Chemical composition and micromorphological structure of cuticular leaf waxes of eight tropical fern species of Mexico

Plants of the humid tropics seal their leaf surfaces with hydrophobic waxes to keep them dry and clean by repelling water and diminishing the adhesion of spores of pathogenic and epiphyllous organisms. We investigated the chemical composition and micromorphological structure of waxes of the upper leaf surfaces of six terrestrial (Diplazium drepanolobium A.R.Sm., Bolbitis portoricensis (Spreng.) Hennipman, Mickelia pergamentacea (Maxon) R.C.Moran, Labiak and Sundue, Danaea nodosa (L.) Sm., Polypodium rhachipterygium Liebm. and Goniopteris rhachiflexuosa (Riba) Salino and T.E.Almeida) and two hemiepiphytic (Mickelia bernoullii (Kuhn ex Christ) R.C.Moran, Labiak and Sundue and Lomariopsis recurvata Fée) tropical fern species. Wax extracts were obtained with CHCl3 and MeOH, analyzed by GC-MS and ultra-high-performance LC-MS, and wax compounds were identified with the library NIST 2.0D, NIST/EPA/NIH (NIST05), and the database Metlin, respectively. In addition, n-alkanes were identified by comparing their retention times and spectrometric means against standards (C10C40). In all taxa, the wax layer consisted of one of three types of coatings, none building wax crystals. With CHCl3 and MeOH extraction, we obtained 21–740 μg/cm2 and 6–33 μg/cm2 of cuticular waxes, respectively. In all eight species, fatty acids were the most abundant wax component (68–91% of total), of which hexadecanoic (30.7–48.2%) and octadecanoic (20.7–32.1%) acids were predominant. In lower abundance, alkanes (6.2–13.2%), aldehydes (<11.3%), esters (<4.05%), ketones (<1.4%) and primary alcohols (<1.0%) were detected. All compounds had carbon chains of C9C36 length, as reported for wax metabolites of mosses and seed plants. Five phenolic compounds were also detected: benzoic acid in five taxa (B. portoricensis, D. drepanolobium, L. recurvata, M. bernoullii and P. rhachipterygium), cinnamic acid and 2-ethylphenol in D. nodosa, ferulic acid in B. portoricensis, and a type of flavonoid in M. bernoullii. A sesquiterpene was found in D. drepanolobium. The composition of leaf waxes in hemiepiphytic species did not vary as much as in terrestrial species, because of their high similarity of fatty acids and alkanes, and the lack of aldehydes.