Somatic Embryogenesis of Carica Papaya Cv. Eksotika

This study was carried out with the main objective of establishing somatic embryo production in Carica papaya cv. Eksotika that can be used for further genetic improvement of the crop. The study included induction of somatic embryogenesis from immature zygotic embryos and anatomical study of the embryo development, establishment of plant regeneration from the somatic embryo, and establishment of cell suspension culture from embryogenic callus. The specific objective of the study on induction of somatic embryogenesis from zygotic embryos was to study the effect of growth regulators on the induction of somatic embryos with or without monthly subculture. The combination of 2,4-dichlorophenoxyacetic acid (2,4-D) and 6-benzylaminopurine (BAP) at various concentrations were assessed. The experiment was conducted in a Randomized Completely Block Design. In the experiment without subculture, MS medium supplemented with 5 mg/L 2,4-D induced the highest percentage (100 %) of somatic embryo formation on the second and third month of culure. At higher 2,4-D concentrations (6-8 mg/L) the percentage of callus formation increased. Monthly subculturing delayed the maturation of somatic embryos and increased callus formation. The percentage of somatic embryo formation was highest (97.5 %) on MS medium supplemented with 5, 6 and 8 mg/L 2,4-D by the third subculture. The specific objectives of the study on plant regeneration from somatic embryos were to determine suitable plant growth regulators, as well as medium formulation on regeneration of somatic embryos. The combination of BAP and NAA at various concentrations were assessed. Different medium formulations evaluated were MS medium (Murashige and Skoog, 1962), LS medium (Linsmair & Skoog, 1965) and B5 medium (Gamborg, 1968) either at full strength or half strength in macronutrients. All experiments were conducted and arranged in a Completely Randomised Design (CRD). MS medium without plant growth regulator induced the highest percentage (59.3 %) of regeneration from the somatic embryos. It was observed that the higher the concentration of BAP and NAA either alone or in combination, the lower the percentage of regeneration and the higher the percentage of callus formation. In the experiment on effect of different medium formulations on regeneration, MS full strength medium produced the highest percentage of regeneration of the somatic embryos. The third study was the establishment of cell suspension culture from embryogenic callus of papaya cv. Eksotika. Four weeks after the transfer of embryogenic callus into liquid medium, single cells were released and after eight weeks pro-embryogenic masses (PEMs) were formed. The highest mean number of cells per ml (56.9) was obtained on liquid MS medium containing 2 mg/L 2,4-D. Maturation of somatic embryos was achieved on transferring the globular or heart-shaped somatic embryos to liquid MS medium without growth regulator. Germination of somatic embryos occurred following transfer of cotyledonary embryos from liquid MS medium onto solid hormone-free MS medium.

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