liESSEL KOK AND GEORGE HOCH 401 



velopcd, all based on the cross-illiiniiiiaiioii of a sample with a 

 monitoring light (/„) and an actinic light (/^) . 



To survey all the i)iienomena observed in light minus dark spectra, 

 especially those occurring at Avavelengths shorter than 050 ni/i,, would 

 be an luidertaking in itsell. Changes in nearly all known pigments: 

 cytochromes, pyridine nucleotides, carotenoids (the latter only in 

 green plants, brown algae, and purple suUur bacteria) and chloro- 

 phylls, have been observed and reviewed on earlier occasions (5, 14, 

 20). Dr. Hill will more specifically review the possible role of cyto- 

 chromes (cf. also Fig. 3) . We therefore will limit oin- present con- 

 sideration to aerobic photosynthesis and to a discussion of phenomena 

 occurring in the far red part of the spectrum. Our own work has been 

 largely concentrated on these, since they seemed to offer the most 

 direct approach to the primary energy conversion step or steps, as 

 w^as explained in the previous sections. 



It woidd be too time-consimiing to give a detailed account of our 

 measurement technique (15, 16) or to describe the difficulties which 

 are often confronted in obtaining very accurate data. Among the be- 

 wildering number of phenomena observable with these sensitive tech- 

 niques, often a rather subjective choice must be made as to what 

 effects are the most significant. The experiments we will present are 

 described, or will be described, in more detail elsewhere. 



The Direct Photochemical Bleaching of an Absorption at 700 M/x 



The difference spectra shown in Fig. 3 (14) were obtained by com- 

 paring (at each wavelength of the detecting beam) the absorbance of 

 a sample before and after a flash of bright white light. The speci- 

 mens studied are representatives of four classes of aerobic photo- 

 synthesizers: green (Scenedesmus and spinach) , brown (Nitzschia), 

 red {Porphyra) , and blue-green (Nostoc) organisms. 



Except for a broad but weak band of increased absorbance at wave- 

 lengths beyond 725 m^x (which is found universally) , all changes are 

 negative. One can correlate the decrease at 650 vap., foimd in green 

 cells only (cf. also Streliler and Lynch, 21, with the disappearance 

 of chlorophyll b. Tlie small decreases around 680 m^ correspond to 

 a minute bleaching of chlorophyll a (Coleman et al., 2) . The most 

 striking shift, which is found universally, occurs slightly beyond 700 

 m/x. It appears quite prominent in blue-green and red algae, as is 

 illustrated for the former in the difference spectrum of Fig. 4. 

 In this experiment the only other pronounced effect occurs in the 

 blue, where an absorption band at about 420 ni/x shifts towards 410 



