Water Relations, Growth, and the Composition of `Braeburn' Apple Fruit under Deficit Irrigation

Abstract

Three-year-old `Braeburn' apple trees (Malus domestica Borkh.) on MM106 rootstock were studied in a glasshouse to assess the effects of deficit irrigation on fruit growth, water relations, composition, and the vegetative growth of the trees. Trees were assigned to one of three treatments. The control (C) was fully watered. The first deficit treatment (D1) was deficit-irrigated from 55 days after full bloom (DAFB) until final harvest at 183 DAFB. The second deficit treatment (D2) was deficit-irrigated from 105 to 183 DAFB. Compared to C, the D1 and D2 trees developed a lower photosynthetic rate, leaf water potential (Ψl), and stomatal conductance (gs) during the stress period. Trunk-circumference growth was reduced in both D1 and D2 trees, but leaf area and shoot length were reduced in D1 only. Total soluble solids increased in both D1 and D2 fruit. Fructose, sorbitol, and total soluble sugar concentrations were higher in D1 fruit than in C and D2. Titratable acidity and K+ levels were higher in D1 fruit than C and D2. For D1, lowering of fruit water potential (Ψw) was accompanied by a decrease in osmotic potential (Ψs), and therefore turgor potential (Ψp) was maintained throughout the sampling period. Regardless of fruit turgor maintenance, the weight of D1 fruit was reduced from 135 DAFB. Weight, sugar concentration, and water relations of D2 fruit were not affected by deficit irrigation. This indicates that fruit water relations and sugar concentration are modified if water deficit is imposed from early in the season. However, if water deficit is imposed later in the season it has less impact on the composition and water relations of the fruit.