MULTIPLICATIVE ACTION OF A UV‐B PHOTORECEPTOR and PHYTOCHROME IN ANTHOCYANIN SYNTHESIS

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Abstract
—: Using 290‐nm light, which excites only a UV‐B photoreceptor, and 385‐ and 660‐nm light, which activate only phytochrome, the fluence rate‐response curves of monochromatic irradiations for anthocyanin synthesis in the first internodes of broom sorghum (Sorghum bicolor Moench, cv. Acme Broomcorn) were analyzed. Although the two photoreceptors absorbed light independently, they multiplicatively increased the action of each other. Accordingly, when the fluence rates of both wavelengths were changed together, the resulting slopes of the fluence rate‐response curves of double‐log plots were steep compared with the slopes obtained with the respective monochromatic irradiations. The slopes of fluence rate‐response curves for monochromatic irradiations at 325 to 345 nm were steeper than those at other wavelengths. This difference was shown to be due to the multiplicative actions of both photoreceptors.