CAROTENOIDS 



bending and chloroplast migrations. He bases this generalization on 

 three main considerations, (1) photosensitive structures contain 

 carotenoids, (2) photokinetic action spectra* correspond closely to the 

 absorption spectra of known carotenoids and (3) no other pigment 

 occurring in the photosensitive structures possesses a comparable 

 spectrum. 



Blaauw ^ * ^ was the first to measure the action spectrum of the photo- 

 tropic bending of the oat (Avena) seedling, but it was not until 1930 

 that the suggestion was put forward that the mediator might be a 

 chromohpid. ^ ^ ^ Since then, Voerkel, ^ * * Castle ^ ^ ^ and Biinning ^ ^ * 

 have confirmed and developed his suggestion. Went ^ ^ ' has examined 

 the spectral sensitivity in pea seedlings of leaf growth, inhibition of 

 stem growth, and phototropic bending ; only the latter response is 

 maximal when the seedlings are irradiated with blue light, i.e.y in the 

 spectral region where light absorption by carotenoids is maximal, the 

 first two responses are minimal under these conditions. This does not 

 imply that phototropic bending is the only photo-reaction into which- 

 the carotenoids enter, for Bottelier^*^ has shown that the action 

 spectrum for protoplasmic streaming in the epidermal cells of Avena 

 coleoptiles is very similar to that for phototropic bending. 



It should be noted, however, that the action spectra for the produc- 

 tion of spikes in barley and for the initiation of flowering in soya beans 

 are identical, with minima in the spectral region 450-480 mfji. ^'* 

 These results strongly suggest that carotenoids do not play any part in 

 these processes. 



Wald's third point that no other pigment in the photosensitive 

 structure possesses a comparable spectrum needs reconsideration in 

 the light of recent work by Galston. Riboflavin occurs throughout the 

 Avena coleoptile ^ « o and Galston and Baker ^oo state that in vivo 

 measurements of action spectra are not sufficiently precise to dis- 

 tinguish between ^-carotene and riboflavin both of which absorb light 

 maximally around 450 mfx. Re-examining the spectral data of Haig,^**^ 

 they concluded that the " tip reaction " in Avena probably involves a 

 carotenoid and the " base reaction " probably riboflavin. If, as Kogl 

 and Schuringa^®* suggest, p-carotene controls phototropic responses 

 by sensitizing the photo -inactivation of auxin-a lactone, then there 

 exists evidence that riboflavin can act in an analogous manner. It has 

 been shown that riboflavin can inactivate indole acetic acid which has 

 claims to be considered a naturally-occurring auxin 2 o 2, 2 o 3 Relevant 



* Action spectra are constructed by plotting the reciprocal of the energy at 

 different wavelengths required to elicit a constant response, against these 

 wave-lengths. 



88 



