Plant Hormones That Enhance Stem Elongation and Fruit Growth

Plant hormones are organic substances produced naturally within the plant, controlling growth and other physiological functions at a site away from its place of production and active in minute amounts. Plant hormones decide the formation of stems, leaves and flowers. They are also responsible for the shedding of leaves and the formation and ripening of fruits. There are many synonymous terms of the word "hormone," namely growth hormone, growth regulator, growth substances, phytohormones and formative substances.

Historical Background

• The discovery of plant hormones dates back at the time of Charles Darwin, when, in 1881, he observed that when any coleoptile tip was subjected to unilateral illumination, the tip bends toward light. This observation led to further research and ultimate discovery and isolation of the hormone called "Auxin a" (auxenotriolic acid) from human urine. Later, scientists discovered "Auxin b" and "Heteroauxin," which is essentially indole acetic acid (IAA). At present, several plant hormones have been identified and isolated from plants.

Auxins

• Auxin hormone is produced by the tip of a coleoptile, i.e. the first leaf of a plant. Auxin occurs in small concentrations at the growing tips, and they act followed by translocation basipetally. Auxin plays an important role in organ differentiation. When a cut surface of a young stem is treated with a mixture of high percentage of auxin, an undifferentiated mass of callus tissues is developed. Auxin also helps in fruiting. It stimulates the ovarian tissue to form fruit. Abscission of leaves, flowers and fruits are controlled by auxin; hence, premature fall of fruit can be checked by the application of auxin just before or during the harvest season.

Gibberellins

• Gibberellins are present in germinating seedlings and also abundantly available in the growing tissues such as expanding cotyledon or leaf. Gibberellins are the natural growth regulating systems in higher plants. They are involved in some types of dwarfism of the plants, particularly in the growth elongation of dwarf corn, dwarf pea and elongation of rice seedlings. Its effect is primarily internode elongation. They are involved in the phenomena of dormancy, flowering, and fruiting. It causes the formation of male flowers whereas its lower concentration favors the formation of female flowers.

Cytokinins

• The hormone cytokinin was previously known as kinin, phytokinin and kinetin. It is universally used as genetic substances that promote plant cell division. They usually seem to influence plant growth by stimulating cell division, although, in some cases, they can only affect cell enlargement. Cytokinins have the ability to induce the formation of organs by undifferentiated plant tissues. In low concentrations, cytokinins cause root development, while higher concentrations lead to bud formation.

Florigen

• Florigen promotes flowering and travel from leave to bud or something that inhibits flowering, which is normally stored in the bud.