74 



Britton Chance and Walter D. Bonner, Jr. 



thereby, a "difference spectrum" for the actinic effect. This is 

 illustrated in Fig. 1|. for Swiss chard leaves and for spinach 

 leaves . In both cases there is a large dimin[ution of absorption 

 which has a maxinrum very near 555 m^^- In spinach leaves, there 

 is a possibility that a satellite band characteristic of cyto- 

 chrome f is observed, although further experimentation is desir- 

 able to ensure this. This satellite band is not observed in 

 Swiss chard leaves, although insufficient data are available at 

 present to substantiate this difference. It is significant, 

 however, that the band at low temperature Isat 555 m\i, whereas the 

 peak in acetone extracted spinach quantasomes is clearly at 552 

 mn (13). It is probable that this difference is a real one, that 

 cj^ochrome exists in a different state in the leaf than in the 

 acetone treated chloroplasts or in the extracted pigment. 



Relative quantxmi efficiency for cytochrome f oxidation . 



As the preceding Figures clearly indicated, the velocity of 

 cytochrome f is under control of the intensity of the sctinic 

 light. It has occurred to us that it would be of considerable 

 interest to determine the effect of the wavelength of light upon 

 cytochrome f oxidation since we have here for the first time, 

 the isolated prim.ary chemical event of the leaf. 



Instead of the fixed wavelength actinic beam a 



Bausch and Lomb 200 mm focus grating monochromator (1200 line 

 grating) is employed, and is ilUuninated with a tungsten lamp. 

 The energy distribution of this canbination is found to be prac- 

 tically flat in the region of interest (620-710 m|i). ?fenual ro- 

 tation of the wavelength knob controls the rate of this chloro- 

 phyll at low temperatures. The monochromator was set with a 2 imn 

 slit width (6 m[i spectral interval) which proved to be adequate 

 to give rates of oxidation of c\rtochrome f (see Fig. 3) f lar::e 

 compared with the rate caused by the measuring light. 



As illustrated by Fig. 5., a Swiss chard leaf cooled at 77 K 

 illuminated first with a measiiring light, it is seen that 

 the rate caused by the measuring light is insignificant. Vfhen 

 the 680 mil. light is turned on an abrupt deflection of the traces 

 is observed which proceeds considerably more rapidly for 

 680 m|jL than it does for 69O mia. In fact, when 700 mil light is 

 employed, the trace is nearly horizontal. The deceleration 

 is, however, reversible, and when 67O and then 68O rau radiation 

 is employed there is rin abrupt acceleration. Since the course 

 of cytochrome f oxidation follows an approximately exponential 

 function, measurements of the rate were restricted approximate- 

 ly to the first third of the course of the reaction. 



