Plant hormones are small molecules that control a plant's life cycle, from the moment the seed is placed in the ground to the day when a bloom turns to seeds. Hormones dictate everything, from the length of the stem to the size of an apple the branch can accommodate. Hormones influence a plant's life cycle, and are influenced by external conditions like climate, rainfall, hours of sunlight, and even soil conditions. For example, if in the first generation a plant (the genotype) is gradually acclimated to cold, the phenotype ? the gene exhibited in the second generation - may produce a cold-hardy plant.



In humans, the thyroid hormone ? produced from a gland in the neck ? regulates metabolism. Plants don't have a circulatory system, so the hormones they produce don't have an immediate effect. Instead, these hormones work gradually, at minute doses, to modify the processes of growth, development and reproduction.

There are five recognized classes of plant hormones (of course, the science is still close to its infancy, like particle physics, and there may be 500 classes for all we know). Some are a single compound; others are made up of several different compounds. All are organic, but they are not plant nutrients even though their effects may mimic the functions of nutrition. Their complete functions are so numerous and scientifically complex they would fill a book, but here is an abbreviated version

Gibberellins In 1930, Japanese scientists discovered a fungus that made plants shoot up like Jack's beanstalk. When they examined the fungus, they discovered that it was producing gibberellins. Since then, more than 70 types of gibberellins have been identified, some more potent than others. As a class, the gibberellins ? working with auxins, another hormone - promote growth, primarily in stalks and leaves, and in the development of fruits. Gibberellins have little effect on root growth, but are abundant in seeds and trigger germination when saturated. The name comes from Greek and means,?to grow.?- Like gibberellins, auxins are compounds, not a single substance. The first recognized auxin was indoleacetic acid. All auxins contribute to growth, or cell elongation in stems. Auxins affect other plant processes, but this ability to stimulate growth is the critical component when classifying an auxin. Unlike gibberellins, auxins also stimulate root growth on cuttings, cause plants to bend toward light sources, promote apical (or central stem growth) and suppress lateral bud growth, set fruit or delay ripening, delay or retard leaf shedding in response to autumn, and promote flowering in Bromeliads and succulents.

Cytokinins

Cytokinins are derived from adenine, a component of nucleic acid, and are ? like the previously mentioned hormones ? complex compounds (more than one substance). They are essential to mitosis, the process a plant uses to produce new cells. Cytokinins are also essential to chloroplast development, and without chloroplast plants could not perform photosynthesis ? the plant equivalent of eating food. Cytokinins also act on meristematic cells (the plant equivalent of human stem cells), telling the plant when to grow a branch or a root, or what kind of branch or root to grow. The growth of leaves, and their eventual loss before winter, is also controlled by cytokinins.

Brassinosteroids

Brassinosteroids, or Brassins, are a group of plant hormones, or plant growth regulators. First discovered in rapeseed pollen, brassins are to plant metabolism what food and air are to humans.

They are released when a plant experiences stress, as in lack of nutrition, pests, root distress, or disease. They look remarkably like cortisol, the hormone mammals produce when faced with stress, and act in similar fashion. Besides metabolism, brassins also enhance stem growth, delay or prevent leaves from falling, and induce plant stems to bend toward light.

Abscisic Acid

Unlike gibberellins, auxins, and cytokinins, abscisic acid is a single compound. It is primarily an inhibitory hormone ? preventing shoots from growing, closing stomata in the absence of water, controlling dormancy ? but can also promote functions like gene transcription if a plant is injured, or the synthesis of proteins in seeds that face adverse environmental conditions.

Jasmonic acid

Jasmonic acid is a growth regulator. It inhibits growth in adverse conditions, and stimulates tuber formation; jasmonic acid is the main reason a potato becomes a potato. Jasmonic acid is the plant equivalent of an amino acid - the building blocks of life in the animal kingdom.

Ethylene

Ethylene is a gaseous hormone, the only one in the plant world (and the only one in the animal world, too, as far as we know). It is hydrocarbon-based, and derived from the amino acid methionine. It is used commercially for ripening fruit. Synthetic compounds based on the same formula are mixed with water and sprayed on fruit to ripen it, on trees to prevent leaf fall, and on seedlings to hasten development.