INTERPRETATION OF LIGHT CURVES OF FLUORESCENCE 1073 



In this special case, photosynthesis reaches half -saturation at the same in- 

 tensity as fluorescence . The rate of photosynthesis according to schemes 

 (28.51CorD)is: 



P = k'k*IC\xU/{k'{C02] + k*I) 



and the saturation rate is P"*''- = A;'Chlo, which gives for !}, I the value 

 (28.51F). 



We can next consider the case discussed on page 1028, in which light 

 saturation is due to the accumulation of chlorophyll in the reduced form, 

 HX-Chl-HZ, because of slow primary hack reaction. (In other words, we 

 assume mechanism 28.21, with the specific assumption that reaction 28.21b 

 is practically instantaneous.) This leads us (for the simplest case when the 

 carboxylation equilibrium is undisturbed by photosynthesis) to equations 

 (28.28) for the rate P, and equations (28.30) and (28.31) for the half- 

 saturating light intensity of photosynthesis. The concentration of chloro- 

 phyll complexes in the inactive form is given in this case by equation 

 (28.26) ; the midpoint of fluorescence transition again coincides with the 

 half-saturation of photosynthesis. 



We can further consider reaction mechanism (28.41) (with the simpli- 

 fying assumption of instantaneous "reloading" with CO2 and H2O), in 

 which light saturation is ascribed to the combined effects of slow restora- 

 tion of the catalyst Eb (assumed to "stabilize" the first reduction product, 

 HCO2), and of the slow primary back reaction (28.41a'). The equation for 

 the concentration of "inactivated" chlorophyll complexes is in this case 

 quadratic, and its one significant solution is: 

 (28.51G) [{Chl|](= [AHCOo-Chl-A'HO]) = 



k'k: + kx^i - u-*7Chio + Kk*! , r/ k'k: + kXEi - kek*ich\o + k k*i \ 



2k.{k' + k*I) ^ LV 2fce(A;' + k*I) J 



+ 



k:k*ICh ]o 

 k,{k' + k*I) 



r 



The midpoint of the fluorescence transition is: 



p k'K + fceA:e'EE + A:efc'Chlo/2 



^^^■^^^) •A-' - k*{k: + A-eChlo/2) 



As could be expected, this midpoint coincides with the half-saturation 

 of photosynthesis (28.47 A) only in the case K > A-eChlo, when (28.5 IH) 

 reduces itself to (28.47Ba). (This extreme case is practically identical 

 with the one derived above from mechanism 28.21 ; in both of them, half- 

 saturation is reached when one-half of all chlorophyll complexes are in the 

 changed state— catalyst Eb not being fully utilized even in the light- 

 saturated state.) 1/, / differs from ./, / when the EB-Hmitation is significant 

 (_/. e., when A'g is not 1^ /fgChlo). 



