EPILOGUE 1993 



above concentration value, the maximum rate of photosynthesis in con- 

 stant Ught, which is eciuivalent (at room temperature) to about 0.025 

 molecules oxygen per chlorophyll molecule per second (1/(2500 X 2 X 10~*) = 

 0.02). However, this satisfactory agreement was spoiled by the more 

 recent observations (by Taraiya et al., and by Kok) of a higher flash yield, ob- 

 tainable by flashes which, though not" instantaneous," still are much shorter 

 than the "Emerson-Arnold period" of 2 X 10"- sec. The yield of these 

 "extended" flashes continues to increase with the length of the dark inter- 

 val, in the region (up to 1 sec. or beyond) in which that of " instantaneous" 

 flashes become constant; furthermore, it depends on temperature, while the 

 yield of the "instantaneous" flashes is independent of the latter. The ex- 

 planation of these complications must be sought in the interaction of two 

 "limiting" reactions. Franck's suggestion tliat one of them (the " Emerson- 

 Arnold reaction") consists in the transformation of the primary photo- 

 chemical products, which "stabilizes them against back reactions, while 

 the second one is concerned with the primary carbon dioxide fixation, may 

 be correct for some of the results, but does not account for similar observa- 

 tions made by Gilmour et al. on the Hill reaction, in which no carboxyla- 

 tion is involved. It seems that more than one "limiting" reaction is in- 

 volved in the oxygen-liberating photochemical process itself. A closer 

 quantitative analysis of flash light results promises important revelations 

 concerning these reactions. The important point to be kept in mind in 

 this analysis is that the maximum yield of a reaction involving a "catenary 

 series" of enzymatic (or other) steps of limited capacity, is not always 

 equal to the maximum rate of the "slowest" of these reactions, but may 

 be dependent on several of them (if their maximum rates are not too dif- 

 ferent), as demonstrated in Chapter 27 (section A7). 



The existence of a second "limiting" reaction, which can utilize effec- 

 tively dark periods of the order of 0.1-1 sec, is indicated, in addition to 

 flashing light experiments, also by observations in alternating light with 

 equal light and dark periods. 



This brief passing in review of some of the problems of photosynthesis in 

 which the most progress has been achieved in the last ten years, or in which 

 unexplained experimental results have posed a clear challenge to further 

 experimentation and theoretical interpretation, shows that at the time this 

 monograph is completed, the field is in great flux. The author hopes that 

 the method he has adopted to describe critically all the experiments, and 

 discuss all the theories suggested to explain them, without (committing him- 

 self too strongly to any one of them (and committing himself against them 

 only when they infringe on general principles of physical chemistry, which 

 — the author believes — cannot be violated by organisms any more than by 

 non-organic chemical systems), will prevent this monograph from becoming 

 obsolete as rapidly as it will inevitably become incomplete. 



