Chapter 31 



THE TEMPERATURE FACTOR 



Photosynthesis is a sequence of photic and catalytic chemical reactions, 

 combined with physical processes of diffusion and convection. The pri- 

 mary photochemical reaction probably is independent of temperature; 

 but all other partial processes of photosynthesis, physical as well as chemi- 

 cal, must be influenced by it. The adsorption and hydration equilibria, 

 which affect the colloidal state of the protoplasm and of the chloroplasts 

 and thus, indirectly, alter the efficiency of the photosynthetic apparatus, 

 also are sensitive to heat or cold. Therefore, the influence of temperature 

 on photosynthesis is a complex phenomenon. The rate of photosynthesis 

 can be expected to be insensitive to temperature changes — at least within 

 a certain range — only in the "light-hmited" state, in which the velocity of 

 the over-all process is equal to that of the primary photochemical reaction. 



Under all other conditions, the rate of photosynthesis will change 

 with temperature, and the character of this change will depend on what 

 factor exercises the strongest influence on the rate under the specific condi- 

 tions of the experiment. 



It was mentioned on page 1137 that the maximum experimentally realizable quan- 

 tum yield of photosynthesis may be smaller than the theoretical photochemical quantum 

 yield because a certain fraction of the products may be lost by back reactions, independ- 

 ently of light intensity. This proportion, and thus the maximum quantum yield, could 

 depend on temperature. In other words, photosynthesis could be somewhat dependent 

 on temperature even in the light-limited state. 



Before considering the "temperature curves" of photosynthesis {i. e., 

 curves in which rate is plotted against temperature, at constant light in- 

 tensity and carbon dioxide concentration) , we will first discuss the differ- 

 ence between the internal temperature of plants and the temperature of the 

 surrounding medium. Obviously, this internal temperature, rather than 

 the temperature of the medium, should be used as the independent vari- 

 able in the analysis of the temperature curves. 



A. Internal Temperature of Plants* 



The internal temperature of plants can be higher or lower than that 

 of the ambient medium. It may be different in different parts of the 



* Bibliography, page 1254. 



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