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The limited water availability in the Mediterranean ecosystems and the current and predicted decrease of water resources are leading to the urgent need to reduce water use for irrigation in the and and semi-arid regions. This study was conducted to evaluate the effects of partial rootzone drying (PRD) irrigation technique on plant growth, yield and water use efficiency of olive tree (Olea europaea) grown under and conditions in southern Morocco. The PRD consists in exposing one-half of the plant root system to drying soil, whereas the other half is kept in wet soil. Field experiments were conducted on adult olive trees and subjected to four irrigation treatments using a localized irrigation system: Control (irrigated with 100% of ETc on both sides of the root system), PRD I (irrigation with 50% of the control, on one side of the root system and switched every 2 weeks), PRD2 (same switched every two irrigations: 4 weeks) and PRD3 (with same amount of water than the control applied on one side of the root system, switched every 2 weeks). The PRD treatments affected significantly olive water relations, starting with an increase in stomatal resistance, and subsequently leaf water potential (psi), with a small non-significant effect on leaf relative water content. The hypothesis of a PRD-induced chemical signal was supported by the observation that stomatal closure was similar in all PRD treatments, including PRD 3, which had exactly the same level and evolution of leaf psi than the control. The PRDI and PRD2 treatments induced a slight reduction of the average shoot length, which was comparable to fruit yield reduction. Yield reduction under PRD1 and PRD2 was mainly due to a decrease in fruit numbers, whereas the average olive fruit diameter was slightly higher under PRD I and PRD2 treatments than the control. Olive yield was significantly higher for the control and PRD3 treatments, compared to PRD1 and PRD2 treatments. The oil percentage in olive fruit and oil acidity did not show any significant differences between PRD treatments and the control. The slight PRD-induced yield reduction (15-20%) compared to the control was achieved with 50% reduction in the total amount of water applied, which resulted in a water use efficiency increase by 60-70% under PRDI and PRD2 treatments, compared to the control and PRD3. (c) 2004 Elsevier B.V. All rights reserved.