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 i n 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 wit hin 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 ye ar 45 days), whereas M. polymorpha cv. Serena and 2 accessions of T. g lomeratum softened mainly during the autumn (half-lives of 126, 104, a nd 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 sm all seeds. The different patterns can be explained using Taylor's 2-st age model of seed softening. The implications of different patterns ar e 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 f alse breaks to the growing season are likely. The pattern of T. subter raneum is considered to be less well adapted to such an environment. H owever, variation within the species indicates the potential for selec tion of better adapted varieties. The inclusion of the short-term seed softening pattern as a selection criterion for pasture legumes is rec ommended.