346 



FACTORS AFFECTING PHOTOSYNTHESIS 



Under natural conditions during the summer months in temperate regions 

 the carbon dioxide concentration of the atmosphere is most frequently the 

 limiting factor in photosynthesis for all photosynthetic tissues which are well 

 exposed to light. Hence if the concentration of this gas in the atmosphere 

 could be increased it would usually result in an increased amount of photo- 

 synthesis. Increased photosynthesis in crop plants usually means a greater 

 yield so there are excellent practical reasons for attempting to raise the con- 

 centration of carbon dioxide in the atmosphere. Artificially increasing the 



947 FOOT CANDLES 



630 FOOT CANDLES 



156 FOOT CANDLES 



0082 0164 0246 0328 



CARBON DIOXIDE CONCENTRATION VOLUME PER CENT 



0410 



Fig. 85. Relation between different carbon dioxide concentrations and rate of photo- 

 synthesis in wheat at three different light intensities. Data of Hoover, et al. (1933). 



carbon dioxide content of greenhouses during seasons when some other factor 

 is not limiting has been found to result in an improved development of green- 

 house crops (Cummings and Jones, 191 8.) Increased yields are also said 

 to have been obtained from field crops which were provided with additional 

 carbon dioxide by means of pipes laid on or in the ground. It is doubtful, 

 however, whether this latter practice can ever be developed upon a com- 

 mercially practical basis. 



Investigations of Lundegardh (1931) and others indicate that a consider- 

 able part of the carbon dioxide utilized by plants in many habitats is pro- 

 duced locally by "soil respiration," which represents largely the respiration of 

 micro-organisms. A part of the favorable effect of the application of ferti- 

 lizers or organic manures on crop production is undoubtedly due to the result- 



