Stomatal control of water use in olive tree leaves

Little is known about the strategies used by olive trees to overcome the long dry periods typical of the areas where they are cropped. This makes it difficult to optimize the water supply in orchards. To study the control of water consumption by olive trees, measurements of leaf water potential (Psi) and stomatal conductance to H2O (g) were made on 26-year-old Manzanilla olive trees under three irrigation treatments. The first treatment provided enough water to cover the crop water demand, the next treatment supplied one third of that rate, and the final treatment was no irrigation at all, typical of dry-farming conditions. Under conditions of high vapour pressure deficit of the air (D-a), the olive trees prevented excessive water loss by closing their stomata. Leaves of the current year showed better stomatal control than did the 1-year-old leaves. The upper-bound functional relationships between g and D-a and photon flux density (I-P) were obtained by boundary-line analysis, based on a technique of non-linear least squares. Maximum values of g were observed at relatively low levels of I-P, from about 500 mu mol m(-2) s(-1), and a proportional decrease in g with increasing D-a was also found, at least for values of up to approximately 3.5 kPa. Higher values of g were observed in the morning than in the afternoon, for similar levels of I-P and D-a. Unirrigated olive trees recovered quickly after the dry season, showing values of Psi and g similar to those of irrigated trees after just two days.