Developmental events in ovules of the ornamental plant Rudbeckia bicolor Nutt

Abstract

Microscopic observations of R. bicolor ovules showed that tetrasporic embryo sacs of Fritillaria type are formed. In the mature female gametophytes modifications in antipodal cell formation and egg apparatus organization were observed, e.g. morphological resemblance was evident of antipode or synergid to the egg cell. In the central cell cytoplasm of the mature gametophytes the presence of small bodies was a characteristic feature. Development of both embryo and endosperm was observed in ∼73% of ovules at the embryo stage, while retarded or arrested development of the endosperm was found in ∼26% of them. Occasionally, two embryos occurred in the embryo sac. This is the first record of polyembryony in this species. Although hemigamy has been previously described in Rudbeckia bicolor Nutt., in the present investigations mosaic structure of embryos was not detected. Measurements of the C-DNA amount (flow cytometry) revealed embryo nuclei with 2C DNA content only, and endosperm nuclei with 5C DNA content in the mature seeds. No peak corresponding to 1C nuclei was detected in the histograms obtained from the nuclear preparation of seeds or seedling parts. These results suggest that hemigamy is not an obligatory phenomenon in R. bicolor. The mean 2C DNA value was determined as 14.51 pg (the first estimation for this species).