CAROTENOIDS IN LAND PLANTS 



As rhodoxanthin is normally considered to be a carotenoid charac- 

 teristic of fruit, and as it never occurs in the leaves of angiosperms (it 

 does, however, exist in the water weed Potamogeton {see p. 57) ) it may 

 be the basis of an important differentiation between angiosperms and 

 gymnosperms. This is an aspect of carotenoid biochemistry which 

 demands much further study. 



CAROTENOIDS IN THE DEVELOPING PLANT; 

 FADING OF LEAVES 



A considerable amount of work has been carried out on the quantita- 

 tive aspects of carotenoid formation in the developing plant, not only 

 because of its intrinsic importance, but because of the importance of 

 green leaves as a source of vitamin A precursors in the nutrition of 

 herbivorous animals. On a wet weight basis representative values for 

 the carotene and lutein (xanthophyll) content of a forage grass are 45 

 and 60 fi-g./g- *^ Variations between the carotene content of the leaves 

 of different species can, however, be considerable ; for example, elm 

 leaves contain twice as much as do willow leaves.** For further 

 details the reader is referred to the appendix (p. 289) where all reported 

 quantitative data on carotenoids in leaves are collected. 



As a source of vitamin A precursors the leaf blade is extremely 

 important, for not only is its carotene concentration 5-50 times greater 

 than that of the mid-rib or the petiole, but also it contains over 90 per 

 cent, of the total leaf carotene. Further, the leaves contain over 90 per 

 cent, of the total plant carotene although they are less than 50 per cent, 

 of the total weight.*^/*' The mid-ribs appear to contain slightly 

 more carotene than do the petioles. * ^ 



Working with turnip tops, Bernstein, Hamner and Parks * * showed 

 that p-carotene is distributed fairly uniformly throughout the leaf 

 blade. Their resufts also indicate that the carotene concentration in 

 leaves picked at midday is less than in those picked either in the 

 morning or at night {see p. 75). Similar diurnal variations are reported 

 for lucerne. * ^ Further data, taking into account variations from plant 

 to plant, are necessary before one can unreservedly accept this pheno- 

 menon of diurnal variation. Markley, ^ " using wheat plants, showed 

 that inter-plant variations can be considerable. 



Carotenoid formation begins immediately after germination takes 

 place ^ 1 and continues rapidly during the early period of active growth ^ ^ 

 {see also p. 66). It is at the period of maximal growth rate that carotene 

 concentration is also maximal ; this has been repeatedly demon- 

 strated for a variety of species growing in a variety of climates in 



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