• air pollution;
  • allocation;
  • dry deposition;
  • evapotranspiration;
  • oxidant;
  • ozone flux;
  • regional ozone concentrations;
  • root–shoot communication;
  • water relations


Ozone (O3) inhibits plant gas exchange and productivity. Vapour phase (gs) 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 gs 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 gs 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 gs that reproduce observations at low ambient O3 concentration and maintain the positive correlation between midday gs and K observed experimentally at higher O3 concentrations. Effects mediated by reduced K may interact with more rapid responses of gs 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.