Abstract:
A study on the effects of light quality and periodicity on the germination of Douglas-fir seed and the growth of Douglas-fir seedlings was conducted. Interest was focused on responses that could be measured and observed without biochemical assay. It was found that .Douglas-fir seed,s would respond to red-light treatments by germinating and that this stimulating effect of red light could be reversed with far-red light. The phytochrome enzyme system is therefore undoubtedly present. The stimulating effects of red light on germination are influenced by the periodicity of the red-light treatment and temperature. Dry Douglas-fir seeds pre-treated with red light have significantly higher germination when compared to seed receiving no red light pretreatment, provided the seeds are germinated in absolute darkness but not if the seeds are allowed to germinate under "standard" conditions or when sown in the soil. Red light given during the dark period will maintain active growth in Douglas-fir seedlings, and far-red light will cause seedlings to go dormant. Red light is effective in breaking dormancy. as opposed to far-red light. Dormancy can be broken with far-red light breaks provided the dormant seedlings do not receive far-red light pre-treatment and/or provided the dormancy period is sufficiently long prior to the far-red light treatment for the seedlings to have a "dormant rest period". Blue light is also effective in breaking dormancy but red light is the most effective. When actively growing seedlings are irradiated with red and farred light for fifteen minutes in the middle of the dark period, red light has a strong influence on maintaining growth and far-red light has a slight influence on maintaining growth. Excessive amounts of far-red light will cause resumption of growth.
