166 Influence of Crop Level on Growth and Quality of `Braeburn' Apple Fruit

Abstract

Crop load and the genetic biological carrying capacity (source–sink relationships) determine the potential for fruit size development on apple; however, the environment within which the fruit grows attenuates this potential. The effects of different crop loads on the growth pattern and the progress of maturity in apples were evaluated at the Comahue National Univ., Argentina (lat. 38 56'S long 67 59'W), during the 1998–99 growing season. Our experiment was conducted on 6-year-old `Braeburn'/Malling Merton 111 apple (Malus domestica Borkh.) trees spaced 4.0 × 2.3 m and trained to palmette leader. Treatments were 1) light crop load (LC), 2.5 fruit/cm2 trunk cross-sectional area (TCSA), 2) moderate crop load (MC), 6.5 fruit/cm2 TCSA (standard commercial crop load) and 3) high crop load (HC), minimum 8 fruit/cm2 TCSA, no fruit removed from tree. Whole trees were hand-thinned 19 days after full bloom (DAFB). Fruit diameter (FD) was taken at two weekly intervals (n = 24 per date and treatment) and maturity indexes were determined at harvest. Analysis of variance was used and mean separations were computed with Student's t test. From 38 DAFB until harvest, fruit size was significantly reduced (P < 0.01) in the HC trees, indicating that they were source-limited during growth. At 166 DAFB, FD was 7.48, 7.14, and 6.89 cm for the LC, MC and HC treatments, respectively. Adequate carbon was apparently available to support a commercial crop load since no differences were found between LC and MC trees. Crop level influenced flesh firmness; at 173 DAFB, it was significantly lower in HC trees than MC and LC trees (84.33, 92.51, and 91.57 N, respectively). These results suggest some goals of thinning for ensuring sizable `Braeburn' fruit.