High density promotes the germination of the recalcitrant seeds of submerged macrophyte Vallisneria natans

Seed germination is a critical process in the seed life cycle, especially for annual submerged macrophytes Vallisneria natans which is a model species for the restoration of degraded shallow lakes. Fresh seeds of V. natans have a very high germination rate but with no significant responses to the aggregation effect; meanwhile, these recalcitrant seeds quickly lose seed viability under dry storage. This study aims to explore the response of seed germination of V. natans after dry storage to aggregation effect by several microcosm experiments. We observed that seed aggregation has a significant positive effect on germination, and among the seven experimental densities between 10 ∼ 640 seeds, higher seed densities resulted in significantly enhanced germination. In contrast, daily removal of germinated seeds led to a significant decrease in the overall germination rate, suggesting that germinated seeds may secrete chemicals that promote the germination of ungerminated seeds. However, further experiments indicated that ethylene and gibberellins had no noticeable impact on seed germination, which was consistent with additional experiment results from spectrophotometer and high-throughput targeted metabolomics. We concluded that the promoting effect of aggregation on the seed germination of V. natans may be an adaptation of this species to its natural habitat but detailed mechanisms warranted further study.