318 RADIATION BIOLOGY 



carbon dioxide acceptor that may have properties of the "reducing 

 agent" is tentatively formulated as vinyl phosphate, and some results 

 suggest that glycolic acid may participate in the formation of the carbon 

 dioxide acceptor. More recent results suggest that the C2 compound is 

 not generated from a Ci — Ci condensation but is derived from C3 and 

 C4 compounds (Calvin et al., 1950, p. 531, and Bassham et al., 1954*). 



The generation of a reducing power under carbon dioxide-free illumi- 

 nation has been questioned by Brown et al. (1948). They interpret the 

 increased uptake, as found by Calvin et al. after preillumination in the 

 absence of carbon dioxide, as being due to a depletion of carbon dioxide 

 "stores" during the illumination. Benson and Calvin (1950) consider 

 this improbable for two reasons: (1) uptake of carbon dioxide after carbon 

 dioxide-free preillumination yields chiefly the saAe compounds as photo- 

 synthesis; (2) cells of Scenedesrmis, preilluminated without carbon dioxide, 

 take up carbon dioxide at a rate one hundred times greater than non- 

 preilluminated ones. According to the view of Brown et al, this would 

 mean that the pressure of carbon dioxide inside the cells had decreased 

 about one hundred times, i.e., to about 0.001 mm Hg during preillumi- 

 nation. This was considered improbable, and the conclusion was drawn 

 "that the chief action of hght was to produce reducing agent(s) and 

 carbon dioxide acceptor(s)" (Benson and Calvin, 1950, p. 37). It should 

 be pointed out, however, that it is difficult to estimate the lower level to 

 which the pressure of carbon dioxide inside a carbon dioxide-free illumi- 

 nated cell may fall. Wassink et al. (1951) observed that illumination of a 

 ChloreUa suspension in the absence of carbon dioxide may easily lead to 

 precipitation of inorganic phosphate. Therefore, the result of Calvin and 

 Benson's experiment may not be fully conclusive as to the generation of 

 reducing power. Nevertheless, it is supported by various other kinds of 

 evidence [collected, for example, by Wassink (1951a)], and it seems to 

 offer the most rational explanation. For a further development of this 

 discussion see Gaffron and Fager (1951). 



Light intensity appears to influence the distribution between 2- and 3-phospho- 

 glyceric acid as first products. Scenedesmus gives the same products in hydrogen- 

 adapted photoreduction, and in photosynthesis. According to Calvin et al. (1950, 

 p. 527), "the major difference between photosynthesis and photoreduction, then, 

 Ues in the mechanisms of hydrogen transfer and not in the path of carbon reduc- 

 tion by the reducing agents produced." As to the relation between photosynthe- 

 sis and the depletion of reservoirs by preillumination, it is interesting that they 

 found the proportion of radiocarbon in malic acid, aspartic acid, and alanine 

 greater in the latter case than in short periods of photosynthesis. " Depletion of 

 the malic acid (C4) and alanine (C3) reservoirs by preillumination (reduction to 

 hexose) resulted in their restoration with labeled compounds as soon as a source 

 of carbon dioxide became available." It should be observed that this conclusion 

 is consistent with the view of Brown et al. (1948) discussed previously. If pre- 



* See Addendum. 



