166 FUNDAMENTALS OF FRUIT PRODUCTION 



Enzymes. — Assimilation is an enzyme reaction. Enzymes are 

 organic catalytic substances that accelerate the rate of a reaction. Their 

 chemical composition is unknown and their existence is shown only 

 by their activity. They are not used up in a reaction, remaining after 

 the process is completed. A minute amount of enzyme can effect the 

 formation of a relatively large amount of end product; however, this 

 is proportional to the amount of enzyme present. The effects of tem- 

 perature described above are characteristic of enzyme reactions. The 

 time factor manifesting itself at temperatures above 25°C. is not due 

 to any direct effect of the temperatures on assimilation, but may be 

 due to the permanent inactivation or decomposition of enzymes at 

 high temperatures. Under these circumstances the rate of assimilation 

 would decrease because the amount of enzyme diminished. The longer 

 the temperature acted, the more enzyme would be decomposed. Simi- 

 larly the narcotic effect of strong concentrations of carbon dioxide and 

 the harmful influences of high light intensities are attributable in part 

 to their effects on the assimilatory enzymes. Such adaptations as 

 different species may show in their ability to assimilate best at higher 

 or lower temperatures or light intensities probably are attributable to 

 differences in their enzymes. 



The principle of limiting factors applies to the six factors determining 

 carbon assimilation.^^ If any one factor is limiting, the rate of assimila- 

 tion cannot be increased by any other. The carbon assimilation of green 

 plants is usually limited by the seasonal variation in temperature and the 

 diurnal variation in light. When temperature and light are both favor- 

 able, the supply of carbon dioxide is probably the limiting factor.^^ 

 Water may be a limiting factor either through a direct effect on assimila- 

 tion or indirectly by closing the stomata and so shutting off the supply 

 of carbon dioxide. 



Products 



The products of photosynthesis are oxygen and carbohydrates. 



Oxygen. — The relation between the amount of oxygen evolved in the 

 process of carbon assimilation and the amount of carbon dioxide taken in 

 is not accurately known. When the respiration of the assimilating 

 tissue is evaluated, it appears that the volume of oxygen evolved is prac- 

 tically equal to, or very slightly greater than, the amount of carbon dioxide 

 absorbed. The path by which oxygen escapes from the leaf is the same as 

 that by which carbon dioxide enters or water vapor is lost, namely, 

 through the intercellular spaces and the stomata. Oxygen is used in 

 respiration, however, and when assimilation proceeds very slowl}^, the 

 oxygen given off by assimilation may be entirely consumed by respiration. 

 Similarly the carbon dioxide evolved by respiration may just about equal 

 that used in assimilation under these circumstances. 



