External leaf waxes and their modification by root-treatment of plants with trichloroacetate

Abstract

The modification of the wax ‘bloom’ of peas and brassicas by trichloroacetate which is used for selective grass killing in agriculture made examination of waxes with reference to their position on the leaf desirable. The application of sequential extraction by hydrocarbon solvents has been developed to maintain morphological separation of lipoidal substances. Physical separation of extracted substances by solvent partition was carried out before hydrolysis. Cold solvent rapidly removes hard waxes containing various parvi-substituted straight saturated paraffins from the cuticular surface. Increase of extraction time and heating releases hydrophilic oils, probably esters of eutin-forming hydroxy acids. Very little release of chlorophyll and complex substances yielding unsaturated fatty acids on hydrolysis occurs until water is removed and cell structure is broken down. The striking reduction of leaf wax content produced by treatment of the soil with trichloro­acetate has been confirmed in kale and, repeatedly, in peas, but not in nettle, where the surface wax is not particulate. The reduction is greatest in the most quickly dissolved fractions and may be confined to the particulate wax. The oils and internal fats are not apparently affected except that the former are more easily released from treated plants. Alteration in the proportions of wax constituents is not great and is influenced by environ­ mental conditions. The results seem at least as consistent with the view that the primary effect is on epidermal structure, leading to interference with wax excretion in parallel with other permeability changes, as with the view that the latter are consequent on a primary interference with the biosynthesis of waxes.