66 PHOTOSYNTHESIS 



of photosynthesis proportional to each increase in light 

 intensity tor all wavelengths, i.e. at no wavelength is the 

 light 'saturating'. Then provided the absorbed energy is 

 equally available to the photosynthetic system the action 

 spectrum should parallel the absorption spectrum. In as far 

 as this is found to be untrue we can deduce from studies 

 comparing the action spectrum of photosynthesis with the 

 absorption spectrum of the plant the relative photosynthetic 

 efficiency of light absorbed by different pigments. 



As we said in Chapter 3 it is not easy to deduce from the 

 absorption of the living cell the relative absorption by each 

 of the constituent pigments even when the absorption 

 curves of the latter are known. Further conclusions can be 

 made only concerning the relative activity of the different 

 pigments. For this reason it is customary to assume that the 

 photosynthetic efficiency of chlorophyll is independent of 

 wavelength, i.e. of quantum size. Throughout most of the 

 visible spectrum this hypothesis appears to be correct but 

 in the far red, beyond 680 mu, there is a rapid decline in 

 photosynthetic activity even within the region where the 

 absorption by chlorophyll is still appreciable. A satisfactory 

 explanation of this observation has not yet been suggested. 



The variation of quantum efficiency with wavelength in 

 Chlorella was investigated by Emerson and Lewis (1943) 

 whose data are shown in Fig. 4. 1 1 . There is a marked drop 

 in the photosynthetic yield in that region where the 

 carotenoids absorb. The decrease is not so great as is to be 

 expected from the relative absorption by the pigments, for 

 where the carotenoids absorb about 50% of the incident 

 energy the decline in photosynthetic yield is only about 

 25%. Thus it may be concluded that light absorbed by 

 carotenoids is about 50% as efficient for photosynthesis 

 as that absorbed by chlorophyll. Whether this decreased 

 efficiency is due to some of the carotenoids in Chlorella 

 being completely inactive or to a generally lower efficiency 

 of all the carotenoids is not yet known. In the diatoms it has 

 been shown that light absorbed by fucoxanthol is almost 

 equally as effective as that absorbed by chlorophyll (Button 

 and Manning, 1941; Tanada, 1951). Of the phycobilins, 



