Somatic Embryogenesis and Plant Regeneration in Paradise Tree (Melia azedarach L.)

Abstract

Several factors affecting somatic embryogenesis induction and conversion in paradise tree (Melia azedarach) were evaluated. Somatic embryogenesis was influenced by plant growth regulators, stage of development of the explant, carbohydrate source, gelling agents, light intensity and induction time. MS medium with 1 mg/l TDZ (thidiazuron) proved to be the best for the induction of embryogenic tissue. Zygotic embryos, 1-1.5 mm long (torpedo and early cotyledonar stage), had a greater embryogenic response than smaller or larger embryos, and resulted in a better conversion of somatic embryos into plants. Embryos formed in media containing 3% sucrose or raffinose had better morphology. Induction and conversion of somatic embryos was superior on medium solidified with agar. The explants maintained under 160 µmol m-2 s-1 or 1 week in darkness and later under 160 µmol m-2 s-1 produced a significantly higher embryogenic index. Only 4 days of treatment on induction medium, with either raffinose or sucrose at 3% as carbohydrate source, were required to induce somatic embryogenesis. However, a longer exposure, up to 18 days, increased the yield and improved the morphology of somatic embryos. After 6 weeks of culture on induction medium, somatic embryos were categorized in four morphological classes based on presence of single or fused embryos and considering if they remained attached or not to the original explant. Somatic embryos of each category were transferred on 1/4 MS medium. Bipolar, more typically normal embryos, had well defined shoot and root apical meristems and produced single plants; when these embryos were subcultured individually, conversion reached 28%. If fused embryos were cultured, only 2.1% conversion was observed and regenerated plants presented fused stems. The lack of conversion of other embryo classes was associated with poorly developed shoot and root meristematic areas or with their complete absence. The converted plants acclimatized and transferred, in a mist, to soil with a 95% survival rate.