Survival of annual ryegrass (Lolium rigidum Gaud.) seed in a Mediterranean type environment. II. Effects of short-term burial on persistence of viable seed.

Two seed components, dark dormant and non-dark dormant, were identified in experiments in which germination and survival of seeds buried in soil were examined. These comprised c. 10-20% and 80-90% of the seed population respectively. About 12% of the dark dormant seeds (1-2% of total seed population) germinated when first buried between 0 and 2 cm depth, and subsequent disturbance of the soil on two occasions caused further germinations. In contrast, all non-dark dormant seed rapidly germinated when buried at 2 cm; however, germination of non-dark dormant seeds decreased progressively as burial depth increased, until at 11 and 14 cm no seed germinated. In one experiment, seed that failed to germinate when buried at 14 cm was found to germinate readily if transferred to a depth of 2 cm without soil disturbance. This indicated that dormancy was enforced, rather than induced, in seeds buried relatively deeply for short periods. In these experiments soil temperature and moisture were favourable for germination, and it is suggested that an unfavourable gaseous environment around deeply buried seeds is the factor enforcing dormancy. The implication of these findings for the persistence of annual ryegrass in the pasture-crop rotation is discussed.