Breaking hard seed dormancy in the perennial legume Lebeckia ambigua E. Mey. to enhance sustainable agricultural production

In this study, seeds of Lebeckia ambigua E. Mey., a South African perennial legume, displayed a high level of physical dormancy, commonly termed ‘hardseededness’. In Australian agricultural systems, this trait is exploited in annual legumes to ensure their regeneration after a cropping phase. While hardseededness in annual legumes has been studied extensively, there have been fewer studies of this feature in perennial legumes. Here, for the first time, we examine the conditions required for hard seed breakdown in L. ambigua, with experiments undertaken in both the field and laboratory. The annual legumes Ornithopus sativus Brot. and Biserrula pelecinus L. were included for comparison. More than 50% of the hard seed of L. ambigua, when buried at 0.5 cm for 87 weeks, remained hard, and 25% were still hard after 188 weeks. We are also the first to demonstrate genetic variation in hard seed breakdown patterns of L. ambigua when buried at 0.5 cm. In the laboratory, L. ambigua seed softened after exposure to a temperature of 80°C for 2–8 days in a dry oven and also after reaching 60°C in a cycling temperature oven, with fluctuating humidity. Seed of L. ambigua produced in two different geographic regions of Western Australia and then buried at 0.5 or 4 cm in the soil, at two softening locations, became differentially soft over 188 weeks. There was a significant three-way interaction between burial depth, site and maternal influences (P<0.01) on hard seed breakdown. The pattern of hard seed breakdown revealed in this perennial legume reflects that described for pyrogenic species and does not fit the models developed by agricultural researchers for annual legumes. Understanding the ecological triggers for release of seeds of L. ambigua from dormancy has provided opportunities for exploitation of this trait in future sustainable agricultural development.