Photoinduced Seed Germination of Oenothera biennis L: II. Analysis of the Photoinduction Period.

The photoinduction period of Oenothera biennis L. seed germination was analyzed by varying the photoinduction temperature and by substituting red light pulses for continuous red light. At 24 degrees C, seeds require 36 hours of continuous red light for maximal percent germination. The optimal photoinduction temperature is 32 degrees C, with higher and lower temperatures being strongly inhibitory. A 30 minute exposure to far-red light, given immediately after a red light period of 1 to 36 hours, reduces germination by about 25%. Seeds escape from far-red inhibition with a half-time of 5 to 10 hours, depending on the length of the red exposure that precedes the far-red light. Periodic 15 minute pulses of red light can substitute for continuous red light in stimulating germination. Ted red light pulses, with 6 hours of darkness between successive pulses, cause maximal germination. The response to periodic red light is fully reversible by far-red light. Probit analysis of the periodic light response shows that as the length of the dark periods between successive pulses increases, less incident light is needed to induce germination but the population variance in light sensitivity remains constant. Probit analysis of the temperature response shows that as the photoinduction temperature increases from 16 to 32 degrees C, less incident light is needed to induce germination and the population variance in light sensitivity also increases.