Relationship between plant hydraulic and biochemical properties derived from a steady-state coupled water and carbon transport model

There is growing evidence that plant stomata have evolved physiological controls to satisfy the demand for CO2 by photosynthesis while regulating water losses by leaves in a manner that does not cause cavitation in the soil-root-xylem hydraulic system. Whether the hydraulic and biochemical properties of plants evolve independently or whether they are linked at a time scale relevant to plant stand development remains uncertain. To address this question, a steady-state analytical model was developed in which supply of CO2 via the stomata and biochemical demand for CO2 are constrained by the balance between loss of water vapour from the leaf to the atmosphere and supply of water from the soil to the leaf. The model predicts the intercellular CO2 concentration (C-i) for which the maximum demand for CO2 is in equilibrium with the maximum hydraulically permissible supply of water through the soil-root-xylem system. The model was then tested at two forest stands in which simultaneous hydraulic, ecophysiological, and long-term carbon isotope discrimination measurements were available. The model formulation reproduces analytically recent findings on the sensitivity of bulk stomatal conductance (g (s)) to vapour pressure deficit (D); namely, g(s) = g(ref) (1 - m x lnD), where m is a sensitivity parameter and g(ref) is a reference conductance defined at D = 1 kPa. An immediate outcome of the model is an explicit relationship between maximum carboxylation capacity (V-cmax) and soil-plant hydraulic properties. It is shown that this relationship is consistent with measurements reported for conifer and rain forest angiosperm species. The analytical model predicts a decline in V-cmax as the hydraulic capacity of the soil-root-xylem decreases with stand development or age.