The photo-inhibition of seed germination and the induction of secondary dormancy.

A study of photoinhibition of seed germination and the induction of secondary dormancy has been made on weedy species that tend to produce large numbers of small, light-sensitive seeds. Both light-requiring (e.g., Chenopodium album) and light-inhibited (e.g., Amaranthus caudatus) species were used but nevertheless were found to show similar responses to high fluence irradiation, differences being largely ones of varying sensitivity. All species tested (except Digitalis purpurea) showed a marked inhibition of germination in continuous fluorescent white light supplemented with far-red compared to the response to fluorescent white light only. Urtica urens and A. caudatus germination, however, could be inhibited by fluorescent white light only and this response was dependent on both time and fluence rate. Interestingly, A. caudatus is a dark germinator whereas U. urens is light requiring (i.e., a small amount of light promotes germination but a large amount is inhibitory). The inhibitory action of prolonged light is dependent on photon fluence rate. In photoinhibition mediated through phytochrome cycling rate of the pigment appears to be important. However, cycling rate per se cannot explain all the observed responses; and the interuption of dark processes is another important factor. Cycling rate effects are increased when photoequilibrium is low. There is also a blue light photoreceptor of unknown form which shows fluence dependent characteristics in its photoinhibitory activity. Secondary dormancy is induced in seeds which have the ability to germinate but are prevented from doing so. In respect to the light environment, the same light conditions which inhibit germination also tend to induce secondary dormancy although there is no direct evidence on the action of blue light operating through the blue photoreceptor. The depth of secondary dormancy increases with the time of incubation in the inhibitory conditions and also is deeper for low photoequilibrium and high photon fluence rate. High photon fluence rate fluorescent white light inhibits the germination of U. urens but the high photoequilibrium established allows the seed to germinate in subsequent darkness. Photoinhibition of germination and induction of secondary dormancy have adaptive advantages for survival of the species since seedlings can avoid the competitive stresses of a stand of vegetation and the desiccating environment of the soil surface in full sunlight. Photoinhibition appears to act by preventing the active processes of germination from occurring, which thus enables the seeds to pass through cycles of hydration and dehydration without being damaged.