Structure of the seed coat and its relationship to seed softening in Mediterranean annual legumes

The softening of the hard seeds of six legumes (five clovers and one medic) were measured over one year and reverse logistic equations were used to measure three parameters (hardseededness after exposure to one year of field conditions, the reduction in hardseededness during that time and the time taken for half of the seeds that softened in one year to soften [half life]). All three parameters were related to seed coat and outer layer thickness (surface to the light line). Histochemistry was used to determine the locations of several chemicals found in the seeds of legumes, including polycarboxylic acids, polyphenols, proteins, lipids and polysaccharides.Seeds of the six species varied in radius with a range from 0.52 to 1.46 mm. The seeds of Trifolium subterraneum and Medicago clypeatum were large, while seeds of T. spumosum, T. lappaceum and T. glanduliferum were small. Seeds of M. polymorpha were kidney-shaped while those of the clovers were round. Seed coat morphology was similar for all six species, but the species differed in seed coat (51-150 mu m) and outer layer thickness (2.2-10.7 mu m) and, in the other layers, shape, size and arrangement of the cells. Lignin, polyphenols and lipids were found mainly in the seed coat. In particular the cuticle on the seed surface stained strongly with Sudan Black B, indicating that lipids were confined to the outer layers of the seed coats. Hardseededness after one year was predicted by outer layer thickness (r(2) = 0.81). This relationship was improved (r(2) = 0.92) by using the ratio of outer layer thickness to seed radius. However, neither outer layer thickness nor its ratio with seed radius predicted half life.The results are discussed in terms of conflicting reports in the literature about the role of the cuticle in the softening of hard seeds. The concentration of lipids in the cuticle lends support to the hypothesis that these compounds may be involved in the process of seed softening. There is a clear need for further research to clarify the roles of different chemical groups and different layers within the seed coat.