NITRATE REDUCTION 447 



method, careful attention must be paid to the carbohydrate factor before 

 any conclusions can be drawn as to a direct light effect on the nitrate 

 consumption. 



It is also premature to generalize from this result, because other plants 

 may behave differently. There is, as a matter of fact, only one other 

 green plant extensively studied in this respect, the alga Chlorella, and it 

 does not behave at all in this manner. Chlorella can undoubtedly assimi- 

 late nitrate in darkness (Warburg and Negelein, 1920; Myers, 1949), but 

 the rate of assimilation appears to be considerably increased in the light. 



The light effect in Chlorella has been ascribed to either one of two 

 indirect actions: the permeability or, as usual, the carbohydrate metabo- 

 lism, besides a direct action on the nitrate reduction. 



2-2. The Permeability Factor. The influence of light on the permea- 

 bility in connection with general structural features of the cytoplasm is 

 dealt with by Stalfelt in Chap. 12, and reference is made to this treatise 

 for the general background of the problem. If there is an increase in 

 the permeability of the cytoplasm in the hght, this may influence the 

 migration of compounds into or between cells and affect the rate of 

 metabolism disguised as a light effect on the reactions themselves. This 

 circumstance must always be considered in interpreting light effects on 

 intact cells. Warburg and Negelein (1920), the first to investigate the 

 nitrate reduction in Chlorella, computed from the gas exchange an at 

 least threefold increase in the nitrate consumption in the light. They 

 assumed, apparently for lack of other possibilities, that this depended 

 upon an increased uptake of nitrate, referring to the light-permeability 

 relation. With regard to the later development of the conception of the 

 active absorption of ions, this explanation is hardly convincing without 

 more positive evidence. It is pertinent to mention in this connection 

 that Pearsall and Billimoria (1939), with excised pieces of Narcissus 

 leaves, found an increased absorption of nitrate in the light which they 

 attributed to a rise in the temperature. This was not much more than 

 a supposition. However, Pearsall and BiUimoria were able to eliminate 

 the carbohydrate factor in the nitrate assimilation by amply supplying 

 the leaves with sugar, but, in spite of this, synthesis of protein took place 

 only in the light. An effect of light on the intake of nitrate by cells may 

 contribute to the light induction of the reduction, but it can scarcely 

 explain the entire phenomenon. 



2-3. The Carbohydrate Factor. Since the final products of the assimi- 

 lation of nitrate are amino compounds, a supply of nitrogen-free organic 

 matter is necessary for the process. In the dark this means that the 

 assimilation takes place at the expense of stored carbohydrates. 



When the reduction is expressed as in Eq. (8-1), it implies the presence 

 of a hydrogen donor. If we still consider only the conditions in the dark, 

 the only source of hydrogen is probably the glycolytic breakdown of 



