Reversible Bleaching of Chlorophyll in Vivo* 



J. W. COLEMAN, A. STANLEY HOLT, and EUGENE I. 



RABINOWITCH, Photosynthesis Research Project, Department of 



Botany, University of Illinois, Urbana, Illinois 



It has often been suggested (L3) that in photosynthesis chlorophyll 

 undergoes a reversible change. It could be either: (1) Transformation 

 into a "hiradical," metastable state (an electronic triplet state, with 

 both free valencies on the same atom, or a tautomeric state, with the 

 two valencies at different atoms); (2) reduction, either to a semi- 

 quinone or to a valence-saturated leuco compound; or (3) oxidation, 

 also either to a radical or to a saturated product. 



Transformation into the metastable state has been suggested as the 

 first step in the internal conversion of excitation energy, which limits 

 the yield of chlorophyll fluorescence to 25% (10) or 33% (7), in vitro, 

 and 2% to 3% in vivo (7). According to Franck (5), photosynthesis 

 probably occurs by reactions of metastable chlorophyll a molecules. 

 According to Livingston and Ryan (12), these molecules are co-re- 

 sponsible for changes in the absorption spectrum of illuminated chloro- 

 phyll solutions in the photostationary state; Livingston and Ryan 

 (12) and Livingston, Porter, and Windsor (11), using condenser 

 flashes with s3Tichronized absorption measurements, found that 

 during an intense flash, up to 90% of chlorophyll (in a IQ-^M solution) 

 can be present in the metastable state, Livingston and Ryan's (12) 

 steady-state experiments indicated bleaching at 403 mn and enhanced 

 absorption at 439.5 to 524.5 mn; whereas their flash results showed 

 bleaching at 468, 470.5, and 477.5 mju, and enhancement of color only 

 at 524.5 m/i. However, according to the newer flash data of Livingston, 

 Porter, and Windsor (11), analyzed by Livingston (9), enhancement 

 extends over the range 450 to 560 m^, with a sharp peak at 475 m/x 

 and a shoulder at 520 m^u. 



Evstigneev and Gavrilova (4) found that reduced chlorophyll a, 

 obtained by illumination of phenylhydrazine-containing solution in 



* This work was carried out with the assistance of the Office of Naval Research. 



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