Seed bank and germination ecology of sub-tropical Vallisneria americana

Vallisneria americana is a broadly distributed North American macrophyte with growth characteristics that vary across regions. While its reproductive traits may also differ with geography, most investigations have occurred in its northern extent, resulting in uncertainty regarding life history and germination strategies in sub-tropical populations. We conducted field monitoring and greenhouse and growth chamber experiments to investigate sub-tropical V. americana seed production, seed banking, and germination cues in central Florida, USA. We observed seed production nearly year-round, which produced a persistent seed bank (where present), with variable, but sometimes high densities (0–6209 seeds/m²). Unlike northern populations that germinate readily, these sub-tropical seeds were dormant, as germination was minimal across temperatures 20–30ºC in light or dark conditions. We found germination was not initiated by individual factors including temperature shifts, warm or cold stratification, high or low dissolved oxygen, or chemicals common in submerged sediments (ethylene or ethanol). However, germination did occur when organic material (sediments and/or vegetative extracts) was present combined with light or following scarification; and germination was enhanced by ethanol and cold-stratification. We also found that germination occurs in organic sediments exposed to high-oxygen conditions but is further improved in low-oxygen organic sediments, possibly mediated by the release of ethanol during oxygen reduction. This enhanced germination in light, decaying organic material, low-oxygen conditions, and ethanol suggests a germination strategy focused on exploiting openings in existing vegetation following disturbance. Without tubers that are present in northern ecotypes, sub-tropical V. americana may rely on seed germination for population maintenance.