Plant trichomes play important protective functions and may have a major influence on leaf surface wettability. With the aim of gaining insight into trichome structure, composition, and function in relation to water-plant surface interactions, we analyzed the adaxial and abaxial leaf surface of holm oak (Quercus ilex) as a model. By measuring the leaf water potential 24 h after the deposition of water drops onto abaxial and adaxial surfaces, evidence for water penetration through the upper leaf side was gained in young and mature leaves. The structure and chemical composition of the abaxial (always present) and adaxial (occurring only in young leaves) trichomes were analyzed by various microscopic and analytical procedures. The adaxial surfaces were wettable and had a high degree of water drop adhesion in contrast to the highly unwettable and water-repellent abaxial holm oak leaf sides. The surface free energy and solubility parameter decreased with leaf age, with higher values determined for the adaxial sides. All holm oak leaf trichomes were covered with a cuticle. The abaxial trichomes were composed of 8% soluble waxes, 49% cutin, and 43% polysaccharides. For the adaxial side, it is concluded that trichomes and the scars after trichome shedding contribute to water uptake, while the abaxial leaf side is highly hydrophobic due to its high degree of pubescence and different trichome structure, composition, and density. Results are interpreted in terms of water-plant surface interactions, plant surface physical chemistry, and plant ecophysiology.

As superficial structures, non-glandular trichomes, protect plant organs against multiple biotic and abiotic stresses. The protective and defensive roles of these epidermal appendages are crucial to developing organs and can be attributed to the excellent combination of suitable structural traits and chemical reinforcement in the form of phenolic compounds, primarily flavonoids. Both the formation of trichomes and the accumulation of phenolics are interrelated at the molecular level. During the early stages of development, non-glandular trichomes show strong morphological similarities to glandular ones such as the balloon-like apical cells with numerous phenolics. At later developmental stages, and during secondary wall thickening, phenolics are transferred to the cell walls of the trichomes. Due to the diffuse deposition of phenolics in the cell walls, trichomes provide protection against UV-B radiation by behaving as optical filters, screening out wavelengths that could damage sensitive tissues. Protection from strong visible radiation is also afforded by increased surface light reflectance. Moreover, the mixtures of trichome phenolics represent a superficial chemical barrier that provides protection against biotic stress factors such as herbivores and pathogens. Although the cells of some trichomes die at maturity, they can modulate their quantitative and qualitative characteristics during development, depending on the prevailing conditions of the external biotic or abiotic environment. In fact, the structure and chemical constituents of trichomes may change due to the particular light regime, herbivore damage, wounding, water stress, salinity and the presence of heavy metals. Hence, trichomes represent dynamic protective structures that may greatly affect the outcome of many plant–environment interactions.

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The surface of peach (Prunus persica 'Calrico') is covered by a dense indumentum, which may serve various protective purposes. With the aim of relating structure to function, the chemical composition, morphology, and hydrophobicity of the peach skin was assessed as a model for a pubescent plant surface. Distinct physicochemical features were observed for trichomes versus isolated cuticles. Peach cuticles were composed of 53% cutan, 27% waxes, 23% cutin, and 1% hydroxycinnamic acid derivatives (mainly ferulic and p-coumaric acids). Trichomes were covered by a thin cuticular layer containing 15% waxes and 19% cutin and were filled by polysaccharide material (63%) containing hydroxycinnamic acid derivatives and flavonoids. The surface free energy, polarity, and work of adhesion of intact and shaved peach surfaces were calculated from contact angle measurements of water, glycerol, and diiodomethane. The removal of the trichomes from the surface increased polarity from 3.8% (intact surface) to 23.6% and decreased the total surface free energy chiefly due to a decrease on its nonpolar component. The extraction of waxes and the removal of trichomes led to higher fruit dehydration rates. However, trichomes were found to have a higher water sorption capacity as compared with isolated cuticles. The results show that the peach surface is composed of two different materials that establish a polarity gradient: the trichome network, which has a higher surface free energy and a higher dispersive component, and the cuticle underneath, which has a lower surface free energy and higher surface polarity. The significance of the data concerning water-plant surface interactions is discussed within a physiological context.