Variation and Adaptive Role of Seed Dormancy in Some Annual Grassland Species

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Abstract
A total of 87 seeds collected from various parts of central and northern California [USA] were tested for variability in seed germination behavior of 5 grass species (Avena fatua, A. barbata, Bromus mollis, B. rigidus and B. rubens) and 2 legume species (Medicago polymorpha and Trifolium hirtum). Laboratory germination occurred after nearly 6 and 12 wk of storage at 21.degree. C. Seed dormancy breaks down more rapidly in the 5 grass species than in the 2 legumes. There is a great deal of interpopulational variation within each species. Seasonal dormancy in Avena and Bromus was highly correlated with the patterns of a probable rainstorm at different sites during the summer months; this suggested a direct adaptive role of seed dormancy in the timing of germination so as to ensure continued moisture availability in the fall. Estimates of genetic variation between families of A. barbata, B. mollis and T. hirtum suggested dormancy to have low heritability in A. barbata and B. mollis but high heritability in T. hirtum (rose clover). Tests of survivorship, using different families in reciprocal seeding experiments during the summer, established germination rates as an important fitness component but with a low resolution of genetic variance effects. These findings are briefly discussed in terms of the population genetic analyses of seed dormancy, its mode of evolution and its adaptive demographic aspects.