THE CONTROL OF TILLERING IN GRASSES BY AUXIN

Abstract

THE PHENOMENON of tillering, which is the development of axillary buds of grass plants into branch shoots or tillers, has been shown to be affected by nutrition, soil moisture, seed size, competition, injury, day-length, temperature, depth of planting and transplanting (cf. review of Gardner, 1942). In the light of the work of Thimann and Skoog (1933, 1934) on the inhibition of buds of Vicia Faba by auxin, it would seem likely that tillering in grasses would be controlled by auxin just as branching in dicotyledonous plants is. The following experiments represent an attempt to test this view. METHODS.-Experiments were carried on with plants of two grasses, Wintex barley and Chalco teosinte, grown in a rich garden soil in a greenhouse during the fall and winter months in conjunction with experiments on flowering reported elsewhere (Leopold and Thimann, 1949). Both of these grasses form tillers abundantly in the field. The plants were germinated and grown in a 16hr. day except where other day-lengths are specified. Daylight was extended to 16 hr. by the use of filament lights and in some cases shortened to 10 hr. by the use of double cloth curtains. Night temperatures were kept above a minimum of 18?C. At the age of 3-5 weeks, groups of ten plants were given each of the treatments described. Various water-soluble chemical agents were applied to the plants through the leaves. The youngest mature leaf was cut off about half way down the blade, and the stump was introduced into a small vial containing 1 ml. of the solution being applied. The solution was generally entirely taken up by the plant within 7 days. Control plants in each experiment were treated with similar amounts of water alone. Three or 4 weeks after treatment each plant was dissected and the number of tillers counted. For the purposes of this study, a tiller is defined as an elongated lateral bud which has developed sufficiently to turn leaf-green. Ordinarily not more than two tillers were formed per plant under the conditions of the experiments. EXPERIMENTS.-The role of the apical bud.Since the apical buds are normally the source of the auxin which inhibits lateral buds in dicotyledonous plants, it would seem likely that apical buds might play the same role in grasses. If this is the case, then removal of the apices should cause tillering, and substitution of auxin for the apices should prevent tillering. Apical buds were therefore destroyed in situ with a needle in twenty plants each of teosinte and of barley. Half of these were then injected in the position of the apex by means 1 Received for publication October 8, 1948. It is a pleasure to acknowledge the help of Professor K. V. Thimann in the organization of the study and preparation of the manuscript. of a hypodermic needle with about 0.1 ml. of solution containing 400 mg. per liter of a-naphthaleneacetic acid. The injection was repeated at weekly intervals for 3 weeks. The number of tillers produced after the fourth week is shown in table 1.