1616 PHOTOCHEMISTRY OF CHLOROPHYLL CHAP. 35 



changed by preillumination. Krasnovsky suggested that Mehler's results 

 were affected by high temperature and presence of air, which favored back 

 reactions. 



Gerretsen (1950^ believed he found potentiometric evidence that 

 anaerobically preilluminated, crude chloroplast suspensions maintain 

 in the dark, for more than an hour, a capacity to form a peroxide upon ad- 

 mission of oxygen or addition of Mn++ salt. 



6. Mutations 



Davis (1952) apphed induced mutations to the study of the relation of 

 Hill reaction to photosynthesis. He subjected Chlorella cells to ultraviolet 

 irradiation and isolated three strains which differed from the wild type by 

 being unable to grow in an inorganic nutrient solution in light. These 

 strains grew well, however, if supplied with glucose. They were green 

 and contained chlorophyll. Two strains (322 and 349) did not evolve oxy- 

 gen in light when suspended in a carbonate bicarbonate buffer, and did not 

 take up carbon dioxide. One strain (332) liberated oxygen in light, but did. 

 not take up carbon dioxide. Intact cells of strains 322 and 349 did not liber- 

 ate oxygen when suspended in Hill's solution in the absence of external car- 

 bon dioxide and illuminated. They thus seemed to have a block in the 

 oxygen-liberating mechanism. Strain 332 liberated oxygen in light in the 

 absence of carbon dioxide, and in the presence as well as in absence of Hill's 

 solution. In the latter case, hydrogen from water must have been donated 

 to some cellular component. Strain 322, which is unable to photosynthe- 

 size, also has a subnormal respiration rate. Assuming single gene muta- 

 tion, this correlation can be considered as indicative of the presence of two 

 respiratory systems, one of which is associated with the chlorophyllous 

 mechanism. With strains 322 and 349, the effect of light on respiration 

 could be studied; experiments showed the absence of such an effect. The 

 wild type was inhibited when grouTi in the presence of sufficient quantities 

 of mutant cells. The inhibition could be caused either by chlorellin (c/., 

 page 880) being produced in greater quantities by the mutants than by the 

 wild type, or by compounds which accumulate within the mutant cells as a 

 result of the blocked reactions and diffuse into the medium. 



C. Photochemistry of Live Cells* 



The first observation that a "Hill reaction" (i. e., sensitized oxidation 

 of water to oxygen, with oxidant other than carbon dioxide) can occur in 

 live cells was made by Fan, Stauffer and Umbreit in 1943 (Vol. I, page 541). 

 They found that oxygen is liberated in light from carbon dioxide-free 

 Chlorella suspensions supplied with ferric phosphate or other ferric salts. 



* Bibliography, page 1629. 



