DIRECT SOMATIC EMBRYOGENESIS AND PLANT-REGENERATION FROM LEAF EXPLANTS OF NICOTIANA-TABACUM-L

Direct somatic embryogenesis was induced in leaf disc-cultures of three cultivars of Nicotiana tabacum L. on MS medium supplemented with various concentrations of cytokinins and auxins. The first somatic embryos were formed on the surfaces of discs without callus proliferation, within 10-12 days. Optimal conditions for induction of embryogenic development were found to be a medium containing a combination of BAP (1 mg.L-1) and NAA (0.1 mg.L-1). Development of somatic embryos progressed through typical embryogenic stages, nemely globular, heart-shaped, torpedo-shaped, and cotyledonary stages similar to those of zygotic embryos. Histological analysis strongly indicated that somatic embryos were formed on leaf discs as anatomically independent units with closed radicular ends, and free of vascular connection with the original explant. The sequences of zygotic embryogenesis as well as pollen embryogenesis are presented as the morphological patterns with which to compare the events occurring during somatic embryogenesis from leaf explants. Somatic embryos and microspore-derived embryos are larger in size than zygotic embryos.