900 CONCENTRATION FACTORS CHAP. 27 



We assume that the carbon dioxide curves of photosynthesis, if properly 

 corrected for respiration, continue smoothly below the compensation point 

 and reach zero when the carbon dioxide concentration is zero. However, 

 their exact determination in the region of very low carbon dioxide concen- 

 trations is difficult because of the production of carbon dioxide by respira- 

 tion. It is difficult to remove this carbon dioxide completely (e. </., by 

 an alkaline absorber) , before some of it is utilized for photosynthesis, since 

 this may occur even before the carbon dioxide has left the interior of the 

 cells (c/. Vol. I, chapter 19, page 529). Some intermediates of respiration, 

 such as certain carboxylic acids, may perhaps be utilized for photosyn- 

 thesis without conversion to free carbon dioxide. One would then obtain 

 small positive values of "true" photosynthesis (i. e., of the difference be- 

 tween the gas exchange in light and in the dark), even when the concen- 

 tration of carbon dioxide is zero, not only in the medium, but also inside 

 the cell. 



Experimental investigation of the relation between photosynthesis and 

 respiration at low [CO2] values is further complicated by the observation 

 that, if carbon dioxide is removed very effectively, photoxidation is apt to 

 occur upon exposure to light, and oxygen consumption becomes larger in 

 light than in the dark (instead of being decreased in consequence of re- 

 assimilation of respiration products). 



One is thus caught on the horns of a dilemma: (a) either the respira- 

 tory carbon dioxide is not removed effectively enough — in which case il- 

 lumination produces an apparent reduction in the volume of respiration, 

 and it appears as if photosynthesis can proceed at a positive rate even at 

 [CO2] — (fig. 28.5 gives an extreme example of this kind); (h) or the 

 removal of carbon dioxide is fully effective — then, photoxidation sets in, 

 and the rate of photosynthesis appears to become negative at [CO2] = 0. 

 In chapter 19 (page 528) it was noted that Noack (1925, 1926) had o))- 

 served mainly the first phenomenon (i. e., an apparent "light inhibition of 

 respiration" in a CO2 free atmosphere); whereas van der Paauw (1932) 

 had discovered, and Franck and French (1941) further explored, the second 

 effect — the photoxidation in carbon dioxide starved leaves. 



Photoxidation can be prevented by using low light intensity and short 

 exposuies. Whether it is possible to avoid the reassimilation of a part of 

 respiratory carbon dioxide (or of its precursors) is a controversial matter. 



Gabrielsen (1949) noted that in the comparatively thick sun leaves of 

 Samhucus, in streaming, carbon dioxide free air, as much as 56% of respira- 

 tory carbon dioxide were reassimilated ; but that in the thinner shade 

 leaves, reassimilation was negligible. Reassimilation could be reduced by 

 lowering the temperature (e. g., to 5° C), and by increasing the rate of flow 

 of the gas, e. g., to 33 cc./m.^ min. Gabrielsen concluded from these ob- 



