Effects of short-term ozone fumigation on tobacco plants: response of the scavenging system and expression of the glutathione reductase

The role that the constituents of the ascorbate-glutathione cycle play in t he mechanism of contrasting ozone sensitivities was examined in mature and old tobacco leaves after acute ozone-fumigation (150 p.p.b., 5 h). Levels o f the enzyme activities associated with the detoxifying system were lower i n ozone-sensitive Bel W3 control plants than in unfumigated ozone-tolerant Bel B plants. In particular, the endogenous activities of ascorbate peroxid ase (APX) and glutathione reductase (GR), and the metabolites ascorbic acid (AA) and reduced glutathione (GSH) were more abundant in Bel B than Bel W3 control plants. These results suggest that the higher tolerance of Bel B t o O-3 is associated with a greater initial content of the antioxidant enzym es or metabolites. Only in the mature leaves of the ozone-tolerant Bel B cv . did fumigation trigger activation of APX and, weakly, of dehydroascorbate reductase (DHAR). The activity of these enzymes was significantly lower af ter ozone treatment in both mature and old leaves of Bel W3 than in control plants. Fumigation had little effect on the ascorbate content. Its main ef fects on the glutathione pool were that it boosted the oxidized form and lo wered the reduced form, particularly in mature Bel W3 leaves. Extractable G R activity remained unchanged in both Bel B and Bel W3 immediately after fu migation, but increased slightly 24 h later, particularly in mature leaves of Bel W3. Exposure to O-3 caused a sharp decline in chloroplastic GR mRNA levels in both cultivars. However, as Western blot analysis failed to detec t any major changes in GR protein content at this time, the protein must be highly stable. There is therefore a good correlation between tolerance to O-3 and high endogenous levels of antioxidant metabolites such as AA and GS H in tobacco. In addition, the degree of inducibility of the system discrim inates the two cultivars investigated.