446 RADIATION BIOLOGY 



in the morning until 2 p.m. (Fig. 8-1) is consequently high, and the com- 

 paratively slight reduction of the nitrate content at this time of the day 

 must correspond to a considerably higher rate of nitrate consumption 

 than in the evening with closed stomata and, nevertheless, increasing 

 content of nitrate. The importance of the stomatal movements for the 

 nitrate consumption is obvious. The cause of the second minimum of 

 the nitrate content around 2 a.m. has not been disclosed. It coincides 

 with the minimum of stomatal aperture and may illustrate some other 

 factor that ought to be considered for a full interpretation of the diurnal 

 rhythm. 



The effect of the stomatal factor on the transpiration was eliminated 

 by Dittrich (1930) in experiments with wheat leaves. In leaves kept in 

 ordinary dry air, the nitrate content increased between 4 and 11 a.m. 

 from 1.86 to 2.16 mg per gram of fresh matter; if, however, the transpi- 

 ration under the same conditions was kept down by spraying the plants 

 with water, the content of nitrate decreased from 1.52 to 1.29 mg/g. 

 The increase in the nitrate content observed in this instance in the morn- 

 ing can accordingly be ascribed to the transpiration. From 11 a.m. to 

 7 p.m., on the contrary, the nitrate content in ordinarily treated leaves 

 decreased from 2.16 to 1.30 mg/g, and in water-sprayed leaves, from 

 1.29 to 0.88 mg/g. This decrease obviously cannot depend upon a 

 change in transpiration and the ensuing nitrate supply but must involve 

 a consumption of nitrate. It may, as Dittrich has suggested, indicate a 

 direct light action on the nitrate reduction ; there is, however, still to be 

 considered the importance of the carbohydrate metabolism for the nitrate 

 consumption, and even here the stomatal movement may be the decisive 



factor. 



A more reliable method of eliminating the transpiration and the absorp- 

 tion of nitrate is to study the reduction of nitrate accumulated in leaves 

 in its relation to light, provided that a migration of nitrate within the 

 leaf does not limit the reduction. Such experiments have likewise been 

 carried out with wheat leaves (Burstrom, 1937, 1943a,b), and they have 

 shown the reduction to be much more intimately connected with the 

 illumination than would be expected from the rather slight diurnal vari- 

 ations of the nitrate content under ecological conditions. The reason, 

 as has been exemplified in the foregoing, is that an increased absorption 

 in the hght seemingly counteracts the increased consumption. In one 

 pair of experiments (Burstrom, 1937), for instance, the assimilation in 

 the light amounted in 48 hr to 0.084 mmole of an available 0.240 mmole, 

 or 35 per cent, whereas in darkness it did not exceed 7 per cent. 



Repeated experiments of this kind (Burstrom, 1943a,b) have failed to 

 disclose any significant reduction of nitrate in wheat leaves in darkness, 

 but they show a measurable consumption even at fairly low light 

 intensities, as will be discussed fully in the next section. Even with this 



