240 ARIE ALTMAN AND RAPHAEL GOREN Physiol. Plant. 30: 240-245. 1974 Growth and Dormancy Cycles in Citrus Bud Cultures and Their Hormonal Control By ARIE ALTMAN and RAPHAEL GOREN Department of Horticulture, The Hebrew University of Jerusalem, P.O. Box 12, Rehovot, Israel (Received September 10, 1973) Abstract 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 develop- ment 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 G A enhanced it and caused shoot elong- ation. Cytokinins specifically induced the formation of numerous adventitious buds, whereas ABA completely inhibited sprout- ing; this inhibition, however, was reversible. A marked decrease in total protein and in therateof 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 dor- mancy 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. Introduction Although the problem of vegetative growth and flowering cycles in perennial plants is of great interest, the specific mechanism of its control is not yet well understood. Thus, even the definition of "bud dormancy" in evergreen trees is debatable, while in deciduous trees dormancy is closely related to changes in the endogenous balance of growth substances and other natural inhibitors (Hendershott and Bailey 1955, Wareing and Saunders 1971). In Citrus it has been established that the new bud is anatomically completed when the new flusb is formed, although its development to either a vegetative or a genera- tive shoot is determined, under natural conditions, only several months later (Nir et al. 1972). Environmental conditions and horticultural treatments such as tempera- ture changes (Monselise and Halevy 1964, Cassin et al. 1969), water stress (Monselise and Halevy 1964, Nir et al. 1972), and girdling (Goren and Monselise 1971), can modify and shorten the time to differentiation. In addition, it has been established that growth regulators like GA can affect growth and inhibit flower formation when applied during flower induction (Goldschmidt and Monselise 1972), while growth retardants and antimetabolites of nucleic acid and protein synthesis can promote flowering and sprouting of buds under certain conditions (Goren and Monselise 1969). The understanding of the endogenous control of vegeta- tive growth and flower formation in trees, and citrus is no exception, is far from being complete, partially due to methodology. Until now, the annual growth rhythm was mainly studied by following anatomical and physiological changes in intact buds on the tree, as correlated with en- vironmental changes. However, this method does not allow isolation of the specific physiological factors which afl"ect bud development. Ringing treatments applied to the branch just below (or above) the bud may help overcome some of these difficulties, but their efl'ect is limited to phloem- transportable factors only. Since the bud of the tree may be regarded as a distinct physiological unit which is dependent on tbe tree but, on the other hand, is able to afl"ect some physiological events occurring in it {e.g. cambium activation by developing buds; Wareing et al. 1964), it was postulated that culture experiments might help elucidate more precisely the role of growth regulators in bud development. In vitro culturing techniques were widely used in recent years in annual