Genotype Affects the Morphogenic Response in vitro of Epicotyl Segments of Citrus Rootstocks

Abstract

The inuence of light, hormones and explant orientation on in vitro regeneration in epicotyl cuttings was compared in four Citrus species (C. aurantium, C. macrophylla, C. reshni and C. sinensis) and the hybrid Troyer citrange (C. sinensis Poncirus trifoliata). In all cases, explants planted vertically regenerated shoots at the apical end by a process of direct organogenesis without callus formation. When the Troyer citrange explants were incubated horizontally, regeneration at the apical end occurred by an indirect organogenic pathway after callus formation. This change in the pathway of regeneration did not occur in any of the Citrus species, and incubation horizontally resulted in a reduction in the number of buds and shoots formed through the direct organogenic pathway. Shoot formation through the direct organogenic pathway was inhibited by darkness, and this inhibitory eect was counteracted by the cytokinin benzyladenine in Troyer citrange and, partly, in C. sinensis, but not in C. macrophylla. A non-organogenic callus formed at the basal end of most of the cuttings of C. reshni. In C. sinensis and C. aurantium, a nonorganogenic callus formed only in a low proportion of explants. Troyer citrange formed an organogenic callus in which buds or roots dierentiated depending on the auxin/cytokinin balance. C. macrophylla formed callus in the dark but not in the light. Root formation occurred both in the presence of the auxin naphthaleneacetic acid or low concentrations (2 . 2 to 4 . 4 mM) of the cytokinin benzyladenine, but no buds were formed. These qualitative and quantitative dierences in the organogenic response indicate that the conditions for regeneration must be optimized for each genotype.