Yield and water relations of two apple cultivars under irrigation

Because of an increased incidence of drought, irrigation has become an important agricultural practice in formerly mesic regions. Efficient irrigation scheduling depends on a good knowledge of tree water relations. For three growing seasons, we monitored stem water potential (Ψ stem ) in two apple tree cultivars ( Malus × domestica cv. ‘Red Jonaprince’ and ‘Gala Brookfield’) with and without irrigation. We also determined xylem potentials at 12% and 50% percent loss of conductivity (Ψ 12 , Ψ 50 ) and vessel diameters in current-year shoots. To evaluate if trees are capable of osmotic adjustment, we measured turgor loss point (TLP), osmotic potential at full turgor, and the concentrations of organic osmolytes (proline, glucose, sucrose, and sorbitol) in leaves throughout the growing season. We found that Ψ stem did not drop below − 1.6 MPa, which is well above the Ψ 50 and TLP. Irrigated trees (ETc-100) had slightly higher Ψ stem than trees without irrigation (ETc-0). The observed conditions in one of the 3 years resulted in similar yields but smaller fruit sizes of the non-irrigated trees. The triploid cultivar ‘Red Jonaprince’ had typically more negative Ψ stem than the diploid cultivar ‘Gala Brookfield’, but ‘Gala Brookfield’ exhibited higher limitations in fruit growth during drought and shoot growth during wet periods. Concentrations of all measured osmolytes were higher in leaves of non-irrigated trees of ‘Gala Brookfield’ and increased during the season, while the pattern was more variable in ‘Red Jonaprince’. In summary, our results indicate that ‘Red Jonaprince’ favours hydraulic efficiency against safety, while ‘Gala Brookfield’ adopts a more conservative growth strategy.