Estimation of the solubility parameters of model plant surfaces and agrochemicals: a valuable tool for understanding plant surface interactions

Background Most aerial plant parts are covered with a hydrophobic lipid-rich cuticle, which is the interface between the plant organs and the surrounding environment. Plant surfaces may have a high degree of hydrophobicity because of the combined effects of surface chemistry and roughness. The physical and chemical complexity of the plant cuticle limits the development of models that explain its internal structure and interactions with surface-applied agrochemicals. In this article we introduce a thermodynamic method for estimating the solubilities of model plant surface constituents and relating them to the effects of agrochemicals. Results Following the van Krevelen and Hoftyzer method, we calculated the solubility parameters of three model plant species and eight compounds that differ in hydrophobicity and polarity. In addition, intact tissues were examined by scanning electron microscopy and the surface free energy, polarity, solubility parameter and work of adhesion of each were calculated from contact angle measurements of three liquids with different polarities. By comparing the affinities between plant surface constituents and agrochemicals derived from (a) theoretical calculations and (b) contact angle measurements we were able to distinguish the physical effect of surface roughness from the effect of the chemical nature of the epicuticular waxes. A solubility parameter model for plant surfaces is proposed on the basis of an increasing gradient from the cuticular surface towards the underlying cell wall. Conclusions The procedure enabled us to predict the interactions among agrochemicals, plant surfaces, and cuticular and cell wall components, and promises to be a useful tool for improving our understanding of biological surface interactions

Study of plant-air transfer of PCBs from an evergreen shrub: Implications for mechanisms and modeling

The depuration of gas-phase polychlorinated biphenyls (PCBs) from a slow-growing evergreen shrub, Skimmia japonica Thunb., was studied to investigate the reversibility of uptake and the compartmentalization of PCB congeners within leaves with respect to air-plant exchange processes. Depuration of PCBs was monitored over periods of hours, days, and weeks. Equilibrium had not been attained between air and leaves during the uptake phase after many weeks. Depuration followed two-phase clearance kinetics, with phase 1 occurring over the order of hours and phase 2 continuing slowly over weeks. In phase 1, a substantial part (ca. 40%) of the PCB burden that the plants had accumulated over weeks was lost in 2-3 h. This observation is further evidence for the close dynamic coupling of air and vegetation compartments. In the second phase,very slow depuration over 28 d only removed a further similar to25% of the accumulated PCB burden. Depuration rates in phase 2 varied between compounds and were not influenced by growth dilution. Depuration rates for both phases were not correlated with K-0A, indicating that plant-air mass transfer coefficients were proportional to plant-air partition coefficients and, therefore, probably dominated by the plant-side resistance to diffusion. Photolysis and metabolism are unlikely to have influenced the rates of congener disappearance. Pathways into the leaf and possible storage locations within the plant are discussed with respect to the observed differences between uptake and clearance rates. Uptake and depuration are not mirror image processes, with a fraction of accumulated PCBs effectively stored in the leaves. This has important implications for terrestrial food chain transfer and global cycling with leaf concentrations remaining elevated long after a contamination event