EMERSON ENHANCEMENT EFFECT AND TWO LIGHT REACTIONS 



IN PHOTOSYNTHESIS 



Govindjee 

 Dedicated to the memory of late Professor Robert Emerson 



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 During the 1940 's Robert Emerson discovered ' that the 



quantum efficiency of photosynthesis, plotted as a function 

 of wavelength, drops appreciably long before one reaches the 

 long-wave limit of absorption by chlorophyll a. This decline 

 is referred to as the "red drop." Recently it has become 

 clear that in green cells, this "drop" begins when the form of 

 chlorophyll a, absorbing at the longer wavelength, . becomes the 

 prime absorber. Persistence of a certain photosynthetic 

 activity to wavelengths up to 740 mji could be interpreted either 

 in terms of fractional absorption by the short-wave form of 

 chlorophyll a (Chi a 670) extending into the far red, or by 

 ascribing to the "long-wave form" of chlorophyll a, the capacity 

 of bringing about complete photosynthesis but with a very low 

 yield. Excitation of the "long-wave form" of chlorophyll a 

 alone in green cells certainly is insufficient to bring about 

 all photochemical steps involved in photosynthesis with a high 

 yield. In the case of the red and blue-green algae, the drop 

 occurs at shorter wavelengths-^' apparently, when chlorophyll 

 a takes over the role of the main absorber from the phycobilins. 

 Yt seems that all (or almost all) chlorophyll a in red and blue- 

 green algae has the same photochemical function as do the long- 

 wave forms of chlorophyll a in green plants. 



In the photosynthesis laboratory at Urbana, a long series of 

 experiments'^"^ revealed that by exciting one of the "short-wave 

 pigments" (chlorophyll b and Chi a 670 in green algae; phycoerth- 

 rins and phycocyanins in red and blue-green algae; fucoxanthol, 

 chlorophyll c and Chi a 670 in diatoms), simultaneously with 

 the long-wave form of chlorophyll a, the "red drop" could be 

 avoided. Simultaneous excitation of two "pigment systems" 

 permits the plants to use efficiently the energy absorbed in 

 the "long-wave" form of chlorophyll a. This synergistic effect 

 is referred to as the "Emerson enhancement effect". Studies 



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