Plant Hormones

The growth and development of a plant are influenced by genetic factors, external environmental factors, and chemical hormones inside the plant. Plants respond to many environmental factors such as light, gravity, water, inorganic nutrients, and temperature.

Plant hormones are chemical messengers that affect a plant's ability to respond to its environment. Hormones are organic compounds that are effective at very low concentration; they are usually synthesized in one part of the plant and are transported to another location.  They interact with specific target tissues to cause physiological responses, such as growth or fruit ripening. Each response is often the result of two or more hormones acting together.

Because hormones stimulate or inhibit plant growth, many botanists also refer to them as plant growth regulators. Many hormones can be synthesized in the laboratory, increasing the quantity of hormones available for commercial applications. Botanists recognize five major groups of hormones: auxins, gibberellins, ethylene, cytokinins, and abscisic acid.

Auxins

Auxins are hormones involved in plant-cell elongation, apical dominance, and rooting. A well known natural auxin is indoleacetic acid, or IAA which is produced in the apical meristem of the shoot.

Before a cell can elongate, the cell wall must become less rigid so that it can expand. IAA triggers an increase in the plasticity, or stretch ability, of cell walls, allowing elongation to occur.

Auxin moves to the darker side of the plant, causing the cells there to grow larger than corresponding cells on the lighter side of the plant. This produces a curving of the plant stem tip toward the light, a plant movement known as phototropism.

Gibberellins

In the 1920's scientists in Japan discovered that a substance produced by the fungus Gibberella caused fungus-infected plants to grow abnormally tall. The substance, named gibberellin, was later found to be produced in small quantities by plants themselves.

Gibberellins has many effects on a plant, but primarily stimulates elongation growth. Spraying a plant with gibberellins will usually cause the plant to grow to a larger than expected height, i.e. greater than normal.

Gibberellins are also used to treat seeds of some food crops because they will break seed dormancy and promote uniform germination.

Ethylene

The hormone ethylene is responsible for the ripening of fruits. Unlike the other four classes of plant hormones, ethylene is a gas at room temperature. Ethylene gas diffuses easily through the air from one plant to another.

Ethylene is usually applied in a solution of ethephon, a synthetic chemical that breaks down and releases ethylene gas. It is used to ripen bananas, honeydew melons and tomatoes.  

Cytokinins

Cytokinins promote cell division in plants. It is produced in the developing shoots, roots, fruits and seeds of a plant, cytokinins are very important in the culturing of plant tissues in the laboratory.  A high ratio of auxins to cytokinins in a tissue-culture medium stimulates root formation. A low ratio promotes shoot formation. Cytokinins are also used to promote lateral bud growth in flowering plants.

Abscisic Acid

Abscisic acid, or ABA, generally inhibits other hormones, such as the auxin IAA. It was originally thought to promote abscission, hence its name. ABA helps to bring about dormancy in a plant's buds and maintains dormancy in its seeds. ABA causes the closure of a plant's stomata in response to drought. Water stressed leaves produce large amounts of ABA, which triggers potassium ions to be transported out of the guard cells. This causes stomata to close, and water is held in the leaf. It is too costly to synthesize ABA for commercial agriculture use.  

Source: http://www.emc.maricopa.edu/faculty/farabee/biobk/biobookplanthorm.html 
http://www.biology-online.org/11/10_growth_and_plant_hormones.htm