Limitations to CO₂ assimilation in ozone-exposed leaves of Plantago major

•  The potential limitations on net leaf carbon assimilation imposed by stomatal conductance, carboxylation velocity, capacity for ribulose 1,5-bisphosphate regeneration and triose phosphate ultilization rate were derived from steady-state gas exchange measurements made over the life-span of two leaves on plants of an 'O₃ -sensitive' population of Plantago major grown at contrasting atmospheric O₃ concentrations. •  Parallel measurements of chlorophyll fluorescence were used to monitor changes in the quantum efficiency of PSII photochemistry, and in vitro measurements of Rubisco activity were made to corroborate modelled gas exchange data. •  Data indicated that a loss of Rubisco was predominantly responsible for the decline in CO₂ assimilation observed in O₃ -treated leaves. The quantum efficiency of PSII was unchanged by O₃ exposure. •  Stomatal aperture declined in parallel with CO₂ assimilation in O₃ -treated plants, but this did not account for the observed decline in photosynthesis. Findings suggested that O₃ -induced shifts in stomatal conductance result from 'direct' effects on the stomatal complex as well as 'indirect effects' mediated through changes in intercellular CO₂ concentration. Leaves on the same plant exposed to equivalent levels of O₃ showed striking differences in their response to the pollutant.