Abstract:
The length of the growing period in the spring months appears to be a critical factor in the development of hardseededness in subterranean clover (Trifolium subterraneum L.). Environments with relatively long spring growing periods cause a higher proportion of hard seeds to form at field maturity, and increase the resistance which these hard seeds are capable of offering to the softening effects of the following summer environment. During the dry summer period the rate of softening of hard seeds is determined, not only by the previous growing season but also by the summer environment itself. Hot summer environments with wide soil surface temperature fluctuations are conducive to a relatively rapid rate of softening. Grazing or removal of the dry topgrowth from a pasture during the summer increases the daily soil surface temperature fluctuations, and results in the hard seeds softening at an increased rate. Differences in the overall environment manifest themselves in terms of site and seasonal variation in the proportion of hard seeds which survive beyond the opening of the following growing season. The scope for variation is wide, and this has agronomic significance from the aspect of long-term persistence of the species.