Large-scale production of microprotoplasts from `Ruby Red' grapefruit (
Citrus paradisiMacf.) and from the CitrusL. sp. relative Swinglea glutinosa(Blanco) Merr., was performed after treatment of suspension cells with APM. An average of 75.2% of the microprotoplasts contained a single chromosome, followed by 17.1% with two, 4.6% with three, and 2.0% with four. Only 1.1% had more than five chromosomes. Maximum chromosome number observed was eight and the average yield was 2 × 106 of total microprotoplasts per gram of suspension cells. Flow cytometry analysis confirmed low DNA content. The polyethylene glycol fusion method was used to fuse microprotoplasts from `Ruby Red' grapefruit with protoplasts of `Succari' sweet orange [ Citrus sinensis(L.) Osbeck], and microprotoplasts from S. glutinosawith protoplasts from sour orange ( C. aurantiumL.). Embryos or suspension cells from the recipient species with a few additional chromosomes were obtained; however, embryogenesis of the fusion products was reduced or inhibited. Chemical name used: amiprophos-methyl (APM).