Survival and Changes in Germination Response of Rumex obtusifolius Polygonum longisetum and Oenothera biennis during Burial at Three Soil Depths

To investigate patterns in seed germination and survival at three different soil depths over a long time span, seeds of Rumex obtusifolius, Polygonum longisetum and Oenothera biennis were examined as a function of season (spring, autumn) and soil depth (3, 10, 30 cm). Buried seeds were exhumed after 3 years and subjected to two different temperature treatments. These were gradually increasing and decreasing temperatures, starting at 5 and 35°C, respectively. Treatments commenced after a 5-day acclimation to the initial starting temperature with light. With increasing burial depth, diurnal temperature fluctuations decreased and seed survival increased. Seed dormancy patterns changed with season and burial depths for all the species. Polygonum seeds germinated between 15 and 35°C in spring, but not in autumn. Rumex seeds germinated between 15 and 25°C in both spring and autumn. The optimal germination temperature for Oenothera seeds differed between spring (>30°C) and autumn (ca 30°C). Seed dormancy and survival are controlled by soil temperature fluctuations that differ with soil depth and seeds appear to have season-sensing and burial-depth detecting mechanisms based upon temperature fluctuations.