Stomatal penetration by aqueous solutions--an update involving leaf surface particles

Selective capture of PM2.5 by urban trees: The role of leaf wax composition and physiological traits in air quality enhancement

Human health risks from particles with a diameter of less than 2.5 µm (PM2.5) highlight the role of urban trees as bio-filters in air pollution control. However, whether the size and composition of particles captured by various tree species differ or not remain unclear. This study investigates how leaf attributes affect the capture of PM2.5, which can penetrate deep into the lungs and pose significant health risks. Using a self-developed particulate matter (PM) resuspension chamber and single-particle aerosol mass spectrometer, we measured the size distribution and mass spectra of particles captured by ten tree species. Notably, Cinnamomum camphora (L.) J.Presl and Osmanthus fragrans Lour. are more effective at capturing particles under 1 µm, which are most harmful because they can reach the alveoli, whereas Ginkgo biloba L. and Platanus × acerifolia (Aiton) Willd. tend to capture larger particles, up to 1.6 µm, which are prone to being trapped in the upper respiratory tract. Leaf physiological traits such as stomatal conductance and water potential significantly enhance the capture of larger particles. The Adaptive Resonance Theory neural network (ART-2a) algorithm classified a large number of single particles to determine their composition. Results indicate distinct inter-species variations in chemical composition of particles captured by leaves. Moreover, we identified how specific leaf wax compositions-beyond the known sticky nature of hydrophobic waxes-contribute to particle adhesion, particularly highlighting the roles of fatty acids and alkanes in adhering particles rich in organic carbon and heavy metals, respectively. This research advances our understanding by linking leaf physiological and wax characteristics to the selective capture of PM2.5, providing actionable insights for urban forestry management. The detailed exploration of particle size and composition, tied to specific tree species, enriches the current literature by quantifying how and why different species contribute variably to air quality improvement. This adds a crucial layer of specificity to the general knowledge that trees serve as bio-filters, offering a refined strategy for planting urban trees based on their particulate capture profiles.

Hyperspectral imaging reveals small-scale water gradients in apple leaves due to minimal cuticle perforation by Venturia inaequalis conidiophores

Effects of Venturia inaequalis on water relations of apple leaves were studied under controlled conditions without limitation of water supply to elucidate their impact on the non-haustorial biotrophy of this pathogen. Leaf water relations, namely leaf water content and transpiration, were spatially resolved by hyperspectral imaging and thermography; non-imaging techniques-gravimetry, a pressure chamber, and porometry-were used for calibration and validation. Reduced stomatal transpiration 3-4 d after inoculation coincided with a transient increase of water potential. Perforation of the plant cuticle by protruding conidiophores subsequently increased cuticular transpiration even before visible symptoms occurred. With sufficient water supply, cuticular transpiration remained at elevated levels for several weeks. Infections did not affect the leaf water content before scab lesions became visible. Only hyperspectral imaging was suitable to demonstrate that a decreased leaf water content was strictly limited to sites of emerging conidiophores and that cuticle porosity increased with sporulation. Microscopy confirmed marginal cuticle injury; although perforated, it tightly surrounded the base of conidiophores throughout sporulation and restricted water loss. The role of sustained redirection of water flow to the pathogen's hyphae in the subcuticular space above epidermal cells, to facilitate the acquisition and uptake of nutrients by V. inaequalis, is discussed.

Nanowonders in agriculture: Unveiling the potential of nanoparticles to boost crop resilience to salinity stress

Soil salinization significantly affects crop production by reducing crop quality and decreasing yields. Climate change can intensify salinity-related challenges, making the task of achieving global food security more complex. To address the problem of elevated salinity stress in crops, nanoparticles (NPs) have emerged as a promising solution. NPs, characterized by their small size and extensive surface area, exhibit remarkable functionality and reactivity. Various types of NPs, including metal and metal oxide NPs, carbon-based NPs, polymer-based NPs, and modified NPs, have displayed potential for mitigating salinity stress in plants. However, the effectiveness of NPs application in alleviating plant stress is dependent upon multiple factors, such as NPs size, exposure duration, plant species, particle composition, and prevailing environmental conditions. Moreover, alterations to NPs surfaces through functionalization and coating also play a role in influencing plant tolerance to salinity stress. NPs can influence cellular processes by impacting signal transduction and gene expression. They counteract reactive oxygen species (ROS), regulate the water balance, enhance photosynthesis and nutrient uptake and promote plant growth and yield. The objective of this review is to discuss the positive impacts of diverse NPs on alleviating salinity stress within plants. The intricate mechanisms through which NPs accomplish this mitigation are also discussed. Furthermore, this review addresses existing research gaps, recent breakthroughs, and prospective avenues for utilizing NPs to combat salinity stress.

Insights into growth stages and genotypes in airborne Pb accumulation in Oryza sativa L. grains: Utilizing isotope fingerprinting alongside a model study

Atmospheric exposure is an important pathway of accumulation of lead (Pb) in Oryza sativa L. grains. In this study, source contributions of soil, early atmospheric exposure, and late atmospheric exposure, along with their bioaccumulation ratios were examined both in the pot and field experiments using stable Pb isotope fingerprinting technology combined with a three-compartment accumulation model. Furthermore, genotype differences in airborne Pb accumulation among four field-grown rice cultivars were investigated using the partial least squares path model (PLS-PM) linking rice Pb accumulation to agronomic traits. The findings revealed that during the late growth period, the air-foliar-grain transfer of Pb was crucial for rice Pb accumulation. Approximately 69-82% of the Pb found in polished rice was contributed by atmospheric source, with more than 80% accumulating during the late growth stage. The air accumulation ratios of rice grains were genotype-specific and estimated to be 0.364-1.062 m3/g during the late growth. Notably, grain size exhibited the highest standardized total effects on the airborne Pb concentrations in the polished rice, followed by leaf Pb and the upward translocation efficiency of Pb. The present study indicates that mitigating the health risks associated with Pb in rice can be achieved by controlling atmospheric Pb levels during the late growth stage and choosing Japonica inbred varieties characterized by large grain size.

Comprehensive framework for overcoming scientific challenges related to assessing radioactive ultra-fine (nano/micro) particles transfer at the atmosphere-leaf interface

Food products are prone into contamination after a nuclear emission of radionuclides. While the mechanisms of emission and deposition of ultrafine radioactive particles are well documented, the transfer of these species from the atmosphere into plants is poorly assessed. This is evident in the lack of quantification of particles distributed within plants, especially regarding particles physical-chemical criteria to plant of different properties. Such knowledge gaps raise the concern about the representativeness of risk assessment tools designed for the transfer evaluation of ionic/soluble species to be qualified for simulating insoluble species exposure and proposes a possible underestimation. This highlights the possible need for special particle codes development to be implemented in models for future emissions. In addition, the later tools utilize transfer factors aggregating relevant sub-processes, suggesting another weak point in their overall reliability. As researchers specialized in the nuclear safety and protection, we intend in this perspective, to develop a compressive analysis of the interaction of ultrafine particles with plants of different specificities at different level processes starting from particles retention and gradual translocation to sink organs. This analysis is leveraged in providing insights for possible improvements in the current modeling tools for better real-life scenarios representation.

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.

Detection and attribution of an anomaly in terrestrial photosynthesis in Europe during the COVID-19 lockdown

Carbon dioxide (CO2) uptake by plant photosynthesis, referred to as gross primary production (GPP) at the ecosystem level, is sensitive to environmental factors, including pollutant exposure, pollutant uptake, and changes in the scattering of solar shortwave irradiance (SWin) - the energy source for photosynthesis. The 2020 spring lockdown due to COVID-19 resulted in improved air quality and atmospheric transparency, providing a unique opportunity to assess the impact of air pollutants on terrestrial ecosystem functioning. However, detecting these effects can be challenging as GPP is influenced by other meteorological drivers and management practices. Based on data collected from 44 European ecosystem-scale CO2 flux monitoring stations, we observed significant changes in spring GPP at 34 sites during 2020 compared to 2015-2019. Among these, 14 sites showed an increase in GPP associated with higher SWin, 10 sites had lower GPP linked to atmospheric and soil dryness, and seven sites were subjected to management practices. The remaining three sites exhibited varying dynamics, with one experiencing colder and rainier weather resulting in lower GPP, and two showing higher GPP associated with earlier spring melts. Analysis using the regional atmospheric chemical transport model (LOTOS-EUROS) indicated that the ozone (O3) concentration remained relatively unchanged at the research sites, making it unlikely that O3 exposure was the dominant factor driving the primary production anomaly. In contrast, SWin increased by 9.4 % at 36 sites, suggesting enhanced GPP possibly due to reduced aerosol optical depth and cloudiness. Our findings indicate that air pollution and cloudiness may weaken the terrestrial carbon sink by up to 16 %. Accurate and continuous ground-based observations are crucial for detecting and attributing subtle changes in terrestrial ecosystem functioning in response to environmental and anthropogenic drivers.

Leaf wettability is the main driver for foliar P uptake in P-deficient maize

Foliar fertilisation is an alternative form of nutrient application, which is of particular interest for phosphorus (P), where the efficiency of soil fertilisation is low. However, the uptake of foliar-applied nutrients is insufficiently characterised. The aim of this study was to investigate the individual and combined significance of wettability, foliar fertiliser properties and surfactant on foliar P uptake in P-deficient maize (Zea mays L.). Sorption and desorption properties of two P salts used as foliar fertilisers (KH2PO4, K2HPO4) were determined with dynamic vapor sorption isotherms. Leaf surfaces and foliar spray depositions of two differently wettable maize cultivars were investigated by scanning electron microscopy and contact angle measurement. Phosphorus uptake was then linked to leaf and fertiliser solution properties and its effect on cell ultrastructure was characterised by transmission electron microscopy. Wettability was the key factor for P absorption, as all foliar fertilisers were taken up reaching a tissue-P level of adequately nourished plants. For unwettable leaves, only solutions with surfactant, especially the combination of surfactant and hygroscopic P salt (K2HPO4) were taken up. This study provides novel insights into the significance of leaf surface and fertiliser properties, which can thus contribute to an improvement of P fertilisation strategies.

Ambient aerosols increase stomatal transpiration and conductance of hydroponic sunflowers by extending the hydraulic system to the leaf surface

Introduction

Many atmospheric aerosols are hygroscopic and play an important role in cloud formation. Similarly, aerosols become sites of micro-condensation when they deposit to the upper and lower surfaces of leaves. Deposited salts, in particular can trigger condensation at humidities considerably below atmospheric saturation, according to their hygroscopicity and the relative humidity within the leaf boundary layer. Salt induced water potential gradients and the resulting dynamics of concentrated salt solutions can be expected to affect plant water relations.

Methods

Hydroponic sunflowers were grown in filtered (FA) and unfiltered, ambient air (AA). Sap flow was measured for 18 days and several indicators of incipient drought stress were studied.

Results

At 2% difference in mean vapor pressure deficit (D), AA sunflowers had 49% higher mean transpiration rates, lower osmotic potential, higher proline concentrations, and different tracer transport patterns in the leaf compared to FA sunflowers. Aerosols increased plant conductance particularly at low D.

Discussion

The proposed mechanism is that thin aqueous films of salt solutions from deliquescent deposited aerosols enter into stomata and cause an extension of the hydraulic system. This hydraulic connection leads - parallel to stomatal water vapor transpiration - to wick-like stomatal loss of liquid water and to a higher impact of D on plant water loss. Due to ample water supply by hydroponic cultivation, AA plants thrived as well as FA plants, but under more challenging conditions, aerosol deposits may make plants more susceptible to drought stress.

Optimizing foliar N-fertilization in sugarcane depends on plant genotype and nitrogen concentration

Foliar N-fertilization (FNf) has emerged as a promising approach to synchronize plant nitrogen (N) demands and application timing, reducing the N losses to the environment associated with traditional soil-based fertilization methods. However, limited information exists regarding the effectiveness of FNf in sugarcane. This study aimed to optimize FNf in sugarcane by evaluating N-fertilizer recovery by the plant (NRP) and assessing potential toxicity effects. Four sugarcane genotypes were subjected to FNf using 15 N-urea at five nitrogen concentrations. NRP was assessed at five time points for roots, stalk, old leaves, 15 N-urea-fertilized leaves (15 NL), and unexpanded leaves (UEL). Leaf scorching, indicating FNf toxicity, was analyzed using morpho-anatomical and histochemical techniques. The results showed that FNf promoted high NRP, with an average recovery of 62.3%. Surprisingly, the redistribution of 15 N-urea did not follow the nitrogen uptake rate by sugarcane leaves, with an average of 41.3% of the total-NRP. The stalk emerged as the primary sink for 15 N-urea, followed by the UEL. Genotypes differed in the leaf scorching intensity, which increased with higher concentration of 15 N-urea. Genotypes also differed in the 15 N-urea uptake rate, down-regulated by the N content in the 15 NL. These findings emphasize that by carefully choosing the appropriate genotype and nitrogen concentration, FNf can significantly enhance N-fertilizer uptake, resulting in potential environmental and economic benefits.

Evaluating Leaf Wettability and Salt Hygroscopicity as Drivers for Foliar Absorption

The objective of this study was to evaluate the rate of foliar absorption of magnesium (Mg) salts with different deliquescence and efflorescence relative humidity values (DRH and ERH, also known as point of deliquescence (POD) and point of efflorescence (POE), respectively) when supplied to leaves of model plants with different wettability properties. For this purpose, a greenhouse pot experiment was conducted with lettuce (very wettable), broccoli (highly unwettable) and leek (highly unwettable). Foliar sprays contained 0.1% surfactant plus 100 mM Mg supplied as MgCl₂·6H₂O, Mg(NO₃)₂·6H₂O or MgSO₄·7H₂O. Leaf Mg concentrations were determined 1 and 7 days after foliar application. Anion concentrations were also measured in lettuce where a significant foliar Mg absorption was detected. Leaf wettability, leaf surface free energy and fertilizer drop deposit appearance onto the foliage were assessed. It is concluded that despite including a surfactant in the spray formulation, leaf wettability plays a major role in foliar Mg absorption.

Kinetics of Lipophilic Pesticide Uptake by Living Maize

We report the uptake of a lipophilic fungicide into the cuticle of living leaves of young maize from droplets of a suspension concentrate. The action of a "coffee-ring" effect is demonstrated during fungicide formulation drying, and the fungicide particle distribution is quantified. We develop a simple, two-dimensional model of uptake leading to a "reservoir" of cuticular fungicide. This model allows inferences of physicochemical properties for fungicides inside the cuticular medium. The diffusion coefficient closely agrees with literature penetration experiments (D_cut ≈ 10^-18 m² s^-1). The logarithm of the inferred cuticle-water partition coefficient log₁₀ K_cw = 6.03 ± 0.04 is consistent with ethyl acetate as a model solvent for the maize cuticle. Two limiting kinetic uptake regimes are inferred from the model for short and long times, with the transition resulting from longitudinal saturation of the cuticle beneath the droplet. We discuss the strengths, limitations, and generalizability of our model within the "cuticle reservoir" approximation.

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.

Use of non-intrusive laser exfoliation to improve substance uptake into citrus leaves

Background: Despite the presence of stomata in leaves, foliar application of agrochemicals can be extremely inefficient due to the low permeability of leaf cuticular surfaces to polar compounds. Methods: This study introduced a laser-based “wax exfoliation” method to facilitate the penetration of substances into the leaf and, together with enhancing their uptake into the phloem and subsequent transport across tissue. This investigation demonstrated the effectiveness and non-invasive properties of laser exfoliation to improve the penetration of foliar-applied substances into citrus leaves. Results: This work presents the use of laser energy to exfoliate the cuticle of a leaf, with the highest energy density of 0.76 J/ cm2 resulting in 85-90% exfoliation across the entire laser-spot area. The infrared wavelength of the erbium laser is specifically chosen to target the wax cuticle without causing damage to the underlying epidermal cells. This selective ablation allows for increased penetration of therapeutic compounds into the leaf and transportation throughout the plant's vasculature. This is demonstrated using a fluorescent glucose analog applied to the laser treated leaves, showing increased penetration and transport throughout the leaf. Conclusions: Our findings demonstrate that the use of laser technology for the foliar application of agrochemicals provides significant advantages, including improved foliage uptake of therapeutic compounds. The method of cuticle exfoliation presented in this study is highly effective and non-intrusive, limiting its effects to the cuticle only. Future work should focus on the development of prototypes for in-field applications, including testing at longer distances as the Er:YAG laser does not require a lens for this application.

Use of non-intrusive laser exfoliation to improve substance uptake into citrus leaves

Background: Despite the presence of stomata in leaves, foliar application of agrochemicals can be extremely inefficient due to the low permeability of leaf cuticular surfaces to polar compounds. Methods: This study introduced a laser-based "wax exfoliation" method to facilitate the penetration of substances into the leaf and, together with enhancing their uptake into the phloem and subsequent transport across tissue. This investigation demonstrated the effectiveness and non-invasive properties of laser exfoliation to improve the penetration of foliar-applied substances into citrus leaves. Results: This work presents the use of laser energy to exfoliate the cuticle of a leaf, with the highest energy density of 0.76 J/ cm2 resulting in 85-90% exfoliation across the entire laser-spot area. The infrared wavelength of the erbium laser is specifically chosen to target the wax cuticle without causing damage to the underlying epidermal cells. This selective ablation allows for increased penetration of therapeutic compounds into the leaf and transportation throughout the plant's vasculature. This is demonstrated using a fluorescent glucose analog applied to the laser treated leaves, showing increased penetration and transport throughout the leaf. Conclusions: Our findings demonstrate that the use of laser technology for the foliar application of agrochemicals provides significant advantages, including improved foliage uptake of therapeutic compounds. The method of cuticle exfoliation presented in this study is highly effective and non-intrusive, limiting its effects to the cuticle only. Future work should focus on the development of prototypes for in-field applications, including testing at longer distances as the Er:YAG laser does not require a lens for this application.

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.

Nocturnal Transpiration May Be Associated with Foliar Nutrient Uptake

Aerosols can contribute to plant nutrition via foliar uptake. The conditions for this are best at night because the humidity is high and hygroscopic, saline deposits can deliquesce as a result. Still, stomata tend to be closed at night to avoid unproductive water loss. However, if needed, nutrients are on the leaf surface, and plants could benefit from nocturnal stomatal opening because it further increases humidity in the leaf boundary layer and allows for stomatal nutrient uptake. We tested this hypothesis on P-deficient soil by comparing the influence of ambient aerosols and additional foliar P application on nocturnal transpiration. We measured various related leaf parameters, such as the foliar water loss, minimum leaf conductance (g_min), turgor loss point, carbon isotope ratio, contact angle, specific leaf area (SLA), tissue element concentration, and stomatal and cuticular characteristics. For untreated leaves grown in filtered, aerosol-free air (FA), nocturnal transpiration consistently decreased overnight, which was not observed for leaves grown in unfiltered ambient air (AA). Foliar application of a soluble P salt increased nocturnal transpiration for AA and FA leaves. Crusts on stomatal rims were shown by scanning electron microscopy, supporting the idea of stomatal uptake of deliquescent salts. Turgor loss point and leaf moisture content indicated a higher accumulation of solutes, due to foliar uptake by AA plants than FA plants. The hypothesis that deliquescent leaf surface salts may play a role in triggering nocturnal transpiration was supported by the results. Still, further experiments are required to characterize this phenomenon better.

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.

Use of non-intrusive laser exfoliation to improve substance uptake into citrus leaves

Background: Despite the presence of stomata in leaves, foliar application of agrochemicals can be extremely inefficient due to the low permeability of leaf cuticular surfaces to polar compounds. Methods: This study introduced a laser-based "wax exfoliation" method to facilitate the penetration of substances into the leaf and, together with enhancing their uptake into the phloem and subsequent transport across tissue. This investigation demonstrated the effectiveness and non-invasive properties of laser exfoliation to improve the penetration of foliar-applied substances into citrus leaves. Results: This work presents the use of laser energy to exfoliate the cuticle of a leaf, with the highest energy density of 0.76 J/ cm2 resulting in 85-90% exfoliation across the entire laser-spot area. The infrared wavelength of the erbium laser is specifically chosen to target the wax cuticle without causing damage to the underlying epidermal cells. This selective ablation allows for increased penetration of therapeutic compounds into the leaf and transportation throughout the plant's vasculature. This is demonstrated using a fluorescent glucose analog applied to the laser treated leaves, showing increased penetration and transport throughout the leaf. Conclusions: Our findings demonstrate that the use of laser technology for the foliar application of agrochemicals provides significant advantages, including improved foliage uptake of therapeutic compounds. The method of cuticle exfoliation presented in this study is highly effective and non-intrusive, limiting its effects to the cuticle only. Future work should focus on the development of prototypes for in-field applications, including testing at longer distances as the Er:YAG laser does not require a lens for this application.

A high-resolution study of PM2.5 accumulation inside leaves in leaf stomata compared with non-stomatal areas using three-dimensional X-ray microscopy

Plant leaves retain atmospheric particulate matter (PM) on their surfaces, helping PM removal and risk reduction of respiratory tract infection. Several processes (deposition, resuspension, rainfall removal) can influence the PM accumulation on leaves and different leaf microstructures (e.g., trichomes, epicuticular waxes) can also be involved in retaining PM. However, the accumulation and distribution of PM on leaves, particularly at the stomata, are unclear, and the lack of characterization methods limits our understanding of this process. Thus, in this study, we aimed to explore the pathway through which PM_2.5 (aerodynamic diameter ≤ 2.5 μm) enters plant leaves, and the penetration depth of PM_2.5 along the entry route. Here, an indoor experiment using diamond powder as a tracer to simulate PM_2.5 deposition on leaves was carried out. Then, the treated and non-treated leaves were scanned by using three-dimensional (3D) X-ray microscopy. Next, the grayscale value of the scanned images was used to compare PM_2.5 accumulation in stomatal and non-stomatal areas of the treated and non-treated leaves, respectively. Finally, a total PM_2.5 volume from the abaxial epidermis was calculated. The results showed that, first, a large amount of PM_2.5 accumulates within leaf stomata, whereas PM_2.5 does not accumulate at non-stomatal areas. Then, the penetration depth of PM_2.5 in stomata of most tree species was 5-14 μm from the abaxial epidermis. For the first time, 3D X-ray microscope scanning was used to confirm that a pathway by which PM_2.5 enters the leaves is through the stomata, which is fundamental for further research on how PM_2.5 translocates and interacts with tissues and cells in leaves.

Nano-enabled agriculture: How do nanoparticles cross barriers in plants?

Nano-enabled agriculture is a topic of intense research interest. However, our knowledge of how nanoparticles enter plants, plant cells, and organelles is still insufficient. Here, we discuss the barriers that limit the efficient delivery of nanoparticles at the whole-plant and single-cell levels. Some commonly overlooked factors, such as light conditions and surface tension of applied nano-formulations, are discussed. Knowledge gaps regarding plant cell uptake of nanoparticles, such as the effect of electrochemical gradients across organelle membranes on nanoparticle delivery, are analyzed and discussed. The importance of controlling factors such as size, charge, stability, and dispersibility when properly designing nanomaterials for plants is outlined. We mainly focus on understanding how nanoparticles travel across barriers in plants and plant cells and the major factors that limit the efficient delivery of nanoparticles, promoting a better understanding of nanoparticle-plant interactions. We also provide suggestions on the design of nanomaterials for nano-enabled agriculture.

Leaf surface traits contributing to wettability, water interception and uptake of above-ground water sources in shrubs of Patagonian arid ecosystems

BACKGROUND AND AIMS

The ecohydrological significance of leaf wetting due to atmospheric water in arid and semiarid ecosystems is not well understood. In these environments, the inputs of precipitation or dew formation resulting in leaf wetting have positive effects on plant functioning. However, its impact on plant water relations may depend on the degree of leaf surface wettability. In this study we evaluated leaf wettability and other leaf traits and its effects on foliar water uptake and canopy interception in plant species of a Patagonian steppe. We also studied how leaf traits affecting wettability vary seasonally from growing to dry season.

METHODS

Contact angle of a water droplet with the leaf surface, water adhesion, droplet retention angle, stomatal density, cuticular conductance, canopy interception and maximum foliar water uptake were determined in six dominant shrub species.

KEY RESULTS

All species increased leaf wettability during the dry season and most species were considered highly wettable. The leaf surface had very high capacity to store and retain water. We found a negative correlation between foliar water uptake and leaf hydrophilia.

CONCLUSIONS

Despite the diversity of life forms, including cushion shrubs and tall shrubs, as well as phenological variability, all species converged in similar seasonal changes in leaf traits that favour wettability. Intercepted water by crowns and the extremely high capacity of retention of droplets on leaf surfaces can have a significant impact on eco-hydrological process in water limited ecosystems where most of water sources during the growing and the dry season may be small rainfall events or dew, which do not always increase soil water availability.

Hydrogel-extraction technique for non-invasive detection of blue fluorescent substances in plant leaves

This paper reports a new hydrogel extraction technique for detecting blue fluorescent substances in plant leaves. These blue fluorescent substances were extracted by placing a hydrogel film on the leaf of a cherry tomato plant infected with Ralstonia solanacearum; herein, chlorogenic acid was confirmed to be a blue fluorescent substance. The wavelength at the maximum fluorescence intensity of the film after the hydrogel extraction was similar to that of the methanolic extract obtained from the infected cherry tomato leaves. Chlorophyll was not extracted from the hydrogel film because no fluorescence peak was observed at 680 nm. Accordingly, the blue fluorescence of the substances extracted from the hydrogel film was not quenched by the strong absorption of chlorophyll in the blue light region. This hydrogel extraction technique can potentially detect small amounts of blue fluorescent substances and the changes in its amount within the leaves of infected plants. These changes in the amount of blue fluorescent substances in the early stages of infection can be used to detect presymptomatic infections. Therefore, hydrogel extraction is a promising technique for the noninvasive detection of infections before onset.

Materials on Plant Leaf Surfaces Are Deliquescent in a Variety of Environments

Materials on plant leaf surfaces that attract water impact penetration of foliar-applied agrochemicals, foliar water uptake, gas exchange, and stomatal density. Few studies are available on the nature of these substances, and we quantify the hygroscopicity of these materials. Water vapor sorption experiments on twelve leaf washes of sample leaves were conducted and analyzed with inductively coupled plasma-optical emission spectroscopy (ICP-OES) and X-ray diffraction. All leaf surface materials studied were hygroscopic. Oils were found on the surface of the Eucalyptus studied. For mangroves that excrete salt to the leaf surfaces, significant sorption occurred at high humidity of a total of 316 mg (~0.3 ml) over 6-10 leaves and fitted a Guggenheim, Anderson, and de Böer sorption isotherm. Materials on the plant leaf surface can deliquesce and form an aqueous solution in a variety of environments where plants grow, including glasshouses and by the ocean, which is an important factor when considering plant-atmosphere relations.

Foliar-applied manganese and phosphorus in deficient barley: Linking absorption pathways and leaf nutrient status

Foliar fertilization delivers essential nutrients directly to plant tissues, reducing excessive soil fertilizer applications that can lead to eutrophication following nutrient leaching. Foliar nutrient absorption is a dynamic process affected by leaf surface structure and composition, plant nutrient status, and ion physicochemical properties. We applied multiple methods to study the foliar absorption behaviors of manganese (Mn) and phosphorus (P) in nutrient-deficient spring barley (Hordeum vulgare) at two growth stages. Nutrient-specific chlorophyll a fluorescence assays were used to visualize leaf nutrient status, while laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) was used to visualize foliar absorption pathways for P and Mn ions. Rapid Mn absorption was facilitated by a relatively thin cuticle with a low abundance of waxes and a higher stomatal density in Mn-deficient plants. Following absorption, Mn accumulated in epidermal cells and in the photosynthetically active mesophyll, enabling a fast (6 h) restoration of Mn-dependent photosynthetic processes. Conversely, P-deficient plants developed thicker cuticles and epidermal cell walls, which reduced the penetration of P across the leaf surface. Foliar-applied P accumulated in trichomes and fiber cells above leaf veins without reaching the mesophyll and, as a consequence, no restoration of P-dependent photosynthetic processes was observed. This study reveals new links between leaf surface morphology, foliar-applied ion absorption pathways, and the restoration of affected physiological processes in nutrient-deficient leaves. Understanding that ions may have different absorption pathways across the leaf surface is critical for the future development of efficient fertilization strategies for crops in nutrient-limited soils.

Aerosol Impacts on Water Relations of Camphor (Cinnamomum camphora)

Major parts of anthropogenic and natural aerosols are hygroscopic and deliquesce at high humidity, particularly when depositing to leaf surfaces close to transpiring stomata. Deliquescence and subsequent salt creep may establish thin, extraordinary pathways into the stomata, which foster stomatal uptake of nutrients and water but may also cause stomatal liquid water loss by wicking. Such additional water loss is not accompanied by a wider stomatal aperture with a larger CO₂ influx and hypothetically reduces water use efficiency (WUE). Here, the possible direct impacts of aerosols on physical and physiological parameters of camphor (Cinnamomum camphora) were studied (i) in a greenhouse experiment using aerosol exclusion and (ii) in a field study in Taiwan, comparing trees at two sites with different aerosol regimes. Scanning electron microscopy (SEM) images showed that leaves grown under aerosol exclusion in filtered air (FA) were lacking the amorphous, flat areas that were abundant on leaves grown in ambient air (AA), suggesting salt crusts formed from deliquescent aerosols. Increasing vapor pressure deficit (VPD) resulted in half the Ball-Berry slope and double WUE for AA compared to FA leaves. This apparent contradiction to the wicking hypothesis may be due to the independent, overcompensating effect of stomatal closure in response to VPD, which affects AA more than FA stomata. Compared to leaves in a more polluted region in the Taiwanese Southwest, NaCl aerosols dominated the leaf surface conditions on mature camphor trees in Eastern Taiwan, while the considerably lower contact angles and the 2.5 times higher minimum epidermal conductances might have come from organic surfactants. Interpretations of SEM images from leaf surface microstructures should consider amorphous areas as possible indicators of aerosol deposition and other hygroscopic material. The amount and type of the material determine the resulting impacts on plant water relations, together with the surrounding atmosphere and ecophysiological traits.

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.

RNAi as a Foliar Spray: Efficiency and Challenges to Field Applications

RNA interference (RNAi) is a powerful tool that is being increasingly utilized for crop protection against viruses, fungal pathogens, and insect pests. The non-transgenic approach of spray-induced gene silencing (SIGS), which relies on spray application of double-stranded RNA (dsRNA) to induce RNAi, has come to prominence due to its safety and environmental benefits in addition to its wide host range and high target specificity. However, along with promising results in recent studies, several factors limiting SIGS RNAi efficiency have been recognized in insects and plants. While sprayed dsRNA on the plant surface can produce a robust RNAi response in some chewing insects, plant uptake and systemic movement of dsRNA is required for delivery to many other target organisms. For example, pests such as sucking insects require the presence of dsRNA in vascular tissues, while many fungal pathogens are predominately located in internal plant tissues. Investigating the mechanisms by which sprayed dsRNA enters and moves through plant tissues and understanding the barriers that may hinder this process are essential for developing efficient ways to deliver dsRNA into plant systems. In this review, we assess current knowledge of the plant foliar and cellular uptake of dsRNA molecules. We will also identify major barriers to uptake, including leaf morphological features as well as environmental factors, and address methods to overcome these barriers.

Leaf rolling and leaf angle improve fog capturing and transport in wheat; adaptation for drought stress in an arid climate

BACKGROUND

Plants use different mechanisms to transport the collected fog water. Leaf traits of wheat play an important role in directing fog water through leaf rolling and leaf angle into the root zone, where it can be stored for consumption. Wheat leaf traits can enhance fog capturing under drought stress. To examine this, 200 wheat genotypes were characterized for leaf rolling and leaf angle under optimal conditions in the field using a randomized complete block design. Seven different phenotypic combinations for leaf traits were observed. A core set of 44 genotypes was evaluated under drought stress.

RESULTS

Results show that variability for leaf traits existed among genotypes. An association was found between leaf rolling and leaf angle, moisture capturing, physiological parameters, and yield contributing traits using correlation. Physiological parameters, especially water use efficiency, were positively correlated with grain yield and moisture capturing at both growth stages. The genotypes (G11 at tillering and G24 at booting phonological phases) with inward to twisting type rolling and erect to semi-erect leaf angle capture more water (12-20%) within the root zone. Twenty-one genotypes were selected based on moisture capturing efficiency and evaluated for leaf surface wettability. Association was found between fog capturing and wettability. This shows that it was due to the leaf repellency validated from static contact angle measurements.

CONCLUSION

These results will give insights into fog capturing and the development of drought-tolerant crops in the semi-arid and arid regions.

Interactive Effects of Drought and Saline Aerosol Stress on Morphological and Physiological Characteristics of Two Ornamental Shrub Species

Effects of drought and aerosol stresses were studied in a factorial experiment based on a Randomized Complete Design with triplicates on two ornamental shrubs. Treatments consisted of four levels of water container (40%, 30%, 20%, and 10% of water volumetric content of the substrate) and, after 30 days from experiment onset, three aerosol treatments (distilled water and 50% and 100% salt sea water concentrations). The trial was contextually replicated on two species: Callistemon citrinus (Curtis) Skeels and Viburnum tinus L. ‘Lucidum’. In both species, increasing drought stress negatively affected dry biomass, leaf area, net photosynthesis, chlorophyll a fluorescence, and relative water content. The added saline aerosol stress induced a further physiological water deficit in plants of both species, with more emphasis on Callistemon. The interaction between the two stress conditions was found to be additive for almost all the physiological parameters, resulting in enhanced damage on plants under stress combination. Total biomass, for effect of combined stresses, ranged from 120.1 to 86.4 g plant−1 in Callistemon and from 122.3 to 94.6 g plant−1 in Viburnum. The net photosynthesis in Callistemon declined by the 70% after 30 days in WC 10% and by the 45% and 53% in WC 20% and WC 10% respectively after 60 days. In Viburnum plants, since the first measurement (7 days), a decrease of net photosynthesis was observed for the more stressed treatments (WC 20% and WC 10%), by 57%. The overall data suggested that Viburnum was more tolerant compared the Callistemon under the experimental conditions studied.

Bioengineering of neem nano-formulation with adjuvant for better adhesion over applied surface to give long term insect control

Although safe and eco-friendly botanical pesticides have been intensively promoted to combat pest attacks in agriculture, but their stability and efficacies remain an issue for their wide acceptability as sustained and effective approaches. The purpose of this work was to develop stable neem oil based nano-emulsion (NE) formulation with enhanced activity employing suitable bio-inspired adjuvant. So, Neem NEs (with and without) natural adjuvants (Cymbopogon citratus and Prosopis juliflora) in different concentrations were prepared and quality parameters dictating kinetic stability, acidity/alkalinity, viscosity, droplet size, zeta potential, surface tension, stability and compatibility were monitored using Viscometer, Zetasizer, Surface Tensiometer, High Performance Liquid Chromatography (HPLC) and Fourier Transform Infrared Spectroscopy (FTIR). Nano-emulsion biosynthesis optimization studies suggested that slightly acidic (5.9-6.5) NE is kinetically stable with no phase separation; creaming or crystallization may be due to botanical adjuvant (lemongrass oil). Findings proved that Prosopis juliflora, acted as bio-polymeric adjuvant to stabilize NE by increasing Brownian motion and weakening the attractive forces with smaller droplets (25-50 nm), low zeta potential (-30 mV) and poly-dispersive index (<0.3). Botanical adjuvant (30%) based NE with optimum viscosity (98.8cPs) can give long term storage stability and improved adhesiveness and wetting with reduced surface tension and contact angle. FT-IR analysis assured azadirachtin's stability and compatibility with adjuvant. With negligible degradation (1.42%) and higher half-life (t_1/2) of 492.95 days, natural adjuvant based NE is substantially stable formulation, may be due to presence of glycosidic and phenolics compounds. Neem 20NE (with 30% adjuvant) exhibited remarkable insecticidal activity (91.24%) against whitefly (Bemisia tabaci G.) in brinjal (Solanum melongena) as evidenced by in-vivo assay. Results thus obtained suggest, bio-pesticide formulation may be used as safer alternative to chemical pesticides to minimize pesticide residues and presence of natural adjuvant may improves the stability and efficacy of biopesticides for safe crop protection in organic agriculture and Integrated Pest Management.

Wheat Cultivar Growth, Biochemical, Physiological and Yield Attributes Response to Combined Exposure to Tropospheric Ozone, Particulate Matter Deposition and Ascorbic Acid Application

In the present study wheat (Triticum aestivum) cultivar HD 2967 was exposed to ambient and elevated levels of O₃ and PM deposition, with and without exogenous application of ascorbic acid (AA). Cultivar HD 2967 exposed to eight treatments in free air O₃ enrichment facility and the assessed results showed that wheat cultivar, growth, biochemical, physiological and yield attributes were variably but adversely affected by combined exposure to O₃ and PM deposition. PM deposition clogged stomata and enhanced leaf temperature. However, plants exposed to O₃ and PM deposition and treated with AA exhibited less reduction in yield as compared to plants exposed to O₃ and PM deposition without AA treatment. The decline in grain yield of HD 2967 due to combined exposure of O₃ and PM deposition were in the range of 4%-17%. AA spray partially mitigated ozone and PM deposition adverse impact and enhanced wheat yield by 16%.

Critical Review: Role of Inorganic Nanoparticle Properties on Their Foliar Uptake and in Planta Translocation

There is increasing pressure on global agricultural systems due to higher food demand, climate change, and environmental concerns. The design of nanostructures is proposed as one of the economically viable technological solutions that can make agrochemical use (fertilizers and pesticides) more efficient through reduced runoff, increased foliar uptake and bioavailability, and decreased environmental impacts. However, gaps in knowledge about the transport of nanoparticles across the leaf surface and their behavior in planta limit the rational design of nanoparticles for foliar delivery with controlled fate and limited risk. Here, the current literature on nano-objects deposited on leaves is reviewed. The different possible foliar routes of uptake (stomata, cuticle, trichomes, hydathodes, necrotic spots) are discussed, along with the paths of translocation, via the phloem, from the leaf to the end sinks (mature and developing tissues, roots, rhizosphere). This review details the interplays between morphological constraints, environmental stimuli, and physical-chemical properties of nanoparticles influencing their fate, transformation, and transport after foliar deposition. A metadata analysis from the existing literature highlighted that plant used for testing nanoparticle fate are most often dicotyledon plants (75%), while monocotyledons (as cereals) are less considered. Correlations on parameters calculated from the literature indicated that nanoparticle dose, size, zeta potential, and affinity to organic phases correlated with leaf-to-sink translocation, demonstrating that targeting nanoparticles to specific plant compartments by design should be achievable. Correlations also showed that time and plant growth seemed to be drivers for in planta mobility, parameters that are largely overlooked in the literature. This review thus highlights the material design opportunities and the knowledge gaps for targeted, stimuli driven deliveries of safe nanomaterials for agriculture.

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.

Adsorption of nanoparticles suspended in a drop on a leaf surface of Perilla frutescens and their infiltration through stomatal pathway

Particulate matter (PM) has become a severe environmental issue, and ultrafine PM particles such as PM2.5 or PM1 can cause various complications and respiratory diseases to human beings. In particular, heavy metals contained in PM particles can contaminate edible plants; for example, plant leaves are exposed to PM particle-laden raindrops. The contaminated edible plants can injure the human health by ingestion, so a detailed understanding on the accumulation of PM particles inside edible plants is essential. In this study, we investigate the infiltration of PM particles in plant tissues with a hypothesis that ultrafine PM particles are absorbed through stomatal pathways. As an edible test plant, Perilla frutescens is selected. Drops of gold nanoparticle (AuNP) suspension are deposited on a leaf of P. frutescens to simulate the scenario where PM particle-laden raindrops fall on patulous stomata of the test plant. To examine AuNP adsorption on the P. frutescens foliar surface and diffusional AuNP absorption through stomatal apertures, we investigate three physical dynamics of AuNPs suspended in a sessile drop: sedimentation, evaporation-driven convective flow, and shrinkage of the drop interface. Quantitative information on the 3D spatial distribution of AuNPs in plant tissues was measured by X-ray imaging and two-photon excitation microscopy.

Unravelling foliar water uptake pathways: The contribution of stomata and the cuticle

Plants can absorb water through their leaf surfaces, a phenomenon commonly referred to as foliar water uptake (FWU). Despite the physiological importance of FWU, the pathways and mechanisms underlying the process are not well known. Using a novel experimental approach, we parsed out the contribution of the stomata and the cuticle to FWU in two species with Mediterranean (Prunus dulcis) and temperate (Pyrus communis) origin. The hydraulic parameters of FWU were derived by analysing mass and water potential changes of leaves placed in a fog chamber. Leaves were previously treated with abscisic acid to force stomata to remain closed, with fusicoccin to remain open, and with water (control). Leaves with open stomata rehydrated two times faster than leaves with closed stomata and attained approximately three times higher maximum fluxes and hydraulic conductance. Based on FWU rates, we propose that rehydration through stomata occurs primarily via diffusion of water vapour rather than in liquid form even when leaf surfaces are covered with a water film. We discuss the potential mechanisms of FWU and the significance of both stomatal and cuticular pathways for plant productivity and survival.

Idioblasts and peltate hairs as distribution networks for water absorbed by xerophilous leaves

Capparis odoratissima is a tree species native to semi-arid environments of South America where low soil water availability coexists with frequent night-time fog. A previous study showed that water applied to leaf surfaces enhanced leaf hydration, photosynthesis and growth, but the mechanisms of foliar water uptake are unknown. Here, we combine detailed anatomical evaluations with water and dye uptake experiments in the laboratory, and use immunolocalization of pectin and arabinogalactan protein epitopes to characterize water uptake pathways in leaves. Abaxially, the leaves of C. odoratissima are covered with peltate hairs, while the adaxial surfaces are glabrous. Both surfaces are able to absorb condensed water, but the abaxial surface has higher rates of water uptake. Thousands of idioblasts per cm² , a higher density than stomata, connect the adaxial leaf surface and the abaxial peltate hairs, both of which contain hygroscopic substances such as arabinogalactan proteins and pectins. The highly specialized anatomy of the leaves of C odoratissima fulfils the dual function of minimizing water loss when stomata are closed, while maintaining the ability to absorb liquid water. Cell-wall related hygroscopic compounds in the peltate hairs and idioblasts create a network of microchannels that maintain leaf hydration and promote water uptake.

Life in a Droplet: Microbial Ecology in Microscopic Surface Wetness

While many natural and artificial surfaces may appear dry, they are in fact covered by thin liquid films and microdroplets invisible to the naked eye known as microscopic surface wetness (MSW). Central to the formation and the retention of MSW are the deliquescent properties of hygroscopic salts that prevent complete drying of wet surfaces or that drive the absorption of water until dissolution when the relative humidity is above a salt-specific level. As salts are ubiquitous, MSW occurs in many microbial habitats, such as soil, rocks, plant leaf, and root surfaces, the built environment, and human and animal skin. While key properties of MSW, including very high salinity and segregation into droplets, greatly affect microbial life therein, it has been scarcely studied, and systematic studies are only in their beginnings. Based on recent findings, we propose that the harsh micro-environment that MSW imposes, which is very different from bulk liquid, affects key aspects of bacterial ecology including survival traits, antibiotic response, competition, motility, communication, and exchange of genetic material. Further research is required to uncover the fundamental principles that govern microbial life and ecology in MSW. Such research will require multidisciplinary science cutting across biology, physics, and chemistry, while incorporating approaches from microbiology, genomics, microscopy, and computational modeling. The results of such research will be critical to understand microbial ecology in vast terrestrial habitats, affecting global biogeochemical cycles, as well as plant, animal, and human health.

Contribution of PM2.5-Pb in atmospheric fallout to Pb accumulation in Chinese cabbage leaves via stomata

Foliar uptake of Pb is especially important when Chinese cabbage (Brassica rapa spp. pekinensis), having a large leaf surface area, is cultivated in North China during seasons with heavy haze. However, the mechanisms of foliar Pb uptake via stomata by Chinese cabbage exposed to atmospheric fallout are unclear. A field experiment was conducted to explore the impacts of Pb in particulate matter with sizes ≤ 2.5 µm (PM_2.5-Pb) from atmospheric fallout to Pb accumulation in cabbage leaves through stomata. Cabbage varieties with low-Pb-accumulation (LPA) and high-Pb-accumulation (HPA) were examined using inductively coupled plasma-mass spectrometry and scanning electron microscopy/energy-dispersive X-ray analysis. The ²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁷Pb ratios of PM_2.5, plants, and soil demonstrated that the major source of Pb in cabbage leaves was PM_2.5. The average width and length of the stomatal apertures were 7.14 and 15.61 µm for LPA cabbage and 8.10 and 16.64 µm for HPA cabbage, which are large enough for PM_2.5-Pb to enter the leaves. The HPA cabbage had significantly higher stomatal width-to-length ratios than the LPA cabbage, indicating that the former trapped much more PM_2.5-Pb and accumulated more Pb. These results clarify the contributions of the stomatal characteristics to PM_2.5-Pb accumulation in the edible parts of Chinese cabbage.

Silica nanoparticles enhance disease resistance in Arabidopsis plants

In plants, pathogen attack can induce an immune response known as systemic acquired resistance that protects against a broad spectrum of pathogens. In the search for safer agrochemicals, silica nanoparticles (SiO2 NPs; food additive E551) have recently been proposed as a new tool. However, initial results are controversial, and the molecular mechanisms of SiO2 NP-induced disease resistance are unknown. Here we show that SiO2 NPs, as well as soluble Si(OH)4, can induce systemic acquired resistance in a dose-dependent manner, which involves the defence hormone salicylic acid. Nanoparticle uptake and action occurred exclusively through the stomata (leaf pores facilitating gas exchange) and involved extracellular adsorption in the air spaces in the spongy mesophyll of the leaf. In contrast to the treatment with SiO2 NPs, the induction of systemic acquired resistance by Si(OH)4 was problematic since high Si(OH)4 concentrations caused stress. We conclude that SiO2 NPs have the potential to serve as an inexpensive, highly efficient, safe and sustainable alternative for plant disease protection.

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.

Effects of soil moisture, needle age and leaf morphology on carbon and oxygen uptake, incorporation and allocation: a dual labeling approach with 13CO2 and H218O in foliage of a coniferous forest

The carbon and oxygen isotopic composition of water and assimilates in plants reveals valuable information on plant responses to climatic conditions. Yet, the carbon and oxygen uptake, incorporation and allocation processes determining isotopic compositions are not fully understood. We carried out a dual-isotope labeling experiment at high humidity with 18O-enriched water (H218O) and 13C-enriched CO2 (13CO2) with attached Scots pine (Pinus sylvestris L.) branches and detached twigs of hemiparasitic mistletoes (Viscum album ssp. austriacum) in a naturally dry coniferous forest, where also a long-term irrigation takes place. After 4 h of label exposure, we sampled previous- and recent-year leaves, twig phloem and twig xylem over 192 h for the analysis of isotope ratios in water and assimilates. For both species, the uptake into leaf water and the incorporation of the 18O-label into leaf assimilates was not influenced by soil moisture, while the 13C-label incorporation into assimilates was significantly higher under irrigation compared with control dry conditions. Species-specific differences in leaf morphology or needle age did not affect 18O-label uptake into leaf water, but the incorporation of both tracers into assimilates was two times lower in mistletoe than in pine. The 18O-label allocation in water from pine needles to twig tissues was two times higher for phloem than for xylem under both soil moisture conditions. In contrast, the allocation of both tracers in pine assimilates were similar and not affected by soil moisture, twig tissue or needle age. Soil moisture effects on 13C-label but not on 18O-label incorporation into assimilates can be explained by the stomatal responses at high humidity, non-stomatal pathways for water and isotope exchange reactions. Our results suggest that non-photosynthetic 18O-incorporation processes may have masked prevalent photosynthetic processes. Thus, isotopic variation in leaf water could also be imprinted on assimilates when photosynthetic assimilation rates are low.

Penetration of foliar-applied Zn and its impact on apple plant nutrition status: in vivo evaluation by synchrotron-based X-ray fluorescence microscopy

The absorption of foliar fertilizer is a complex process and is poorly understood. The ability to visualize and quantify the pathway that elements take following their application to leaf surfaces is critical for understanding the science and for practical applications of foliar fertilizers. By the use of synchrotron-based X-ray fluorescence to analyze the in vivo localization of elements, our study aimed to investigate the penetration of foliar-applied Zn absorbed by apple (Malus domestica Borkh.) leaves with different physiological surface properties, as well as the possible interactions between foliar Zn level and the mineral nutrient status of treated leaves. The results indicate that the absorption of foliar-applied Zn was largely dependent on plant leaf surface characteristics. High-resolution elemental maps revealed that the high binding capacity of the cell wall for Zn contributed to the observed limitation of Zn penetration across epidermal cells. Trichome density and stomatal aperture had opposite effects on Zn fertilizer penetration: a relatively high density of trichomes increased the hydrophobicity of leaves, whereas the presence of stomata facilitated foliar Zn penetration. Low levels of Zn promoted the accumulation of other mineral elements in treated leaves, and the complexation of Zn with phytic acid potentially occurred owing to exposure to high-Zn conditions. The present study provides direct visual evidence for the Zn penetration process across the leaf surface, which is important for the development of strategies for Zn biofortification in crop species.

Towards a coupled paradigm of NH3 -CO2 biosphere-atmosphere exchange modelling

Stomatal conductance, one of the major plant physiological controls within NH₃ biosphere-atmosphere exchange models, is commonly estimated from semi-empirical multiplicative schemes or simple light- and temperature-response functions. However, due to their inherent parameterization on meteorological proxy variables, instead of a direct measure of stomatal opening, they are unfit for the use in climate change scenarios and of limited value for interpreting field-scale measurements. Alternatives based on H₂ O flux measurements suffer from uncertainties in the partitioning of evapotranspiration at humid sites, as well as a potential decoupling of transpiration from stomatal opening in the presence of hygroscopic particles on leaf surfaces. We argue that these problems may be avoided by directly deriving stomatal conductance from CO₂ fluxes instead. We reanalysed a data set of NH₃ flux measurements based on CO₂ -derived stomatal conductance, confirming the hypothesis that the increasing relevance of stomatal exchange with the onset of vegetation activity caused a rapid decrease of observed NH₃ deposition velocities. Finally, we argue that developing more mechanistic representations of NH₃ biosphere-atmosphere exchange can be of great benefit in many applications. These range from model-based flux partitioning, over deposition monitoring using low-cost samplers and inferential modelling, to a direct response of NH₃ exchange to climate change.

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.

Ecophysiological Traits of Invasive C3 Species Calotropis procera to Maintain High Photosynthetic Performance Under High VPD and Low Soil Water Balance in Semi-Arid and Seacoast Zones

The evergreen C3 plant Calotropis procera is native to arid environments. Thus, it grows under high vapor pressure deficit (VPD), intense light, and severe drought conditions. We measured several ecophysiological traits in C. procera plants growing in semi-arid and seacoast environments to assess the attributes that support its photosynthetic performance under these contrasting conditions. Gas exchange analysis, primary metabolism content, nutrients, the antioxidant system, and leaf anatomy traits were measured under field conditions. In the semi-arid environment, C. procera was exposed to a prolonged drought season with a negative soil water balance during the 2 years of the study. Calotropis procera plants were exposed to a positive soil water balance only in the rainy season in the seacoast environment. The leaves of C. procera showed the same photosynthetic rate under high or low VPD, even in dry seasons with a negative soil water balance. Photosynthetic pigments, leaf sugar content, and the activity of antioxidant enzymes were increased in both places in the dry season. However, the anatomical adjustments were contrasting: while, in the semi-arid environment, mesophyll thickness increased in the driest year, in the seacoast environment, the cuticle thickness and trichome density were increased. The ability to maintain photosynthetic performance through the seasons would be supported by new leaves with different morpho-anatomical traits, with contrasting changes between semi-arid and seacoast environments. Furthermore, our results suggest that an efficient antioxidative system and leaf sugar dynamics can contribute to protecting the photosynthetic machinery even under severe drought.

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.