DISCUSSION 419 



comparison. It docs not make sense to pnt it on a per milligram chlorophyll 

 basis. 



Dr. |,\(;i:mk)Ri: H you apply monochronialic light or white light at equal 

 energies under the same experimental conditions, you would have a common 

 basis. 



Dr. Hoch: VV^e have not done that experiment. 



Dr. Arnon: May I ask this question? Do you think that the phosphoryla- 

 tion vou get at 70.'} ni/u represents a substantial proportion of the total 

 phosphorylation you get in white light? Is this a lair question? 



Dr. Kok: Let's run an experiment tomorrow. 



Dr. Strkhlkr: I recall that Dr. Duysens' findings, some later work that 

 Lynch and I did (in which we used circulating Chlorella rather than measure- 

 ment during illumination) and certain experiments which Chance and I 

 reported all ga\e essentially negative results for the region 660 to 750 m/i. 

 Certainly, there were not changes of nearly the order of magnitude of those 

 occurring in the regions of 480 to 515 m^. In view of these other negative 

 findings. I want to ask a question concerning the technique of measurement. 

 It seems to me that if you have extremely dense chloroplasts of Chlorella 

 as you ha\e in your chamber and you pass light through it so that only 

 0.0001 of the total light comes through at the maximum of the chlorophyll 

 peak, then such fluorescence excitation by the measuring beam as occurs at 

 the side of the cuvette nearest the photomultiplier will be much more pro- 

 nounced at wavelengths where chlorophyll does not absorb so strongly, 

 i.e., out at the long wavelength end of the spectrum. Now, if the exciting 

 beam, but not the measuring beam, produces a change in the fluorescence 

 yield of the chlorophyll at the far end of the tube, one would expect to 

 find changes in the fluorescence intensity produced by the measuring beam 

 on the red edge of the band which would not be apparent when you are 

 measuring at the peak of the chlorophyll (where most of the fluorescence 

 would be reabsorbed) . I am sure you have some experiments which w'ould 

 rule out this possibility. 



Dr. Kok: No, I haven't. All we have done is our best to go down to the 

 weakest suspensions w^e can use. 0.01 optical density is a big shift for us. 

 You can use very dilute suspensions and still get this shift. This also holds 

 for the chemicallv induced shifts, which one can even observe with a standard 

 Cary instrument. However, the signal to noise ratio may be very poor. 



A^ote added in proof: After the meetings we made a few experiments 

 specifically designed to meet Dr. Strehler's doubts (with his consultation) . It 

 appeared that fluorescence induced by the detecting beam, although under 

 conditions contributing significantly to the total photosignal, could not 

 account for the light-induced changes of this signal. 



Dr. .Strehi.kr: I was quite convinced by the phosphate esterification action 

 spectrum, until Dr. Hoch mentioned you are alisorijing all the light. The 

 explanation there might be somewhat similar: namely, that in the region 



