Michael Noy , Amos Naor , Menashe Levi , Ami Keinan , George Chudi

The objective was to investigate the combined effect of irrigation regime and fruit load on the development ‘Hass’ avocado trees grown in lysimeters, at different phenological periods. The effects of the fruit sink on water uptake and gas-exchange properties were assessed by comparing the performance of fruiting and de-fruited avocado trees. The role of leaf-carbohydrate concentrations was emphasized by fruit removal. Fruit presence provided a significant sink for carbohydrates, which vanished when they were removed. Rapid accumulation of carbohydrates in leaves of de-fruited trees induced a reduction in stomatal aperture early in the morning, which led to reductions in stomatal conductance (gs) and photosynthesis per unit leaf area (A) rates in the course of the day, in spite of the increasing irradiance and vapor pressure deficit (VPD). The strong carbohydrates sink formed by the presence of the fruits slowed down carbohydrates accumulation, enabling additional factors, such as irradiance, VPD, and root-zone water regime to affect gs and A. Higher stomatal aperture in fruiting tree induced a significant increase in water uptake of compared with de-fruited trees and consequently, their daily water uptake was 40% higher than that of de-fruited trees, thus despite the higher vegetative growth of de-fruited trees. The traditional ‘KCET0’ approach might be insufficient as a basis for proper irrigation management during the productive period and irrigation management should be adjusted therefore for the actual crop load.