INDUCTION OF DARK DORMANCY IN SEEDS - ITS IMPORTANCE FOR THE SEED BANK IN THE SOIL

Exposure to light breaks dormancy in seeds of many species. Dark dormancy prevents germination of seeds in the soil, which could otherwise be fatal. However, seeds may not be dark dormant upon dispersal, or exposure to stimulatory doses of light before burial may still cause the seeds to germinate in the soil. The induction of dark dormancy when germination was inhibited during incubation in polyethylene glycol has been investigated in three species. The process was faster at high than at low temperatures. In Origanum vulgare L. dark dormancy was re-established rather slowly. Disappearance of phytochrome in the far-red absorbing form (Pfr) and re-establishment of phytochrome control, from which the seeds had escaped, were involved. Induction of dark dormancy was much faster in Plantago major L., due to a reduction in responsiveness to light, absence of escape from phytochrome control and a high threshold level of Pfr for breaking dormancy. Not all seeds of O. vulgare and P. lanceolata L. were initially dark dormant; a light requirement was induced in the former, but not in c 25% of the seeds of the latter species. The importance of these findings for the formation of a seed bank in the soil is discussed.