AUXINS AND THE INHIBITION OF PLANT GROWTH
Authors: THIMANN KENNETH V. 
Published: January 2008

Inhibition of growth is widespread in the plant kingdom, Just as the promotion of growth is controlled by the growth hormones, comprising principally the group known as the auxins, so also inhibitions are, in many cases at least, due to the action of auxins.

The application of very high concentrations of auxin inhibits the growth of shoots directly. Such concentrations retard the rate of protoplasmic streaming and are close to the range at which these substances are definitely toxic. Another effect which results from very high concentrations of auxins, and is perhaps of a more indirect nature, is the inhibition of the growth of parts morphologically above the point of the auxin application. Whether or not these phenomena of very high auxin concentration have any bearing upon normal growth inhibitions is still not clear. They may play a part in pathological inhibition.

The inhibiting effect of the root tip upon the growth of the root may be readily imitated by application of very low concentrations of auxin, probably of the order of those present in the tip. Hence this inhibition, where it occurs, is due to the auxin coming from the root tip, A somewhat lower range of auxin concentrations accelerates root growth. These effects are observable on isolated roots. The response of roots to auxin can thus be represented by an optimum curve with the peak at very low concentration. The growth of roots which have been inhibited may become accelerated when the auxin is removed; this may lead to a definite acceleration of shoot growth.

The inhibition of the development of lateral buds by the terminal bud of growing shoots can also be quantitatively imitated by the application of auxin in concentrations not much greater than those which are produced by the terminal bud. Hence this inhibition, the release of which comprises the basis of pruning, is due also to auxin, and in fact auxin is produced in rather large amount in young terminal buds of the majority of plants. The inhibition of buds by the leaves in whose axil they stand is similarly caused.

Not only buds, but also young developing shoots may be inhibited by another shoot or by auxin in suitable (physiological) concentration. The adventitious outgrowths on fern prothallia are subject to a similar auxin inhibition. In tubers the inhibiting action of one bud upon another is, at least in part, also an auxin effect.

Differences in bud inhibition between sympodially and monopodially growing trees, between normal and dwarf forms, or between related species of different growth habit, may be, and in some instances have been satisfactorily explained by, differences in the rates of auxin production or consumption. This concept unifies a scattered and diverse group of observations on plant behaviour.

However, the paradox that auxin, which typically promotes growth by cell enlargement in shoots, should inhibit growth of buds and of roots, has not been satisfactorily explained. The divergent views on bud inhibition have engendered nine theories. The principal point at issue is whether the inhibition is due to the auxin itself or to some effect of auxin on the production or movement of other substances. None of these theories is entirely adequate to explain both the inhibition of buds and that of young growing shoots, though it is still possible that one of four may be established by further study. It is pointed out that the inactivation of auxin in inhibited parts may play an important role which has not yet been considered.