Significance of post-germination buoyancy in Helmholtzia glaberrima and Philydrum lanuginosum (Philydraceae)

Post-germination buoyancy has been proposed to reduce seedling establishment in amphibious plants if seeds germinate under flooded conditions. We tested this hypothesis in two amphibious species, Helmholtzia glaberrima (Hook.) and Philydrum lanuginosum (Banks & Sol.). We tested whether seed germination was affected by different levels of inundation, whether seedlings germinating underwater floated and whether seedlings established after prolonged floatation at rates comparable to seedling establishment in waterlogged soil. Germination underwater and in waterlogged soil was similar for both species. Seeds germinating underwater exhibited post-germination buoyancy and established at similar rates to seedlings in waterlogged soil. The results demonstrated that if seeds germinated underwater, post-germination buoyancy conferred the potential to avoid inundation and promote establishment, when waters recede and/or in areas of high soil moisture that are not submerged. More generally, this finding showed that post-germination buoyancy can promote seedling dispersal and the deposition of seedlings in conditions more favourable for establishment if seeds germinate under flooded conditions or in permanent wetlands.