FUNCTION OF CAROTENOIDS IN PHANEROGAMS 



There is nothing inherently improbable in this suggestion and such a 

 sensitization seems to occur in the case of the fluorescence of chlorophyll 

 in green algae and diatoms^^' (see p. 141). A relevant chemical 

 experiment was carried out by Karrer and Straus, ^ ^ ^ who found that 

 colloidal solutions of carotene sensitized the autoxidation of benzidine. 

 Of even more interest is the recent demonstration by Kogl and Schur- 

 inga 1 8 ^ that (3-carotene, a-carotene, and lycopene sensitize the photo- 

 chemical inactivation of the plant hormone auxin-a-lactone by con- 

 verting it into lumiauxin. 



CH3 /CH2. CHj CH3 CH2. CH, 



I / \ I I / \ I 



CH3 CH^-CH-CH CH.CH.CHj.CHa GHjCHj.CH.CH CHCH. CHj.CHj 



.CH2— CHOH— > .CH*CH 



CH=C-CH CHOH CH C«C. ^HOH 



\o— c-0 \— e-0 



Auxin-a-lactone > Lumiauxin. 



As might be expected the efficiency of this action is maximal at the 

 wavelength corresponding to the main peak of the absorption spectra 

 of the carotenes. Whether this inactivation occurs in vivo remains to 

 be demonstrated. 



The implications in the important study of Frank^^° must not be 

 overlooked. She has shown that in etiolated Avena seedlings carotenoids 

 do not act as filters for blue hght, and, as first postulated by Noack ^ ^ ^^^ 

 suggests that the carotenoids are located behind the protochlorophyll 

 in the plastids. 



In the aquatic plant Potamogeton fluitans, submerged and shaded 

 leaves are green ; on exposure to strong light they become red owing 

 to the replacement of chlorophyll in the plastids by carotenoids ; ^ * ^ 

 this is reversed on removal of the light source. It is claimed that this 

 observation, which requires confirmation, demonstrates the protective 

 action of carotenoids when photosynthesis is inhibited by strong light. 



Herisset has recently made the somewhat startling claim that mixed 

 solutions of carotene and chlorophyll stimulate photosynthesis, alde- 

 hydes being produced in the presence of CO 2. 



Neither pigment alone possesses this property. ^ ^ ^^ 



CAROTENOIDS IN PHOTOKINETIC RESPONSES 



Wald * ' considers the primary function of carotenoids in plants to 

 be the stimulation of photokinetic responses such as phototropic 



87 



