CAROTENOIDS 



The reason for this stabihty is that probably the carotenoids exist as 

 protein complexes in the same way as does chlorophyll, ^ ^ ' they may 

 even be attached to the same protein as is chlorophyll to form what 

 Lubimenko^*^ called ** natural chlorophyll" (see also Smith). ^^* 

 Although it has not yet been isolated, this carotene protein complex is 

 more stable than are carotene solutions ; further examples of caroteno- 

 proteins will be dealt with in later sections of this book [see p. 171). 



Chailakhyan ^ * *^ has recently examined the effect of introducing 

 lanolin containing 0*2% of carotene into incisions made in the stems 

 of Chrysanthemum^ Pertlla and Rudbuckia spp. In Perilla and Rud- 

 beckia growth of the main stem was retarded but side runners developed 

 abnormally well. In Chrysanthemum there was decreased growth of the 

 main stem (less leaf-tier spacing) accompanied by the development of 

 thicker, stiffer, and darker leaves. Powdered saffron (rich in caroten- 

 oids, including carotene) had the same action, whilst a chlorophyll 

 preparation, obtained from nettles, was inactive. 



CAROTENOIDS IN PHOTOSYNTHESIS 



As long ago as 1844 Draper i^*^ showed that the blue and violet 

 regions of the spectrum (in which chlorophyll action is minimal) are 

 almost completely ineffective in producing photosynthesis as measured 

 by oxygen liberation and Willstatter and StoU ^ ^ ' have stated categoric- 

 ally that carotenoids play no part in photosynthesis. Engelmann^^' 

 using a different technique, came to the opposite conclusion. He 

 irradiated plant material with the visible spectrum and noted the regions 

 of the leaf corresponding to definite wave-lengths, at which mobile 

 aerobic bacteria accumulated, these accumulations denoting areas of 

 rapid oxygen evolution. The variations which occurred in the points 

 of maximum accumulations when materials of different colour were 

 used led him to conclude that pigments other than chlorophylls {i.e.y 

 carotenoids) enter into the photosynthetic mechanism. 



It is clear that carotenoids cannot play a primary role in photo- 

 synthesis for no case has been recorded of their ability to promote 

 photosynthesis in the absence of chlorophyll ; Rabinowitch ^ « ^ believes 

 that carotenoids participate in photosynthesis by transferring their 

 excitation energy to chlorophyll : 



CAR + hv > CAR* 



CAR* + CHL ► CHL* + CAR 



86 



