124 



J. L. Rosenberg and Tevfik Bigat 



believe that Ms raaxim-um might be a normal induction outb\arst, 

 and that its occurrence might require a longer elapsed period 

 at low photosynthetic activity (dark or blue light) before 

 switching to green than was allowed in ovor experiments . 



TIME (seconds) 



Fig. 2. Chlorophyll a fluorescence of Porphyridium 

 showing the chromatic transient accompanying the shift 

 from blue excitation (l = 128) to green. Intensity of 

 green^ in imits of 10"11 einstein/cm^-sec: A, I = l^Sj 

 B, I = 79; C, I = ko. 



Discussion 



(1) 



The above results, like those of Duysens with red algae and 

 of Govindjee et al. with Chlorella( 2 ) , may be interpreted on the 

 basis of two pigment systems. System 1, containing the biolk of 

 the chlorophyll a in the red algae and the long-wave absorbing 

 component in the gredn algae, gives very little chlorophyll a 

 fluorescence; while System 2, containing the phycobilins in the 

 red algae and the bulk of the chlorophyll a in the green algae, 

 is capable of a strong chlorophyll a fluorescence. 



To accommodate the experimental results we start with a model 

 different from that of Duysens in one very important respect. 

 He postulates that System 1 is req.uired for the sensitization of 

 photochemical Step (l) and that System 2 can sensitize only 

 photochemical Step (2). The substrate for Step (2), Q is 



