Explant Orientation and Polarity Determine the Morphogenic Response of Epicotyl Segments of Troyer Citrange

The morphogenic pathway of adventitious bud and shoot regenerationat the ends of Troyer citrange epicotyl cuttings is determinedby polarity and explant orientation. In explants planted verticallywith the basal end inserted in the medium, bud formation atthe apical end occurs by direct organogenesis. Bud growth andsubsequent shoot formation is markedly increased by the additionof 6-benzyladenine (BA) to the medium. This growth regulatoralso increases the number of buds formed. When they come intocontact with the culture medium, both the apical end and thebasal end of the cuttings form a vigorous callus with many xyllaryelements, more numerous in the calli from the basal end. Inthese calli, buds differentiate by a process of indirect organogenesis.This indirect regeneration pathway requires the addition of6-benzyladenine to the medium, and the number of buds formedis higher at the apical end than at the basal end of the cuttings.This pathway of regeneration is reduced as the position of thecuttings during incubation deviates from the normal uprightvertical position. Thus, for the basal end of the cuttings,the number of buds and shoots formed is higher when the explantsare placed vertically than when they lie on the surface of themedium. For the apical end, this number is higher in explantsplaced horizontally than when inserted vertically in the mediumin an inverted position. Copyright 1999 Annals of Botany Company Troyer citrange, Citrus sinensis x Poncirus trifoliata, explant orientation, histology, hormone dependence, morphogenesis, organogenesis, polarity, xylogenesis.