CAROTENOIDS 



Frank '^^ has constructed the action spectrum for the destruction 

 of carotenoids and the synthesis of chlorophyll during the early stages 

 of illumination of etiolated oat seedlings. They are identical and are 

 similar to the absorption spectrum of a porphyrin. Frank puts forward 

 the following stimulating suggestion : The porphyrin mediating in 

 these reactions could be magnesium vinyl phaeophorphyrin, which by 

 stimulating the conversion of carotene into (?) phytol is itself supplied 

 with essential unit for its own conversion into magnesium vinyl 

 phaeophorphyrinphytyl ester (protochlorophyll), which is probably 

 the penultimate step in the biogenesis of chlorophyll. It will be in- 

 teresting to see how this stands up to experimental attack, but it is 

 important to note that Granick * ^^ has already found that, in a mutant 

 of the unicellular alga Chlorella, the conversion of magnesium vinyl 

 phaeoporphyrin into protochlorophyll is accompanied by the dis- 

 appearance of carotene when the alga is kept at room temperature ; 

 at 0°, hoAvever, carotene destruction still occurs although there is no 

 concomitant increase in protochlorophyll. 



Beck, *^ however, accepts Rudolph's suggestion and further con- 

 siders that carotenoids may be precursors of auxin. When the effect of 

 light on the synthesis of the carotenes and xanthophyll fractions is 

 considered separately the results are somewhat contradictory. Whit- 

 more has reported that the carotene concentration in bean leaves was 

 greatest when plants were grown in green light, whilst the concen- 

 tration of xanthophylls was greatest in either red light. or in the dark. 

 When the yields of xanthophylls were considered, it was noted that 

 plants grown in the dark only produced half as much as did those 

 grown in red light. * * 



Seybold and Egle, ' ^ Beck ' ^ and Nagel ' ^ have found that xantho- 

 phylls develop more quickly than carotenes when etiolated seedlings 

 are developed in white light ; on the other hand, Barrenscheen, Pany 

 and Srb ^ ^ indicated that in wheat seedlings the increase is mainly in 

 the carotene fraction. This has recently been confirmed by Blaauw- 

 Jansen, Komen and Thomas ' ^ who exposed 8-day seedlings of Avena 

 sativa to light of varying intensities. At the beginning of the investiga- 

 tion the xanthophyll concentration was relatively high, but the amount 

 did not increase on illumination. The amount of the carotene fraction 

 did increase on illumination and the increase was proportional to the 

 intensity of the illumination. 



(ii) Normally growing plants 



In normally growing plants most of the work which has been 

 described has dealt with the effects of light on colour of developing 



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