386 LIGHT AND LIFE 



17. Govindjee, and Rabinowitch, E., Science, 132, 355 (1960). 



18. Krasnovsky, A. A., and Kosobutskaya, L. M., Doklady Akad. Nauk. SSSR, 104, 



440 (1955). 



19. Tanada, T.. Am. J. liotmiy, 38. 276 (1951). 



20. Vorobyova, L. M., and Krasnovsky, A. A., liinkliiniixa, 21, 126 (1956). 



21. Vorobyova, L. M., and Krasnovsky, A. A., Biokhimiya, 23, 760 (1958). 



DISCUSSION 



Dr. Franck: I am grateful for the opportunity to compare the basic 

 differences of Dr. Calvin's concept of the photochemical steps of photo- 

 synthesis with mine. That I accept his biochemical resuhs on the pathway 

 of the carbon in photosynthesis, is self-evident. Our main difference is 

 that Dr. Cabin, as practically all biochemists, takes it for granted that the 

 reduction of the final photosynthetic oxidant proceeds by dark reactions 

 with a photochemically produced reducing agent. On the other hand, accord- 

 ing to my view, physical evidence indicates direct photochemical reduction 

 acts with direct contact between the oxidants and the excited chlorophyll 

 molecules. In the last years Dr. Calvin emphasized the hypothesis that the 

 only light reaction of photosynthesis consists in a photoionization of 

 chlorophyll which by migration of electrons and positixe holes, produces a 

 charge of a 'solar battery' sufficient for electrolysis of water. The original 

 model of this battery is, according to my opinion, based on very improbable 

 assumptions; the new one presented today avoids some difficulties, but others 

 remain. I enumerate the main ones briefly: 



1) I still have grave doubts whether the quantum yield of photoionization 

 can be high enough to match the high yield of photosynthesis. That a 

 limited photoionization occurs, is shown by the beha\ ior of the after- 

 glow of chlorophyll, found by Strehler and .Arnold, and explained as recom- 

 bination luminosity of positive chlorophyll ions with electrons, by .Arnold. 

 That this hypothesis fits all observations of afterglow, lias been discussed 

 in a paper of Brugger and Franck. 



2) The assumption that light energy is fully utilized for photoionization 

 has failed to explain the very characteristic relations between chlorophyll 

 fluorescence intensity and utilization of light energy for photosynthesis. 

 .All factors which lower the jjhotosynthetic utilization of excitation energy, 

 as low temperature, absence of CO,, induction phenomena, addition of 

 enzymatic poisons, etc., enhance the fluorescence, but not more than by a 

 factor of ca. 2, under conditions where piiotosynthetic rates are zero during 

 illumination. A detailed up-to-date description of such observations and 

 their interpretation based on the concept of direct contact between oxidants 

 and e\(ilcd chlorophyll, will Ix' found in an article on chlorophyll after- 

 glow in the Hnndbiuh fiir Pflauzenbiologie, Vol. V. (The article was 

 originally written several years ago and is finally due to appear this spring) . 



3) .Another point not considered so far in Dr. Calvin's concept is an 



