Hardseededness in Mediterranean annual pasture legumes in Australia: a review

Abstract

Hardseededness (seed coat impermeability) is the main seed dormancy mechanism for regulating germination of annual pasture legumes both within and between years. Progress made in Australia over the past 30 years towards an understanding and better utilisation of the mechanism is the subject of this review, together with relevant overseas studies. Although some legumes produce virtually no hard seeds, newly ripened seeds of most cultivated annual pasture legumes are >90% hard when produced under favourable seed maturation conditions. The pattern of seed softening (loss of impermeability) varies widely among legumes both within and between years and is one of the more important considerations in selection programs. The many factors that influence the longevity of seed hardness are described. Differences among legumes in patterns of summer and autumn seed softening, which may influence the extent of seed losses through false breaks of season, are explained in terms of a 2-stage conceptual model of the seed softening process. This model has led to the development of laboratory techniques that effectively simulate field softening behaviour in a wide range of legumes. Different rates of seed imbibition, which may be attributable to a previously unrecognised stage in the seed softening process, and which can offer some further protection against false breaks of season are also described. The wide range of seed softening characteristics that are now recognised provides opportunities for better adapting pasture legumes to particular management systems, including rotations with crops.