28 



PHOTOCHEMICAL PRINCIPLES 



focused on a platform holding the leaves. The intensity was measured 

 by a thermopile. After exposures of various times at each wavelength 

 the leaves were removed, the pigments extracted, and the amounts of 

 chlorophyll and protochlorophyll determined spectrophotometrically in 

 solution. The intensity of light necessary to transform 20% of the 

 protochlorophyll into chlorophyll was plotted against wavelength. 



0.9 



o.s 



■ 0.7 



- 0.6 



0.5 



0.4 





0.3 



- 0.2 



a 



at 



oc 



0.1 



400 



450 



500 550 600 



Wavelength mfL 



650 



300a 



200» 



3 



100 » 



700 



Fig. 7. The action spectra for chlorophyll formation in normal and in a 

 carotenoid-free corn mutant as compared with the absorption spectrum of 

 another carotenoid-free corn leaf (Smith, 1957). 



The resulting action spectrum for an albino corn mutant lacking carot- 

 enoids is shown as the upper dotted curve of Fig. 7 (Smith, 1957). 

 A corresponding set of measurements for normal corn containing carot- 

 enoids is indicated by the lower curve showing reduction of the blue 

 light effectiveness by internal screening. The carotenoids, however, 

 do not absorb in the red part of the spectrum, so that the two curves 

 are identical there. The red action peak comes at 650 ni/x, where the 



