Apical dominance is a common growth pattern in plants where the main, central stem grows more vigorously and taller than its side branches. This phenomenon results in a plant that prioritizes upward growth, often creating a distinct conical or upright shape. The plant’s resources are directed towards the primary shoot, allowing it to extend vertically.
The Hormonal Control of Growth
The mechanism behind apical dominance involves a balance of plant hormones, particularly auxin. The apical bud, located at the tip of the main stem, is the primary site of auxin production. This hormone is then transported downwards through the stem. As auxin moves down, its high concentration actively inhibits the growth of lateral buds, which are located where leaves meet the stem.
Cytokinins, another group of plant hormones, promote the growth of lateral buds and are primarily produced in the root tips. However, high auxin levels from the apical bud suppress the effect of cytokinins on lateral bud growth. This interaction allows the apical bud to maintain control over lateral growth below it.
Observable Growth Patterns in Plants
Apical dominance shapes the architecture of plants, reflecting an evolutionary advantage for survival. By directing energy towards vertical growth, plants can gain height, allowing them to compete more effectively for sunlight against neighboring vegetation. This strategy ensures the plant’s leaves are exposed to optimal light for photosynthesis. Without this upward drive, a plant might be outcompeted and shaded by others, hindering its ability to thrive.
Plants exhibit varying degrees of apical dominance, leading to diverse growth forms. Conifers, such as fir trees, display strong apical dominance, resulting in their characteristic pyramidal or “Christmas tree” shape with a single, prominent central trunk and well-differentiated horizontal branches. In contrast, plants with weak apical dominance, like many bushes and shrubs, develop a bushier, more branched appearance. Examples include basil and certain fuchsia varieties, where side branches grow more freely, contributing to a denser canopy.
Modifying Plant Shape Through Pruning
Gardeners can manipulate apical dominance to achieve desired plant shapes and growth habits through pruning. A common technique is “pinching” or “heading back,” which involves removing the apical bud at the tip of a stem. This simple action removes the primary source of auxin, thereby lifting the inhibitory effect on the lateral buds located below the cut. With the auxin suppression gone, cytokinins are then able to stimulate the previously dormant lateral buds, causing them to grow into new side stems.
This process results in a more compact, fuller, and bushier plant with increased branching. For instance, pinching the tips of basil plants encourages more leaf production and a denser form, rather than a tall, spindly stalk. Similarly, for flowering plants like petunias, dahlias, or chrysanthemums, removing the apical bud promotes the development of multiple flower-producing stems, leading to a more abundant display of blooms. This horticultural practice effectively redirects the plant’s growth energy from vertical extension to lateral expansion, enhancing its overall appearance and productivity.