EFFECT OF FINISHING DARK REACTIONS 1039 



The two expressions become identical if A"e ^ A-gChlo ; in which case we write : 



(28.47Ba) :// = (k' + hEl)/k* 



This was stated before to mean that the bottleneck reaction is (28.41b), 

 while the catalyst, Eb, is present in excess. According to (28.47) it also 

 means that the maximum yield Pniax! is proportional to Chlo; and the 

 observ^ations on aurea leaves, quoted above on page 1031, indicate that this 

 condition is not realized in nature. The reverse relation, AgChlo ^ A"e, 

 which allows Eb to be fully utilized and the maximum rate (equation 28.46) 

 to be independent of chlorophyll concentration, appears the more likely ap- 

 proximation to natural conditions. This inequality reduces (28.47A) to: 



(28.47C) v/ = ^"'^^'l^teCht^"^ ^^^' << ^"^^^^"^ 



Furthermore, we have seen above that, for the utilization of practically 

 all the absorbed light quanta at low light intensities, k' must be >C A-eEs; 

 we thus have approximately : 



(28.47D) ./J - ^'°2iiichl^° (A-; <C A-eChlo; k' <^ k.El) 



(nonrectangular hyperbola), and: 



(28.47E) vJ-^ (A-; » A-eChJo; k'<^k,El) 



(rectangular hyperbola). It thus appears that the hyperbola (28.42) 

 ascends more steeply and reaches saturation more suddenly than a rec- 

 tangular hyperbola with the same parameters n and Pmax.'- This deviation 

 from the shape of a rectangular hyperbola is in the direction actually noted 

 in many experimental light curves (c/. the speculations of Brackett and 

 Smith in the next section). 



It was stated above that the empirical relation between the parameters 

 n, i/J, and Pmll'. is approximately that required for a rectangular hyperbola. 

 However, the calculation which led to this conclusion was one of order of 

 magnitude only ; more precise determinations of the light curves may show 

 whether a deviation from the rectangular hyperbola by a factor of 1/2 in 

 the i//-value is compatible with the facts. Such a determination should not 

 be beyond the precision with which kinetic measurements of photosynthesis 

 can be carried out at the present time (although the persistent disagreement 

 about a factor of about 1/2 in the value of n, to be described in the next 

 chapter, indicates the difficulty of such measurements). 



