The flowering characteristics and reproductive biology in variegated leaf Rhododendron ‘President Roosevelt’

• Variegated leaf phenotype represents an ornamental characteristic of Rhododendron . • ‘President Roosevelt’ is a variegated leaf Rhododendron cultivar. • The mechanisms of dichogamy and herkogamy result in low fruit-setting rate. • Artificial assisted pollination improves fruit set in the variegated leaf cultivar. The cultivar ‘Roosevelt President’ is currently the most outstanding of the variegated Rhododendron in parental selection for Rhododendron hybrid breeding owing to its large inflorescences, diverse flower colors, and distinctive variegated leaves. However, due to a lack of understanding of its flowering and reproductive biology characteristics, the reasons behind the “low fruit-setting rate despite abundant flowering” phenomenon remains unclear, limiting its application in breeding. Thus, it was investigated of the floral phenology, floral morphology, development of pistils and stamens, and breeding systems of ‘President Roosevelt’ in this study. The results showed that blooms from mid-March to late April. The ‘President Roosevelt’ racemose umbels presented clustered blooms; petals obovate, white inside with red corrugated margins; leaf blades predominantly display a variegated phenotype during the ornamental period. The microstructure exhibited that the surface of the pollen grains had sticky filaments and gathered in clusters in the pollen sacs; the stigma was a wet stigma, which provided natural conditions for cross-pollination; the ovary was seven carpels and seven compartments; the ovules inserted on the median placenta. Furthermore, the pollen-ovule ratio was 640.42 ± 33.64, the outcrossing index was four, when combined with artificially controlled pollination experiments indicated that the breeding system of ‘President Roosevelt’ is mainly outcrossing, with partial self-compatible. Observation under fluorescence microscope artificial assisted pollination revealed that pollen tubes successfully reached the ovary within 15 d The findings offer a theoretical foundation for the breeding of new variegated leaf cultivar of Rhododendron and the regulation of floral management.