GRAVIMORPHISM IN FRUIT TREES, ESPECIALLY APPLE: FACTS AND HYPOTHESES

The term "gravimorphism" is here taken to mean those modifications in branching habit brought about by the displacement of a more or less erect shoot with respect to gravity. Better understanding of the physiological grounds of this phenomenon might suggest new ways, including the use of growth regulators, for substituting time- and work-consuming practices like arching or palisading.

Some features are emphasized which bring new facts to light and lead to new insights on the mechanisms involved:-

1. Experiments on time of arching show that the factors of gravimorphism are acting long before the start of growth and that they need that time-lapse to exert their effects. Therefore, gravimorphism cannot be explained simply as a diversion of factors involved in apical dominance.
2. Gravimorphic changes are superimposed on the normal acrotonic branching of the shoots. They appear late in spring or summer and dominate only after a growth arrest or slow-down. Arching at the beginning of dormancy is thus likely to be the most effective.
3. From experiments based on tests with "single-node cuttings", the first effect of bending is a general stimulation of bud activity all over the arched part of the shoot. This effect seems rather independent of any gravity influence and would seem to be a straight-forward stress response, possibly mediated through ethylene.
4. Later, differences arise between buds on the upper and lower sides of the shoot in their capacity to grow out. These are comparable with the differences between buds at different positions along the length of the shoot, and seem to be mediated through the effects of gravity, which could act by modifying the transport of root-produced factors needed for bud burst and early shoot growth.