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

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.