apical dominance

Correlative inhibition of the growth of lateral (axillary) shoots exerted by the growing apical bud of the plant stem. Partial or complete apical dominance can also occur in the initiation of lateral roots and in the orientation of lateral organs such as branches, leaves, and stolons.

In the apical meristem, cell division occurs in the young forming leaves and along the flanks of the apical bud. As the number of cells increases, they elongate, pushing the apical meristem upward and leaving a small portion of the dividing cells behind the axil of each of the laterally forming leaves. This meristem, called the lateral meristem, may remain a small group of cells with little organization or may develop into an axillary bud possessing short internodes, unexpanded leaves, and an apical meristem. In roots, branching does not directly involve the apical meristem. Lateral roots do not form from organized buds but originate in the layer of cells between the endodermis and root vascular system (pericycle). See also Lateral meristem; Root (botany).

The degree of apical dominance over the lateral buds varies with the plant species. Some plants, such as pea and sunflower, exhibit strong apical dominance, causing the formation of single branchless shoots. Other plants, such as tomato, have weak apical dominance, allowing axillary bud growth and a bushy growth habit. Apical dominance can be broken by several factors, including apical bud removal (decapitation), horizontal positioning of the plant's main axis (gravistimulation), shoot inversion, low light intensity, or short-day photoperiods. In some situations, apical dominance is weakened as the plant becomes older. See also Photoperiodism; Plant movements.

Plant organs other than the main shoot are under the control of apical dominance. In roots, strong apical dominance causes tap-root growth, whereas weak apical dominance results in a fibrous root system. Leaves and branches which grow at characteristic angles to the main axis of the stem will grow more upright after removal of the apical bud of the main stem. The growth form of some organs is also controlled by the presence of the apical bud. Rhizomes (underground axillary shoots lacking chlorophyll and having rudimentary leaves) will grow upright, forming green, leafy shoots if the apical bud and all the aboveground axillary buds are removed. Lower plants, such as mosses and ferns, as well as fungi and algae, also exhibit apical dominance.

One hypothesis for the mechanism of apical dominance is based on the competition of nutrients between centers of growth. According to this hypothesis, nutrients are preferentially transported to the apical bud, causing deficiencies in the axillary buds. A second hypothesis for the mechanism of apical dominance proposes that one or more plant hormones, such as auxin, cytokinin, and gibberellin, acts as correlative signals. See also Auxin; Cytokinins; Gibberellin; Plant growth.

Apical dominance is a phenomenon in which the terminal bud produces hormones inhibiting the growth of axillary buds. This allows the plant to grow taller, increasing its exposure to light. Under certain conditions, the axillary buds begin to grow, producing branches. When the terminal bud is pruned ("pinched back") on houseplants and fruit trees, axillary bud growth is stimulated, producing bushy, full-looking plants.

The tendency of the central or terminal shoot that grows upward to inhibit the development of branching side shoots. To make a plant bushier or shorter, gardeners pinch the terminal shoots, which then permits the side shoots to develop.

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Many conifers show particularly strong apical dominance, strongest of all in the family Araucariaceae, showing a single erect central trunk with strongly differentiated horizontal branching. Cuttings of Araucariaceae species taken from a side branch will not develop erect growth. Araucaria heterophylla, South Durras, New South Wales, Australia.

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 ; on a branch the main stem of the branch is further dominant over its own side branchlets.

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 to 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.

weeping larch showing growth habit when lacking apical dominance.

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

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