Summary The phototactic orientation of Volvox is the result of periodic shading of the photoreceptors, due to the rotation of the colony during the movement. As a consequence, the action spectrum of phototaxis is assumed to represent the absorption of the photoreceptor pigment and that of the shading pigment(s) in a complex way. This paper tries to disentangle this complex with the aim of finding out the contribution of both these systems to the phototactic orientation and to identify the pigments in question. Phototactic Volvox colonies do not swim precisely to the light source but deviate always to the left. This deviation angle is a function of the efficiency of the periodic shading mechanism, hence it depends on the wavelength; with stronger periodic shading the angle decreases. Thus, the action spectrum of the deviation has been used to characterize the absorption properties of the shading system. This action spectrum has a maximum near 434 nm; it is matched very nicely by the combined absorption spectra of stigma and chloroplast with their maxima at 465 and 440 rim, respectively. It is concluded, therefore, that both these organelles contribute to the periodic shading. Sudden increase of intensity results in a typical stop response of those cells which suffer this increase. This response, being the base of phototactic orientation, is due to absorption changes in the photoreceptor pigment proper. Hence, the action spectrum of the stop response represents the absorption spectrum of the photoreceptor pigment. This action spectrum has its maximum at 480-490 nm and extends to the red region until about 626 nm. Nearly no effect has been found below 400 rim. The photoreceptor pigment, therefore, is assumed to be a carotenoprotein. The action spectrum of positive phototaxis has been obtained by using a threshold method. It has its maximum at 491 nm and a shoulder around 433 nm. If, on a proper scale, the action spectrum of the photoreceptor and that of the absorption pigments are added to each other, this combined spectrum fits well with the phototactic action spectrum. Even the very low phototactic effect of red light (626 nm) does not contradict this interpretation. From the knowledge of the shading organelles and the cell's anatomy, one can predict the localization of the photoreceptor structure. This is assumed to be located in the plasmalemma covering the concave side of the stigma, hence in a rather far distance to the basal body of the flagella. Possible mechanisms are discussed by which excitation may be conducted from the photoreceptor to the flagella.
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