GERMINATION REQUIREMENTS OF OENOTHERA-BIENNIS SEEDS DURING BURIAL UNDER NATURAL SEASONAL TEMPERATURE CYCLES

Buried seeds of Oenothera biennis, which have the potential to form long-lived seed banks, were investigated to determine whether or not they (i) undergo seasonal changes in their dormancy states and (ii) require light for germination. Seeds were buried in soil and exposed to natural seasonal temperature changes. Samples of seeds were exhumed at monthly intervals for 31 months and tested for germination in light and darkness at 12-h daily thermoperiods of 15:6, 20:10, 25:15, 30:15, and 35:20 degrees C. At maturity in autumn, seeds germinated to 84-95% in light at 30:15 and 35:20 degrees C, but to 0-69% at other test conditions. By late winter, seeds germinated to 95-100% at the five thermoperiods in light and in darkness. In summer and autumn, germination in light decreased at 15:6 degrees C, and in darkness it dropped to 0% at 15:6 degrees C and decreased at 20:10, 25:15, 30:15, and 35:20 degrees C. Following the second winter of burial, seeds germinated to near 100% at all thermoperiods in light and darkness. Thus, seeds exhibited an annual nondormancy - conditional dormancy cycle, being nondormant from midwinter to late spring and conditionally dormant in summer and autumn. Oenothera biennis is 1 of 10 species whose seeds live for 39-40 years or longer in soil and also have an annual conditional dormancy - nondormancy cycle. Seeds of six of these species, including O. biennis, can germinate in darkness in spring or summer at simulated habitat temperatures. Therefore, a light requirement for germination is not necessarily a prerequisite for long-term survival of buried seeds, and something other than darkness prevents germination of seeds of some species buried in soil.