338 



FACTORS AFFECTING PHOTOSYNTHESIS 



(1905) enunciated the "principle of limiting factors." This principle is 

 essentially an elaboration of Liebig's "law of the minimum" (Chap. XXXIII) 

 and was stated by its author as follows: "When a process is conditioned as 

 to its rapidity by a number of separate factors, the rate of the process is limited 

 by the pace of the 'slowest' factor." 



The explanation of this principle can best be presented in terms of the 

 illustration given by Blackman (Fig. 83). Assume the intensity of light to 

 be just great enough to permit a leaf to utilize 5 cc. of carbon dioxide per 

 hour in photosynthesis. If only i cc. of carbon dioxide can enter the leaf 

 in an hour the rate of photosynthesis is limited by the carbon dioxide factor. 

 As the carbon dioxide supply is increased the rate of photos}^! thesis is also 



CARBON DIOXIDE CONCENTRATION 



Fig. 83. Diagram to illustrate Blackman's interpretation of the principle of limiting 



factors. 



increased until 5 cc. of carbon dioxide enter the leaf per hour. Any further 

 increase in the supply of carbon dioxide will have no influence upon the rate 

 of photosynthesis, unless a sufficient concentration is present to bring about 

 toxic effects, because insufficient light energy is available to permit its utiliza- 

 tion. Light has now become the limiting factor and further increase in the 

 rate of photosynthesis can only be brought about by an increase in the intensity 

 of light. These results are indicated graphically as J 5 C in Fig. 83. This 

 theory assumes a progressive increase in the rate of process with a quantitative 

 increase in the limiting factor (in this example, carbon dioxide) until the 

 point is reached at which some other factor becomes limiting (in this example, 

 light intensity). At this point the increase stops abruptly (point B in Fig. 

 83), and the rate of photosynthesis becomes constant {B C oi Fig. 83). Ac- 

 cording to this concept when the magnitude of photosynthesis is limited by 

 one of a set of factors only a shift in that factor towards a condition more 

 favorable for the process will result in an increase in the rate of photosyn- 

 thesis. 



