How plant hormones affect growth.

Auxins The name comes from the Greek word auxein which means “to grow” These are produced in the tips of shoots and roots. Auxin is water soluble Auxin causes elongation of cells promoting growth

Auxin Cont It is auxin which allows plants to move or bend in response to light. The main auxin is called Indole Acetic Acid or IAA. Artificial auxins can be used as weed killers.

The Effect of Auxin on Shoots and Roots. This depends on the concentration of auxin. Low concentrations stimulate roots, while high concentrations inhibit growth. Low concentrations stimulate growth of lateral buds, while high concentrations inhibit growth. Low concentrations do not stimulate the stems, while high concentrations do.

The Effect of Auxin on Shoots and Roots. Ref pg 16

Apical Dominance The auxin from the top of the plant, the apical meristem is produced in relatively high conc, so the shoot is stimulated. However the lateral buds remain dormant as high concs of auxin inhibit their growth. As the auxin diffuses down the plant it becomes less concentrated, and is inactivated by enzymes and hormones in the lower part of the plant.

Apical Dominance Cont Near the bottom of the plant the concentration of auxin is low enough to stimulate the growth of lateral buds. This causes the triangular shape of trees such as conifers. The process is known as Apical Dominance Ref pg17

Geotropism in Seedlings When the shoot and root emerge from a seed, gravity receptors come in to play. These cause an uneven movement of auxin from the top of the shoot or root to the bottom, so the conc of auxin on the underside is greatest. Ref pg 18

Geotropism in Seedlings Higher conc of auxin stimulates the shoot, so the underside elongates the most and causes the shoot to curve upwards. Higher conc of auxin inhibits the root, so the top side grows faster and so the root curves downwards.

Other Effects of Auxin Auxin encourages root development even on stems and leaves. This is important in the propagation of new plants. IAA, along with other hormones is involved in abscission. IAA appears to delay this abscission – used by fruit growers to kepp fruit on trees until it is ripe.

Other Effects of Auxin Auxin stimulates the growth of cambium when a tree stem is under stress – if a branch is in danger of breaking due to heavy fruit or leaves, the cambium lays down more wood on one side to strengthen the branch. Suppression of root elongation. Initiation of flowering in some plants.