Yield Responses of a Mature Olive Orchard to Water Deficits

Irrigation is one of the most important means of increasing olive oil production but little information exists on the responses of olive to variable water supply. Five different irrigation strategies, full irrigation, rain fed, and three defi cit irrigation treatments were compared from 1996 to 1999, in Cordoba, southern Spain, to characterize the response of a mature olive (Olea europaea L. 'Picualʼ) orchard to irrigation. Crop evapotranspiration (ETc) varied from less than 500 mm in the rain fed to ≈900 mm under full irrigation. The defi cit irrigation treatments had ETc values that ranged from 60% to 80% of full ETc depending on the year and treatment. Water relations, and oil content and trunk growth measure- ments allowed for the interpretation of yield responses to water defi cits. In a defi cit irrigation treatment that concentrated all its ETc defi cit in the summer, stem water potential (Ψx) decreased to -7 MPa but recovered quickly in the fall, while in the treatment that applied the same ET defi cit progressively, Ψx was never below -3.8 MPa. Minimum Ψx in the rain fed treatment reached -8 MPa. Yield (Y) responses as a function of ETc were calculated for biennial yield data, given the alternate bearing habit of the olive; the equation are: Y = -16.84 + 0.063 ET -0.035 × 10-3 ET2, and Y = -2.78 + 0.011 ET - 0.006 × 10-3 ET2, for fruit and oil production respectively, with responses to ET defi cits being similar for sustained and regulated defi cit irrigation. The yield response to a defi cit treatment that was fully irrigated during the bearing year and rain fed in the nonbearing year, was less favorable than that observed in the other two defi cit treatments. The olive (Olea europaea L.) is, among fruit trees, an important source of both fruit and edible oil. Increased appreciation of olive oil as part of the Mediterranean diet (International Olive Oil Council, 2001) has raised olive oil prices, leading to an important increase in new plantings in many world areas. Additionally, there have been trends to intensify olive production, primarily via high-density plantings and irrigation. Because olive has been traditionally a rain-fed crop in the Mediterranean region, little information exists on its irrigation requirements, and even less on the responses to variations in irrigation water supply (Orgaz and Fereres, 1997). New irrigation developments are under close scrutiny at present and the economic advantages of shifting olive plantations from traditional rain fed to irrigation must be clearly demonstrated before new water is allocated for irrigation. Many olive growing regions suffer chronic water shortages and increased competition from other sectors for the available water resources (Fereres and Cena, 1997). A basic tool to assess the value of irrigation water is the crop water production function (Vaux and Pruitt, 1983). Crop biomass and yield are linearly related to evapotranspiration (ET) but the relations with applied irrigation water are usually curvi- linear (Stewart and Hagan, 1973). Sometimes, nonlinear relations between yield and ET have been observed (i.e., cotton; Orgaz et al., 1992). It is important to characterize the production function of olive, a crop that has a reputation of being drought resistant. If the response to variation in ET is nonlinear, optimal ET levels can be defi ned for different oil and water prices and defi cit irrigation programs to achieve such ET levels could be formulated. Reports on the responses of olive yield to variable amounts of applied water differ according to different sources. Lavee et al.