THE CHLOUOPLAST KKACTION 265 



dium light intensities. However, their observed maximal rates were 

 low (in the case of photosynthesis by a factor of at least 5) and it is 

 possible that maximal rates for the Hill reaction will be dependent on 

 the oxidant used. Davenport, Hill and Whittingham (unpublished) 

 have obtained light curves for pea chloroplasts in presence of the fac- 

 tor and methemoglobin, and it may be of interest to compare these 



5 cf 

 o 



lOO XX3 300 4O0 500 6O0 



RELATIVE LIGHT INTENSITY 



Fig. 1. The relationship between rate of oxygen production and Hght intensity 

 for Chlorella pyrenoidosa (+, X) and chloroplasts in presence of methemoglobin 

 (• O D)- Data for two independent chloroplast preparations are shown. 



with the light curve for photosynthesis of Chlorella for the same ab- 

 sorption of light energy by chlorophyll. 



The rate of oxygen production at different light intensities was com- 

 pared for a suspension of Chlorella pyrenoidosa and chloroplasts iso- 

 lated from Pisum sativum. Both rates were determined by the for- 

 mation of oxyhemoglobin. With Chlorella, the cells were suspended 

 in 0.033 M phosphate buffer (pH 6.7) with 3% carbon dioxide and 

 the procedure was that described by Hill and Whittingham (7). 

 The chloroplasts were suspended in 0.033 M phosphate buffer (pH 



