Irrigation effects were investigated on an 8-year-old olive (Olea europaea L., cv. Cobrançosa) 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 estimated local evaporative demand by a drip irrigation system. Irrigation increases the photosynthetic activity of olive trees, in association with increases in water status, and reduces the midday and afternoon depression in gas exchange. The closely association between photosynthetic rate (A) and stomatal conductance (g s ) revealed that the decline in net photosynthesis over the course of the day was largely a consequence 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 , suggesting that non-stomatal limitations of photosynthesis also occur when environmental conditions become more stressful. The occurrence of perturbations at chloroplastic level in rainfed plants was demonstrated by a lower maximum photochemical efficiency of photosystem II during the afternoon. Chlorophyll fluorescence measurements also revealed the occurrence of a dynamic photoinhibition in irrigated trees, mainly in T2 and T3, which seemed to be effective in protecting the photosynthetic apparatus from photodamage. Irrigation enhances 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 levels (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 peroxidation. Accordingly, leaves in T3 treatment had high levels of -SH compounds and the highest

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RESUMEN Efecto de diferentes regímenes de riego sobre los índices de calidad del aceite de oliva de cv. "Co brançosa"El objetivo del estudio fue evaluar el efecto de distintas estrategias de riego en la composición y la calidad del aceite de oliva virgen de cv. Cobrançosa, integrado en una denominación de origen protegida de «Azeite de Trás-osMontes», en el noreste de Portugal. Tres tratamientos de riego fueron aplicados: T2-riego total, el olivo recibió el equivalente estacional de agua al 100% de la evapotranspiración estimada del cultivo (ETc), T1-riego continuo deficitario (30% de la ETc) y el tratamiento T0-de secano. Los datos fueron recolectados a partir de dos campañas agrícolas consecutivas (2005)(2006). Los regímenes de riego tuvieron un efecto menor en los índices de calidad estándar (ácidos grasos libres, índice de peróxidos, K 232 y K 270 ) del aceite de oliva y en la composición de ácidos grasos. Los polifenoles totales disminuyeron hasta el tratamiento T2, y se relacionaron fuertemente con el integral de estrés hídrico, lo que sugiere que el efecto del riego sobre esta variable se produce a lo largo de la temporada de cultivo y no sólo durante la fase de acumulación de aceite. Una estrategia de riego deficitario continuo con sólo 30% de la ETc máxima puede tener un efecto beneficioso, ya que aumentó la producción de aceite de oliva a más del doble que en condiciones de secano, mientras que la calidad del aceite de oliva fue similar. PALABRAS CLAVE: Aceite de oliva virgen -cv. Cobrançosa -Índices de calidad -Riego. ABSTRACT Effect of different irrigation regimes on the quality attributes of monovarietal virgin olive oil of cv. "Cobrançosa"The aim of this study was to assess the effect of different irrigation strategies in virgin olive oil (VOO) composition and quality of cv. Cobrançosa, integrated in a protected denomination of origin of "Azeite de Trás-os-Montes" in the Northeast of Portugal. Three irrigation treatments were applied: T2-full irrigation that received a seasonal water equivalent to 100% of estimated crop evapotranspiration (ET c ), T1-continuous deficit irrigation (30% ET c ) and T0-rainfed treatment. Data were collected from two consecutive crop years (2005)(2006). Irrigation regimes had a minor effect on standard quality indices (free fatty acids, peroxide value, K 232 and K 270 ) of VOO and in fatty acid composition. Total polyphenols decreased up to treatment T2, and were strongly related to the water stress integral, suggesting that the effect of irrigation on this variable occurs along the crop season and not just during the oil accumulation phase. A strategy of continuous deficit irrigation with only 30% of maximum ETc may have an advantageous effect, as it increased oil yield to more than double that of rainfed conditions while VOO quality was similar.

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