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 C-i increased significantly. This is also suggested by an uncoupling between G(w) and A(max), 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 O-3 exposure. Thus, the absence of G(w) 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 A(max) 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 O-3-induced photoinhibition. No significant differences are reported in the parameters of chlorophyll a fluorescence and in quenching components of NC-R in response to O-3. 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. (c) 2006 Elsevier B.V. All rights reserved.