Phytohormones (A.K.A. plant hormones) are specific chemicals that regulate the growth processes of plants. Hormones produced by plants are different from the hormones found within the bodies of animals, however.

Animals have a central nervous system that provides internal coordination of all physiological activities. Plants lack this central nervous system, as well as a hormone transport system efficient as the circulatory system found within animals.

This efficient transport system is what allows for an animal's greater complexity in hormones (most animal hormones are very specific in both action and location). Although still very complex, plant hormones tend to have multiple functions and their receptors are spread widely throughout the plant. Like the hormones found in animals, plant hormones play intricate roles in virtually all aspects of biological function.

Based on their molecular structure, plant hormones are broken down into five major classes. These hormones affect a wide variety of plant processes, including the formation of stems, leaves and roots, as well as the initiation and development of fruits and flowers.

Understanding how each group of hormones affects plant growth allows horticulturists to manipulate their plants for a desired purpose. The five major classes of hormones are auxins, cytokinins, gibberellins, ethylene and abscisic acid.

Auxins

Auxins are the plant hormones associated mainly with physical structure. These hormones stimulate upward growth, suppress side branching and stimulate root growth. As the plant grows larger and the growth rate slows down, auxins affect the reduction in apical dominance (the plant having one dominate stem), which gives the plant a more rounded, uniform canopy.

Auxins are also the plant hormones associated with tropism (moving toward the light); it is the concentration of auxins on the shaded side of the stem that stimulates cell elongation, which then turns the stem toward the sun. Auxins are transported cell to cell through a complex and organized process known as polar auxin transport. It is this intricate transporting of plant hormones that allows plants to react to external conditions without requiring a central nervous system.

Auxins play a large role in horticulture as growth and root stimulators. For the indoor horticulturist, auxins are most commonly found in rooting concentrates. Rooting gels or solutions designed for propagation usually contain at least one, if not multiple, auxins. These particular auxins help initiate root growth and aid in root establishment. Some of the new, innovative products that utilize auxins are focused on increasing vegetative growth and manipulating the structural integrity during the fruiting and flowering stage.

Cytokinins

Cytokinins are the plant hormones associated with cellular division. These hormones are also associated with embryo development, seed germination and flower development (mainly the promotion of auxiliary bud sites). Cytokinins have also been shown to have some influence on apical dominance, the most fascinating of which is cytokinins' effect on delaying leaf senescence (aging). It is believed that cytokinins can slow down the aging process of plants by inhibiting protein breakdown and aiding in the acquisition of nutrients from nearby tissue.

Cytokinins and auxins act in concert with each other. In other words, the ratio of cytokinins to auxins directly affects the way each hormone influences plant growth. If the ratio has more auxins than cytokinins, root formation will be stimulated. If there are more cytokinins than auxins, the growth of shoot buds will be stimulated. On a related note, cytokinins are popular in modern horticulture as growth and flower stimulators due to their stimulation of cell division.

Products containing cytokinins have been shown to increase yields on a variety of crops even in adverse conditions. Foliar applications of cytokinins during the early weeks of the flowering stage will increase fruit or flower sites, which leads to higher yields for most indoor horticulturists.

Recently, some cytokinins have been shown to increase a plant's pathogenic resistance. This discovery could lead to hormone treatments that would make monoculture crops equally or more resistant to pathogens than the GMO plants designed for that purpose.

Gibberellins

Gibberellins are the plant hormones connected with developmental processes, including germination, dormancy, establishing sex, flowering, and leaf and fruit senescence. Gibberellins are specifically involved in breaking dormancy and multiple aspects of germination.

It is believed that gibberellins trigger the synthesis of specific enzymes responsible for the breakdown of the stored starch into usable glucose, such as those that break down stored starches in the endosperm when the seed is exposed to moisture (the resulting glucose can then be used to produce energy for the seed embryo). Due to their crucial role in germination, gibberellins could be viewed as the plant hormones responsible for the initiation of plant life.

Gibberellins have been used in horticulture to stimulate flowering, alter sex and initiate germination. Since many hormones fit in the classification of gibberellins, their use in horticulture is widespread.

In some cases, horticulturists will use a hormone antagonist instead of the hormone itself. Paclobutrazol is a well-known antagonist to gibberellins and is commonly used on ornamental plants. (A helpful hint, paclobutrazol should never be used on any consumable plants.) Paclobutrazol inhibits gibberellins, causing the plant growth to slow and induce early flower onset.

Ethylene

Ethylene is the plant hormone related to stimulating and regulating the ripening process. Aside from ripening, ethylene has also been shown to be influential on the opening of flowers and the shedding of leaves.

In some plants (such as chrysanthemums), however, ethylene causes a delayed flowering response. Ethylene is produced in practically every part of the plant, including the leaves, stems, roots, fruits or flowers, tubers and seedlings.

The production of ethylene is regulated by environmental and developmental factors. Environmental stresses—including flooding, drought, frost or pathogenic attack—can stimulate ethylene production. It has also been shown that certain chemicals, including auxins, can induce or inhibit ethylene production.

Ethylene has been used in horticulture since the ancient Egyptians deliberately damaged their figs in order to stimulate ripening. Presently, catalytic generators create ethylene gas in commercial chambers designed to ripen fruits and vegetables. Chrysanthemum growers, conversely, will deliberately raise the ethylene concentration in their greenhouses to slow the flowering process when necessary.

Abscisic acid

Abscisic acid is the plant hormone most associated with stresses. This hormone is synthesized by plants in response to environmental stresses and has influence in many of the plant's developmental processes. In times of decreased soil moisture the abscisic acid produced by the roots translocates to the leaves, where it rapidly causes the stomata to close.

This reduces further loss of moisture through transpiration and protects the plant from suffering additional damage. Abscisic acid is also the hormone responsible for initiating seed dormancy in harsh conditions.

For perennial plants, abscisic acid plays a vital role in bud dormancy during the winter months; abscisic acid produced by the plant slows down plant growth and helps form a protective barrier around the dormant buds as the cold season approaches.

The most common use of this hormone in indoor horticulture is in anti-wilting solutions. During the propagation process growers can spray their cuttings or seedlings with a solution containing abscisic acid, which will close the leaves' stomata. This will allow the cuttings or seedlings to withstand lower humidity conditions without wilting due to moisture loss.

Plant hormones play crucial roles in virtually every aspect of plant physiology. The plant hormone-based products currently available have proven their effectiveness and their importance in the indoor gardening industry.

Horticulturists trying to increase their propagation success, stimulate rapid growth, accelerate cellular division and reap bountiful yields are utilizing or stimulating plant hormones within their gardens.

The emergence of new, innovative hormonal-based products will define the future of plant hormones in horticulture. As we discover more about the plant functions that are initiated by hormones (or their antagonists), we get that much closer to developing the ultimate hormonal treatments for optimizing all stages of growth.