75 



Britton Chance and Walter D. Bonner, Jr. 



Furthermore, relative rates on ad.^acent segments of the cvirve 

 were employed. 



A summary of the resxolts of the nujtiber of experiments is plot- 

 ted in Fig. 7 . On the ordinates are represented the rates of 

 ahsorbancy decrease at 55O-5UO mn (as measiired fiom the slope of 

 the curve similar to that of the preceding Figure), the abcissa 

 are values of vavelenijth of the actinic light. The dashed line 

 at the bottom of the Figure is the rate obtained by the measure- 

 ment light only, and this is seen to be negligible for all wave- 

 lengths employed. The graph shows a plateau in the region 

 of 66O-68O mn with an abrupt decrease at 69O and TOO mu. The 

 curve may be interpreted as a decrease in quantiom efficiency of 

 electron transfer between chlorophyll and cytochrome f; the 

 efficiency falls to half its value at approximately 695 m^. A 

 similar fall-off is fotuid with frozen chloroplast suspensions. 

 One measiirement of a decrease of efficiency at 6k0 m\x report- 

 ed in the oral presentation was foiind to be in error. 



Absorbancy changes at 700-70^ m^i 



In order to observe the response of the 700 m\i pigment under 

 conditions where the response of cytochrome f can also be mea- 

 sured, we have employed actinic excitation at k'iS m\i from a 

 medium pressure mercury arc . With this 



ill\xmination, a rate of cytochrome f oxidation corresponding to 

 1.0 X 10"^ OD units/sec was obtained as is indicated in Fig. 7^ 

 where illumination occurs for a period of 20 seconds . A second 

 period of 20 second illumination carries the reaction nearly to 

 completion. 



If the measuring wavelengths are now changed to those appro- 

 priate to the 700 mp. pigment (705-635 mp) Fig. 8, ve find a more 

 rapid rate of change of absorbancy when the meastiring light is 

 on, and note a considerable increase of rate during the 20 sec- 

 ond period of actinic illumination at 14-36 m|i. Since the sensi- 

 tivity is lower than in the recordiiig of cytochrome f, the rate 

 corresponds to 2.7 x 10"^ OD units/sec. A second interval of 

 illumination causes completion of the reaction. 



A number of features of the reaction are of importeince. First, 

 both the reactions proceed when the measuring light is turned on, 

 and are accelerated simultaneously when illuminated with actinic 

 light. This point will be taken up in the Discussion. 



By repeating the experiment of Fig. ,3 with different leaves 



