1860 



SPECTROSCOPY AND FLUORESCENCE OF PIGMENTS CHAP. 37C 



The apparent absence of an equivalent loss of absorption in the main 

 chlorophyll bands (cf., however, page 1989). 



Witt (1954) applied to photosynthesizing leaves and algae a flash illu- 

 mination technique, similar to that used by Livingston and Ryan (cf . chap- 

 ter 35, part A) on chlorophyll solutions. The apparatus he used permitted 

 measurement of absorption changes of <1%, produced by flash illumina- 

 tion. Witt found a new absorption band which arose, in light, at 515 m/x; 

 it disappeared, in the dark, within about 10"^ sec. The absorption at 475 

 m^ was reversibly weakened during the flash; but no mention was made 



+ 1 



■4- 



-2-J^ 



550 



500 



450 



400 



Fig. 37C.39. Changes in absorption spectrum of illuminated Porphyri- 



dium compared to the difference between the spectra of oxidized and 



reduced cytochrome / (after Duysens 19543). 



of a similar change at 670-680 m^, so that an uncertainty remains as to 

 whether the above-quoted objection against attributing the 515 m^u band 

 to a chlorophyll-derived radical remains valid in the case of Witt's experi- 

 ments. Witt himself suggested such an attribution, and identified the 

 decay period of the 515 m/i band with the " Emerson- Arnold period" in 

 photosynthesis. However, it was pointed out in the discussion of that 

 period in chapters 32 and 34, that the catalytic agent requiring 10"^ sec. 

 for recovery must be present in a much smaller concentration than chloro- 

 phyll to account for the observed saturation yield in flashing light and 

 saturation rate in steady light. 



Witt found a light saturation of the photochemical processes generating 

 the 515 mjLt band at flash intensities of the same order of magnitude as 

 those known to cause flash saturation of photosynthesis. 



We return now to those absorption changes which, Duysens suggested, 

 were due to reversible photoxidation in the cytochrome system. 



Lundegardh (1954) observed such changes by a flow method, in 



