The Control of Tillering in the Barley Plant II. The Control of Tiller-Bud Growth During Ear Development

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Abstract
The interaction between grain number and the level of mineral nutrient supply in the control of tiller-bud elongation has been examined in two varieties of barley (cv. Piroline, free tillering cv. CI. 3576, restricted tillering). It was found in Piroline that the level of the nutrient supply did not alter the effect of the grains, although a considerable increase in tillering at all levels of grain removal resulted from an increase in the mineral nutrient supply. In CI. 3576, increasing the nutrient supply decreased the influence of the grains although it required an extremely high nutrient supply to completely suppress the grain effect. The hypothesis that tillering in this variety was primarily controlled by competition between the grains and tiller buds for a limited nutrient supply was supported by an examination of the nitrogen contents of the grains and emerging tillers. The leaves were found to be without influence on the number of tiller buds which elongated except under conditions of reduced light intensity. Removing green leaves on the main shoots reduced tiller dry weight, however, indicating that carbohydrates from the mature leaves contribute to the growth of the tillers, at least in the early stages of growth. The mature shoots on an intact plant have been shown to influence the elongation of tiller buds on other shoots on the same plant. This effect is dependent upon a vascular connection between the tillers and is additional to intershoot competition for light or nutrients in the rooting medium. The nature of the substances moving between shoots in this manner is unknown, but indirect evidence indicated that foliar-applied nitrogen could be so translocated. The data are discussed in the light of current theories of apical dominance.