1038 THE LIGHT FACTOR. I. INTENSITY CHAP. 28 



(28.45) PZl: = nhCh\ok:El/(hCh]o + K) 



(the lower index being justified by the assumption that reaction with 

 carbon dioxide is practically instantaneous). If k^ <^ fcgChlo, equation 



(28.45) is reduced to: 



(28.46) PSS: = nk:El 



which obviously is the maximum possible rate of restoration of the free 

 catalyst Eb by reaction (28.41c). It will be noted that, if A^gChlo is not 

 •C kl, this rate is not reached even in saturating light intensity; in other 

 words, the catalj^st Eb is never utilized to the maximum of its capacity. 

 In the opposite extreme case, A^e ^ A^eChlo, the saturation value becomes: 



(28.47) PTsl: = nkeChloEl 



in other words, the limiting rate is the maximum rate of reaction (28.41b), 

 reached when, in the photostationary state, practically all chlorophyll 

 complexes are in the "tautomeric" form, AHC02-Chl-A'H0. 



This last conclusion reminds us of the fact that the Franck-Herzfeld 

 variant of the mechanism of EB-limitation of photosynthesis does not 

 strictly belong under the heading of "limitation by finishing dark reac- 

 tions," since we have defined finishing reactions as those whose slowness 

 does not affect the composition of the photosensitive complex (and thus 

 the rate of the primary photochemical process) . The reason why the reac- 

 tion catalyzed by catalyst Eb in scheme (28.41) does not qualify as finish- 

 ing reaction is obvious: it is the assumed stable association of the sub- 

 strate of this reaction, { AHCOo}, with chlorophyll. As long as this photo- 

 chemical product has not reacted with Eb, it "blocks" the return of the 

 chlorophyll complex into the photosensitive form, AC02-Chl-A'H20. 



The half-saturating light-intensity according to equation (28.42) is: 



,_«(*' + « Wili^') 



(28.47A) v/ k*WTWm 



(It will be recalled that equation 28.42 was derived by assuming complete 

 and instantaneous saturation of the acceptor with carbon dioxide; there- 

 fore no [CO2] -proportional factor appears in equation 28.47A.) 



The hyperbola represented by equation (28.42) is not rectangular; it 

 therefore cannot be represented in the form F/(Pmax. — P) = const. X /• 

 The degree of its deviation from the shape of a rectangular hyperbola, with 

 the same initial slope and same saturation value, can be seen from a com- 

 parison of the half-saturating intensity (28.47 A) with the half-saturating 

 intensity derived from (28.43) and (28.45) by means of equation (28.48C) : 



