Abstract Irrigation effects were investigated on an 8-year-old olive (Olea europaea L., cv. Cobranc ¸osa) commercial orchard located in northeast Portugal. Trees were subjected to a rainfed control (T0) and three treatments (T1, T2, T3) that received a seasonal water amount equivalent to 30%, 60% and 100% of the esti- mated local evaporative demand by a drip irri- gation system. Irrigation increases the photosynthetic activity of olive trees, in associa- tion with increases in water status, and reduces the midday and afternoon depression in gas ex- change. The closely association between photo- synthetic rate (A) and stomatal conductance (g s ) revealed that the decline in net photosynthesis over the course of the day was largely a conse- quence of stomatal limitation. However, the ratio of intercellular to atmospheric CO 2 concentration increased markedly from morning to midday in non-irrigated plants, in spite of lower g s , sug- gesting that non-stomatal limitations of photo- synthesis also occur when environmental conditions become more stressful. The occur- rence of perturbations at chloroplastic level in rainfed plants was demonstrated by a lower maximum photochemical efficiency of photosys- tem II during the afternoon. Chlorophyll fluo- rescence measurements also revealed the occurrence of a dynamic photoinhibition in irri- gated trees, mainly in T2 and T3, which seemed to be effective in protecting the photosynthetic apparatus from photodamage. Irrigation en- hances antioxidant protection and decreases the oxidative damage at leaf level. Leaves grown under rainfed conditions revealed symptoms of oxidative stress, like the reduction (14%) in chlorophyll concentration and the increased lev- els (57%) of lipid peroxidation. We also found that the scavenging function of superoxide dismutase was impaired in rainfed plants. In contrast, the low thiobarbituric acid reactive substances concentration in T3 indicates that irrigation enhances the repairing mechanisms and decreases the oxidative damage by lipid peroxi- dation. Accordingly, leaves in T3 treatment had high levels of –SH compounds and the highest E. A. Bacelar D. L. Santos J. M. Moutinho-Pereira B. C. Gonc ¸alves C. M. Correia (&) Centre for Technological, Environmental and Life Studies (CETAV), Department of Biological and Environmental Engineering, University of Tra ´ s-os- Montes e Alto Douro, Apartado 1013, 5001-801 Vila Real, Portugal e-mail: ccorreia@utad.pt J. I. Lopes Regional Direction of Agriculture of Tra ´ s-os-Montes, Quinta do Valongo, 5370-347 Mirandela, Portugal T. C. Ferreira Division of Climatology, University of Tra ´ s-os- Montes e Alto Douro, Apartado 1013, 5001-801 Vila Real, Portugal Plant Soil (2007) 292:1–12 DOI 10.1007/s11104-006-9088-1 123 ORIGINAL PAPER Physiological behaviour, oxidative damage and antioxidative protection of olive trees grown under different irrigation regimes Eunice A. Bacelar Dario L. Santos Jose ´ M. Moutinho-Pereira Joa ˜o I. Lopes Berta C. Gonc ¸alves Timo ´ teo C. Ferreira Carlos M. Correia Received: 18 May 2006 / Accepted: 17 July 2006 / Published online: 22 February 2007 Ó Springer Science+Business Media B.V. 2007