615 



Mordhay Avron and Noun Shavit 



I atfer observation may explain some of the variance in the results of different v/orkers, using 

 markedly different light intensities, as to the inhibition by CMU derivatives on different 



UgN imtnviy -LwiO 



Fig. 6. The effect of light intensity on the extent of inhibition of phenazine 

 methosulfate or ferricyanide dependent photophosphorylations by several 

 uncouplers or inhibitors. 

 Details as described under Fig. 1. 



photoreactions of chloroplasts (see for example, reference 21). The only exception found 

 was the inhibition produced by 2-nonyl-4-hydroxyquinoline N-oxide (NQNO), which 

 decreased, rather than increased, as the intensity was lowered. This is the only basic 

 difference found to-date between the inhibitory effect of this compound and that of the 

 CMU derivatives on the photoreactions of chloroplasts(22). 



The dependence of the rate of electron transport on the osmotic concentration 



Another effect which leads to a large variation in the rate of electron transport in the 

 various photoreactions of chloroplasts was recently discovered in our laboratory. Figure 7 

 describes the effect of lowering the salt concentration normally used in the reaction mixture. 

 It is evident that the rate of electron flow in all of the photoreactions illustrated was marke- 

 dly decreased as the salt concentration decreased. This effect was not due to an uncoupling, 

 since within the concentration used, the salt had no depressive effect on photophosphory- 

 lation. Table 1 shows that this was a general osmotic effect with NaCI, KNO3, or sucrose 

 having a similar effect at the same osmotic concentration. Maximal increase in rate by 

 about twenty fold have often been observed. The table also indicates that with the osmotic 

 concentration employed by most workers, only a slight further stimulation of rate can be 

 achieved by an additional increase in the osmotic concentration. 



Figure 8 shows that the extent of stimulation in the rate of reduction of ferricyanide is 

 also affected by the osmotic concentration of the reaction mixture. Thus whereas at low 

 osmotic concentration approximately a five fold stimulation was produced by the addition of 

 the phosphorylation-components or of an unooupler, less than two fold stimulation was 

 produced at higher osmotic concentration. 



The osmotic effect described is reminiscent of the earlier observations on the effect of 

 salts on the rate of the Hill reaction which were performed in attempt to prove or disprove 

 the function of chloride ions as a coenzyme of the Hill reaction(23/ 24)_ || does not seem 



