Effects of deficit irrigation in different fruit growth stages on ‘Star Ruby’ grapefruit trees in semi-arid conditions

Grapefruit (Citrus paradisi Macf.) has increased in importance as a crop species in the south-east of Spain in recent years. In spite of the fact that grapefruit is well adapted to semi-arid conditions, the irrigation necessities for fresh fruit production continue to be very high. The scarcity of water resources forces citrus growers to optimise their water use by using deficit irrigation (DI) strategies. The aim of this work was to evaluate the sensitivity to DI applied during different fruit growth stages of 14-year-old ‘Star Ruby’ grapefruit grafted on ‘Cleopatra’ mandarin (Citrus reshni Hort.), regarding water relations, trunk growth, yield and fruit quality. The experiment was carried out over two years in an experimental orchard located in Torre Pacheco (Murcia, south-eastern Spain). There were four irrigation treatments; Control (100% crop evapotranspiration – ETc) and three DI treatments (50% ETc) applied only during different fruit growth stages; DIPh-I (Phase I – cell division), DIPh-II (Phase II – cell elongation) and DIPh-III (Phase III – final fruit-growth period, ripening and harvest). The midday stem water potential (Ψmd) values of DIPh-I and DIPh-III were influenced by the rainfall regime in both years, whereas the Ψmd of DIPh-II was decreased and remained lower throughout the study period. Annual trunk growth was reduced only by the DIPh-I treatment; although the DIPh-II treatment decreased trunk growth during phase II it was relieved after the recovery period (during phase III). The main effects of both the DIPh-I and DIPh-III treatments were related with changes in fruit quality parameters; DIPh-I reduced the percentage of juice and DIPh-III affected negatively the peel colour when the water stress was moderate. However, the effects of DIPh-II were more drastic, decreasing yield due to smaller fruits, altering fruit composition, increasing the titratable acidity much more than the total soluble solids and affecting peel colour, therefore delaying fruit maturation.