FUNCTION OF CAROTENOIDS IN PHANEROGAMS 



This has the merits of having a close analogy in the generally 

 accepted hypothesis of the formation of fats from carbohydrates in 

 mammals, ^ and further, it should be noted that Smedley ^ and Kuhn, 

 and Grundmann and Trischmann ^ ° have obtained by chemical means 

 long chain polyene aldehydes starting from crotonaldehyde. 



Recent work by Bonner and Arreguin ^ ^- ^ ^ on the related problem 

 of the biogenesis of rubber indicates that, using the stem-culture 

 technique, neither isoprene nor [3-methylcrotonaldehyde was a 

 precursor of rubber ; nor were other obvious possibilities such as 

 pyruvate and citrate. Acetone, acetate and acetoacetate (probably 

 degraded to acetate) and glycerol were, however, active. Acetone and 

 acetate can, theoretically condense to form ^-methylcrotonic acid and 

 it was found that this substance was active but not to the same extent 

 as acetate and acetone. Acetate, acetone and (3-methylcrotonic acid, 

 but not glycerol, were also active in stimulating rubber synthesis in 

 aseptic experiments with isolated stem fragments. ^ ^ 



Me Me Me Me 



y ^^ 



,c c 



CH CH-Ch=CH-C.CH 



CH2 C. S 



\ / Me 

 CHj 



+ H7O 



H,0 



CH^ CH.CH=CH.C-CH3 



\ / ^Me 

 _^ CHj 



postulated intermedi'ate 



-H,0 



Me Me 



\/ 



C 

 / \ 



CH2 C.CH = CH-C.CH, 



1 II 



CH2 C-Me 



p-ionone 



Inherent in all these hypotheses of carotenogenesis is the assumption 

 that lycopene is the parent carotenoid from which other carotenoids 

 are formed by isomerization. Facts which militate against this assump- 

 tion are that (1) lycopene is never detected even in minute traces in 

 green material, (2) in the tomato the production of lycopene and caro- 

 tene is probably carried out by two separate processes^* {see p. 40) 

 and (3) in the case of a- and [3-carotene, at least, interconversion by 

 asymmetrical isomerization does not appear to occur. ^ ^ However, 



65 



