CAROTENOIDS 



maize at the " medium dent " stage of maturity, although the carotene 

 concentration is highest at the pollen-shedding stage ; * '• ^ ' as soon as 

 flowering begins there is a rapid drop in carotene concentration. « '. « * 

 In pasture grasses which produce a second cutting the carotene con- 

 centration generally recovers during the early autumn ; "^ «. « 2 excep- 

 tions to this are " big blue stem " (Andropogon furcatus) and buffalo 

 grass (Buchloe dactyloides), autumn samples of which were almost 

 devoid of carotene.*' An observation which may become important 

 in practical animal nutrition is that late cuttings may actually have a 

 higher carotene concentration than have the first cuttings,*''*' 

 although the evidence is not unequivocal.®*'®' It is further claimed that 

 frequent clippings increase the yield per acre.** However, in some 

 vegetables which can be " wintered,'* e.g.y sprouting broccoli, kale, 

 coUards, there is no doubt that a marked drop in carotene concentration 

 occurs iuring the winter ; for example, the concentration in collards 

 fell from 7-9 mg. per cent, (wet wt.) in August to 2-8-4-6 mg. per cent, 

 in mid-winter.'"'®' Chard does not show this drop.®' Winter wheat, 

 on the other hand, is a good source of carotene for cattle.®® 



Fading of Leaves 



Although the carotene concentration of leaves varies during growth 

 there are apparently no great variations in the relative distribution of 

 carotenoids. The ratio of total carotene concentration to total xantho- 

 phyll concentration, which can vary considerably with species (the 

 normal ratio lies between 1 : 1 and 1 : 8 but can rise to 1 : 15 in alpine 

 plants) ' 4. 8 9. » does not seem to alter much during growth ; when 

 leaves die, however, extensive qualitative and quantitative changes 

 occur. 



The yellowing of leaves in the autumn is due to the preferential 

 destruction of chlorophylls which unmasks the colour of the 

 carotenoids, but these themselves are undergoing changes. This was 

 first appreciated by Tswett, who found that the yellow leaf carotenoids 

 were epiphasic in a light petroleum — ^90 per cent, aqueous methanol 

 partition.* This is a characteristic property of carotenes, but as the 

 pigments behaved on chromatography similarly to xanthophylls he 

 called them " autunrn carotenes." 



Much later von Euler, Demole, Weinhagen and Karrer^^ showed 

 that these " autumn carotenes " were without appreciable vitamin A 

 activity and agreed with the suggestion of Kuhn and Brockmann" 



♦ When a solution of carotenoids in light petrolexim is shaken with methanol containing 10 

 per cent, (v/v) of water, the carotenes and their mono-hydroxy and mono-keto-derivatives 

 remain in the petrol (the epiphase), whilst the di- and poly-hydroxy-carotenoids are transferred 

 to the methanol (the hypophase). 



22 



