High-efficiency plant production via direct somatic single embryogenesis from preplasmolysed cotyledons of Panax ginseng and possible dormancy of somatic embryos

Cotyledon explants of immature ginseng zygotic embryos cultured on Murashig e and Skoog medium lacking growth regulators formed somatic embryos directl y, most in a multiple state, fused together and to the parent cotyledon exp lants. When the cotyledon explants of ginseng were pretreated with 1.0 M su crose for 24-72 h, all the somatic embryos developed individually from all surfaces of the cotyledons and the number of somatic embryos per explant wa s enhanced fourfold. Histological observation revealed that all the single somatic embryos from preplasmolysed cotyledons originated from epidermal si ngle cells, whereas all the multiple embryos from cotyledons without pretre atment originated from epidermal and subepidermal cell masses. When the som atic embryos matured to the cotyledonary stage, further growth ceased and t hey remained white, probably indicating dormancy. Gibberellic acid (GA(3)) (over 1.0 mg/l) or chilling treatment (-2 degrees C for over 8 weeks) were prerequisites for the germination of somatic embryos. Ultrastructural obser vation revealed that the cotyledon cells of somatic embryos without chillin g or GA(3) treatment contained numerous lipid reserves, dense cytoplasm, pr oplastids and non-activated mitochondria, whereas the cotyledon cells of so matic embryos after chilling or GA(3) treatment were highly vacuolated and contained well-developed chloroplasts and active-state mitochondria enclosi ng numerous cristae, indicating that in-vitro-developed somatic embryos of P. ginseng may be dormant after maturing in a manner similar to zygotic emb ryos.