Tree Hormones: Regulating Growth and Health

Ross Tree, a plant healthcare company, offers tree hormone treatments to promote or sometimes slow plant growth, helping trees better fit their landscapes or heal faster from stress or damage. The application of tree hormones and growth regulators is a specialized task. Due to their high cost and activation at very low concentrations, Ross Tree takes great care in their preparation. Ross Tree has three ISA-certified arborists on staff with expertise in tree hormonal applications. These compounds offer an advanced approach to nurturing and managing the health of valuable urban tree assets.

Only naturally occurring compounds can be described as plant hormones. However, manufactured compounds that mimic hormones are called plant growth regulators. We have found that a relatively small group of plant hormones can control an extensive range of plant growth developments. Hormones that promote growth are called growth promoters, while those that inhibit it are called growth inhibitors. Given the sensitivity of plant tissues to low hormone concentrations, it is crucial to understand both hormone synthesis and its destruction. These factors together sometimes achieve the desired tree management outcome.

Five Tree Hormones and Growth Regulators

Auxins – The Growth Drivers

Perhaps the best-known and most significant group, auxins have been studied since the 1880s. Well-known auxins include naphthalene acetic acid and indolebutyric acid, widely used in the nursery industry.

Key Plant Functions:

• Cell Elongation and Growth: Crucial for the lengthening of stems and roots.
• Apical Dominance: Inhibits side branch growth, promoting a dominant central leader.
• Root Formation: Promotes root development and speeds healing of damaged root systems.
• Fruit Development: Encourages flowering and fruit development after pollination.
• Phototropism: Causes stems to bend towards light, with higher auxin concentrations on the shaded side leading to faster cell elongation.
• Gravitropism: Enables roots and limbs to grow downwards by inhibiting growth on the lower side and promoting it on the upper side.
• Leaf Fall: Plays a role in regulating leaf abscission.

Cytokinins – The Cell Multipliers

Cytokinins are powerful growth promoters that specifically stimulate cell division and expansion. The meristems of plant roots produce Cytokinins.

Key Plant Functions:

• Cell Division and Growth: Essential for mitosis in both roots and shoots.
• Embryo Development: Critical for proper embryo development, including the formation of cotyledons.
• Meristem Maintenance: Helps maintain stem cells that create adult plant cells.
• Leaf Development: Promotes cell division and expansion, contributing to leaf size and shape.
• Senescence Delay: Delays the aging process in leaves and other plant parts, prolonging their functional lifespan.
• Nutrient Allocation: Regulates the distribution of nutrients throughout the tree.
• Stress Response: Involved in responses to environmental stresses like drought and salinity.
• Interaction with other Hormones: Their ratio with auxins, in particular, determines the type of tree tissue or organ that develops.

Gibberellins – Accelerating Growth and Development

First discovered in Japan in the 1930s, young leaves primarily synthesize gibberellins. These hormones elongate cells, with many growth-promoting effects, often similar to auxins.

Key Plant Functions:

• Stem Elongation: Promotes the elongation of plant cells, increasing overall plant height.
• Floral and Fruit Development: Critical for initiating flowering and increasing fruit size.
• Plant Response to Stress: Helps trees respond to environmental stresses like shading and submergence by lengthening stems and petioles.
• Triggers Reproductive Growth: Regulates the shift from vegetative to reproductive phases, leading to flowers and seeds.
• Modulating Cell Wall Properties: Changes cell wall properties to increase water uptake and cell expansion.

Abscisic Acid – The Growth Inhibitor and Stress Responder

Abscisic acid is a highly effective inhibitor of plant growth and plays a crucial role in abscission – the natural detachment of leaves, flowers, and fruit. Its inhibitory role counteracts the stimulatory effects of growth-promoting hormones above.

Key Plant Functions:

• Stomatal Regulation: Helps trees respond to water scarcity by signaling stomata (small pores on leaves) to close, preventing water loss through transpiration.
• Seed Dormancy and Germination: Inhibits seed germination and promotes dormancy, ensuring seeds sprout only under favorable conditions.
• Stress Response: Helps trees respond to nutrient scarcity, UV radiation, and pathogen attacks.
• Fruit Ripening: Plays a role in the fruit ripening process.
• Root Growth: Stimulates root growth during periods of low humidity or salty conditions, helping the plant find water and nutrients.

Ethylene – The Ripener and Ager

Ethylene production occurs in various plant tissues, including flowers, leaves, fruits, roots, and seeds. Many homeowners do not realize it but modern refrigerator vegetable bins use Ethylene absorbers to keep produce fresh longer by absorbing ethylene gas, a natural gas that promotes ripening and spoilage.

Key Plant Functions:

• Fruit Ripening: Signals and accelerates the ripening process in many fruits by converting starch into sugars.
• Senescence (Aging): Promotes aging, causing flowers to wilt and leaves/fruits to drop.
• Germination and Sprouting: In some plants, it encourages seed germination and the sprouting of bulbs and potatoes.
• Stress Response: Plays a role in helping plants respond to stress.

Homeowners who want their trees to fit their landscape should consider tree hormonal treatments. For an appointment call 303-871-9121 or click here to fill out a request service form