving stomatal movement. This study tested this hypothesis by aiming to induce WWRs through both a sudden increase in the leaf-to-atmosphere vapor-pressure deficit and leaf excision. We investigated the WWR kinetics in five Arabidopsis thaliana lines with varying epidermal characteristics. Stomatal conductance was measured at high temporal resolution using infrared gas analysis and epidermal anatomical traits were characterized using scanning electron microscopy. Interestingly, no WWR was detectable following a rapid decrease in air humidity. By contrast, leaf excision triggered a WWR across the majority of leaves examined, enabling robust analysis of response kinetics. While variation in magnitude and duration of the WWR among different lines was small, the duration and the amplitude of the WWR was strongly correlated with pavement cell size. No association between these WWR metrics and stomatal traits or shape-related characteristics such as cell eccentricity or lobe density were detected. These findings indicate that pavement cell size is a key determinant of the mechanical coupling underlying the WWR.