Plasmalemma protection by the apoplast as assessed from above-zero ozone concentrations in leaf intercellular air spaces

To evaluate reactive absorption of ozone (O₃) in the leaf apoplast, amphistomatous leaves of Phaseolus vulgaris L. were allowed to take up O₃ through the stomata on the lower leaf surface at high rates for 3–5 min. Up to 5% of the O₃ taken up diffused through the leaf and emerged from the stomata on the upper surface, suggesting above-zero O₃ concentrations in the leaf intercellular air space, [O₃]ᵢ. Moreover, measurements revealed that [O₃]ᵢ increased during exposure to the pollutant. Time patterns of O₃ fluxes through the gas phase and into the aqueous apoplast indicated that the increase in [O₃]ᵢ was the result of a decrease in the diffusion-reaction conductance of the aqueous apoplast, g ₐq. Under an intense O₃ pulse, g ₐq approached the value of the pure diffusional conductance within 2.5 min of exposure, suggesting the exhaustion of protective resources in the leaf apoplast. Toward the end of the exposure g ₐq tended to increase, suggesting either a recovery in the protective resources in the leaf apoplast and/or the induction of new defences. The possibility of estimating the degree of protection afforded by apoplast constituents and the rate of recovery of these protective systems in intact leaves using brief O₃ pulses is discussed.