The five most important chemicals or phytohormones that are produced by plants to facilitate various functions are as follows: 1. Auxins 2. Cytokinins 3. Gibberellins 4. Ethylene 5. Abscisic Acid.
Phytohormone # 1. Auxins:
Auxin, indole-3-acetic acid (IAA), was the first plant hormone identified. It is produced primarily in the shoot tips and in developing flowers and seeds. Its transport from cell to cell occurs through the parenchyma. Auxins alone or in combination with other hormones are responsible for many factors of plant growth.
It activates the differentiation of vascular tissue in shoot apex, initiates division of the vascular cambium in the spring; promotes the growth of vascular tissue in healing of wounds. It activates cellular elongation by increasing the plasticity of the cell wall.
It promotes initiation and growth of roots and the growth of fruits. It suppresses abscission (separation from the plant) of fruits and leaves (lowered production of auxin in the leaf is correlated with the formation of the abscission layer). It controls aging and senescence, and dormancy of seeds.
Senescence is the biological aging or the gradual deterioration of function characteristic.
Phytohormone # 2. Cytokinins:
They derive their name from their function in cell division (cytokinesis). Cytokinins are found in sites of active cell division in plants—for example, in root and shoot tips, seeds, fruits, and leaves.
They are transported by xylem cells and work in the presence of auxins to promote cell division. Lateral bud development is promoted by cytokinins. Cytokinins also delay the senescence of leaves and promote the expansion of cotyledons.
Phytohormone # 3. Gibberellins:
The gibberellins are widespread throughout the plant kingdom, and more than 75 have been isolated. Gibberellic acid three (GA3) is the most widespread and most thoroughly studied hormone.
The gibberellins are abundant in seeds and young shoots, where they control stem elongation by stimulating both cell division and elongation. The gibberellins are carried by the xylem and phloem tissues.
Phytohormone # 4. Ethylene:
Ethylene is a simple gaseous hydrocarbon produced from an amino acid and appears in most plant tissues in large amounts when the plants face stressful conditions. It diffuses from its site of origin into the air and affects surrounding plants as well.
Large amounts are produced by roots, senescing flowers, ripening fruits, and the apical meristem of shoots. It regulates opening of a flower, ripening of fruits, shredding of leaves, induces seed germination and root hair growth.
Phytohormone # 5. Abscisic Acid:
Abscisic acid (ABA), despite its name, does not initiate abscission (shredding). In 1960s, it was named as Abscisic acid as the botanists thought that it initiates abscission. It is synthesized in plastids and diffuses in all directions through vascular tissues and parenchyma. Its principal effect is inhibition of cell growth. ABA increases in developing seeds and promotes dormancy.
The movement can be autonomic that is instant response or paratonic which arises in higher plants.