High Frequency Callus Initiation, Somatic Embryogenesis and Plantlet Regeneration in Carica papaya L. cv. COORG HONEYDEW

Two month old stem explants of Carica papaya L. cv. Coorg Honeydew showed 80 per cent callus initiation on Murashige-Skoog (MS) nutrient medium supplemented with 3.0 μM of 2,4-dichloro phenoxyacetic acid (2,4-D). Treatment with phytohormones like Kinetin (Kin) or Benzyl adenine (BA) (@ 0.2 to 2.0 mg l^-1) were found to have no role with regard to callus initiation. However, these initiating calli when subcultured on MS + 2,4-D (3.0 μM) + Kin (0.5 mg l^-1) showed a two-fold growth by proliferation within 21 days after the date of sub-culture. During this period, 30 per cent of the callus tissue underwent necrosis. Thereafter, the best of 70 per cent friable, healthy calli were recultured on MS + 2,4-D (3.0 μM) + Napthalene acetic acid (NAA, 2.0 mg l^-1) + Kin (0.5 mg l^-1), also supplemented with casein (50 mg l^-1). This combination for reculture resulted in vigorous callus growth on fresh weight basis. Best somatic ernbryogenesis was achieved when callus tissue so obtained was further recultured in MS + NAA (1.0 mg l^-1) + Kin (0.5 mg l^-1) + Gibberelic acid (GA3 1.0 mg l^-1) + L- Ascorbic and (Asc, 50 mg l^-1) alongwith glycine (1.0 mg l^-1) + thiamine (Thia, 1.0 mg l^-1) as adjuvants. The pH of such culture media was maintained at 5.7, incubated under a 16/8-hr light/dark cycle at 25°±1°C in the culture room. This protocol resulted in 80 per cent somatic embryogenesis out of which about 20 per cent yielded regenerants. The plantlets were carefully transferred to half-strength MS medium for further growth and hardening.