Effects of extended (4-12 years) burial on seed softening in subterranean clover and annual medics

Abstract

The effect of burial of seeds of 3 cultivars of subterranean clover (Trifolium subterraneum) and 1 cultivar each of burr medic (Medicago polymorpha) and barrel medic (M. truncatula) that had experienced 1 summer at the soil surface was continued from 4 to up to 12 years. Seeds were situated at 2, 6 and 10 cm depth, as well as at the soil surface. Numbers 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 depth 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 residual hard seeds germinated readily after nicking with a razor blade. Seeds 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 cm, or of burial to 6 cm in the case of Cyprus barrel medic. Species differences in response to seed burial are explained in terms of the effects of soil temperatures on the 2-stage seed softening process. The absence of an effect of shallow burial on the softening of medic seeds appears to be attributable to a lower optimum temperature for the first stage of seed softening than is the case for subterranean clover. Unfavourable temperatures for the final stage of seed softening can result in the accumulation of latent soft seeds, particularly in subterranean clover. These latent soft seeds will soften during the first summer/autumn after their return to close to the soil surface as a consequence of tillage. Whereas increased seed longevity as a consequence of burial in crop years can be advantageous in terms of legume persistence, particularly of clover, it can also be distinctly disadvantageous when it comes to cultivar replacement.