CAROTENOIDS IN PLANTS 



If a well-formed mycelial mat of Phycomyces is placed on a medium 

 containing only leucine or valine but no glucose, little or no carotene 

 synthesis takes place : ^ ^ this is taken to mean that the energy required 

 for absorption of the amino acids into the cells and for condensation 

 and/or dehydrogenation of the repeating units must cpme from the 

 dissimilation of glucose. 



Mery^^ describes qualitative experiments which suggest that 

 tyrosine stimulates considerably both lipogenesis and carotenogenesis 

 in red yeasts. 



Significance of the Carbon : Nitrogen Ratio in Carotenogenesis. 

 Schopfer's pioneer experiments on carotene production in Mucor 

 hiemalis and Phycomyces blakesleeanus indicated that the carbon/ 

 nitrogen ratio (C/N) is an important factor controlling the extent of 

 carotenogenesis, the higher the ratio the more carotene produced. 2 4, 5 o 

 Similar suggestions have been put forward to explain the quantitative 

 aspects of lipogenesis in micro-organisms {see Kleinzeller ^ * for a 

 review). Garton et al, 1 ». 2 found that the C/N ration can often assume 

 a spurious significance when applied to carotenogenesis. Provided 

 enough nitrogen is available to allow maximal mycelial growth, the 

 controlling factor is not the C/N ratio but the amount of assimilable 

 carbon available after growth has finished. An example will make this 

 clear : the concentration of carotene in a mycelium containing sufficient 

 nitrogen for maximal growth (0-2 per cent, asparagine) and just suffi- 

 cient carbon (1-5 per cent, glucose), is 20-30 mg./lOO g. (dry wt.). 

 When the asparagine concentration is kept constant and the glucose 

 concentration increased to 2*5 per cent, the carotene concentration 

 increases to 120-150 mg.,/100 g. If, however, the glucose concentration 

 is kept at 2-5 per cent, increasing the nitrogen concentration to 1-0 

 per cent {i.e., decreasing the C/N ratio) does not reduce the carotene 

 concentration, which remains at 120-150 mg./lOO g. 



As previously stated Goodwin and Willmer ^ ^ have found that caro- 

 tenogenesis only proceeds rapidly when the mycehal mat is fully 

 formed, i.e., when nitrogen assimilation has stopped. It was at 

 first found difficult to reconcile these results with the previous results 

 of Garton et al.,'^^'^^ who noted that when fully grown mats were 

 transferred to a medium containing glucose but no nitrogen no carotene 

 synthesis occurred ; from these experiments it was tentatively con- 

 cluded that the fungus had to be metabolizing exogenous nitrogen in 

 order to synthesize carotene. It has now been shown that the failure 

 to produce carotene in the absence of nitrogen in the original experi- 

 ments was due to the fact that the medium used was buffered at pH 7. 

 Using non-buffered media fully grown mats will synthesize carotene 



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