Somatic Embryogenesis in Cassava: The Anatomy and Morphology of the Regeneration Process

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Abstract
Anatomical and morphological studies demonstrated that somatic embryos developed similarly on mature seed and clonal leaf explants of cassava (Manihot esculenta Crantz) cultured for 20–24 d on Murashige and Skoog (MS2) basal medium supplemented with 4.0 mg l−1 2,4-D (Stage 1) before transfer to MS2 basal medium supplemented with 0–01 mg l−1 2,4-D and 0–1 mg l−1 6-benzylaminopurine (Stage II medium). Within 7 d of inoculation onto Stage I medium, cell divisions occurred in the adaxial tissues of cotyledon-piece and leaf-lobe explants, and associated with this was the development of embryogenetic protusions and ridges on the adaxial surface. Foliose structures and somatic embryo initials developed from these tissues on cotyledon, embryonic axis and leaf-lobe explants and, when cultures were transferred to Stage II medium, further somatic embryo development occurred. Somatic embryos apparently originated from groups of cells and were identified by the presence of a closed root axis, a shoot axis and cotyledons of similar shape and venation to those of zygotic embryos. Somatic embryos had no vascular connection with parental cultures.