Plant Growth continued

Water Plants require water for growth, temperature regulation, and for support. Remember that water is one of the reactants needed for photosynthesis. A plant that does not have sufficient amounts of water will wilt. Plants that receive too much water actually prevent the roots from taking in oxygen. The root cells require oxygen for cellular respiration and they get the oxygen from tiny pores in the soil. If the soil is drenched with water, the root hairs are not able to take in oxygen.

Activity Why Are my Plants turning Yellow?

Plant Hormones You have already learned the effect that the plant hormone gibberellin has on germination. There are two other groups of plant hormones that control the cell division and growth in plants: auxins and cytokinins. Two additional hormones called abscisic acid and ethylene also alter rate of cell division and growth.

Auxins You probably have noticed that plants grow and gravitate towards light. You can see that effect if you have ever placed a plant near a window or walked through a forest and noticed the shapes of stems and branches of trees. Auxin is a plant hormone that is released from a plant tip when the cells are exposed to light. Auxin is released and it travels to the cells of the stem that are not exposed to light.

This causes the cells on the dark side to elongate and as a result, the stem bends towards the light. In the root, auxin is released from the apical meristems and it causes the roots to grow downwards away from the light. If a root cell is exposed to light, auxin will cause these cells to elongate downward away from the cell. The cells that are not exposed to sunlight do not elongate.

The height of trees is also affected by the presence of auxins. Some trees release auxin from the highest bud which inhibits the growth of buds that appear below. Plants that have this feature usually grow tall and straight. Plants that do not release auxin from their terminal bud are usually shorter and bushier. Some trees show apical dominance through the early development and then lose the feature as they mature. The result is a beautiful shade tree that is tall and provides a nice reprieve from the sun.

In the spring, auxins are present in the leaves of deciduous trees, but the tree stops producing auxin once the growing season has ended. Without the presence of auxin, the leaves can no longer stay attached to the tree and fall off. Auxin is also commercially available and can be applied to promote root growth in cutting, prevent fruits from falling off of trees, inhibit stored potatoes from sprouting, and to produce seedless tomatoes and watermelons.

Gibberellins are produced in the apical meristems and work with auxins to encourage the growth of roots and stems. By itself, gibberellin stimulates the growth of leaves, flowering, and the development of vascular tissue. Commercial gibberellin promotes the growth of fruits and vegetables, it delays the ripening of citrus fruits, and it can speed up the flowering of strawberries.

Cytokinins This hormone is important in the development of specialized plant cells and tissue. As well, it also promotes rapid cell division. This hormone is produced in the roots of plants and is transported throughout the plant. Commercially cytokinins are used by florists to keep cut flowers looking fresh. Cytokinins prevent plant cells from aging.

Other Hormones Abscisic acid is a hormone that is produced in green leaves, fruits, and root caps. It prevents the germination of seeds, inhibits the growth of buds, and prevents the stomata from taking in carbon dioxide. Ethylene is gas that is naturally produced by fruits. It stimulates the aging of plant tissue and increases the ripening of fruit. This gas is a problem when fruits are picked and are shipped to other areas. The ethylene gas causes the fruit to ripen quickly.

Homework plant-an-annuals-garden4.htm plant-an-annuals-garden4.htm Look through each of the series of animations and complete the exercises: mations/content/plantgrowth.html mations/content/plantgrowth.html htm htm