The Role of Light and Nitrate in Seed Germination

Abstract

The minimal requirement for seed germination is the imbibed state. If seeds germinate independently of external factors such as light and soil components, and without a need for pretreatments, either dry or imbibed, at certain temperatures, they can be considered non-dormant in an absolute sense. However, most seeds require one or more external stimulants to germinate. This requirement depends on the ‘ecological history’ of the seed, including the developmental period on the motherplant. By far the most important component of the seed’s ecological history is temperature. The ‘temperature history’ determines the number and magnitude of environmental factors, including temperature, that are required for germination. In other words, it determines the dormancy status of the seeds. Therefore, over the year, dormancy of seeds in the field may alternate, giving rise to dormancy cycles (Bouwmeester and Karssen, 1989). Figure 1 shows that the expression of dormancy not only depends on the temperature history but also on the germination test conditions. Dormancy of seeds of Sisymbrium officinale was induced by a prolonged dark incubation in water at 15° C. At a germination temperature of 24 °C the seeds did not germinate in the dark, irrespective of the pretreatment period. Thus, under these conditions a change in dormancy could not be expressed. However, when the seeds were irradiated with red light (R) and germinated at 24 °C, the influence of the pretreatment became visible by the changing germination level. Addition of nitrate to the germination medium in combination with R irradiation again changed the germination pattern: induction of dormancy appeared to be delayed. This pattern has been shown for a number of species in our laboratory (Bouwmeester and Karssen, 1989) and can be deduced from several germination studies described in the literature.