FUNCTION OF CAROTENOIDS IN PHANEROGAMS 



inhibition of the photosynthetic dark reaction (with narcotics, ether, 

 etc.) causes a decrease in this ratio. In neither case does the total 

 amount of carotenoids present alter. 



Starch production in plants is, according to Seybold^^'', generally 

 associated with the presence of chlorophyll b. Investigating some 

 members of the Allium family, which do not produce starch, he found 

 a deficiency of chlorophyll b, but no significant variation from normal 

 in the xanthophyll : carotene ratio although as would be expected the 

 ratio chlorophyll b : chlorophyll a was very low. There was also no 

 significant departure from normal in the total carotenoid or total 

 chlorophyll content. Similarly, in developing mango fruit there is no 

 correlation between carbohydrate synthesis and carotenoid synthesis. ^ ' ^ 



CAROTENOIDS AS OXYGEN TRANSPORTERS 



This conception of carotenoid function also follows from Amaud's ^ * * 

 pioneer work in 1889 which demonstrated that carotenoids are easily 

 autoxidisable. It was assumed that carotenoids form unstable peroxides 

 which can transfer Og to other substances. The in vitro ** pro-oxidant " 

 activity of carotenoids appears to favour the assumption, i ' 2- 1 7 e with 

 this in mind the suggestions made at various times that carotene has a 

 catalytic function in the binding of oxygen in plants can be 

 appreciated ; 1 ' '. 1 7 s however, it must also be emphasized that all 

 these suggestions are speculative and that no experimental evidence is 

 available. Further, as Rabinowitch ^ * 2 ^^s pointed out the reverse 

 process of liberation of oxygen during photosynthesis is equally 

 feasible. Lazar^'® claims that carotenoids stimulate root formation 

 and general development of plants in cultures of Impatiens balsamina. 

 If this is confirmed the action may well be related to oxygen transfer, 

 for he later found that carotene and sucrose produced a response 

 similar to that of glucose + oxygen. ^ ^ ^ Giroud ^ ^ ^ maintains that 

 carotene has an antioxidant rather than a pro-oxidant action ; he claims 

 that it is not fortuitous that plants richest in carotenoids are also 

 richest in ascorbic acid, for the ascorbic acid is protected by the 

 carotenoids. There is, however, no real evidence that this suggested 

 correlation between vitamin C and carotenoid levels is a reality. ^ * ® 

 Taking a viewpoint similar to that of Giroud, Herisset^^* considers 

 that the carotenoids act by inhibiting plant and animal oxidases. 



An apparently insuperable objection in the way of theories based on 

 the autoxidisibility of carotenoids is their undoubted stability in situ 

 in the plant chloroplast compared with instability in extracted solutions. 



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