HIGH-FREQUENCY SOMATIC EMBRYOGENESIS AND PLANT-REGENERATION FROM PAPAYA HYPOCOTYL CALLUS

High frequency somatic embryogenesis in papaya (Carica papaya L.) tissue cultures was achieved by culturing hypocotyl sections from ten-day-old seedlings on half-strength Murashige and Skoog salts (MS) medium containing modified MS vitamins, 2.3 to 112.5 muM 2,4-dichlorophenoxyacetic acid (2,4-D), 400 mg l-1 glutamine, and 6% sucrose. Four hermaphroditic Hawaiian cultivars produced embryogenic calluses after ten to 14 weeks of culture at 27-degrees-C in the dark. Efficiency in embryogenic response of genotypes differed, 'Kapoho' > 'Sunset' > 'Sunrise' > 'Waimanalo'. The frequency of embryogenesis in induction medium containing 4.5 muM 2,4-D was lowest with 3% sucrose and highest with 7% sucrose. Somatic embryos developed directly from embryogenic calluses on induction medium, or, more often, they differentiated from calluses subcultured on a medium devoid of growth regulators. Between 50 and 500 embryos were produced from each 2-mm hypocotyl section after at least two months on induction medium and two months on maturation medium. Embryos subsequently developed into normal-looking plants on MS medium. Shoot cuttings from germinated embryos and micropropagated plants were rooted with 5.0 muM indole-3-butyric acid (IBA), grown in the greenhouse, and transferred to the field.