PHYSIOLOGICAL STUDIES OF WEED SEED GERMINATION

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The persistence of many annual and winter annual weeds on the grain farms of Western Canada is closely related to the tendency of their seeds to remain viable in the soil under conditions favorable for the germination of most crop species. Irrespective of competing ability, prolific seeding, and other plant habits, it is only by the relatively narrow germinative responses of its seeds that an annual species can persist under frequent summer-fallowing, spring and fall cultivation, and the hazards of low winter temperatures, drought, and plant competition. Darlington (9) and Toole and Brown (23) have reported two of the classical experiments on the longevity of weed seeds. They showed that seeds of many species may live in a moist soil for several decades without germinating. Toole and Brown point out that while the seeds of many Leguminoseae had to be scarified at the completion of the test before they would germinate, seeds of a number of species that had remained in the soil for 39 years had seed coats which were pervious to and had taken in water during the test. A viable seed which is not in active growth is considered as being in a dormant condition. A seed may be in a state of dormancy because of the lack of one or more essential conditions for its development, a condition termed in this paper environmental'' dormancy. Or the seed may be under a satisfactory germinative environment but be in a state of dormancy because of some physiological condition. This is considered as inherent'' dormancy. The terms environmental and as applied to dormancy are synonymous with the terms induced and natural as used by Brenchley and Warington (3). They are preferable, however, since they are more descriptive of the condition. Also, the term induced was used previously by Kidd and West (16) in discussing the type of dormancy which developed in seeds of Brassica alba in high partial pressures of carbon dioxide. The two types of dormancy, inherent and environmental, are not always as distinct as they would at first appear, since the requirements for the germination of a particular seed may vary somewhat with its degree of inherent dormancy. Crocker (8) divides inherent dormancy into primary and secondary types. Primary dormancy usually exists from maturity of the seed until it is ready to germinate for the first time. This type of dormancy is reported as being due to a seed coat which interferes with water adsorption, gaseous exchange, or expansion by the embryo, or to a direct