10 PLANT GROWTH SUBSTANCES 



CH3 -CHg CH3 



CHg-CHj-CH-CH CH-CH-CH2-CH3 



HC= C-CHOH-CH2-CHOH-CHOH-COOH 



AUXIN-a 



CH3 ^CHj CH3 

 CHrCH,-CH-CH \h-CH-CH,-CH, 



Ig Wl 12 



HC= C-CHOH-CH2-CO-CH2-COOH 



AUXIN-b 

 Figure i. Formulae for auxins-a and -b. 



contains only one hydroxyl and a ketone group. The relation between 

 the two compounds is well established since it is possible to convert 

 auxin-a to auxin-b by the use of dehydrating agents. It has been observed 

 that the auxins gradually lose their activity, and this change is explained 

 by an allyl rearrangement. (See Fig. 2). The positions of the hydroxyl 

 and carbonyl groups are such that lactone formation takes place easily. 

 This lactone in equilibrium with the free acid undergoes, upon ultra- 

 violet radiation, rapid conversion to an inactive product named lumi- 

 auxone. (See Fig. 3). This conversion also takes place by irradiation in 

 the visible range of the spectrum when carotenoid pigments are present 

 (30). While it was not yet possible to synthesize the entire molecule, 

 a major part of the proposed structure was confirmed by synthesis of 

 one of the degradation products, auxin-a glutaric acid. (See Fig. 4). 

 The synthesis is compHcated by the presence of four asymmetric carbon 

 atoms. Auxins-a and -b, with 7 and 5 asymmetry centers respectively, 

 will present an even greater problem. 



The substances isolated fulfill all the requirements necessary for ac- 

 tivity in the Avena test. They are transported from the top of the 

 coleoptile to the base in a polar fashion. It has been shown that the 



AUXIN- a CHjCHj-CH-CH "^CHCHCHjCH, 



j HC=C-CHOH-CH,-CHOH-CHOH-COOH 



* Oris uHj (-'Ha 



PSEUDOAUXIN-CIi CH3-CH2-CH-CH ):h-chch2-ch3 



hh:-c=ch-chj-choh-choh-cooh 



OH 

 Figure 2. Comparison of auxin-a with pseudo auxin-a. 



