Olive trees are able to take up, transport and transpire water at soil moisture contents which are usually too low to maintain these processes active in other fruit tree species. There are several mechanisms that allow the olive tree to withstand long periods of drought, high temperature and high irradiance regimes. Stomata close slowly as water deficit increases so that photosynthetic rate can be maintained over a wide range of leaf water potential. The stomatal response to vapour pressure deficit is attenuated in highly-stressed plants. Osmotic adjustment induced by water deficit can reach 1.5 MPa. As for fruit growth and development, an early water deficit (between between 4 and 9 weeks after full bloom) decreases fruit fresh weight, volume, and transverse area both at pit hardening and harvest. Water deficit also decreases mesocarp area and, within the mesocarp, affects cell size but not cell number. In this paper the main aspects of the water relations of the olive tree are reviewed to highlight the links between physiology and irrigation management. Irrigation increases fruit size, flesh-to-pit ratio, and yield. The effect of irrigation on yield is higher at low crop evapotranspiration than at high values. The current trend is to develop regulated deficit irrigation (RDI) strategies, whereby applied water is less than the evapotraspirative needs of the t r e e . The main advantages of RDI are the saving of water, the maintenance of high yields, and effects on olive oil quality. Fatty acid composition is usually unaffected by irrigation, whereas the concentration of phenolic compounds in the oil decreases as soil water availability increases, regardless of the irrigation strategy.
Keywords: fruit, irrigation, oil, stomatal conductance, water potential