The biphasic fluence response of carotenogenesis in Neurospora crassa: Temporary insensitivity of the photoreceptor system.

Whether or not illumination is continuous or interrupted during the span in which increasing illumination time periods (i.e., increasing fluences) have no different effect on carotenogenesis is optional, in regard to the amount of carotenoids produced (revealing the plateau of the biphasic fluence response curve). This indicates temporary insensitivity. When the time delay between the onsets of the initial and second illumination is extended beyond the expanse of the plateau, the amount of carotenoids induced by the second illumination depends on the time elapsed following the first exposure; after ca. 2 h, maximum competence for a second induction is completely restored. On the other hand, the amount of carotenoids induced by a second illumination also depends on the duration of this second illumination, but, unlike the dependence in a single illumination, results in a different fluence response curve for the second exposure. When UV-A is used for induction, the refractory period which follows the first exposure seems to be the same as for blue light, suggesting vision of UV-A and blue light by the same photoreceptor system.