Observations and model simulations link stomatal inhibition to impaired hydraulic conductance following ozone exposure in cotton

Ozone (O-3) inhibits plant gas exchange and productivity. Vapour phase (g(s )) and liquid or hydraulic phase (K) conductances to water flux are often c orrelated as both change with environmental parameters. Exposure of cotton plants to tropospheric O-3 reduces g(s) through reversible short-term mecha nisms and by irreversible long-term disruption of biomass allocation to roo ts which reduces K. We hypothesize that chronic effects of O-3 On gas excha nge can be mediated by effects on K without a direct effect of O-3 on g(s) or carbon assimilation (A), Experimental observations from diverse field an d exposure chamber studies, and simulations with a model of mass and energy transport, support this hypothesis, O-3 inhibition of K leads to realistic simulated diurnal courses of g(s) that reproduce observations at low ambie nt O-3 concentration and maintain the positive correlation between midday g (s) and K observed experimentally at higher O-3 concentrations. Effects med iated by reduced K may interact with more rapid responses of g(s) and A to yield the observed suite of oxidant impacts on vegetation. The model extend s these physiological impacts to the extensive canopy scale. Simulated magn itudes and diurnal time courses of canopy-scale fluxes of H2O and O-3 match observations under low ambient concentrations of O-3. With greater simulat ed concentrations of O-3 during plant development, the model suggests poten tial reductions of canopy-scale water fluxes and O-3 deposition. This could represent a potentially unfavourable positive feedback on tropospheric O-3 concentrations associated with biosphere-atmosphere exchange.