The effect of deficit irrigation on seasonal variations of plant water use in Olea europaea L. Roberto Tognetti 1,3 , Riccardo d’Andria 2 , Giovanni Morelli 2 & Arturo Alvino 1 1 Dipartimento di Scienze Animali, Vegetali e dell’Ambiente (SAVA), Universita ` degli Studi del Molise, via De Sanctis, I-86100 Campobasso, Italy. 2 Istituto per i Sistemi Agricoli e Forestali del Mediterraneo (ISAFOM), Consiglio Nazionale delle Ricerche, via Patacca 85, I-80056 Ercolano, Italy. 3 Corresponding author* Received 29 September 2004. Accepted in revised form 6 December 2004 Key words: conductance, heat-pulse, olive, transpiration, water relations Abstract A field experiment on olive trees (Olea europaea L.) was designed with the objective to search for an optimum irrigation scheduling by analyzing the possible effects of deficit irrigation. Treatments were: a non-irrigated control (rainfed) and three treatments that received seasonal water amount equivalent to 33 and 66% of crop evapotranspiration (ET C ) in the period August–September (respectively 33II and 66II), and 66% of ET C from late May to early October (66I-II). Atmospheric evaporative demand and soil moisture conditions were regularly monitored. Irrigation effects on plant water relations were char- acterized throughout a growing season. Whole-plant water use, in deficit irrigated (66I-II) and rainfed olive trees, was determined using a xylem sap flow method (compensation heat-pulse technique). The magnitude of variations in water use and the seasonal dynamic of water relations varied among treat- ments, suggesting that olive trees were strongly responsive to both irrigation amount and time. Physio- logical parameters responded to variations in tree water status, soil moisture conditions and atmospheric evaporative demand. All measurements of tree water status were highly correlated with one another. There was a considerable degree of agreement between daily transpiration deduced from heat-pulse velocity and that determined by calibration using the water balance technique. Deficit irrigation during the whole summer (66I-II) resulted in improved plant water relations with respect to other watering regimes; while, severe regulated deficit irrigation differentiated only slightly 33II treatment from rainfed plants. Nevertheless, regulated deficit irrigation of olive trees after pit hardening (66II) could be recom- mended, at least in soil, cultivar and environmental conditions of this study. Introduction In Mediterranean-type agro-ecosystems olive tree (Olea europaea L.) has traditionally been culti- vated in marginal areas with no irrigation, despite rainfall does not meet evapotranspirative demand (Ferna´ ndez and Moreno, 1999). To improve yield and maintain low water consump- tion, modern olive plantations with relatively high number of trees per hectare require drip irri- gation. Growth conditions that maximize crop growth in general optimize production, though a positive yield response to mild water deficits has been demonstrated for several fruit trees in Medi- terranean environments (Girona et al., 1993; Marsal et al., 2002). Indeed, a sagacious irriga- tion approach is a major task in modern olive growing due to the limited water resources avail- able in Mediterranean countries (Villalobos et al., 2000). Olive is a well-known drought avoidance species, though its degree of control over water * FAX No: +39-0874-404855. E-mail: tognetti@unimol.it Plant and Soil (2005) 273: 139–155 Ó Springer 2005 DOI 10.1007/s11104-004-7244-z