Study on the Flower Biology of Camellia luteoflora—A Species with an Extremely Small Population

The aim of this study was to elucidate the reproductive strategy of Camellia luteoflora, an endangered evergreen endemic to karst ecosystems. We observed and recorded its flowering phenology and flower-visiting insects, observed pollen morphology, determined pollen viability, and assessed stigma receptivity. The results showed that the flowering period of C. luteoflora started from early September to late December, with the average flowering period of individual flowers being 10–12 days. The pollen morphology of C. luteoflora was subprolate and prolate, with three germinal apertures and the fossulate exine ornamentation. Pollen viability was the highest at the initial opening stage (80.30%). In the process of pollen in vitro, the order of influence on the germination rate and pollen tube length was temperature > sucrose > calcium chloride (CaCl2) > boric acid (H3BO3). The best combination for the germination rate was 24 °C, 75 g/L sucrose, 0.2 g/L CaCl2, 0.15 g/L H3BO3, while that for the pollen tube length was 24 °C, 100 g/L sucrose, 0.2 g/L CaCl2, 0.25 g/L H3BO3. Stigma receptivity was the strongest at the full blooming stage. The pollen/ovule ratio (P/O) was 2240, suggesting a facultative outcrossing breeding system. The outcrossing index (OCI) was 4, suggesting that the exogamous breeding system is the cross-pollination type, partially self-compatible and insect pollinator-dependent. The flower-visiting insects included bees, weevils, and ants. In summary, C. luteoflora exhibits an extended flowering period, with a prolonged overlap of stable pollen viability and stigma receptivity, suggesting a potential strategy to cope with pollination uncertainty. However, field observations recorded only a few species of potential pollinators, while the occurrence frequency of non-pollinating insects was relatively high. It is thus hypothesized that this apparent lack of effective pollinators may act as a potential barrier to successful fertilization and natural regeneration, which might also be one of the factors contributing to its endangered status. Future studies, particularly pollinator exclusion and hand-pollination experiments, are critically needed to verify whether pollinator limitation is indeed a key factor.