NITRATE REDUCTION 449 



open question whether it can be quantitatively expressed by the proposed 

 equation, Eq. (8-3). The unsettled problem is how nitrate is linked with 

 the breakdown of sugar and how far it is reduced before it is assimilated. 



It ought to be mentioned in this connection that several authors dis- 

 cuss an extra gas formation in darkness or in light from inadequate 

 observations, in so far as they lack determinations of either oxygen or 

 carbon dioxide, but present figures that do not necessarily denote more 

 than an increase in the respiratory intensity (Wehner, 1928; Hamner, 

 1936; Lovell, 1938; Gilbert and Shive, 1945). Such an increase can, of 

 course, be caused merely by the presence of nitrate or a higher nitrogen 

 content, apart from an extra gas production. 



It is, furthermore, of importance to observe that nitrate can hardly 

 function simply as a hydrogen acceptor in the terminal oxidation instead 

 of oxygen, which could be inferred from Eq. (8-1). Thermodynamic 

 objections may be raised against such a simplification of the problem, 

 and there are two physiological reasons as well: First, the addition of 

 nitrate seems to cause a production of extra carbon dioxide mainly by 

 increasing the liberation of carbon dioxide, whereas the consumption of 

 oxygen may remain unchanged or decrease slightly (Ruhland and Ullrich, 

 1929; Cramer and Myers, 1948). This means that nitrate must increase 

 the glycolytic breakdown of sugar and not simply replace oxygen. It is 

 known that oxygen tensions corresponding to the normal content of the 

 air do not limit the respiration to any great extent. Thus it seems 

 unlikely that an increased supply of hydrogen acceptors can augment 

 the consumption of sugar. Second, the normal assimilation of nitrate, 

 in contrast to the bacterial denitrification, seems to be an aerobic process 

 (Warburg and Negelein, 1920; Yamagata, 1934; Burstrom, 1939); this 

 does not quite agree with the idea that nitrate can replace oxygen as an 

 oxidizer. Nance (1948, 1950) assumes, on the other hand, that the step 

 nitrate to nitrite is really an anaerobic reaction and that only the subse- 

 quent steps are aerobic. 



These reflections lead to the conclusion that nitrate can hardly act 

 strictly as assumed in Eqs. (8-1) to (8-3); it seems even more probable, 

 on the contrary, that nitrate reacts with some intermediary product of 

 the glycolysis, presumably at such an early stage of the breakdown of 

 sugar that this is accelerated by nitrate removing the intermediary com- 

 pound. This may be written 



nCHaO ^ X + 2CO2, .^ .. 



HNO3 + X ^ NH3 + Y. ^ ^^ 



The reduction of nitrate to the ammonia level involves several steps 

 performed with the aid of different hydrogen donors, but only the first 

 one has as yet been successfully studied in green plants. Evans and 

 Nason (1952, 1953) have clearly demonstrated in different plant mate- 



