FUNCTION OF CAROTENOIDS IN PHANEROGAMS 



fruit and this has already been discussed {see p. 40). In the green tissues 

 variation in intensity of light affects all pigments equally ' * and, unless 

 the intensity is too great, carotenoid production is proportional to the 

 light intensity.'*^''* There is, in practice, an optimal intensity of 

 illumination ; for example, in isolated bean leaves this is 600 foot- 

 candles over the temperature range 25-35°.^* Whether this is truly 

 the case or whether it is the result of the interplay of thermal and photic 

 effects is very difficult to decide ; as has already been emphasised 

 (see p. 40) the separation of these two effects is extremely difficult. 



The outcome of varying the photoperiod is by no means clear cut. 

 Very early experiments indicated a marked variation in colour of 

 carrots exposed to different day lengths. ' ^ Murneek ^ ^ stated that 

 foliage of soya bean, cosmos, and salvia plants grown under 7 hours 

 of daylight had higher carotene contents than had those grown under 

 14 hours of light. Barnes, ' ' however, in a well-documented report 

 concludes that a somewhat smaller variation in photoperiod (9-14 

 hours) has no effect on carotenoid production when conditions of 

 temperature and moisture were optimal. More recent work by 

 Roberts ^^ suggests that pigmentation is altered by variation in the 

 photoperiod, for branches of the same plant when grown under differ- 

 ent photoperiods contain different amounts of carotenoids. Roberts, 

 however, did not consider the age factor which operates from leaf 

 to leaf (see p. 20). Photoperiod and temperature may be connected. ^ * ' 



EFFECT OF TEMPERATURE 



Although seedlings grown at low temperatures may contain at least 

 as much carotene as those grown at normal temperatures, ' ^ the 

 existence of an optimal temperature range (60-70°) for the production 

 of carotene in roots such as carrots 3 ». ' 9 ^nd beet ^ ® cannot be doubted. 

 Carrots grown at lower temperatures are visibly less coloured owing to 

 the absence of pigment from the peripheral cells. ^ ° Recently, using 

 isolated bean leaves, Bandurski^* found the temperature coefficient 

 for carotenoid synthesis to be 2-9 in the dark and 1-4 in the light. 



Work on the relation between temperature and ripening of fruit 

 has already been discussed {see p. 40). 



EFFECT OF SOIL NUTRIENTS 

 (i) General 



A large literature exists on this subject and although a number of 

 investigations have been insufficiently controlled to warrant the con- 

 clusions drawn, the general picture is reasonably clear. Short reviews 



75 



