464 



LIGHT AND LIFE 



2000 



3000 



4000 



Light in+ensiiy, f.c. 



Fig. ir>. A typical intensity curve for photosynthesis. From data of H. W. Milner 

 for Miiiiulus cardinnlis 7119-1 (Baja Calif.). COo uptake measured at 30°C. 



McLeod (47) has recently measured the action spectrum of photo- 

 synthesis using high intensities that, at each wavelength, were shown 

 to be saturating. The results of such an experiment, when plotted as 

 rate against wavelength, should give a perfectly straight and horizon- 

 tal line over the wavelength region effective in photosynthesis. How- 

 ever, it turned out that the beautifully simple concept of a single 

 saturation rate independent of wavelength is inadequate. The results 

 obtained were not at all like the expected horizontal line but gave a 

 curve with significant peaks and valleys. These data themselves show 

 that there is not one photochemical first step, but two different 

 ones that are promoted by different pigments. Fig. 16 compares the 

 low intensity conventional action spectrum of Chlorella photosynthesis 

 with the action spectrum measured at light saturation lor the red part 

 of the spectrum. The low intensity spectrum shows a peak at 680 n\jx, 

 (chlorophyll a) , while the high intensity spectrum has a peak at 

 650 m^ where chlorophyll b also absorbs light. 



We believe the shape of the high intensity action spectrum is in 

 some way caused by the Emerson enhancement effect. At 650 m^x, 

 both chlorophyll a and /; are activated, at 680 m^i. the absorption is 

 almost entirely by chlorophyll a. Some difference in the relation of 

 intensity to rate for the Cu680-promoted photochemical reaction and 



