EUGENE RAIiLXOVVJTCH AND i.OVlNDJEE 383 



effects not only in die action spectrum ol Aiutcystis, but also in those 

 ol NiwicnUi and Porlyliyridium. 



The negative effect in the region r>7()-7()() ny in Anacystis suggests 

 that this alga contains relatively little, if any, of the "activating" form, 

 Chi a ()70, and that the main chlorophyll a component in it, Chi 

 (I 680, not only does not complement the photochemical action of 

 Chi a 690, but counteracts it! (The existence of such photochemical 

 countereffects is well known in various photobiological phenomena.) 



Another approach to the interpretation of the negative effects is 

 through the study of light saturation in monochromatic light. So far, 

 it has been presumed, on theoretical grounds — and confirmed by 

 observations in filtered colored light — that light saturation of photo- 

 synthesis is independent of the spectral nature of the absorbed light, 

 being imposed by the limited amount of some enzyme, and a con- 

 secjuent "ceiling" on the overall rate of photosynthesis. Some pre- 

 liminary observations suggest, however, that the saturation rate may 

 be much lower at 690 and 700 m^ than at the shorter wavelengths. 

 If this is true, then the superposition of two beams, both belonging 

 to the far red region, may produce a negative effect simply by bring- 

 ing the process into the light-saturation range, even though one would 

 not expect this to happen from the known extension of the "linear 

 range" in white light. Such low saturation level may be itself con- 

 ceivably due to some kind of a "photochemical counter effect." 



Difference Spectra 



The different fates of several chlorophyll a components in photo- 

 synthesis are suggested also by the shape of the "difference spectrum" 

 of Chlorella (cf. Fig. 5, based on observations by Coleman in our 

 laboratory; 5) . The reversible bleaching of chlorophyll in strongly 

 illuminated Chlorella cells is shown by this curve to affect Chi a 

 680 more strongly than Chi a 670 or Chi a 690; in fact, it seems possi- 

 ble that a reversible transformation of Chi a 680 into Chi a 670 and 

 Chi a 690 takes place in light. Since, however, the whole difference 

 spectrum remains negative above 600 m^, this conversion must be 

 superimposed on the transformation of Chi a into a compound with 

 no red absorption band at all. (This compound may absorb in the 

 520 mfi band, and may perhaps be identical with Krasnovsky's "pink" 

 reduced chlorophyll, for which the name "eosinophyll" was suggested 



(H).) 



Krasnovsky (18, 20, 21) has observed preferential bleaching of Chi 

 a 670 in light; but this was not confirmed by Brown and French (4) , 



