The rules of identifying the occurrence of chromosome doubling or the ploidy levels of the regenerated plants are as follows: if the channel number of G0/G1 peak position measured for the regenerated plant is the same or closely the same with the datum value of a particular species, this means that the DNA content of the leaf cell nuclei of the regenerated plant remains in the same ploidy level as diploid (FIG. 3A); if the channel number of G0/G1 peak position measured for the regenerated plant has doubled or quadrupled that of the datum value, this means that the DNA content of the leaf cell nuclei or the chromosome of the regenerated plant is doubled once or twice, and the regenerated plant can be identified as a polyploid plant or a plant having chromosome doubled (FIG. 3B).