200 FIXATION OF CARBON DIOXIDE CHAP. 8 



this substrate. Arens (1933, 1936^-2) asserted, for example, that Elodea, 

 Potomageton, and other aquatic plants can continue photosynthesis for a 

 considerable length of time after having been transferred from a bicar- 

 bonate solution into distilled water, and attributed this phenomenon to 

 the formation of carbonate reserves. Gessner (1937) confirmed the 

 existence of such reserves in many (although not all) aquatic plants, but 

 found that they are much less extensive than could be gathered from 

 Arens' observations. Figure 20 shows the rapid decline in the rate of 

 oxygen evolution by Hydrilla and Cabomha which follows a transfer of 

 their twigs into distilled water. This figure indicates that the carbonate 

 reserves of aquatic plants are not larger than those of land leaves (Tables 

 8.IX and 8.X), i. e., of the order of 0.1 to 1% of the dry weight of the 

 leaves. Since intense photosynthesis leads to an hourly increase in dry 

 weight by several per cent (Vol. II, Chapter 28), carbonate stores of this 

 magnitude cannot maintain photosynthesis at its full rate for more than 

 a few minutes. 



4. Carboxylation and the ICO2} Complex 



Except for the observations of Schafer on the increase in carbon 

 dioxide content upon illumination (page 193), the experiments described 

 above do not reveal any relationship between the reversible carbon 

 dioxide absorption by plants in the dark and the reduction of carbon 

 dioxide in light. We shall now describe experiments which indicate that 

 a different (although also a reversible and nonphotochemical) absorption 

 of carbon dioxide is closely associated with photosynthesis — presumably 

 as a preliminary step in this process (as assumed in Chapter 7). The 

 quantities of carbon dioxide involved in this absorption are twenty or 

 fifty times smaller than those which can be accounted for by the carbon 

 dioxide-bicarbonate equilibria, i. e., of the order of 2 X 10~^ mole per 

 liter of cell volume, or 5 X 10"^% CO2 relative to the dry weight of the 

 cells, or 0.5 ml. carbon dioxide gas per 10 grams of fresh cells. On the 

 other hand, the affinity of the acceptor responsible for this absorption 

 to carbon dioxide must be higher than that of the phosphate or carbonate 

 buffers, since its saturation occurs at carbon dioxide pressures of the 

 order of 1 mm. This value is derived from the "carbon dioxide" curves 

 of photosynthesis (representing rate vs. concentration of carbon dioxide). 

 These curves (c/. Vol. II, Chapter 27) show "half-saturation" at [CO2] 

 values of the order of 0.03% in the air. One explanation of this satura- 

 tion, which will be discussed in chapter 27 (Vol. II), is that the carbon 

 dioxide curves are equilibrium isothermals of the acceptor-carbon dioxide 

 complex. According to this hypothesis these curves may be distorted 

 by supply and disposal limitations which prevent the maintenance of the 

 carboxylation equilibrium during intense photosynthesis, or cause the 



