584 LIGHT AND LIFE 



quence. This selectivity appears to correlate well with the selectivity 

 seen by Kok for the spectral changes in illuminated cells (17) and 

 w^ould indicate that he is observing more directly what we have in- 

 ferred to be reaction 4. 



Finally, we can point out the relatively large number of inhibitors 

 or conditions that affect either a Hill reaction or an oxygen exchange 

 reaction, but not the closed cycle, and which therefore appear to 

 work on reaction 3. This sort of response is obtained with o-phenan- 

 throline as an inhibitor (22, 12), as well as the relative of CMU, 

 DCMU (dichlorophenyl dimethyl urea) (19, 22) . When chloroplasts 

 are treated with an excessive concentration of Tris, they appear to be 

 permanently damaged in this same reaction 3 (22) . A deficiency 

 of chloride ions, especially at a high pH, brings on the same symptoms 

 (1). Slightly different experimental criteria (inhibition of photo- 

 synthesis, but not of photoreduction by hydrogen-adapted algae) in- 

 dicate that hydroxylamine poisoning (8) and manganese deficiency 

 (16) , as well as o-phenanthroline (9) and DCMU (2) , act at about 

 the same site. These agents or conditions are so diverse that one be- 

 comes suspicious that reaction 5, again, may be a disguise for a 

 series of reactions rather than a single one. 



A very unusual sort of inhibition of reaction 3 was discovered by 

 Dr. Margulies, in this laboratory. He found that if the primary leaves 

 of the Black Valentine Bean plant were stored for 24 hours in the 

 dark at 0°C, their ability to photosynthesize was impaired (20) . 

 Chloroplasts from these leaves could accomplish a cyclic electron flow, 

 but not a Hill reaction or any oxygen exchange reaction. A re- 

 versal of the inhibition could be accomplished by bringing the leaves 

 to room temperature and illuminating them for 15 minutes. Un- 

 fortunately, this reversal was not possible in vitro. Although we do 

 not understand the mechanism of this inhibition or of its reversal, 

 the function of light in the reversal and the speed with which it 

 happens is reminiscent of the efEect observed by Brody and Brody, 

 in which illumination of red algae, grown in blue light, with green 

 light induced in 20 minutes to 1 hour a tighter coupling between the 

 accessory pigments and chlorophyll a (3) . 



The concept that accessory pigment light absorption may be speci- 

 fically necessary for reaction 3 received its first and most direct sup- 

 port from the experiments of Barbara Petrack (25) . She was able to 

 prepare chloroplast particles or chromatophores from blue-green algae 

 which had lost their phycobilin pigments and retained only chloro- 

 phyll a. These particles were still able to catalyze a cyclic electron 



