381 LIGHT AND LIFE 



^00 500 600 700 



A 



m 



'/ 



Fig. 5. Dlfrerence .spectrum of CliloreUa (after Coleman and Rabinowitch, h\. 

 The three ciir\c.s correspond to tliree light intensities (.S.4, 12.0, and 31.4 X '^'^ 

 hp/ (sec X tin-). Reversible bleaching in the red chlorophyll a band is observed 

 only at the higher intensities. It is strongest at 680 m/x. weaker at (570 m^ and 

 690 ni^. Maximum cHects are of the order of 0.29^. 



who found that the bleaching affects most strongly the 690 niyu, form, 

 next the 680 ni/^ form and least of all the 670 m^ form. In any case, 

 these observations applied to more extensive and not immediately 

 reversible bleaching, rather than to the small (-~0,1%) instantaneous 

 and fully reversible effect shown in Fig. 5. 



Conclusions 



All these results suggest that, in addition to the "one-dimensional" 

 action spectra of i)hotosynthesis, one should investigate systematically 

 also "two-dimensional action spectra," i.e., the effect on the yield of 

 photosynthesis of (ondnnations of two monochromatic light beams of 

 systematically varied wavelengths. I'hat certain combinations of light 

 colors have specific effects on photosynthesis has been asserted in 

 the past, e.g., by Danilov (6, 7) ; but conclusions were unconvinc- 

 ing because of the primitive techniques used. It seems that these 



