ENHANCEMENT 

 Jack Myers 



The photosynthetic phenomenon which has come to be known as 

 enhancement arose, not from any theoretical insight, but simply 

 from the attempt to understand an experimental anomaly. On the 

 long wavelength side of the in vivo absorption spectrum, the quan- 

 tum yield drops rapidly even at wavelengths (i.e., 690 im) where 

 chlorophyll a absorption is still high."^'-""^ Study of the anomaly 

 led Emerson and co-workers to the discovery of enhancement.^ When 

 imposed upon a constant background of shorter wavelength illumina- 

 tion, long wavelength light (690 mj-i) gave a higher rate of photo- 

 synthesis and a quantum yield approaching maximum. Further, it 

 turned out that a significant intensity and maintained rate of 

 photosynthesis was needed in the short wave background. The phe- 

 nomenon is not one of "catalytic" or triggering action, not a sec- 

 ondary light effect, but an intrinsic character of photosynthesis. 

 Hence a more useful description of enhancement becomes the follow- 

 ing: a light beam of wavelength >^i and another beam of properly 

 chosen wavelength '^z, when presented together, elicit a rate of 

 photosynthesis greater than the sum of the rates when presented 

 separately. 



THE HYPOTHESIS 



Enhancement observed between alternated light beams and other 

 more direct lines of evidence, which are discussed in other papers 

 of this symposium, support the most obvious hypothesis for en- 

 hancement : that two photoreactions (l and II ) are necessary for 

 green plant photosynthesis. The spectral character of actinic 

 light by which the two photoreactions are partially distinguish- 

 able implies participation of two pigment systems (l and 2, fol- 

 lowing the convention of Duysens and Amesz^). So far we have not 

 discovered in any plant a wavelength at which quanta can be in- 

 jected exclusively into either pigment system and thence to the 

 corresponding photoreaction. However, we can identify wavelength 

 regions in which quanta are absorbed in excess by either one of 

 the two pigment systems. 



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