-j.54 RADIATION BIOLOGY 



rises to a nearly constant and, in the two instances, equal figure; whether 

 this depends upon the supply of nitrate cannot be inferred from the data. 



The interpretation presupposes that the oxygen values in darkness 

 show the normal respiration, to which the extra carbon dioxide is added. 

 Furthermore in high-carbohydrate plants in the light the extra carbon 

 dioxide that is not assimilated causes a reduction in the apparent uptake 

 of carbon dioxide, but the real assimilation could be computed from the 

 oxygen values (column 6 of Table 8-1). In low-carbohydrate plants, on 

 the contrary, the carbon dioxide values in the light may show the real 

 assimilation, and nitrate causes an additional production of oxygen. The 

 striking result is that the two series of values for the real assimilation of 

 carbon dioxide, computed in different ways for the two sets of plants, 

 tally very well. This was to be expected if the computations are correct, 

 because it is hardly probable, with 4 per cent carbon dioxide in the air, 

 that additional gas exchange could influence the actual rate of carbon 

 dioxide fixation. 



It is also obvious that the interpretation of Myers, involving two dif- 

 ferent mechanisms, alone can explain the surprising fact that both oxygen 

 evolution and carbon dioxide fixation are higher in the low-carbohydrate 

 than in the high-carbohydrate algae — the oxygen values because extra 

 oxygen is produced, and the carbon dioxide values because extra carbon 

 dioxide is produced in the latter algae. 



The nature of the photoreaction in the low-light algae is not clear. 

 Myers eliminates the possibility of a photorespiration on the ground that 

 it should not exist in Chlorella (cf. Burk et al, 1949). Further experi- 

 ments of Myers have revealed that nitrogen-deficient cells, which ought 

 to be rich in carbohydrates, have an extra carbon dioxide production in 

 the dark of 1.2 units but, at saturating light intensity, an extra oxygen 

 evolution of 10.3 units. If the nitrate reduction was linked with respi- 

 ration in this instance, this would imply an eightfold increase in the 

 glycolysis in light, which is incompatible with all experience. 



This way of reasoning should, as a matter of fact, exclude every possi- 

 bility except a connection between nitrate reduction and photolysis if we 

 refrain from as.suming an independent photochemical splitting of nitrate. 

 There is, however, no evidence for or against the existence of such a 

 mechanism. 



3-2. Experiments on Wheat. The investigations by the author (Bur- 

 strom, 1943a, b, 1945) have likewise led to the assumption of two paths 

 of nitrate assimilation. In wheat plants they should be locally separated. 

 Assimilation of nitrate takes place in both roots and leaves; in the former 

 it is confined to darkness and is independent of light, and in the leaves 

 it is almost entirely dependent upon light. Only the second process need 

 be considered here. 



There is no significant assimilation of accumulated nitrate in wheat 



