THE NATURE OF PHOTOSYNTHESIS \77 



longer the determining or limiting factor. At this point (still provided 

 there is ample carbon dioxide) temperature becomes the determining fac- 

 tor and an increase in temperature increases the rate of photosynthesis. 



The results of Plaetzer on the compensation point must be regarded 

 in the light of these facts. The observation that photosynthesis de- 

 creases vi^ith increasing temperature can hold only for a limited range of 

 light intensity. When this is increased so that it no longer is a limiting 

 factor at the lower temperature, but rather the temi^erature constitutes 

 the limiting factor, then photosynthesis will increase with increasing 

 temperature. 



Perhaps a clearer insight of the phenomenon of apparent decreasing 

 photosynthesis with increasing temperature when light is the limiting fac- 

 tor can be obtained from Plaetzer's quantitative data. Let i represent the 

 light intensity which at 5" produces compensation between respiration and 

 photosynthesis. It will be recalled that at 20° i no longer is the com- 

 pensation point and that with light intensity i at 20°, carbon dioxide is 

 given ofif by the plant, i.e., oxygen is taken up and respiration overbalances 

 photosynthesis. The question then arises, is the absolute amount of 

 oxygen produced in photosynthesis with light intensity / at 5° the same 

 as at 20°. The following simplified data answer the question: 



a = respiration (CO, emitted) at 20° in the dark. 



b = respiration (CO2 emitted) at 5° in the dark, i.e., the respiration which 



at the compensation point i is equal to photosynthesis, hence also 



b = photosynthesis at 5°. 

 c = oxygen consumed at 20° under the influence of light intensity i. 



Then a = b + c if the light intensity is the limiting factor. If this equa- 

 tion does not hold the action of / is different at 5° than at 20°, i.e., greater 

 or less. Plaetzer's results are : 



a = 4.3 

 b = 1.38 

 c = 2.97 



b + c = 4.35. 



In the equation a r= b + c the assumption is, of course, made that the 

 rate of respiration is the same in the dark as in the light. This is still a 

 debated question and no conclusive evidence is as yet available. If i 

 has the same influence, x, on respiration at 5° and at 20°, then c would 

 remain unchanged, for (a -f x) — (b + x) = c. The nature of x is 

 still unknown, though it is possible that the difference between a == 4.3 

 and b + c = 4.35 indicates a slight stimulation of respiration. The main 

 result shows quite definitely, however, that with light constant and the 

 limiting factor, a change in temperature does not alter the absolute rate 

 of photosynthesis, while respiration is directly influenced. 



