Estimation of the degree of self-incompatibility reaction during flower bud development and production of self-fertilized seeds by bud pollination in self-incompatible Citrus cultivars

Bud pollination in seven self-incompatible Citrus cultivars was carried out to produce self-fertilized seedlings with genotypes homozygous for the self-incompatibility gene. Flower buds of the seven self-incompatible Citrus cultivars, 'Banpeiyu' (C. grandis), 'Hirado Buntan' (C. grandis), 'Hassaku' (C. hassaku), 'Shishiyuzu' (C. pseudogulgul), 'Clementine' (C. clementina), 'Tosa Buntan' (C. otachibana) and 'Hyuganatsu' (C. tamurana), were emasculated at various stages of flower bud development, and self-pollinated with their fresh pollen from flower buds collected just before anthesis. Seeded fruits were obtained from flower buds of 6-8 mm length for 'Clementine', 7-11 mm for 'Hyuganatsu', 11-15 mm for 'Shishiyuzu', 6-14 mm for 'Hassaku', 12-19 mm for 'Hirado Buntan' and 12-22 mm for 'Banpeiyu'. The optimum length of flower buds for self-fertilized seed production was 6-8 mm for 'Clementine', 8-12 mm for 'Hassaku', 9-13 mm for 'Hyuganatsu', 11-14 mm for 'Shishiyuzu', 12-17 mm for 'Hirado' and 14-19 mm for 'Banpeiyu'. Flower buds producing the largest number of perfect seeds in each self-pollination were about half the length of those just before anthesis. At or just after this stage of flower bud development, stigmatic exudate production occurred. The largest number of perfect seeds in self-pollinated fruits was almost the same as the average number of perfect seeds in open- or cross-pollinated fruits in six cultivars except for 'Shishiyuzu' in which self-pollinated fruits contained a few seeds. These results suggested that half-sized flower buds in their development were the most optimum stage for the production of self-fertilized seeds by bud pollination in self-incompatible Citrus cultivars, and that self-incompatibility reaction increased after this stage. It was also suggested that in 'Shishiyuzu' the self-incompatibility reaction is different from that in the other six Citrus cultivars. No difference was found in size between self-fertilized and cross-fertilized seeds. The self-fertilized seeds germinated and grew normally without any inbreeding depression.