A hormone is a chemical that affects the ways in which an organism functions; it is produced in one part of the plant (or animal) body but affects many other parts of the body as well. Hormones work by coming in contact with target cells, causing the organism to respond in various ways to the chemical signal. In plants, hormones usually work by influencing cell differentiation, elongation, and division. They also affect the timing of reproduction and germination. These hormones are divided int o five categories: auxins, cytokinins, gibberellins, inhibitors (or abscisic acid), and ethylene.
The primary function of the auxin hormones is to elongate plant cells in the stem. For instance, auxins are the hormones responsible for phototropism, the growth of a plant toward the light. Phototropism results from the rapid elongation of cells on the dark side of the plant, which causes the plant to bend in the opposite direction. The acid growth hypothesis explains this occurrence by speculating that auxins trigger proton pumps in cell membranes, lowering the pH in the cell wall to such an extent that the hydrogen bonds holding its cellulose fibers together break apart. These broken bonds give the cell wall greater flex ibility and expandability, so that more water can enter the cell by diffusion, causing the cell itself to elongate.
Auxins are also responsible for adventitious root development, secondary growth in the vascular cambium, inhibition of lateral branching, and fruit development. In fact, seedless fruits can be artificially created by applying synthetic auxin to plants; this causes fruit to develop even though fertilization (and thus seed formation) has not occurred.
The cytokinin hormones promote cell division and tissue growth, and depend upon the presence of auxins to determine the extent of their activity. When the ratio of cytokinins to auxins is relatively high, stem and leaf growth is stimulated. When, on the other hand, the ratio of cytokinins to auxins is relatively low, root growth is stimulated instead. The balance between these hormones ensures that the plant invests in both root and shoot growth, so that neither becomes too large or small for the other. Cytokinins are also involved in the development of chloroplasts, fruits, and flowers. In addition, they have been show n to delay senescence (aging), especially in leaves, which is one reason that florists use cytokinins on freshly cut flowers.
Gibberellins stimulate growth, especially elongation of the stem, and can also end the dormancy period of seeds and buds by encouraging germination. Additionally, gibberellins play a role in root growth and differentiation, and produce an enzyme that pro motes the conversion of an embryo's starchy food supply into utilizable sugars. In some plants, these hormones are essential for flowering and fruit development.
True to their name, inhibitors restrain growth and maintain the period of dormancy in seeds and buds. The most important of this type of hormone is abscisic acid, which in addition to restraining growth, causes the guard cells, of the stomata to close when the plant is losing too much water.
Ethylene is best known for controlling the ripening of fruits. It also contributes to the senescence (aging) of plants by promoting leaf loss (termed leaf abscission) and other changes. Ethylene can bring buds and seeds out of dormancy, initiate flo wer development, and promote radial (horizontal) growth in roots and stems. Interestingly, in certain circumstances ethylene can also act as a growth inhibitor, most often in conjunction with auxin.