Ozone and water stress: effects on the behaviour of two white clover (Trifolium repens L.) biotypes.

Since ozone penetration in vegetation occurs mainly through stomata, all factors which alter plant-atmosphere relations could be able to modify plant response to ozone. Interaction between ozone and water stress in Mediterranean environment was studied on ozone resistant (R) and sensitive (S) biotypes of white clover, which were grown in charcoal filtered and not-filtered Open Top Chambers. Measurements of biomass, leaf area and stomata conductance were made during the growth period. Ozone injuries were estimated by the ratio between biomass yield gained in not-filtered and in filtered OTC; ozone uptake was estimated by the stomata conductance and ozone hourly concentrations. In presence of ozone, biomass yield was higher in R biotype than in S biotype, while no difference between the two biotypes was recorded in filtered air. Water stress partially reduced ozone injuries, as well as water stress effect was partially reduced in S biotype grown in the presence of ozone, while no interaction between ozone and water stress was noticed in R biotype. Yield losses of S was significantly correlated to cumulated ozone uptake, while yield losses of R was not influenced by variations in ozone fluxes. Flux based models could better estimate yield losses due to ozone in Mediterranean environments in which other stresses could be contemporary present; therefore, the new European directives might modify the actual thresholds based on ozone concentration with others based on models of stomata flux of ozone.