P. Batini

Photoinhibition and oxidative stress : effects on xanthophyll cycle, scavenger enzymes and abscisic acid content in tobacco plants

Summary The effects of short-term ozone pollution at high doses (300 ppb for 30 min) on the xanthophyll cycle under photoinhibitory conditions (1800 μmol 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 -sensi-tive (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 H 2 O 2 , · O 2 − , · OH 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 anthera-xanthin and zeaxanthin synthesis, particularly during exposure to the superoxide ion and the · OH radical. In addition, we determined the activities of the scavenger enzymes, i.e. ascorbate peroxidase, dehydroas-corbate 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.

Physiological characteristics of tobacco cultivars with contrasting sensitivity to ozone

Abstract Two cultivars of tobacco with contrasting sensitivity to ozone have similar photosynthesis at low light intensity, but at high light, photosynthesis of the sensitive cultivar Bel-W3 was lower than that of the resistant cultivar Havana. In contrast, the electron transport rate at photosynthesis saturated light intensity was higher in Bel-W3 than in Havana. This excess may require efficient mechanisms to dissipate energy and oxygen radicals. We found that the non-radiative dissipation was lower in Bel-W3 than in Havana and that the conversion between violaxanthin and anther-axanthin + zeaxanthin was not complete in the ozone sensitive cultivar. Violaxanthin may have been partly converted in ABA. The high content of ABA may have caused stomatal closure and explain why photosynthesis at high light was reduced in Bel-W3 in comparison to Havana. β-Carotene content was also lower in Bel-W3 than in Havana. A low β-carotene content coupled with an inefficient use of the xanthophyll mechanism may contribute to determine the ozone sensitivity of Bel-W3 leaves