ブドウ‘ヒムロッド•シードレス’果粒の無核性発現に関する解剖組織学的研究

Abstract

To clairfy the mechanism of seedlessness in ‘Himrod Seedless’ grape, histological comparisons were made between unseeded ‘Himrod Seedless’ and seeded ‘Campbell Early’ berries. The development of berry and seed, pollen fertility, pollen tube elongation in the ovary, internal morphology of the ovule, and the nuclear division in the endosperm and fertilized egg (zygote) were studied.1. Berry and seed (ovule) development: The berries of ‘Campbell Early’ clearly exhibited a double-sigmoidal growth curve, whereas those of ‘Himrod Seedless’ lacked the second stage of berry growth and, therefore, did not reveal the same growth pattern. Seeds of ‘Campbell Early’ increased rapidly in size from about 1 week through 4 weeks after flowering which coincided with the first stage of fruit growth. The size remained constant thereafter until harvest. Seeds of ‘Himrod Seedless’ grew slightly for 2 weeks after anthesis and then stopped growing.2. Parthenocarpic ability: The percent sets of parthenocarpic berries resulting from emasculated flowers were 23.3 for ‘Himrod Seedless’ and 15.9 for ‘Campbell Early’, which demonstrate that both cultivars can set berreis parthenocarpically. The weights of seedless berries of ‘Himrod Seedless’ and ‘Campbell Early’ were 60% and 15% lighter, respectively, compared to those naturally pollinated berries.3. Pollen fertility: The percentages of pollen which germinated on agar medium (20% sucrose, 1% agar) were 14.0 and 22.2 for ‘Himrod Seedless’ and ‘Campbell Early’, respectively. The percentages of pollen stained by acetocarmine were 90.4 and 45.6 for ‘Himrod Seedless’ and ‘Campbell Early’, respectively.4. Pollen tube elongation in the ovary: The rate of pollen tube elongation in ‘Himrod Seedless’ was slightly slower than that of ‘Campbell Early’. However, in both cultivars, 80% of the pollen tubes reached the ovules within 48hr after flowering.5. Internal morphology of ovule and embryo sac: At anthesis, ‘Himrod Seedless’ ovules had an abnormally long inner integument and a larger micropyle compared to those of ‘Campbell Early’. The embryo sac of many ‘Himrod Seedless’ ovules lacked the egg cell or the synergids, or exhibited degeneration of the egg apparatus. The percentage of ovules with these abnormalities was 88.4%.6. Nuclear division in the endosperm and embryo formation: In ‘Campbell Early’, freenuclear division of the endosperm began 3 days after anthesis, whereas nuclear division of the zygote began about 18 days later. In ‘Himrod Seedless’, 93.3% of the endosperm did not exhibit free-nuclear division but degenerated. In the remaining 6.7% of the ovules, the endosperm nuclei divided only 2 or 3 times beginning 6 days after flowering. Cell division of the zygote was not observed.We speculate that seedlessness in ‘Himrod Seedless’ berries is the result of abnormal double fertilization based on the high percentage of abnormal development of the embryo sac and egg apparatus and the limited nuclear division occurring in the endosperm. Polar nuclei may become fertilized but subsequent neclear divisions are abnormally low. This may hamper or limit the nutrient supply and lead to the degeneration of the zygote.