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

Ozone (O₃) inhibits plant gas exchange and productivity. Vapour phase (g_s) and liquid or hydraulic phase (K) conductances to water flux are often correlated as both change with environmental parameters. Exposure of cotton plants to tropospheric O₃ reduces g_s through reversible short-term mechanisms and by irreversible long-term disruption of biomass allocation to roots which reduces K. We hypothesize that chronic effects of O₃ on gas exchange can be mediated by effects on K without a direct effect of O₃ on g_s or carbon assimilation (A). Experimental observations from diverse field and exposure chamber studies, and simulations with a model of mass and energy transport, support this hypothesis. O₃ inhibition of K leads to realistic simulated diurnal courses of g_s that reproduce observations at low ambient O₃ concentration and maintain the positive correlation between midday g_s and K observed experimentally at higher O₃ concentrations. Effects mediated 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 extends these physiological impacts to the extensive canopy scale. Simulated magnitudes and diurnal time courses of canopy-scale fluxes of H₂O and O₃ match observations under low ambient concentrations of O₃. With greater simulated concentrations of O₃ during plant development, the model suggests potential reductions of canopy-scale water fluxes and O₃ deposition. This could represent a potentially unfavourable positive feedback on tropospheric O₃ concentrations associated with biosphere–atmosphere exchange.