STEM WATER POTENTIAL AND APPLE SIZE

The sensitivity of leaf (psi(leaf)) and stem (psi(stem)) water potenti al and stomatal conductance (g(s)) to soil moisture availability in ap ple (Malus domestica Borkh.) trees and their correlation with yield co mponents were studied in a field experiment. Two drip irrigation treat ments, 440 mm (H) and 210 mm (L), were applied to a 'Golden Delicious' apple orchard during cell enlargement stage (55-173 days after full b loom). Data collected included psi(stem), psi(leaf), g(s)) and soil wa ter potential at 25 (psi(soil-25)) and 50 (psi(soil-50)). NO differenc es in midday psi(leaf)'s were found between irrigation treatments. Ste m water potential was higher in the H treatment than in the L treatmen t in diurnal measurements, and at midday throughout the season. Stomat al conductance of the H treatment was higher than the L treatment thro ughout the day. Stomatal conductance between 0930 and 1530 HR were hig hly correlated with psi(stem). The H treatment increased the percentag e of fruit >65 mm, and increased the proportion of earlier harvested f ruit reaching marketable size compared to the L treatment. Fruit size in the first harvest and the total yield were highly correlated with p si(stem). The degree of correlation between plant water stress indicat ors 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.