Responses of the xanthophyll cycle pool and ascorbate-glutathione cycle to ozone stress in two tobacco cultivars.

Plants of Nicotiana tabacum (O3-tolerant cv Bel-B and O3-sensitive cv Bel-W3) were exposed to 150 ppb of ozone for 5 h; the fumigation produced visual injury in mature leaves, particularly in Bel-W3. After O3-treatment the pigments of the xanthophyll cycle pool decreased in both cvs, with a strong reduction in violaxanthin content, while antheraxanthin and zeaxanthin increased slightly. Under these conditions the content of leaf abscisic acid (ABA) markedly increased, particularly in O3-sensitive cv, indicating that the violaxanthin may have been partially converted into ABA. The control plants of Bel-B showed an ascorbic acid content four times greater than Bel-W3 and the ozone treatment did not produce significant differences in the ascorbic acid content and in the redox state. The two tobacco cvs were found to have similar total glutathione content, however the redox state was lower in O3-sensitive cv and decreased after ozone exposure. Ozone fumigation caused an increase in oxidized glutathione, particularly in Bel-W3, associated with a reduced glutathione reductase (GR) activity and a reduced GR protein content.