47 Determining Irrigation Levels for a Modern Israeli Olive Orchard: Towards Maximum Yields of High Quality Oil A. Ben-Gal, U. Yermiyahu, I. Zipori, E. Hanoch E. Presnov, N. Agam and A. Dag The Extension Service Gilat Research Center Ministry of Agriculture and Rural Development Agricultural Research Organization Israel Israel Z. Kerem and L. Basheer Faculty of Agriculture The Hebrew University of Jerusalem Israel Keywords: Olea europea, ‘Barnea’, ‘Souri’, water application, oil quality, regulated deficit irrigation Abstract Irrigators of modernized olive orchards are challenged to determine regional and cultivar specific water regimes that increase yield while not jeopardizing olive oil quality. We have investigated olive irrigation regimes, including severe deficit strategies, on two Israeli cultivars (‘Souri’ and ‘Barnea’) in a two year field exper- iment. Trees received 25, 50, 75, 100 or 125% of potential evapotranspiration through- out the dry seasons. Increasing irrigation increased stem water potential, vegetative growth, and olive fruit yield with the increases tapering off at application rates above the 75-100% treatments. Increases in oil yield as a function of increased irrigation were measured only after the second year of treatments, and only for the cultivar ‘Barnea’ that had been in an ‘off’ year during the first season in which the treatments lead to higher vegetative growth. Increasing irrigation lead to fruits with greater water content (lower oil percentage) as well as to relative decreases in total polyphenol content and to increased free fatty acids in the ‘off’ years. These preliminary results suggest a number of methods that could be beneficial to irrigation management including cultivar specific irrigation regimes and irrigation systems. Stress levels and water requirements are highly dependent on fruit load and best irrigation management should account for biennial bearing effects. Of specific interest is that oil quality of trees in ‘off’ years is particularly negatively affected by excess water. More work is necessary in order to determine optimum water stress scheduling (stress levels and timing) for best yield-quality combinations and to further develop methods for monitoring and maintaining water stress levels. INTRODUCTION Olive production has historical importance throughout the Mediterranean. Traditionally, olives are not irrigated, but recently water application has been recognized as constructive in order to a) increase yields and quality of olives in regions with traditional olive production, b) allow high-density olive orchards and c) expand olive production into regions where there is not enough rainfall to otherwise support the crop. The literature shows that, under conditions like those in Israel where growing season rainfall is insufficient, irrigation can allow olive fruit and oil yields to be increased by up to 400% (Spiegel, 1955; Samish and Spiegel, 1961; Lavee et al., 1990; Moriana et al., 2003; Grattan et al., 2006). Oil accumulation in fruits can be reduced when fruit load on trees is high and stress levels are extreme. Increased oil yields appear to be highly a function of increased numbers of fruit per tree. Oil content on a per fruit basis is not usually found to be effected by water application (d’Andria et al., 2004; Lavee et al., 2007; Grattan et al., 2006; Moriana et al., 2003). In fact, oil percentage is commonly found to decrease as a function of increased water applied. Additionally, increased yields Proc. IS on Olive Irrigation and Oil Quality Eds.: U. Yermiyahu et al. Acta Hort. 888, ISHS 2011