1032 THE LIGHT FACTOR. I. INTENSITY CHAP, 28 



It was noted that, according to equation (28.32), half saturation of 

 photosynthesis with light should take place, in the presence of excess car- 

 bon dioxide, when chlorophyll is distributed equally between the forms 

 X- Chi -HZ and HX-Chl-HZ. This state should also correspond to the 

 halfway point in the transition of fluorescence from the "low light" yield, 

 <Pi to the "high light" yield, ^2 (c.f. page 1049). It has been argued that ac- 

 cumulation of one half the total quantity of chlorophyll in a changed state 

 during intense photosynthesis is unlikely, since no reversible change in the 

 spectrum of strongly illuminated green plants has ever been noticed. An 

 exact experimental re-examination of this statement remains desirable; 

 but even if it were confirmed, this might only mean that the spectrum of 

 the chlorophyll complex in the form which accumulates in strong light is 

 practically identical with that of the form present in darkness. This is 

 quite possible, since the difference in composition is supposed to lie not in 

 the chlorophyll molecule itself, but in the associated hydrogen donors and 

 acceptors. 



One has to distinguish between this hypothesis and the assumption 

 (favored, among others, by Franck and Herzfeld) that the conversion of 

 X- Chi -HZ to HX-Chl-Z (or of ACOs-Chl-A'H.O to AHCO^-Chl- A'HO 

 to use Franck's picture) requires two quanta, with an intermediate, 

 X • ChlH • Z, formed by the first quantum. According to this picture, when 

 photosynthesis proceeds most efficiently, the rate of light absorption by 

 ChlH must be equal to that by Chi (because only under these conditions 

 can all quanta be utilized). If this assumption is made, we must postulate 

 a close similarity between the spectra of the compounds Chi and ChlH. 

 This, too, is not impossible, but more remarkable than a spectroscopic 

 similarity between X- Chi -HZ and HX-Chl-HZ. 



If the changed form of the chlorophyll complex is green, the question 

 arises as to the photochemical effect of light absorbed by this form. One 

 possibility is that the quanta absorbed by the changed form produce no 

 photochemical effect at all; the other that they cause photochemical back 

 reactions, such as : 



HX-Chl-Z ^^X-ChlHZ or HX-ChlHZ + A'HO ^^ 



HX-Chl-Z + A'HsO 



Kinetic considerations show that photochemical back reactions would not 

 in themselves cause Ught saturation, but merely reduce the quantum yield 

 by the same factor at all light intensities. Franck and Herzfeld, in one of 

 their earlier speculations on the mechanism of photosynthesis (1987), sug- 

 gested that light saturation may be caused by photochetnically induced chain 

 back reactions: but they abandoned this hypothesis later (1911) in favor of 



