Dependence of the Phototropic Response of Arabidopsis thaliana on Fluence Rate and Wavelength

In the phototropic response of Arabidopsis thaliana seedlings, the shape of the fluence-response relation depends on fluence rate and wavelength. At low fluence rates, the response to 450-nm light is characterized by a single maximum at about 0.3 μ mol· m-2. At higher fluence rates, the response shows two distinct maxima, I and II, at 0.3 and 3.5 μ mol· m-2, respectively. The response to 500-nm light shows a single maximum at 2 μ mol· m-2, and the response to 510-nm light shows a single maximum at 4.5 μ mol· m-2, independent of fluence rate. The response to 490-nm light shows a maximal at 4.5 μ mol· m-2 and a shoulder at about 0.6 μ mol· m-2. Preirradiation with high-fluence 510-nm light from above, immediately followed by unilateral 450-nm light, eliminates maximum II but not maximum I. Preirradiation with high-fluence 450-nm light from above eliminates the response to subsequent unilateral irradiation with either 450-nm or 510-nm light. The recovery of the response following high-fluence 450-nm light is considerably slower than the recovery following high-fluence 510-nm light. Unilateral irradiation with low-fluence 510-nm light followed by 450-nm light results in curvature that is approximately the sum of those produced by either irradiation alone. Based on these results, it is proposed that phototropism in A. thaliana seedlings is mediated by at least two blue-light photoreceptor pigments.