704 



Walter F. Bertsch, J. B. Davidson, and J. R. Azzi 



Cyanide is known to inhibit the dark reactions of photosynthetic COr> 

 reduction without affecting the quantum conversion stepsU-*/ ■^^/^^z ^^i. We 

 found no effect of KCN (lO'^ to 10*2 m) on the 1 to 20 msec delayed light 

 emission. 



Two poisons changed the intensity of delayed light emission without 

 significantly changing the time course of decay; azide and 2-chloro-6- 

 nitrophenol. Figure 2 gives the effect of NoN-^ (10"^ to 10"' M) on 

 delayed light emission at various times. The curves for the various times 

 are approximately proportional to each other. Implying that the poison had 

 the same relative effect at each time of emission. Azide may therefore be 

 said to Inhibit delayed light without changing the shape of the decay curve. 

 2-chloro-6-nitrophenol also had a relatively small effect on the time course 

 of delayed light emission, but this poison increased the intensity of emission. 

 Figure 3 shows that at optimal concentrations of 2-chloro-6-nitrophenol 

 (7 to 1 OX 10"5 M) the emission at 1 msec was increased by a factor of about 

 3.5, and at 20 msec by a factor of about 7. Sweetser et al .\'^"/ also reported 

 that 2-chloro-6-nitrophenol increased delayed light emission, and also that 

 it was a powerful inhibitor of photosynthesis. 



Several poisons that are believed to act very close to photochemical 

 steps were tested. These compounds had a marked effect on the time course 

 of delayed light emission, as well as on the intensity of emission. These 

 poisons may be put in two categories, according to their effect on the time 

 course. 



1. Diuron (DCMU), monuron (CMU), orthophenanthroline (all at lO'^to 

 10"2 M) had similar effects on the emission. At certain concentrations they 

 resulted in a very flat decay curve in which no fast decay components were 

 present In the 1 to 20 msec time range. As the concentration of each poison 

 was increased, the decay remained flat but was reduced in intensity. The 

 lowest concentration at which the fast decay components were strongly 

 inhibited without reducing the intensity of emission were different for the 

 individual compounds: SXIO'^ M for DCMU, IO'Sm for CMU, and 10-3 M 

 for orthophenanthroline. The delayed light decay of unpolsoned cells is 

 compared to that of CMU-treated cells in Figure 4 (top and bottom). The 

 effect of various concentrations of orthophenanthroline is shown in Figure 5 

 for various times of emission. At 10"^ M this poison slightly inhibited the 

 1 msec emission while increasing the 20 msec emission by a factor ot 15. 



