CHLOROPHYLL PARTICIPATION IN THE PRIMARY PROCESS 



549 



chlorophyll has two modifications, "similar to hemoglobin and oxy- 

 hemoglobin," and that, in the course of photosynthesis, the pigment is 

 first bleached (by oxidation or by reduction) and then restored to its 

 colored form. He pointed to the increased transparency of leaves in 

 intense light, "usually attributed to the re-alignment of chloroplasts" 

 (c/. Vol. II, Chapter 22) as an argument in favor of this "reversible 

 bleaching theory." Reinke (1886) took issue with Timiriazev. He saw 

 in the comparative photostability of chlorophyll in the leaves killed by 

 boiling, a proof that light does not cause a reversible decolorization of 

 this pigment — since, in his opinion, the capacity for restoration of the 

 color should be lost after boiling. This argument against Timiriazev's 

 theory is not very convincing; but the merit of Reinke's paper lies in a 

 remarkably clear presentation of the alternative "physical" theory, 

 visualizing a transfer of vibrational energy from the light-excited colored 

 sensitizer to the colorless reaction substrate. The triple alternative 

 faced by light-excited chlorophyll molecules — energy transfer to a 

 reactive system, self-destruction of the pigment, or re-emission of ab- 

 sorbed energy as fluorescence — was described by Reinke with a clear 

 understanding of the physical problems, remarkable in a botanical paper 

 in the year 1886. 



The controversy between the proponents of "physical" and "chemi- 

 cal" theories has not ceased since the time of Timiriazev and Reinke. 

 Willstatter and Stoll (1918) thought it could be resolved by the determi- 

 nation of the effect of photosynthesis on the concentration of chlorophyll 

 in plants. As shown by table 19.11, they found that even intense photo- 



Table 19.11 



Effect of Photosynthesis on Chlorophyll Concentration 

 (after Willstatter and Stoll) 



synthesis at elevated temperatures, does not change this concentration. 

 Their results prove that chlorophyll is not permanently affected by 

 photosynthesis. But, contrary to the original intention of Willstatter 

 and Stoll, they do not solve the problem of a reversible chemical trans- 

 formation of the pigment. In this case, the concentration of chlorophyll 

 could be reduced to a certain extent during photosynthesis (as envisaged 



