CARBON DIOXIDE COMPENSATION POINT 899 



temperatures, this gas composition remained unchanged for several hours; 

 at 35-37° C, after a short period of constancy, the carbon dioxide pressure 

 began to increase again — probably because photosynthesis suffered slow 

 thermal inhibition (c/. chapter 31), while respiration remained constant. 



Thomas, Hendricks and Hill (1944) found that in beet plants, at 15° C, 

 photosynthesis compensated respiration at fC02] — 0.003% — about one 

 third of the value found by Miller and Burr. Gabrielsen (1949) found a 

 value of 0.009 vol.% for the CO2 compensation point of Sambucus leaves 

 at 10 klux. 



In submerged plants, one has to distinguish between the compensation 

 point at constant pH and the steady state reached after prolonged photo- 

 synthesis with limited carbon dioxide supply: In the latter case, both 

 [CO2] and pH change with time (OH" ions being left behind when CO2 is 

 withdrawn from HCO3-), and the final steady state may be determined by 

 either one or both of these factors. We have referred above to the experi- 

 ments of Dahm (1926), Shutov (1926) and Ruttner (1947, 1948), which 

 were interpreted by Ruttner (1948) and Osterlind (1948, 1949) as indicating 

 that some aquatic phanerogams and algae can use bicarbonate ions so 

 efficiently that the presence of a minimum concentration of free CO2 

 molecules is not needed to maintain their photosynthesis; these plants 

 are able to continue -net synthesis even after [CO2] has been reduced to 

 10-^ mole/1, or less, and pH had risen above 10 or 11. (The pH of the 

 sap inside the cells remains approximately neutral.) Aquatic mosses, such 

 as FontinaJis, on the other hand, cease to liberate 0? when [CO2] is 

 reduced to some such value as 0.4 X 10"^ mole/1. (Ruttner 1948). This 

 corresponds to 0.01 vol. % CO2 in the atmosphere, and indicates a compen- 

 sation point similar to that found with land plants. In the steady state 

 reached by photosynthesis of aquatic plants of this type, the reaction of 

 the medium is below, or about equal to pH 9. 



A rather striking observation of Miller and Burr was that the carbon 

 dioxide compensation point did not depend on temperature. This contrasts 

 with the strong dependence on temperature of the light compensation point 

 (c/. page 984). The reason for this difference is that temperature has a 

 strong influence on respiration, as well as on photosynthesis in strong light, 

 but only a weak effect (or none at all) on photosynthesis in light of low 

 intensity (c/. chapters 29 and 31). In the measurement of the "light com- 

 pensation point," photosynthesis is in the "light-limited" and therefore 

 temperature-independent state; while in the measurement of the carbon 

 dioxide compensation point, it is in the "carbon dioxide limited" state and 

 therefore depends on temperature. However, the exact coincidence of 

 the temperature coefficients of respiration and photosynthesis, implied in 

 the results of Miller and Burr, is unlikely to be more than an accident. 



