46 



Bessel Kok 



given, the responsible pigment molecules are in the fluores- 

 cent state. The exponential build-up of part of the 698 

 emission in fresh leaves or chloroplasts implies that the 

 responsible pigment is not in the fluorescent state to begin 

 with, but converted into it by light. We therefore, must 

 assume the presence of trapping centers which cease to be 

 traps as soon as they have received a quantum. 



One can conceive two possible mechanisms: (a) the trapping 

 pigment bleaches upon excitation and ceases to absorb light 

 from surrounding pigment so that the latter is free to fluo- 

 resce, until the trap is again restored in a consecutive pro- 

 cess, (b) the trapping pigment does not bleach but converts 

 an associated molecule. Unless the latter conversion is re- 

 stored in a consecutive reaction the next quantum cannot be 

 used in photochemistry and will be re-emitted by the trapping 

 pigment. 



In either case, one can determine the number of traps by 

 measuring the number of quanta required to raise the fluo- 

 rescence from Eo to Emax* Assuming a quantum requirement of 

 one per trapping molecule and absence of restoration reactions 

 at 77*K, our measurements indicate a concentration of 1 trap 

 per about —50 chlorophyll molecules. Though still preliminary, 

 our data probably show the correct order of magnitude; Kaut- 

 sky et al. (2), observed at room temperature a similar but 

 much faster initial rise of fluorescence and computed the 

 presence of 1 quencher per --^OO chlorophylls. Using Porphy- 

 ridium, Duysens and Sweers (3) arrived at a ratio 1:150. V.'e 

 indeed observed at 77°K for F7OO (and probably F685 as well) 

 a 10 times slower rise than at 300*'K for F685. 



This detrapping at low temperature indicates a photochemi- 

 cal phenomenon probably correlated with a primary act of 

 photosynthesis. Since as far as we know no corresponding 

 color change accompanies this detrapping, the second of the 

 two mechanisms discussed above might be involved. 



The number of traps in this system is much (^^lOx) greater 

 than expected on the basis of the classical photosynthetic 

 unit. Under appropriate conditions the rise time of F685 at 

 room temperature reveals the same small photosynthetic unit. 

 A trapping pigment present in so high a concentration ( — '2-5% 

 of Chi.) might be detectable by rather direct methods. 



In the following we will discuss some further observations 

 concerning long wave absorption and emission bands which could 



