Differential effects of sucrose, abscisic Acid, and benzyladenine on shoot growth and callus formation in the abscission zone of excised citrus buds.

The omission of sucrose from the basal medium stimulated callus formation in bud explants of Citrus sinensis (L.) Osbeck. Moreover, it increased the abscisic acid-induced callus proliferation reported earlier in the presence of 5% sucrose (Altman and Goren, Physiol. Plant. 32: 55, 1974). The inhibition of callus formation by the addition of sucrose was not due to the high osmotic potential of the medium. Benzyladenine induced callus formation slightly, in all sucrose concentrations up to 5%. The high level of sucrose was required, however, for the growth of shoots from buds cultured on both basal and benzyladenine-containing media.The uptake of (14)C-sucrose by bud explants was linear for at least 98 hr, and was enhanced significantly by both abscisic acid and benzyladenine during the initial 24-hour period. Abscisic acid enhanced the absorption of (14)C-sucrose and the accumulation of sugars in buds cultured at high levels of sucrose. More than 50% of the total label accumulated in the callus of abscisic acid-treated explants whereas only 16 and 23% were observed in the growing shoots of control and cytokinin-treated explants, respectively.Results suggested that while sucrose "starvation" induced initial callus formation, high levels retarded further proliferation of the callus.