107G THE LIGHT FACTOR. I. INTENSITY CHAP. 28 



(28.52) AHCO2 + A'lIO > ACO2 + A'11,0 



or: 



(28.53) AHCOa-Chl-A'HO— — ^ ACOo-Chl-A'H.O 



and if no exhaustion of the photosensitive form has occurred at the time 

 hght saturation has been produced by Eb deficiency, there is no reason why 

 such exhaustion should occur if the hght intensity is stepped up still further, 

 since the products of the back reaction are wiuly to participate again in the 

 primary photochemical reaction. Experimentally, an increase of fluores- 

 cence at "supersaturating" light intensities has been noted in several cases 

 discussed above. This could be explained by assuming, with Franck, that 

 the back reaction liberates so much energy as to cause the reversal not only 

 of the first oxidation-reduction step, but also of the carboxylation reaction : 



(28.54) AHCO, + A'HO > A + CO. + A'H^O (or A + COo + A' + H.O) 



or : 



(28.55) AHC02- Chi -A'HO > AChlA'HaO + CO2 (or A-ChlA' + CO2 + H.O) 



In this way, the products of the back reaction are added to the pool of 

 free carbon dioxide and water rather than to the immediately available 

 substrates of the primary photochemical process, ACO2 and A'H20. 



We recall that this hypothesis was first suggested by Franck to ex- 

 plain an entirely different observation — the "carbon dioxide burst" some- 

 times observed in the first minutes of illumination {cf. Vol. I, page 207, and 

 chapter 29, page 1093). 



When the yield of fluorescence goes up with increasing light intensity, 

 as in figure 28.25, and reaches a new steady value, ^2, in the region of the 

 light saturation of photosynthesis (fig. 28.26), this can be taken as a sign 

 that saturation is due to a preparatory dark reaction; it is thus under- 

 standable why, in McAlister's experiment represented in figure 28.25, this 

 change was observed in a C02-deficient medium and not in 5% CO2. 



The results obtained by Wassink and Kersten with Nitzschia (fig. 28.28) 

 are puzzling. The fact that above 50 kerg/cm.^ sec. tp decreases rather than 

 increases with light intensity could be formally explained by assuming 

 that, in this organism, the form of the chlorophyll complex that accumu- 

 lates during intense photosynthesis has a higher value of ki {i. e., dissipates 

 energy more rapidly), so that the sum h -\- h increases in strong light even 

 if kt declines to zero. AVhat is more difficult to explain is that in the 

 absence of carbon dioxide the diatoms retain the high yield of fluorescence 

 {(Pi) in strong light, while one would offhand expect that, in this case, the 

 lower value (^2) would prevail from the very beginning. (It was mentioned 



