Plant hormones are naturally occurring chemicals that influence plant development and growth. Often called plant growth regulators to distinguish them from animal hormones, they are similar to animal hormones in that they function as chemical messengers. There are three major groups of plant hormones, as well as two other hormones that do not fit in any group.
Plant hormones were developed by plants as one way of assuring their survival. Since a plant cannot move away from a threatening situation, it is important that it have an internal messenger system that ensures that the entire plant is able to react in a proper way to its environment. This role is filled by the messenger plant hormones.
Besides influencing growth rate, plant hormones also control a plant's response to its environment. Like their animal counterparts, plant hormones are effective in small amounts and tend to be made in one place within the plant and transported somewhere else where they do their work. Unlike animal hormones, however, they are not produced by special glands nor do they only work on specialized target cells. Instead, plant hormones can have an effect on any part of the plant that produces them. Plant hormones were discovered in 1926 by the Dutch botanist (a person specializing in the study of plants), Frits W. Went (1903–1990), who isolated the first plant hormone, which he called "auxin." He chose this name from the Latin word meaning "to increase" since that word describes its result. Now known to be indoleacetic acid (IAA), this hormone is transported to the roots of young plants where it stimulates growth. Besides IAA, other growth-stimulating auxins besides IAA have been identified, and auxins are now considered to be one of the three major hormone groups.
In addition to auxins, there are two other major hormone groups: cytokinins and gibberellins. Cytokinins are an important group of plant hormones since they stimulate cell division and delay aging in older tissues. Cytokinins are thought to be produced in the tips of roots from where they travel upwards through the plant and stimulate branching rather than the lengthy growth promoted by auxins. Gibberellins are a chemically complex family of plant hormones that stimulate the growth of shoots (that part of a beginning plant that first pops out of a seed and reaches for the light). Gibberelins are important for plant embryos and seedlings, and stimulate the beginnings of root growth.
ABSCISIC ACID AND ETHYLENE
The other plant hormones that do not fall under any of the major three groups are abscisic acid and ethylene. While most plant hormones usually involve stimulating growth in one part or another, the hormone abscisic acid is actually an inhibitor since it turns off growth or development when conditions are not right for it. Sometimes certain environmental conditions, such as a drought, make water conservation a necessity. For a plant to survive in these conditions, it must slow down or stop its growth with the release of abscisic acid. Abscisic acid got its name because it was believed to play a key role in "abscission" or the seasonal loss of leaves. It is now know, however, that other hormones are more important in causing a tree to drop its leaves in autumn. Even though abscisic acid is not solely responsible for trees losing their leaves, it is extremely important given that the total lack of this hormone results in the inability of a plant's embryos to stop growing inside their seeds. Without abscisic acid premature eruption of a shoot through the seed coat may occur at a time and place where the seedling may not be able to grow.
The final and possibly best-known plant hormone, ethylene, plays a major role in the ripening of fruit. Ethylene is an unusual plant hormone in that it is released outside the plant and into the atmosphere. One plant is therefore able to influence its neighbors. It is ethylene gas that explains the old saying, "One bad apple spoils the whole bunch." Since apples continue to release ethylene after they are picked, any wound in its skin will stimulate extra production of ethylene gas, which in turn speeds up the ripening or aging of any apples nearby.
COMMERCIAL USES FOR PLANT HORMONES
Greater understanding of plant hormones has led to their increased commercial use. Farmers and gardeners are now able to use certain hormones regularly to achieve desired effects. For example, a well-known use of ethylene is by farmers who use ethylene gas to ripen tomatoes that are mistakenly harvested while they are still green. Plant hormones have also been used for military purposes. An example is IAA, which if used in very high doses, can have the opposite effect of not only slowing down growth, but it also may prove poisonous to plants. This was the case when the United States used one of these auxin-related compounds during the Vietnam War (1954–75). Called "Agent Orange," this chemical spray caused a plant's leaves to dry and fall off, thus supposedly denying the military enemy any hiding places in the jungle. Unfortunately, a chemical by-product of Agent Orange is dioxin, a cancer-causing agent.