898 CONCENTRATION FACTORS CHAP. 27 



remain in an approximately constant ratio; saturation is approached very 

 gradually, and is not quite reached at 32 X 10 ~^ mole/1., even on the curve 

 that corresponds to an illumination of only 2000 lux. Harder's curves 

 indicate that, in his plants, the rate of oxygen liberation did not become 

 limited entirely by carbon dioxide supply even at the lowest used bicar- 

 bonate concentrations, and did not become independent of [CO2] even at 

 the highest used concentrations. The range studied, 0.03 to 0.3% KHCO3, 

 i. e., from 4 X IQ-^ to 40 X 10-^ mole/1. CO2 (c/. Vol. I, page 178), was, 

 however, a rather narrow one. The curves in figs. 27.2-4 clearly tend to 

 coincide only below 1 X 10 ~^ mole/1. 



In the theoretical discussion later in this chapter, we will see that carbon 

 dioxide curve? that diverge from the origin can be expected if the carbon 

 dioxide-acceptor complex, ACO2, is not fully saturated with carbon di- 

 oxide, at low [CO2] values, even in the equilibrium state; while carbon 

 dioxide curves that coincide at low [CO2] values can be predicted if the 

 carbon dioxide dependence of photosynthesis is due entirely to the limita- 

 tion of the rate of processes by which carbon dioxide is made available for 

 photosynthesis (such as liberation of CO2 from HCOa", diffusion, and car- 

 boxylation of an "acceptor"). 



Table 27.1 shows that, with increasing light intensity, the "half satura- 

 tion point," which we will designate by 1/JCO2], generally shifts toward 

 the higher concentrations; so that, in very intense light, it may fall con- 

 siderably beyond 10 X 10 ~^ mole/1. This fact can be significant, indi- 

 cating certain kinetic conditions (as will be shown later in this chapter, 

 see p. 934 ff); often, however, it merely means increasing depletion of 

 carbon dioxide in the neighborhood of the cells when photosynthesis pro- 

 ceeds at a faster rate. 



3. Carbon Dioxide Compensation Point 



For each light intensity, there must exist a carbon dioxide concentration 

 at which photosynthesis just compensates respiration and the net gas ex- 

 change is zero, and below which respiration exceeds photosynthesis. This 

 "carbon dioxide compensation point" has not been studied in the same sys- 

 tematic way as was the "light compensation point" (c/. Table 28. Ill); 

 Miller and Burr (1935) first devoted an investigation to it. In their 

 experiments, a large variety of potted plants were enclosed in vessels filled 

 with gas mixtures of different composition and illuminated with white 

 light of about 20,000 lux, until all observable gas exchange stopped, i. e., 

 until the carbon dioxide concentration had declined to the compensation 

 point. It was found that this occurred — at temperatures from 5° to 35° 

 C. — when the carbon dioxide content was down to about 0.01%. At low 



