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The earliest reports of somatic embryogenesis were based on the use of seedling stem tissues (DeBruijne et al., 1974; Yie and Liaw, 1977). DeBruijne’s group induced embryogenic cultures from petiole sections on MS and White (1963) semisolid media containing 0.001–10 μM auxins and cytokinins in a multistep protocol (Table 6.1.4). Plants were not recovered. Subsequent studies involved media containing 6 μM or higher concentrations of 2,4-dichlorophenoxyacetic acid (2,4-D) or NAA (Yie and Liaw, 1977; Mehdi and Hogan, 1979; Chen et al., 1987; Fitch and Manshardt, 1990; Fitch, 1993; Monmarson et al., 1994; Fitch, 1995; Monmarson et al., 1995; Mahon et al., 1996) were used.

Table 6.1.4. Summary of somatic embryogenesis of papaya.

Although embryogenic cultures were induced from various explants in the presence of NAA and kinetin (Table 6.1.4; Yie and Liaw, 1977; Mehdi and Hogan, 1979; Chen et al., 1987), a dependable protocol derived from a seed-based protocol (Litz and Conover, 1981) was developed by culturing immature zygotic embryos on semisolid MS medium containing 0.5–125 μM 2,4-D (Fitch and Manshardt, 1990). Proliferation of embryogenic cultures using media without 2,4-D to minimize somaclonal variation (Larkin and Scowcroft, 1981) was described by Chen et al. (1987) and Yang et al. (1996).

Most somatic embryos matured and germinated on MS or other medium formulations containing no growth regulators or reduced or different growth regulators.