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.