Studies on Foliar Penetration: IV. MECHANISMS CONTROLLING THE RATE OF PENETRATION OF 2,4-DICHLOROPHENOXYACETIC ACID (2,4-D) INTO LEAVES OFPHASEOLUS VULGARIS

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Abstract
The effects of a wide range of metabolic inhibitors on the penetration of 2,4-dichlorophenoxyacetic acid (2,4-D) into the leaf disks of Phaseolus vulgaris have been studied. While recognizing the lack of specificity of most inhibitors, compounds were chosen which are known to affect respiration, phosphorylation, photosynthesis, membrane permeability, protein synthesis, and the binding capacity of membrane systems. They were: fluoride, azide, arsenite, iodoacetate, arsenate, 2,4-dinitrophenol (DNP), 3-(3,4-dichlorophenyl), -I, I-dimethylurea (DCMU), phenylmercuric chloride, octenylsuccinic acid, decenylsuccinic acid, dimethyl sulphoxide, actinomycin-D, chloramphenicol, streptomycin, 5-fluorouracil, cycloheximide, and cetyltrimethylammoniumbromide (CTAB).At sub-toxic levels all compounds had little or no influence on penetration in darkness save for iodoacetate and decenylsuccinic acid, which caused some enhanced entry at 10-4M and 10-3M respectively, and CTAB which promoted penetration at concentrations known tolower the surface tension of water.The much greater rate of penetration of 2,4-D into disks exposed to bright light (16 000 lx) is unaffected by fluoride, azide, DNP, octenylsuccinic acid, decenylsuccinic acid, dimethyl sulphoxide, or actinomycin-D. It is, however, progressively inhibited by increasing concentrations of arsenite, iodoacetate, arsenate, streptomycin, and 5-fluorouracil. Chloramphenicol, cycloheximide, and CTAB lower the rate of penetration at intermediate concentrations but at high concentrations the affect is reversed. The most active inhibitors of light-induced penetration are DCMU and phenylmercuric chloride, compounds which block the production of ATP.These results are discussed in relation to mechanisms of transport, in particular the structureand stability of barriers likely to impede penetration.

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