The Effect of Water Deficit during Early Fruit Development on Olive Fruit Morphogenesis

ADDITIONAL INDEX WORDS. Olea europaea, cell number, cell size, endocarp, mesocarp, water potential ABSTRACT. Water defi cit was applied between 4 and 9 weeks after full bloom by withholding irrigation from 3-year-old Olea europaea L. ('Leccinoʼ) plants grown in 2 L containers in a greenhouse. At 6, 8, and 22 weeks after full bloom (AFB), fruit were sampled for fresh weight and volume determinations, and then fi xed for anatomical studies. Struc- tural observations and measurements were performed on transverse sections at the point of widest fruit diameter us- ing image analysis. Water defi cit applied between 4 and 9 weeks AFB produced a signifi cant decrease in predawn leaf water potential, which reached minimum values of -3.1 MPa. The applied water defi cit reduced fruit fresh weight and volume at 8 and 22 weeks AFB. Fruit transverse area of the water defi cit treatment was 50%, 33%, and 70% of the irrigated one at the 6-, 8-, and 22-week sampling dates, respectively. Mesocarp growth occurred for both irrigated and water defi cit plants between 8 and 22 weeks AFB. At 22 weeks AFB differences between treatments were signifi cant for mesocarp transverse area, but not for endocarp area. Mesocarp cell size, indicated by area, was signifi cantly different between treatments at 8 and 22 weeks AFB. However, the mesocarp cell number was similar for both treatments at all times, and most mesocarp cells were produced by 6 weeks AFB. The growth of endocarp area showed the greatest shift in timing in response to the early water defi cit. Ninety percent of endocarp growth had occurred by 8 weeks AFB in the irrigated treatment, but only 40% when the defi cit irrigation treatment was imposed. The olive (Olea europaea L.) tree is well adapted to arid environments, as it can withstand long periods of drought, high temperature, low humidity and high radiation regimes (Bongi and Palliotti, 1994). Olive trees have been traditionally grown under dryland conditions in the Mediterranean basin. In recent years, irrigation has been shown to increase yield, fruit size, fl esh-to-pit ratio and oil content (Goldhamer et al., 1994; Inglese et al., 1996; Lavee et al., 1990; Proietti and Antognozzi, 1996). Therefore, irrigation of olive trees is becoming very popular worldwide. Because of the scarce water supply in areas where olive trees are usually grown, there is a need to save water or increase water use effi ciency. Regulated defi cit irrigation (RDI) consists of applying an amount of water less than the evapotranspirative requirement of a crop at selected phenological stages (Mitchell et al., 1986). In deciduous fruit trees there are several reports showing that RDI strategies can effectively save irrigation water, while yield or crop quality are similar or increased with respect to fully irrigated trees (Behboudian and Mills, 1997). Regulated defi cit irrigation has only recently been explored for olive trees (Goldhamer et al., 1994). To optimize RDI management, precise knowledge about the effects of selected periods of drought on the developmental processes that determine fi nal fruit size and crop yield is critical. Although formed from a bicarpelate ovary, the olive fruit has a typical drupe structure consisting of internal stony endocarp or pit, fl eshy mesocarp, and external exocarp (King, 1938). In drupes, the