Effects of Nitrogen Supply on Source-sink Balance and Fruit Size of ‘Gala’ Apple Trees

Abstract

The objective of this study was to determine how nitrogen (N) supply affects the source-sink balance and fruit size of ‘Gala’ apple when crop load was controlled at a moderate level. Five-year-old ‘Gala’/‘M.26’ trees grown in sand culture and trained in tall spindle received a total of 3.3, 10.0, 20.0, or 40.0 g actual N through fertigation using Hoagland's solution from bloom to 3 weeks before harvest. The crop load of these trees was adjusted to 6.5 fruit/cm2 trunk cross-sectional area by hand thinning when the diameter of the largest fruit was 10 mm. As N supply increased, total shoot leaf area in the canopy increased, whereas total spur leaf area remained unchanged. Both single leaf and whole canopy net CO2 assimilation rates increased with increasing N supply. The net dry matter gain of the whole tree from budbreak to fruit harvest increased ≈74% from the lowest N supply to the highest N supply, but the proportion of net dry matter gain partitioned to fruit (harvest index) decreased from 83% to 70%. Both leaf area to fruit ratio and average final fruit size increased with increasing N supply, and a linear relationship was found between leaf area to fruit ratio and final fruit size. The number of cells per fruit increased with increasing N supply, whereas average cell size remained unchanged. As N supply increased, fruit soluble solids concentration increased, whereas fruit firmness decreased slightly. These results indicate that 1) apple trees grown under low N supply are source-limited; and 2) within the range of N supply used, increasing N supply improves leaf N status, leaf and whole tree photosynthetic capacity, and leaf area to fruit ratio, leading to more cells per fruit, larger fruit, and higher soluble solids.