Auxins are produced by plant apices, i.e. root apex and shoot apex. They bring about cell elongation resulting in growth. They are diffusible substances which effect growth when in very small amounts.
Roots require lower concentrations than shoots. The effect of auxins on the growth of roots and shoots has already been discussed. Auxins also exert other effects on plant growth and development. There are various other chemical substances which have been shown to influence plant growth and development.
Effects of Auxin on Plant Growth
Apical Dominance
Auxins inhibit the growth of side branches. This is referred to as apical dominance. If the terminal bud is removed, side branches develop from the lateral buds. This knowledge is applied in pruning.
As long as the main stem is allowed to remain intact, the development of side branches is suppressed. Pruning the terminal bud removes the main sources of auxin, thus allowing side branches to sprout.
Growth of adventitious roots
Adventitious roots develop from the stem. Auxins stimulate the growth of such roots.
Parthenocarpy
This refers to the formation of fruits without fertilisation. This can be induced by treating unpollinated flowers with auxin. This phenomenon is applied in the development of seedless fruit varieties. Auxins, together with other plant hormones, are involved in secondary growth, falling of leaves and ripening of fruits.