Flowering phenology and cross-compatibility analysis of Rhododendron genotypes to support hybrid breeding and cultivar development

Rhododendron latoucheae Franch. and Rhododendron molle (Blume) G. Don are two plants of significant ornamental value in the genus Rhododendron , but their reproductive characteristics and hybrid compatibility remain unclear. To broaden the selection range of breeding parents and enhance hybridization efficiency, this study examined floral biology, pollen storage, and hybridization compatibility through field observations and microscopic analyses. The results showed that the flowering periods of the two species partially overlapped by 15–20 days. Pollen viability peaked at the full-bloom stage in R. latoucheae (96.23 %) and at the initial blooming stage in R. molle (89.33 %), and "short-term forced dormancy" was observed under low-temperature storage. Stigma receptivity in both species increased initially and then decreased, coinciding with peak pollen viability. Both species were protandrous, with pistils maturing subsequently. Significant asymmetry was observed in reciprocal crosses: when R. latoucheae served as the female parent, fruit set rate reached 84.38 % and seed germination rate 48.14 %; when R. molle served as the female parent, crossing failed due to pollen tube growth arrest in the style, indicating a pre-fertilization barrier. This study clarified the hybridization direction and optimal pollination period of the two rhododendrons, providing key parameters for efficient breeding. It also provides cytological evidence for unidirectional reproductive disorders among different subgenera of rhododendrons, offering theoretical and technical support for hybrid breeding and germplasm innovation, and laying a foundation for in-depth analysis of the molecular mechanism of reproductive isolation among subgenera.