Most aerial plant surfaces are covered with a lipid-rich cuticle, which is a barrier for the bidirectional transport of substances between the plant and the surrounding environment. This review article provides an overview of the significance of the leaf cuticle as a barrier for the deposition and absorption of water and electrolytes. After providing insights into the physico-chemical properties of plant surfaces, the mechanisms of foliar absorption are revised with special emphasis on solutes. Due to the limited information and relative importance of the leaf cuticle of herbaceous and deciduous cultivated plants, an overview of the studies developed with Alpine conifers and treeline species is provided. The significance of foliar water uptake as a phenomenon of ecophysiological relevance in many areas of the world is also highlighted. Given the observed variability in structure and composition among, for example, plant species and organs, it is concluded that it is currently not possible to establish general permeability and wettability models that are valid for predicting liquid-surface interactions and the subsequent transport of water and electrolytes across plant surfaces.

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.

Abies alba and Abies pinsapo are closely related species with the same ribulose 1·5-bisphosphate carboxylase/ oxygenase (Rubisco) large subunit (rbcL) but contrasting hydraulic traits and mesophyll structure occurring in the Iberian Peninsula under contrasting conditions. As photosynthesis and hydraulic capacities often co-scale, we hypothesize that these species differ in mesophyll conductance to CO2 (gm). gm and key anatomical traits were measured in both species. Drought-adapted population of A. pinsapo has higher photosynthesis than the more mesic population of A. alba, in agreement with its higher hydraulic capacity. However, A. alba exhibits the largest stomatal conductance (gs), and so water use efficiency (WUE) is much higher in A. pinsapo. The differences in photosynthesis were explained by differences in gm, indicating a correlation between hydraulic capacity and gm. We report a case where gm is the main factor limiting photosynthesis in one species (A. alba) when compared with the other one (A. pinsapo). The results also highlight the discrepancy between gm estimates based on anatomical measurements and those based on gas exchange methods, probably due to the very large resistance exerted by cell walls and the stroma in both species. Thus, the cell wall and chloroplast properties in relation to CO2 diffusion constitute a nearfuture research priority.

Leaf mass per area (LMA) has been suggested to negatively affect the mesophyll conductance to CO (g ), which is the most limiting factor for area-based photosynthesis (A ) in many Mediterranean sclerophyll species. However, despite their high LMA, these species have similar A to plants from other biomes. Variations in other leaf anatomical traits, such as mesophyll and chloroplast surface area exposed to intercellular air space (S /S and S /S), may offset the restrictions imposed by high LMA in g and A in these species. Seven sclerophyllous Mediterranean oaks from Europe/North Africa and North America with contrasting LMA were compared in terms of morphological, anatomical and photosynthetic traits. Mediterranean oaks showed specific differences in A that go beyond the common morphological leaf traits reported for these species (reduced leaf area and thick leaves). These variations resulted mainly from the differences in g , the most limiting factor for carbon assimilation in these species. Species with higher A showed increased S /S, which implies increased g without changes in stomatal conductance. The occurrence of this anatomical adaptation at the cell level allowed evergreen oaks to reach A values comparable to congeneric deciduous species despite their higher LMA.