739 



John D. Spikes and Dennis C. Hall 



would aid in iinder standing the latter process. This follovs 

 since both systems absorb light in approximately the same energy 

 range and then transfer this energy to other molecules throiigh 

 the mediation of some long-lived state of the pigment. Finally^ 

 photodynaraic action might be used as a specific inhibitor in 

 studying sub-reactions of photosynthesis. Chlorella photo- 

 synthesis is more sensitive than the Hill reaction to photo- 

 dynamic inactivation in the presence of Rose bengal. Different 

 dyeS;, because of different chemical and physical properties, 

 might be expected to localize at different sites in the photo- 

 synthetic reaction sequences. On subsequent illumination, dif- 

 ferent sites could then be preferentially inactivated. As an 

 example of this approach, guanine residues are found to be des- 

 troyed preferentially when DNA is illuminated in the presence of 

 methylene blue; the other bases are little affectedv^^/ . 



In contrast to earlier studies ^-'-^^, our data on the time 

 course of inactivation of Chlorella Hill reaction and photosyn- 

 thesis by ultraviolet radiation did not fit first-order kinetics. 

 The dose required for 50^ inhibition of photosynthesis fell with- 

 in the range previously observed' 17) . It is not clear from our 

 data whether the ultraviolet-inactivation of Hill reaction and 

 photosynthetic activities involves the same "sensitive" molecule. 

 With low radiation doses, Hill reaction activity is stimulated 

 significantly while photosynthetic activity immediately de- 

 creases. Hov;-ever, after the initial stimulation. Hill reaction 

 activity decreases with increasing dose in much the same way as 

 photosynthetic activity. 



Packard^ -'-^'' reported that photosynthetic organisms were less 

 sensitive to damage by ionizing radiation when irradiated in the 

 light. The present work shows that Chlorella cells illuminated 

 during gamma irradiation retained more than twice as much Hill 

 reaction and photosynthetic activity as algae irradiated with 

 the same dose in the dark. Another interesting observation was 

 that oxygen partially protected these activities against inacti- 

 vation by gamma radiation. Such a "reverse" oxygen effect has 

 been observed only rarely' 19J. Chlorophyll in situ was much 

 less sensitive to destruction by gamma radiation than chlorophyll 

 dissolved in organic solvents (also see C^OJ)^ jt rnay be that 

 certain constituents of the chloroplast protect the pigment in 

 some v/ay, or the pigment may be present in the chloroplast in a 

 physical form (such as micro-crystals) much more resistant to 

 ionizing radiation than chlorophyll molecules in solution. The 

 increase in extractability of chlorophyll from Chlorella with 



