REDUCTION OF ORGANIC OXIDANTS 541 



observations on the effect of violet and ultraviolet light on the assimilation 

 of nitrate by green plants {cf. Tottingham and Lease 1934) bear any 

 relation to the photochemical nitrate reduction. 



Lovell (1938) thought that the increase in oxygen evolution by Elodea, which he 

 observed upon the addition of potassium nitrate (but not of potassium chloride), was 

 due to nitrate reduction. However, according to Warburg and Negelein, "nitrate 

 photosynthesis" occurs only in strongly acid solutions; comparison of Lovell's results 

 with those of Pirson, described in chapter 13 (page 339), makes it probable that he ob- 

 served merely a stimulation of normal photosynthesis by a removal of nitrate deficiency. 



Developing the hj^pothesis introduced on page 540, we may suggest 

 that the photochemical reduction of nitrate (or other substitute oxidants) 

 occurs whenever the concentration of the normal oxidants in the primary 

 photochemical process, i. e., of the complex, {CO2}, or of the intermediate 

 hydrogen acceptor, X {cf. Eq. 7.10a), becomes depleted (while that of 

 substitute oxidants is high). It may be worth recalling in this connection 

 that some autotrophic bacteria can substitute nitrate for oxygen in 

 chemosynthesis (cf. Chapter 5, pages 115 and 116). 



2. Reduction of Other Inorganic Oxidants 



In chapter 4 (section A), we described the interesting experiments 

 of Hill on the chlorophyll-sensitized oxidation of water by ferric salts 

 in aqueous suspensions of chloroplasts. Fan, Stauffer, and Umbreit 

 (1943) were able to obtain some oxygen also from suspensions of live 

 Chlorella cells deprived of carbon dioxide but provided with ferric phos- 

 phate or other ferric salts as oxidants. However, they were unable to 

 prevent the back reaction (reoxidation of ferrous iron by oxygen) in the 

 way used by Hill (i. e., by reaction with ferricyanide), since the ferri- 

 cyanide itself was reduced by the cells. This caused the reduction of 

 ferric phosphate to come to an early end. 



The so-called "Molisch reaction" (precipitation of silver from silver 

 nitrate in the chloroplasts) was described in chapters 10 (page 270) and 

 14 (page 360). Gauteret (1934) noticed that this reaction is accelerated 

 by light. This, too, may be an example of a chlorophyll-sensitized 

 reduction in vivo (although it is doubtful whether water plays the part 

 of reductant in this reaction). 



3. Reduction of Organic Oxidants 



Noack (1922) interpreted the occasional red coloration of green leaves 

 as a chlorophyll-sensitized photochemical reduction of flavonols to 

 anthocyanins. Similarly to photoxidation, this reaction occurs only when 

 photosynthesis is inhibited (e. g., by an excess of sugars, or by the absence 

 of carbon dioxide). 



