Short-term patterns of seed softening in Trifolium subterraneum, T. glomeratum and Medicago polymorpha

The short-term (within-year) dynamics of the softening of hard seeds in a number of accessions of Trifolium subterraneum L., T. glomeratum L., and Medicago polymorpha L. were monitored in the field. There were distinct differences in the patterns of seed softening between and within species and between years. Seed softening was accurately described by logistic curves with calculated half-lives of hard seeds (within a given year) a good indicator of differences in the softening patterns between species and accessions. T. subterraneum cv. Nungarin softened most rapidly over summer, ceasing by March (half-life in the first year 45 days), whereas M. polymorpha cv. Serena and 2 accessions of T. glomeratum softened mainly during the autumn (half-lives of 126, 104, and 136 days, respectively) First year half-lives of 4 other accessions of T. subterraneum ranged from 64 to 79 days. The results showed that large seeds were more likely to soften in the first year than were small seeds. The different patterns can be explained using Taylor's 2-stage model of seed softening. The implications of different patterns are discussed in terms of adaptation to a Mediterranean environment. T. glomeratum and M. polymorpha cv. Serena are considered to have a short-term pattern of seed softening well adapted to an environment where false breaks to the growing season are likely. The pattern of T. subterraneum is considered to be less well adapted to such an environment. However, variation within the species indicates the potential for selection of better adapted varieties. The inclusion of the short-term seed softening pattern as a selection criterion for pasture legumes is recommended.