There are a number of applications of plant hormones in agriculture, horticulture, and biotechnology. For instance, the synthetic cytokinin kinetin is considered a hormone. The term plant growth regulator is usually used to denote a synthetic plant hormone, but most of the synthetic compounds with structures similar to those of the natural hormones have also been called hormones. Furthermore, hormonal interactions may be cooperative, antagonistic, or in balance. Such interactions may occur at various levels, including the synthesis of hormones, hormone receptors, and second messengers, as well as at the level of ultimate hormone action. Each hormone performs its specific functions however, nearly all of the measurable responses of plants to heredity or environment are controlled by interaction between two or more hormones. A number of natural or synthetic substances such as brassin, morphactin, and other growth regulators not considered to be hormones nevertheless influence plant growth and development. Circumstantial evidence suggests that flower initiation is controlled by hypothetical hormones called florigens, but these substances remain to be identified. The commonly recognized classes of plant hormones are the auxins, gibberellins, cytokinins, abscisic acid, and ethylene. Also, the effects of different hormones overlap and may be stimulatory or inhibitory.

Each plant hormone evokes many different responses. They are also active in tissues where they are produced. Plant hormones, which are active in very low concentrations, are produced in certain parts of the plants and are usually transported to other parts where they elicit specific biochemical, physiological, or morphological responses. Organic compounds other than nutrients that regulate plant development and growth.