418 LIGHT AND LIFE 



not involve only chlorophyll b. Chlorophyll a is also involved. Among 

 several hypotheses one can conceive the presence of two pigment groups: 

 (1) chlorophyll a and (2) accessory pigment combined with a small fraction 

 of the total chlorophyll a. The first pigment group cannot perform complete 

 photosynthesis by itself since it can only transfer light to P700. In doing 

 so it generates the reaction center for the second pigment group. The second 

 group will preferentially transfer to this latter center, but if not there it 

 can pass the light on to the first group (whose chlorophyll a should there- 

 fore absorb at slightly longer wavelengths) and so reconvert P700— the other 

 trapping center. Via such a mechanism, light received by the second pigment 

 group would not only be automatically distributed over the two paths but 

 assist the conversion of radiation falling in the first group (enhancement) . 



Dr. Arnon: Do I understand you have measured phosphorylation in mono- 

 chromatic light at 705 m^^i with a chloroplast system that was catalyzed by 

 PMS. Is that correct? 



Dr. Kok: Yes. 



Dr. Arnon: Would you tell us something about the magnitude of this 

 phosphorylation. How does it compare with phosphorylation in white light? 



Dr. Kok: Would you care to answer that question, Dr. Hoch? 



Dr. Hoch: It doesn't make much sense to put it on a chlorophyll basis, 

 because we are totally absorbing the light. 



Dr. Arnon: On any basis. 



Dr. Hoch: Well, for instance, we measure in 0.3 ml about 0.8 micromoles 

 of phosphate disappearing in five minutes. Yet we have run experiments in 

 white light and gotten up to four to five hundred micromoles of phosphate 

 per milligram of chlorophyll per hour, and we have never tried to get the 

 maximum rate. 



Dr. Arnon: Did I understand that the comparison was between 400 and 

 0.8? 



Dr. Hoch: There's about one micromole in five minutes. 



Dr. Arnon: Let's put it on the same basis. 



Dr. Hoch: You can't put it on the same basis. 



Dr. Arnon: My question is, is there any basis on which we can compare 

 them? 



Dr. Hoc:h: All 1 can say is that under the conditions that we run the 

 monochromatic ligiit, we have also run white light experiments in which the 

 rate compared— that is, with data available in the literature. 



Dr. Arnon: You say they do compare? 



Dr. Hoch: That is, on a milligram chlorophyll basis. That is, with white 

 iigiit of high intensity. 



Dr. Arnon: Well, if there are no figures, may I ask for an opinion as to 

 the order of magnitude of phosphorylation in red light as compared to white 

 light? 



Dr. Hoch: J cannot give an opinion because I cannot find any l)asis for 



