356 



FACTORS AFFECTING PHOTOSYNTHESIS 



(i) Accumulation of the end products of the reaction causes a gradual 

 decrease in the rate of photosynthesis. This effect is considered in more 

 detail later in this chapter. 



(2) Inactivation of enzymes essential to photosynthesis occurs more rapidly 

 at higher temperatures. The temperature relations of enzymatic reactions 



90 



80 



70 



10 



^60 



I- 



Z 



> 



in 



O 



l- 



O 50 



I 



[^40 

 < 



30 



20 



10 



40° C. 



^ ^ n ^ „ n ^ n n n n n ^ n ^^ 



25C. 



10 IS 20 



TIME IN MINUTES 



25 



30 



Fig. 91. Relative rates of apparent photosynthesis in Elodca canadensis at different 

 temperatures over a period of thirty minutes. 



in general are similar to those of the photosynthetic reaction which seems to 

 be evidence in support of this view. 



(3) Diffusion of carbon dioxide into the cells of the chlorenchyma may 

 not occur as rapidly as it could be used in photosynthesis, even though the 

 external supply is not limiting. 



(4) Respiration increases more rapidly than photosynthesis with increase 

 in temperature, thus resulting in a rapid decrease in the apparent photosyn- 

 thetic rate at high temperatures. That this explanation cannot entirely ac- 

 count for this phenomenon is shown in the subsequent discussion. 



