434 LIGHT AND LIFE 



be connected into a complete sequence by a chemical reaction between 

 ( ytochromes / and /;,j. 



The only evidence ior the above hypothesis, in higher plants, 

 is the observation that illumination of pale yellow-green leaves causes 

 an oxidation of / and the reduction of /;,;. As yet, this sequence of 

 reactions has not been observed, in our laboratories, with normal 

 green leaves or chloroplasts. 



There are a few observations in the literature that are pertinent 

 to this discussion. One of these observations is that of Lundegardh 

 (13), who reported that in Chlorella, following illumination, a re- 

 duced peak disappeared and an oxidized peak appeared in a spectral 

 region that was ascribed to cytochrome /. Similarly, James and Leech 

 (II) reported spectral changes following illumination of chloro- 

 plasts derived from Caulerpa prolifera and Vicia faba. Interestingly 

 enough, the light-induced spectral changes reported by James and 

 Leech correspond exactly to those suggested here, viz., disappearance 

 of 555 m/x absorption followed by appearance of 563 mfx absorption. 

 The time relations of the above two observations leave much to be 

 desired; an interpretation of such results would be facilitated greatly 

 if the observations could be made over very short time intervals. The 

 importance of the time interval between observations, as well as 

 other possible soiuces of error, in experiments of the kind just de- 

 scribed has been discussed by Smith and Chance (15) . 



In algae also there has been no clear-cut demonstration of changes 

 in cytochrome absorption following illumination of normal cells. 

 Interestingly enough. Chance and Sager (3) have shown the oxida- 

 tion of cytochrome / following illumination of a pale-green mutant 

 of Chlamydomonas, a mutant that contains unusually low concen- 

 trations of chlorophylls and carotenoids and yet retains efficient 

 photosynthesis. The situation in the Cldamydomonas mutant is ex- 

 actly analogous to that in the pale yellow-green leaves mentioned 

 above. An unequivocal demonstration of a light-induced cytochrome 

 absorption change in normal green cells or chloroplasts must be made 

 before the role of cytochromes in the photosynthetic process can be 

 assessed properly. 



It is our hope that futine investigations will be able to elucidate 

 more fully the role of cytochromes in the photosynthetic process of 

 green plants. The suggestions given here are made only as a work- 

 ing hyjjothesis and to serve as models for further experimentation. It 

 is hoped that the spectral proj^erties of the chloroplast cytochromes 

 which are described clearly here will help in such experimentation. 



