400 



LIGHT AND LIFE 



700s 

 ^700 



1.8n 



1.6 



1.4- 



1.2- 



1.0 



'638 

 ^700 



Fig. 2. "Enhancement effect" in Anacystis nidulans. Calculated from rates of 

 oxygen evolution. Abcissa: ratio of rates induced by the individual beams (I700 con- 

 stant throughout). Ordinate: Rt,,,,,, ecjuals total rate with both beams concurrently 

 minus the rate in less separately. R^ou equals the rate in I;oo alone. 



less it is excited via transfer from some accessory pigment, or is as- 

 sisted in some indirect way as in the enhancement experiment. 



Light Minus Dark Difference Spectra 



A powerful approach toward understanding the effects brought 

 about by illumination of photosynthetic material — first applied by 

 Duysens (3) — is the application of difference spectroscopy. 



A rather obvious question to ask is the following: do one or more 

 of the sensitizing dyes or the pigment-electron carriers undergo a 

 change of absorption spectrum as a direct or indirect consequence of 

 the photochemical act? 



Although, in algae and chloroplasts, chlorophyll may amount to 

 4 per cent of the total dry weight, it turns out that illumination yields 

 only minuscule changes of absorption (as long as we disregard the 

 non-specific photooxidative bleaching in excessive light) . Additional 

 difficidties are encoiuitered in these measiuements, due to the strong 

 pigmentation of the material and the fkiorescence of chlorophyll, 

 excited or altered by the actinic illumination. Various rather special 

 technicjues to study photosynthetic difference spectra have been de- 



