REDUCTION OF NITRATES IN PLANT TISSUES 441 



organic nitrogenous compounds in plants whenever nitrates are the source 

 of nitrogen. Nitrates are first reduced to nitrites and these are further re- 

 duced to the NH2 and NH3 groups found in organic compounds. 



Each of the steps in the reduction of nitrogen requires energj\ The 

 observation that nitrates disappear more rapidly from leaves exposed to full 

 sunlight than from leaves in shade or darkness has led to the suggestion that 

 the energy of sunlight can be used directly in the reduction of nitrates. This 

 process can occur in the total absence of light, however, and it is generally 

 agreed that the necessary energ>' is supplied by the process of respiration. 

 Upon the initiation of nitrate reduction in plant tissues their rate of respiration 

 may increase several fold. Light probably exerts only an indirect effect on 

 this process and the relatively rapid reduction of nitrates in illuminated green 

 tissues is presumably due to the higher carbohydrate content of such tissues. 

 Carbohydrates are not only consumed in respiration during nitrate reduction 

 but are also used in the construction of the organic nitrogenous compounds 

 which are built up during this process. 



Eckerson (1924) has succeeded in following microchemically some of 

 the steps in the reduction of nitrates in the tomato plant. Rapidly growing 

 tomato plants were transplanted from good soil to quartz sand when about 

 eight inches tall. The plants were watered with a solution lacking nitrogen 

 until the tissues showed no tests for nitrates, nitrites, ammonia, and amino 

 acids, although they still contained an abundance of carbohydrates. Calcium 

 nitrate was then added to the sand. The nitrate ions were rapidly absorbed 

 and could be detected in all parts of the plant within twenty-four hours. 

 In thirty-six hours nitrites were present in considerable amounts at the tips 

 of the stems and in certain other tissues. Traces of ammonia could also be 

 detected. By the end of forty-eight hours the quantity of nitrite had de- 

 creased, the amount of ammonium ion in the plants had increased, and a small 

 quantity of asparagine was also found to be present. Three to five days after 

 the addition of the nitrate to the sand around the roots amino acids were 

 present in abundance in the plant tissues and they continued to increase in 

 quantity for three weeks. During the synthesis of these amino acids the 

 carbohydrate content of the cells decreased. The reduction of nitrates within 

 plant tissues is catalyzed by an enzyme known as reducase. 



Temperature exerts a marked effect on the nitrate reducing capacity of 

 plants. In the tomato plant, for example, although nitrates are almost in- 

 stantaneously absorbed, their reduction and the synthesis of organic nitrogen 

 compounds occur very slowly at 13° C. At 21° C, on the other hand, both 

 absorption and reduction of nitrate ions occur very rapidly (Nightingale, 

 1933). 



