FUNCTION OF CAROTENOIDS IN PHANEROGAMS 



weight) whilst in the ripe fruit the lycopene concentration is 7-75 mg. 

 per 100 g. dry weight. Further, Smith ^^ has found that tomatoes 

 produced in complete darkness possess no chlorophyll when unripe but 

 do contain lycopene when mature. Miller, Winston and Schomer ^ ^ 

 found that the carotene content of the rinds of maturing oranges 

 continues to increase after all the chlorophyll has disappeared. 



There is thus no reason to believe that, in maturing fruit, any 

 appreciable amount of carotene can be produced from chlorophyll ; 

 however, on the evidence so far available, there is also no reason to 

 assume that phytol is not an intermediate in carotenoid formation, for 

 the investigations just considered do not rule out the possibility that the 

 plant calls on phytol reserves other than those provided by chlorophyll. 



Possible intermediates in carotenoid synthesis in plants are the 

 strongly fluorescent materials which can be separated from the carote- 

 noids by chromatography. Those detected in green leaves by 

 Strain ^2, 3 3 showed absorption spectra with ** sharp fine structure" 

 about 320 m^x. Phytofluene, a partly saturated fluorescing carotenoid, 

 has been noted by Zechmeister and his colleagues ^ *■ ^ ^ in fruit and 

 petals {see p. 28) and has been suggested as a possible intermediate 

 in carotenoid biogenesis. If this suggestion turns out to be true, then 

 the route of carotenoid biogenesis in green leaves may be diflFerent 

 from that in fruit and petals because phjrtofluene is never observed in 

 green leaves. 



It is important to note that the sky-blue fluorescing material 

 associated with phytofluene in petals of Tagetes erecta is not a 

 carotenoid derivative but a-terthienyl (CjaHsSg)^'. 



Following their extensive investigations on carotenoid production 

 in tomatoes. Porter and Lincoln ^^ have proposed a scheme for the 

 biogenesis of carotenoids in tomatoes which, in essence, involves the 

 stepwise dehydrogenation (four H atoms at a time) of tetrahydro- 

 phytoene to lycopene which is then isomerized to a-, P-, and y- 

 carotenes and oxidized to lycoxanthin and lycophyll. The mechanism 

 envisaged is shown on the next page. 



This interesting hypothesis is based on genetical studies and on the 

 fact that all these compounds do occur in tomatoes. The main objec- 

 tion which Porter and Lincoln have to combat is '* may not the reactions 

 go in exactly the opposite direction to that which they have suggested ? " 

 In other words, may not lycopene (or a- and ^-carotene, etc.) be the 

 primary product which is then hydrogenated ? 



Porter and Lincoln have carried out inheritance studies involving 

 crosses of commercial varieties of tomatoes with high lycopene content 

 with selections with low lycopene and high tetrahydrophytoene content 



67 



