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 30 min) on the xanthophyll cycle under photoinhibitory conditions (1800 mu mol m(-2) s(-1)) were studied. The experiments were conducted on two different tobacco (Nicotiana tabacum L.) cultivars, one O-3-tolerant (Havana) and the other O-3-sensitive (Bel-W3). Given that the phytotoxicity of O-3 is due to the formation of active oxygen species, the plants were also subjected to direct treatment with H2O2, O-.(2)-, (OH)-O-. and paraquat, a herbicide that, under certain conditions, produces the above-mentioned toxic forms. Our results show that in both cultivars oxidative stress inhibits the function of the xanthophyll cycle with a consequent decrease of antheraxanthin and zeaxanthin synthesis, particularly during exposure to the superoxide ion and the (OH)-O-. radical. In addition, we determined the activities of the scavenger enzymes, i.e. ascorbate peroxidase, dehydroascorbate reductase, glutathione reductase, and glutathione-S-transferase involved in the ascorbate metabolism, the basic substrate for the de-epoxidation reaction of violaxanthin. The quantity of abscisic acid was also determined after ozone exposure. The results showed a greater presence of this hormone in the leaves of the O-3-treated plants, a simultaneous degradation of violaxanthin was observed, which was not compensated by the transformation into antheraxanthin + zeaxanthin. It is hypothesized that, under photoinhibitory conditions combined with strong oxidative stress, violaxanthin is used in large part not for the xanthophyll cycle reaction but for the synthesis of growth inhibitory substances such as abscisic acid.