Seed dormancy characteristics in six weed species as affected by after-ripening temperatures and field conditions
Dormant seeds of Amaranthus retroflexus L. (AMARE), Chenopodium album L. (CHEAL), Digitaria sanguinalis (L.)Scop. (DIGSA), Echinochloa crus-galli (L.)Beauv. (ECHCG), Polygonum pensylvanicum L. (POLPY), and Setaria glauca (L.)Beauv. (SETLU) were collected in both 1983 and 1984 at the Curtiss Experimental Farm, Ames, Iowa. They were planted both on the soil surface, and at 2 cm in pots, and then exposed to field conditions from January to June, 1984, and from October, 1984 to July, 1985. Five replications of each treatment were removed from the field every month and carried to the greenhouse to test germination capacity, along with seeds that had been kept at 10 C and 40% RH. Significant differences in dormancy characteristics occurred between buried seeds and seeds on the soil surface for all six species. Germination of AMARE, CHEAL and ECHCG seeds planted on the soil surface, was apparently associated with the photoperiod prevailing in the greenhouse, while the conditions previously experienced played a minor role. When buried at 2 cm, AMARE seeds remained dormant or acquired secondary dormancy; part of the CHEAL and ECHCG seeds germinated after experiencing low temperatures during January and February. DIGSA seeds did not germinate while buried in the soil, and exhibited delayed germination until July when on the soil surface. Buried non-dormant seeds of ECHCG and DIGSA entered into a secondary dormancy when subjected to field conditions from October to January. Both POLPY and SETLU seeds were released from primary dormancy by the prevailing field conditions; this occurred at different times according to their location in the soil. POLPY seeds entered secondary dormancy if conditions were not suitable for germination when primary dormancy was broken;Seeds were also stored for 15 months at -20 to 40 C, and germination was periodically tested in the greenhouse and the growth chamber. Temperatures above 0 C caused accelerated after-ripening in all species except POLPY. AMARE, ECHCG and SETLU seeds germinated better in the dark, while CHEAL and DIGSA seeds germinated better in the light. These differences, however, were less evident for seeds stored at 20 to 40 C;Most of these responses are thought to be phytochrome-mediated and may have practical implications regarding the behavior of weed seeds under different soil management methods.