458 RADIATION BIOLOGY 



supply of light energy is limited, nitrate and carbon dioxide compete for 

 the available photolytically formed hydrogen. In the former case the 

 result is an increased production of oxygen, and in the latter, a reduced 

 carbon dioxide assimilation in the presence of nitrate. It should be possi- 

 ble to decide this question experimentally. 



Under ordinary conditions, even at a moderate light intensity, the 

 assimilation of carbon dioxide can be increased considerably by increas- 

 ing the supply of this gas. This means that light under such circum- 

 stances does not limit the reductions, and hydrogen ought to be formed 

 photolytically in excess of that which is used for photosynthesis at the 

 normally low content of carbon dioxide in the air. This excess could, 

 of course, be utilized for the reduction of nitrate; ordinarily it must be 

 reoxidized by oxygen to water. If this is the case, the extra oxygen pro- 

 duction must, as emphasized by Pirson and Wilhelmi (1950), consist of 

 an increase in the oxygen evolution. They claim that their own experi- 

 ments have shown a deficit in the consumption of carbon dioxide but 

 admit that "die 02-Entwicklung .... bei einsetzender Nitratverarbei- 

 tung offenbar nicht ganz unbeeinflusst bleibt." This judgment does not 

 quite agree with their own diagrams, from which the following figures 

 can be drawn. At a moderate deficiency of nitrogen the gas exchange, 

 with a respiratory quotient of around 1, amounted to about 38 units and 

 after 4 hr to about 33 ; with the addition of nitrate the oxygen production 

 after 4 hr had increased to 47 units, and the carbon dioxide consumption 

 had decreased to 36. With a strong deficiency the corresponding figures 

 for controls without nitrate were 28 and 30 units, respectively, and with 

 nitrate added the oxygen production after 4 hr was 37 units, and the 

 carbon dioxide consumption 24 units [all figures approximately estimated 

 from diagrams (cf. Pirson, 1937)]. Myers (1949) likewise found a simul- 

 taneous increase in oxygen and a decrease in carbon dioxide at a high light 

 intensity; without nitrate the oxygen/carbon dioxide ratio is 31.6/31.4, 

 compared with an oxygen/carbon dioxide ratio of 39.0/28.7 in the pres- 

 ence of nitrate. It is difficult to escape the impression that, at saturating 

 light intensities, nitrate mainly causes an increased oxygen evolution and 

 a definitely less pronounced decrease in the carbon dioxide consumption. 

 This would imply an increased utilization of light energy by the reduc- 

 tion of nitrate if we could disregard the possibility of a simultaneous 

 decrease in the carbon dioxide consumption through the competition with 

 nitrate and a general increase in the level of photosynthesis owing to the 

 improved nitrogen status of the plant (cf. Pirson and Wilhelmi, 1950). 



The case of Pirson and Wilhelmi with strong nitrogen deficiency and 

 an obviously reduced carbon dioxide value ought to correspond to Myers's 

 high-light plants with assumed high content of carbohydrates. Here the 

 respiratory reduction of nitrate predominates and causes a decrease in 

 the carbon dioxide intake. 



