CSIRO PUBLISHING www.publish.csiro.au/journals/ajar Australian Journal of Agricultural Research, 2006, 57, 65–74 Variation in seed softening patterns and impact of seed production environment on hardseededness in early-maturing genotypes of subterranean clover H. C. Norman A,B,F , F. P. Smith A,B , P. G. H. Nichols B,C , P. Si D , andN. W. Galwey B,D,E A CSIRO Centre for Environment and Life Sciences, Private Bag 5, Wembley, WA 6913, Australia. B Centre for Legumes in Mediterranean Agriculture, University of Western Australia, Crawley, WA 6009, Australia. C Department of Agriculture Western Australia, 3 Baron Hay Court, South Perth, WA 6151, Australia. D Faculty of Natural and Agricultural Sciences, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia. E Present address: New House, George Green, Little Hallingbury, Bishop’s Stortford, Hertfordshire, CM22 7PP, UK. F Corresponding author. Email: Hayley.Norman@csiro.au Abstract. This paper describes variation in the dynamics of seed softening (loss of impermeability) in 20 early- maturing genotypes, including 6 cultivars, of subterranean clover (Trifolium subterraneum L.). It reports the effect of 3 sites of seed production in south-western Australia on the pattern of softening in the first summer–autumn and on total softening over the subsequent 2 years. Seeds were softened at a single field location and in a diurnally fluctuating cabinet (60 ◦ C/15 ◦ C). There was significant variation among genotypes in the pattern of seed softening over the first 5 months after senescence. Cultivars Nungarin, Dwalganup, and Geraldton softened most rapidly in late February, whereas cultivars Dalkeith, Urana, and Izmir softened most rapidly in late March. The duration of field exposure required in order for 50% of the first season’s softening to occur ranged from 44 to 108 days among the 20 genotypes. Persistence of hard seeds into the second and third years also varied among genotypes. Of the cultivars, Nungarin and Izmir had the highest levels of residual hard seed after 30 months (5.3% and 3.9%, respectively), whereas Dalkeith had the lowest (0.9%). Site of seed production had a small but significant effect on both the pattern of softening in the first summer– autumn and the persistence of hard seeds in subsequent years. Seeds produced in a relatively high-rainfall site (768 mm of growing-season rainfall plus supplementary irrigation) had a slower rate of hard seed breakdown than those from either of 2 sites located in the wheatbelt (217 and 423 mm growing-season rainfall). Seed softening through exposure in the field and in a 60 ◦ C/15 ◦ C fluctuating-temperature cabinet was compared for all genotypes. The cabinet treatment was fairly successful in ranking genotypes for relative between-season hardseededness, although it underestimated total softening by an average of 16%. However, the cabinet treatment was a poor predictor of the within-season pattern of seed softening. Additional keywords: Trifolium subterraneum, dormancy, seedbank ecology, pasture legumes, persistence, fluctuating-temperature cabinet. Introduction Subterranean clover (Trifolium subterraneaum L.) is one of the most widely distributed annual legumes in southern Australia (Fortune et al. 1995) and plays an integral role in farming systems, grown either in rotation with crops (ley-farming systems) or as permanent or short-term phase pastures (Reeves and Ewing 1993; Howieson et al. 2000). First distributed in 1909 (Symon 1961), subterranean clover occupied approximately 20 million ha by the 1970s (Donald 1970; Cocks and Phillips 1979). By the 1980s, it appeared that the legume content of pastures was declining in many ley- farming systems and this was likely to be partially associated with an inability of the legume seedbank to persist through longer cropping rotations (Ewing 1982; Carter and Challis 1987; Taylor et al. 1991). Hardseededness (impermeability of the seed coat to water) is the most important form of seed dormancy in annual clovers and is essential to persistence in farming systems in the Mediterranean-type climate of south-western Australia. When clovers are grown in rotation with crops © CSIRO 2006 10.1071/AR05116 0004-9409/06/010065