Role of Carotenoids in the Phototropic Response of Corn Seedlings

The herbicide, 4 chloro-5-(methylamino)-2-(alpha,alpha,alpha,-trifluoro-m-tolyl)-3 (2H)-pyridazinone (SAN 9789), which blocks the synthesis in higher plants of colored carotenoids but not of flavins, was used to examine the involvement of carotenoids in corn seedling phototropism. It was concluded that "bulk" carotenoids are not the photoreceptor pigment based on the results that increasing concentrations of SAN 9789 (up to 100 micromolar) did not alter the phototropic sensitivity to 380 nanometers light (using geotropism as a control) and did not increase the threshold intensities of fluence response curves for both 380 and 450 nanometers light even though carotenoid content was reduced to 1 to 2% of normal. SAN 9789 treatment, however, did reduce seedling sensitivity toward 450 nanometers light indicating that carotenoids are involved in phototropism. Carotenoids, which are located mainly in the primary leaves, may act in phototropism as an internal screen, enhancing the light intensity gradient across the seedling and thus contributing to the ability of the seedling to perceive light direction. These results indicate that the action spectra for phototropic responses can be significantly affected by the absorbance of screening pigments in vivo thus altering its shape from the in vitro absorption spectrum of the photoreceptor pigment.