Chapter 12 



INHIBITION AND STIMULATION OF PHOTOSYNTHESIS 



I. CATALYST POISONS AND NARCOTICS 



Photosynthesis is strongly affected by many so-called inhibitors or 

 stimulants, substances which change the rate without participating 

 directly in the reaction. Recently it has become increasingly clear, 

 mainly through the work of Gaffron, that certain poisons inhibit specific 

 steps in the photosynthetic process, and that a skillful use of such selective 

 inhibitors may help to differentiate between the component reactions of 

 photosynthesis, to retard at will some of them, and to direct the process 

 into alternative channels. Thus, the use of specific poisons promises to 

 become an important tool in the disentanglement of the complex chem- 

 istry of photosynthesis. 



The addition of almost any new substance to the medium in which a 

 plant lives (or the removal of a substance usually present in it) is likely 

 to affect its photosynthetic efficiency. The list of effective agents ranges 

 from poisons, through narcotics, to aldehydes, sugars, organic and inor- 

 ganic acids and salts, oxygen, and water. The action of some substances 

 is highly specific; they obviously possess affinities with certain com- 

 ponents of the photosynthetic apparatus. Other substances act in a 

 less specific way, as, for example: all urethans, by their surface activity; 

 all acids, by their common constituent, the hydrogen ion; and all solutes 

 in general, by their effect on osmotic pressure. The present chapter is 

 concerned, in the first part, with specific "catalj^st poisons" (hydrocyanic 

 acid, hydroxylamine, hydrogen sulfide, etc.) and, in the second part, with 

 "narcotics," of the type of chloroform, ether, and urethan. Chapter 13 

 will deal with the effects of oxygen, carbohydrates, salts and other 

 miscellaneous physical and chemical inhibitors and stimulants. 



Many typical catalyst poisons (e. g., cyanide) owe their toxicity to the 

 formation of a complex with metal atoms, contained in the prosthetic 

 groups of many enzymes. Others (e. g., dinitrophenol or acetyl iodide) 

 probably react with specific groups in catalytically active proteins. 

 Narcotics, on the other hand, are supposed to act by blocking active 

 surfaces rather than by attaching themselves to individual atoms or 

 groups. 



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