CARBON DIOXIDE FERTILIZATION 001 



servations that there is no evidence of photochemical reduction of precur- 

 sors of free carbon dioxide formed in respiration (since the lattci- would 

 manifest itself in a general reduction of oxygen consumption in light). 



Warburg, Burk and co-workers (1949) reached the same conclusion in 

 experiments with strongly stirred, dense ChloreUn pyrenoidosa suspensions: 

 With alkali present in the side arm of the reaction vessel, illumination with 

 Aveak red light (lielow compensation) had practically no effect on the con- 

 sumi)tion of oxygen. This showed that all the carbon dioxide produced by 

 respiration was conveyed to the alkali and absorbed there (and none re- 

 assimilated in light), and that no intermediate products of respiration were 

 used up as substitute oxidants in the photosynthetic process in the absence 

 of the normal oxidant, external carbon dioxide. 



We \\\\\ have to return to these observations in chapter 29, because of 

 their significance for the calculation of the quantum yield of photosynthesis 

 in weak light. We will note there that the results of Warburg et at. may 

 have been associated with the intermittency of illumination, caused in their 

 experiments by the rapid stirring of dense cell suspensions. Most of the 

 respiratory carbon dioxide was produced while the cells were in the shade and 

 could escape into the medium before entering the small illuminated zone. 



Supporting Kostj^chev's concept of indirect, physiological regulation of photosynthe- 

 sis (cf. chapter 25), Chesnokov and Bazyrina (1932) concluded, from experiments on 

 higher plants, that the rate of photosynthesis is not a smooth function of the external 

 carbon dioxide concentration at all. They asserted that photosynthesis drops to zero 

 when the external carbon dioxide concentration declines below 0.2 X 10 ~^ M, while above 

 1 X 10-5 ji/ the rate is not affected by changes in [COj]. They considered this be- 

 havior a proof of the admirable adaptation of plants to natural conditions — a "trigger 

 action," which puts the photos3Tithetic mechanism to work when conditions are "nor- 

 mal" and stops it completely when the conditions become unfavorable. A direct in- 

 fluence of external carbon dioxide concentration on the reaction rate, subordinated to 

 the law of mass action, could not, they argued, produce such an "all or nothing" response. 

 However, the alleged discontinuity of the carbon dioxide curve, and the consequent 

 assumption of the existence of a "carbon dioxide threshold" of photosynthesis, is not 

 confirmed by kinetic investigations under well-controlled laboratory conditions (e. g., 

 by the measurements presented in figs. 27.2A-27.4). 



4. Carbon Dioxide Fertilization and Inhibition 



Practically all carbon dioxide curves show that neither the normal car- 

 bon dioxide concentration of the air (0.03%, or approximately 1 X 10 ~^ 

 M) nor the content of this gas in water equilibrated with the free atmos- 

 phere is sufficient for complete saturation of photosynthesis in moderate 

 or strong light — at least, without exceedingl}^ strong stirring. The curves 

 indicate that it should be possible to improve, perhaps by as much as 50- 

 100%, the yield of photosynthesis under natural conditions, by means of 

 "carbon dioxide fertilization," and one may expect that this will lead to a 



