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The impact that different regulated-deficit irrigation (RDI) treatments exert on a 12-year-old orange orchard (Citrussinensis L. Osbeck, cv. salustiano) was studied from 2004 to 2007. The experiment consisted of a control irrigation treatment which was irrigated at 100% of the crop evapotranspiration (ETc) values for the whole season, and three deficit treatments imposed as a function of the water-stress index (WSI), which is defined as the ratio of the actual volume of water supply to the ETc rate. In our case, these WSI values were 0.75, 0.65, and 0.50, respectively. The stem-water potential at noon (?Stem) was used as a parameter to estimate the water status of the plant. Yield and fruit quality was evaluated at harvest in each treatment (taking into account the temporal variability of the results due to the climatic characteristics of each of the years of this study) and an overall analysis was made using the whole dataset. Significant differences were found in fruit quality parameters (total soluble solids and titrable acidity), which also showed significant regression coefficients with the values of the integrated stem-water potential. These results led us to conclude that in mature orange trees grown under these conditions, regulated-deficit irrigation has important and significant effects on the final fruit quality, but the effects are not so clear-cut in tree yield, where the differences in the case of reducing a 50% of the crop ETc, were not considered to be statistically significant despite an approximate 10% decrease in fruit yield. A global rescaled distance cluster analysis was performed in order to summarize the main relationships between the variables evaluated and to establish a different correlation matrix. Finally, a classification tree was derived and principal-component analysis was undertaken in order to identify and evaluate the variables which had the strongest effect on the crop response to different irrigation treatments
