Naphthyloxy compounds as plant growth regulators 



expected, was active as a growth regulator, as was also 2:4:5-trichloro- 

 phenoxy acetic acid, but y-(2:4:5-trichlorophenoxy) -butyric acid, which 

 should be active, was not. 



Wain and Wightman (1954) studying the same compounds also found 

 y-(2:4:5-trichlorophenoxy) butyric acid and higher homologues to be 

 inactive in the pea and tomato leaf epinasty tests, though the homologues 

 with an even number of carbon atoms in the side-chain were active in the 

 wheat coleoptile cylinder test. This would imply that the /5-oxidase capable 

 of degrading the higher homologues of 2:4:5-trichlorophenoxyacetic acid is 

 absent from pea and tomato tissue but present in wheat. The absence of the 

 appropriate /j-oxidase from tomato ovaries might therefore provide an 

 alternative explanation for the lack of activity of the homologues of 3-chloro- 

 2-naphthyloxyacetic acid in the present investigation. 



a- (2-naphthyloxy) -h-alkylcarboxylic acids 



The introduction of alkyl groups of gradually increasing chain length on the 

 a-carbon atom of 2-naphthyloxyacetic acid causes a rapid decrease in 

 activity, the first homologue to be completely inactive being a- (2-naphthyl- 

 oxy) -;2-valeric acid. In a similar series starting with 3-chloro-2-naphthyloxy- 

 acetic acid activity is again rapidly lost {Table 3). 



Resolution of a-(3-chloro-2-naphthyloxy)-propionic acid using cinchonine 

 (Pope and Woodcock, 1955) has given a highly active (-f ) form and an 

 inactive optical antipode. Examination of this compound by Matell (1955) 

 has shown that it most probably has the D-configuration, thus supporting the 

 view that optically active plant growth regulators of the a-aryloxyalkyl 

 carboxylic acid type which have greater auxin activity than their respective 

 antipodes belong to the D-series. It is also noteworthy that the ( — ) form here 

 shows competitive antagonism with the (-f ) form, resulting in an activity of 

 only 60 for the {^) mixture instead of an expected value of 125. This 

 agrees with the findings of Aberg (1951) with a- (2-naphthyloxy) -propionic 

 acid in root inhibition, and of Smith, Wain and Wightman (1952) with 

 a-(2:4-dichloro-phenoxy)-, a-(2 :4:5-trichlorophenoxy)- and a- (2-naph- 

 thyloxy) -propionic acids in cell elongation promoting activity. 



In general the activity of these a-substituted acids is much lower than the 

 corresponding 2-naphthyloxyacetic acid and the fairly rapid decrease with 

 increasing chain-length would seem to indicate a size or, rather less likely, a 

 solubility effect. The close relationship between activity and the position of 

 nuclear substituents will be more obvious after the results in the next section 

 have been reviewed. 



SUBSTITUTED 2-NAPHTHYLOXYACETIC ACIDS 



In one of the most recent and critical i^eviews of structure-activity relation- 

 ships in the field of plant growth regulating substances, Jonsson (1955) has 

 reformulated the requirements for appreciable auxin activity as follows: 



(i) An unsaturated ring nucleus. 



(ii) A side-chain carrying a — COOH group which must not be situated 

 at a quaternary carbon atom (or otherwise highly sterically hindered). 



(iii) A high interface activity of the extended ring system. 



(iv) The side-chain must be able to assume a spatial orientation in which 



199 



