1246 THE TEMPERATURE FACTOR CHAP. 31 



process that supplies reactants (oxidants or reductants) to the chlorophyll 

 complex to keep pace with the consumption of these reductants by the 

 photochemical process proper; this failure leads to "denudation" of chloro- 

 phyll, or its association with substances other than the normal reactants in 

 photosynthesis. Franck suggested an additional cause of fluorescence 

 changes: failure of the oxygen-Hberating enzyme to keep pace with the 

 production of "photoperoxides" by the primary photochemical process, 

 resulting in the formation of a "narcotic" poison, that displaces the 

 normal reactants from association with the chlorophyll complex. 



Of the processes supplying the reactants, Franck considered, in par- 

 ticular, the formation of the oxidant ACO2 from carbon dioxide and the 

 acceptor A, catalyzed by the enzyme Ea- He saw in the transition from 

 the low light yield, 991, to the (higher) high light yield, <P2, evidence of ex- 

 haustion of the reactant ACO2. The fact that this exhaustion occurred 

 earlier at the lower temperatures can then be looked upon as evidence that 

 the AC02-forming catalytic process has considerable activation energy. 

 The observation that, at high temperatures, saturation of photosynthesis 

 may occur at a lower light intensity than the transition <pi — > ^, while at 

 lower temperatures the two phenomena occur simultaneously, then indi- 

 cates that the activation energy of the carbon dioxide fixation process is 

 higher than that of the enzymatic process (presumably, according to Franck, 

 the stabilization of the primary photoproducts by the catalyst Eb), which 

 is responsible for light saturation at high temperatures. Franck was thus 

 led to the hypothesis that the (higher) temperature coefficient of photo- 

 synthesis observed in measurements in the lower temperature range (such 

 as 5-15° C.) is characteristic of the carbon dioxide-fixation process, while 

 the (lower) temperature coefficient observed around room temperature (15- 

 25°) is determined by the "finishing" reaction, catalyzed by Eb- 



In confirmation of this hypothesis, one may quote the observation 

 (noted on page 1057) that the effect on fluorescence of lowering the tem- 

 perature is quite similar to that of carbon dioxide deprivation. 



The results of Wassink and Kersten with diatoms do not fit well into 

 this picture, because, apart from the reversal of the relationship between 

 <Pi and <P2, the effect of carbon dioxide deprivation on the hght curve of 

 fluorescence was found, in this case, to be similar to that of an increase, 

 rather than of a decrease, in temperature. The effect of carbon dioxide 

 deprivation on the light curves of fluorescence of Chromatium (fig. 28.30) 

 also proved to be different and more complex than from the effect of low 

 temperature (fig. 28.40) . Altogether, it is not at all clear to what extent the 

 transition from the "EA-limited" state to the "Efi-limited" state, postu- 

 lated by Franck, can be held responsible for the observed decrease of 

 the temperature coefficient of photosynthesis with increasing temperature. 



