The contribution of seeds to the recruitment of a Nymphoides peltata population

Abstract

The Nymphoides peltata population expands quickly in Lake Taihu, China. One question addressed in this study is whether the seeds of N. peltata contribute to this expansion. The buoyancy and germination of N. peltata seeds and the development of N. peltata seedlings were studied, using seeds collected from Lake Taihu. The results indicated that a low wind velocity of 2.4–3.0 m s −1 had a slightly negative effect on seed buoyancy. After 19 and 67 h of gentle stirring, 50% and 90%, respectively, of the N. peltata seeds had sunk. Few seeds floated again after sinking, but these refloating seeds sank soon with the disturbance. The N. peltata seeds did not germinate without stratification, but the stratification of seeds for a two-week period resulted in a high germination rate (63.3%) at a light intensity of 20 μmol photons m −2 s −1 . Both the light and stratification treatments stimulated the seeds germination. The seeds did not germinate in sediment at depths greater than 0.25 cm. A high germination rate (74%) was observed for the seeds that laid on the water–sediment interface; however, nearly all of the germinated seeds floated on the water surface after germination. Only a small fraction (14%) of the buoyant seedlings could re-establish in shallow water (less than 3 cm). In the eighth week of the experiments, the buoyant seedlings that failed to re-establish rotted. When grown in low light intensity conditions, the N. peltata seedlings had smaller cotyledons, shorter primary roots, and weak development of adventitious roots. Sufficient light was important for both seed germination and seedling development. It was found that sexual reproduction is likely to have little direct contribution to the rapid expansion of N. peltata towards the centre of this large shallow lake.