Apple fruit cell numbers in relation to cropping alternation and certain treatments

Fruit cell numbers in a block of Jonathan apple trees with an alternate cropping rhythm were studied over the period 1955–1962. The number of cells per fruit showed a rhythm in phase with the crop level, while the percentage dry matter showed a rhythm counter to this. Seed numbers per fruit showed no rhythm. The rhythm in cell numbers proved very persistent, continuing for some years under severe thinning treatments. Against this background four types of treatment designed to affect cell numbers were imposed over the period 1957–1962: (1) Decreasing competition between fruits, by thinning by hand, by DNOC, and by NAA. (2) Attempts to increase competition, by delaying abscission with 2,4-D. (3) Attempts to stimulate cell division by kinetin plus synergistic compounds. (4) Attempts to supplement nutrition which forms the background to cell division. No significant effects resulted from the 2,4-D, kinetin, or nutritive supplements, but reducing competition by thinning, either by hand or by DNOC, resulted in an increase in cell numbers which was cumulative over several years. Thinning with NAA did not increase cell numbers. The implications of these results are discussed.