Plant hormones are natural chemicals produced in minute quantities in one part of a plant and have a physiological effect when moved to another part of the plant; a chemical messenger. Hormones control growth or other physiological functions such as root initiation, flowering, fruit drop, etc. Unlike animal hormones, phytohormones are not produced in individual organs, but are produced by every cell at various times during a plant’s growth cycle. They are necessary for communication in plants because plants do not contain nervous systems. Naturally-produced hormones are properly called hormones or phytohormones. There are three major groups of plant hormones:
Auxins are plant hormones that influence cell enlargement, root initiation, and bud formation. They suppress the initiation of lateral buds.
These messengers cause cells to elongate. When cells elongate on one side of a plant only, they cause the plant to bend. The elongation is irreversible and widely used in horticulture and agriculture. Perhaps one of the most familiar uses is in the elongation of green grapes, which provide growers with more fruit substance per bunch.
The common rooting hormones available in most garden stores all contain auxins in minute concentrations. Auxins, usually indole-butric acid, are formed naturally in fruits, seeds, pollen, growing points, young leaves and developing buds. Auxins cause cells to grow and especially to elongate & auxins promote apical dominance.
Keikiroot with a minute concentration of the auxin has been found advantageous for orchid root initiation and development.
Cytokinins are plant hormones that are involved in cell division, shoot multiplication and axillary bud proliferation (buds developing in the leaf axils). They help delay senescence (aging). If plants are too spindly, increased cytokinin will help foster shorter, thicker stems.
Rather than affect cell growth, such as elongation or inhibiting dormancy, cytokinins promote the change in a standard plant cell to a cell that becomes a stem, a root, a leaf or a flower.
It is this group of hormones that is of special interest to orchid growers. Cytokinins cause cells to differentiate into stems, leaves and eventually flowers.
Cytokinins suppress this tendency of plants to reach for the light.
c) Gibberelic Acid or Gibberellins
Gibberellins are a group of naturally occurring substances that influence cell enlargement and stem elongation. Gibberellins also supplement the actions of auxins and cytokinins.
These hormones are isolated from many different plants, but primarily fungi and are used to increase the growth of many plants and seedlings because they function like plant hormones in stimulating the growth of roots, leaves and stems, the germination of seeds, etc. Now over fifty gibberellins have been isolated and are used commercially, primarily for stem elongation and seed germination. They have been used to induce flowering by breaking dormancy and they can be used to offset a plant’s requirements of long day exposure or periods of low temperature in order for it to flower.
d) Minor Groups of Hormones / Plant Regulators
There are now many compounds, including enzymes which have been isolated and identified. Little experimental data is available about them and certainly little experimentation has been conducted on orchids. A few have achieved a status where more experimentation should be attempted. A few examples may illustrate their effects on plant growth.
Florigens: These are thought to stimulate the flowering of plants. They are now usually referred to as anthesins. Dormins: These compounds are thought to inhibit growth. Perhaps they have use in producing miniature plants for light culture. Ethylene: This is probably the best known member of this minor category because it has many uses including many not related to plant growth. It is a colorless, flammable, gaseous hydrocarbon obtained from petroleum and as a plant regulator it is useful for the ripening of fruits. Green bananas, for example, are sprayed by the shipload so that they ripen by the time the ship arrives at its destination. Many tropical gardeners will be familiar with the “trick” of getting bromeliads to bloom by confining the plant in a plastic bag with a well- ripened apple. The apple, as it ripens produces ethylene and the bromeliad “ripens” or produces a flower.