FUNCTION OF CAROTENOIDS IN PHANEROGAMS 



perbenzoic acid in the laboratory, may be achieved in plant cells by a 

 HgOg-peroxidase system. They further consider that conditions in 

 the plant are such as to bring about the isomerization of 5 : 6- epoxides. 

 Thus lutein (xanthophyll)-5 : 6-epoxide may well be the precursor of 

 flavoxanthin or chrysanthemaxanthin, shown on previous page. 



With regard to this isomerization, it is brought about chemically 

 by addition of traces of HCl to chloroform solutions of 5 : 6-epoxides ; 

 previously, however, Strain * ° had suggested that the isomerization of 

 carotenoids by plant acids observed chemically is prevented in vivo by 

 the presence of plant organic bases. 



INFLUENCE OF LIGHT 



(i) Etiolated seedlings 



Before discussing the effect of light on carotene synthesis by nor- 

 mally growing plants it will be interesting to consider the result of 

 illuminating etiolated seedlings. 



Grown under the most stringent conditions of light exclusion 

 etiolated seedlings of maize, wheat, barley, ^ ^ and sunflowers ^ ^ contain 

 carotenoids ; xanthophylls predominate and are similar to those 

 obtained from the corresponding green leaves. ^^'^^ Strain ^^ also 

 demonstrated that xanthophylls in etiolated seedlings are very suscepti- 

 ble to oxidation by atmospheric oxygen, and this probably accounts 

 for the contradictory findings previously reported. In his fundamental 

 investigations Strain proved unequivocally the synthesis of carotenoids 

 in the dark and a small but definite synthesis has also been demon- 

 strated in detached bean leaves. ^ * The production of carotenoids 

 by a non-photosynthetic mechanism is also suggested by Holman's ^ * 

 work, which indicates that in germinating soya beans, carotenogenesis 

 precedes chlorophyll formation. 



On illurainating etiolated seedlings with blue or white light there is 

 a transient drop in both carotenes and xanthophylls*^-**^ accom- 

 panied by a sharp rise in chlorophylls. * ^ In red light Rudolph * " 

 claims that there is a steady increase in all pigments but that the 

 chlorophylls increase more slowly ; Franck, however, * *^ found the 

 same results with both red and blue light. From his experiments 

 Rudolph was led to entertain the idea that carotenoids were pre- 

 cursors of the phytol necessary for the synthesis of chlorophyll ; 

 but as Wald * ' observed ** the later changes [in the growth of plants 

 in light of different wavelengths] are complicated, perhaps due to the 

 opening of new channels of production by photosynthesis." 



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