FUNCTION OF CAROTENOIDS IN PHANEROGAMS 



carotenoid increase. They consider that this indicates the probable 

 formation of leaf carotenoids from deaminated amino acids (possibly 

 leucine or valine) mobilized from the hydrolysis of some stored 

 protein. The iodine values of benzene extracts of developing w^heat 

 seedlings were measured but no variations were recorded. This appears 

 to rule out saturated lipids (? fatty acids) as a potential source of caro- 

 tenoids — formed by dehydrogenation followed by subsequent methyla- 

 tion. Also against the idea of the formation of carotenoids by dehydro- 

 genation of saturated lipids, is the recent work of Holman, ^ ® which 

 demonstrates that the unsaturation of soya bean lipids decreases during 

 germination. 



Wilson ^2 has attempted to correlate nitrogen metabolism and 

 carotenoid production from the fact that the variation in the carotenoid 

 levels in plants on dull and sunny days inversely follows the nitrite 

 variation under these conditions. It is suggested that the accumulating 

 nitrite oxidises the carotenoids to colourless compounds, for this oxida- 

 tion rapidly occurs in vitro. The main objection to this suggestion is 

 that the diurnal variation in carotenoid levels of plants is by no means 

 fully established. Although recently Roberts ^ ^ has produced what is, 

 up to the time of writing, the best evidence that such a variation does 

 occur. 



General considerations 



Two final general observations concerning carotenoid production 

 in leaves can be noted before turning to the recent work of Bandurski : 

 in box leaves carotenoid synthesis appears to be associated with 

 increased lipid production and the disappearance of starch,^* and 

 barley seeds rich in nitrogen and aneurin (thiamin) produce plants 

 giving a greater yield of carotenoids than do seeds less rich in these 

 constituents. ^ ^ 



Studies with isolated tissues 



The study of carotenoid synthesis in detached bean leaves which 

 has been carried out recently by Bandurski,*^* can be considered the 

 first attempt to elucidate the fundamental biochemical problem con- 

 cerning carotenogenesis in higher plants ; even so this important 

 investigation has only scratched the surface of the problem. Detached 

 bean leaves cultured on a " 3-salt " medium and exposed to light can 

 synthesize considerable amounts of carotenoids in 24 hours, much 

 more than can leaves attached to their petioles. This difference is 

 probably due to the fact that when a leaf is severed from its stem no 

 translocation of food reserves can take place. Carotenoid formation 



71 



