CAROTENOIDS 



Amaud^®* originally suggested that because carotene, a highly labile 

 substance, remained stable in the leaf it must take part in some redox 

 system. This idea was expanded in 1913 by Willstatter and StolP^* 

 who suggested that as chlorophylls and carotenoids are so intimately 

 connected in plants, they are probably involved in a coupled redox 

 system thus : 



+O3 



Chlorophyll a 



Carotene 



chlorophyll h 



xanthophyll (? lutein) 



Willstatter and StolP*' later carried out a series of experiments 

 to test this hypothesis. They examined the relative concentration of 

 the chloroplast pigments before and after exposure to extreme con- 

 ditions of heat and light. One would expect changes in the ratios 

 chlorophyll a : chlorophyll b, and carotenes : xanthophylls under 

 such extreme conditions if the pigments were part of a balanced 

 system. Generally, no such change in ratio occurred and when they 

 did they could in no way be interpreted as implying interconversion. 



(It should also be pointed out that the function of leaf carotenoids in 

 a redox system would almost certainly involve an isomerization of a 

 [3-ionone residue to an a-ionone residue for (3-carotene is the major 

 carotene in leaves whilst lutein is the predominant xanthophyll ; such 

 an isomerization has not yet been observed in vivo.) During the fading 

 of leaves, although the total chlorophyll content diminishes rapidly, 

 the ratio of chlorophyll a : chlorophyll b remains constant throughout ; 

 the disappearance of chlorophylls is not accompanied by a rise in 

 carotenoid content which remains constant until the extreme stages of 

 necrosis are reached ; they do, however, undergo qualitative changes 

 (see p. 19). 



In 1938 Seybold and Egle, ^^^ investigating shade- and sun-plants 

 found that the ratios xanthophylls : carotenes and chlorophyll b : 

 chlorophyll a are both greater in shade plants. This work needs con- 

 firmation but as it stands it rules out a coupled redox system for the 

 " oxidised " forms of both pigments appear to increase simultaneously. 

 Recently, two Russian scientists, Sapozhnikov and Lopatkin^®* have 

 taken up this problem and have claimed that under favourable photo- 

 synthetic conditions the carotene : xanthophyll ratio does increase ; 



84 



