274 RADIATION BIOLOGY 



6. BIOLOGICAL FACTORS 



One of the greatest uncertainties in the determination of the quantum 

 yield in photosynthesis Hes in the correction for respiration. Living 

 plants and algal cells take in oxygen and release carbon dioxide in the 

 process of respiration. This respiration goes on in the light as well as 

 in the dark, and in most of the past researches the rate of photosynthesis 

 has been obtained by adding the oxygen taken up in respiration to the 

 observed net evolution of oxygen. Usually the oxygen taken up in respi- 

 ration is determined by measuring the oxygen consumption while the 

 plants or algae are in the dark. It has usually been assumed that the 

 respiration thus determined in the dark is the same as the respiration 

 going on in the presence of the Hght, and practically all quantum yields 

 in photosynthesis have been calculated on this basis. 



Kok (1951a) has reported evidence indicating that the respiration goes 

 considerably faster in the presence of light than in the dark. Below 

 the compensation point, where at low light intensities respiration exceeds 

 photosynthesis, he has found that the apparent quantum requirement 

 is smaller than it is above the compensation point, where photosynthe- 

 sis exceeds respiration. Thus, quantum requirements of 8 photons per 

 molecule were found above the compensation point. He explains his 

 observations by assuming that the respiration below the compensation 

 point is considerably less and the over-all apparent photosynthesis corre- 

 spondingly greater. Calvin (1949) also has evidence that respiration is 

 increased by the introduction of Hght. 



On the other hand. Brown et al. (1952) carried out significant experi- 

 ments with isotopic oxygen, O^^ using a mass spectrometer for analysis 

 of the gases in equihbrium with a rapidly stirred suspension of algae. 

 The decrease in concentration of O^* is a measure of the oxygen taken up 

 in respiration, and this decrease goes on in both dark and hght. The 

 oxygen released in photosynthesis is released from the water and is not 

 isotopically labeled with 0^^ They found that in some types of algae 

 the respiration rate is greater in the light than in the dark, but that in 

 Chlorella it is the same. The evidence concerning Chlorella, then, is con- 

 flicting, and it is not yet known whether it is safe to assume that the respi- 

 ration rate obtained under a specified set of conditions in the dark is 

 maintained in the hght. However, Brown's method seems more reliable. 

 Further experiments and new approaches to the problem are needed.* 



Most of the measurements on photosynthetic efficiencies have been car- 

 ried out with algae, and, of these, the large majority have been Chlorella. 

 Three or four other types of algae, such as Scenedesmiis, have been used, 

 and they give about the same values as the Chlorella. Even salt-water 

 diatoms {Nitzschia closterium) give 8-10 photons per molecule. Some 



* Added in proof: See Brackett et al. (1953). 



