Exploitation of polyploids from 39 citrus seedling populations

Polyploid formation often results in trait variation and new phenotypes, and exploitation of new polyploid germplasm will greatly facilitate seedless and polyploid citrus breeding. In this study, first we established a simple procedure for citrus polyploidy exploitation by sowing the seeds in vivo and in vitro on a large scale, followed by polyploidy screening of the germinated seedlings based on leaf morphology and flow cytometry analysis. By this procedure, we obtained 100 tetraploids from 12 diploid rootstock genotypes and 58 tetraploids from 19 diploid scion genotypes while 30 triploids from nine of the 19 diploid scion genotypes were also exploited. In addition, 14 triploids and four tetraploids were generated from four sexual crosses by embryo rescue of small seeds, so were nine tetraploids from another four sexual crosses. The autotetraploid progenies of the diploid early-flowering trifoliate orange flowered as early as fifteen months since seed germination, which indicates their inheritance of the early-flowering trait. The autotetraploid Ponkan mandarin already flowered and set fruits. Most polyploid seedlings produced herein were further examined for genetic origin using simple sequence repeat (SSR) markers. These citrus polyploids hold great value for polyploid rootstock breeding, seedless triploid breeding, and relevant basic researches on genomic variation, environmental adaptability and biotic / abiotic stress resistances of polyploids.