Apical dominance

  In plant physiology, apical dominance is the phenomenon whereby the main central stem of the plant is dominant over (i.e., grows more strongly than) other side stems, and on a branch, the main stem of the branch is further dominant over its own side branchlets.

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The apical bud (or tip) produces the growth hormone auxin, which not only promotes cell division, but also diffuses downwards and inhibits the development of lateral bud growth which would otherwise compete with the apical tip for light and nutrients. Removing the apical tip and its suppressive hormone, allows the lower dormant lateral buds to develop, and the buds between the leaf stalk and stem produce new shoots which compete to become the lead growth. Manipulating this natural response to damage (known as the principle of apical dominance) by processes such as pruning (as well as coppicing and pollarding) allows the horticulturist to determine the shape, size and productivity of many fruiting trees and bushes.

Understanding the principle of apical dominance is helpful when embarking upon any regime of tree or other plant management. Plants form new tissue in an area called the meristem, located near the tips of roots and shoots, where active cell division takes place. Meristem growth is aimed at ensuring that leaves are quickly elevated into sunlight, and that roots are able to penetrate deeply into the soil. Once adequate height and length is achieved by the stems and roots, they will begin to thicken to give support to the plant. On the shoots, these growing tips of the plant are known as apical buds.

  Some fruit trees have strong apical dominance, and young trees can become "leggy", with poor side limb development. One can reduce the apical dominance in this case, or in cases where limbs are broken off by accident, by cutting off the auxin flow above side buds that one wishes to stimulate. This is often done by orchardists for young trees. Select the bud along the leader (stem) where one desires a side branch to develop, or where one already is present, but growing too weakly. With a sharp knife make a horizontal cut about a half inch above it, just deep enough to break the cambium, and only about a quarter of the way around the stem. This breaks the flow of auxins that had suppressed its growth. Later, when a bud breaks, it can be trained or pruned as needed.

Occasionally strong apical dominance is advantageous, as in the "Ballerina" apple trees. These trees are intended to be grown in small gardens, and their strong apical dominance combined with a dwarfing rootstock gives a compact narrow tree with very short fruiting side branches.

See also Fruit trees pruning