Water availability and crop load (number of fruit per tree) affect the fruit size of apple (Malus ×domestica Borkh.), but their interaction in relation to fruit size is not well understood. The objective of the present study was to explore the effect of crop load on water consumption of `Golden Delicious' apple in relation to fruit size. A wide range of irrigation rates and crop loads was applied to mature, field-grown `Golden Delicious' apple trees for two consecutive years, 1995 and 1996. The number of fruit, crop yield, and average fruit diameter were determined for each tree. A model was proposed to describe the combined effect of crop yield and irrigation rate on fruit size. In the model, irrigation waters were divided between two uses: vegetative water use (UV), which enables the tree to produce a steady, long-term yield; and reproductive water use (UR), which supports the production of the dry mass of commercial-size fruit. Potential fruit diameters were 77.1 and 72.2 mm for 1995 and 1996, respectively. Calculated vegetative use values were 300.2 and 323.4 mm for 1995 and 1996, respectively. The response of fruit diameter to reproductive water use per ton fresh weight (specific reproductive water use; URT) was fitted by a hyperbolic model in which the fruit diameter increases with increasing specific reproductive water use and approaches the yearly potential diameter at 60 to 70 m3·t-1, irrespective of the potential fruit diameter. In both years, fruit diameter showed a closer correlation with the specific reproductive water use than with either crop load or irrigation rate. In conclusion, the crop yield and the potential fruit size determine the irrigation rate required to achieve a certain average fruit diameter. The year's potential fruit diameter does not affect the total tree water use or its components. The proposed model can be used by growers for supporting decisions on irrigation and thinning strategies in commercial orchards.
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