406 Mineral Nutrition of Plants 



reaction is exergonic, energy is required which, in the case of nitrate 

 reduction in roots, is derived from respiration. 



In 1920, Warburg and Negelein (54) observed a low assimilatory 

 quotient with Chlorella which they assumed was due to the reduction 

 of nitrates by some photosynthetic process. Burstrom (5, 6) later exten- 

 sively studied nitrate reduction in the wheat plant. He found that the 

 root was capable only of thermal reduction, which was independent of 



NITRATE HEXOSE 



p. MOL >* MO 1 - 



60 300 



40 200 - 



20 100 



OTHER 

 ASSIMILATES 



•20 



100 



2000 



FOOT CANDLES 



3000 



Figure 3. Nitrate reduction in excised wheat leaves as a function of 

 photosynthetic rate over a 24-hour period in an atmosphere of about 10 

 per cent higher than normal air carbon dioxide. — H. Burstrom (5). 



Data are for apparent carbon dioxide assimilation, sugar formed, nitrate 

 lost from leaf tissue, and other assimilates (CN compounds) formed. 



light, but required an external supply of manganese. Kinetical stuJies 

 indicated that the reduction occurred at the very outer surface of the 

 root cells and that, while manganese was essential, iron could partially 

 substitute for manganese. 



In contrast to wheat roots, the leaves were completely unable to 

 carry out nitrate reduction in the dark. In the presence of light and 

 carbon dioxide, however, nitrate reduction occurred and the rate paral- 

 leled very closely the photosynthetic rate as shown in Figure 3. This 

 process does not appear to be the result of the synthesis of carbohydrates 



