CAROTENOIDS 



There is ample evidence (quoted in the previous chapter) that, at 

 maturity, the carotenoid concentration in the green parts of plants 

 begins to decline. It has been suggested that this is due to mobilization 

 of carotenoids into the reproductive structures of the plant. ^®^ 

 Evidence in favour of this mobilization is very meagre, and it has, 

 in fact, been denied. ^^^ Further, it should be noted that in the case of 

 maize (corn) this decline was still evident in plants in which pollination 

 was eliminated by covering the ear shoots with paper bags. ^^<* One 

 possibility is that at maturity the synthetic mechanism rather than the 

 final product, is diverted to the flowers and then to the developing fruit. 

 It is well known that carotenoids are rapidly synthesized in many 

 developing fruit (see p. 39), but it has only recently been reported that 

 the carotene concentration of the anthers and petals of a number of 

 plants (Californian poppy, jasmine, pumpkin, and St. John's Wort) 

 increases through the budding period and reaches a maximum at 

 flowering. ^ i <> 



Zhukovskii and Medvedev ^ ^ ^ consider that microsporogenesis is 

 intimately connected with carotenoid metabolism for they claim to 

 have demonstrated histologically that carotenoids and lipids are con- 

 centrated in the tapetum of the pollen sac and that, during formation 

 of pollen, both migrate into the developing pollen grains, young pollen 

 grains being white whilst mature grains are yellow. This work does not, 

 however, eliminate the possibility that carotenoids are produced in situ 

 in the pollen grains. A somewhat related observation is that pollen 

 tube growth on an agar medium is stimulated by the addition of 

 carotene to the medium. ^ ^ ^ 



Deleano and Dick ^ i » have shown that the amount of carotene per 

 leaf is greater in fully developed male crackwillow (Salix fragilis) than 

 in fully developed female trees ; the concentration in both cases,* is 

 however, the same. It seems then that the increased carotene content 

 of male leaves is merely a reflection of their larger size and has no 

 significance for carotenoids per se. 



The function of carotenoids in pollen, if any, cannot be universal 

 because not all pollen contains carotenoids {see p. 52), and in this 

 connection, the recent work of Kuhn and Low ^ i * on Forsythia inter- 

 media Zabel {F. suspensa X F. viridissima) is relevant. This plant is self- 

 sterile, fertilization only taking place by cross pollination of the R°-type 

 (short-styled flowers with long filamented anthers) and the R+ type 

 (long-styled flowers with short filamented anthers). Kuhn and Low 

 found no qualitative or quantitative differences between the carotenoids 

 in the petals of the two types. No mention is made of the carotenoids 

 in the two pollens and this is rather disappointing because they found 



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