Chapter 26 

 EXTERNAL AND INTERNAL FACTORS IN PHOTOSYNTHESIS 



1. The "Cardinal Points" and the "Limiting Factors" 



Even when studying simple reactions in vitro, the physical chemist is 

 rarely able to control all the conditions that affect the reaction rate. Con- 

 sequently, seldom will two investigations of the velocity of a reaction result 

 in agreement in more than the order of magnitude. Beside the readily con- 

 trollable external factors, such as temperature, pressure and light intensity, 

 the rate often depends on factors as elusive as the state of the walls of the 

 vessel, or the presence of minute impurities. It is therefore easy to judge 

 the difficulties encountered in the kinetic study of a complex chemical 

 process in a living organism. The rate of such a process depends on many 

 physiological factors that do not enter ostensibly into the kinetic equations. 

 Among all life phenomena, photosynthesis is perhaps the most sensitive 

 to slight variations in the structure and composition of the biocatalytic 

 apparatus. No wonder that doubts have been expressed as to the very 

 possibility of deriving significant kinetic relationships from the quantita- 

 tive study of this phenomenon. 



At first, considerable optimism prevailed in this respect. The develop- 

 ment of natural science in the nineteenth century led to the belief that bio- 

 logical processes follow relatively simple mathematical laws, and several 

 attempts have been made to formulate such laws for the production of or- 

 ganic matter by plants. 



The earliest discussions of the dependence of photosynthesis on external 

 factors were based on the concept of the three "cardinal points" (Sachs 

 1860). According to this concept, which was widely accepted until the 

 turn of the century, biological processes get under way at certain minimum 

 values of the relevant external variables (such as temperature, pressure, 

 humidity, light intensity etc.), reach the highest rate at certain optimum 

 values of these variables, then decline, and cease altogether after the max- 

 imum tolerable values have been exceeded. 



Twenty years before the enunciation of the principle of the three car- 

 dinal points, Liebig (1840, 1843) had formulated a simple rule for the effect 

 of various elements on the yield of field crops. He postulated that these 



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