RESPONSE OF 'STAR RUBY' GRAPEFRUIT TREES TO DEFICIT IRRIGATION DURING DIFFERENT STAGES OF FRUIT GROWTH: EFFECTS ON WATER RELATIONS, YIELD AND FRUIT QUALITY

Grapefruit (Citrus paradise Mac.) is an emergent crop in south-eastern Spain. In spite of the fact that grapefruit is well adapted to semiarid regions, the irrigation needs for fresh fruit production are still very high. The scarcity of water resources forces citrus growers to optimise water use using deficit irrigation (DI) strategies. The aim of this work was to evaluate the sensitivity to DI at different fruit growth stages of 14-year-old ‘Star Ruby’ grapefruit grafted on ‘Cleopatra’ mandarin (Citrus reshni Hort.). Water relations, yield and fruit quality responses were studied. The experiment was carried out in an experimental orchard located in Torre Pacheco (Murcia, south-eastern Spain) during two years. There were four irrigation treatments: Control (100% crop evapotranspiration - ETc) and three DI treatments (50% ETc) applied only during certain fruit growth stages: DI_Ph-I (Phase I - cell division), DI_Ph-II (Phase II - cell elongation) and DI_Ph-III (Phase III - final fruit-growth period, ripening and harvest). The midday stem water potential (Ψ_md) of DI_Ph-I and DI_Ph-III were influenced by the rainfall regime in both years, whereas the Ψ_md of DI_Ph-III was decreased and remained lower throughout the study period. The main effects of both the DI_Ph-I and DI_Ph-III treatments were related with changes in fruit quality parameters: DI_Ph-I reduced the percentage of juice and DI_Ph-III affected negatively the peel colour when the water stress was moderate. However, the effects of DI_Ph-II were more drastic, decreasing yield due to smaller fruits, altering fruit composition, increasing much more the titratable acidity than the total soluble solids and reducing peel colour, therefore delaying fruit maturation. Based on these results, the periods less sensitive to DI were Ph_-I and Ph_-III, which should be considered for future studies of regulated DI under Mediterranean climates.