Robert B. Withrow 407 



as such, since it will be noted that even when the light intensity drops 

 below the compensation point and respiration is consuming more car- 

 bohydrates than are being formed, nitrate reduction still proceeds. 

 Neither does the process appear to be a simple photochemical reduction 

 of nitrate as a substitute for carbonate since carbon dioxide is likewise 

 essential as exhibited in Figure 4, where nitrate reduction was carried 

 out in an atmosphere containing only about 10 per cent normal atmos- 



HEXOSE 

 NITRATE 



/IMOL 



40 



20 



-20 



■40 



OTHER 

 ASSIMILATES 



1000 2000 



FOOT CANDLES 



3000 



Figure 4. Nitrate reduction in excised wheat leaves in a low carbon 

 dioxide atmosphere, about 10 per cent of normal air. — H. Burstrom (5). 



pheric carbon dioxide concentration. When the carbon dioxide con- 

 centration is very low, nitrate reduction appears to take precedence over 

 sugar formation; possibly some intermediate product of carbon dioxide 

 reduction is utilized in the reduction of nitrate, thus blocking sugar 

 formation. At high light intensities, sugar formation practically ceases 

 in a low carbon dioxide atmosphere. These experiments at low carbon 



