STEM WATER POTENTIAL AND APPLE SIZE

The sensitivity of leaf (psi(leaf)) and stem (psi(stem)) water potential and stomatal conductance (g(s)) to soil moisture availability in apple (Malus domestica Borkh.) trees and their correlation with yield components were studied in a field experiment. Two drip irrigation treatments, 440 mm (H) and 210 mm (L), were applied to a 'Golden Delicious' apple orchard during cell enlargement stage (55-173 days after full bloom). Data collected included psi(stem), psi(leaf), g(s)) and soil water potential at 25 (psi(soil-25)) and 50 (psi(soil-50)). NO differences in midday psi(leaf)'s were found between irrigation treatments. Stem water potential was higher in the H treatment than in the L treatment in diurnal measurements, and at midday throughout the season. Stomatal conductance of the H treatment was higher than the L treatment throughout the day. Stomatal conductance between 0930 and 1530 HR were highly correlated with psi(stem). The H treatment increased the percentage of fruit >65 mm, and increased the proportion of earlier harvested fruit reaching marketable size compared to the L treatment. Fruit size in the first harvest and the total yield were highly correlated with psi(stem). The degree of correlation between plant water stress indicators and yield component decreased in the following order: psi(stem) > psi(soil-25) > psi(soil-50) > psi(leaf). The data suggest that midday psi(stem) may serve as a preferable plant water stress indicator with respect to fruit size.