Hormones are organic compounds that govern a plant's ability to grow, reproduce and adapt to changes in its environment. Plant hormones both stimulate and inhibit plant growth depending on the season and weather, hence they are often called growth regulators; two or more hormones acting together are often required to bring about a response. Plants produce growth five major groups of hormones naturally; botanists are now able to produce some of these synthetically.
Auxins govern phototropism, in which plants bend toward the light; it causes geotropism in which roots, in response to gravity, grow downward. It promotes apial dominance in which the top of a plant grows more than lateral branches. Auxin also governs the formation of flowers and the setting and growth of fruit. It promotions adventitious roots, those that form on stems, leaves and branches. Auxins produced in the terminal buds of a canopy stimulate the growth of roots. Cuttings are often dipped in rooting compounds containing auxin before they are planted.
A synthetic auxin, naphthalene, sprayed on tomato plants will increase the amount of tomatoes grown. Naphthalene sprayed on very young apples causes some drop off the trees and the remainder grow larger, yet when it is sprayed on maturing citrus fruits, apples and pears it prevents them from falling off the trees. A synthetic auxin 2,4-D kills dandelions and pigweed without harming grasses and grain crops. Agent Orange, the controversial herbicide used in Vietnam was a synthetic auxin.
Absicisic acid is a hormone that inhibits plant growth, inducing dormancy in the fall and preventing seeds from germinating. It causes flowers to drop in the spring and summer. Absicisic acid and another hormone, ethylene, work in combination to cause fruit and leaves to drop in the autumn. This hormone causes leaf pores called stomata to close to prevent the loss of water through transpiration, a critical function during periods of drought.
Cytokinins stimulate the division of growing cells. These hormones are often part of the media used to grow plants from tissue culture. If a growing media has more cytokinins than auxin, the growing plant will produce more shoots; if a growing media has more auxin than cyptokinin, the plant will grow more roots.
Ethylene is a hormone that exists as a gas. The shorter, cooler days of late summer and early autumn triggers ethylene that causes fruit to ripen and leaves to change color, droop and drop. A rotting apple produces ethylene gas which causes nearby apples to ripen, hence the saying “One bad apple will spoil the barrel.” Ethylene is used commercially to ripen fruits from bananas to melons and tomatoes. It produces more vibrant oranges and yellows in citrus fruit.
Gibberellins are hormones that break seed dormancy. Seeds that are difficult to germinate are often soaked in a solution containing giberellins. Gibberellins speed the germination of seeds and stimulate the division and elongation of growing cells. Gibberellins produced in the growing tips of roots stimulate the growth of the canopy of a tree. When they are sprayed on a plant it will usually grow taller.