SPECIFIC TROPISM CAUSED BY ULTRAVIOLET-C RADIATION IN PHYCOMYCES

The giant sporangiophores of Phycomyces blakesleeanus turn towards blu e and away from ultraviolet C sources (wavelength under 310 nm). We ha ve isolated fifteen mutants with normal blue tropism but defective ult raviolet tropism. Wild-type sporangiophores described a double turn wh en exposed successively to blue and ultraviolet beams coming from the same side; under certain conditions, the mutants turned only to the bl ue. The new uvi mutations modified the behaviour in heterokaryosis and were lethal in homokaryosis, i.e., they affected essential cellular c omponents. The responses of the wild type and one of the mutants were registered and evaluated with a computer-aided device. The mutant beha ved normally under blue light, but took longer than the wild type to t urn away from the ultraviolet source. With very weak ultraviolet stimu li (10(-8) and 10(-9) W . m(-2)), the wild type turned towards the sou rce, but the mutant did not respond. Calculations of absorbed-energy d istributions in the sporangiophore showed that Phycomyces responds dif ferently to similar spatial distributions of blue and ultraviolet radi ations. Wild-type and mutant sporangiophores had the same high ultravi olet absorption due to gallic acid. We conclude that ultraviolet tropi sm is not just a modification of blue phototropism due to the high ult raviolet absorption of the sporangiophores. Phycomyces has a separate sensory system responsive to ultraviolet radiation, but not to blue li ght.