i 



EFFECTS OF PHOTOSYNTHETIC POISONS ON DELAYED LIGHT IN THE 



MILLISECOND TIME RANGE 



Walter F. Bertsch, J. B. Davidson, and J. R. Azzi 



The initial steps in photosynthesis Involve conversion of energy from light 

 quanta into some form that is available for utilization in the biochemical 

 reduction of carbon dioxide. Since about eight light quanta are needed to 

 reduce one carbon dioxide molecule^'°^ and since the absorption cross section 

 for photosynthesis is known to be the size of several hundred chlorophyll 

 molecules^'^'^', it is clear that some type of cooperation exists between 

 chlorophyll molecules in the photosynthetic apparatus. A great deal of 

 evidence indicates that this cooperation occurs within groups of chlorophylls, 

 known as photosynthetic units (1^' ^^' ^^' ^9, 22-24)^ The energy from a 



photon absorbed within a photosynthetic unit presumably migrates by resonance 

 transfer^^' °' '^' '^/ until It reaches a reaction center. At the reaction center 

 the energy is somehow made available to run biochemical dark reactions. 



It is now known that these early steps in quantum conversion involve 

 purely electronic, as well as enzymatic, processes^"^' '"'^K Furthermore, the 

 photosynthetic aoporatus of higher plants and photosynthetic bacteria, when 

 dried, has electrical properties that are usually associated with organic 

 semiconductors— photoconductlon, thermolumlnescence, and Increased 

 conductivity at higher temperatures^^' °' '^^ It therefore seems that the 

 delayed light emission of living plants'"^"' might provide a direct physical 

 measurement of electron transitions involved In photosynthetic quantum 

 conversion. 



The dim glow from plants is emitted by the first excited singlet transition of 

 chlorophyll ("^z ^'*^^ '^2) and has decay characteristics that maybe Interpreted 

 in terms of untrapping of electrons In a semiconductor^' ' ' ' ^ '/. However, 

 it is not certain that the delayed light is actually produced by a solid state 

 physical phenomenon of this sort. 



Regardless of the physical mechanism of delayed light emission, if the 

 delayed light is associated with photosynthesis, it is presumably emitted 

 from very early steps of quantum conversion. The argument is simple: the 



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