Abstract
In an earlier paper (2) it was concluded that a broad correlation exists between the resistance of a plant to 2-methyl-4-chlorophenoxyacetic acid (MCPA) and its ability to adsorb the growth regulator. This lent support to the suggestion that the foundation of species resistance may be partly based on the extent of MCPA adsorption to sites not concerned in the physiological response. This work has been extended since it follows that the biological activity of different growth regulator molecules on a given plant species may also be limited by the extent of their adsorption, and a correlation may exist between biological activity and the adsorption of growth regulators to plant constituents. This has already been implied by Veldstra and Booij (9) when noting that chlorination of the phenoxy compounds generally appeared to increase their activity but chlorination of naphthoxy compounds reduced their activity. Similarly, these authors consider that certain trichlorophenoxy acetic acids have exceeded the optimum size and hence optimum interaction with plant membranes, for high biological activity. Three series of acids were investigated. The phenoxyacetic and benzoic acids were selected in view of the importance of some members of these series as herbicides. The phenylacetic acids were studied as a result of earlier observations at Jealott's Hill in 1953 that 2,3-dichloro and 2,6-dichlorophenylacetic acids were compounds with biological activity similar in character but somewhat inferior to MCPA. This paper presents physical measurements of the adsorption of the acids to monolayers from the squashes prepared from oat leaves using the Langmuir trough. No biological data are presented in this paper but it is hoped it will be possible to seek a correlation when the results of biological tests on phenylacetic acids being made by various research teams become available. It is proposed to present these in Part IV of this series. MATERIALS AND METHODS
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