Responses of apple fruit size to tree water status and crop load.

The combined effects of irrigation rate and crop load on apple yield and fruit size were examined in two commercial apple orchards (cv. Golden Delicious) in a semi-arid zone. The irrigation rates applied were 1, 3 and 7 mm day(-1), and the two fruit thinning treatments involved adjusting crop load to 100 and 300 fruits per tree at Ortal and 50 and 150 fruits per tree at Matityahu. Unthinned trees served as the control. The fruit from each tree was picked separately, and fruit size distribution was determined with a commercial grading machine. Midday stem water potentials varied from -0.9 to -2.8 MPa, crop load varied from 80,000 to 1,900,000 fruit ha(-1) and crop yield varied from 10 to 144 Mg ha(-1). Midday stem water potential decreased with increasing crop load in all irrigation treatments at Matityahu, but only in the 1 mm day(-1) treatment at Ortal. The extent of the lowering of midday stem water potential by crop load decreased with increasing soil water availability. At both orchards, a similar response of total crop yield to crop load on a per hectare basis was observed. Mean fruit mass and relative yield of fruit > 70 mm in diameter increased with midday stem water potential, with the low crop loads having similar but steeper slopes than the high crop load. The responses of mean fruit mass and relative yield of fruit > 70 mm in diameter to midday stem water potential were similar at both orchards, perhaps indicating that thresholds for irrigation scheduling are transferable to other orchards within a region. Factors that may limit the transferability of these thresholds are discussed.