Salinity-induced changes in growth, superoxide dismutase activity, and ion content of two olive cultivars

Olive trees (Olea europaea L.) are considered moderately tolerant to salinity, with clear differences found among cultivars. One-year-old self-rooted olive plants of the Croatian cv. Oblica and Italian cv. Leccino were grown for 90 d in nutrient solutions containing 0, 66, or 166 mM NaCl, respectively. The shoot length and the number of nodes and leaves for both cultivars were not affected by salinity up to 66 mM NaCl. However, at 166 mM NaCl, growth of Leccino was reduced earlier and to a higher extent than growth of Oblica. After 10 d of exposure to 66 and 166 mM NaCl, increased activity of superoxide dismutase (SOD) was observed in Leccino, whereas there was almost no response in Oblica. Reduced SOD activity in Leccino at 166 mM NaCl was observed after prolonged stress (90 d), whereas in Oblica SOD was increased at 66 mM compared to control or 166 mM NaCl. Electrolyte and K+ leakage were increased and relative water content decreased as NaCl concentration increased with similar intensity of response measured in both cultivars. Oblica exhibited an ability to keep a higher K+ : Na+ ratio at all salinity levels compared to Leccino, but since no difference was found in leaf K+ concentration, this was mainly achieved by less Na+ ions reaching the younger leaves. The antioxidative system represents a component of the complex olive salt-tolerance mechanism, and it seems that the role of SOD in protection from oxidative stress depends on sodium accumulation in leaves.