EXTERNAL SUPPLY AND EXHAUSTION EFFECTS 909 



The initial inhibition of photosynthesis in low [CO2], shown by cells 

 previously exposed to high [CO2] values, could be related to the photoxida- 

 tion phenomena observed in CO2 starved plants (c/. Vol. I, chapter 19). 

 Using Franck's picture, it can be suggested that CO2 satiated cells, placed 

 in CO2 deficient medium and exposed to light, develop a large quantity of a 

 "narcotic" (perhaps, because they were full of metabolites), which settles 

 on chlorophyll and holds photosynthesis down. The autocatalytic removal 

 of this inhibition seems to require 2-3 hours (as against a few minutes in 

 ordinary induction, cf. chapter 33). That such cells in fact are inhibited 

 is confirmed by the observation that if, after brief exposure to light in low 

 [CO2], they are brought back into a medium of high [CO2] (such as buffer 

 No. 9), they show a reduced rate of photosynthesis in this medium as well. 



Since these experiments were carried out in carbonate buffers, the ob- 

 served effects can be attributed either to changes in [CO2], or to those in 



If all carbon dioxide activity gradients between the outside medium 

 and the site of photosynthesis could be avoided, we would still anticipate, 

 on theoretical grounds, that carbon dioxide concentration will retain an 

 influence on the rate of photosynthesis: first, because of dissociation, 

 under low partial pressure of carbon dioxide, of the carbon dioxide-acceptor 

 compound that we assume is formed as an intermediate in photosynthesis 

 (cf. chapter 8) ; and second, because of the dependence of the rate of forma- 

 tion of this compound ("carboxylation") on the factor [CO2]. These two 

 relationships will be discussed theoretically in sections 7b and c ; but there 

 can be no certainty, until much more precise measurements have been 

 carried out, that any of the observed carbon dioxide curves actually reflect 

 one or both of these intrinsic kinetic relationships, rather than the more 

 incidental diffusion phenomena. As long as a [CO2] effect can be made to 

 disappear by improved stirring, it reveals itself as due to external diffusion; 

 but, when no further improvement in rate can be achieved in this way, this 

 does not mean that the remaining [CO2] effect is not caused by diffusion in 

 those parts of the gas path were external stirring can do no good. 



In estimating the supply of carbon dioxiile to plants under natural conditions, the 

 possibility of caibon dioxide siipplij through, llir rools nuist not be overlooked. It was 

 mentioned in chapter 2 (N'ol. I) that the doclrine of the aerial nijurishment of plants was 

 the second accomplishment of the discoverer or, more exactly, co-discoverer of ])hot()- 

 synthesis, Ingen-Housz. Since the time of Liebig, this doctrine has become the basis 

 of the science of plant nutrition. However, under certain conditions, Senebier's concept 

 that carbon dioxide can be supplied by the soil water to the roots and thence to the 

 leaves, scorned l)\' Tngcn-TTousz, may be correct. This may affect, field determinations 

 of the rate of pliotosynthcsis, based on measui'ements of the carbon dioxide consumption 

 from the air, and may also influence the results obtained by other methods, if the ob- 

 served rates are considered in relation to the external carbon dioxide concentration. For 



