CAROTENOIDS 



is stimulated by addition of sucrose or glucose to the medium and can 

 also occur in the dark, but only to a limited extent (about 1 /5 of that in 

 light). This indicates a non-photosynthetic pathway. Inhibition of 

 photosynthesis by culturing leaves in an atmosphere free from CO 2 or 

 by the addition of hydroxylamine to the culture medium, reduced the 

 synthesis of carotenoids almost to nil in the salt medium ; synthesis 

 was, however, resumed if glucose or sucrose was added to the medium. 

 It seems from these experiments that carotenoid synthesis depends 

 only indirectly on the presence of light, in so far as the pigments are 

 produced from photosynthesised substrates. 



Under Bandurski's experimental conditions, neither glycerol nor 

 pyruvate can replace glucose or sucrose in stimulating pigment syn- 

 thesis. Fluoride, but not sulphanilamide, inhibits synthesis from 

 glucose. 



Formation in petals 



Knowledge concerning the biosynthesis of carotenoids in flowers is 

 limited, although possible intermediates have been detected. Monkey 

 flowers (Mimulus longiflorus) developed under natural conditions 

 contain no ciy-carotenoids, but those developed by keeping stems with 

 buds in water for several days exposed only to diffuse light, produce 

 considerable amounts of /)rolycopene {see p. 30) and pro- y-carotene 

 {see p. 30) as well as other stereoisomers. These results suggested that 

 cw-isomers might be precursors of the naturally-occurring trans- 

 isomers. * ' 



Me Me Me 



(?in plant cells) 



Me Me Me 



Karrer and his colleagues, ^^'^^ who have demonstrated the natural 

 occurrence of 5 : 6- and 5 : 8- carotenoid epoxides in petals, and to a 

 lesser extent in leaves {see p. 15), consider that the production of 5 : 6- 

 epoxides from the parent carotenoid, which requires reagents such as 



72 



