INFLUENCE OF IRRIGATION SCHEDULING ON FRUIT QUALITY OF YOUNG POTTED 'MANZANILLA DE SEVILLA' OLIVE TREES

Abstract

The aim of this work was to analyse the influence of soil water content and distribution on fruit quality in 5-year-old ‘Manzanilla de Sevilla’ olive trees in pots subjected to different irrigation treatments for 3 years (2007-2009). The experiment was carried out near Seville, in southwest Spain. Four water treatments were considered: 1) T0, trees were under dry farming conditions except for small amounts of water supplied to ensure their survival; 2) T1, irrigation dose was about 50% of the crop’s water needs (ETc); 3) T2, irrigated at 100% of ETc. Some of the roots of the T1 and T2 trees were left in drying soil during the irrigation season; 4) T3, irrigated to non-limiting soil water conditions in the whole rhizosphere throughout the irrigation season. All treatments were fertilized under non-limiting conditions. Results from the 2009 season showed no differences between treatments in either fruit or endocarp shapes. In all cases, the longitudinal diameters of both fruits and endocarps increased with the amount of water applied, as did equatorial diameters, except without significant differences between irrigation treatments. Fruit weight, volume and the mesocarp/endocarp ratio also increased with the amount of water applied. Those increments were mainly related to those of fresh and dry mesocarp weights. The endocarp weight, both fresh and dry, was lower in T0 than in the irrigation treatments, with no differences between irrigation treatments. Oil content on a fresh weight basis decreased significantly with the amount of irrigation and no differences between T2 and T3 were found. These results show both a positive response of fruit quality to regulated deficit irrigation and the fact that wetting the whole rhizosphere to around field capacity influences little, if any, the fruit quality. INTRODUCTION The olive tree (Olea europaea L.) is well adapted to dry conditions, which are usual in areas in which it is cropped. Water supplied during the dry season, however, improves olive yield, fruit weight, volume, mesocarp/endocarp ratio and oil yield, among others parameters related to table olive and oil quality (Lavee and Wodner, 1991; Patumi et al., 2002; Moriana et al., 2003). Not only amount, but also distribution of the water in the soil is important. There is evidence to suggest that olive tree transpiration is markedly increased when water supplied by irrigation wets the whole rhizosphere (Fernandez et al., 2003). There is a lack of information, however, on the influence of water distribution in the soil on fruit quality and oil content. The aim of this work was to analyse the extent to which fruit quality in 5-year-old ‘Manzanilla de Sevilla’ olive trees is modified by soil water content and distribution. MATERIALS AND METHODS The experiment was carried out at ‘La Hampa’ experimental farm, close to Coria del Rio, Seville, in southwest Spain (37o 17’ N, 6o 3’ W), in 2009. In 2007, when the trees were 3 years old, a completely randomized design with 3 trees per plot and 4 plots per treatment was established. Each tree was planted in the middle of two 50-L pots, with Proc. IS on Olive Irrigation and Oil Quality Eds.: U. Yermiyahu et al. Acta Hort. 888, ISHS 2011 178 about half of the root system in each pot. The growing media was sandy loam soil (14.8% clay, 7.0% silt, 4.7% fine sand and 73.5% coarse sand). Drainage was favoured by a 0.05 m gravel layer at the bottom of the pots. Four water treatments were considered: 1) dry farming conditions, except for small amounts of water supplied to ensure the survival of the trees (treatment T0); 2) regulated deficit irrigation in which the irrigation dose varied between 100% and 30% of the crop’s water needs (ETc), depending on phenological stage (treatment T1); 3) daily irrigation with 100% of ETc (treatment T2); 4) pond irrigation, in which the whole rhizosphere was wetted to around field capacity throughout the irrigation season (treatment T4). Each irrigation season, the T1 trees received a total of ca 50% of ETc. In the T1 and T2 trees, some of the roots were left in drying soil during the irrigation season, to emulate the local irrigation systems normally used in olive orchards. The T0 and T3 trees had three 2 L/h drippers per pot. The T2 and T1 trees had three drippers in one pot, and just one dripper in the other, to ensure that part of the root system would be left in drying soil during the irrigation season. The T1, T2 and T3 trees were irrigated daily from May to September. Irrigation doses were calculated by the crop coefficient approach, as described by Fernandez et al. (2006). Basically, ETc was calculated as ETc = Kc Kr ETo, with crop coefficient (Kc) values of 0.76 in May, 0.70 in June, 0.63 in July and August, 0.72 in September and 0.77 in October. The coefficient related to the percentage of ground covered by the crop (Kr) was 0.71. In the 2009 season, when trees were 5 years old, the irrigation amounts were 176.8 L/tree in T0, 341.6 L/tree in T1, 650 L/tree in T2 and 1189.8 L/tree in T3. In 2009, volumetric soil water contents (θv) were measured every 7 to 10 days with a time domain reflectrometry (TDR) system (FOM, Institute of Agrophysics, Lublin, Poland), consisting of two 0.15 m long TDR probes inserted in each of the two pots of one tree peer plot, at the depths of at 0.05 and 0.20 m. The canopy volume of each tree was calculated in May (at the beginning of the irrigation period), from the measurements of the two perpendicular diameters at the height of maximum canopy width, plus the canopy height. A fruit sample of 200 g per plot was picked on 3 September, at maturity index 1 (Beltran et al., 2004), as is common with olives picked for ‘Spanish-style’ green processing. The average fruit weight, volume, mesocarp/endocarp ratio (calculated as the difference between fruit and mesocarp weight) and average fresh and dry weights both of the mesocarp and endocarp were determined. Fruit and endocarp shapes were calculated from measurements of the major longitudinal and equatorial diameters in 50 fruits per plot. The harvesting was performed by hand on 14 October, when the maturity index was about 3.5. The number of fruits per tree was determined, and the oil content in fruits of each treatment was extracted and analysed by the standard Soxhlet method (UNE 55030). Analyses of variance were performed on the data to evaluate differences among treatments. Separation of the means was obtained by least significant difference (LSD) test at the 0.05% probability level. RESULTS AND DISCUSSION Canopy volume at the beginning of the irrigation period was similar for T0 (the dryfarming treatment) and T1 (the regulated deficit-irrigation treatment), but increased significantly for T2 (daily irrigation with 100% ETc) and T3 (the pond irrigation treatment) (Table 1). No differences between treatments were found in the number of fruits per tree. This result may have been influenced by a rainfall event of 14 mm that occurred at full bloom, which might have affected flowering and fruit set. Other parameters related to fruit quality were, however, modified by the water treatments. Thus, fruit weight, volume and the mesocarp/endocarp ratio increased significantly in T2 as compared to T0 (Fig. 1). These increases were related specifically to those of the mesocarp and endocarp tissues. Table 1 shows, in fact, that both the fresh and