Regulation of Stem Abscission and Callus Growth in Shoot Explants of Sweet Orange [Citrus sinensis (L.) Osbeck]

Two-node explants from Sweet Orange cv. St Ives Valencia orange shoots produced prolific callus and formed secondary abscission zones within internodes when cultured in vitro with abscisic acid (ABA, 5 μM) or α-naphthaleneacetic acid (NAA, 5 μM). Benzyladenine (BA, 1 μm) induced callus but had little effect on abscission. Secondary abscission zone formation was associated with ABA-induced and auxin-induced ethylene formation. Treatment of explants with inhibitors of ethylene synthesis [aminoethoxyvinyl glycine (AVG), Co2+, PO43−] prevented formation of secondary abscission zones but had variable effects on callus formation. Newly made explants contained high concentrations of endogenous ABA (up to 6000 ng g−1 f.wt), as measured by GC/MS/SIM. Long-term subculture of explants (two years) in media containing BA (1 μm) led to a reduction in endogenous ABA level (40 ng g−1 f. wt) and to loss of capacity to form extensive callus and secondary abscission zones.