Geographical Differentiation in Leaf Thickness between Coastal and Freshwater Populations of the Coastal Plant Lathyrus japonicus (Fabaceae)

The beach pea, Lathyrus japonicus, is a coastal plant that is also found along the shore of Lake Biwa, an ancient freshwater lake in Japan. These plants occur in two fundamentally differ- ent types of habitats, seacoast and freshwater lakeshores, suggesting the possibility of ecological adaptation to habitats (e.g., salt tolerance). We focused on morphological differentiation between coastal and inland populations and measured leaf blade thickness in a total of ten Japanese popula- tions of L. japonicus from Lake Biwa, Ise Bay on the Pacific Ocean side, and Wakasa Bay on the Sea of Japan side. In addition, we compared transverse leaf sections between inland and coastal populations using light microscopy. An analysis of variance with a post-hoc test revealed that the inland and coastal populations comprised three major groups, with the inland populations account- ing for one group and the coastal populations being assigned to the other two groups regardless of locality. Inland individuals tended to have thinner leaf blades, due to differences in the cell sizes of the palisade and spongy tissues. The succulence in L. japonicus at Lake Biwa may have been lost since salinity tolerance was not essential for their survival in a freshwater environment.