908 CONCENTRATION FACTORS CHAP. 27 



• 



"bottleneck" may well have contributed, c. gr., to the decline in rate ob- 

 served by Warburg (1919) in Chlorella at [CO2] < 9 X 10"^ M. (As 

 mentioned before, Emerson and Green have noted no such decline until 

 [CO2] was down to 0.7 X 10~* M, and have suggested that damage caused 

 by increased alkalinity of the lower carbonate buffers could provide an 

 explanation of Warburg's results.) 



Carbon dioxide exhaustion effects are not restricted to experiments in 

 liquid media, but affect also measurements made with land plants in a car- 

 bon dioxide atmosphere, if it is stationary (c/. Lundegardh 1921), or in- 

 sufficiently agitated (Kreusler 1885,1887; Singh and Kumar 1935); this 

 was demonstrated by Kostychev ef aZ. (1927) and Chesnokov and Bazyrina 

 (1932). Here again, not only the rate of gas circulation, but also the size 

 and shape of the plants may be of importance and the opening of the 

 stomata constitutes an additional complication. 



To sum up it seems safe to assume that, whenever the rate of photo- 

 synthesis was found to continue its increase with the external concentra- 

 tion of carbon dioxide much above [CO2] = 10 X 10 "^ M, the reason was 

 slow outside supply of carbon dioxide to the photosynthesizing cells, and 

 consequent exhaustion of the reduction substrate. Experiments in vigor- 

 ously stirred solutions, or in rapidly circulating gas mixtures, regularly 

 showed the photosynthetic apparatus to become saturated with carbon 

 dioxide at concentrations not much higher, or even lower, than 1 X 10 ~^ 

 M. Even in experiments of this type, one cannot be certain whether all 

 diffusion effects have been eliminated, particularly in higher plants, where 

 the diffusion resistance of the stomata, epidermis and air channels cannot 

 be destroyed by stirring or gas circulation. The diffusion resistance of the 

 cell walls or protoplasmic layers also remains unaffected by all me- 

 chanical means (although it may perhaps be changed by chemical agents). 



Another source of distortion of the carbon dioxide curves of photosyn- 

 thesis was noted by Howies (unpublished) and Whittingham (1949) in 

 Brigg's laboratory. They observed that the photosynthesis of Chlorella 

 in carbonate buffers with low [CO2] values was time-dependent, if the cells 

 had been transferred into the CO2 deficient medium from a culture medium 

 of higher concentration (such as 4% CO2). The initial rate was low; it 

 increased by a factor of 3 in the course of two or three hours, and then be- 

 came constant. If the cells were cultured in air (0.03% CO2), the rate 

 was high and constant from the beginning. Obviously then, with cells 

 "incubated" at high [CO2], the shape of the carbon dioxide curve will de- 

 pend on the duration of the measurement. 



If the carbon dioxide curve of Chlorella is determined at low [COu] 

 values, with cells "adapted" to low carbon dioxide concentration, the value 

 of VJCO2] is as low as 0.5 to 1.0 X 10-« mole/1. 



