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Many weeds produce dormant seeds that are unable to complete germination under favourable conditions. There are two types of seed dormancy: primary dormancy (innate dormancy), in which seeds are in a dormant state upon release from the parent plant, and secondary dormancy (induced dormancy), in which dormancy develops in seeds through some experience after release from the parent plant. Mechanisms of seed dormancy are categorized as embryo dormancy and coat-imposed dormancy. In embryo dormancy, the control of dormancy resides within the embryo itself, and in coat-imposed dormancy, it is maintained by the structures enclosing the embryo. Many factors can influence seed dormancy during development and after dispersal; they can be abiotic, biotic, or a combination of both. Most weeds deposit a large number of seeds in the seed bank, which can be one of two types—transient or persistent. In the transient type, all viable seeds in the soil germinate or die within one year, and there is no carry-over until a new crop is deposited. In the persistent type, at least some seeds survive in the soil for more than one year and there is always some carry-over until a new crop is deposited. Some dormant seeds require after-ripening—changes in dry seeds that cause or improve germination. Nondormant, viable seeds can germinate if they encounter appropriate conditions. In the face of climate change, including global warming, some weeds produce a large proportion of nondormant seeds, which germinate shortly after dispersal, and a smaller, more transient seed bank. Further studies are required to explore this phenomenon. |
