Difese antiossidanti in olivo in condizioni di deficit idrico

The olive tree (Olea europaea L.) is commonly grown in the Mediterranean basin and is able to resist severe and prolonged drought. The effects of drought stress and water recovery on the activities of superoxide dismutase (SOD; EC 1.15.1.1), catalase (CAT; EC 1.11.1.6), ascorbate peroxidase (APX; EC 1.11.1.11), guaiacol peroxidase (POD; EC 1.11.1.7), polyphenol oxidase (PPO; EC 1.30.3.1) and lipoxygenase (LOX; EC 1.13.11.12), and on malondialdehyde (MDA) levels were investigated in two-year-old olive (cv. “Coratina”) plants grown in environmental conditions characterized by high temperatures and irradiance levels and gradually subjected to a controlled water deficit. After 20 days without irrigation, mean predawn leaf water potential fell from -0.37 to -5.37 MPa and this was accompanied by a decrease in net photosynthesis and transpiration. The activities of SOD, APX, CAT and POD increased according to the severity of drought stress in both leaves and roots. In particular, a significant five-fold increase in APX activity was found in leaves of plants at the maximum level of stress. POD and IAA oxidase activities showed a similar pattern and CAT activity increased during severe water deficit conditions in leaves and thin roots. In contrast, PPO activity decreased during the progression of stress in all the tissues studied. After reaching the maximum level of water stress, plants were subjected to a rewatering treatment for 30 days, under both environmental irradiance and semi-shade conditions. The activities of SOD, CAT, APX, POD and LOX, and MDA levels decreased during the rewatering period in both leaves and roots and these decrements were faster in plants rewatered in semi-shade conditions (SHP) than in plants under environmental light (NSHP). In contrast, PPO activity increased during rewatering in both leaf and root tissues. The results show that the ability of olive trees to increase antioxidant system activity, thus limiting cellular damage caused by active oxygen species, might be an important attribute linked to drought tolerance. Moreover, the lower expression of the enzymatic antioxidant system in SHP with respect to NSHP may be due to a reduced need of activated oxygen species removal. On the contrary, in NSHP, higher enzyme activities are required for a better protection against a more pronounced oxidative stress.