EFFECTS OF IRRADIANCE AND SPECTRAL QUALITY ON THE GERMINATION OF VERBASCUM THAPSUS L. AND OENOTHERA BIENNIS L. SEEDS

S ummary Germination of Verbascum thapsus L. and Oenothera biennis L. seeds was investigated over a range of irradiance levels and under different canopy types in the laboratory. Both species colonize early successional and disturbed habitats, and their seedlings establish only in bare soil. In the laboratory, Verbascum seeds germinated to equivalent levels (70 to 80%) over a broad range of irradiance levels and required only brief exposure to light (5 s) to germinate subsequently in darkness. In contrast, germination of Oenothera seeds increased with irradiance to a maximum of approximately 80% at 13 W m −2 . Oenothera seeds also required a longer duration of exposure (1 h) to induce germination, and percentage germination increased directly with exposure time to a maximum of 80 % after 22 h of exposure. Germination of both species was significantly reduced in light filtered through leaves, but the responses differed under different canopy types. Exposure to red and blue light had no effect on germination of Verbascum seeds but had an inhibitory effect on that of Oenothera seeds. The results are discussed in relation to the observed patterns of seedling emergence and survival of these species in the field; a possible evolutionary basis is suggested for their differences in response to light.