SEED AND SEEDLING DYNAMICS OVER 4 CONSECUTIVE YEARS FROM A SINGLE SEED SET OF 6 ANNUAL MEDICS (MEDICAGO SPP) IN NORTH SYRIA

The fate of seed of six annual medics produced in spring 1987 and used in the pasture phase of cereal-pasture rotations was followed over four years. The medics were sown at low and high density, and seed set beyond 1987 was prevented by cultivation in 1988, and by herbicides in 1989 and 1990. The breakdown of hard seed was measured in the summer and autumn of each year, and the number of seedlings was counted after the first seasonal rains in the final three years. The survival of seeds buried at three depths was monitored. Seed of Medicago noeana lived longer than that of M. rotata and M. polymorpha. Survival of M. rigidula and M. truncatula seed was intermediate and there was no effect of planting density on survival. Breakdown of hard seed was least in the first year and increased in the second and third years. Within any year breakdown began in late summer or early autumn, beginning earlier as the seed aged. Seed burial enhanced germination in the year following seed set, depressed it in the second and third years, and had little effect in the fourth year. The net effect of seed burial on survival was therefore slight. The results suggest that the use of medics that have harder seeds than currently available cultivars, and medics in which germination is delayed until late in the autumn, will increase the sustainability and productivity of cereal-pasture rotations in semi-arid areas.