July 14, 1911] 



SCIENCE 



63 



THE RELATION BETWEEN PHOTOSYNTHESIS OF 

 CARBON DIOXIDE AND NITRATE REDUCTION 



The first step in the utilization of nitrates 

 for protein synthesis by the plant leaf is no 

 doubt a reduction of the nitrate to the nitrite 

 and finally to ammonia. The role played by 

 sunlight in this action has been investigated 

 from several sides. While Schimper tried to 

 demonstrate that it was by means of light 

 only, and in the cell containing chlorophyll, 

 that nitrate reduction takes place, this theory 

 has been somewhat modified by the researches 

 of Zaleski, of Suzuki and especially of God- 

 lewski to the effect that while nitrate reduc- 

 tion and concomitant protein synthesis take 

 place to a much greater extent in the light, 

 these reactions do also take place in the dark. 



The writer has found that perfectly sterile 

 aqueous solutions of potassium or calcium 

 nitrate will keep in the dark, even at 95", in- 

 definitely without forming a trace of nitrite. 

 If, however, a small quantity of colloidal 

 platinum is added, a reduction of the nitrate 

 soon sets in with the formation of nitrites 

 and of ammonia, as made very evident with 

 the Gries's and ISTessler's reagent, respectively. 

 This observation is quite in harmony with 

 that of Schoenbein, who noticed a reduction 

 of nitrates by means of juices of certain 

 fungi, confervse, etc. As most of the com- 

 mon forms of bacteria possess the property, 

 to some extent at least, of reducing nitrates, 

 it is of coarse very important to exclude all 

 bacteria from the mixture. 



That this reaction is brought about by 

 means of hydrogen peroxide I am very much 

 inclined to doubt, for absolutely no reduction 

 takes place in the presence of hydrogen per- 

 oxide, with or without colloidal platinum; 

 nor is the reaction in the least inhibited in 

 vacuo. More probable does it appear that the 

 platinum acts as follows, if there is something 

 present to take up the hydrogen formed:' 

 Pt -f- 2K,0 -> Pt(0H)2 + 2H. 



The hydrogen is kept in the active, or nascent, 

 state by the platinum, and reduces the ni- 



' See Mond and Ramsay, Zeit. f. phys.-chem., 

 25, 657. Bredig and von Berneck, Hid., 31, 254. 



trate directly. The platinum hydroxide in 

 turn splits into platinum and hydrogen 

 peroxide; the latter decomposes to water with 

 the liberation of oxygen: 



Pt(0H)2 ^ Pt -I- H,0,; H,0, -^ H^O + 0. 



The increased reduction of nitrates and of 

 protein synthesis in the leaf in the light can 

 be understood from the following observa- 

 tions. It was found that the reduction of 

 potassium and calcium nitrates takes place 

 with remarkable ease in the ultra-violet light 

 of the quartz mercury vapor lamp, forming 

 nitrite, ammonia and oxygen. The same 

 action takes place, though more slowly, in the 

 sunlight and even in diffuse or sky light. 

 The action is greatly accelerated by colloidal 

 platinum, though the latter is not necessary. 

 Of special importance is the fact that the 

 reduction of these nitrates results in a de- 

 cidedly alkaline solution. 



The theory that formaldehyde is the first 

 product of reduction in the carbon dioxide 

 appropriation has, of late, gained much sub- 

 stantiation. It must be remembered, how- 

 ever, that the next step, the polymerization 

 of formaldehyde to carbohydi-ates takes place 

 in alkaline solution only. Now while of 

 course it is true that nitrate reduction in the 

 leaf takes place in the dark, it can not be de- 

 nied that this action is much greater in the 

 light, and, as Schimper has shovsnn, in the 

 neighborhood of the chloroplasts; which must 

 then result in a local aU^alinity at these 

 points. Thus not only are the proper condi- 

 tions for carbohydrate synthesis established, 

 but nitrogen is also produced in the best form 

 and at the most available place .for protein 

 synthesis. No other explanation has been 

 given so far as my information goes, and I 

 can conceive no other way in which these very 

 necessary conditions can be produced in the 

 midst, it might be said, of all the acid pro- 

 ducts of sugar and protein metabolism. In 

 this connection mention might also be made 

 of a micro-chemical observation of MacCal- 

 lum' who found potassium localized in the 

 immediate neighborhood of the chloroplasts. 



-Jour, of Phys., 32, 95, 1905. 



