FUNCTION OF CAROTENOIDS IN PHANEROGAMS 



to parasitization by fungi or insects is due to excessive carotenogenesis 

 accompanied by excessive lipid production and a fall in starch con- 

 tent. ^ ^ ^ Proof of excessive carotenogenesis has, however, not been 

 adequately presented. 



Griffith, Valleau and Jeffrey^''* studied 18 varieties of tobacco plant 

 but could find no relationship between carotene content and mosaic 

 resistance factors. 



It has been found recently that the pycnidial lesions on crab-apple 

 leaves (Malus ioensis) infected with the common rust fungus Gymno- 

 sporangium juniperi-virginianaey contain ycarotene to the extent of 

 34-5 per cent, of the total carotenes present. Unaffected regions of 

 the leaves produce no y-carotene. ^^^ (See also p. 108). 



Booth and Dark ^ ° ^ have noted that carrots infested with the larvae 

 of the carrot fly (Psila rosae) have a slightly higher carotene concen- 

 tration than have healthy roots. Recently it has been reported that 

 treatment of kidney bean seedlings with 2 : 4 -dichlorophenoxyacetic 

 acid reduces carotenoid synthesis. ^ ^ ® Treatment of germinating seeds 

 with streptomycin retards carotenoid synthesis, ^ ® ^ whilst spraying 

 with isopropylphenylcarbamate has no effect. ^ * ® 



FUNCTION 



Theories advanced to explain the function of carotenoids in plants 

 are many but are generally based on one or two characteristic pro- 

 perties : {a) the ability to absorb oxygen and {b) the ability to absorb 

 light energy in the blue region of the visible spectrum. Nevertheless, 

 rigorous proof of any specific function is still lacking, but as will 

 become apparent one theory appears much more plausible than any 

 other. The problems of carotenoid function in photo-reception and 

 photosynthesis have been critically reviewed by WaW and Rabino- 

 witch^^^ respectively. Frey-Wyssling ^ ^ ^ considers carotenoids to be 

 excretory products of unknown metabolites ; their insolubility in 

 water removing them from active participation in the functioning of 

 the cells ; later investigations have, however, shown that carotenoids 

 are almost always solubilized by attachment to proteins ^ * * (see p. 6). 

 Some of the more outre suggestions concerning carotenoid function, 

 which will not be discussed here, have been summarized by Wenzinger. ^ 



CAROTENOIDS IN OXIDATION-REDUCTION SYSTEMS 



In theory carotenes and xanthophylls appear to be components of a 

 perfect redox system and this is also true of chlorophylls a and b. 



83 



