SEASONAL CHANGES IN THE GERMINATION RESPONSES OF FALL PANICUM TO TEMPERATURE AND LIGHT

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Abstract
Seeds of fall panicum (Panicum dichotomiflorum Michx.) were buried in fine-mesh nylon bags in soil and exposed to the annual temperature cycle. Fresh seeds and seeds exhumed after 1–15 mo were tested in light and darkness at five temperature regimes simulating those in the field from early spring through late autumn. Freshly matured seeds were dormant, but they came out of dormancy during late autumn and winter. Thus, by spring (April) seeds germinated to near 100% in light at 20/10, 25/15, 30/15 and 35/20 °C. However, except at 35/20 °C during June, July and August, the majority of the seeds required light for germination. In the field, germination does not begin until April when temperatures come within the range of those required for germination. Seeds retained the ability to germinate to a high percentage at 30/15 and 35/20 °C throughout the summer, but they lost this ability at 20/10 °C in early summer and at 25/15 °C in late summer. Thus, seeds stop germinating in early autumn when diurnal field temperatures decrease below about 20–25 °C maximum and 10–15 °C minimum. In autumn, seeds lost the ability to germinate even at the high temperatures, but they regained it by the following January.Key words: Germination, dormancy, after-ripening, fall panicum, summer annual, buried seeds