CHAPTER XXI 

 FACTORS AFFECTING PHOTOSYNTHESIS 



The process of photosynthesis is conditioned by a number of different 

 factors, some external, others internal. Four environmental factors are of 

 primary importance in influencing the rate of photosynthesis: (i) light, (2) 

 carbon dioxide concentration of the atmosphere, (3) temperature, and (4) 

 water. Other environmental conditions which can be shown to influence 

 the rate of this process include (1) oxygen, (2) various chemicals, (3) 

 wounding, (4) mineral salt supply, and, in the case of water plants, (5) the 

 osmotic pressure of the aqueous medium. Some of the factors in this latter 

 group rarely if ever have any effect upon photosynthesis under natural con- 

 ditions, and their influence is only known from laboratory experimentation. 

 The internal conditions influencing the rate of photosynthesis are much less 

 completely understood than the environmental factors. Several such factors 

 have been very definitely recognized, however, as follows : ( i ) chlorophyll 

 content of the leaves, (2) hydration of the protoplasm, (3) leaf anatomy, 

 (4) protoplasmic factors, including enzymes, and (5) accumulation within 

 the cells of the products of photosynthesis. 



The Principle of Limiting Factors. — Earlier investigators of the effects 

 of various conditions upon the rate of photosynthesis attempted to distinguish 

 among miiiiinuin, opti?ninn, and jnaximiun values for each factor in relation 

 to photosynthesis. In evaluating the effect of temperature upon photosynthesis, 

 for example, it was generally considered that there was a minimum tem- 

 perature below which no photosynthesis occurred, an optimum at which the 

 process takes place most rapidly, and a maximum above which photosynthesis 

 ceases. Advocates of this point of view, however, soon found themselves 

 confronted with the anomalous situation of a fluctuating "optimum." The 

 "optimum" carbon dioxide concentration was found to be greater at high 

 light intensities than at low ones, the "optimum" temperature was found to 

 vary with the light intensity, the "optimum" light intensity was different 

 for plants well supplied with water than for those which were inadequately 

 supplied, etc. 



The first important step in the clarification of this problem of the in- 

 fl[uence of various factors upon photosynthesis was taken when Blackman 



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