Underdevelopment of Embryos in Japanese Plum Cultivars and Its Improvement by Fruit Thinning

ニホンスモモ品種（P. salicina）にはメスレーのように胚の発育が不良で交雑育種の効率，特に種間雑種の育種効率が悪い場合がある．そこで、比較的完全種子の割合が高い‘オオイシ早生’，‘陽光’，‘ソルダム’及び完全種子の割合が著しく低い‘メスレー’を供試し，摘果による胚の発育促進効果の程度を調査した．‘大石早生’およびその異名同品種の‘陽光’の摘果区では無摘果区に比べて完全種子の割合が10％以上，平均果重が30g以上増加した．‘ソルダム’の摘果区では無摘果区に比べて平均果重は2倍に増加したが，完全種子の割合に増加は見られなかった．一方，‘メスレー’の無摘果区では完全種子が得られなかったが，摘果区では完全種子の割合が約40％に増加し，平均果重が2.2倍になった．摘果区と無摘果区を比較した場合，完全種子に含まれる胚の大きさ（胚長）は摘果区でわずかにおおきくなる程度（1.05－1.18倍）であった．これらのことから，果実の早期摘果は完全種子の発生割合の低いニホンスモモ品種を種子親とする交配において特に有効であると考えられた．Of Japanese plum cultivars, some cultivars such as‘Measly\u27 produce seeds that contain underdeveloped embryos, and thus it is necessary to enhance the embryo development to improve the efficiency of breeding, especially in that of interspecific hybrid plants. In this study, three cultivars, ‘Oishiwase\u27, ‘Yoko\u27 and ‘Sordum\u27, which produce perfect seeds with relatively high frequencies and one cultivar ‘Measly\u27 producing imperfect and empty seeds, were examined for promotional effect of thinning on the embryo development. In ‘Oishiwase\u27 and ‘Yoko\u27, the same plant as ‘Oishiwase\u27, the rate of fruits containing perfect seeds and the fruit weight increased more than 100% and about 30g respectively in the trees that thinning treatment was carried out, as compared with those that it was not. In ‘Sordum\u27, fruits weight increased two times in the branches that thinning treatment was carried out, but the rate of fruits containing perfect seeds did not. Although no fruits with perfect seeds were obtained in ‘Measly\u27 trees without thinning treatment, the rate of fruits with perfect seeds increased in the branches that thinning treatment was carried out. In each cultivar, the mean size (length) of embryos included in the perfect seeds was slightly larger in the non-thinning treatment than the thinning one, i.e., mean length of embryos in fruits on treated branches was 1.05-1.18 times as long as that on non-treated ones. These results suggested that thinning treatment is effective for the development of embryos, especially in those of Japanese plum cultivars with low rate of perfect seed formation