Auxin
Auxin, any of a group of plant hormones, substances naturally produced in actively growing parts of plants that regulate many aspects of plant growth and development. These substances affect the growth of the stem tip, leaves, and roots, and the development of side branches and fruit. Auxins affect the growth of these plant parts by stimulating certain cells to elongate, or lengthen, and by inhibiting the growth of other cells, depending on the amount and distribution of auxin in the plant tissue.
The effect of auxin on plant cells is important in controlling plant functions called tropisms. A tropism is a plant’s response to external stimuli that causes a change in the direction of the plant’s growth, such as bending, turning, or curving. When an indoor plant is placed in a sunlit window, the plant appears to bend or grow toward the sun. This response to the stimulus of light is called phototropism. It is believed that light destroys auxin where it strikes the stem, causing an imbalance in which the side of the stem that receives less light has more auxin. Because more auxin is present, the cells on the darker side are able to elongate more than the cells on the lighted side, causing the plant to bend toward the light.
Geotropism is the response of plants to gravity. If a growing plant is placed on its side, the stem will tend to bend upward and the roots will tend to bend downward. As with phototropism, this is caused by an imbalance in the distribution of auxin. When the plant is horizontal, the force of gravity causes the auxin to move to the underside of the stem. Because of the increased amount of auxin, the cells on the underside of the stem elongate more than the cells on the upper part, causing the stem to turn upward. In the roots, gravity also causes auxin to move to the underside. However, in roots, the increased auxin inhibits the elongation of cells. Thus the cells on the upper side elongate more and the roots turn downward.
Indoleacetic acid, the most common auxin, is usually formed near the new stems at the top of a plant and flows downward to stimulate the elongation of newly formed leaves. Scientists have developed chemical compounds, called growth substances, based on naturally occurring auxins. These synthetic growth substances, in the form of sprays or powders, are used to slow the sprouting of eyes, or buds, on stored potatoes, kill broad-leaved weeds, and prevent the premature falling of fruit and flower petals. Growth substances are also used to produce seedless fruits, such as tomatoes, figs, and watermelons, and to stimulate root growth in plant cuttings.
The effect of auxin on plant cells is important in controlling plant functions called tropisms. A tropism is a plant’s response to external stimuli that causes a change in the direction of the plant’s growth, such as bending, turning, or curving. When an indoor plant is placed in a sunlit window, the plant appears to bend or grow toward the sun. This response to the stimulus of light is called phototropism. It is believed that light destroys auxin where it strikes the stem, causing an imbalance in which the side of the stem that receives less light has more auxin. Because more auxin is present, the cells on the darker side are able to elongate more than the cells on the lighted side, causing the plant to bend toward the light.
Geotropism is the response of plants to gravity. If a growing plant is placed on its side, the stem will tend to bend upward and the roots will tend to bend downward. As with phototropism, this is caused by an imbalance in the distribution of auxin. When the plant is horizontal, the force of gravity causes the auxin to move to the underside of the stem. Because of the increased amount of auxin, the cells on the underside of the stem elongate more than the cells on the upper part, causing the stem to turn upward. In the roots, gravity also causes auxin to move to the underside. However, in roots, the increased auxin inhibits the elongation of cells. Thus the cells on the upper side elongate more and the roots turn downward.
Indoleacetic acid, the most common auxin, is usually formed near the new stems at the top of a plant and flows downward to stimulate the elongation of newly formed leaves. Scientists have developed chemical compounds, called growth substances, based on naturally occurring auxins. These synthetic growth substances, in the form of sprays or powders, are used to slow the sprouting of eyes, or buds, on stored potatoes, kill broad-leaved weeds, and prevent the premature falling of fruit and flower petals. Growth substances are also used to produce seedless fruits, such as tomatoes, figs, and watermelons, and to stimulate root growth in plant cuttings.
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