Abstract: Leaf isozymes of glutamate oxaloacetate transaminase, phosphoglucose isomerase, phosphoglucose mutase, malic enzyme, peroxidase, superoxide dismutase and tetrazolium oxidase from 105 biotypes of Citrus, FortuneUa and Poncirus were analyzed by polyacrylamide gel electrophoresis to demonstrate the phylogenetic relationship among the biotypes. The study on genetic control of seven enzyme systems was conducted based on the segregation of isozymic phenotypes in one self-crossing and two Fl populations. They were controlled by at least 14 loci which displayed polymorphism. Genotypic comparison among the biotypes showed that there were remarkable differences among the three genera, and each of them possessed its unique genes. The affinity between two biotypes could be inferred by comparing the differences of their isozymic genes. The results suggest that there is a close correlation between the number of homozygous loci and the evolution of citrus plants. Biotypes with a higher number of homozygous loci are primitive types, while those with a lower number of homozygous loci are of hybrid origin. Fuming trifoliate orange could be considered as a new species of Poncirus—P. polyandra S. Q. Ding. It seems to be more reasonable to consider Ichang papeda as the third subgenus of Citrus, i. e. subgenus Ichang papeda. The small-fruited mandarins are primitive forms, and it is better to consider “Daoxian wild mandarin”, “Horse-nose mandarin” or “Goushigan mandarin” as original species of mandarins originated in China, The origins of F. obovata, F. crassifolia, C. aurantifolia and C. limonia are discussed.

Key words: Citrus, Isozymes, Taxonomy, Genetic analysis