1454 PHOTOSYNTHESIS IN INTERMITTENT LIGHT CHAP. 34 



Table 34.11 scatter rather badly, but show an unmistakable downward 

 trend with increasing temperature, and indicate a temperature coefficient 

 of approximately 2. Comparison of the figures in the last two rows shows 

 that the Emerson- Arnold reaction is about five times slower in heavy than 

 in ordinary water, a fact that can be explained by the assumption that 

 the "stabilizing" reaction (which may be a dismutation) involves the 

 transfer of a hydrogen atom. 



The exponential curves in figures 34.10 and 34.11 can be defined by two 

 parameters, e. g., the half-time, iy,, and the maximum yield per flash, 

 pmax.^ The latter constant was discussed in chapter 32; its values, 

 determined by Emerson and Arnold (1932'''), Arnold and Kohn (1934) 

 and Emerson, Green and Webb (1940), were listed in Table 32.1 (expressed 

 in the form of a factor r, the ratio flash yield/chlorophyll content). In 

 healthy specimens of plants from various phyla, P™^^-. was found to vary 

 between 5 X 10-^ [Chi Jo and 2 X 10-^ [Chl]o; it declined below 1 X lO"'' 

 [Chl]o in aged cultures of Chlorella. (Similar decline can be caused, ac- 

 cording to section 4, by narcotization and by ultraviolet illumination.) 



The yield P™"^- was identified, in equation (34.3), with [I-Eb]o. Since 

 we now apply this equation to saturating flashes, in which all molecules of 

 the catalyst Eb are occupied at the beginning of the dark period, we can 

 write : 



(34.6) P""""- = E^w 



where Eb is the total available amount of Eb, and l/n the number of oxygen 

 molecules produced by a single reaction of a molecule of 1/Eb ; l/n may be 1, 

 but is more likely to be M, or even %. 



Franck and Herzfeld's (1941) interpretation of the maximum yield per 

 flash as a measure of the available quantity of the limiting catalyst, Eb, 

 imphed in (34.6) was considered the most plausible interpretation of this 

 constant in chapter 32. As an alternative, one could consider the hy- 

 pothesis that the maximum yield per flash is equal to the amount of 

 the reaction substrate present in the cells at the beginning of the flash, in a 

 form suitable for immediate transformation (cf. Arnold, 1935) . Because of 

 the order of magnitude of P'"^^-, it is impossible to identify this substrate 

 with the A . CO2 complex. (It was estimated in chap. 8 that the concentra- 

 tion of the acceptor A is of the same order of magnitude as that of chloro- 

 phyll, while P'^^''- is about a thousand times smaller.) An attempt can be 

 made (as in the hypothesis of Briggs 1941; cf. sect. A2) to identify the 

 "limiting" substrate mth the intermediate oxidant, X (intervening be- 

 tween chlorophyll and A-C02), but this assumption does not explain the 

 results of fluorescence experiments, (the latter indicating that all chloro- 

 phyll molecules — and not only one in several thousands — are parts of 



