Direct and indirect shoot organogenic pathways in epicotyl cuttings of Troyer citrange differ in hormone requirements and in their response to light

Bud differentiation by direct organogenesis at the apical end of Troyer citrange (Cirrus sinensis [L]. Osbeck x Poncirus trifoliata [L]. Raf.) epicotyl cuttings inserted vertically in a semi-solid culture medium did not require hormone additions. The number of buds regenerated was slightly, but significantly, increased when the incubation was performed in the light as compared to the dark, and by the addition of benzyladenine (BA; 2.2 to 22 mu M) to the medium. Bud sprouting and subsequent shoot formation required the addition of BA and was increased by light to a higher extent than bud formation. The best response was obtained with the highest BA concentration rested (22 mu M). Regeneration through the indirect organogenic pathway at the two edges of the epicotyl cuttings when in contact with the culture medium did not occur in the absence of benzyladenine, which was an absolute requirement for callus development. The best regeneration response was obtained when the explants were incubated in the light in the presence of 4.4 mu M BA and an auxin. Indole-3-acetic acid (IAA; 5.8 mu M) was more effective in increasing shoot formation than naphthaleneacetic acid (NAA; 0.54 mu M). Higher NAA concentrations inhibited shoot formation. Incubation in the dark or increasing the BA concentration (22 mu M) increased markedly callus growth, but inhibited both bud differentiation and sprouting, almost completely suppressing shout formation. The conditions during regeneration affected the rooting of the regenerated shoots. Rooting of 86 % of the shoots was achieved in a medium with 2.7 mu M NAA and 2.6 mu M indole-3-butyric acid. All the rooted explants acclimated and survived transplanting. Under the optimal conditions tested, the proliferation rate obtained through the indirect regeneration pathway ranged from 60 to 86 plants per seedling. (C) 2000 Annals of Botany Company.