Stem Water Potential and Apple Size

Abstract

The sensitivity of leaf (ψ leaf ) and stem (ψ stem ) water potential and stomatal conductance (g s ) to soil moisture availability in apple (Malus domestics 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 ψ stem , y leaf, g s , and soil water potential at 25 (ψ soil-25 ) and 50 cm (ψ soil-50 ). No differences in midday ψ 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 ψ 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 ψ stem . The degree of correlation between plant water stress indicators and yield component decreased in the following order: ψ stem >ψ soil-25, >ψ soil-50 >ψ leaf . The data suggest that midday ψ stem may serve as a preferable plant water stress indicator with respect to fruit size.