B. Chemistry of Chloroplast Pigments* 

 (Excluding Photochemistry) 



(Addenda to Chapters 15 and 16) 



No important progress has been achieved, since the pubUcation of 

 Volume I in 1945, in the synthesis of chlorophyll or in further elucidation of 

 its structure. Important observations have been made, however, concern- 

 ing the biogenesis of chlorophyll and its oxidation-reduction reactions. 



1. Biosynthesis of Chlorophyll ; The Protochlorophyll 

 (Addendum to Chapter 15, section B2) 



In Vol. I (p. 405), after the description of protochlorophyll and its 

 possible function as chlorophyll precursor in plants, the remark was made 

 that "the whole problem of chlorophyll development in seedlings is in need 

 of renewed study." 



Since then, the subject has been taken up by Smith and co-workers at 

 the Stanford Laboratory of the Carnegie Institution. Through quantita- 

 tive determination of protochlorophyll, chlorophyll a and chlorophyll h in 

 etiolated seedlings after exposure to light, they proved that up to 90% of 

 the protochlorophyll, accumulated in the dark, are quantitatively con- 

 verted, within a minute or less of moderately strong illumination, into 

 chlorophyll a. After this initial period, the formation of chlorophyll a 

 continues (and that of chlorophyll h gets under way) at a much slower rate, 

 to reach saturation in a day or two; during this second stage, the rate is 

 governed by thermal reactions, involving other precursors besides proto- 

 chlorophyll — e.g., those containing no magnesium in ether-soluble form. 

 Whether this slow synthesis goes through protochlorophyll as intermediate, 

 or by-passes this compound (as was assumed in Lubimenko's scheme, illus- 

 trated on p. 405), remains an open question. The presence of small 

 amounts of protochlorophyll in fully green barley plants, noted by Koski 

 and Smith (1948^), could be explained by protochlorophyll's being a side 

 product, as well as by its being a necessary intermediate in chlorophyll 

 synthesis. 



Following is a brief summary of the studies of Smith and co-workers. 



Smith (1947) determined the changes in the total magnesium content, 

 ether-soluble magnesium, and chlorophyll magnesium, caused by exposure 

 to light of etiolated barley seedHng, in attached as well as in excised leaves. 

 The results are summarized in Table 37B.I. 



* Bibliography, page 1790. 



17.59 



