AN ACTION SPECTRUM IN THE BLUE and ULTRAVIOLET FOR PHOTOTROPISM IN ALFALFA*

Abstract

Abstract Phototropism is a common property of plants, but it is not known if different species use the same photoreceptor for their response. We have determined fluence‐response relations for phototropism in response to brief, broad‐band blue irradiation for four plant species grown under red light (Amaranthus paniculatus, Linum usitatissimum, Vigna radiata and Medicago sativa) and compared these to ones previously obtained for Pisum sativum and Zea mays, grown under similar conditions. Curves for all species showed a bell‐shaped dependence on fluence, a characteristic of first positive curvature as originally defined for the Avena coleoptile, and had a similar optimal fluence, near 3 H.mol m−2. We have obtained an action spectrum in the blue and UV spectral regions for first positive phototropism of the hypocotyl of alfalfa grown under red light. Fluence‐response curves at wavelengths between 300 and 500 nm were nearly identical in shape and magnitude; whereas below 300 nm, their slopes and maximum curvatures were reduced. The action spectrum showed that activity rose sharply at wavelengths below 500 nm, peaked at 450 nm with shoulders on either side of that peak, and had lesser peaks at 380 and, in the far ultraviolet, at 280 nm. This action spectrum was very similar to ones in the literature (obtained between 350 and 500 nm) for first and second positive phototropism of oat coleoptiles. We conclude that the same photoreceptor mediates phototropism in oat and alfalfa.