Action and Transmission Spectra of Phycomyces

Abstract

In this paper measurements are presented of action spectra for the photoresponses of sporangiophores of Phycomyces an(d of the transmission spectra of the growing zone of single sporangiophores in stage IV b (7) and of the cell wall. Compared to other organisms , Phyconmyces offers several definite advantages for such measurements. There are two distinct photoresponses, the growvth response and the tropic response, and there are many indications, particularly the action spectra presented here, that these two responses are mediated by the same pigment. The growth response occurs under conditions of symmetric irradiation when the intensity varies with time. and the tropic response occurs with an irradiation program which may be constant in time, but asymmetric with respect to azimuth angle, i.e., the angle around the axis of the sporangiophore. Crudely speaking. the growth response measures the azimuthal summa-tion of photoeffects, while the tropic response measures the azimuthal asymmetry. The combination of these two measurements permits an estimate of the influence of screening pigments lying betwzeen the receptors proximal to the incident light andl those distal to it. Similarly, the measurement of transmission spectra of the intact sporangiophore and of the cell wall permits some conclusions with respect to the absorbing materials present in various locations. Another advantage of Phycomyces is that the optics of the system are relatively simple. The sensitive region closely approximates a cylindrical lens with uniform refractive index. The importance of the di-optric properties of this lens for the tropic effect was clearly demonstrated 40 years ago by Buder (2), who showed that immersion of the specimens in a medium with refractive index higher than that of the cell contents reversed the sign of the tropic response. Thus, the dioptric properties, both refractive and reflective , and not scattering or absorption have the decisive influence in producing the azimuthal asymmetry in the sporangiophore. Two years ago, Curry and Gruen (10) discovered that the tropic sensitivity of the sporangiophores extends deep into the ultraviolet region and that the tropic response reverses sign aroundl 300 mu. The reversal has a simple explanation. There are screening substances in the growing zone which, for wave-Receive(June 22, 1959.