Development of a protocol for the micropropagation of mature Eucalyptus grandis clones through somatic embryogenesis
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Biotechnology techniques such as micropropagation VIa somatic embryogenesis offer potential significant advances in the improvement of forest species, which could sustain forest production in South Africa, as well as globally, without increased use of land. In order to apply such techniques to commercial breeding and clonal programmes of E. grandis species, it is necessary to develop reliable and efficient protocols applicable to explants of proven superior genotypes. Most of the research on E. grandis somatic embryogenesis has used the genetically variable embryos or seedlings as explant sources, which results in the propagation of material of unproven genetic value. In order to exploit somatic embryogenesis maximally for cloning of superior trees, somatic embryos have to be induced from highly selected and, hence, mature trees. The aim of this investigation was to develop such a protocol for E. grandis and to test its applicability to various E. grandis hybrids. Somatic embryos were induced from buds, stems, leaves and petioles, with petioles and buds giving the best results. Thus, these were selected for further studies which involved testing the effect of medium composition on embryogenic callus induction. Media used for this purpose contained MS or B5 nutrients, 1 mg.l' 2,4-D, 0.5 g.r! glutamine, 0.5 g.r! casein hydrolysate, 4 g.r! Gelrite and 30 or 50 g.rl sucrose. All the media tested were able to support induction of embryogenic callus, although the number of explants producing embryogenic calli was affected significantly by the media composition (10-91 %). Callus induction media with B5 nutrients seemed to have a significant effect onn the developmental stage of embryos in the callus induction medium. Presence of 50 g.r! sucrose in the callus induction medium reduced the embryo yield, but the progress of embryo development was enhanced. The callus induction medium containing B5, 1 mg.l' 2,4-D, 0.5 g.rl glutamine, 0.5 g.r! casein hydrolysate, 4 g.r! Gelrite and 30 g.l' sucrose was chosen for subsequent studies. Of all the media tested for embryo development, the medium with B5, 2.5 mg.l' 2iP, 0.5 g.r! glutamine, 0.5 g.r! casein hydrolysate, 4 g.r! Gelrite and 50 g.r! sucrose was found to be the most suitable for embryo development to the cotyledonary stage. Experiments involving incorporation of both ABA and 2iP aiming at maturation of E. grandis somatic embryos led to an increase in size of the cotyledonary embryos formed but not to germination.