154 



Bacon Ke 



FLASH 



.1 .2 .3 .4 

 TIME, SECONDS 



Fig. 1. Light- induced absorption change at 430 mp in: 



a. week- old chloroplasts. 



b. a + 2 X 10'^ M ascorbate. 



c. b + 3 X 10" 6 M PMS. 



chloroplasts already containing ascorbate, the absorption change was im- 

 mediately converted to one composed of a rapidly decaying portion (i/> 10"^ 

 sec) superimposed on a portion having the same decay time as before PMS 

 addition (Fig. 1, curve c). 



By varying the measuring- beam wavelength between 400 and 450 mp, a 

 difference spectrum was obtained as shown in Fig. 2. The spectrum shows a 

 broad negative peak covering the 420-430 mja region and a smaller positive 

 peak in the 405 mp region. 



Examination at 703 mp revealed an absorption decrease with kinetic be- 

 havior very similar to that of the slow portion of the 430 m;j change. No 

 rapidly decaying portion was observed in the 703 m;j absorption change. 



The composite absorption change shows an interesting dependency on the 

 intensity of the excitation flashes (Fig. 3). Starting from a given saturating 

 intensity (Imax^ ^^^ gradually decreasing it, the signal remained practically 

 unchanged down to 50% intensity. Decreasing the intensity to below 50%, the 

 rapidly decaying portion started to disappear. Between 30 and 15% intensity. 



