Seed dormancy release in three common pasture grasses from a Mediterranean-type environment under contrasting conditions

Pastures in the Mediterranean region of Australia are typically comprised of a mixture of winter annuals, including grasses and legumes, which utilise a seed bank to survive the long, dry summer. The risk of out-of-season summer rain requires species from such environments to adopt appropriate strategies to protect their seed banks. The seed bank strategies of the grasses from these environments have, so far, received little attention. We conducted an experiment to observe the dormancy release characteristics of 3 grasses common to these environments when stored under contrasting thermal regimes. The grasses studied were great brome grass (Bromus diandrus Roth), annual ryegrass (Lolium rigidum Gaud.), and barley grass (Hordeum leporinum Link). The different species displayed contrasting dormancy release characteristics over the summer. Two of the species examined (great brome grass and annual ryegrass) displayed relatively late release from dormancy which gave high levels of protection from false breaks. The other (barley grass) displayed earlier release from dormancy which would enable it to better exploit earlier true breaks but make it more sensitive to false breaks. Unlike pasture legumes, high and fluctuating temperatures were not necessary for the seeds of these grasses to become germinable. In fact, exposure to high temperatures appeared more likely to suppress release from dormancy.