Softening of temperate annual pasture legume hard seeds on the soil surface and with shallow burial at three contrasting sites in southern Australia

Context Self-regenerating annual pasture legumes generally have hard seeds (impermeable to water) at maturity. The extent and timing of hard-seed softening over summer–autumn is crucial for seedling regeneration and long-term persistence. Aims This study examined diversity for the magnitude and timing of seed softening among annual pasture legumes. Methods In total, 42 cultivars in 15 species were grown at Perth, Adelaide and Canberra (with 20 common entries). Freshly ripened seeds or pods were either placed in nylon mesh pockets on the soil surface or buried at 2 cm depth and sampled for germination testing every 28 days until early winter. Key results The rate and extent of seed softening was greatest in Perth and least in Canberra, with Adelaide being intermediate. Considerable diversity was evident among and within species. Subterranean clover (Trifolium subterraneum) had the greatest and most rapid seed softening, whereas annual medic (Medicago spp.) and most yellow serradella (O. compressus) cultivars had the least, with a range of responses in other species. Burial reduced seed softening in subterranean clover, balansa clover (T. michelianum), biserrula (Biserrula pelecinus), arrowleaf clover (T. vesiculosum), eastern star clover (T. dasyurum) and purple clover (T. purpureum), but increased softening in yellow serradella and had little effect on French serradella (O. sativus) and the annual medics. An important finding was significant cultivar × site and cultivar × site × burial treatment interactions. Conclusions Residual hardseededness, seed-softening rates and response to shallow burial differ among and within species and are dependent on environment. Implications There is sufficient diversity to select annual legumes with appropriate hard seed traits for different regions and farming systems. However, cultivar evaluations need to be conducted under local conditions to ensure that they have the intended adaptation.