An attempt to prove the ecological significance of red-far red control mechanisms in seed germination was made. The seeds of 30 species were exposed beneath the plant canopies. All the normally light-stimulated seeds, and also seeds of 14 (out of 19) “insensitive’ species and seeds of 1 (out of 4) light-inhibited species, were inhibited or significantly retarded in their germination, as compared with seeds exposed to diffuse light in an artificial construction.
Further experiments with “insensitive’ seeds of Lactuca sativa L. cv. Cud Vorburgu showed that after prolonged plant-shadow treatment the seeds became light-sensitive in the usual phytochrome-mediated manner. Seeds exposed under the plant canopies during a few days were extremely sensitive to red or white light, but this sensitivity diminished slowly in the course of treatment.
The spectral composition of light filtered through the leaves shows great preponderance of far red radiation. The red-far red reversion can be simply obtained with the natural light and a leaf. In open stands bright weather retards considerably the germination of lettuce, cloudy weather brings about full germination.
Some considerations on the ecological significance of seed behaviour, particularly as connected with plant competition, are given.