A shift of the response thershold to red irradiation in dormant lettuce seeds

Seeds of lettuce of the cultivars Noran and Meikoningin germinate readily in darkness, but a requirement for red light can be induced by long exposure to far-red irradiation or by exposure to temperatures above 30 °C. After the first treatment the response threshold to red irradiation is several orders of magnitude higher than after the second treatment.