Quantifying the Influence of Physiological Processes on Crop Water Status and Consumption.

Physiological processes including reproductive activity are expected to influence source-sink carbon relationships and to affect water status and water consumption.  We tested the hypothesis that whole tree water consumption of olives is fruit-load dependent. Fruit-load was manipulated in 14 mature olive trees grown in large weighing-drainage lysimeters. Fruit was alternatively thinned or removed from trees immediately following fruit set and at three additional stages of growth; early, mid and late in the season. Tree-scale transpiration, calculated from lysimeter water balance, was found to be a function of the presence of fruit in addition to canopy size and weather conditions. Removal of fruit brought about an immediate decline in tree water consumption, measured as transpiration normalized to tree size, which persisted until the end of the season. The later the fruit removal was executed, the greater was the response, indicating that factors such as fruit size or stage of development influence the governing of water consumption. The amount of water transpired by a fruit-loaded tree was found to be ~30% greater than that of an equivalent low- or non-yielding tree. The tree-scale response to fruit was not mirrored in stem or leaf-scale physiological measurements including plant water potential, stomatal conductance, or carbon exchange rate. Trees with low or no fruit-load had higher vegetative growth rates. However, no significant difference was observed in the overall aboveground dry biomass among groups, when fruit was included. The most likely mechanisms explaining the role of fruit on water consumption are found in response of gas exchange processes to carbon source-sink relationships, dynamic progression from isohydric to anisohydric stomatal regulation, and by fruits signaling and changing hydraulic properties of vascular tissues and tree organs. Practical outcomes of the results include need for fruit-load/phenological specific crop factors or set points for irrigation scheduling.