Seed germination ecology in Trapa natans L., a widely distributed freshwater macrophyte

Trapa natans is an aquatic annual plant that grows in stagnant water and occupies an extremely wide, yet discontinuous native range across temperate Europe, Asia, and Africa, and is naturalized in North America and Australia. Despite its wide distribution and invasion success, its seed germination ecology is poorly known. We hypothesized that, due to its wide distribution range, T. natans seeds should germinate under a broad spectrum of environmental conditions. In the present study, freshly matured seeds were cold stratified and tested for germination over a range of temperatures in light/dark, and dark under aerobic and hypoxia conditions. Seed and seedling tolerance to freezing and desiccation also were determined. Freshly matured seeds were physiologically dormant, requiring 9 weeks of cold stratification to overcome the dormancy. Seeds tolerated freezing down to -14 degrees C, whereas those with an emerged hypocotyl exhibited a high sensitivity to freezing down to -4 degrees C. The non-dormant seeds germinated (75-100%) under almost all environmental conditions, i.e. constant 22 degrees C and alternating temperature of 14/6 degrees C to 30/22 degrees C and 30/10 degrees C, light and darkness, oxic and hypoxia, thus supporting our proposed hypothesis. Finally, seeds were found to be highly desiccation-sensitive, even at high relative humidity. Our findings suggest that the wide germination niche could help explain its broad distribution and invasion success at local and global scales.