Different irrigation levels corresponding to 30, 60 and 100% of estimated ETc were applied to a 'Gala' apple orchard from 50 days after full bloom (DAFB) until harvest. Treatments were applied both to trees in full light conditions and under a white 50% shading net. Fruit diameter variations were continuously monitored on 6 trees per treatment from 64 to 112 DAFB, using automatic fruit gauges connected to a wireless data-logger system. During this period, midday stem water potential (Ψstem) was determined weekly. Due to frequent rainfall the irrigation treatments affected tree water status only occasionally. Fruit growth-related parameters such as fruit shrinkage and fruit net daily growth were affected by irrigation and shading treatments mainly during the first period of measurements, when fruit had a higher surface conductance and xylem functionality. Shading and high water availability tended to increase fruit daily growth which was linearly related to midday Ψstem. Later in the season, fruit stopped shrinking during the day, while their daily growth showed a curvilinear relation with midday Ψstem. Due to the small range of Ψstem obtained within the trial, slight or no relationships were found between the other fruit-related parameters (fruit shrinkage; maximum and minimum hourly growth rates) and tree water status. These results suggest how monitoring fruit growth could potentially be used for the early detection of drought stress in decision support systems aimed at optimizing irrigation scheduling.
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