Assessment of Seasonal Changes in the Germination of Heteromorphic Achenes of Bidens pilosa

In weed species, asynchronous emergence and lengthy emergence periods are important factors that increase the difficulty of weed management. Seed heteromorphism and polymorphism are one of the causes of asynchronous emergence of weed species. Bidens pilosa is a noxious weed species. Its capitulum are heteromorphic achenes divided into central and peripheral achenes. Central achenes are longer, with thorns facing outward from the capitulum, whereas peripheral achenes are shorter, with thorns facing inward. In the present study, we hypothesized that dimorphic achenes of B. pilosa have different seed dispersal strategies, and we compared the difficulty of their detachment and seasonal changes in their germination in order to identify the differences in these strategies. Our results revealed that central achenes were significantly easier to detach than peripheral achenes, suggesting that central achenes may be used for dispersal to farther areas, and peripheral achenes may be used for dispersal to neighboring areas. Furthermore, central achenes showed relatively higher germination rates than peripheral achenes. Only the central achenes showed high germination in dark conditions at 20°C in May. In contrast, peripheral achenes showed high germination in dark conditions at 20 C and 25 C in July. This suggested that central achenes germinated first (in May), and peripheral achenes germinated later in the summer. Our results also indicated that the achenes of B. pilosa showed seasonal changes in photoblastic germination, i.e., both central and peripheral achenes showed light-induced germination in March and dark-induced germination from May to September. Dimorphic achenes and seasonal change in dormancy and photoblastic germination cause asynchronous emergence of plants, making it difficult to control the spreading of B. pilosa.