REVERSIBLE BLEACHING OF CHLOROPHYLL in VIVO 7 1 



lengths to see whether the cells showed the normal response to illumi- 

 nation. 



The apparatus reproduced with excellent agreement the earlier 

 work of Duysens; in addition, it clearly showed absorption changes in 

 the red. 



A typical differential absorption spectrum is shown by solid line in 

 Fig. 1. At 680 m/i, the optical density of illuminated cells can be 0.25% 

 lower. Exact comparison of this decrease with the increase at 515 mju 

 is not possible because we had to use different exciting Ughts in the 

 two regions. However, since the two effects are of the same order of 

 magnitude, the assumption is permitted that they are both caused by 

 a reversible change in chlorophyll a. Spectroscopically, this change is 

 most similar to that observed by Krasnovskii et al. (and by Evstigneev 

 and Gavrilova) upon reversible reduction of chlorophyll a in vitro. 

 The smaller changes farther in the red (decline of absorption at 710 

 to 715 niju, and increase at 730 mn), as well as the bleaching at 478 

 mn (already noted by Duysens), remain to be interpreted. Several 

 reversible changes of chlorophyll may occur at once in the cell, e.g., 

 the formation of metastable triplet molecules may be superimposed 

 on that of the semiquinone. It will be noted, however, that the effect 

 observed in vivo at 475 mju is opposite in sign to that expected from 

 the formation of metastable chlorophyll a. 



Discussion 



Duysens: I, too, made some experiments in red while I was working in your 

 laboratory. I used illumination of presumably lower intensity than you did — the 

 same illumination at 515 and 680 ran. It was rather difficult to exclude completely 

 any change at 680 mn, but I think the changes there were less than one-fourth of 

 those at 515 rrifi. So it is possible that the changes which you find occur only at 

 higher light intensity and are not correlated with those at 515 m/x- 



Rabinowitch: That is a pos.sibility, and I would not say that there is here a di- 

 rect expeiimental disagreement of the kind we are only too familiar with in pho- 

 tosynthesis. However, according to Coleman and Holt, the effects are of the same 

 order of magnitude in the green and in the red; but, since two different kinds of 

 illuminating light were used in these two regions, we as yet cannot compare 

 them exactly. 



Strehler: I would like to make two points: One, that using Dr. French's ap- 

 paratus we were not able to find any changes on the red side of about 670 mp, 

 while we did get large changes around 648 m^u. Second, apparent changes in trans- 

 mission could be due to fluorescence. How can you rule out the possibility that 



