Productivity of olive trees with different water status and crop load By R. GUCCI 1 * , E. LODOLINI 1 and H. F. RAPOPORT 2 1 Dipartimento di Coltivazione e Difesa delle Specie Legnose, Università di Pisa, Via del Borghetto 80, 56124 Pisa, Italy 2 Instituto de Agricultura Sostenible, C.S.I.C., P.O. Box 4084, 14080 Cordoba, Spain (e-mail: rgucci@agr.unipi.it) (Accepted 6 March 2007) SUMMARY A field experiment was conducted over two growing seasons to determine the combined effect of crop load and irrigation on yield components of olive trees (Olea europaea L. ‘Leccino’) planted at 6 m  3.8 m in a sandy-clay soil. Different crop loads were established by manual thinning of fruits. Drip irrigation was managed to maintain pre-dawn leaf water-potentials (PLWP) within the following ranges: (i) higher than –1.1 MPa (FI; fully irrigated); (ii) between –1.0 and –3.3 MPa (DI; deficit irrigated); or (iii) below –1.2 MPa, but not lower than –4.2 MPa (SI; severe deficit irrigated). The irrigation period lasted from 6 – 16 weeks after full bloom (AFB) in 2003, and from 5 – 19 weeks AFB in 2004. In 2003, full bloom was on 26 May; in 2004, it was on 3 June. Neither irrigation regime nor crop load affected flowering or flower quality the following Spring. The combined fruit yields [on a fresh weight (FW) basis] over both years in SI and DI trees were 49.0% and 81.6% of FI trees, respectively.The oil yields of SI and DI trees were 52.5% and 81.2% of FI trees, respectively. Fruit FWs in FI trees were greater than those of DI or SI trees at 8 weeks AFB.At harvest, FI trees bore the largest fruits, and SI trees the smallest fruits.The FWs of individual fruits at harvest in the FI and DI treatments decreased as crop load increased, but no such relationship was apparent for SI trees. The oil content of the mesocarp increased as PLWP increased from approx. –3.5 MPa to –1.5 MPa.The oil content of FI trees at harvest decreased from 53.1% to 45.7% dry weight as fresh fruit yield increased from 5 – 25 kg dm –2 trunk cross- sectional area. However, crop load did not have any effect on the oil content of the mesocarp in DI trees. Fruit maturation was delayed by irrigation. Maturation index also decreased (indicating delayed maturation) as the crop load on FI or DI trees increased, but did not vary with crop level in SI trees. T here are a number of reasons why it is important to elucidate the response of olive trees (Olea europaea L.) to soil water availability and crop level. First, most olive trees are still cultivated under rainfed conditions and, consequently, yields and economic returns are strongly affected by the year-to-year variability in rainfall. Second, the use of irrigation is expanding rapidly, both in traditional olive orchards and in modern plantings, yet this practice is still relatively novel. Irrigation has been shown to increase yield, fruit size, the flesh-to-pit ratio, mesocarp cell size, and oil content (Costagli et al., 2003; Goldhamer et al., 1994; Inglese et al., 1996; Lavee et al., 1990; Proietti and Antognozzi, 1996). Moriana et al. (2003) determined the yield response of a mature olive orchard (‘Picual’) to crop evapotranspiration in Andalusia and reported that water productivity varied from 22 kg fruit ha –1 mm –1 water at low evapotranspiration (450 – 550 mm), to approx. 9 kg ha –1 mm –1 at high values (750 – 850 mm). A third reason lies in the natural alternate bearing habit of olive trees, with crop loads varying, sometimes dramatically, from year-to-year. Heavy crops reduce fruit size, growth rate, soluble solids and leaf starch contents, delay fruit maturation, and alter the relationship between midday and pre-dawn leaf water potential in deciduous fruit trees (Marsal and Girona, 1997; Naor et al., 1997; 1999; Wunsche et al., 2000). As for the olive tree, there is little quantitative information on the effect of crop load on yield variables such as fruit growth and oil accumulation. Lavee and Wodner (2004) suggested that olive oil production is primarily a function of the amount of mesocarp available, and that the mesocarp:endocarp ratio is closely related to fruit size, which is partially a function of crop load. The varying sensitivity of the growth processes of different organs or tissues to water deficit is the basis for regulated deficit irrigation (RDI), a management practice aimed at reducing water volumes at specific phenological stages, without affecting yield (Behboudian and Mills, 1997). Deficit irrigation strategies have long been used in deciduous fruit crops to control vegetative growth, alter dry matter partitioning, save water and, in many instances, improve fruit quality (Li et al., 1989; Mitchell et al., 1986), but the responses of evergreen tree crops are less known. Recent evidence suggests that it is possible to reduce the amount of water applied during the growing season without negative effects on olive fruit (Goldhamer, 1999) or oil yields (Alegre et al., 2002; Motilva et al., 2000). However, it still remains unclear which deficit irrigation strategies are best to optimise water use for this crop. For instance, Moriana et al. (2003) found no differences in the yield responses of olive trees grown under RDI or sustained deficit irrigation over a 3-year period, although both strategies proved better than full irrigation. *Author for correspondence. Journal of Horticultural Science & Biotechnology (2007) 82 (4) 648–656