Interrelationship of Abscisic Acid and Gibberellic Acid in the Promotion of Callus Formation in the Abscission Zone of Citrus Bud Cultures

The mechanism of ABA-induced callus formation was studied in sterile bud cultures of Citrus Citrus sinensis (L.) Osbeck] on defined media. ABA was found to promote callus formation in the abscission zone between the petiole and the branch while inhibiting bud growth. The promoting effect of ABA was dependent on the physiological state of the shoot from which buds were excised, and on the size of the explant. Callus formation was highest in autumn and summer (i.e. younger) buds, and lowest in older buds excised from previous summer flush. GA was only slightly active in promoting callus formation when applied separately, but showed a highly synergistic effect when applied with ABA: maximal callus formation was attained at a combination of 10^?5M ABA and 10^?6 MGA in the medium. Subcultures of ABA-induced callus revealed that ABA inhibited the growth of isolated subcultured callus, while IAA and kinetin, and especially GA, promoted its rapid proliferation. A general decrease in protein synthesis was found in the abscission zone during the first 5 days of induction, while total protein content changed only slightly. The results suggest that ABA-induced callus formation in Citrus bud explants is a multiphasic phenomenon involving, at least, two stages: (1) activation of certain cells in the abscission zone by ABA, resulting in the formation of callus layers, and (2) subsequent proliferation of the callus tissue, which is dependent on the hormonal balance in the explant. This growth-promoting effect of ABA seems to be a general phenomenon in explants exposing a cut-surface.