1406 INDUCTION PHENOMENA CHAP. 33 



3. Changes in Absorption Spectrum during Induction 



It was mentioned on p. 1376 that all measurements of fluorescence- 

 time curves are predicated on the assumption that the fluorescence spec- 

 trum remains the same, and only the yield of fluorescence changes. This is 

 not certain; but the low intensity of fluorescence makes an experimental 

 check difficult. Somewhat easier is the measurement of the absorption 

 spectrum as a function of time ; this could be achieved either (if the changes 

 in the extinction coefficient are greater than about 10%) by means of one 

 of the now available rapid spectrometers, which registers a complete spec- 

 trum in fractions of a second, or, in a more cumbersome way, by determin- 

 ing the extinction as a function of time for each wave length in a separate 

 run. Experiments of the second type have been carried out by Duysens 

 (1952, 1954^"^). The measuring time of his apparatus was of the order of 

 magnitude of one second; the changes in extinction coefficient recorded 

 were of the order of 0.1%. In as far as his experiments deal with the ab- 

 sorption spectra in the stationary state in light, they will be discussed in 

 chapter 37C. Here, we will merely mention some preliminary results 

 concerning the transition from darkness to light. 



In Rhodos-pirillurn ruhrum (in peptone, under anaerobic conditions), 

 conspicuous changes have been noted by Duysens both in the infrared, 

 where they indicate some kind of chemical change in the bacteriochloro- 

 phyll (or molecules complexed with it), and in the violet, where the changes 

 in weak light (10'' erg/cm. ^ sec.) seem to indicate the transformation of a 

 compound of the type of cytochrome c from reduced into the oxidized 

 state in the first 10 seconds of illumination. In very strong light (10^ 

 erg/cm. 2 sec), there is a second spectroscopic change, opposite in direction, 

 following closely upon the cytochrome change; superimposed upon these 

 fast changes there is a much slower one, taking a minute or more. All 

 three changes are reversed in the dark. 



Duysens (1954*) observed similar changes in Chlorella and Porphyri- 

 dium, between 330 and 530 mju. Some of them could be attributed to the 

 oxidation of a cytochrome or reduction of a pyridine nucleotide in Hght; 

 one new band in the green remained unexplained (c/. chapter 37C, section 

 6f). No kinetic measurements have been as yet made on these transforma- 

 tions. 



This may be the place to mention the observations of Kandler (1950) 

 and Strehler (1953) on the changes of ATP concentration in Chlorella. The 

 ATP level estabHshed in the dark under anaerobic conditions, rises (and 

 that established under aerobic conditions, falls) upon exposure to light. 

 The rate of rise is proportional to light intensity, 7, but the stationary level, 

 reached after about 1 minute in light, has a maximum at a certain intensity. 

 The change is independent of the presence of carbon dioxide. 



