STUDIES ON PLANT GROWTH‐REGULATING SUBSTANCES

The cress‐root test and the maize‐root segment tests, both of which depend on the measurement of root‐growth inhibition, have been used to assess the growth‐regulating activity of a wide range of phenoxyalkanecarboxylic acids. The results are presented and compared with those obtained with the same compounds in other tests in which shoot tissues were employed. The results are also discussed in relation to current theories on mode of action and structure/activity relationship. These studies included the ‘α‐hydrogen’ effect, the activity of stereoisomers and activity of homologues of seventeen series of ω‐phenoxyalkanecarboxylic acids, considered in relation to their degradation within the tissues by β‐oxidation. Earlier findings were largely confirmed. Compounds which had previously been shown to be active in shoot tests such as the wheat‐cylinder, the pea‐curvature and the pea‐segment tests, were found to be active in the root‐growth inhibition tests and, furthermore, the degree of activity was usually similar. It is concluded that the two root tests investigated are of real value in assessing the activity of phenoxy acids in studies on the relationships between chemical structure and plant growth‐regulating activity.