Revealing the hidden role of capacitance in the water flow through plants to the atmosphere.

Identical physical principles dictate the formulation of transpiration and sap flow. Yet, a hiatus separates the ways the conduction of water in the vapor and liquid phases are dimensioned, leading to the elaboration of conflicting response mechanisms to water stress. An analysis prompted by the discrepancy between the dimensionality assigned to water vapor conductance for transpiration and that assigned to hydraulic conductance for sap flow reveals that capacitance combines with conductance to determine the respective transport rates. For transpiration, the capability of air to accept or release water vapor qualified as pneumatic capacitance, derived from theory, is equal to the reciprocal of the barometric pressure. The conductance is set by the network of diffusive and convective air paths where stomatal aperture is the ruling biological component. For sap flow the current view is that the proportionality between flux density and motive water potential difference is the hydraulic conductance, disregarding that its physical dimensions lump conductance and capacitance. Plant hydraulic capacitance derives from measured water retention curves. It varies according to water potential which is determined by the metabolic regulation of living cell osmolarity. Hydraulic conductance is set by abiotic factors: friction and water column cohesion in dead xylem conduits. Lumping the two properties into a single parameter erroneously defined as plant hydraulic conductance confounds the factors affecting sap flow, precluding a scientifically sound investigation of plant hydraulics.