Flowering characteristics and mating system of Fritillaria cirrhosa (Liliaceae), an endangered plant in China

Fritillaria cirrhosa D. Don is an endangered species of the family Liliaceae that has economic, medicinal, and ecological values, as well as the source of the traditional Chinese medicine ‘Fritillariae Cirrhosae Bulbus.’ Fritillariae Cirrhosae Bulbus is one of the traditional precious Chinese medicines in China, which has the medicinal value of clearing away heat and moistening the lungs, resolving phlegm and relieving cough, dispelling knots, and eliminating carbuncle. In this study, the phenology, flowering dynamics, and floral traits of F. cirrhosa were observed in the field, and the related morphological indicators were measured. The pollen viability and stigma receptivity during flowering of F. cirrhosa were measured by MTT method and benzidine–hydrogen peroxide method, respectively, and the microstructures of pollen grains were observed using scanning electron microscopy. At the same time, the mating system of F. cirrhosa was evaluated by measuring the pollen and ovule ratio (P:O), determining the outcrossing index (OCI), and performing bagging experiments. Based on these observations, we found that the phenology of F. cirrhosa was from late April to mid-August, the average duration of 117 days, divided into eight stages from germination to fructescence, of which the population flowering lasted 31 days. The flowering period of F. cirrhosa is divided into initial budding stage, middle budding stage, late budding stage, initial bloom stage, full bloom stage, and final bloom stage. Most of the anthers of F. cirrhosa were longitudinally split and released pollens from the proximal end of the anthers to the top from the second day of flowering, and the anthers corresponding to the outer corolla began to release pollens first. As the anthers dehisced and pollens were released, and the stamens were gradually lower than the pistils. The pollen grains of F. cirrhosa were prolate, which had the reticulated surface and various mesh sizes. The pollen viability and stigma receptivity of F. cirrhosa increased at first and then decreased with the flowering stage. Combined with P:O = 2891.6 ± 373.4, OCI = 4 and the results of the bagging experiments, we determined that F. cirrhosa was generally outcrossing. These results provide the basis for the cultivation, breeding, and conservation of this rare and charismatic plant, helping to reveal the nature of its sexual reproduction process, explore the possible endangerment reasons from the perspective of reproductive bio