Ozone and salinity combined stress effects on olive leaf antioxidant enzyme activities
The presence of high ambient ozone concentrations is common in the Mediterranean region, while saline water is often used for olive irrigation. The effects of this combined stress were studied in young olive trees. Two-year-old 'Konservolea' and 'Kalamata' olive plants grafted on olive seedlings were subjected to ambient O 3 levels or to charcoal-filtered air in open top chambers and irrigated with half strength Hoagland's solution with or without 100 mM NaCl during the growing period in 2006 and 2008. Superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX) activities were measured periodically from July to October in this year's (young) and last year's (old) leaves. Enzyme activities increased gradually during summer followed by a sharp decrease in October, except APX in 2008, when the activity did not change significantly with time. This increase in antioxidant enzyme activities must be related to progressive stress during summer from high temperatures, salinity and ozone. Young leaves always had higher antioxidant enzyme activities than old leaves. 'Konservolea' leaves (especially the young ones) often had higher antioxidant enzyme activities than 'Kalamata' leaves, thus the former could be more resistant to salinity than the latter cultivar. The most photosynthetically active young leaves from both 'Konservolea' and 'Kalamata' were mainly affected by salinity. Old leaves from both cultivars were not significantly affected by either of the stresses.