Abstract
The effects of water deficit on photochemical parameters and activities of superoxide dismutase, catalase, ascorbate peroxidase and glutathione reductase were investigated in two olive cultivars differing in drought tolerance - 'Chemlali' and 'Chetoui'. After 30 days without irrigation, leaf water potential fell to -5.5 MPa that was accompanied by a marked decrease in net photosynthesis in 'Chetoui' olive cultivar. Maximal efficiency of PSII photochemistry (F-v/F-m) decreased slightly in 'Chemlali' (28 %) and substantially in 'Ch,toui' (47 %). Both cultivars showed a similar decline (about 25 %) in the photochemical quenching coefficient, but only the drought-sensitive olive cultivar exhibited an enhancement (31 %) of non-photochemical fluorescence quenching under water deficit conditions. The quantum yield of electron transport decreased in both olive cultivars. 'Chemlali' showed a higher protection against oxidative stress, as judged from the lower levels of the malondialdehyde production. Catalase activity was higher in 'Chetoui'. Glutathione reductase activity was increased similarly in both olive cultivars under water stress. Ascorbate peroxidase activity was enhanced in 'Chemlali' under water stress, but was unaffected in 'Chetoui'. While, superoxide dismutase activity was inhibited in both cultivars under water stress, but higher activity was detected in 'Chemlali'. Thus, the ability to increase ascorbate peroxidase and a higher superoxide dismutase activity might be an important attribute linked to the drought tolerance in 'Chemlali' olive cultivar.