PHOTOINHIBITION AND OXIDATIVE STRESS - EFFECTS ON XANTHOPHYLL CYCLE, SCAVENGER ENZYMES AND ABSCISIC-ACID CONTENT IN TOBACCO PLANTS

The effects of short-term ozone pollution at high doses (300 ppb for 3 0 min) on the xanthophyll cycle under photoinhibitory conditions (1800 mu mol m(-2) s(-1)) were studied. The experiments were conducted on t wo different tobacco (Nicotiana tabacum L.) cultivars, one O-3-toleran t (Havana) and the other O-3-sensitive (Bel-W3). Given that the phytot oxicity of O-3 is due to the formation of active oxygen species, the p lants were also subjected to direct treatment with H2O2, O-.(2)-, (OH) -O-. and paraquat, a herbicide that, under certain conditions, produce s the above-mentioned toxic forms. Our results show that in both culti vars oxidative stress inhibits the function of the xanthophyll cycle w ith a consequent decrease of antheraxanthin and zeaxanthin synthesis, particularly during exposure to the superoxide ion and the (OH)-O-. ra dical. In addition, we determined the activities of the scavenger enzy mes, i.e. ascorbate peroxidase, dehydroascorbate reductase, glutathion e reductase, and glutathione-S-transferase involved in the ascorbate m etabolism, the basic substrate for the de-epoxidation reaction of viol axanthin. The quantity of abscisic acid was also determined after ozon e exposure. The results showed a greater presence of this hormone in t he leaves of the O-3-treated plants, a simultaneous degradation of vio laxanthin was observed, which was not compensated by the transformatio n into antheraxanthin + zeaxanthin. It is hypothesized that, under pho toinhibitory conditions combined with strong oxidative stress, violaxa nthin is used in large part not for the xanthophyll cycle reaction but for the synthesis of growth inhibitory substances such as abscisic ac id.