EFFECT OF REGULATED DEFICIT IRRIGATION ON OLIVE OIL YIELD AND QUALITY AND ON LEAF RESPONSES OF OLIVE CULTIVAR 'PICUAL' (OLEA EUROPAEA L.)

Irrigation of olive-trees (Olea europaea L.) increases yields and oil production, although high yields can be achieved with a limited water supply. Under stress conditions, an accumulation of soluble sugars contributes to osmotic adjustment. The effect of irrigation treatments on changes in carbohydrates concentration, yield and quality of fruits and oil in a commercial ‘Picual’ olive orchard was evaluated. The experimental orchard was 10-year-old, planted at 10×5 m in sandy loam soil and drip irrigated. Irrigation treatments were: T1=100%, T2=77%, T3=55% and T4=30% of crop evapotranspiration (ETc) during fruit growth phases II and III through veraison with normal budget irrigation during growth stages I and IV. During the water-deficit period the available water remaining percentage (%AWr) and midday stem water potential (SWP) were monitored. Fruit growth was measured weekly. At harvest yield, weight, pulp:pit ratio, moisture and fat content of the fruits were measured. In oils, acidity, peroxide index, oxidative stability and total polyphenols were evaluated. At the end of deficit period, the concentration of mannitol, glucose, fructose and saccharose in mature leaves were determined. During the deficit period, average SWP (-1.24, -1.53, -1.91 and -2.29 MPa) was significantly different between treatment levels (T1, T2, T3 and T4 respectively), which reflected decreasing average %AWr values (89, 74, 54 and 40%). In T3, fruit weight and pulp:pit ratio was 10% less than in T1, and in T4, 33 and 24% respectively. Irrigation treatments did not affect industrial oil yield. Oil in T2, maintained a higher oxidative stability than in T3 and T4 (52 and 43% respectively). SWP was highly correlated with glucose concentration in leaves (r2=0.79). In the leaves of T4 the glucose concentration increased 24% in response to water stress. Using regulated deficit in summer could be an alternative way to increase oil quality while maintaining yields. Furthermore, it is an efficient irrigation strategy for arid regions.