Changes in the physiological response between leaves and fruits during a moderate water stress in table olive trees

Pit hardening period is the phenological stage when water stress is recommended in regulated deficit irrigation (RDI) in olive trees. In table olive trees, fruit growth is a very important process which could affect the final profit of the yield. RDI scheduling based on water status measurements could improve water management, but accurate threshold values are needed. Previous works in low fruit load conditions suggested −1.8MPa of midday stem water potential as “first step” of water stress level where no variations of fruit growth have been detected. The aim of this work is to describe the physiological response of table olive trees with a significant yield in a moderate water stress conditions during pit hardening period. Water relations of Control (no water stress) trees and Stressed trees were studied in a mature table olive orchard in Seville (Spain). Control trees were irrigated with 100% of ETc and values around field capacity were measured. Irrigation in Stressed trees was withdrawn during pit hardening period, and they were irrigated as Control in the rest of the experiment. Fruit growth was not affected until the last days of the deficit period, though midday stem water potential and maximum leaf conductance measurements reached minimum values a few days after the beginning of the water stress period. Such responses suggest two phases in the water stress period. At the beginning of the experiment, the physiological response of the trees (osmotic adjustment and trunk dehydration in the present work) compensated the decrease in water potential. In this phase, leaves and fruits are similar water sink in the shoots. During the last days of the drought period, the reduction of the osmotic adjustment and the greater decrease of fruit water potential transform fruits in more strength water sink than leaves. These changes produced a decrease in the fruit growth. The recovery, though it was not complete, increase fruit size as the same level than Control.