Genome Size, Flowering, and Breeding Compatibility in Osmanthus Accessions

Abstract

Extending the range of Osmanthus species into more cold-hardy climates would open new opportunities for adoption and use of these species by growers, landscapers, and the public. Breeding improvement is hindered by few available cultivars and a lack of female or perfect flowers. The objectives of this study were to evaluate floral morphology and pollination biology of Osmanthus species available in the U.S. market. Thirty-three genotypes representing four species were evaluated in McMinnville, TN, USA for genome size, floral morphology, pollen viability, and self- and cross-compatibility. All genotypes were diploid with 2C genome sizes ranging from 2.89 to 3.21 pg. Perfect flowers were observed in all O. armatus and 82% of O. heterophyllus genotypes. All observed O. fortunei and O. fragrans genotypes had male-only flowers. Pollen viability based on staining and in vitro germination ranged from 8% to 98% and 6% to 53%, respectively. Pollen germination was observed on stigmas of 94.2% of O. armatus and O. heterophyllus flowers collected 24 hours after cross- or self-pollination. There was a significant association between cross type and percentage of flowers with pollen tubes reaching the ovaries; after 72 h, pollen tubes had reached the ovaries of in 67% of intraspecific crosses, 78% of interspecific crosses, and 0% of self crosses (χ2 = 26.5, p < 0.001). This study provides evidence of a self-incompatibility system in O. armatus and O. heterophyllus and provides insights into opportunities and challenges for Osmanthus hybrid breeding.