564 PHOTOSYNTHESIS AND RESPIRATION CHAP. 20 



of photosynthesis are incapable of serving as substrates of respiration, 

 particularly when the supply of the normal substrates is insufficient. 



An interesting example of a partial identity of the enzymatic appa- 

 ratus of respiration and photosynthesis is provided by certain bacteria. 

 As suggested on page 111, these bacteria use the same enzymatic channels 

 to convey hydrogen from organic donors to oxygen in the dark (respi- 

 ration) and to carbon dioxide in light (photosynthesis). However, this 

 duplication of enzymatic functions takes place in a part of the photo- 

 synthetic apparatus which is not usually active in green plants (the 

 "hydrogenase system," cj. Chapter 6). 



While a direct chemical link between the intermediates of photo- 

 synthesis and the substrates of respiration is possible, but not yet proved, 

 a stimulation of respiration by the end products of photosynthesis — carbo- 

 hydrates—has been established beyond doubt, and appears natural, in 

 consideration of the fact that respiration can also be stimulated by 

 externally supplied sugars. 



The acceleration of respiration after a period of illumination was first 

 observed by Borodin (1881) and Palladin (1893), and again by Warburg 

 and Negelein (1922). The extent of this effect must depend on the 

 capacity of the cells for disposing of the synthesized carbohydrates by 

 translocation or by formation of insoluble polymers. This explains 

 widely varying figures given by different authors for the respiration 

 increases which followed a period of photosynthesis (c/. Table 20.1). 



The experiments of Spoehr and McGee (1923) showed that respiration 

 increases particularly strongly in leaves which have been previously kept 

 in darkness for several days and have thus been depleted of carbohydrates; 

 the effect on the respiration of nonstarved leaves is much weaker. 



Gessner (1939) too, found that a "dark adaptation period" of several 

 days is necessary to produce a stimulation of respiration by an illumi- 

 nation of 40,000 lux. He noticed, however, that a similar effect can be 

 produced also by ultraviolet light, and concluded that it is not caused by 

 photosynthesis, but represents a direct stimulation of the respiratory 

 system. Ranjan (1940) found that ultraviolet light inhibits the respira- 

 tion of Eugenia jamholana. 



It seems probable that the same was true also of other results in 

 Table 20.1, particularly those of Fockler (1938). Probably, a direct stim- 

 ulation of respiration by short-wave hght (ultraviolet, violet, and blue), 

 which will be discussed on pages 567-569, is superimposed on the effect of 

 accumulated photosynthates in all experiments except those performed 

 in yellow or red light. 



Although it seems plausible that respiration should be stimulated by 

 the accumulated products of photosynthesis, it is by no means certain 

 that all of the increased oxygen consumption, observed after a period of 



