Lateral bud outgrowth and its control by the apex

In many plants, the outgrowth of lateral buds can occur after the apex is removed. In order to analyze how the apex communicates with and controls the development of the bud below, the pertinent literature from the last 5 to 10 years has been reviewed. Decapitation results in an increase in bud length (after a lag period of only 6 to 8 hours in peas) as well as an acceleration of cell division. Similar changes can be induced on intact plants by applying cytokinins and, in certain circumstances, by gibberellins. It has been further shown that within a few hours after cytokinin application, lipid, RNA and DNA synthesis are all accelerated. Growth stimulations are also seen under a variety of environmental modifications such as nitrogen supplements, high light intensities, and the transition from water stress to adequate water levels. Growth of lateral buds on decapitated plants can be inhibited by indoleacetic acid, ethylene, and abscisic acid. The wide variety of factors affecting bud growth makes it difficult to determine which factor, if any, might be an endogenous regulator. Removal of the roots, a source of cytokinins and gibberellins, does not markedly affect bud growth, and cytokinin levels actually decrease after decapitation. Retardation of purine biosynthesis by hadacidin, however, inhibited bud growth and the effect could be reversed by adding cytokinins. It is presumed that auxin from the terminal organs participates in the control of apical dominance. While the inhibition of bud growth by auxin can be mimicked by ethylene, the evolution of this gas is not retarded during bud growth. Abscisic acid levels, however, markedly decrease as the lateral bud begins to grow. Because the apex or applications of auxin can be shown to mobilize important nutrients away from the lateral buds, the nutrient supply of the buds has been implicated as a regulatory factor. Levels of nitrogen, for example, increase in the bud after decapitation on a per bud basis but not, however, when expressed as per mg dry weight. A possible relationship between nutrients and hormonal levels is discussed. A sequence of events between decapitation and lateral bud outgrowth can not yet be proposed. It is suggested that each factor capable of affecting apical dominance be investigated individually while other factors are controlled. Measurements should be made at short time intervals so that agents involved with the onset of bud growth can be separated from events contributing to the