OZONE STIMULATES APOPLASTIC ANTIOXIDANT SYSTEMS IN PUMPKIN LEAVES

The phytotoxiticity of ozone is due to its high oxidant capacity and t o its ability to generate toxic molecular species. It is well known th at intracellular peroxidases play an important role in eliminating tox ic forms of oxygen but little evidence has been reported on the role o f peroxidases in the apoplastic compartment. The detoxification system s located in the foliar extracellular matrix and in the intracellular fluid of sensitive pumpkin plants (Cucurbita pepo L. cv. Ambassador) e xposed to ozone (150 ppb, 5 days, 5 h day(-1)) in a fumigation chamber , were analyzed The analyses were carried out on both young and mature leaves. Ascorbate peroxidase (EC 1.11.1.11) was found in the extracel lular matrix of the pumpkin tissues. Its activity increased in both yo ung and mature leaves as a consequence of the treatment, while at intr acellular levels its effect was most prominent in mature leaves. Analy sis of the ascorbic-dehydroascorbic acid system revealed an enhancemen t of the pool content in the extracellular matrix of both kinds of lea ves as a consequence of fumigation, while at the intracellular level s mall variations were found. Very little variation was observed in the glutathione pool as a consequence of fumigation. The analysis of a lip id peroxidation marker, malondialdehyde, showed the significant effect of ozone on membrane lipids. Following fumigation, the free phenols i n the extracellular matrix decreased in both young and mature leaves, while the free and glycoside-bound phenols of the intracellular fluid showed little increase. The results support the hypothesis that ozone stimulates the antioxidant systems mainly in the apoplast and that asc orbic peroxidase activity, ascorbic acid levels and cell wall stiffeni ng are the most influenced parameters.