Loss of conductance between mesophyll symplasm and intercellular air spaces explains nonstomatal control of transpiration

This research provides experimental and theoretical evidence that stomates are not the sole regulators of transpiration in plants. We use a nanoreporter of water potential (AquaDust) to document significant nonstomatal control of transpiration, with significant gains in water-use efficiency and large local disequilibrium within leaf tissue under moderate drought stress. With the methods and biophysical model introduced here, we quantitatively explain this nonstomatal control of water loss based on loss of conductance of plasma membranes in the leaf. These developments open paths to investigate this phenomenon and pursue its implications for the design of crops with high water-use efficiency and for our understanding of water stress responses across both agricultural and natural ecological contexts. The conventional assumption is that stomatal conductance (gs) 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 (goxz) at moderate xylem water stress, without stomatal closure or turgor loss. The ratio of the resistances (1/goxz)/(1/gs) serves as a predictive phenotype of undersaturation, nonstomatal 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 paths of inquiry into the molecular basis and functional consequences of nonstomatal regulation of transpiration.