Biology Super Review by Editors of REA;

Author:Editors of REA;
Language: eng
Format: epub
Publisher: Research & Education Association
Published: 2013-08-15T00:00:00+00:00

8.10 Plant Hormones

A) Auxins – These plant growth regulators stimulate the elongation of specific plant cells and inhibit the growth of other plant cells.

B) Gibberellins – In some plants, gibberellins are involved in the stimulation of flower formation. They also increase the stem length of some plant species and the size of fruits. Gibberellins also stimulate the germination of seeds.

C) Cytokinins – Cytokinins increase the rate of cell division and stimulate the growth of cells in a tissue culture. They also influence the shedding of leaves and fruits, seed germination, and the pattern of branch growth.

Problem Solving Examples:

What are plant hormones? Discuss the important effects of the auxins.

Plant hormones, like animal hormones, are organic substances which can produce striking effects on cell metabolism and growth even though present in extremely small amounts. They are produced primarily in actively growing tissues. Plant hormones usually affect parts of the plant body somewhat removed from the site of their production. Movement of plant hormones to target regions of the plant is made possible by the presence of phloem.

Auxins may be regarded as the most important of the plant hormones, since they have the most marked effects in correlating growth and differentiation to result in the normal pattern of development. The differential distribution of auxin in the stem of the plant as it moves down from the apex (where it is produced) causes the plant to elongate and bend toward light. Auxin from seeds induces the maturation of the fruit. The auxins, in addition, determine the growth correlations of the several parts of the plant. They inhibit development of the lateral buds and promote growth of the terminal bud. Finally, auxins control the shedding of leaves, flowers, fruits, and branches from the parent plant.

Discuss the important effects of gibberellins and cytokinins on plant growth.

Gibberellins and cytokinins are two major types of plant hormones that have been identified in addition to auxins. They interact with auxins and with each other to regulate biological activities in plants. Gibberellins and cytokinins have dominant roles in controlling the early phases of growth and development; auxins become dominant later in controlling cell elongation.

Gibberellins function to lengthen stems, stimulate seed germination, induce flower formation, and increase the size of fruits in some species of plants. The former two are their most vital functions. Seed germination is examined here: Just before germination, the embryo of a seed secretes a gibberellin which induces the production of an enzyme which hydrolyzes the stored starch for energy. The hormone also activates other enzymes of the seed which break down the materials of the seed coat.

Cytokinins function in stimulating growth of cells and accelerating their rate of division. Cytokinins can also change the structure of plant tissue: at certain concentrations, cytokinins are shown to cause root and shoot formation in plant tissue cultures. Cytokinins are also found to oppose auxins by causing lateral buds to develop and thus modifying apical dominance. In addition, cytokinins have been demonstrated to prevent leaves from yellowing and hence play a role in delaying senescence.

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