312 



RADIATION BIOLOGY 



does lowering of the concentration of the hydrogen donor. This led to 

 the conclusion that the hydrogen donor is not active in the process of 

 energy transfer as such but influences this transfer only via a tempera- 

 ture-sensitive dark reaction. 



If photosynthesis is observed by measuring gas exchange at various 

 incident intensities of light, limitation of the concentration of the hydro- 

 gen donor yields curves showing a typical Blackman inhibition; i.e., 

 photosynthesis is strongly inhibited at high intensities and only slightly 

 or not at all at low light intensities. A similar situation is found if carbon 

 dioxide is available in only limited amounts (Fig. 5-lOa, 6). This means 



250 



c; 200 

 e 



E 



llj 



^ 150 

 < 

 I- 

 a. 



CJ 



8 100 



50 



O 5% COg 

 A ~ l%C02 



c/ 



/ 



/ 



/ 



/ 



/ 



i-i I I 1 L 



I I 



_] I I I L 



I 2 3X10" 



INCIDENT INTENSITY , ergs /cm2-sec 



I 2 3X10" 



INCIDENT INTENSITY , ergs/cm^sec 



{a) ib) 



Fig. 5-10. (a) The influence of the concentration of the hydrogen donor (sodium 

 thiosulfate) upon the rate of photosynthesis in Chromatium, strain D (N2 + 5 per cent 

 COo, phosphate buffer pH 6.3, 29°C). {From Wassink et al, 1942.) 



(b) The influence of the concentration of CO 2 upon the rate of photosynthesis in 

 Chromatium, strain D (N. -F CO 2, sodium thiosulfate 1 per cent, phosphate buffer 

 pH 6.3, 29°C). {From. Wassink et al., 19-12.) 



that both the hydrogen donor and carbon dioxide react in dark chemical 

 reactions and not specifically in the photoactive part of the process. In 

 this connection it is of special interest that the way in which fluorescence 

 of bacteriochlorophyll reacts upon the attainment of light saturation in 

 photosynthesis depends on the cause of the hght saturation. Limited 

 availability of the hydrogen donor causes a strongly increased fluorescence 

 yield at high light intensities. On the other hand, limited availability of 

 carbon dioxide leaves the fluorescence yield practically unaltered (Fig. 

 5-11). There is only a slight increase in yield at medium light intensities, 

 and at high intensities the yield often is even somewhat decreased. 

 These observations reveal the important fact that the dark reactions 



