Cytosolic water potential as a mechanistic driver of leaf airspace unsaturation and non-stomatal control of transpiration

1 Abstract Mesophyll cells exhibit a previously underappreciated capacity to regulate water loss via low plasma membrane conductance ( L p ), offering a non-stomatal mechanism for transpiration control. However, the structural basis and regulation of L p remain poorly understood, limiting its integration into predictive models. In this study, we show that L p responds dynamically to changes in cytosolic water potential ( ψ cy ), decreasing as ψ cy approaches the turgor loss point. This identifies ψ cy as the primary physiological signal regulating L p . We introduce a predictive, physiologically grounded model linking L p to ψ cy . Our model establishes a mechanistic connection between internal water status, leaf hydraulics, substomatal cavity unsaturation, and gas exchange. This framework opens new avenues for understanding and modelling plant water use under stress.