Metabolic changes in white clover clones exposed to ozone

The effects of ozone (200 ppb, 5 h) on metabolic activity of NC-S (sensitive) and NC-R (resistant) white clover clones have been investigated at the end of fumigation and then 24 h after. NC-S exhibited foliar injury and membrane permeability was significantly increased, indicating that it was subjected to membrane damage. No alterations were observed in NC-R. After the end of treatment, both clones decreased their CO2 fixation ability. In NC-S, strong negative effects on photosynthetic ability were observed due mainly to mesophyllic limitation, since Ci increased significantly. This is also suggested by an uncoupling between Gw and Amax, which came up at the end of fumigation. NC-R showed a peculiar response: after the end of the fumigation, Ci increased. This could indicate that the limitation to photosynthetic processes can be due to non-stomatal components. Photosynthesis was completely recovered when stress was removed. WUEi was highest in NC-R since it increased its efficiency under O3 exposure. Thus, the absence of Gw changes after the treatment and the open stomata ability during the recovery period observed in NC-R clone may be the strategy used to restore Amax to control values and stimulate protective mechanisms. In ozonated NC-S, the significant increase of the minimal fluorescence and the concomitant decrease in the maximal fluorescence have determined the reduction of photochemical efficiency of PSII. This result is a reliable sign of O3-induced photoinhibition. No significant differences are reported in the parameters of chlorophyll a fluorescence and in quenching components of NC-R in response to O3. During the recovery period, a slight increase was observed in chlorophylls a and b and carotenoid contents of NC-S. The exposure induced the activation of the phenylpropanoid pathway, which was in agreement with PAL activity measured, but in NC-S a strong increase in these compounds was observed, leading us to suppose that they were not used as antioxidants and/or as substrates of phenol-consuming enzymes such as peroxidase isoforms, phenolases and polyphenoloxidase.