EFFECTS OF EXTENDED (4-12 YEARS) BURIAL ON SEED SOFTENING IN SUBTERRANEAN CLOVER AND ANNUAL MEDICS

The effect of burial of seeds of 3 cultivars of subterranean clover (T rifolium subterraneum) and 1 cultivar each of burr medic (Medicago pol ymorpha) and barrel medic (M. truncatula) that had experienced I summe r at the soil surface was continued from 4 to up to 12 years. Seeds we re situated at 2, 6 and 10 cm depth, as well as at the soil surface. N umbers of residual seeds were determined after each sampling occasion, the timing of which was varied between cultivars and depths of burial according to the progress of seed softening. The marked reduction in the rate of seed softening in all 3 clover cultivars with increasing d epth of burial established during the first 4 years of the experiment was maintained. After 12 years, 37% of the Nungarin clover seeds that had been buried at 10 cm were still present as hard seeds. All residua l hard seeds germinated readily after nicking with a razor blade. Seed s of both medic cultivars, that were slower to soften than the clovers at the soil surface, continued to show little effect of burial at 2 c m, or of burial to 6 cm in the case of Cyprus barrel medic. Species di fferences in response to seed burial are explained in terms of the eff ects of soil temperatures on the 2-stage seed softening process. The a bsence of an effect of shallow burial on the softening of medic seeds appears to be attributable to a lower optimum temperature for the firs t stage of seed softening than is the case for subterranean clover. Un favourable temperatures for the final stage of seed softening can resu lt in the accumulation of latent soft seeds, particularly in subterran ean clover. These latent soft seeds will soften during the first summe r/autumn after their return to close to the soil surface as a conseque nce of tillage. Whereas increased seed longevity as a consequence of b urial in crop years can be advantageous in terms of legume persistence , particularly of clover, it can also be distinctly disadvantageous wh en it comes to cultivar replacement.