The effects of temperature and light on the dormancy of velvetleaf, common waterhemp, and giant foxtail seeds were studied under controlled growth chamber conditions. Seeds were either kept chilled at 4 C for 12 wk under wet conditions or nonchilled at 4 C in dry storage. Then, seeds were germinated under increasing and decreasing temperatures and under continuous red light (R) and far-red light (FR). In addition, chilled and nonchilled seeds were germinated in the dark after being exposed to alternating R and FR flashes. Velvetleaf germination was increased by exposure to high temperatures (36 C) immediately after exposure to low temperatures (4 C), but light had no effect. Chilling increased common waterhemp seed germination and sensitivity to light and temperature. R promoted common waterhemp seed germination, whereas FR inhibited germination and maintained dormancy. In addition, the effect of light was reversible. Therefore, common waterhemp dormancy was phytochrome regulated. However, high temperatures (36 C) promoted the germination of chilled seeds, even when exposed to FR. The germination of chilled giant foxtail seeds was reduced by FR. Giant foxtail seed dormancy was partially phytochrome regulated, but dormancy regulation was more dependent on mean temperature.

Nomenclature: Common waterhemp, Amaranthus tuberculatus (Moq.) J.D. Sauer. syn. Amaranthus rudis Sauer. AMATA; giant foxtail, Setaria faberi Herrm. SETFA; velvetleaf, Abutilon theophrasti Medicus. ABUTH.