Temperature dependence of the diffusion of organic compounds across plant cuticles

Plant surfaces are exposed to a wide range of temperatures even under a temperate climate. The effect of temperature on the diffusion of three model compounds differing in molar volumes and lipophilicity was studied using radiolabelled 2,4-D, chlorfenvinphos, 1-(3-fluoromethylphenyl)-5phenoxy-1,2,3,4-tetrazole and cuticular membranes isolated from pear (Pyrus communis) and bitter orange (Citrus aurarntfum) leaves. Increasing temperatures from 17 to 25 °C and 25 to 34 °C caused an increase in solute mobility by up to one order of magnitude. The corresponding activation energies ED of diffusion ranged from 119 to 168 kJ/mol, resulting in Q10-values of 5 to 10. Activation energies did not depend on lipophilicity of compounds, but they were higher with cuticles of low permeability and with compounds having higher molar volumes. It follows from our data that rates of diffusion and penetration of organic chemicals into leaves and fruits increase greatly with increasing temperature and equilibrium will be obtained faster at higher temperatures. In modelling rates of sorption and penetration these facts should be taken into consideration.