CARBON DIOXIDE EXHAUSTION EFFECTS 905 



atmosphere contained 0.1% CO2, and so on. Bicarbonate solutions con- 

 tain, for each GO2 molecule, about 100 HCOs" ions; they should therefore 

 last one hundred times longer than acid solutions with the same value of 

 [CO2]. Finally, 0.1 M carbonate-bicarbonate buffers containing from 2 X . 

 10^ (buffer No. 1) to 330 (buffer No. 11) carbonate and bicarbonate ions 

 for each CO2 molecule provide sufficient reserves to maintain full photosyn- 

 thesis, in suspensions containing 0.1 to 1% cells by volume, for from 500 

 to 5000 minutes, or S to 80 hours. 



These figures lead to several conclusions. First, measurements of the 

 rate of photosynthesis at low carbon dioxide concentrations (e. g., less than 

 1% CO2 in the air, or 30 X 10 -^ M in solution), if they are to last for more 

 than a few seconds, require an ample supply of carbon dioxide, either in 

 situ, in the form of carbonate and bicarbonate ions, or from outside, in the 

 form of large amounts of circulating liquid or gas, which must be kept well 

 supplied with fresh carbon dioxide to replace losses. Second, whenever 

 leaves or multicellular algae are used, very intense stirring or circulation is 

 required to prevent the establishment of a carbon dioxide concentration 

 gradient around the plants. The required stirring depends on the ratio of 

 surface to volume. This is well illustrated by the following example: 

 Gessner (1938) measured the oxygen liberation by two varieties of Proser- 

 pinaca palustris — one with large leaves and one with finely divided, feather- 

 like leaves. In stagnant water, the first variety produced much less oxygen 

 than the second one; stirring improved strongly the efficiency of the large- 

 leafed, but did not affect the oxygen production by the feather-leafed 

 variety. In other words, in the absence of circulation, external carbon 

 dioxide supply must have been the rate-limiting factor for the large-leafed, 

 but not for the feather-leafed, variety. 



It seems that, with multicellular objects, even the provision of a 

 strongly stirred bicarbonate-buffered medium does not always guarantee 

 the absence of carbon dioxide exhaustion effects. Wassink (1946) found, 

 for example, that the photosynthesis of 5 mm. discs cut out of leaves, sus- 

 pended in carbonate buffer No. 9 (7.9 X 10 -^ mole/1. CO2) and shaken in a 

 Warburg apparatus, still was largely "carbon dioxide limited." The 

 equilibrium concentration of carbon dioxide in the atmosphere above this 

 buffer is 0.25% (cf. Tables 8.V and 8.II). By increasing the initial carbon 

 dioxide content in the air space to 2% in some cases, and to as much as 9% 

 in others, Wassink was able to obtain carbon dioxide saturation. Without 

 exact knowledge of the dimensions of the apparatus, it is difficult to esti- 

 mate the final carbon dioxide concentration and the pH of the solutions 

 treated in this way. 



Closure of the stomata in punched leaf discs may have been one of the 

 reasons for apparent extreme carbon dioxide requirements observed in 



