DISCUSSION . 421 



weak li^lit, yon sec practically nothiiit? in the red. However, if you incre.nse 

 the h\<;lit intensity Aud l)rin«; it into the saturation re<>;ion of photosynthesis, 

 the nc<;ati\e ellec t at ()8() nxfi develops, and other negative bands appear, 

 e.g., at 650 and 700 ni/i. In the strongest light we used, the cytochrome 

 elTects were ])ractically gone. The strongest remaining effects were those at 

 480 and r)20 ni/u (which Dr. Chance, and we, too, ascrii)ed to a transformation 

 of carotenoids) : and the negative effects at 680 and 130 ni/t, which are most 

 likely due to the bleaching (reduction?) of chlorophyll, I would like to note 

 here that the bleaching of the red chlorophyll band does not occur uni- 

 formly over its width. The several components of chlorophyll a appear 

 to respond differently. However, the important point at the moment is that 

 bleaching of the red chlorophyll band is not a linear function of light in- 

 tensity, but follows a sigmoid curve. There is no significant change as long 

 as you are in the linear part of the photosynthesis light curve. Oidy when 

 you get into the saturation region, something begins to happen to chlorophyll. 



Dr. Franck: I was very interested to hear that changes in the absorption 

 spectrum of chlorophyll become strong as soon as the irradiation intensity 

 surpasses saturation. Here photo-oxidation of chlorophyll might be responsi- 

 ble for the bleaching. If the spectral region around 7000 A is, as we are 

 inclined to believe, the region of light absorption by water-exposed chloro- 

 phylls, the bleaching in this region is supposed to be the strongest. 



Dr. Kok: I am sorry but on this point I cannot agree. The discussed effects 

 don't show sigmoid light curves. Other things besides activating intensity 

 determine their sign and magnitude. With the short darktime arrangement 

 the effects in the red began only to make sense after we found that the wave- 

 length of activating light mattered. Still, green cells are confusing, maybe 

 because their pigments overlap so closely and (or) photosynthetic sensiti- 

 zation is so well balanced over the entire spectrum until 680 m/i. 



Possibly we notice the light-induced shifts at 700 m/z only if the system is 

 out of balance: in continuous illumination at saturating intensities, as Dr. 

 Rabinowitch showed, or in our long darktime flashing light ("fast effect") . 

 In the latter case photosynthesis still runs at normal integrated rate and 

 efficiency, but the involved intermediates can be easier observed since they 

 undergo cyclic changes. We also found earlier (12), that the (fast) shifts 

 at 520 ni/i saturated earlier than the red and blue shifts. This does not 

 necessarily imply that photooxidation underlies the latter ones. As I said, 

 it might be explained on the basis that the oxygen evolution step (correlated 

 with the slow effect) saturates earlier and that not until then does a negative 

 effect become noticeable. The situation might be analogous in the case of 

 cytochrome / oxidation. In green material the 555 nifi shift is not per- 

 ceptible with continuous actinic light, but we found it repeatedly with long 

 darktime flashing light. 



In Anacystis, where the quantum yield is normally poor in the far red, 

 the system might always be off balance and the 700 m/t shifts (as well as 



