DIFFERENT OXIDANTS 



1581 



state of the sample. This, and the failure of PGA to increase oxygen yield 

 of chloroplasts which had been separated from the cell sap, indicate that 

 PGA does not serve directly as a Hill oxidant, but contributes to the reac- 

 tion in a more complex way — perhaps by dark reaction mth the reduced 

 form of a "natural" Hill oxidant. 



Addition of adenosine triphosphate (ATP), glyceric acid or glucose to 

 the chloroplast-bearing leaf juice did not increase the oxygen burst in light; 

 but addition of TPN leads to strong stimulation of the oxygen production, 

 as shown by figure 35.19C. At low TPN concentrations (e. g., 2.5 X 10~^ 



}0 40 90 «0 



TIME (MINUTES) 



Fig. 35.19C. Photochemical Oo evolution from 1.5 mm.' chloroplast sus- 

 pension (1.5 ng. Chi) to which were added: (A) 1.5 mm.' water; (B) 1.5 mm.' 

 1 X 10-' M TPN; (C) 1.5 mm.' 1 X IQ-^ M TPN. 10-12° C; 9.4 cm.' N2/ 

 min. (Tolmach 1951). 



M) the amount of extra oxygen evolved was up to 17 times the stoichio- 

 metric equivalent of the TPN added; at the higher concentrations (e. g., 

 5 X 10"* M) this ratio declined to or below 1, but the initial rate of oxygen 

 production continued to increase with [TPN] (e. g., from 41 mm.^ O2 per 

 milligram chlorophyll per hour in the absence of TPN, to 163 mm.* at 

 2.5 X 10-^ mole/1, and 282 mm.» at 5 X lO"" mole/1. TPN. 



Addition of TPN had no effect on oxygen liberation from washed chloro- 

 plasts, indicating that this compound, too, entered into secondary oxida- 

 tion-reduction reactions between the "natural" Hill oxidant and some 

 reducible substances present in cell sap, and did not act directly as Hill 

 oxidant (in agreement with the earlier observations of Mehler, and Holt 

 and French). The above-suggested attribution of the incapacity of TPN 

 to serve as such ultimate Hill oxidant, to rapid reoxidation of TPNH2, was 



