The Effects of Rootstock on the Response of Cox’s Orange Pippin Apple to Fruit-Setting Hormone Sprays

SummarySpraying Cox’s Orange Pippin trees on M.9 rootstock at petal-fall with a mixture of 200 mg H GA3+3OO mg 1−1 DPU+50 mg 1−1 NOXA increased both initial and final fruit set although the same treatment applied to similarly open-pollinated Cox trees on M.26, M.7 and MM. 106 increased initial set only. In a second experiment the application of 200 mg I−1 GA3+100 mg 1−1 DPU+15 mg 1−1 NAA at petal-fall similarly increased the final set of Cox on M.9 but not of Cox on M.26, M.7 or MM. 106. Seed counts showed that control trees on M.9 had relatively more fruits with low seed numbers at harvest than did the trees on the other rootstocks. This tendency was accentuated by the hormone spray treatments, which led to a reduction in the number of seeds per fruit in Cox on all rootstocks and to a particularly large proportion of few-seeded fruits on M.9. It is suggested that the hormone fruit-setting sprays lead to the initial retention of few- seeded or seedless fruits on all rootstocks and that these are retained to harvest more on M.9 than on other rootstocks, possibly because of less severe competition from shoot growth. It is thus conjectured that the effect of the hormone spray is to supplement natural seed-produced hormones, not necessarily to replace them totally, by giving parthenocarpic fruits although some of these were observed especially on hormone- sprayed trees on M.9. These results may partially explain the apparent variability in response of Cox to fruit-setting hormone sprays in different experiments reported in the literature.