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Water scarcity in the Mediterranean basin in addition to the extension of irrigated lands is one of the main factors limiting agricultural development. The need for supplementary irrigation of the Chemlali olive cultivar (Olea europaea L.) during summer and autumn periods was investigated. Leaf water content, gas exchange parameters, fruit development and yield in rain-fed and in irrigated plants have been monitored in 12-year-old olive trees grown under environmental conditions in semi and regions characterized by high temperatures and high light intensity. Trees were subjected to three irrigation treatments, T0, T1 and T2 corresponding respectively to 0, 33 and 66% of crop evapotranspiration (ETc) by a drip irrigation system. The water deficit during the summer (from June to August) led to the decrease of soil moisture, leaf water content and gas exchange parameters. Irrigated trees showed the same slow activity in the three summer months as the rain-fed trees. For all treatments, net CO, assimilation. stomata] conductance and transpiration rates were markedly decreased by environmental conditions (high air temperature and high light intensity) during the summer period. At the partial active growth phase of the Chemlali olive cultivar (September-November), a significant reincrease in all parameters was observed. However, net photosynthesis and stomatal conductance of control plants (TO) were, respectively, 57 and 40% lower than those of plants conducted under milder water contribution (TI). The decrease of physiological activity in irrigated plants during hot and dry (summer time) and cold (winter) seasons was a clear evidence that water supply during such periods will be without a great benefit for photosynthetic activity, and thus growth, if applied under critical conditions inducing the rest phase of the plant. The non-statistically significant slight differences as well in photosynthetic performances activities (Pn, Gs and E rates), as in olive production between the two irrigated treatments will not cover the expenses of water loss when applying irrigation at 66% of ETc especially in and region characterized by scant and irregular rainfall. On the light of these results, we can conclude that the irrigation of this species during the vegetative growth phase (in spring and autumn), and stopping it during the olive rest phase (in summer and winter) could be recommended at least under the experimental conditions of this study; and that the contribution of 600 nim of water per year (33% of ET,) can respond to the needs of the Chemlali olive cultivar in a semi and region without impairing photosynthetic activity and olive production. (C) 2007 Elsevier B.V. All rights reserved.