HYDROGEN SULFIDE AND OTHER INORGANIC POISONS 317 



the de-adaptation of Scenedesmus. An inhibition of hydrogenase by 

 carbon monoxide also was found by other investigators, e. g., in experi- 

 ments with Azotohader. 



(c) Sulfur Dioxide, and Nitrous Oxides 



The poisoning of the photosynthetic apparatus by sulfur dioxide and 

 by the oxides of nitrogen was investigated by Noack and coworkers from 

 a point of view somewhat different from that adopted in the investigations 

 discussed so far. Noack (1925) started his research with the concept 

 that the iron contained in the chloroplasts plays a catalytic part in 

 photosynthesis — not (or not only) the small part of it which is contained 

 in organic complexes, but also that larger part which gives the hema- 

 toxylin color test and is therefore present in the form of noncomplex 

 organic or inorganic salts (cf. Chapter 14, page 376). A second assump- 

 tion of Noack was the hypothesis (cf. Chapter 19, part A) that, when 

 photosynthesis is inhibited, the light energy absorbed by chlorophyll is 

 diverted towards destructive "photodynamic" reactions which oxidize 

 and destroy both the protoplasm and the pigment. Starting from these 

 two concepts, Noack set out to study the destructive effect on plant 

 cells and pigments of reagents known to react with noncomplex iron. 

 The effect of sulfurous and nitrous gases on vegetation is well known, 

 and it is also known that the assimilating tissues are the first to be 

 damaged. Noack found that, by proper caution (e. g., in experiments 

 with the water moss Fontinalis, by using a 5 X 10~'*% solution of so- 

 dium bisulfate), the damage caused by sulfur dioxide can be restricted 

 to the chloroplasts without affecting the protoplasm. The moss was 

 treated with the bisulfate solution in the dark, then washed and illumi- 

 nated. It showed a gradual deterioration of photosynthetic eflSciency; 

 after 24 hours of illumination, oxygen evolution was replaced by oxygen 

 consumption, and the chloroplasts began to show decoloration, which 

 Noack attributed to a "photodynamic" oxidation. 



In a subsequent investigation by Wehner (1928), similar treatments 

 by nitrous, sulfurous and hydrochloric acid gases, and ammonia, were 

 applied not only to Fontinalis, but also to whole land plants (clover) 

 and detached leaves (tobacco and spinach). A treatment with sulfur 

 dioxide or nitrogen oxides in the dark made all these plants liable to 

 photoxidation, even though the poison was washed out before illumi- 

 nation. Hydrogen chloride acted less strongly, while ammonia showed 

 no effect at all (that is, its damaging influence was not enhanced by 

 subsequent illumination). Very small quantities of sulfur dioxide and 

 of nitrogen oxides acted as stimulants. 



Since Noack ascribed the effect of all these poisons to the binding of 

 iron, Wehner attempted to "cure" the poisoned plants by the adminis- 



