Effects of light intensity and plant density on growth and reproduction of the amphicarpic annual Persicaria thunbergii

To understand the advantages of amphicarpy in the early life history of Persicaria thunbergii, we compared plants from aerial seeds (AS) and subterranean seeds (SS) in two experiments using mesocosms. In full light, the growth of AS plants (75.5 ± 10.8 g/plant) and SS plants (80.2 ± 9.6 g/plant) was similar. However, in shade, SS plants (81.1 ± 8.5 g/plant) grew significantly more than did AS plants (40.3 ± 8.5 g/plant). SS plants produced more subterranean seeds in shade than they did in full light. An experiment comparing three seedling densities revealed no effect on total biomass of SS plants or AS plants, but the relative biomass of seeds produced by SS plants (12.6%) was higher than that produced by AS plants (6.4%). These traits of SS seedlings were more advantageous when competition for light was intensified as seedlings density increased. We suggest that SS plants play a key role in population formation in mature communities with dense canopies.