1232 THE TEMPERATURE FACTOR CHAP. 31 



the order of 1.2 or 1.3. If the supply of all reactants, as well as of Hght, 

 is abundant, the over-all rate will be determined by the efficiency of a non- 

 photochemical "bottleneck" reaction, and the temperature coefficient 

 may reach or exceed 2 (which is the common value for enzyme reactions 

 in vitro and in vivo). Obviously, by working in intermediary regions, one 

 can obtain values of Qio ranging all the way from 1 to 2 or more (as illus- 

 trated by figs. 28.6 and 28.7). 



Most of the earlier investigators worked under "natural" conditions, 

 which means partial or complete hght saturation, but usually incom- 

 plete saturation with carbon dioxide. No wonder a controversy arose as to 

 whether the temperature coefficient of photosynthesis is about 1.3 or about 

 2. Prjanishnikov (1876), Kreusler (1887, 1888, 1890) and Lubimenko 

 (1906), among others, found Qio values of less than 1.5, whereas Matthaei 

 (1904), Blackman and Matthaei (1905) and Blackman and Smith (1911) 

 obtained values close to 2. Blackman was the first to realize that high 

 temperature coefficients were associated with high light intensities. He 

 took this as an indication that photosynthesis involves, in addition to the 

 photochemical reaction proper, also an enzymatic process, which becomes 

 rate-determining when light and carbon dioxide are supplied in abun- 

 dance. 



This conclusion was criticized by Brown and Heise (1917), who referred to the above- 

 mentioned papers of Prjanishnikov and others for proof that the temperature coefficient 

 of photosynthesis is lower than that of typical enzymatic reactions; this criticism was 

 rejected by Smith (1919) as showing a lack of understanding of the dependence of the 

 temperature coefficient on external conditions. 



The designation of the nonphotochemical stages of photosynthesis as 

 the "Blackman reaction," first used by Willstatter and Stoll in 1918 and 

 Warburg in 1919, was based on this argument of Blackman. We know 

 now that photosynthesis involves not one, but many nonphotochemical 

 catalytic reactions; therefore, references to the Blackman reaction should 

 be avoided. Whenever possible, one should specify which of the several 

 known or suspected catalytic reactions one has in mind. 



1. Absence of Temperature Influence in Weak Light 



Primary photochemical processes ordinarily are independent of tem- 

 perature. This is so because light usually provides more than the required 

 minimum activation energy, so that no additional thermal activation is 

 needed. Exceptional situations occur, however, in which light energy 

 alone is insufficient to accomplish a certain transformation that becomes 

 possible if a certain amount of thermal energy is supplied during the ex- 

 citation period. 



