Physiological responses to cumulative ozone uptake in two white clover (Trifolium repens L. cv. Regal) clones with different ozone sensitivity

Critical ozone levels must be determined to assess ozone damage on plants, but the cumulative ozone exposure concept of 'AOT40' fails to consider actual ozone uptake via the stomata. While the use of ozone fluxes to assess ozone-induced plant responses is mechanistically appropriate, few studies have examined the relationship between cumulative ozone fluxes and physiological dysfunction. Physiological differences between a widely used ozone-sensitive (NC-S) and ozone-resistant (NC-R) bio-monitor clone of white clover (Trifolium repens L. cv. Regal) were studied and related to cumulative ozone fluxes (CUO3). Generally, no physiological effect of ozone uptake was detected in the NC-R clone, whereas there were negative responses in the NC-S clone for most leaf gas exchange parameters, including net photosynthesis (Asat) and carboxylation capacity (Vcmax). Stomatal conductance (gs) was not significantly different between clones in ozone-free conditions, but gs decreased significantly for the NC-S clone during ozone exposure. The NC-S clone showed higher electron transport rates but lower non-photochemical quenching under high photon flux densities and elevated ozone compared to NC-R clone. Our results suggest that avoiding ozone-induced damage depends on the ability of different genotypes to reduce O3 uptake through stomatal closure and on the capacity for non-photochemical quenching to scavenge reactive oxygen. Relating key physiological parameters to cumulative ozone fluxes contributes to refining flux-based ozone uptake models used in setting critical ozone levels to alleviate the detrimental impact of O3 exposure on vegetation.