A Preliminary Study On Mechanism Of Somatic Embryogenesis In Longan [Dimocarpus Longan]

In this paper the mechanism of somatic embryogenesis was studied through the observations of optical microscope of paraffin section, scanning and transmission electron microscopes, the examinations of endogenous phytohormones and polyamines and the analyses of isozymatic patterns and protein components by IEF and SDS-PAGE in the synchronic materials of the different developmental stages of somatic embryogenesis obtained by controling the medium in Dimocarpus longan Lour. cv. Honghezi.The microscropic observation of paraffin section showed that the somatic embryogenesis originated from internal cells of the callus and the somatic embryos formed from single cells. The observations under the scanning and transmission electron microscopes indicated that the cells with embryogenetic capacity kept independent from other cells; and the ultrastructure in the process of somatic embryogenesis changed greatly, especially the cells were crinkling in wall and less in inclusion in the early-stage vitrified cotyledonary embryoid but regular in wall and rich in inclusion in the matured cotyledonary embryoid.The levels of endogenous phytohormones and polyamines in embryogenic calli were higher than those in nonembryogenic calli. The changes of endogenous hormones and PAs in the process of somatic embryogenesis showed an "M"curve. Embryogenic calli in the MS medium with 2,4-D 0.8 mg/l and globular embryoids were the 2 turning-point stages in the changes of endogenous phytohormones and polyamines insomatic embryogenesis. The changes of endogenous phyhormones and polyamines were earlier than those of morphogenesi s.The trends of isozymatic patterns and protein components in the deffierent developmental stages of somatic embryogenesis were similar. However, some significant differences were observed. The isozyme and protein components of non-embryonic calli were less than those of embryogenic calli. The specific 13.9 KD protein was detected in non-embryogenic calli by SDS-PAGE. Some new isozymatic and protein components emerged in embryogenic calli, and the changes from embryonic calli to globular embriod were quite a few. The speicific p14.4 protein in the early- stage vitrified cotyledonary embryoid was examined by IEF. It is very interesting that two small molecular weight proteins, 6.9KD~ 6.2KD specific proteins, emerged in the matured cotyledonary embryoids, which suggested that they play an important role in the development of somatic embryos and be related to the characteristics of the woody plants with large seeds such as longan.