Changes in gas exchange, proline accumulation and antioxidative enzyme activities in three olive cultivars under contrasting water availability regimes

Changes in photosynthetic performance, osmolyte accumulation and the activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT) and polyphenol oxidase (PPO) were investigated in one-year-old olive cultivars (Chemlali, Meski and Picholine) subjected to contrasting water availability regimes under arid climatic conditions in Tunisia. Shoot elongation rates (SER) and photosynthetic performance were markedly reduced by the water deficit regime (WD) in all cultivars except for Chemlali, which proved to be superior to the other two cultivars with respect to drought tolerance. Higher photosynthetic performance (net photosynthesis (P_n), stomatal conductance (g_s) and transpiration rates (E)) in the Chemlali and Meski cvs. compared to Picholine olive allowed them to maintain better plant water status and shoot elongation rates. Under WD conditions, Chemlali showed a greater capability for proline accumulation. Leaves grown under WD conditions showed signs of oxidative stress such as reduced chlorophyll and carotenoid concentrations. Nevertheless, different cultivars developed certain antioxidative defense mechanisms, including elevated SOD, APX and CAT activities. In contrast, PPO activity decreased under WD circumstances. Comparatively, Chemlali olive displayed better antioxidative enzyme activity, and thus better protection against oxidative stress. These results show that the ability of olive trees to up-regulate the enzymatic antioxidative system might be an important attribute linked to drought tolerance. These findings demonstrate that the association of higher P_n, proline accumulation and antioxidative defenses could be effective in a water-limited environment and may be useful selection criteria in breeding programs with the objective of improving drought tolerance and growth of olive trees, at least under the described environmental conditions.