botany Botany, also called plant science, is the branch of natural science and biology studying plants, especially Plant anatomy, their anatomy, Plant taxonomy, taxonomy, and Plant ecology, ecology. A botanist or plant scientist is a scientist who s ...

, 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 A branch, also called a ramus in botany, is a stem that grows off from another stem, or when structures like veins in leaves are divided into smaller veins. History and etymology In Old English, there are numerous words for branch, includ ...

the main stem of the branch is further dominant over its own side twigs.

Plant physiology Plant physiology is a subdiscipline of botany concerned with the functioning, or physiology, of plants. Plant physiologists study fundamental processes of plants, such as photosynthesis, respiration, plant nutrition, plant hormone functions, tr ...

describes apical dominance as the control exerted by the terminal bud (and shoot apex) over the outgrowth of lateral buds.

Overview

Apical dominance occurs when the shoot apex inhibits the growth of lateral buds so that the plant may grow vertically. It is important for the plant to devote energy to growing upward so that it can get more light to undergo

photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...

. If the plant utilizes available energy for growing upward, it may be able to outcompete other individuals in the vicinity. Plants that were capable of outcompeting neighboring plants likely had higher fitness. Apical dominance is therefore most likely adaptive. Typically, the end of a shoot contains an apical bud, which is the location where shoot growth occurs. The apical bud produces a

plant hormone Plant hormones (or phytohormones) are signal molecules, produced within plants, that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development, including embryogenesis, the regulation of Organ (anat ...

auxin Auxins (plural of auxin ) are a class of plant hormones (or plant-growth regulators) with some morphogen-like characteristics. Auxins play a cardinal role in coordination of many growth and behavioral processes in plant life cycles and are essent ...

( IAA), that inhibits growth of the lateral buds further down on the stem towards the axillary bud. Auxin is predominantly produced in the growing shoot apex and is transported throughout the plant via the

phloem Phloem (, ) is the living tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as ''photosynthates'', in particular the sugar sucrose, to the rest of the plant. This transport process is ...

and diffuses into lateral buds which prevents elongation. That auxin likely regulates apical dominance was first discovered in 1934. When the apical bud is removed, the lowered IAA concentration allows the lateral buds to grow and produce new shoots, which compete to become the lead growth.

Apex removal

Plant physiologists have identified four different stages the plant goes through after the apex is removed (Stages I-IV). The four stages are referred to as # lateral bud formation, # "imposition of inhibition" (apical dominance), # initiation of lateral bud outgrowth following decapitation, and # elongation and development of the lateral bud into a branch. These stages can also be defined by the hormones that are regulating the process which are as follows: Stage I,

cytokinin Cytokinins (CK) are a class of plant hormones that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in Cell (biology), cell growth and cellular differentiation, differentiation, but also affect apical ...

promoted, causing the lateral bud to form since cytokinin plays a role in

cell division Cell division is the process by which a parent cell (biology), cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukar ...

; Stage II, auxin is promoted, resulting in apical dominance ("imposition of inhibition"); Stage III, cytokinin released resulting in outward growth of the lateral bud; and Stage IV, auxin is decreased and gibberellic acid is promoted which results in cell division, enabling the bud or branch to continue outward growth. More simply stated, lateral bud formation is inhibited by the shoot apical meristem (SAM). The lateral bud

primordium A primordium (; : primordia; synonym: anlage), in embryology, is an Organ (anatomy), organ or tissue in its earliest recognizable stage of development. Cell (biology), Cells of the primordium are called primordial cells. A primordium is the simp ...

(from which the lateral bud develops) is located below SAM. The shoot tip rising from the SAM inhibits the growth of the lateral bud by repressing auxin. When the shoot is cut off, the lateral bud begins to lengthen which is mediated by a release of cytokinin. Once the apical dominance has been lifted from the plant, elongation and lateral growth is promoted and the lateral buds grow into new branches. When lateral bud formation prevents the plant from growing upward, it is undergoing lateral dominance. Often, lateral dominance can be triggered by decapitating the SAM or artificially decreasing the concentration of auxin in plant tissues.

Applications

When the apical bud is removed, the lowered IAA concentration allows the lateral buds to grow and produce new shoots, which compete to become the lead growth. Pruning techniques such as

coppicing Coppicing is the traditional method in woodland management of cutting down a tree to a tree stump, stump, which in many species encourages new Shoot (botany), shoots to grow from the stump or roots, thus ultimately regrowing the tree. A forest ...

and

pollarding Pollarding is a pruning system involving the removal of the upper branches of a tree, which promotes the growth of a dense head of foliage and branches. In ancient Rome, Propertius mentioned pollarding during the 1st century BCE. The practice h ...

make use of this natural response to curtail direct plant growth and produce a desired shape, size, and/or productivity level for the plant. The principle of apical dominance is manipulated for espalier creation, hedge building, or artistic sculptures called topiary. If the SAM is removed, it stimulates growth in the lateral direction. By careful pruning, it is possible to create remarkable designs or patterns. Some fruit trees have strong apical dominance, and young trees can become "leggy", with poor side limb development. Apical dominance can be reduced 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. Occasionally, strong apical dominance is advantageous, as in the "Ballerina"

apple An apple is a round, edible fruit produced by an apple tree (''Malus'' spp.). Fruit trees of the orchard or domestic apple (''Malus domestica''), the most widely grown in the genus, are agriculture, cultivated worldwide. The tree originated ...

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.

References

* * {{Botany Plant physiology Auxin action

Overview

Apex removal

Applications

See also

References