The roles of fruit sink in the regulation of gas exchange and water uptake: A case study for avocado

The effects of drip irrigation frequency on ‘Hass’ avocado trees grown in lysimeters were examined. The experimental design comprised three irrigation frequencies: (a) pulsed irrigation (10–20min every 30min) throughout the day (Irg1); (b) one daily irrigation event beginning at night and terminated in the morning every day (Irg2); and (c) one irrigation event every two days (Irg3). Irrigation treatments induced significant differences in water availability in the root zone and in plant water uptake. The effects of the fruit sink on gas-exchange properties and water uptake were assessed by comparing the performance of fruiting and defruited avocado trees. Despite the higher vegetative growth of defruited trees, their daily water uptake was 40% lower than that of fruiting trees and therefore, crop load should play an important role on irrigation scheduling. Measurements of stomatal conductance (gs) and photosynthesis per unit leaf area (A) during two vegetative years were not in accordance with irrigation treatments or with diurnal changes in atmospheric conditions. Similar pattern was observed for the defruited trees. Leaf-carbohydrate concentrations in trees with and without fruits were lowest before sunrise, and increased during the day in different patterns. In defruited trees the carbohydrate concentrations increased steeply to a maximum around 09:00, while in fruiting trees, it increased monotonically until midday. Our findings may indicate that leaf-carbohydrate plausibly play a role in the complex framework of stomata aperture.