Does invasion involve alternation of germination requirements?: A comparative study between native and introduced strains of an annual Brassicaceae, Cardamine hirsuta

Cardamine hirsuta is a European annual weed that has been naturalized in Japan. Although the species is a widespread weed in Europe, its introduction to the Japanese Islands occurred recently. We hypothesized that the introduction of Cardamine hirsuta required adaptation that has delayed its spread in Japan. The aim of this study is to determine whether there is a difference in temperature requirements for seed germination between Japanese and European strains of Cardamine hirsuta. We compared temperature requirements for seed germination, because it is known to be a critical determinant of phenology and thus is expected to be important in the success of introduction into different climates. Seeds of six and five strains from Japan and Europe, respectively, were used in seed germination experiments under five different temperature regimes. Japanese strains generally showed stronger initial seed dormancy and were characterized by suppressed germination in higher temperature regimes. European strains showed variable patterns of temperature-dependent seed germination. It turned out that these temperature-dependent dormancies are mediated by abscisic acid (ABA), because dormant seeds germinated under the presence of an inhibitor of ABA synthesis. Seed germination characteristics shared by Japanese strains presumably enhance long transport and autumn germination, while these characteristics are not necessarily common among European strains. The results are supportive for the idea that adaptation has altered germination characteristics of Cardamine hirsuta through the introduction process into Japan.