Stomatal response to VPD is not triggered by changes in soil-leaf hydraulic conductance in Arabidopsis or Callitris.

Stomatal closure under high VPD (leaf to air vapour pressure deficit) is a primary means by which plants prevent large excursions in transpiration rate and leaf water potential (Ψ) that could lead to tissue damage. Yet, the drivers of this response remain controversial. Changes in Ψ appear to drive stomatal VPD response, but many argue that dynamic changes in soil-to-leaf hydraulic conductance (K) 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) in response to typical changes in daytime VPD is influenced by dynamic changes in K. 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 and g were continuously monitored by combining concurrent in situ measurements of Ψ using an open optical dendrometer and whole plant transpiration using a balance. Large changes in VPD were imposed to induce stomatal closure and observe the impact on K. In both species, g was observed to decline substantially as VPD increased, while K remained stable. Our finding suggests that stomatal regulation of transpiration is not contingent on a decrease in K. Static K 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.