Stomatal response to <scp>VPD</scp> is not triggered by changes in soil–leaf hydraulic conductance in <i>Arabidopsis</i> or <i>Callitris</i>
Ibrahim Bourbia, Timothy J. Brodribb
Abstract
Summary Stomatal closure under high VPD L (leaf to air vapour pressure deficit) is a primary means by which plants prevent large excursions in transpiration rate and leaf water potential (Ψ leaf ) that could lead to tissue damage. Yet, the drivers of this response remain controversial. Changes in Ψ leaf appear to drive stomatal VPD L response, but many argue that dynamic changes in soil‐to‐leaf hydraulic conductance ( K s‐l ) make an important contribution to this response pathway, even in well‐hydrated soils. Here, we examined whether the regulation of whole plant stomatal conductance ( g c ) in response to typical changes in daytime VPD L is influenced by dynamic changes in K s‐l . We use well‐watered plants of two species with contrasting ecological and physiological features: the herbaceous Arabidopsis thaliana (ecotype Columbia‐0) and the dry forest conifer Callitris rhomboidea . The dynamics of K s‐l and g c were continuously monitored by combining concurrent in situ measurements of Ψ leaf using an open optical dendrometer and whole plant transpiration using a balance. Large changes in VPD L were imposed to induce stomatal closure and observe the impact on K s‐l . In both species, g c was observed to decline substantially as VPD L increased, while K s‐l remained stable. Our finding suggests that stomatal regulation of transpiration is not contingent on a decrease in K s‐l . Static K s‐l provides a much simpler explanation for transpiration control in hydrated plants and enables simplified modelling and new methods for monitoring plant water use in the field.