Immature cotyledons within nucleoulus of young fruits, 40-50 days old, were cultured on various media containing different concentrations of plant growth regulators. MS medium was most effective for callus formation, but cytokinins added to MS media containing 1.0 mg/l 2,4-D inhibited callus formation. Combination of 2,4-D and BAP was more effective than 2,4-D or BAP alone for somatic embryo formation from callus. Highest percent of somatic embryogenesis was observed on MS medium supplemented with 1.0mg/l 2,4-D and 50% sucrose. Cobalt and nickel, inhibitors of ethylene synthesis, reduced significantly the number of somatic embryos. Plantlets germinated freely from somatic embryos on MS medium containing 1.0mg/l zeatin and 0.01mg/1 2,4-D. Segments of hypocotyl, cotyledon and leaf blade were observed for callus formation, somatic enbryogenesis and plant regeneration. Shoots were directly differentiated from the callus induced from segments of hypocotyl on MS medium containing 0.1mg/l 2,4-D after 4 weeks of culture.
From one week through 7 weeks after artificial pollination, immature ovules of yooza(Citrus junos Sieb. et Tanaka) were excised and cultured in vitro on MT media. Even though there was only a little difference in percentage of somatic embryo formation depending upon the time of excision, immature ovules of 4-week-old showed the highest ratio of somatic embryo formation without callus outgrowth. Various growth regulators or other stimulators were added to the MT media to increase the somatic embryogenesis, In general, BAP was more effective than 2,4-D for somatic embryo formation and the combinations of 0.01mg/l 2,4-D and 0,01 or 0.1mg/l BAP were particularly effective in stimulating somatic embryo formation. When 500mg/l malt extract was added to the medium, the percentage of somatic embryo formation increased reaching as high as 86.7%. Plant regeneration from somatic embryos reached to 66.7% on the medium containing 1.0mg/l zeatin. Isozyme banding patterns were also analyzed to confirm the variations of characteristics of the plantlets derived from direct somatic embryos.
The genetic relationships among 96 peach and nectarine [Prunus persica (L.) Batsch.] genotypes and botanical varieties originating from different ecogeographical regions of China, Japan, North America, and South Korea were evaluated with 33 SSR markers screened from 108 published SSR markers developed for peach or sweet cherry (P. avium L.). The 33 SSRs detected polymorphisms among 96 genotypes and revealed a total of 283 alleles with an average of 8.6 alleles per locus. The polymorphism information content (PIC) value ranged from 0.40 (BPPCT041) to 0.98 (BPPCT009) with an average of 0.80. Unweighted pair group method average (UPGMA) cluster analysis based on Nei's genetic distances classified genotypes into six groups, corresponding to their ecogeographical origin. Group I consisted of northern Chinese and northwestern Chinese local cultivars, and was divided into two subgroups, white and yellow peaches. Group II contained mainly southern Chinese local, Japanese, and North American cultivars and can be divided into four subgroups: Japanese white, Chinese flat, North American yellow, and some Chinese local ornamental peach cultivars. Groups III, IV, and V were comprised of Chinese local ancient cultivars, and contained `Xinjiangdatianren' and `Renmiantao', Chinese dwarf cultivars, and `Fenshouxing', respectively. Group VI had only `Baishanbitao', a Chinese ornamental cultivar. Northern and northwestern Chinese local cultivars clustered together with a greater diversity than southern Chinese local cultivars, indicating that the northern and northwestern Chinese local cultivars are similar ecotypes, and southern Chinese local cultivars are a subset of the northern Chinese group. Moreover, the Japanese and North American genotypes had a close phylogenetic relationship with southern Chinese local cultivars. The taxonomic placement of P. ferganensis (Kost. et Kiab) Kov. et Kost. and the phylogenetic relationship of `Baishanbitao' with peaches are discussed.