Plant regeneration from embryogenic cells of two Rosa hybrida cultivars, Kardinal and Classy, was increased by dispersing embryogenic callus in liquid medium for 3 h followed by size-fractionation to isolate proembryogenic masses that were smaller than 530 microm. Dispersed callus of three cultivars, Kardinal, Classy, and Tineke, produced 61-135 cotyledonary-stage embryos/100 mg fresh weight (FW) as compared to intact callus that had not been dispersed, which produced only zero to three cotyledonary-stage embryos/100 mg FW. Over 500 cotyledonary-stage embryos/100 mg FW callus developed from proembryogenic masses of Kardinal, Classy, and Tineke following 2 months of culture on solidified Murashige and Skoog's basal salts medium supplemented with 0.25% activated charcoal. Cotyledonary-stage embryos of Classy that developed from both dispersed callus and fractionated cells of various sizes showed a significantly higher conversion frequency to plants (28%) than cotyledonary-stage embryos isolated from intact callus (9%). The highest conversion frequencies for Kardinal (50-58%) occurred from cotyledonary-stage embryos that developed from dispersed callus and from the fraction of cells smaller than 850 microm.
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