Abstract
Availability of cross-pollination with soft X-ray-irradiated pollen for seedless fruit production in Hyuganatsu (Citrus tamurana hort. ex Tanaka) was estimated and compared with conventional methods inducing seedlessness by pollination with pollens of ‘Kankitsu Kuchinotsu 41 Gou’, a tetraploid cultivar, and ‘Nishiuchi Konatsu’, a self-compatible mutant Hyuganatsu cultivar. Pollen of ‘Tosa Buntan’ pummelo, which is a conventional pollinizer for Hyuganatsu production, ‘Kankitsu Kuchinotsu 41 Gou’, and ‘Nishiuchi Konatsu’ was irradiated with 500, 1,000, and 2,000 Gy soft X-rays. The germination rates using irradiated pollen decreased with the dose. Irradiated or non-irradiated pollen was used to pollinate flowers of ‘Sukumo Konatsu’ Hyuganatsu, and natural pollination was prevented by covering the whole tree with a net. The fruit setting rates using irradiated pollen decreased with the dose. Perfect seed numbers in harvested fruit pollinated with non-irradiated ‘Tosa Buntan’, ‘Kankitsu Kuchinotsu 41 Gou’, and ‘Nishiuchi Konatsu’ pollens were 23.8, 0.4, and 1.5 respectively; seeds scarcely remained using irradiated pollen. In fruits following pollination with non-irradiated ‘Kankitsu Kuchinotsu 41 Gou’ and ‘Nishiuchi Konatsu’ pollen, a few large empty imperfect seeds over 10 mm long remained in 3.9 and 2.3, respectively; 500 Gy soft X-ray irradiation of pollen decreased large imperfect seeds, and 1,000 Gy irradiation led to the disappearance. On microscopic observation, although a few embryos survived in seeds following pollination with non-irradiated ‘Kankitsu Kuchinotsu 41 Gou’ and ‘Nishiuchi Konatsu’ pollen even 8 weeks after pollination, no active embryo was observed in seeds following pollination with irradiated ‘Tosa Buntan’ pollen; pollination with irradiated pollen can be considered to inhibit seed development more markedly than with non-irradiated ‘Kankitsu Kuchinotsu 41 Gou’ and ‘Nishiuchi Konatsu’ pollen. Pollination with irradiated pollen decreased the harvested fruit weight; however, that would not be so deleterious because consumers actually prefer smaller fruits with fewer seeds at Hyuganatsu market in Kochi. These results indicate that pollination with soft X-ray-irradiated pollen is a useful technique for seedless Hyuganatsu fruit production without the development of a few large imperfect seeds, and the suitable irradiation dose is 500 to 1,000 Gy.
