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An irrigation experiment involving pistachio (Pistacia vera cv Kerman on Pistacia terebinthus L. rootstocks) was performed over a four-year period in central Spain to determine the effect of regulated deficit irrigation (RDI) on nut yield and quality. The growth season was divided into three phenological stages: stage I � from sprouting until the end of rapid nut growth; stage II � from maximum nut size until the beginning of kernel growth; and stage III � from the beginning of kernel growth until harvest. Control trees were irrigated to supply their full water needs throughout the growth season, except for the post-harvest period. Sustained deficit irrigation at 65% (DI65) and 50% (DI50) of control irrigation was provided to two other groups of trees. The RDI provided to a further group was designed to provide a stress period during stages I and II but no water stress during stage III; the aim was to reduce water use and increase the percentage of split nuts. A fifth group of trees was maintained under rain fed conditions. Water potential and leaf conductance were affected in the DI65, DI50 and rain fed treatments mainly during stages II and III, with midday water potentials below -2.0 MPa. The RDI trees were only significantly water stressed during stage II, showing midday water potentials of around -1.4 MPa. On most days, leaf conductance was not significantly affected in any of the irrigation treatments. The nuts of the DI65 and DI50 trees were smaller in diameter and their total yield was reduced compared to the controls. However, no significant differences in kernel dry weight were observed. The RDI trees showed a total yield and percentage of split nuts similar to those of the controls, even though they received around 20% less water. The split nut yield showed a linear relationship with crop evapotranspiration. However, since the percentage of split nuts was similar in all treatments this variation was likely related to the total yield. The RDI trees did not show the normal alternate bearing pattern (which was clearly maintained in the control trees). Early splitting, a process that decreases the quality of the yield, was not related to water status but to temperatures lower than 13 °C. The results suggest that P. terebinthus L. rootstocks confer P.vera scions a degree of drought-resistance, reducing the likelihood of water stress and, therefore, allowing more severe RDI scheduling
