Loss of plasma membrane conductance in outside-xylem zone explains non-stomatal control of transpiration

The conventional assumption is that stomatal conductance ( g _s ) dominates the regulation of water and carbon dioxide fluxes between leaves and the atmosphere. Here, a nanoreporter of water status at the mesophyll cell surface and local xylem within intact maize leaves documents significant undersaturation of water vapor in the outside-xylem zone (OXZ) and a large loss of conductance of this zone ( g _oxz ) at moderate xylem water stress (no turgor loss). The ratio of the resistances (1/ g _oxz )/(1/ g _s ) serves as a predictive phenotype of undersaturation, non-stomatal regulation of transpiration, errors in standard gas exchange analysis, and an increase of intrinsic water use efficiency ( iWUE ). Cell-scale access to water status reveals symplasmic-apoplasmic disequilibrium and informs a biophysical model that can explain experimental observations quantitatively based on localization of variable conductance to the plasma membrane. This work opens new paths of inquiry into the molecular basis and functional consequences of non-stomatal regulation of transpiration.