Apical dominance and the effect of gravity on nutrient distribution.

The effect of orientation with resepct to gravity on the accumulation of phosphorus-32 into lateral and leader apices has been investigated in seedling trees of Betula pubescens. In upright, well-branched seedlings, showing only weak apical dominance, basally injected (32)P is distributed more or less equally among the lateral and leader apices. Re-orientation treatments which result in the assumption of strong dominance by a lateral apex also result in a marked accumulation of (32)P into that apex. This effect of orientation on nutrient transport is manifested within two to four days after application of the gravimorphic treatments.The results are discussed in the light of modern concepts of hormone-directed nutrient transport and a working hypothesis for the role of gravity in apical dominance of woody plants is suggested.