Flowering characteristics and reproductive biology of Nymphaea hybrid, a precious water lily

Abstract

Nymphaea hybrid is a precious aquatic plant of the family Nymphaeaceae that has esthetic, economic, medicinal, and ecological values. Owing to its spectrum of color, especially that of the rare blue strain of the genus Nymphaea, it has gained importance in the breeding of water lilies. However, the lack of research on its flowering characteristics and reproductive biology has severely hindered the reproduction and breeding of superior water lily varieties. In this study, the flowering phenology, floral characteristics, and flower-visiting insects of N. hybrid were assessed. The microstructures of stigmata and pollen grains were observed and the stigma receptivity was determined. Pollen viability was measured by an in vitro germination experiment. We also evaluated the breeding system by measuring the pollen/ovule (P/O) ratio, determining the outcrossing index (OCI), and performing field pollination experiments. The N. hybrid population flowered from late May to mid-November, with a flowering period of approximately 160 days. The florescence of a single flower ranged for 3‒5 days from June‒August and for 5‒10 days from September‒November. Inverse herkogamy and protogyny were exhibited by N. hybrid flowers. Stigmata were wet with papillae distributed on their surface, and the pollen grains were oblate with granular ornamentation of different sizes on their surface. The pollen germination rate was highest when they were deposited in a medium containing 5% sucrose, 0.001% boric acid, 1% agar, and 20 mg/L calcium chloride and cultured at 30 °C. Stigma receptivity was the strongest on the 1st day of floral anthesis, and pollen viability was the highest between 9:30 a.m. and 11:30 a.m. on the 2nd day of floral anthesis. The P/O ratio suggests that xenogamy is the main reproductive system. Artificial pollination results indicated that N. hybrid favored outcrossing and required the participation of pollinators, such as Bombus lucorum L. and Xylocopa s.str. valga Gerstaecker, both of which are effective bee pollinators. These results can aid in formulation and implementation of strategies for N. hybrid hybridization programs, and can also improve the controlled pollination efficiency for agronomic and ornamental purposes.