Action spectra for photogravitropism of Phycomyces wild type and three behavioral mutants (L150, L152, and L154).

We have measured fluence rate-response curves and action spectra for photogravitropism in Phycomyces wild type and in three recently isolated mutants with elevated phototropic thresholds. The action spectra were determined from least-squares fits of a sigmoidal function to the fluence rate-response data for each wavelength. The action spectrum for wild type has maxima near 383, 413, 452, and 490 nm and minima near 397, 425, and 469 nm. This photogravitropism action spectrum is very similar to the Phycomyces phototropic balance action spectrum between 413 but has significantly higher effectiveness below 400 nm and above 490 nm. These differences may be caused by dichroic effects of the oriented receptor pigment and/or by multiple receptor pigments. The action spectra of the three mutants differ significantly from one another and from that of wild type. Relative to the wild type spectrum, all three mutants exhibit a suppression in effectiveness near 425 nm, which is near the transmission peak of the broadband blue filter used to isolate the mutants.