Seasonal control of seed germination

Seeds commonly develop on the mother plant with a level of intrinsic dormancy such that they do not germinate immediately upon shedding. Instead, germination is delayed both to avoid competition between the mother plant and its seedlings, and to coordinate seedling emergence with favourable environmental conditions. In annuals such as Arabidopsis thaliana, this primary or intrinsic dormancy is largely imposed by the parent plant during seed development and is influenced by the environmental factors to which the plant is exposed. After shedding from the plant the level of dormancy in seeds can fluctuate in response to environmental factors such as water, light, temperature and nutrients. Such factors tend to reflect variables in the spatial environment (‘Is this the right environment in which to germinate?’). Against this background of post-shedding fluctuations in dormancy, ecotypes behaving either as winter or summer annuals exhibit contrasting patterns of seasonal seed dormancy, ensuring that germination occurs in autumn or spring, respectively (‘Is this the right time to germinate?’). Such seasonal dormancy cycling is essential to species or ecotype fitness. A major question is how seeds in the soil recognize autumn and spring, since they cannot measure daylength if they are not exposed to light. Much evidence indicates that temperature is very important in conveying such temporal information, leading to the concept of ‘thermal time’ as a means by which seeds could distinguish autumn and spring.