CAROTENOIDS 



it should be noted that ^'-ionone can be converted into a mixture of a- 

 and p-ionones by the action of H2SO4. ^ ^ 



It must be emphasized then that Uttle evidence exists in favour of 

 any of the hypotheses outhned above and that conjecture has out- 

 stripped experimentation. 



In considering the pattern of carotenoid formation it will be con- 

 venient to start with the seed and follow the production as the plant 

 develops. Seeds contain small amounts of carotenoids, probably 

 qualitatively the same as those in the corresponding leaves (see p. 42). 

 Synthesis generally begins within 3-5 days of germination ; 1 '. 1 « fQj- 

 example, although no increase in the carotene content of a soya bean 

 occurs during the first 24 hours, 72 hours later a three-fold increase has 

 occurred. 1' Wheat seedlings take rather longer (10 days) to treble 

 their content ^^ and in some cases there may be a slight drop at first 

 until, coinciding with the appearance of the first leaf, there is a rapid 

 synthesis. ^ « The increase is not at the same rate in all parts of the 

 seedling, for after 54 hours there is three times more carotene in the 

 cotyledon than in the hypocotyl. ^ ' In a number of pulses and cereals 

 the carotene content is approximately doubled 7 days after germi- 

 nation, ^o^ The amount of pigment produced during the first 40 hours 

 is proportional to the weight of the seed. ^ 1 



Miller and Jablonski^s have shown that the germination of grape 

 fruit {Citrus paradisi) until the radicle was 1-2 in. long, increases the 

 carotene content of the embryo from 3-36 to 44-8 mg. per 100 g. (dry 

 wt.) and of the whole seed from 0*02 to 0-27 mg. per 100 g. (dry wt.). 



Immediately following germination carotenoid synthesis precedes 

 that of chlorophyll ^^-^^ but as the plant develops the formation of leaf 

 carotenoids runs roughly parallel with that of the chlorophylls. 24-26 

 At the approach of maturity, however, the chlorophylls disappear 

 before the carotenoids. These observations on immature plants in no 

 way suggest conversion of chlorophylls into carotenoids and further, 

 Guthrie ^ ' has shown that in the foliage of tomato and soya bean plants 

 grown in the dark for several days, the carotenoid content remained 

 constant although that of chlorophyll decreased. The significance of 

 Beck's 2 3 claim that carotene is formed more readily from young than 

 from old seeds is not yet obvious. 



In ripening of fruit the possibility of an inter-conversion does arise, 

 for on maturation the fruit chlorophylls disappear and the carotenoids 

 accumulate. However, the chlorophylls destroyed do not liberate 

 sufficient phytol to account for the carotenoids produced in ripened 

 Physalis alkekengi^^ and Lycopersicum spp. ^ » In the latter the chloro- 

 phyll-phytol concentration in the green fruit is only 1 mg. per lOOg. (dry 



66 



