444 RADIATION BIOLOGY 



assimilation renders a chemical formulation of the nitrate reduction 

 impossible. Thus it is necessary for the following discussion to exclude 

 the assimilation in the restricted sense and to regard the nitrate reduc- 

 tion preliminarily as an entirely inorganic reaction leading to ammonia, 



HNO3 + 8H ^ NH3 + 3H2O, (8-1) 



with the reservation that NH3 actually stands for nitrogen at this level 

 of oxidation in the organic or inorganic form. 



No appreciable amounts of inorganic or organic intermediary products 

 of the nitrate reduction appear normally or have been shown to appear 

 under ordinary conditions in higher plants, but the reduction schemati- 

 cally expressed by Eq. (8-1) practically equals the consumption of nitrate. 

 Most plants, however, are able to accumulate nitrate in the unreduced 

 form in larger or smaller amounts, and therefore the consumption cannot 

 simply be considered equal to the absorption of nitrate from the external 

 nutrient medium, a circumstance that must not be overlooked. 



It has long been known that the consumption of nitrate in the 

 green parts of plants is considerably accelerated by light. Ever since 

 Schimper's work (1888), this fact has given rise to discussions of a possi- 

 ble direct participation of light in the reduction of nitrate, perhaps analo- 

 gous to the function of hght in the photosynthetical assimilation of carbon 

 dioxide. This possibility has been denied as often as it has been con- 

 firmed, or perhaps more often. There are, however, surprisingly few 

 rehable quantitative data in the literature directly illustrating the con- 

 sumption of nitrate in relation to the light conditions which can be 

 brought forward as conclusive arguments in this discussion. They will 

 be dealt with in detail in Sect. 3. 



Diurnal fluctuations of the nitrate content of green leaves have been 

 observed by Chibnall (1922) ; bean leaves contained 0.019 per cent nitrate 

 in the evening, and in the night the figure rose to 0.025 per cent. How- 

 ever, such figures can by no means be interpreted simply as an increase 

 in the nitrate content in the dark and a consumption of nitrate in the 

 daytime. A more complete picture has been obtained with leaves of 

 Helianthus annuus in experiments (unpublished) carried out at the 

 author's institute by H. Rufelt (Fig. 8-1). These observations were 

 made under ecological conditions, i.e., with a normal periodicity of the 

 meteorological factors. This causes, of course, some irregularity in the 

 diurnal variation of the nitrate content. The example recorded in the 

 figure shows a distinct minimum of the nitrate content after noon, which 

 seems to recur regularly. Another was observed 12 hr later, before dawn, 

 so that the nitrate content showed one maximum after sunset and another 

 in the morning. Thus the periodicity does not follow to any appreciable 

 extent the change between light and darkness. 



The amount of nitrate accumulated at any moment is only the net 



