51 



Bessel Kok 



700 720 740 



WAVELENGTH m/j. 



Figure 5 



10 20301O5O607OBO9OIOO 



RELATIVE LIGHT INTENSITY 



Figure 6 



IV, Fluorescence yield and electron transport. 



Figure 6 shows measurements (made at room temperature) of 

 fluorescence as a function of intensity, run concurrently 

 with measurements of the rate of dye reduction by fresh 

 chloroplasts. In confirmation with the earlier analysis of 

 Lumry et al. (12), the data show that efficient electron 

 transport corresponds to a low yield of fluorescence. In the 

 absence of a Hill oxidant the yield is high at all intensi- 

 ties. In the presence of ferricyanide the yield is low in 

 weak light. Addition of NH4CI — which by uncoupling phosphor- 

 ylation accelerates the rate by about a factor two — lowers 

 the fluorescence yield even further (to 25% of the control 

 value). In higher intensities electron transport approaches 

 its saturation rate and the fluorescence yield rises again. 



Effect of Subsequent Additions upon the Fluorescence 

 Yield of Fresh Chloroplasts in Weak Light 



Addition 



Rel. Yield 



None 



PPNR (Saturating) 



10"% TPN 



io-5m j)cm 



10""^M DCPIP + Ascorbate 



100 

 98 

 52 

 86 

 82 



The above table shows that addition of PPNR (which does 

 not induce significant electron transport), has no effect, 

 whereas successive addition of TPN lowers the yield. Addi- 

 tion of DCMU again brings the fluorescence close to the con- 

 trol value — indicating that electron transport stops. A 



