Seedling shrubs in the Mediterranean semi‐arid climate are subjected to intense droughts during summer. Thus, seedlings often surpass their limits of tolerance to water stress, resulting in the loss of hydraulic conductivity due to xylem cavitation. The response in terms of stomatal conductance, vulnerability to cavitation, leaf dieback, and survival were analysed in two co‐occurring seedlings of mastic tree (Pistacia lentiscus L.) and kermes oak (Quercus coccifera L.) during an intense drought period. Both species reacted to drought with steep decreases in stomatal conductance before the critical water potential brought about the onset of cavitation events. Q. coccifera showed wider safety margins for avoiding runaway embolism than P. lentiscus and these differences could be related to the particular drought strategy displayed by each species: water saver or water spender. The limits for survival, resprout capacity and leaf dieback were also analysed in terms of loss of conductivity. By contrast with previous studies, the species showing higher seedling survival in the presence of drought also showed higher susceptibility to cavitation and operated with a lower safety margin for cavitation. Both species showed a leaf specific conductivity (LSC) threshold below which leaf biomass had to be regulated to avoid runaway embolism. However, each species displayed a different type of response: P. lentiscus conserved total leaf area up to 100% loss of LSC, whereas Q. coccifera continuously adjusted leaf biomass throughout the drought period in order to maintain the LSC very close to the maximum values recorded without loss of conductivity. Both species maintained the capacity for survival until the loss of conductivity was very nearly 100%.

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.

SUMMARY: The absorption of water and solutes by plant leaves has been recognised since more than two centuries. Given the polar nature of water and solutes, the mechanisms of foliar uptake have been proposed to be similar for water and electrolytes, including nutrient solutions. Research efforts since the 19th century focussed on characterising the properties of cuticles and applying foliar sprays to crop plants as a tool for improving crop nutrition. This was accompanied by the development of hundreds of studies aimed at characterising the chemical and structural nature of plant cuticles from different species and the mechanisms of cuticular and, to a lower extent, stomatal penetration of water and solutes. The processes involved are complex and will be affected by multiple environmental, physico‐chemical and physiological factors which are only partially clear to date. During the last decades, the body of evidence that water transport across leaf surfaces of native species may contribute to water balances (absorption and loss) at an ecosystem level has grown. Given the potential importance of foliar water absorption for many plant species and ecosystems as shown in recent studies, the aim of this review is to first integrate current knowledge on plant surface composition, structure, wettability and physico‐chemical interactions with surface‐deposited matter. The different mechanisms of foliar absorption of water and electrolytes and experimental procedures for tracing the uptake process are discussed before posing several outstanding questions which should be tackled in future studies.

Plant functioning and survival in drylands are affected by the combination of high solar radiation, high temperatures, low relative humidity, and the scarcity of available water. Many ecophysiological studies have dealt with the adaptation of plants to cope with these stresses in hot deserts, which are the territories that have better evoked the idea of a dryland. Nevertheless, drylands can also be found in some other areas of the Earth that are under the Mediterranean-type climates, which imposes a strong aridity during summer. In this review, plant species from hot deserts and Mediterranean-type climates serve as examples for describing and analyzing the different responses of trees and shrubs to aridity in drylands, with special emphasis on the structural and functional adaptations of plants to avoid the negative effects of high temperatures under drought conditions. First, we analyze the adaptations of plants to reduce the input of energy by diminishing the absorbed solar radiation through (i) modifications of leaf angle and (ii) changes in leaf optical properties. Afterwards, we analyze several strategies that enhance the ability for heat dissipation through (i) leaf size reduction and changes in leaf shape (e.g., through lobed leaves), and (ii) increased transpiration rates (i.e., water-spender strategy), with negative consequences in terms of photosynthetic capacity and water consumption, respectively. Finally, we also discuss the alternative strategy showed by water-saver plants, a common drought resistance strategy in hot and dry environments that reduces water consumption at the expense of diminishing the ability for leaf cooling. In conclusion, trees and shrubs living in drylands have developed effective functional adaptations to cope with the combination of high temperature and water scarcity, all of them with clear benefits for plant functioning and survival, but also with different costs concerning water use, carbon gain, and/or leaf cooling.