Histological Studies on the Relationship between the Process from Fertilization to Embryogenesis and the Low Seed Set of Sweet Potato, Ipomoea batatas ( L. ) Lam.

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To determine the direct cause of the low seed set in sweet potato, 14 cross-compatible combinations among 11 varieties were histologically examined for the process from fertilization to embryogenesis at 4, 7 and 48 hours after pollination. Pollen fertility on the flowering day exceeded 80% in most of the varieties examined and pollen germination 4 hours after pollination reached 10〜20% in most of the crosses. These results show that neither the fertility of mature pollen nor the pollen germination on stigma is the principal cause of the low seed set. The mean number of pollen tubes passing through the conducting tissue of the ovary 7 hours after pollination largely differed among the cross combinations. However, it did not show a close relationship with the number of seeds per capsule. Among the embryosacs examined 48 hours after pollination, those that had not been penetrated by pollen tubes were mostly abnormal, in which the development of the egg apparatus had been interrupted halfway. On the other hand, the number of normal young embryos was closely correlated with the number of seeds per capsule. These results suggest that a large number of such abnormal embryosacs is the principal cause of the low seed set of sweet potato.