Coordination in Plants

Plant growth responses are coordinated by hormones such as auxins, gibberellins, cytokinins and florigens.

Tropisms are growth responses, which are mostly influenced by the hormone auxin.

Auxins

Auxins are a group of plant growth hormones.

One of the commonest auxins is Indole-Acetic Acid (IAA).

Auxins are produced at the apical meristems of shoots and roots.

The auxins move backwards to the region of cell elongation where they exert their effects.

The movement of auxins is by diffusion from cell to cell in one direction. They are translocated in the phloem from the shoots towards the roots.

There is higher production of auxins in shoots compared to the roots.

IAA stimulates cell elongation in both shoots and roots. They also stimulate cell division in cambium.

The auxin concentration that will stimulate shoot growth inhibits root growth. This is illustrated in the figure below;

Hence:

• Low auxin concentration stimulates growth of roots but inhibits shoot growth.
• High auxin concentration stimulates growth in shoots but inhibits root growth.