STUDIES WITH FLASH ILLUMINATION ON THE ENHANCEMENT EFFECT IN 



CHLOROPLASTS 



C.P. Whittingham and P.M. Bishop 



Emerson and Lewis (1) were the first to observe in Chlorella 

 that the quan*-.um efficiency of photosynthesis in far-red light 

 was very low. This was particularly surprising because whilst 

 the absorption curve was more or less symmetrical about the 680 

 mp peak, the action spectrum was asymmetrical. Many years later 

 Emerson et al . (2) showed that the efficiency in the far-red 

 could be increased if the cells v;ere simultaneously illuminated 

 with a second shorter wavelength. The action spectrum for the 

 increased rate of photosynthesis resulting from a second wave- 

 length superimposed on a beam of light of 697 mp showed two peaks, 

 one at 650 and one at 670 mp. The peak at 650 mp is character- 

 istic for absorption by chlorophyll b, and Emerson concluded that 

 the simultaneous excitation of chlorophyll b must improve the 

 photosynthetic efficiency of the light absorbed in the far-red by 

 chlorophyll a. It was observed later that the enhancement effect 

 resulted from excitation of a second pigment throughout its spec- 

 trum. For example, excitation in Chlorella of either the blue or 

 the red absorption bands of chlorophyll b was equally effective 

 for enhancement. The general conclusion was that it was necessary 

 to have simultaneous excitation of both chlorophyll a and of some 

 other pigment for efficient photosynthesis. In studies with mono- 

 chromatic light, the relative inefficiency of absorption by chlor- 

 ophyll a alone appears only at the far-red end of the spectrum 

 because this is the only region in the visible where chlorophyll 

 a is the sole absorbing pigment. 



Emerson made no comment on the second peak at 670 mp in the 

 enhancement action spectrum observed by him. A peak (Govindjee , 

 et al. (3)) or shoulder (Myers and French (4)) at 670 mp in the 

 action spectra for the Emerson effect in Chlorella was inter- 

 preted by the first mentioned authors as an indication of the par- 

 ticipation in the enhancement effect of a chlorophyll a type with 

 a maximum absorption at 670 mp. Emerson and Rabinowitch (5) (c.f. 

 also Franck (6)) proposed the following hypothesis. Two photo- 

 chemical reactions occur in photosynthesis; one of these is 

 caused by non- or weakly- fluorescent chlorophyll a, the second 



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