330 VARIOUS CHEMICAL AND PHYSICAL AGENTS CHAP. 13 



of their photochemical transformation; consequently, some of the sensi- 

 tizer becomes idle and can supply energy for photoxidations; and the 

 catalyst Ep, may become the first victim of this photoxidation. Thus, 

 the inhibition effect grows "autocatalytically," and, in sufficiently strong 

 light, photosynthesis may come to a complete standstill. In moderate 

 light, the inhibition remains incomplete, despite its autocatalytic ac- 

 celeration, because catalyst Ex is continuously restored by the organism, 

 so that, after some time, an equilibrium is reached between the rate of 

 its destruction by photoxidation and of its restoration by metabolic proc- 

 esses. From then on, photosynthesis can run steadily at a rate corre- 

 sponding to the stationary concentration of the active catalyst, Ek- 



B. Effect of Excess Carbon Dioxide * 



The effect on photosynthesis of^he partial pressure of carbon dioxide 

 will be discussed in volume II, chapter 27. We shall find there that the 

 rate increases with the concentration of carbon dioxide, until the latter 

 reaches about 0.1% (corresponding to a partial pressure of about 0.8 

 mm.), and then becomes constant. However, photosynthesis has often 

 been found to decrease at very high concentrations of carbon dioxide. 

 This was first observed by de Saussure (1804), then by Boussingault 

 (1868), Bohm (1873), and Ewart (1896), and interpreted by Chapin 

 (1902) as a case of narcotic poisoning. However, if narcotization is 

 attributed to a competition between the reactants and the narcotic for 

 the catalytically active surfaces in the photosynthetic apparatus, one 

 may ask how carbon dioxide can^ "compete with itself." An answer is 

 that the full efficiency of photosynthesis is achieved when the different 

 catalysts are occupied by their respective substrates, which include 

 carbon dioxide, water, and various intermediates. An excessive concen- 

 tration of carbon dioxide can cause a "squeezing out" of the inter- 

 mediates, and thus hinder the completion of the transformation. An- 

 other possible effect of high concentrations of carbon dioxide — which also 

 may affect the rate of photosynthesis — is the acidification of the cell 

 fluids, whose buffering capacity is not unhmited. 



The sensitivity to excess carbon dioxide varies widely from species to 

 species. Blackman and Smith (1911) found no decline in the rate of 

 photosynthesis of Elodea and Fontinalis, even in water equilibrated with 

 an atmosphere containing 35% carbon dioxide. Jaccard and Jaag (1932) 

 found some leaves to be indifferent to carbon dioxide concentrations 

 of 15% or more; and Ewart (1896) observed that mosses can survive in 

 an atmosphere of pure carbon dioxide (which is lethal to other plants). 

 Singh and Kumar (1935) reported, on the other hand, that a maximum 

 of the photosynthesis of radish leaves occurs at 5% carbon dioxide, and 



* Bibliography, page 348, 



