A naturally occurring biochemical in plants, sometimes called a plant growth substance, plant regulator or plant hormone. Auxin is involved in the regulation of numerous life processes. It is made principally in the shoot apex and is transported towards the base of the shoot, influencing cell extension and the direction of shoot growth, inhibiting the growth of axillary buds and, in concert with cytokinin, regulating apical dominance. It can also promote adventitious rooting.

This definition is abridged from A – Z of tree terms: A companion to British arboriculture.
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