286 RADIATION BIOLOGY 



variety of conditions. The quantum requirements of photosynthesis as 

 determined by him and his associates show considerable variation, with 

 a median of 8.5 photons per molecule. Plotted against chlorophyll con- 

 centration, the yields fall in a band about 2 quantum units wide, the 

 logarithm of the yield increasing with chlorophyll concentration and 

 extrapolating to limits of 6 and 8 photons per molecule for the highest 

 concentration. Emerson and Lewis (1939) had also found that the effi- 

 ciency increased with chlorophyll content — thus the greener the algae, 

 the more efficient the storage of energy. Chlorophyll density and high 

 respiration are probably both indicative of healthy algal cells, which 

 should give normal photosynthesis. 



In a study of the dependence of quantum yield on light intensity, 

 Brackett (personal communication) * found that the evaluation of the res- 

 pirational contribution played an important role. Using average dark 

 values of respiration, he confirmed the findings of Kok (1951a) that high 

 apparent efficiencies of 4 photons per molecule or less appeared below the 

 compensation point. However, marked increases in respiration were 

 found following illumination. For a given culture, respiration can be 

 reduced to a very low rate by dark adaptation or raised to a high rate 

 by light adaptation without any effect on the quantum efficiency. If 

 light-respiration values determined by interpolation are used, the photon 

 requirements and light efficiency become independent of light intensity. 

 Thus the 4 photons per molecule below the compensation point can 

 become 8, as suggested by Kok, if proper correction is made for respi- 

 ration. Brackett believes that this may explain many of the observa- 

 tions of higher efficiency at low light intensities. The photon require- 

 ment ranged from 6 to 13.5 molecules per photon, depending on the 

 intensity of light, the time factor, and the previous history of the algae 

 both in their cultural conditions and in their immediately preceding 

 exposure to light. 



Brackett concludes that, after dark adaptation, the initial rate of photo- 

 synthesis is very small and that it increases to a constant value in 1-3 min. 

 The induction effect decreases after several alternations of dark and light, 

 and this adaptation appears to carry over several minutes of darkness. 



Kok (1951a, 1951b) has carried out an extensive set of experiments and 

 concludes that the minimum quantum reciuirement is 7-8 photons per 

 molecule, but that at low light intensities, where respiration exceeds 

 photosynthesis, the changes in respiration lead to an apparent (juantum 

 requirement of 4 photons per molecule. He reports a definite change in 

 the quantum requirement at the compensation point. 



Warburg and Burk and their associates have published many papers 

 in the period 1950-1952 and have made improvements in apparatus and 



* Added in proof: Brackett el al., 1953b. 



