Abstract
Phototaxis of plasmodium of true slime mold Physarum polyce-phalum was studied as a function of light wavelength and intensity. The changein the motive force of protoplasmic streaming (ΔPt) or isometric tension (ΔF/Fo) was taken as the phototactic response. Three active spectral regionsexist in the action spectrum of phototaxis. The blue light region (λmax = 490nm)or far-red light region (λmax = 720nm) induced negative taxis, and the red lightregion (λmax = 650nm) led to positive taxis. The absorption spectrum of plas-modial yellow pigments did not coincide with the maxima of the action spec-trum. The time course of the phototactic response ΔPt was independent ofintensity for a given wavelength so far as the intensity of stimulating lightexceeded the threshold value, suggesting that light stimulation acted as atrigger for initiating the tactic movement. The mode of phototaxis resembledthat of chemotaxis for motive force, i.e., the steady value of phototactic motiveforce ΔP changed from zero to 8-12 cm H2O in rather narrow intensity regions, and ΔP values were independent of the intensity of light so far as the intensityexceeded the threshold value Ic. When the slime mold was illuminated withblue (500nm, 510nm) and far-red (720nm) light, isometric tension increasedwith the intensity of light (I>Ic) while for red light (650nm) isometric tensiondid not show an appreciable change on illumination.
