404 Mineral Nutrition of Plants 



Chlorella and found that it was a very rapid reaction, even at low 

 partial pressures of atmospheric carbon dioxide, indicating a low free 

 energy per mol of about -2 kilogram-calories. The free energy of reac- 

 tion of known carboxylations involving aliphatic and aromatic com- 

 pounds is very much higher, being of the order of 10 kilogram-calories. 

 Ruben postulated that high-energy phosphate bonds which have a free 

 energy of 8 to 12 kilogram-calories per bond might be involved in this 

 reaction. 



Emerson, Stauffer, and Umbreit (10) and Ochoa (41) have postu- 

 lated that the later steps following fixation of the carbon dioxide and 

 involving the transfer of energy of the absorbed light quanta probably 

 also occur through the agency of high-energy phosphate bonds. In 

 endeavoring to test these hypotheses, Aronoff and Calvin (2) were 

 unable to show any significant correlation between phosphate turnover 

 and photosynthesis in Chlorella using P 32 as a tracer. Gest and Kamen 

 (/6), however, also used P 32 in Scenedesmus and Chlorella and found 

 that such turnover studies could be carried out with consistent results 

 only when low phosphate cells were used. In such cells, the soluble 

 phosphate, readily exchangeable during washing processes, is reduced 

 to a minimum and errors resulting from such manipulations are con- 

 siderably reduced. They found that phosphate turnover in this case 

 could be correlated directly with photosynthetic activity. 



In Figure 2 are plotted the effects of light intensity on the trichlorace- 

 tic acid-insoluble phosphate in Scenedesmus. The cells were grown on 

 a low phosphate medium and the trichloracetic acid-insoluble fraction 

 was found to be relatively stable as far as leaching was concerned. 

 These data show that light markedly accelerates the formation of 

 insoluble phosphate by the cells. Cyanide in Chlorella depressed the 

 phosphate uptake; since the "ground respiration" of plant cells is 

 cyanide resistant, these results indicate that the accelerated uptake is 

 not correlated with respiratory activity that results directly from in- 

 creased photosynthates. These experiments indicate that ester phos- 

 phate may be formed as a result of light absorption, but the evidence is 

 insufficient to determine whether such esterification is directly coupled 

 with the reactions of photosynthesis. 



Wassink et al. {56, 57) have obtained somewhat similar results with 



