| Manchrian ash (Fraxinus mandshurica Rupr.) is a hardwood species in the genus Fraxinus in Oleaceae and one of the three valuable hardwood species in Northeast China. It is important for timber production and landscape establishment as well. By using cotyledons of zygotic embryo in different collecting periods as explants, the satisfied collecting time of explants for somatic embryogenesis was determined. At the same time, the influencing factors to reppenning, germination and rooting of somatic embryos such as media, concentration and types of hormones, concentration of sucrose and agar, were tested for exploring the possible approach for changing somatic embryos to plantlets. Based on our former studies, systematic study for the first time on collecting periods of explants were conducted for improving system of somatic embryogenesis of F. mandshurica and provided some bases for realize plant regeneration. The conclusions are as follow:(1) The collecting time of explants was one of the key influence factors on somatic embryogenesis. The frequency of somatic embryo induction was much higher and the quality of somatic embryos was better when the explants was collected from immature zygotic embryos in the middle of July (about nine weeks after pollination), the frequency was very low when the explants were collected in the early July (about eight weeks after pollination) and the middle of September (about seventeen weeks after pollinate) as well. So, we ascertain the optimal collecting stage of explants was in middle July.(2) The cotyledon explants colleted from zygotic embryos in different periods were cultured on MS1/2 media supplement with NAA and BA in different concentrations could induce somatic embryos. Direct somatic embryogenesis was the main pattern when the proportion and concentration of NAA and BA was proper. In this case, the quantity and quality of the somatic embryos was well. Secondary somatic embryos can be produced easyly after premary somatic embryos regenerated.The optimal somatic embryo induction condition was MSI/2 medium containing BA 0.5 mg.L-1and NAA1.5, 2.5mg.L-1, the induction frequency can be reached 35.32%.(3) MS and MS1/2 were used as the germination media supplement with NAA in different concentrations, the result show that the germination percentage was higher in MS1/2, and a lot of secondary somatic embryo produced from radicels and hypocotyls;the optimal germination condition was MS1/2 medium with NAA 0.01mg.L-1, the germination percentage reached 24.8%.(4)Rooting of Manchrian ash somatic embryos was a difficult problem. The optimal rooting condition was MS1/2 medium with NAA 0.01mg.L-1, sucrose 20g.L-1 and agar 7 g.L-1.(5)The study validated further that Manchrian ash somatic embryogenesis had two originate ways: single cell and multi-cell. The somatic embryos from the surface of cotyledon or callus epidermis were single cell originate, and from internal cells of callus were multi-cell originate. Somatic embryogenesis passed through proembryo, heart, torpedo and cotyledonary stages. At late of globular stage to early of heart stage can observed suspensors and with heart embryo develop, suspensors gradually disappear;at the same time, vascular bundle differentiated at heart embryo stage that took on the character " V " at late of torpedo, and did not contact with mother tissues, obvious bipolarity was detected.
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