EFFECT OF ULTRAVIOLET LIGHT 



1923 



line, indicating a superposition, upon the main first-order inactivation 

 process, of higher order effects, particularly noticeable at low exposures. 

 Fig. 37D.11 (to be compared with fig. 36 on p. 345) shows this deviation, 

 and incidentally illustrates once again the steadiness of endogenous respira- 

 tion. With Chlorella, the solid exposure curves deviate from linearity 

 much stronger than those obtained by interrupted exposure. 



# RESPIRATION 



8 16 24 32 40 



MINUTES ULTRAVIOLET 



Fig. 37D.n. Inhibition of photosynthesis by ultraviolet light (253.7 m^) (after 

 Holt, Brooks and Arnold 1951). 20 mm.s cells of Scenedesinus Di, 20% C. R = rate. 

 (D), Cells from fresh culture, in buffer No. 9. (O), same culture stored at 4° C. for 6 

 days and resuspended in 0.05 M KH2PO4 under 5% CO2 in air. ( • ), same culture stored 

 at 4° C. for 7 days; resuspended in buffer No. 9. Interrupted exposures. 



In isolated chloroplast fragments (from Phytolacca americana), the 

 capacity for the Hill reaction (measured by the initial rates of oxygen libera- 

 tion from ferricyanide solution, and by decoloration of phenolindophenol 

 solution) also was strongly affected by irradiation with 253.7 m/x. It 

 disappeared after 25 min. of irradiation. The catalase and the poly- 

 phenol oxidase activity of the same preparation was, on the contrary, 

 unchanged even after 60 min. of irradiation; the cytochrome oxidase 

 activity was down by only 12% after 25 min., and by 55% after 60 min. 

 None of these three enzymes is thus responsible for the ultraviolet injury to 

 the chloroplasts, nor is chlorophyll, since it showed no spectroscopic change 

 after irradiation. 



The inactivation of the chloroplast fragments was found to affect in the 

 same proportion the rates of the Hill reaction in light of all intensities, 



