Quantitative and qualitative analysis of cutin in maize and a maize-cropped soil: Comparison of CuO oxidation, transmethylation and saponification methods

To evaluate the qualitative and quantitative information obtained through three commonly used methods for cutin analysis in plants and soils (saponification, CuO oxidation and transmethylation) and understand the chemical mechanisms involved during the depolymerisation, we compared the three methods using a sample of maize leaves, where cutin might be more intact and quantitatively more important than in soil. Among the aliphatic monomers identified with the different methods, special attention was paid to the hydroxy acids and diacids with a chain length of 16 and 18 carbons, which are considered to be specific for cutin. CuO oxidation afforded seven cutin structural units in low concentration (0.41 mg g−1 dry leaves) possibly because of chemical alteration of the monomers under the drastic conditions. Transmethylation afforded only four structural units (0.37 mg g−1 dry leaves), possibly because of steric exclusion of the reagent from the more cross-linked parts of the macromolecules. Saponification, affording the highest concentration (1.7 mg g−1 dry leaves), including eight different compounds, was also successfully applied to a loamy maize-cultivated soil with low carbon content (12.9 mg OC g−1 soil). Except for 9,10-dihydroxyoctadecanoic acid, released from maize leaves, all the structural units specific for maize cutin could be identified and reached a concentration of 19.6 μg g−1 soil. The concentrations of the monomers were estimated in soil using this method, with the coefficient of variation ranging from 2% to 9%.