Apical dominance is a biological phenomenon where a plant prioritizes the upward growth of its main stem over the horizontal growth of its side branches. This developmental strategy helps plants compete in crowded environments, allowing them to rapidly gain height and access sunlight more efficiently. The control exerted by the topmost growing point dictates the overall architecture and shape of the plant, determining whether it will be a tall column or a low, sprawling shrub. This system governs how a plant allocates its energy and resources.
The Centralized Growth Pattern
The physical manifestation of apical dominance is visible in the distinct difference between the apical bud and the lateral buds on a plant stem. The apical bud, located at the very tip of the main shoot, is the primary growth point driving vertical extension. Lateral or axillary buds are located further down the stem, typically in the angle between a leaf and the main shoot. These lateral buds are genetically capable of developing into side branches, but they remain suppressed and dormant under the influence of the apical bud.
This suppression creates a classic growth habit, exemplified by many conifers like pine or fir trees that display a distinct cone shape. The single, dominant central trunk, known as the leader, grows uninhibited. The side branches below it are progressively shorter towards the top, forming the characteristic pyramid structure. Suppression of lateral growth is strongest near the apex and gradually lessens further down the stem, which is why lower branches tend to be longer and more vigorous.
The Auxin and Cytokinin Mechanism
The mechanism underlying apical dominance is a balancing act between two classes of plant hormones: auxins and cytokinins. Auxin, primarily indole-3-acetic acid (IAA), is synthesized in the actively dividing cells of the apical meristem, the growth tissue within the apical bud. From this source, auxin is transported downward through the stem in a process called polar transport.
This downward flow of auxin creates a high concentration gradient near the shoot apex. This high local concentration directly or indirectly inhibits the activation and outgrowth of the nearby lateral buds. The lateral buds are highly sensitive to auxin; even a concentration that promotes growth in the apical meristem can be inhibitory to the lateral buds. Auxin is considered the primary signal that enforces the dominance of the main shoot.
In contrast, cytokinins are a class of hormones largely produced in the plant’s roots and are transported upward through the xylem tissue. Cytokinins act as an antagonist to auxin, actively promoting cell division and stimulating the development and outgrowth of the lateral buds. The constant interaction between the high, downward-moving concentration of inhibitory auxin and the upward-moving, stimulatory cytokinins determines the degree of lateral bud suppression. When the ratio of auxin to cytokinin is high, apical dominance is strong, resulting in a tall, unbranched plant. When the ratio shifts, the lateral buds are released from dormancy.
Manipulating Plant Shape and Yield
Understanding the hormonal mechanism allows for the deliberate manipulation of a plant’s structure, a practice widely used in horticulture and agriculture. The most direct way to disrupt apical dominance is by physically removing the apical bud, a technique known as pruning, pinching, or topping. Removing the apical meristem immediately eliminates the primary source of auxin production.
The sudden drop in auxin concentration releases the lateral buds from their suppression. This hormonal shift allows the cytokinins, which are still being supplied from the roots, to stimulate the now-activated lateral buds, causing them to grow out as new side branches. The result is a change in the plant’s growth pattern from vertical to horizontal, leading to a much bushier, denser form.
Horticulturists use this technique to achieve specific goals, such as maintaining the dense, uniform shape of ornamental hedges by shearing the tops of the plants. For fruit trees and shrubs, breaking apical dominance encourages the formation of more branches, which increases the overall surface area for flower and fruit production, thereby enhancing crop yield. Similarly, the selective removal of buds is a fundamental technique used in shaping plants like bonsai, where controlling the balance between height and lateral spread is paramount.