Growth and Dormancy Cycles in Citrus Bud Cultures and Their Hormonal Control

An in vitro bud culture method was devised in order to better understand the control mechanism of Citrus bud development. This technique offers a new approach to the study of hormonal control of growth, dormancy and flowering cycles in perennial plants. Buds were excised from orchard trees throughout the year, cultured on defined media for prolonged periods, and their vegetative growth responses to various growth hormones were determined. The buds proceeded with their vegetative development in vitro and achieved sprouting on a basal medium. The various growth regulators affected both the time required for sprouting (TRS) and the type of growth. In summer buds, IAA delayed sprouting, while GA enhanced it and caused shoot elongation. Cytokinins specifically induced the formation of numerous adventitious buds, whereas ABA completely inhibited sprouting; this inhibition, however, was reversible. A marked decrease in total protein and in the rate of its synthesis was evident during the first 20 days of sprouting induction and early bud growth. The annual growth rhythm was determined in spring buds sampled and cultured throughout the year, and an innate dormancy of citrus buds was revealed. Both the dormancy and the sprouting periods of buds in vitro corresponded to the natural periods occurring under field conditions. The effect of exogenous IAA, GA and cytokinins on the TRS varied at different periods along the season, suggesting the concept of “critical levels” in the endogenous balance of hormones.