Auxins are a group of plant hormones that are involved with the growth of many different types of tissues, roots, new shoots, flowers and fruit. The name auxin comes from the Greek word "auxein," which means "to grow." The rooting hormone powder that is used to promote root growth in cuttings is an example of an auxin. The first auxin to be identified by scientists was indole-3-acetic acid (IAA).
Auxin and Cellular Elongation
When a new shoot bends in the direction of the sun or a bean plant curves around a pole, the cells on one side of the stem grow longer because the pressure of the liquid inside the cell increases. The auxin IAA acidifies the cell by creating more positive hydrogen ions (H+) within the cell, causing the cell wall to weaken, allowing it to elongate. When all the cells on one side of the shoot are stimulated to lengthen, the stem curves toward the other side.
Auxin and Root Growth
Auxin stimulates the growth of roots, both the creation of new root tissue from the nodes of a stem and the branching of these roots. It causes roots to grow downward in response to the pull of gravity. Auxin also plays a role in promoting cell division in the cambium, the green layer between the bark of roots and branches and the wood inside. In addition, it plays a role in directing cells to become either xylem, water-conducting tissue, or phloem, nutrient-conducting tissue.
Auxin and Flowers
Auxin plays a role in the development of flower tissue and promotes flowering in bromeliads, as well as the growth of female flowers in the plants where individuals are either male or female. The seeds in a fruit produce an auxin that stimulates the ovary of a flower to develop into a fruit, but auxin also can delay fruit ripening.
Auxina and Apical Bud Dominance
The auxin produced by the bud at the tip of a shoot suppresses the growth of the buds at the base of the leaves along that shoot. When the tip is removed and the auxin production stopped, two or more new side shoots will develop along that branch. This process, often called "pinching back," creates a more compact, bushier plant.
Auxin and Fall Leaf Drop
Growing leaves produce auxin that is transported to the stem and retards aging. When a plant is stressed by drought or lack of nutrients, or receives less light because of the shortening of the days in the fall, the leaves produce less auxin, enzymes are produced at the base of the leaf that dissolves the cell walls, the cells separate and the leaf falls.
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