Response of mature ‘Shamouti’ orange trees to irrigation of different soil volumes at similar levels of available water

Summary

Different proportions of the soil volume of ‘Shamouti’ orange (Citrus sinensis L. Osbeck) trees were irrigated at similar levels of available water during 1979 to 1981 in the coastal plain of Israel. Two treatments were compared: one in which 100% of the soil volume was irrigated (fully irrigated plot), and the other in which 40% of the volume was irrigated (partially irrigated plot) by sprinklers and sprayers, respectively. Irrigations were given independently in the two treatments whenever the moisture content in the wetted soil dropped to a fixed predetermined level (Table 2). Moisture depletion was measured by the neutron scattering method, transpiration by the heat pulse method and soil evaporation by microlysimeters. Averaged for the three years, water depletion from the partially irrigated plot was 66% of that of the fully irrigated one (Table 1). Transpiration in 1980 from trees of the partially irrigated plot was 72% of the transpiration from trees of the fully irrigated plot (Fig. 3 and Table 6), and the evaporation from the soil surface was 58% (Fig. 4). The density of roots less than 1 mm in diameter in the dry soil region of the partially irrigated plot was 45% of the equivalent region of that in the fully irrigated plot. Underneath the tree row (the wetted region) the root density ratio was 77% (Fig. 2). Extraction of stored water from the unirrigated soil volume of the partially irrigated plot ceased in July (Fig. 1). Transpiration from the partially irrigated plot recovered during the spring period following the rainy winter season (Table 7). The total production of flowers per tree was 120,000 in the partially irrigated plot during 1981, as compared with 79,000 per tree in the fully irrigated plot (Table 3). There was a considerable difference in flower abscission between treatments especially at the beginning of the flowering season. The flower abscission rate in the partially irrigated trees was higher than in the fully irrigated trees (Fig. 5). In contrast, abscission of fruitlets was less in the partially irrigated treatment (Fig. 5).