Light environment, phytochrome and germination of Datura stramonium L. seeds

The role in germination of the maternal pre-existing far-red absorbing form of phytochrome (Pfr) was studied in seeds harvested at different developmental stages. Datura stramonium seeds acquired marked dormancy in the later ripening stages, independently of phytochrome phototransformation. However, the balance between stimulators, such as Pfr, and inhibitors (probably abscissic acid and/or phenolic substances) in the stage prior to complete dehydration appeared to play a crucial role in the degree of progeny dormancy. Enrichment of the far-red (FR) wavelength (simulated canopy light) induced light-dependent dormancy if stage IV progeny were incubated in darkness in suboptimal conditions. In contrast, natural storage of seeds in soil led to a uniform degree of dormancy. An increase in photosensitivity owing to storage in soil appeared to be linked to simultaneous loss of inhibitors. It should be noted, however, that this extreme sensitivity to very low Pfr levels (724 nm monochromatic light pulse) was acquired only by seeds stored deep in the soil (typical very low fluence response). It is postulated that these reactions are induced by an elevated quantity of phytochrome typical of etiolated tissue, in conditions of near-absence of inhibitors. (C) 1997 Elsevier Science B.V.