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