The multiple roles of trichomes in two Croton species

Trichomes are common in plants from dry environments, and despite their recognized role in protection and defense, little is known about their role as absorptive structures and in other aspects of leaf ecophysiology. We combine anatomical and ecophysiological data to evaluate how trichomes affect leaf gas exchange and water balance during drought. We studied two congeneric species with pubescent leaves which co-occur in Brazilian Caatinga: Croton blanchetianus (dense trichomes) and Croton adenocalyx (sparse trichomes). We found a novel foliar water uptake (FWU) pathway in C. blanchetianus composed of stellate trichomes and underlying epidermal cells and sclereids that interconnect the trichomes from both leaf surfaces. The water absorbed by these trichomes is redistributed laterally by pectin protuberances on mesophyll cell walls. This mechanism enables C. blanchetianus leaves to absorb water more efficiently than C. adenocalyx. Consequently, the exposure of C. blanchetianus to dew during drought improved its leaf gas exchange and water status more than C. adenocalyx. C. blanchetianus trichomes also increase their leaf capacity to reflect light and maintain lower temperatures during drought. Our results emphasize the multiple roles that trichomes might have on plant functioning and the importance of FWU for the ecophysiology of Caatinga plants during drought.

Dew benefits on alpine grasslands are cancelled out by combined heatwave and drought stress

Increasing frequencies of heatwaves combined with simultaneous drought stress in Europe threaten the ecosystem water and carbon budgets of alpine grasslands. Dew as an additional water source can promote ecosystem carbon assimilation. It is known that grassland ecosystems keep high evapotranspiration as long as soil water is available. However, it is rarely being investigated whether dew can mitigate the impact of such extreme climatic events on grassland ecosystem carbon and water exchange. Here we use stable isotopes in meteoric waters and leaf sugars, eddy covariance fluxes for H₂O vapor and CO₂, in combination with meteorological and plant physiological measurements, to investigate the combined effect of dew and heat-drought stress on plant water status and net ecosystem production (NEP) in an alpine grassland (2000 m elevation) during the June 2019 European heatwave. Before the heatwave, enhanced NEP in the early morning hours can be attributed to leaf wetting by dew. However, dew benefits on NEP were cancelled out by the heatwave, due to the minor contribution of dew in leaf water. Heat-induced reduction in NEP was intensified by the combined effect of drought stress. The recovery of NEP after the peak of the heatwave could be linked to the refilling of plant tissues during nighttime. Among-genera differences of plant water status affected by dew and heat-drought stress can be attributed to differences in their foliar dew water uptake, and their reliance on soil moisture or the impact of the atmospheric evaporative demand. Our results indicate that dew influence on alpine grassland ecosystems varies according to the environmental stress and plant physiology.

Populus euphratica counteracts drought stress through the dew coupling and root hydraulic redistribution processes

BACKGROUND

In arid and semi-arid areas, plants can directly absorb and use dew through their leaves, and some plants have the ability for hydraulic redistribution of their roots. Therefore, in arid areas, plants may redistribute dew to the soil, using the soil as a reservoir for short-term dry seasons, i.e. dew may participate in the hydraulic redistribution process of plants. This process plays an important role in plant survival and community stability.

METHODS

To verify this hypothesis, we investigated the water use mechanism of Populus euphratica through a comprehensive observation of sap flow, water potential and soil water content using a heavy water tracer experiment under in situ field conditions.

RESULTS AND DISCUSSION

Dewdrops contributed 28.3 % of soil moisture near the roots, and applying dew on leaves for several days significantly improved soil moisture status. Hydraulic redistribution in the roots mainly occurred from 2200 h at night to 800 h the following day and mainly occurred in the 20- to 80-cm soil layer. Water storage in the trunk is the intermediate link in the coupling process of foliar water uptake and hydraulic redistribution; water storage in the trunk is mainly replenished from May to July and consumed throughout the rest of the year. In conclusion, dew redistributes water into soil through the coupling process of foliar water uptake and hydraulic redistribution. Populus euphratica uses the trunk and soil for water storage to cope with water stress during short-term drought periods. Our findings provide a scientific basis for the restoration of different species in water-deficient areas, which is conducive to maintaining vegetation ecosystem stability in areas of desertification and improving the soil water balance.

The Brazilian Cerrado is becoming hotter and drier

The Brazilian Cerrado is a global biodiversity hotspot with notoriously high rates of native vegetation suppression and wildfires over the past three decades. As a result, climate change can already be detected at both local and regional scales. In this study, we used three different approaches based on independent datasets to investigate possible changes in the daytime and nighttime temperature and air humidity between the peak of the dry season and the beginning of the rainy season in the Brazilian Cerrado. Additionally, we evaluated the tendency of dew point depression, considering it as a proxy to assess impacts on biodiversity. Monthly increases of 2.2-4.0℃ in the maximum temperatures and 2.4-2.8℃ in the minimum temperatures between 1961 and 2019 were recorded, supported by all analyzed datasets which included direct observations, remote sensing, and modeling data. The warming raised the vapor pressure deficit, and although we recorded an upward trend in absolute humidity, relative humidity has reduced by ~15%. If these tendencies are maintained, gradual air warming will make nightly cooling insufficient to reach the dew point in the early hours of the night. Therefore, it will progressively reduce both the amount and duration of nocturnal dewfall, which is the main source of water for numerous plants and animal species of the Brazilian Cerrado during the dry season. Through several examples, we hypothesize that these climate changes can have a high impact on biodiversity and potentially cause ecosystems to collapse. We emphasize that the effects of temperature and humidity on Cerrado ecosystems cannot be neglected and should be further explored from a land use perspective.

Fungal Communities on Standing Litter Are Structured by Moisture Type and Constrain Decomposition in a Hyper-Arid Grassland

Non-rainfall moisture (fog, dew, and water vapor; NRM) is an important driver of plant litter decomposition in grasslands, where it can contribute significantly to terrestrial carbon cycling. However, we still do not know whether microbial decomposers respond differently to NRM and rain, nor whether this response affects litter decomposition rates. To determine how local moisture regimes influence decomposer communities and their function, we examined fungal communities on standing grass litter at an NRM-dominated site and a rain-dominated site 75 km apart in the hyper-arid Namib Desert using a reciprocal transplant design. Dominant taxa at both sites consisted of both extremophilic and cosmopolitan species. Fungal communities differed between the two moisture regimes with environment having a considerably stronger effect on community composition than did stage of decomposition. Community composition was influenced by the availability of air-derived spores at each site and by specialization of fungi to their home environment; specifically, fungi from the cooler, moister NRM Site performed worse (measured as fungal biomass and litter mass loss) when moved to the warmer, drier rain-dominated site while Rain Site fungi performed equally well in both environments. Our results contribute to growing literature demonstrating that as climate change alters the frequency, magnitude and type of moisture events in arid ecosystems, litter decomposition rates may be altered and constrained by the composition of existing decomposer communities.

How do leaf wetting events affect gas exchange and leaf lifespan of plants from seasonally dry tropical vegetation?

Foliar uptake of dew is likely an important mechanism of water acquisition for plants from tropical dry environments. However, there is still limited experimental evidence describing the anatomical pathways involved in this process and the effects of this water subsidy on the maintenance of gas exchange and leaf lifespan of species from seasonally dry tropical vegetation such as the Brazilian caatinga. We performed scanning electron, bright-field and confocal microscopic analyses and used apoplastic tracers to examine the foliar water uptake (FWU) routes in four woody species with different foliar phenology and widely distributed in the caatinga. Leaves of plants subjected to water stress were exposed to dew simulation to evaluate the effects of the FWU on leaf water potentials, gas exchange and leaf lifespan. All species absorbed water through their leaf cuticles and/or peltate trichomes but FWU capacity differed among species. Leaf wetting by dew increased leaf lifespan duration up to 36 days compared to plants in the drought treatment. A positive effect on leaf gas exchange and new leaf production was only observed in the anisohydric and evergreen species. We showed that leaf wetting by dew is relevant for the physiology and leaf lifespan of plants from seasonally dry tropical vegetation, especially for evergreen species.

Effects of Wilting and Dew on the Water Isotope Composition of Detached Grass in Temperate Grassland

Understanding the water isotopes in feed products derived from grass is fundamental for tracing domestic animal products. Grass silage water was reported to have fewer heavy isotopes than fresh grass, but it is still unknown whether dew formation (either dewfall or dewrise), exchange with soil water, or other processes override the expected enrichment of heavy isotopes due to wilting. The isotopic variations of water (δ²H, δ¹⁸O) in fresh grass and cut grass during wilting on soil and on plastic were compared in this study. Drying enriched heavier isotopes, but this was overridden by three processes that finally caused low δ²H and δ¹⁸O values: (i) the adsorption of humidity from the surroundings, (ii) the exchange with humidity, and (iii) the depletion of heavy water isotopes close to organic surfaces, called the surface effect, which was the most dominant effect at the end of drying when the water content became low.

Foliar water uptake in Amazonian trees: Evidence and consequences

The absorption of atmospheric water directly into leaves enables plants to alleviate the water stress caused by low soil moisture, hydraulic resistance in the xylem and the effect of gravity on the water column, while enabling plants to scavenge small inputs of water from leaf-wetting events. By increasing the availability of water, and supplying it from the top of the canopy (in a direction facilitated by gravity), foliar uptake (FU) may be a significant process in determining how forests interact with climate, and could alter our interpretation of current metrics for hydraulic stress and sensitivity. FU has not been reported for lowland tropical rainforests; we test whether FU occurs in six common Amazonian tree genera in lowland Amazônia, and make a first estimation of its contribution to canopy-atmosphere water exchange. We demonstrate that FU occurs in all six genera and that dew-derived water may therefore be used to "pay" for some morning transpiration in the dry season. Using meteorological and canopy wetness data, coupled with empirically derived estimates of leaf conductance to FU (k_fu ), we estimate that the contribution by FU to annual transpiration at this site has a median value of 8.2% (103 mm/year) and an interquartile range of 3.4%-15.3%, with the biggest sources of uncertainty being k_fu and the proportion of time the canopy is wet. Our results indicate that FU is likely to be a common strategy and may have significant implications for the Amazon carbon budget. The process of foliar water uptake may also have a profound impact on the drought tolerance of individual Amazonian trees and tree species, and on the cycling of water and carbon, regionally and globally.

Fog and Dew as Potable Water Resources: Maximizing Harvesting Potential and Water Quality Concerns

Fog and dew are often viewed as economic nuisances causing significant financial losses in the transportation industry and agricultural sector. However, they are also critical components of the hydrological cycle, especially in water scarce environments. Water scarcity is one of the major threats to mankind in the 21st century, and this can be due to development pressures, pollution, and/or expanding populations. In water scarce environments, fog and dew represent potentially exploitable ancillary water resources that could ameliorate the water scarce situation, if efficiently harvested. However, two important issues are often overlooked in relation to fog and dew harvesting and potability. First, current fog and dew harvesting technologies are low yielding with great potential for improvements. Second and more importantly, the potability of these water resources is often based on simple analyses that often omit trace metal and biological analyses. The few studies that report trace metal or biological measurements suggest elevated trace metal concentrations or biological contamination that could be of concern to public health. We discuss the potential for fog and dew harvesting technologies and the need for trace metal and biological analyses of these waters before use.

Dynamic Shaping of Femtoliter Dew Droplets

Herein, we show that wetting properties such as giant wetting anisotropy and dynamic shaping can be observed when femtoliter (submicron scale) dew droplets are condensed on nanopatterned mildly hydrophilic surfaces. Large-scale, optically transparent, nanopatterned TiO₂ surfaces were fabricated by direct nanoimprinting lithography of sol-gel-derived films. Square, infinitely elongated, or circular droplets were obtained with square, line, or concentric patterns, respectively, and were visualized in situ during formation and recession using optical microscopy and environmental scanning electronic microscopy. We first describe how extremely elongated droplets could form on mildly hydrophilic surfaces, naturally contaminated in real environmental conditions. In this configuration, the dew droplet shape can be dynamically and reversibly varied by controlling the out-of-equilibrium conditions associated with condensation/evaporation kinetics. As an example of the application, we propose a "morphological" sensor that exploits the shape of the dew droplets as a transduction mode for detecting organic vapors in the outer atmosphere. Importantly, this study is underlining that environmentally stable, purely hydrophilic surfaces can be smartly engineered to induce wetting phenomena at very small scale never observed so far for hydrophobic or heterogeneous surfaces. Our versatile approach based on nanoimprinted, transparent sol-gel films could open great perspectives for the implementation of environmentally stable, mildly hydrophilic materials for "dew engineering" applications such as open microfluidics, fuming for fingerprints, vapor sensing, or water harvesting on glass windows, for instance.

Patterned Polymer Coatings Increase the Efficiency of Dew Harvesting

Micropatterned polymer surfaces, possessing both topographical and chemical characteristics, were prepared on three-dimensional copper tubes and used to capture atmospheric water. The micropatterns mimic the structure on the back of a desert beetle that condenses water from the air in a very dry environment. The patterned coatings were prepared by the dewetting of thin films of poly-4-vinylpyridine (P4VP) on top of polystyrene films (PS) films, upon solvent annealing, and consist of raised hydrophilic bumps on a hydrophobic background. The size and density distribution of the hydrophilic bumps could be tuned widely by adjusting the initial thickness of the P4VP films: the diameter of the produced bumps and their height could be varied by almost 2 orders of magnitude (1-80 μm and 40-9000 nm, respectively), and their distribution density could be varied by 5 orders of magnitude. Under low subcooling conditions (3 °C), the highest rate of water condensation was measured on the largest (80 μm diameter) hydrophilic bumps and was found to be 57% higher than that on flat hydrophobic films. These subcooling conditions are achieved spontaneously in dew formation, by passive radiative cooling of a surface exposed to the night sky. In effect, the pattern would result in a larger number of dewy nights than a flat hydrophobic surface and therefore increases water capture efficiency. Our approach is suited to fabrication on a large scale, to enable the use of the patterned coatings for water collection with no external input of energy.

Biotic degradation at night, abiotic degradation at day: positive feedbacks on litter decomposition in drylands

The arid and semi-arid drylands of the world are increasingly recognized for their role in the terrestrial net carbon dioxide (CO₂ ) uptake, which depends largely on plant litter decomposition and the subsequent release of CO₂ back to the atmosphere. Observed decomposition rates in drylands are higher than predictions by biogeochemical models, which are traditionally based on microbial (biotic) degradation enabled by precipitation as the main mechanism of litter decomposition. Consequently, recent research in drylands has focused on abiotic mechanisms, mainly photochemical and thermal degradation, but they only partly explain litter decomposition under dry conditions, suggesting the operation of an additional mechanism. Here we show that in the absence of precipitation, absorption of dew and water vapor by litter in the field enables microbial degradation at night. By experimentally manipulating solar irradiance and nighttime air humidity, we estimated that most of the litter CO₂ efflux and decay occurring in the dry season was due to nighttime microbial degradation, with considerable additional contributions from photochemical and thermal degradation during the daytime. In a complementary study, at three sites across the Mediterranean Basin, litter CO₂ efflux was largely explained by litter moisture driving microbial degradation and ultraviolet radiation driving photodegradation. We further observed mutual enhancement of microbial activity and photodegradation at a daily scale. Identifying the interplay of decay mechanisms enhances our understanding of carbon turnover in drylands, which should improve the predictions of the long-term trend of global carbon sequestration.

Nonrainfall water origins and formation mechanisms

Dryland ecosystems cover 40% of the total land surface on Earth and are defined broadly as zones where precipitation is considerably less than the potential evapotranspiration. Nonrainfall waters (for example, fog and dew) are the least-studied and least-characterized components of the hydrological cycle, although they supply critical amounts of water for dryland ecosystems. The sources of nonrainfall waters are largely unknown for most systems. In addition, most field and modeling studies tend to consider all nonrainfall inputs as a single category because of technical constraints, which hinders prediction of dryland responses to future warming conditions. This study uses multiple stable isotopes (²H, ¹⁸O, and ¹⁷O) to show that fog and dew have multiple origins and that groundwater in drylands can be recycled via evapotranspiration and redistributed to the upper soil profile as nonrainfall water. Surprisingly, the non-ocean-derived (locally generated) fog accounts for more than half of the total fog events, suggesting a potential shift from advection-dominated fog to radiation-dominated fog in the fog zone of the Namib Desert. This shift will have implications on the flora and fauna distribution in this fog-dependent system. We also demonstrate that fog and dew can be differentiated on the basis of the dominant fractionation (equilibrium and kinetic) processes during their formation using the ¹⁷O-¹⁸O relationship. Our results are of great significance in an era of global climate change where the importance of nonrainfall water increases because rainfall is predicted to decline in many dryland ecosystems.

Removal mechanisms of dew via self-propulsion off the gecko skin

Condensation resulting in the formation of water films or droplets is an unavoidable process on the cuticle or skin of many organisms. This process generally occurs under humid conditions when the temperature drops below the dew point. In this study, we have investigated dew conditions on the skin of the gecko Lucasium steindachneri. When condensation occurs, we show that small dew drops, as opposed to a thin film, form on the lizard's scales. As the droplets grow in size and merge, they can undergo self-propulsion off the skin and in the process can be carried away a sufficient distance to freely engage with external forces. We show that factors such as gravity, wind and fog provide mechanisms to remove these small droplets off the gecko skin surface. The formation of small droplets and subsequent removal from the skin may aid in reducing microbial contact (e.g. bacteria, fungi) and limit conducive growth conditions under humid environments. As well as providing an inhospitable microclimate for microorganisms, the formation and removal of small droplets may also potentially aid in other areas such as reduction and cleaning of some surface contaminants consisting of single or multiple aggregates of particles.

"Breath figures" on leaf surfaces-formation and effects of microscopic leaf wetness

"Microscopic leaf wetness" means minute amounts of persistent liquid water on leaf surfaces which are invisible to the naked eye. The water is mainly maintained by transpired water vapor condensing onto the leaf surface and to attached leaf surface particles. With an estimated average thickness of less than 1 μm, microscopic leaf wetness is about two orders of magnitude thinner than morning dewfall. The most important physical processes which reduce the saturation vapor pressure and promote condensation are cuticular absorption and the deliquescence of hygroscopic leaf surface particles. Deliquescent salts form highly concentrated solutions. Depending on the type and concentration of the dissolved ions, the physicochemical properties of microscopic leaf wetness can be considerably different from those of pure water. Microscopic leaf wetness can form continuous thin layers on hydrophobic leaf surfaces and in specific cases can act similar to surfactants, enabling a strong potential influence on the foliar exchange of ions. Microscopic leaf wetness can also enhance the dissolution, the emission, and the reaction of specific atmospheric trace gases e.g., ammonia, SO2, or ozone, leading to a strong potential role for microscopic leaf wetness in plant/atmosphere interaction. Due to its difficult detection, there is little knowledge about the occurrence and the properties of microscopic leaf wetness. However, based on the existing evidence and on physicochemical reasoning it can be hypothesized that microscopic leaf wetness occurs on almost any plant worldwide and often permanently, and that it significantly influences the exchange processes of the leaf surface with its neighboring compartments, i.e., the plant interior and the atmosphere. The omission of microscopic water in general leaf wetness concepts has caused far-reaching, misleading conclusions in the past.

Harmful effects of radicals generated in polluted dew on the needles of Japanese Red Pine (Pinus densiflora)

•  The effects of free radicals, ·OH and ·NO, generated in polluted dew water on needles of Pinus densiflora (Japanese Red pine) were investigated. •  ·OH-generating solutions (HOOH with Fe(III) and oxalate ion; ·OH treatment) and ·OH- and ·NO-generating solutions (NO₂ ^- ; ·OH/·NO treatment) were regulated at 25, 50 and 100 µmol and pH 4.4. HOOH only (HOOH treatment) was used as a control solution. Solutions were applied as a mist to the needle surface of P. densiflora seedlings before dawn twice a week for 3 months. •  Within a month, net photosynthesis at near saturating irradiance (Pn) and stomatal conductance (gl) of ·OH-treated needles decreased with increasing solution concentration. The HOOH treatment had no effects on any of the measured parameters. Therefore, ·OH in the artificial dews caused the decreases in Pn and gl. In ·OH/·NO-treated needles, gl increased during the experiment, but Pn was unchanged. In all experiments, the characteristics of PSII were not significantly altered. •  Free radicals in polluted dew water have harmful effects on the photosynthesis of P. densiflora and compound effects of ·OH and ·NO are different.