LIGHT ABSORPTION BY PIGMENTS m VIVO 



1857 



other — variation in intensity of chlorophyll fluorescence associated with 

 variations in the rate of photosynthesis {cf. chapter 24, section 4, and 

 chapter 33, part B)— has been explained by changes in the composition 

 of compounds associated ^\dth chlorophyll in the "pigment complex" 

 (oxidation, reduction, formation and deposition of "narcotics"). 



Bell (1952) found the transmission of leaves to increase rapidly, by 

 3-5% (in the region 350-670 m/x) upon exposure to strong light [intensity 

 up to 185 kerg/(cm.2 sec.)], in the absence of carbon dioxide; the trans- 

 mission decreased again (by about 0.6%) when carbon dioxide was admitted. 

 The increase in transparency was ascribed by Bell to reversible chlorophyll 



RHODOSPIRILLUM RUBRUM 



Od 



0.03 



002 



^ 



V 



, absorption spectrum 

 of 8% fraction 



absorption spectrum 

 of changed fraction 



008 



006 



OO'i' 



002 



—9 



850 



ROO 



900 



850 



800 



750 



Fig. 37C.36. Absorption sj>octra of bacteriochlorophyll in vivo during illumination, 

 calculated from fig. 37C.35 by assuming that 8% (left) or 20% (right) of bacteriochloro- 

 phyll (B 890) is transformed by u-radiation (after Duysens 1952). 



bleaching through transfer into a long-lived (> 0.02 sec.) excited state; 

 he suggested that the back reaction is accelerated when the stored energy 

 can be utihzed for photosynthesis {i. e., when carbon dioxide is present). 



No evidence of a change in spectroscopic composition (of the trans- 

 mitted or fluorescent light) has been sought in all these studies. Duysens 

 (1952) found, however, that the absorption spectrum of bacteriochloro- 

 phyll changes when the bacteria are photochemically active. By a com- 

 pensation method, permitting measurement of very small, sudden changes 

 in transmission, Duysens found that illumination of a suspension of purple 

 bacteria by a 500 w. lamp, [about 30 kerg/(cm.2 sec.)], causes an immedi- 

 ate change in the absorption spectrum, which is rapidly reversed in the 

 dark. Figiu'e 37C.35 (left and right) illustrates these changes for a Rhodo- 

 spirillum suspension and in Chromatium extract. In Rhodospirillum 



