Somatic hybrids between navel orange (Citrus sinensis) and grapefruit (C-paradisi) for seedless triploid breeding

Protoplasts from cell suspension cultures of 'Bonnaza' navel orange ( Citrus sinensis (L.) Osbeck) were electrically fused with mesophyll protoplasts isolated from seedless 'Red Blush' grapefruit (Citrus paradisi). After 6 months of culture, a total of 20 plants were regenerated. Root tip chromosome counting revealed that 4 of them were tetraploids (2n = 4x = 36) and the rest were diploids (2n = 2x = 18) morphologically resembling the mesophyll parent. After 6 months of transplantation into the greenhouse, 4 of the diploid mesophyll regenerants unexpectedly flowered, but this phenomenon disappeared in the next year. This is the first report of precocious flowering in citrus via protoplast fusion. RAPD analysis further confirmed that the tetraploid regenerants were somatic hybrids while the diploid regenerants were mesophyll parent type. This somatic hybrid will be utilized as a possible pollen parent for improving the seedy pummelo cultivars in China by producing triploid seedless pummelo hybrid. The mechanism of early flowering was also discussed.