Further evidence is presented on the occurrence of
β
-oxidation within the plant. The growth regulating activities of eighteen homologous series of
ω
-phenoxyalkanecarboxylic acids, RO(CH
2
)
n
COOH with
n
= 1 to 6 and with chlorine and methyl groups substituted in different positions of the ring, have been assessed in the wheat cylinder, pea curvature, pea segment and tomato-leaf epinasty tests. The breakdown of a number of homologues when metabolized in wheat coleoptile or peastem tissues has been investigated using paper chromatography and ionophoresis, identification of metabolites being made by chromogenic methods and bio-assay. Growth-regulating activity is considered in relation to degradation of the side-chain by
β
-oxidation. In certain series the pattern of biological activity and breakdown sequence are shown to be different in wheat and pea tissues, indicating substrate specificity of the enzyme systems concerned. Evidence is presented that in both wheat and pea tissues
β
-oxidation of homologues containing an even-number of side-chain methylene groups is hindered at the propionicacid stage (
n
= 2); this may result in an unusual pattern o f biological activity in series where the propionic homologue possesses growth-regulating properties
per se
. The degradation of homologues with an odd number of side-chain methylene groups normally leads to the production of the corresponding acetic derivative (
n
= 1), and although the results indicate that this breakdown occurs in all series in wheat tissue, with ten of the series there is evidence that
β
-oxidation is hindered at the butyricacid stage (
n
= 3) in pea and tomato tissue. Such hindrance of
β
-oxidation at the butyric stage is shown to be associated with the presence of an
ortho
chlorine or methyl group, though the effect is largely removed by introducing a further chlorine atom at the
para
but not the
meta
position. The fact that the
β
-oxidase enzymes in wheat and pea tissues show specific differences in their reactions with these phenoxy acids, and the importance of ring substitution in influencing the degree to which hindrance of
β
-oxidation occurs at the propionic and butyric acid stages, together provide an explanation for the different types of growth-regulating activity observed in the series examined. All the biological results are discussed in relation to these considerations, and it is concluded that hindrance of side-chain degradation by
β
-oxidation is associated with electronic, in addition to steric factors. Some similarity thus appears to exist between the broad class of aromatic reactions in organic chemistry which are hindered by
ortho
substituents and these hindered enzymatic reactions within the plant.
Synthesis and Biological Activity of Brassinosteroid Analogues with a Nitrogen-Containing Side Chain
