Production of tetraploid forms of acid citrus cultivars by top grafting of shoots with sprouting axially buds treated with colchicine

To induce the tetraploid forms of three diploid acid Citrus cultivars (2n=2x=18), 'Yuzu' (C. junos), 'Kizu'(C. kizu) and 'Hanayu' (C. hanaju), sections of one-year-old spring shoots with sprouting axillary buds were treated with various concentrations of colchicine and immediately top-grafted on 30-year-old satsuma mandarin trees at the beginning of April. Characteristics of the plants developed from the top grafted scions were scored five years after the treatments. Tetraploid forms of the cultivars, which were certified by tetraploid chromosome number in their seedlings, were derived from the shoot sections treated with 0.05-0.4% colchicine solutions for 12 hours for 'Yuzu', 0.05-0.4% for 6 hours for 'Kizu' and 0.1 and 0.2% for 6 hours for 'Hanayu'. As compared with the original plants, these colchicine-induced tetraploid plants produced large flowers, fruits with thick rind, large oil gland and large seeds, and showed high leaf shape index (leaf width / leaf length). The large seeds from open-pollinated tetraploid plants showed the same degree of polyembryony as seeds from their original diploid cultivars, and their seedlings were tetraploid. These results suggested not only that the tetraploid forms of the three cultivars can be directly used for breeding of triploid seedless acid citrus cultivars as seed or pollen parents, but also that the present method is superior to the other methods to produce citrus tetraploid forms directly applicable for triploid breeding at two to four years after colchicine treatments.