Study on Floral Syndrome and Breeding System of an Endangered Species Rhododendron nymphaeoides

Rhododendron nymphaeoides is a plant belonging to the genus Rhododendron (Rhododendron Linn.) and the subsection Fortunea. Although this species is clearly listed as endangered (with an endangered status of EN) by the International Union for Conservation of Nature (IUCN) Red List, the Red List of Rhododendrons, the Red List of China's Higher Plants, and the Threatened Species List of China's Higher Plants, the question of whether its mating system is self-pollinating or cross-pollinating remains a perplexing issue that hinders the conservation and management of wild resources. This study investigated the flowering phenology, floral characteristics, pollen traits, breeding system, and pollination biology of R. nymphaeoides. The results indicate that (1) the peak flowering period for R. nymphaeoides occurs in late April, with the flowering process of a single flower divided into five stages: bud stage, bud swelling stage, initial flowering stage, peak flowering stage, and wilting stage, exhibiting synchronous flowering; (2) the pollen of R. nymphaeoides is tetrahedral, arranged in a regular tetrahedral formation, with an average diameter of 56.47 μm. The individual pollen grains are nearly spherical with a surface covered in sticky threads, featuring three pores with indistinct ornamentation around the grooves, and the outer wall consists of two layers with a faint reticulate sculpture visible on the surface; (3) the pollen vitality of R. nymphaeoides is relatively strong throughout the flowering period, peaking at 11:00 a.m. on the day of flowering at 91.86% and maintaining a 36.67% vitality until the end of the flowering period (9 days after flowering), making it suitable for use as a parent in the hybrid breeding work; (4) the stigma receptivity of R. nymphaeoides lasts for a long time, with the optimal pollination period occurring during the peak flowering stage (days 3-6 after flowering); (5) the most suitable culture medium for pollen is 150 g·L^-1 sucrose + 200 mg·L^-1 H₃BO₃ + 50 mg·L^-1 CaCl₂, with the optimal storage temperature being -80°C; (6) the breeding system of R. nymphaeoides is classified as a facultative cross-fertilization mixed mating system, relying on pollinators, with a low natural fruit set rate (43.33%). The fruit set rate for cross-pollination between different individuals is 83.33%, indicating pollination limitations and weak reproductive assurance (RA); (7) effective pollinators of R. nymphaeoides include Papilio paris, Papilio bianor, Papilio polytes, Papilio protenor, Apis cerana, Vespula koreensis, Vespa velutina, Xylocopa appendiculata, and Xylocopa tranquebarorum, while insects such as Bombus eximius, Anthophora plumipes, Syrphus torvus, Helophilus eristaloidea, Eristalis arbustorum, and Nemoraea pellucida act as nectar robbers, and Osmia taurus serve as pollen thieves.