The effect of deficit irrigation on apple tree performance

As global warming-induced climatological changes may affect limited water resources, the response of full-bearing Golden Delicious apple trees on Merton 793 rootstock to different deficit irrigation treatments (combinations of 50%, 75% or 90% depletion of total plant available water or no irrigation during four phenological phases) is evaluated to supply guidelines for optimal irrigation management.  Limited information is available in South Africa on water use and sensitivity of these apple trees to water stress.  Evapotranspiration from a coarse sandy soil in a semi-arid climate reduced in each phenological phase as depletion levels increased, resulting in different degrees of water saved for fifteen proposed irrigation strategies. Crop coefficients for irrigation scheduling of full bearing Golden Delicious apple trees in a semi-arid climate in South Africa is made available for each of these strategies. Shoot and fruit growth, production variables and water use efficiency were evaluated. Water deficit in phase 1 or 2 reduced shoot growth. Depletion in phase 2 affected fruit growth rate in phase 2 and 3, and final fruit diameter of the current season. The cumulative effect of depletion levels is reflected in the fruit growth rate in phase 1 of the subsequent season. Irrigation strategies with the best yield and fruit mass allowed 50% depletion in phase 2 or in the full season. Income decreased linearly as water savings increased under regulated deficit irrigation.