Gravimorphism in Trees

Abstract

The effect of shoot-orientation on the vegetative growth and apical dominance of rootstocks of apply, cherry, plum, and blackcurrant was investigated. In all the species studied the total annual extension growth of horizontally grown trees was markedly less than that of vertical trees, and the effect was accentuated when the plants were rotated. In cherry and plum the reduced total growth was due both to a smaller number of internodes and also to reduced internode length; in apple and blackcurrant the difference in extension growth was mainly due to differences in internode length. In experiments with laternal shots trained in various positions it was found that the apical dominance normally exhibited by the uppermost shoot is only manifested when it occupies a vertical position. The normal apical dominance relationships can be reversed if the uppermost shoot is grown horizontally and the second shoot vertically. Gravitational effects appear to play a role in the normal apical dominance relationships of the tree. In all species there is a marked tendency for laterals to grow out on the upper side of horizontal and arched plants. The effects of training arched trees into various positions are considered in relation to current concepts regarding the role of auxin in apical dominance and the redistribution of auxin in horizontal organs; it is concluded that the outgrowth of laterals in arched shoots cannot be interpreted primarily in terms of these current concepts. The observed results suggest that the outgrowth of lateral buds occurs at the nearest point to the roots at which the shoot is diverted from the vertical position, and that nutrients are diverted to the highest upwardly directed meristem. It is proposed that the term gravimorphism should be applied to these diverse effects of gravity on plant growth.