Effects of Shade on the Growth and Cropping of Apple Trees. II. Effects on Components of Yield

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Abstract
Cox’s Orange Pippin apple trees on M.26 rootstock were shaded so as to receive 37, 25 or 11 % of full daylight during the post-blossom growing season in either or both 1970 and 1971 and their flowering and fruit development and yield were compared with those on control trees. Shading reduced fruitlet retention and fruit size and percentage dry matter in the year of shading. It also reduced flower bud formation and had a residual adverse influence on the percentage of flowers which set fruit in the following year. As a consequence of these latter effects, that of shade on yield in the year following shading was at least as great as its effect in the year of shading. There was a marked interaction between shading and crop load in their influence on flower bud formation, fruit set and fruit size, which suggests that the effect of shading for one year may partially pre-adapt the tree for such conditions in the following year. The possible roles of substrate competition and hormonal influences are discussed, but further evidence is needed; probably both are important. The pattern of response to shading is such that biennial cropping combined with a low overall yield level seems to be the most probable result of management systems or environmental conditions which impose inadequate illumination.