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Plants, like animals, rely on a number of hormones to regulate their growth and development. Auxins are a class of hormones found in plants that stimulate (and occasionally hinder) development.
What is Auxin?
Plants generate auxins in their meristems (meristems are explained on successive pages). Auxins are important for stimulating cell elongation, which is necessary before a cell can differentiate. It achieves this by increasing the flexibility of the cell, allowing it to deal with the increased amount of water taken in by the cell. Indole acetic acid is one of the most prevalent auxins.
Auxins: Indole Acetic Acid (IAA)
Indole Acetic Acid has an effect on the plant’s root and shoot tips, as explained below.
1. Shoot Tip: IAA stimulates plant development in the shoot region regardless of concentration (though higher concentrations promote growth more).
2. Root Tip: High quantities of auxin restrict root growth, whereas modest levels of indole acetic acid are sufficient to stimulate root growth.
Auxins and Phototropism
Auxins are also involved in phototropism, which occurs when plants bend or move away from light. Because auxins are present in the shoot tip, it is responsible for the plant’s directional movement in reaction to sunlight.
Auxin is destroyed by sunshine, thus the portion of the plant’s shoot tip that receives direct sunlight will have the least amount of auxin.
After this lop-sided growth, the additional auxin present on the shaded side encourages greater cell division and elongation, leading the plant to bend towards the sunshine.
Auxins and Geotropism
Geotropism is a kind of phototropism in which the plant grows in a directed pattern in response to gravity. Negative geotropism (growth against gravity) is shown at the shoot tip, but positive geotropism is seen at the root tip (grows in the same direction as gravity).
Auxins and Apical Dominance
Auxins present in the plant’s lateral regions (between the root and the shoot tip) inhibit lateral growth. The lack of ‘diffusable’ auxins implies that they cannot hinder development in these lateral regions if you chop off a plant’s shoot tip. Apical dominance is the term for this.
Auxins and Leaf Abscission
Leaf abscission is also prevented by the presence of auxins in the lateral regions. Auxin concentrations and photosynthetic rates decrease as the weather becomes cooler. Because of the absence of auxin in the lateral regions, an abscission layer forms at the stem of the leaf, weakening its attachment to the plant and causing it to fall off.
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