CAROTENOIDS IN LAND PLANTS 



that they were xanthophyll esters which are known to exist in small 

 amounts in green leaves.*^ Quantitative considerations indicated 

 that if this were true some carotenes must have been converted into 

 xanthophylls ; this is unlikely from other considerations {see p. 84) 

 and in 1934 Karrer and Walker •* showed that ** autumn carotenes " 

 are not xanthophyll esters but unidentified oxidation products of both 

 carotenes and xanthophylls. Even as late as 1937 these oxidation 

 products were still being mistaken for carotenes. ® ^ During autumn 

 necrosis, then, both types of pigment are oxidized, but the carotenes 

 rather more quickly. ' ^ 



Strain**^ states that the predominating pigment in yellow leaves is 

 zeaxanthtn which persists even after the leaves have fallen. This is 

 attributed to the relatively greater stability of zeaxanthin, for there 

 appears to be no question of its formation from other carotenoids. 



It is interesting to find that normally yellow leaves appear to have a 

 carotenoid system similar to that of necrosed leaves, for it is reported 

 that leaves of the aurea variety of the elder, Sambucus nigrUy contain 

 excess " xanthophylls." The mixture of carotenoids in the young yellow 

 leaves of Euonymus japonica is typical of that of autumn leaves ^ • and 

 Egle'* has demonstrated that carotenoids of the autumn leaves of 

 tropical evergreens are very similar to those found in the necrosed 

 leaves of deciduous plants. 



It will be obvious from what has been stated previously that the 

 carotenoid picture in green leaves is reasonably simple. Very few 

 green leaves differ from this general pattern and, when they do, it is 

 only in minor details, e.g.^ in the proportion of a-carotene present. 

 This lack of species specificity disappears when the carotenoids of 

 other plant tissues are considered. It is from the point of view of 

 the production of characteristic carotenoids that fruit, blossoms, etc., 

 are important, for, with perhaps one or two exceptions {see p. 269), 

 they are relatively poor sources of the nutritionally important 

 carotenoids {see p. 24). 



FRUIT CAROTENOIDS 



(i) ^-carotene. Generally, the preponderant leaf carotenoids, [3- 

 carotene and lutein (xanthophyll), are found in fruit, but they are often 

 only minor constituents ; for example, Le Rosen and Zechmeister, • * 

 found that the red flesh of the fruit of Celastrus scandens contained p- 

 carotene to the extent of only 3 per cent, of the total carotenoid pigment 

 and the presence of lutein (xanthophyll) was not recorded. The fruit 

 of Cotoneaster occidentaUs contain no p-carotene but here, as in the case 



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