Trees, like all plants, possess hormones that regulate their entire life cycle, from sprouting to senescence. These chemical messengers are known as phytohormones, and they operate at incredibly low concentrations to coordinate growth, development, and environmental responses. Unlike animal hormones, which are typically produced in specialized endocrine glands and transported via a circulatory system, many plant cells are capable of producing phytohormones. These hormones often act locally or move through the plant’s vascular tissue. This system allows trees to manage complex biological processes without a central nervous system, using chemical signals to dictate when and how to grow or adapt.
The Primary Growth Regulators
The fundamental shape and vertical structure of a tree are dictated by a trio of growth-promoting hormones. Auxins are the most studied class, primarily synthesized in the shoot tips and young leaves. This hormone is responsible for cell elongation, driving the vertical growth of the trunk and branches by loosening cell walls for expansion. Auxin also enforces apical dominance, where its high concentration at the main growing tip suppresses the growth of lateral buds below it, ensuring the tree prioritizes height over bushiness.
Gibberellins are another class of hormones, known for promoting stem elongation and breaking seed dormancy to initiate germination. They cause cells to divide and stretch, increasing the length of internodes, which are the sections of stem between leaf attachments. In a dormant seed, gibberellins signal the embryo to begin growth by stimulating enzymes that break down stored starches into usable energy.
Cytokinins work in balance with auxins to manage the distribution of growth throughout the tree structure. They are mainly produced in the roots and stimulate cell division, or cytokinesis, which creates new cells for growth. Cytokinins traveling upward partially counteract auxin’s apical dominance, encouraging the development of lateral buds into side branches. The ratio between auxin and cytokinin concentrations helps determine whether the plant grows tall and straight or develops a fuller canopy.
Environmental Response and Adaptation
Trees must adjust their physiology to survive seasonal changes and unpredictable stress, a task managed by two major regulatory hormones. Abscisic acid (ABA) acts as the tree’s primary stress signal and growth inhibitor, particularly during drought or cold. When a tree senses water scarcity, ABA levels rise and trigger the closure of stomata, the small pores on the leaves, to conserve moisture. ABA also induces bud dormancy, preparing the tree for winter and preventing premature opening during warm spells. It maintains seed dormancy, preventing germination until environmental conditions are favorable for seedling survival.
Ethylene is unique among phytohormones because it is a simple gaseous hydrocarbon, allowing it to diffuse through plant tissues and the surrounding air. This gas signals aging and stress, triggering leaf abscission (the shedding of leaves in autumn) and flower wilting. Ethylene is also the primary hormone that promotes fruit ripening, causing climacteric fruits like apples and bananas to soften, change color, and become sweeter. Ethylene production increases in response to injury, flooding, or disease, acting as a warning signal throughout the plant. In fruit, a small amount of ethylene quickly accelerates the process, leading to a rapid spike as the fruit reaches maturity.
Human Utilization of Plant Hormones
The horticultural and agricultural industries harness synthetic and natural phytohormones to control plant development and maximize yield. Synthetic auxins, such as indole-3-butyric acid (IBA), are widely used as “rooting powders” to stimulate rapid root formation on stem cuttings. These applications allow for the efficient vegetative propagation of desirable tree varieties that would otherwise be difficult to grow from seed. Ethylene control is essential in the commercial fruit industry to manage shelf-life and transport. Inhibitors like 1-methylcyclopropene (1-MCP), which blocks ethylene receptors, are sprayed on harvested fruit to delay ripening during long-distance shipping and storage.
Conversely, fruit picked green can be exposed to controlled amounts of ethylene gas in ripening chambers before being sent to market, ensuring optimal color and flavor. Gibberellins are used to increase the size of certain fruits, most notably in the production of large, seedless grapes. Sprays of gibberellic acid cause the cells in the grape berries to enlarge significantly. Other synthetic compounds are employed as growth retardants, which inhibit the tree’s natural gibberellin synthesis, resulting in more compact, shorter ornamental plants that require less pruning.