RECEPTION OF FAR-ULTRAVIOLET LIGHT IN PHYCOMYCES - ANTAGONISTIC INTERACTION WITH BLUE AND RED-LIGHT

Phototropism experiments were done with sporangiophores of the fungus Phycomyces blakesleeanus to characterize the interaction between far-U V, blue and red light. Far-UV light elicits negative phototropism (ben ding away from the light source) while blue light elicits positive pho totropism (bending toward the light source). In contrast, red light ab ove 600 nm is phototropically inert. Phototropism was analyzed with li ght regimens of bilateral or unilateral irradiation with far-UV and bl ue light. Under bilateral irradiation, in which the two light sources were facing each other, blue light partially inhibited the far-UV-elic ited phototropism. A fluence-response curve for this inhibition showed that blue light was maximally effective at fluence rates which exceed ed 3 to 57 times that of the far-UV. Tonic red light, which was given from above, abolished to a large extent the antagonistic action of blu e light. With a regimen of unilateral irradiation, i.e. when far-UV an d blue light were given from the same side, a phototropic balance coul d be achieved with approximately equal fluence rates of blue and UV li ght. Above or below this critical balance point the bending was either negative or positive. In this setup the effect of tonic red light was complex. First, it caused an enhancement of the positive or negative bending, and second, it caused at some fluence rates a sign reversal f rom positive to negative phototropism. The balance point itself was on ly marginally affected. The data cannot be explained on the basis of a single photoreceptor and support the previous notion of separate far- UV and blue-light receptors. The antagonism between these two receptor s probably occurs on the level of a red-light-absorbing receptor inter mediate.