Effect of crop load on phytohormones, sugars, and biennial bearing in apple trees

The amount and composition of phytohormones, sugars, and some other leaf characteristics depending on a crop load were evaluated in apple (Malus domestica Borkh. cv. Ligol grafted on P 60 rootstock) trees in order to prevent biennial bearing. The crop load was adjusted to 12 (control, unthinned), 8, 4, and 0 (non-fruiting) inflorescences (or fruits) per cm(2) of trunk cross-sectional area (TCSA). Inflorescences were removed in May before flowering. Phytohormones were analyzed in axillary buds and leaves in September. Results show that, in contrast to the unthinned trees, thinning to 4 fruits cm(-2)(TCSA) resulted in a significant decrease of yield per tree, but a significant increase of fruit mass, return bloom, and leaf area. The heavy crop load resulted in suppressed bloom in the following year. Composition and content of phytohormones was changed considerably. Moreover, thinning resulted in an increased hexose accumulation. Such data suggest that flowering inhibition depended on the phytohormones that were exported to buds and on sugar-hormone signalling cross-talk.