Foliar Water Uptake in Trees: Negligible or Necessary?

Fungal Hyphae on the Assimilation Branches Are Beneficial for Haloxylon ammodendron to Absorb Atmospheric Water Vapor: Adapting to an Extreme Drought Environment

Research on endophytic fungi in desert plants, particularly the epiphytic or endophytic fungi of leaves, remains limited. In the extremely arid regions of northwest China, the ultra-xerophytic desert plant Haloxylon ammodendron harbors white fungi on its assimilating branches during autumn. The hyphae of these fungi intertwine, both internally and externally, comprising superficial, bridging, and endophytic types. The superficial hyphae attach to the surface of the assimilating branches and continuously grow and intersect, forming a thick layer of felt-like hyphae. This thick, felt-like layer of hyphae facilitates the adsorption of atmospheric water vapor on the surface of the hyphae or the assimilating branches, allowing H. ammodendron to capture atmospheric moisture, even under low humidity. Some superficial hyphae penetrate the cuticle into the epidermis, becoming bridging hyphae, which can rapidly transport water from the outside of the epidermis to the inside. The endophytic hyphae shuttle within the epidermis, achieving rapid water transfer within the epidermis of the assimilating branches. The presence of these three types of hyphae not only enables the assimilating branches of H. ammodendron to achieve rapid water absorption and transmission, but also facilitates the uptake of atmospheric water vapor under low humidity conditions. We discuss the mechanism by which the hyphae promote water absorption from the perspectives of hyphal composition, the formation of felt-like structures, and environmental conditions. We consider the presence of fungal hyphae on the surface of the H. ammodendron assimilating branches as an inevitable ecological process in arid environments. This study provides important theoretical insights into the mechanisms underlying the strong drought resistance of desert plants in extremely arid regions and offers strategies for desertification control.

An Experimental Investigation of the Precipitation Utilization of Plants in Arid Regions

What represents a water source for the ecological restoration of a plant in an arid region is still up to debate. To address this issue, we conducted an in situ experiment in the Ulan Buh Desert of China, to study desert plants absorbing atmospheric water vapor. We selected Tamarisk, a common drought-salt-tolerant species in the desert, for ecological restoration as our research subject, used a newly designed lysimeter to monitor precipitation infiltration, and a sap flow system to track reverse sap flow that occurred in the shoot, branch, and stem during the precipitation event, and observed the precipitation redistribution process of the Tamarisk plot. The results showed that Tamarisk indeed directly absorbs precipitation water: when precipitation occurs, the main stem, lateral branch, and shoot all show the signs of reversed sap flow, and the reversed sap flow accounted for 21.5% of the annual sap flow in the shoot and branch, and 13.6% in the stem. The precipitation event in the desert was dominated by light precipitation events, which accounted for 81% of the annual precipitation events. It was found that light precipitation can be directly absorbed by the Tamarisk leaves, especially during nighttime or cloudy days. Even when the precipitation is absent, it was found that desert plants can still absorb water from the unsaturated atmospheric vapor; even the absorbed atmospheric water vapor was transported from the leaves to the stem, forming a reversed sap flow, as a reversed sap flow was observed when the atmospheric relative humidity reached 75%. This study indicated that the effect of light precipitation on desert plants was significant and should not be overlooked in terms of managing the ecological and hydrological systems in arid regions.

Revisiting an ecophysiological oddity: Hydathode-mediated foliar water uptake in Crassula species from southern Africa

Hydathodes are usually associated with water exudation in plants. However, foliar water uptake (FWU) through the hydathodes has long been suspected in the leaf-succulent genus Crassula (Crassulaceae), a highly diverse group in southern Africa, and, to our knowledge, no empirical observations exist in the literature that unequivocally link FWU to hydathodes in this genus. FWU is expected to be particularly beneficial on the arid western side of southern Africa, where up to 50% of Crassula species occur and where periodically high air humidity leads to fog and/or dew formation. To investigate if hydathode-mediated FWU is operational in different Crassula species, we used the apoplastic fluorescent tracer Lucifer Yellow in combination with different imaging techniques. Our images of dye-treated leaves confirm that hydathode-mediated FWU does indeed occur in Crassula and that it might be widespread across the genus. Hydathodes in Crassula serve as moisture-harvesting structures, besides their more common purpose of guttation, an adaptation that has likely played an important role in the evolutionary history of the genus. Our observations suggest that ability for FWU is independent of geographical distribution and not restricted to arid environments under fog influence, as FWU is also operational in Crassula species from the rather humid eastern side of southern Africa. Our observations point towards no apparent link between FWU ability and overall leaf surface wettability in Crassula. Instead, the hierarchically sculptured leaf surfaces of several Crassula species may facilitate FWU due to hydrophilic leaf surface microdomains, even in seemingly hydrophobic species. Overall, these results confirm the ecophysiological relevance of hydathode-mediated FWU in Crassula and reassert the importance of atmospheric humidity for some arid-adapted plant groups.

Bark water uptake through lenticels increases stem hydration and contributes to stem swelling

Foliar water uptake can recharge water storage tissue and enable greater hydration than through access to soil water alone; however, few studies have explored the role of the bark in facilitating water uptake. We investigated pathways and dynamics of bark water uptake (BWU) in stems of the mangrove Avicennia marina. We provide novel evidence that specific entry points control dynamics of water uptake through the outer bark surface. Furthermore, using a fluorescent symplastic tracer dye we provide the first evidence that lenticels on the outer bark surface facilitate BWU, thus increasing stem water content by up to 3.7%. X-ray micro-computed tomography showed that BWU was sufficient to cause measurable swelling of stem tissue layers increasing whole stem cross-sectional area by 0.83 mm2 or 2.8%, implicating it as a contributor to the diel patterns of water storage recharge that buffer xylem water potential and maintain hydration of living tissue.

Night-time warming in the field reduces nocturnal stomatal conductance and grain yield but does not alter daytime physiological responses

Global nocturnal temperatures are rising more rapidly than daytime temperatures and have a large effect on crop productivity. In particular, stomatal conductance at night (gsn ) is surprisingly poorly understood and has not been investigated despite constituting a significant proportion of overall canopy water loss. Here, we present the results of 3 yr of field data using 12 spring Triticum aestivum genotypes which were grown in NW Mexico and subjected to an artificial increase in night-time temperatures of 2°C. Under nocturnal heating, grain yields decreased (1.9% per 1°C) without significant changes in daytime leaf-level physiological responses. Under warmer nights, there were significant differences in the magnitude and decrease in gsn , values of which were between 9 and 33% of daytime rates while respiration appeared to acclimate to higher temperatures. Decreases in grain yield were genotype-specific; genotypes categorised as heat tolerant demonstrated some of the greatest declines in yield in response to warmer nights. We conclude the essential components of nocturnal heat tolerance in wheat are uncoupled from resilience to daytime temperatures, raising fundamental questions for physiological breeding. Furthermore, this study discusses key physiological traits such as pollen viability, root depth and irrigation type may also play a role in genotype-specific nocturnal heat tolerance.

Towards multivariate functional trait syndromes: Predicting foliar water uptake in trees

Analysis of functional traits is a cornerstone of ecology, yet individual traits seldom explain useful amounts of variation in species distribution or climatic tolerance, and their functional significance is rarely validated experimentally. Multivariate suites of interacting traits could build an understanding of ecological processes and improve our ability to make sound predictions of species success in our rapidly changing world. We use foliar water uptake capacity as a case study because it is increasingly considered to be a key functional trait in plant ecology due to its importance for stress-tolerance physiology. However, the traits behind the trait, that is, the features of leaves that determine variation in foliar water uptake rates, have not been assembled into a widely applicable framework for uptake prediction. Focusing on trees, we investigated relationships among 25 structural traits, leaf osmotic potential (a source of free energy to draw water into leaves), and foliar water uptake in 10 diverse angiosperm and conifer species. We identified consistent, multitrait "uptake syndromes" for both angiosperm and conifer trees, with differences in key traits revealing suspected differences in the water entry route between these two clades and an evolutionarily significant divergence in the function of homologous structures. A literature review of uptake-associated functional traits, which largely documents similar univariate relationships, provides additional support for our proposed "uptake syndrome." Importantly, more than half of shared traits had opposite-direction influences on the capacity of leaves to absorb water in angiosperms and conifers. Taxonomically targeted multivariate trait syndromes provide a useful tool for trait selection in ecological research, while highlighting the importance of micro-traits and the physiological verification of their function for advancing trait-based ecology.

High potential for foliar water uptake in early stages of leaf development of three woody angiosperms

Foliar water uptake (FWU) is a widespread mechanism that may help plants cope with drought stress in a wide range of ecosystems. FWU can be affected by various leaf traits, which change during leaf development. We exposed cut and dehydrated leaves to rainwater and measured FWU, changes in leaf water potential after 19 h of FWU (ΔΨ), minimum leaf conductance (gmin ), and leaf wettability (abaxial and adaxial) of leaves of Acer platanoides, Fagus sylvatica, and Sambucus nigra at three developmental stages: unfolding (2-5-day-old), young (1.5-week-old) and mature leaves (8-week-old). FWU and gmin were higher in younger leaves. ΔΨ corresponded to FWU and gmin in all cases but mature leaves of F. sylvatica, where ΔΨ was highest. Most leaves were highly wettable, and at least one leaf surface (adaxial or abaxial) showed a decrease in wettability from unfolding to mature leaves. Young leaves of all studied species showed FWU (unfolding leaves: 14.8 ± 1.1 μmol m-2  s-1 ), which may improve plant water status and thus counterbalance spring transpirational losses due to high gmin . The high wettability of young leaves probably supported FWU. We observed particularly high FWU and respective high ΔΨ in older leaves of F. sylvatica, possibly aided by trichomes.

Differences in phytohormone and flavonoid metabolism explain the sex differences in responses of Salix rehderiana to drought and nitrogen deposition

Due to global warming and the increase in nitrogen oxide emissions, plants experience drought and nitrogen (N) deposition. However, little is known about the acclimation to drought and N deposition of Salix species, which are dioecious woody plants. Here, an investigation into foliar N deposition combined with drought was conducted by assessing integrated phenotypes, phytohormones, transcriptomics, and metabolomics of male and female Salix rehderiana. The results indicated that there was greater transcriptional regulation in males than in females. Foliar N deposition induced an increase in foliar abscisic acid (ABA) levels in males, resulting in the inhibition of stomatal conductance, photosynthesis, carbon (C) and N accumulation, and growth, whereas more N was assimilated in females. Growth as well as C and N accumulation in drought-stressed S. rehderiana females increased after N deposition. Interestingly, drought decreased flavonoid biosynthesis whereas N deposition increased it in females. Both drought and N deposition increased flavonoid methylation in males and glycosylation in females. However, in drought-exposed S. rehderiana, N deposition increased the biosynthesis and glycosylation of flavonoids in females but decreased glycosylation in males. Therefore, foliar N deposition affects the growth and drought tolerance of S. rehderiana by altering the foliar ABA levels and the biosynthesis and modification of flavonoids. This work provides a basis for understanding how S. rehderiana may acclimate to N deposition and drought in the future.

The Plant Leaf: A Biomimetic Resource for Multifunctional and Economic Design

As organs of photosynthesis, leaves are of vital importance for plants and a source of inspiration for biomimetic developments. Leaves are composed of interconnected functional elements that evolved in concert under high selective pressure, directed toward strategies for improving productivity with limited resources. In this paper, selected basic components of the leaf are described together with biomimetic examples derived from them. The epidermis (the "skin" of leaves) protects the leaf from uncontrolled desiccation and carries functional surface structures such as wax crystals and hairs. The epidermis is pierced by micropore apparatuses, stomata, which allow for regulated gas exchange. Photosynthesis takes place in the internal leaf tissue, while the venation system supplies the leaf with water and nutrients and exports the products of photosynthesis. Identifying the selective forces as well as functional limitations of the single components requires understanding the leaf as an integrated system that was shaped by evolution to maximize carbon gain from limited resource availability. These economic aspects of leaf function manifest themselves as trade-off solutions. Biomimetics is expected to benefit from a more holistic perspective on adaptive strategies and functional contexts of leaf structures.

Foliar water uptake in Pinus species depends on needle age and stomatal wax structures

BACKGROUND AND AIMS

Foliar water uptake (FWU) has been documented in many species and is increasingly recognized as a non-trivial factor in plant-water relationships. However, it remains unknown whether FWU is a widespread phenomenon in Pinus species, and how it may relate to needle traits such as the form and structure of stomatal wax plugs. In this contribution, these questions were addressed by studying FWU in current-year and 1-year-old needles of seven Pinus species.

METHODS

We monitored FWU gravimetrically and analysed the needle surface via cryo-scanning electron microscopy. Additionally, we considered the effect of artificial wax erosion by application of the surfactant Triton X-100, which is able to alter wax crystals.

KEY RESULTS

The results show for all species that (1) FWU occurred, (2) FWU is higher in old needles compared to young needles and (3) there is substantial erosion of stomatal wax plugs in old needles. FWU was highest in Pinus canariensis, which has a thin stomatal wax plug. Surfactant treatment enhanced FWU.

CONCLUSIONS

The results of this study provide evidence for (1) widespread FWU in Pinus, (2) the influence of stomatal wax plugs on FWU and (3) age-related needle surface erosion.

The overlooked functions of trichomes: Water absorption and metal detoxication

Trichomes are epidermal outgrowths on plant shoots. Their roles in protecting plants against herbivores and in the biosynthesis of specialized metabolites have long been recognized. Recently, studies are increasingly showing that trichomes also play important roles in water absorption and metal detoxication, with these roles having important implications for ecology, the environment, and agriculture. However, these two functions of trichomes have been largely overlooked and much remains unknown. In this review, we show that the trichomes of 37 plant species belonging to 14 plant families are involved in water absorption, while the trichomes of 33 species from 13 families are capable of sequestering metals within their trichomes. The ability of trichomes to absorb water results from their decreased hydrophobicity compared to the remainder of the leaf surface as well as the presence of special structures for collecting and absorbing water. In contrast, the metal detoxication function of trichomes results not only from the good connection of their basal cells to the underlying vascular tissues, but also from the presence of metal-chelating ligands and transporters within the trichomes themselves. Knowledge gaps and critical future research questions regarding these two trichome functions are highlighted. This review improves our understanding on trichomes.

Foliar water uptake enables embolism removal in excised twigs of Avicennia marina

Embolism refilling is thought to require relaxation of xylem tension, and it is unclear whether and how tall trees or plants growing in arid or saline soils recover from embolism. We tested whether foliar water uptake could enable embolism refilling in dehydrated twigs of the grey mangrove (Avicennia marina). Four dehydrated twigs were imaged by laboratory-based micro-computed tomography before and after wetting leaves. Emboli were observed in dehydrated stems and leaves. Embolism decreased with increasing distance from the cut end of stems, suggesting that stem emboli were caused by cutting. A significant (P = 0.026) c. 80% reduction in the embolised area was observed in leaves between the start and the end of the experiment (29 ± 10 h after wetting). Embolus diameter was unaffected by wetting. Embolism refilling occurred slowly, in stems embolised by cutting and leaves embolised by cutting and/or dehydration. The lack of response of embolus diameter to wetting suggests that capillarity was not the main mechanism for refilling. Results show that excised twigs of A. marina are able to recover from embolism by absorption of atmospheric water and call for studies under natural conditions.

Response of water and photosynthetic physiological characteristics to leaf humidification in Calligonum ebinuricum

Foliar water uptake (FWU) has increasingly been regarded as a common approach for plants to obtain water under water-limited conditions. At present, the research on FWU has mostly focused on short-term experiments; the long-term FWU plant response remains unclear; Methods: Through a field in-situ humidification control experiment, the leaves of Calligonum ebinuricum N. A. Ivanova ex Soskov were humidified, and the changes of leaf water potential, gas exchange parameters and fluorescence physiological parameters of plants after long-term and short-term FWU were discussed; The main results were as follows: (1) After short-term humidification, the water potential of Calligonum ebinuricum decreased, the non-photochemical quenching (NPQ) increased, and the plant produced photoinhibition phenomenon, indicating that short-term FWU could not alleviate drought stress. (2) After long-term humidification, the leaf water potential, chlorophyll fluorescence parameter and net photosynthetic rate (Pn) increased significantly. That is to say, after long-term FWU, the improvement of plant water status promoted the occurrence of light reaction and carbon reaction, and then increased the net photosynthetic rate (Pn); Therefore, long-term FWU is of great significance to alleviate drought stress and promote Calligonum ebinuricum growth. This study will be helpful to deepen our understanding of the drought-tolerant survival mechanism of plants in arid areas.

Acid waters in tank bromeliads: Causes and potential consequences

PREMISE

The consequences of acidity for plant performance are profound, yet the prevalence and causes of low pH in bromeliad tank water are unknown despite its functional relevance to key members of many neotropical plant communities.

METHODS

We investigated tank water pH for eight bromeliad species in the field and for the widely occurring Guzmania monostachia in varying light. We compared pH changes over time between plant and artificial tanks containing a solution combined from several plants. Aquaporin transcripts were measured for field plants at two levels of pH. We investigated relationships between pH, leaf hydraulic conductance, and CO₂ concentration in greenhouse plants and tested proton pump activity using a stimulator and inhibitor.

RESULTS

Mean tank water pH for the eight species was 4.7 ± 0.06 and was lower for G. monostachia in higher light. The pH of the solution in artificial tanks, unlike in plants, did not decrease over time. Aquaporin transcription was higher for plants with lower pH, but leaf hydraulic conductance did not differ, suggesting that the pH did not influence water uptake. Tank pH and CO₂ concentration were inversely related. Fusicoccin enhanced a decrease in tank pH, whereas orthovanadate did not.

CONCLUSIONS

Guzmania monostachia acidified its tank water via leaf proton pumps, which appeared responsive to light. Low pH increased aquaporin transcripts but did not influence leaf hydraulic conductance, hence may be more relevant to nutrient uptake.

Foliar water uptake as a source of hydrogen and oxygen in plant biomass

Introductory biology lessons around the world typically teach that plants absorb water through their roots, but, unfortunately, absorption of water through leaves and subsequent transport and use of this water for biomass formation remains a field limited mostly to specialists. Recent studies have identified foliar water uptake as a significant net water source for terrestrial plants. The growing interest in the development of a new model that includes both foliar water uptake (in liquid form) and root water uptake to explain hydrogen and oxygen isotope ratios in leaf water and tree rings demands a method for distinguishing between these two water sources. Therefore, in this study, I have devised a new labelling method that utilizes two different water sources, one enriched in deuterium (HDO + D2O; δD = 7.0 × 10 4‰, δ18O = 4.1‰) and one enriched in oxygen-18 (H218O; δD = -85‰, δ18O = 1.1 × 104‰), to simultaneously label both foliar-absorbed and root-absorbed water and quantify their relative contributions to plant biomass. Using this new method, I here present evidence that, in the case of well-watered Cryptomeria japonica D. Don, hydrogen and oxygen incorporated into new leaf cellulose in the rainy season derives mostly from foliar-absorbed water (69% from foliar-absorbed water and 31% from root-absorbed water), while that of new root cellulose derives mostly from root-absorbed water (20% from foliar-absorbed water and 80% from root-absorbed water), and new branch xylem is somewhere in between (55% from foliar-absorbed water and 45% from root-absorbed water). The dual-labelling method first implemented in this study enables separate and simultaneous labelling of foliar-absorbed and root-absorbed water and offers a new tool to study the uptake, transport and assimilation processes of these waters in terrestrial plants.

Foliar water uptake as a source of hydrogen and oxygen in plant biomass

Introductory biology lessons around the world typically teach that plants absorb water through their roots, but, unfortunately, absorption of water through leaves and subsequent transport and use of this water for biomass formation remains a field limited mostly to specialists. Recent studies have identified foliar water uptake as a significant net water source for terrestrial plants. The growing interest in the development of a new model that includes both foliar water uptake (in liquid form) and root water uptake to explain hydrogen and oxygen isotope ratios in leaf water and tree rings demands a method for distinguishing between these two water sources. Therefore, in this study, I have devised a new labelling method that utilizes two different water sources, one enriched in deuterium (HDO + D2O; δD = 7.0 × 10 4‰, δ18O = 4.1‰) and one enriched in oxygen-18 (H218O; δD = -85‰, δ18O = 1.1 × 104‰), to simultaneously label both foliar-absorbed and root-absorbed water and quantify their relative contributions to plant biomass. Using this new method, I here present evidence that, in the case of well-watered Cryptomeria japonica D. Don, hydrogen and oxygen incorporated into new leaf cellulose in the rainy season derives mostly from foliar-absorbed water (69% from foliar-absorbed water and 31% from root-absorbed water), while that of new root cellulose derives mostly from root-absorbed water (20% from foliar-absorbed water and 80% from root-absorbed water), and new branch xylem is somewhere in between (55% from foliar-absorbed water and 45% from root-absorbed water). The dual-labelling method first implemented in this study enables separate and simultaneous labelling of foliar-absorbed and root-absorbed water and offers a new tool to study the uptake, transport and assimilation processes of these waters in terrestrial plants.

Tracheid buckling buys time, foliar water uptake pays it back: Coordination of leaf structure and function in tall redwood trees

Tracheid buckling may protect leaves in the dynamic environments of forest canopies, where rapid intensifications of evaporative demand, such as those brought on by changes in light availability, can result in sudden increases in transpiration rate. While treetop leaves function in reliably direct light, leaves below the upper crown must tolerate rapid, thermally driven increases in evaporative demand. Using synchrotron-based X-ray microtomography, we visualized impacts of experimentally induced water stress and subsequent fogging on living cells in redwood leaves, adding ecological and functional context through crown-wide explorations of variation in leaf physiology and microclimate. Under drought, leaf transfusion tracheids buckle, releasing water that supplies sufficient temporal reserves for leaves to reduce stomatal conductance safely while stopping the further rise of tension. Tracheid buckling fraction decreases with height and is closely coordinated with transfusion tissue capacity and stomatal conductance to provide temporal reserves optimized for local variation in microclimate. Foliar water uptake fully restores collapsed and air-filled transfusion tracheids in leaves on excised shoots, suggesting that trees may use aerial water sources for recovery. In the intensely variable deep-crown environment, foliar water uptake can allow for repetitive cycles of tracheid buckling and unbuckling, protecting the tree from damaging levels of hydraulic tension and supporting leaf survival.

Foliar Water Uptake Capacity in Six Mangrove Species

Foliar water uptake (FWU) is a mechanism that enables plants to acquire water from the atmosphere through their leaves. As mangroves live in a saline sediment water environment, the mechanism of FWU might be of vital importance to acquire freshwater and grow. The goal of this study was to assess the FWU capacity of six different mangrove species belonging to four genera using a series of submersion experiments in which the leaf mass increase was measured and expressed per unit leaf area. The foliar water uptake capacity differed between species with the highest and lowest average water uptake in Avicennia marina (Forssk.) Vierh. (1.52 ± 0.48 mg H2O cm−2) and Bruguiera gymnorhiza (L.) Lam. (0.13 ± 0.06 mg H2O cm−2), respectively. Salt-excreting species showed a higher FWU capacity than non-excreting species. Moreover, A. marina, a salt-excreting species, showed a distinct leaf anatomical trait, i.e., trichomes, which were not observed in the other species and might be involved in the water absorption process. The storage of leaves in moist Ziplock bags prior to measurement caused leaf water uptake to already occur during transport to the field station, which proportionately increased the leaf water potential (A. marina: −0.31 ± 0.13 MPa and B. gymnorhiza: −2.70 ± 0.27 MPa). This increase should be considered when performing best practice leaf water potential measurements but did not affect the quantification of FWU capacity because of the water potential gradient between a leaf and the surrounding water during submersion. Our results highlight the differences that exist in FWU capacity between species residing in the same area and growing under the same environmental conditions. This comparative study therefore enhances our understanding of mangrove species’ functioning.

Impacts of global change on the phyllosphere microbiome

Plants form complex interaction networks with diverse microbiomes in the environment, and the intricate interplay between plants and their associated microbiomes can greatly influence ecosystem processes and functions. The phyllosphere, the aerial part of the plant, provides a unique habitat for diverse microbes, and in return the phyllosphere microbiome greatly affects plant performance. As an open system, the phyllosphere is subjected to environmental perturbations, including global change, which will impact the crosstalk between plants and their microbiomes. In this review, we aim to provide a synthesis of current knowledge of the complex interactions between plants and the phyllosphere microbiome under global changes and to identify future priority areas of research on this topic.

Salix myrtillacea Female Cuttings Performed Better Than Males under Nitrogen Deposition on Leaves and Drought Conditions

Drought and nitrogen (N) deposition are major threats to global forests under climate change. However, investigation into how dioecious woody species acclimate to drought and N deposition and how this is influenced by gender has, so far, been unexplored. We examined the phenotypic and physiological changes in Salix myrtillacea females and males under 60 d drought, and wet N deposition on leaves’ treatments. Drought inhibited their growth by limiting water acquisition, photosynthesis, and increasing oxidative stress, especially in males. However, females exhibited greater drought resistance than males due to their better water acquisition ability and instantaneous water use efficiency (WUEleaf), higher foliar abscisic acid (ABA) and auxin (IAA) levels and greater antioxidase activities. N deposition increased foliar ABA, H2O2 accumulation, and reduced N distribution to the leaves, causing restricted photosynthesis and aerial growth in males. Interestingly, N deposition improved biomass accumulation in both the genders under drought, with greater positive effects on drought-stressed males by increasing their radial growth and causing greater N distribution to the leaves, increased foliar IAA and reduced oxidative stress. Regardless, S. myrtillacea females still showed better growth and drought resistance than males under both drought and N deposition. The females’ superior performance indicated that they are more appropriate for forestation, thus supporting the dominant gender’s selection in the afforestation of unisexual S. myrtillacea in drought and severe N deposition regions.

Foliar water uptake does not contribute to embolism repair in beech (Fagus sylvatica L.)

BACKGROUND AND AIMS

Foliar water uptake has recently been suggested as a possible mechanism for the restoration of hydraulically dysfunctional xylem vessels. In this paper we used a combination of ecophysiological measurements, X-ray microcomputed tomography and cryo-scanning electron microscopy during a drought treatment to fully evaluate this hypothesis.

KEY RESULTS

Based on an assessment of these methods in beech (Fagus sylvatica L.) seedlings we were able to (1) confirm an increase in the amount of hydraulically redistributed water absorbed by leaves when the soil water potential decreased, and (2) locate this redistributed water in hydraulically active vessels in the stem. However, (3) no embolism repair was observed irrespective of the organ under investigation (i.e. stem, petiole or leaf) or the intensity of drought.

CONCLUSIONS

Our data provide evidence for a hydraulic pathway from the leaf surface to the stem xylem following a water potential gradient, but this pathway exists only in functional vessels and does not play a role in embolism repair for beech.

Shoot dimorphism enables Sequoia sempervirens to separate requirements for foliar water uptake and photosynthesis

PREMISE

Trees in wet forests often have features that prevent water films from covering stomata and inhibiting gas exchange, while many trees in drier environments use foliar water uptake to reduce water stress. In forests with both wet and dry seasons, evergreen trees would benefit from producing leaves capable of balancing rainy-season photosynthesis with summertime water absorption.

METHODS

Using samples collected from across the vertical gradient in tall redwood (Sequoia sempervirens) crowns, we estimated tree-level foliar water uptake and employed physics-based causative modeling to identify key functional traits that determine uptake potential by setting hydraulic resistance.

RESULTS

We showed that Sequoia has two functionally distinct shoot morphotypes. While most shoots specialize in photosynthesis, the axial shoot type is capable of much greater foliar water uptake, and its within-crown distribution varies with latitude. A suite of leaf surface traits cause hydraulic resistance, leading to variation in uptake capacity among samples.

CONCLUSIONS

Shoot dimorphism gives tall Sequoia trees the capacity to absorb up to 48 kg H₂ O h^-1 during the first hour of leaf wetting, ameliorating water stress while presumably maintaining high photosynthetic capacity year round. Geographic variation in shoot dimorphism suggests that plasticity in shoot-type distribution and leaf surface traits helps Sequoia maintain a dominate presence in both wet and dry forests.

Transcriptome profiling reveals that foliar water uptake occurs with C3 and crassulacean acid metabolism facultative photosynthesis in Tamarix ramosissima under extreme drought

Tamarix ramosissima is a typical desert plant species that is widely distributed in the desert areas of Northwest China. It plays a significant role in sand fixation and soil water conservation. In particular, how it uses water to survive in the desert plays an important role in plant growth and ecosystem function. Previous studies have revealed that T. ramosissima can alleviate drought by absorbing water from its leaves under extreme drought conditions. To date, there is no clear molecular regulation mechanism to explain foliar water uptake (FWU). In the present study, we correlated diurnal meteorological data, sap flow and photosynthetic parameters to determine the physical and biological characteristics of FWU. Our results suggested that the lesser the groundwater, the easier it is for T. ramosissima to absorb water via the leaves. Gene ontology annotation and Kyoto Encyclopaedia of Genes and Genomes pathway analysis of the transcriptome profile of plants subjected to high humidity suggested that FWU was highly correlated to carbohydrate metabolism, energy transfer, pyruvate metabolism, hormone signal transduction and plant-pathogen interaction. Interestingly, as a C₃ plant, genes such as PEPC, PPDK, MDH and RuBP, which are involved in crassulacean acid metabolism (CAM) photosynthesis, were highly upregulated and accompanied by FWU. Therefore, we proposed that in the case of sufficient water supply, C₃ photosynthesis is used in T. ramosissima, whereas in cases of extreme drought, starch is degraded to provide CO₂ for CAM photosynthesis to make full use of the water obtained via FWU and the water that was transported or stored to assimilating branches and stems. This study may provide not only an important theoretical foundation for FWU and conversion from C₃ plants to CAM plants but also for engineering improved photosynthesis in high-yield drought-tolerant plants and mitigation of climate change-driven drought.

Asymmetric Mass Transport through Dense Heterogeneous Polymer Membranes: Fundamental Principles, Lessons from Nature, and Artificial Systems

Many organisms rely on directional water transport schemes for the purpose of water retention and collection. Directional transport of water and other fluids is also technologically relevant, for example to harvest water, in separation processes, packaging solutions, functional clothing, and many other applications. One strategy to promote mass transport along a preferential direction is to create compositionally asymmetric, multi-layered, or compositionally graded architectures. In recent years, the investigation of natural and artificial membranes based on this design has attracted growing interest and allowed researchers to develop a good understanding of how the properties of such membranes can be tailored to meet the demands of particular applications. Here a summary of theoretical works on mass transport through dense asymmetric membranes, comprehensive reviews of biological and artificial membranes featuring this design, and a discussion of applications, remaining questions, and opportunities are provided.

Cuticular wax composition contributes to different strategies of foliar water uptake in six plant species from foggy rupestrian grassland in tropical mountains

The cuticle is the outermost region of the epidermal cell wall of plant aerial organs. The cuticle acts as a two-way lipid barrier for water diffusion; therefore, it plays a vital role in foliar water uptake (FWU). We hypothesised that the chemical composition of the cuticular waxes influences the FWU strategy that plants adopt in a foggy tropical ecosystem. We analysed the leaf cuticular waxes of six plant species known by their different FWU strategies, in both qualitative and quantitative approaches, to test this hypothesis. We also investigated the fine structure of the plant cuticle by scanning electron microscopy. Neither the total wax loads nor the amounts of single wax compound classes correlated to the FWU. In contrast, the qualitative chemical composition of the cuticular waxes was related to the water absorption speed but not to the maximum water absorbed. The presence of wax crystals might interfere with the FWU. Our findings suggest that a complex three-dimensional network of the cuticular compounds contributes to different strategies of FWU in six plant species from foggy tropical mountaintops.

Foliar water uptake via cork warts in mangroves of the Sonneratia genus

Foliar water uptake (FWU) occurs in plants of diverse ecosystems; however, the diversity of pathways and their associated FWU kinetics remain poorly resolved. We characterized a novel FWU pathway in two mangrove species of the Sonneratia genus, S. alba and S. caseolaris. Further, we assessed the influence of leaf wetting duration, wet-dry seasonality and leaf dehydration on leaf conductance to surface water (K_surf ). The symplastic tracer dye, disodium fluorescein, revealed living cells subtending and encircling leaf epidermal structures known as cork warts as a pathway of FWU entry into the leaf. Rehydration kinetics experiments revealed a novel mode of FWU, with slow and steady rates of water uptake persistent over a duration of 12 hr. K_surf increased with longer durations of leaf wetting and was greater in leaves with more negative water potentials at the initiation of leaf wetting. K_surf declined by 68% between wet and dry seasons. Our results suggest that FWU via cork warts in Sonneratia sp. may be rate limited and under active regulation. We conclude that FWU pathways in halophytes may require ion exclusion to avoid uptake of salt when inundated, paralleling the capacity of halophyte roots for ion selectivity during water acquisition.

Harvesting water from unsaturated atmospheres: deliquescence of salt secreted onto leaf surfaces drives reverse sap flow in a dominant arid climate mangrove, Avicennia marina

The mangrove Avicennia marina adjusts internal salt concentrations by foliar salt secretion. Deliquescence of accumulated salt causes leaf wetting that may provide a water source for salt-secreting plants in arid coastal wetlands where high nocturnal humidity can usually support deliquescence whereas rainfall events are rare. We tested the hypotheses that salt deliquescence on leaf surfaces can drive top-down rehydration, and that such absorption of moisture from unsaturated atmospheres makes a functional contribution to dry season shoot water balances. Sap flow and water relations were monitored to assess the uptake of atmospheric water by branches during shoot wetting events under natural and manipulated microclimatic conditions. Reverse sap flow rates increased with increasing relative humidity from 70% to 89%, consistent with function of salt deliquescence in harvesting moisture from unsaturated atmospheres. Top-down rehydration elevated branch water potentials above those possible from root water uptake, subsidising transpiration rates and reducing branch vulnerability to hydraulic failure in the subsequent photoperiod. Absorption of atmospheric moisture harvested through deliquescence of salt on leaf surfaces enhances water balances of Avicennia marina growing in hypersaline wetlands under arid climatic conditions. Top-down rehydration from these frequent, low intensity wetting events contributes to prevention of carbon starvation and hydraulic failure during drought.

Dew water-uptake pathways in Negev desert plants: a study using stable isotope tracers

Dew is an important water resource for plants in most deserts. The mechanism that allows desert plants to use dew water was studied using an isotopic water tracer approach. Most plants use water directly from the soil; the roots transfer the water to the rest of the plant, where it is required for all metabolic functions. However, many plants can also take up water into their leaves and stems. Examining the dew water uptake pathways in desert plants can lend insight on another all water-use pathways examination. We determined where and how dew water enters plants in the water limited Negev desert. Highly depleted isotopic water was sprayed on three different dominant plant species of the Negev desert-Artemesia sieberi, Salsola inermis and Haloxylon scoparium-and its entry into the plant was followed. Water was sprayed onto the soil only, or on the leaves/stems only (with soil covered to prevent water entry via root uptake). Thereafter, the isotopic composition of water in the roots and stems were measured at various time points. The results show that each plant species used the dew water to a different extent, and we obtained evidence of foliar uptake capacity of dew water that varied depending on the microenvironmental conditions. A. sieberi took up the greatest amount of dew water through both stems and roots, S. inermis took up dew water mainly from the roots, and H. scoparium showed the least dew capture overall.

Photosynthetic heat tolerance in plants with different foliar water -uptake strategies

PREMISE

The distribution and even the survival of plant species are influenced by temperature. In an old climatically buffered infertile landscape (OCBIL) in Brazil, we previously characterized different strategies for foliar water uptake (FWU). It is possible that photosystem II tolerance to heat and excessive light intensity varies among species with different FWU capacities.

METHODS

The relationship between FWU, photoinhibition, and thermotolerance was investigated in seven species from this ecosystem.

RESULTS

The species with slow water absorption and high water absorption are those that presented less photoinhibition. Contrastingly, the species that have fast and low water absorption presented greater thermotolerance when their leaves are totally hydrated. However, when there is greater leaf dehydration, the most thermotolerant species were those with slow but high water absorption.

CONCLUSIONS

Foliar water uptake is an important trait for plants to tolerate excessive light intensity and higher temperatures. Plants in this OCBIL may be differentially affected by future global warming, and the best strategy to deal with this expected climate change is with slow and high absorption of water.

Asymmetric water transport in dense leaf cuticles and cuticle-inspired compositionally graded membranes

Most of the aerial organs of vascular plants are covered by a protective layer known as the cuticle, the main purpose of which is to limit transpirational water loss. Cuticles consist of an amphiphilic polyester matrix, polar polysaccharides that extend from the underlying epidermal cell wall and become less prominent towards the exterior, and hydrophobic waxes that dominate the surface. Here we report that the polarity gradient caused by this architecture renders the transport of water through astomatous olive and ivy leaf cuticles directional and that the permeation is regulated by the hydration level of the cutin-rich outer cuticular layer. We further report artificial nanocomposite membranes that are inspired by the cuticles' compositionally graded architecture and consist of hydrophilic cellulose nanocrystals and a hydrophobic polymer. The structure and composition of these cuticle-inspired membranes can easily be varied and this enables a systematic investigation of the water transport mechanism.

On Change of Soil Moisture Distribution With Vegetation Reconstruction in Mu Us Sandy Land of China, With Newly Designed Lysimeter

BACKGROUND

China's so-called Three North Shelterbelt Program (3NSP) has produced a vast area of lined forest reconstruction in the semi-arid regions. This study uses the lined rain-fed Pinus sylvestris var. mongolica (PSM) sand-fixing forest in the eastern part of Mu Us Sandy Land in Northwestern China as an example to investigate the ecohydrological process in this region. Rain gauges, newly designed lysimeters and soil moisture sensors are used to monitor precipitation, deep soil recharge (DSR) and soil water content, where DSR specifically refers to recharge that can reach a depth more than 200 cm and eventually replenish the underneath groundwater reservoir.

RESULTS

This study shows that there are two obvious moisture recharge processes in an annual base for the PSM forest soil: a snowmelt-related recharge process in the spring and a precipitation-related recharge process in the summer. The recharge depth of the first process can reach 180 cm without DSR occurring (in 2018). The second process results in noticeable DSR in 2018. Specifically, the DSR values over 2016-2018 are 1, 0.2, and 1.2 mm, respectively. To reach the recharge depths of 20, 40, 80, 120, 160, and 200 cm, the required precipitation intensities have to be 2.6, 3.2, 3.4, 8.2, 8.2, and 13.2 mm/d, respectively. The annual evapotranspiration in the PSM forest is 466.94 mm in 2016, 324.60 mm in 2017, and 183.85 mm in 2018.

CONCLUSION

This study concludes that under the current precipitation conditions (including both dry- and wet-years such as 2016-2018), water consumption of PSM somewhat equals to the precipitation amount, and PSM has evolved over years to regulate its evapotranspiration in response to annual precipitation fluctuations in Mu Us Sandy Land of China.

Foliar water and solute absorption: an update

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.

The Neglected Reverse Water Pathway: Atmosphere-Plant-Soil Continuum

The soil-plant-atmosphere continuum (SPAC) describes the continuous water movement from soil via plants to atmosphere. Here, we propose to name the reverse water pathway, driven by foliar water uptake, the atmosphere-plant-soil continuum (APSC). We highlight the different hydraulic resistances this reverse water movement has to overcome.

Wettability, water absorption and water storage in rosette leaves of the dragon tree (Dracaena draco L.)

Leaf surfaces of Dracaena draco are wettable and can absorb water. The thick, basal leaf part could act as a water reservoir that changes in volume with plant hydration. Rosettes of leaves of Dracaena draco play an important role in directing fog water through leaf axils into the stem tissues, where it can be stored for further use. However, how water is intercepted and collected by the leaves remains unclear, especially since leaf blade surfaces are considered hydrophobic. Based on the observations of D. draco individuals growing in Spain and in glasshouse conditions in Poland, we hypothesised that their long leaves (~ 70 cm) are able to absorb water along the whole leaf blade, but leaf age affects this process. We used water droplet contact angle measurements, anatomical analyses of leaf cross sections along the age gradient and dye tracer experiments to test this hypothesis. The data showed that the leaf surfaces of D. draco are wettable. In general, the mature leaves of the rosette are more wettable than the young ones. Water can be absorbed both through the adaxial and abaxial surfaces. The hydrenchyma is not uniformly distributed along the leaf, it is especially abundant towards the leaf base where it forms a massive water reservoir, which changes in volume depending on plant water status. The results of these studies shed light on the role of rosettes in water absorption by D. draco, and broaden our understanding of the functioning of this vulnerable species.

Identifying the pathways for foliar water uptake in beech (Fagus sylvatica L.): a major role for trichomes

Foliar water uptake (FWU), the direct uptake of water into leaves, is a global phenomenon, having been observed in an increasing number of plant species. Despite the growing recognition of its functional relevance, our understanding of how FWU occurs and which foliar surface structures are implicated, is limited. In the present study, fluorescent and ionic tracers, as well as microcomputed tomography, were used to assess potential pathways for water entry in leaves of beech, a widely distributed tree species from European temperate regions. Although none of the tracers entered the leaf through the stomatal pores, small amounts of silver precipitation were observed in some epidermal cells, indicating moderate cuticular uptake. Trichomes, however, were shown to absorb and redistribute considerable amounts of ionic and fluorescent tracers. Moreover, microcomputed tomography indicated that 72% of empty trichomes refilled during leaf surface wetting and microscopic investigations revealed that trichomes do not have a cuticle but are covered with a pectin-rich cell wall layer. Taken together, our findings demonstrate that foliar trichomes, which exhibit strong hygroscopic properties as a result of their structural and chemical design, constitute a major FWU pathway in beech.