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

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.

Foliar nitrogen uptake in broadleaf evergreen Mediterranean forests: Fertilisation experiment with labelled nitrogen

Atmospheric nitrogen (N) deposition in Mediterranean sclerophyllous forests of Holm oak (Quercus rotundifolia, Q. ilex) in Spain often exceeds empirical critical loads established for ecosystem conservation. There are still uncertainties on the capacity of canopy retention and uptake of the atmospheric N deposited of these forests. Studying and analysing all the forest nitrogen-cycle processes is essential to understand the potential effect of N deposition in these ecosystems. This study conducted a year-long short-term fertilisation experiment with labelled ammonium (15N-NH4) and nitrate (15N-NO3) to estimate foliar N absorption rates and assess the influence of leaf phenology and meteorological seasonal variations. Fertilising solutions were prepared to simulate low and high wet N deposition concentration, based on data reported from previous studies. Additionally, ecophysiological and meteorological measurements were collected to explore potential relationships between absorption rates, plant activity, and weather conditions. The results showed that Holm oak leaves were able to absorb both oxidised and reduced N compounds, with higher rates of NH4+ absorption. N recovery of both NH4+ and NO3- was higher in the low concentration treatments, suggesting reduced effectiveness of absorption as concentration increases. Foliar absorption rates were leaf-age dependent, with the highest values observed in young developing leaves. Foliar uptake showed seasonal changes with a clear reduction during the summer, linked to drought and dry weather conditions, and showing also smaller leaf net assimilation and stomatal conductance. During the rest of the year, foliar N absorption was not clearly associated to plant physiological activity but with environmental conditions. Our findings suggest that Holm oak canopies could absorb an important part of the incoming N deposition, but this process is compound, season and leaf phenology dependent. Further research is therefore needed to better understand and model this part of the N cycle.

The Chemical Landscape of Leaf Surfaces and Its Interaction with the Atmosphere

Atmospheric chemists have historically treated leaves as inert surfaces that merely emit volatile hydrocarbons. However, a growing body of evidence suggests that leaves are ubiquitous substrates for multiphase reactions-implying the presence of chemicals on their surfaces. This Review provides an overview of the chemistry and reactivity of the leaf surface's "chemical landscape", the dynamic ensemble of compounds covering plant leaves. We classified chemicals as endogenous (originating from the plant and its biome) or exogenous (delivered from the environment), highlighting the biological, geographical, and meteorological factors driving their contributions. Based on available data, we predicted ≫2 μg cm-2 of organics on a typical leaf, leading to a global estimate of ≫3 Tg for multiphase reactions. Our work also highlighted three major knowledge gaps: (i) the overlooked role of ambient water in enabling the leaching of endogenous substances and mediating aqueous chemistry; (ii) the importance of phyllosphere biofilms in shaping leaf surface chemistry and reactivity; (iii) the paucity of studies on the multiphase reactivity of atmospheric oxidants with leaf-adsorbed chemicals. Although biased toward available data, we hope this Review will spark a renewed interest in the leaf surface's chemical landscape and encourage multidisciplinary collaborations to move the field forward.

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

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

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.

'Follow the Water': Microbial Water Acquisition in Desert Soils

Water availability is the dominant driver of microbial community structure and function in desert soils. However, these habitats typically only receive very infrequent large-scale water inputs (e.g., from precipitation and/or run-off). In light of recent studies, the paradigm that desert soil microorganisms are largely dormant under xeric conditions is questionable. Gene expression profiling of microbial communities in desert soils suggests that many microbial taxa retain some metabolic functionality, even under severely xeric conditions. It, therefore, follows that other, less obvious sources of water may sustain the microbial cellular and community functionality in desert soil niches. Such sources include a range of precipitation and condensation processes, including rainfall, snow, dew, fog, and nocturnal distillation, all of which may vary quantitatively depending on the location and geomorphological characteristics of the desert ecosystem. Other more obscure sources of bioavailable water may include groundwater-derived water vapour, hydrated minerals, and metabolic hydro-genesis. Here, we explore the possible sources of bioavailable water in the context of microbial survival and function in xeric desert soils. With global climate change projected to have profound effects on both hot and cold deserts, we also explore the potential impacts of climate-induced changes in water availability on soil microbiomes in these extreme environments.

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.

Diagnosing the Model Bias in Simulating Daily Surface Ozone Variability Using a Machine Learning Method: The Effects of Dry Deposition and Cloud Optical Depth

Machine learning methods are increasingly used in air quality studies to predict air pollution levels, while few applied them to diagnose and improve the underlying mechanisms controlling air pollution represented in chemical transport models (CTMs). Here, we use the random forest (RF) method to diagnose high biases of surface daily maximum 8 h average (MDA8) ozone concentrations in the GEOS-Chem CTM evaluated against measurements from the nationwide monitoring network in summer 2018 over China. The feature importance results show that cloud optical depth (COD), relative humidity, and precipitation are the top three factors affecting CTM high biases. Such results indicate that the high ozone biases in summer over China mainly occur on wet/cloudy days (∼40% biased high), while biases on dry/clear days are small (within 5%). We link the important features with model parameterizations and variables, identifying model underestimates in the dry deposition velocity and COD on wet/cloudy days. By accounting for the enhanced dry deposition on wet plant cuticles and using satellite observation constrained COD, we find that CTM high ozone biases can be halved with an improved agreement in the temporal variability, highlighting the effects of dry deposition and COD on ozone, as suggested by the RF outcomes.

Droplet size and surface hydrophobicity enhance bacterial plasmid transfer rates in microscopic surface wetness

Conjugal plasmids constitute a major engine for horizontal gene transfer in bacteria, and are key drivers of the spread of antibiotic resistance, virulence, and metabolic functions. Bacteria in terrestrial habitats often inhabit surfaces that are not constantly water-saturated, where microscopic surface wetness (MSW), comprised of thin liquid films and microdroplets, permanently or intermittently occurs. How physical properties of microdroplets, and of the surfaces they reside on, affect plasmid transfer rates is not well understood. Here, building on microscopy-based microdroplet experiments, we examined the relation between droplet properties (size and spread) and plasmid transfer rates at single-cell and individual droplet resolution, using Pseudomonas putida as a model species. We show that transfer rates increase with droplet size, due to higher densities of cells on the surface in larger droplets, resulting from lower ratio between the area of the liquid-solid interface and droplet volumes. We further show that surface hydrophobicity promotes transfer rates via the same mechanism. Our results provide new insights into how physical properties of surfaces and MSW affect plasmid transfer rates, and more generally, microbial interactions mediated by cell-to-cell contact, with important implications for our understanding of the ecology and evolution of bacteria in unsaturated environments.

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.

Could Bacillus biofilms enhance the effectivity of biocontrol strategies in the phyllosphere?

Maize (Zea mays L.), a major crop in Argentina and a staple food around the world, is affected by the emergence and re-emergence of foliar diseases. Agrochemicals are the main control strategy nowadays, but they can cause resistance in insects and microbial pathogens and have negative effects on the environment and human health. An emerging alternative is the use of living organisms, i.e. microbial biocontrol agents, to suppress plant pathogen populations. This is a risk-free approach when the organisms acting as biocontrol agents come from the same ecosystem as the foliar pathogens they are meant to antagonize. Some epiphytic microorganisms may form biofilm by becoming aggregated and attached to a surface, as is the case of spore-forming bacteria from the genus Bacillus. Their ability to sporulate and their tolerance to long storage periods make them a frequently used biocontrol agent. Moreover, the biofilm that they create protects them against different abiotic and biotic factors and helps them to acquire nutrients, which ensures their survival on the plants they protect. This review analyzes the interactions that the phyllosphere-inhabiting Bacillus genus establishes with its environment through biofilm, and how this lifestyle could serve to design effective biological control strategies.

Wet-dry cycles protect surface-colonizing bacteria from major antibiotic classes

Diverse antibiotic compounds are abundant in microbial habitats undergoing recurrent wet-dry cycles, such as soil, root and leaf surfaces, and the built environment. These antibiotics play a central role in microbial warfare and competition, thus affecting population dynamics and the composition of natural microbial communities. Yet, the impact of wet-dry cycles on bacterial response to antibiotics has been scarcely explored. Using the bacterium E. coli as a model organism, we show through a combination of experiments and computational modeling, that wet-dry cycles protect bacteria from beta-lactams. This is due to the combined effect of several mechanisms including tolerance induced by high salt concentrations and slow cell-growth, which are inherently associated with microscopic surface wetness-a hydration state typical to 'dry' periods. Moreover, we find evidence for a cross-protection effect, where lethal doses of antibiotic considerably increase bacterial survival during the dry periods. This work focuses on beta-lactams, yet similar protection was observed for additional major antibiotic classes. Our findings shed new light on how we understand bacterial response to antibiotics, with broad implications for population dynamics, interspecies interactions, and the evolution of antibiotic resistance in vast terrestrial microbial habitats.

Leaf surface microtopography shaping the bacterial community in the phyllosphere: evidence from 11 tree species

Phyllosphere bacteria are an important component of environmental microbial communities and are closely related to plant health and ecosystem stability. However, the relationships between the inhabitation and assembly of phyllosphere bacteria and leaf microtopography are still obscure. In this study, the phyllosphere bacterial communities and leaf microtopographic features (vein density, stomatal length, and density) of eleven tree species were fully examined. Both the absolute abundance and diversity of phyllosphere bacterial communities were significantly different among the tree species, and leaf vein density dominated the variation. TITAN analysis showed that leaf vein density also played more important roles in regulating the relative abundance of bacteria than stomatal features, and 6 phyla and 62 genera of phyllosphere bacteria showed significant positive responses to leaf vein density. Moreover, LEfSe analysis showed that the leaves with higher vein density had more bacterial biomarkers. Leaf vein density also changed the co-occurrence pattern of phyllosphere bacteria, and the co-occurrence network demonstrated more negative correlations and more nodes on the leaves with larger leaf vein density, indicating that higher densities of leaf veins improved the stability of the phyllosphere bacterial community. Phylogenetic analysis showed that deterministic processes (especially homogeneous selection) dominated the assembly process of phyllosphere bacterial communities. The leaf vein density increased the degree of bacterial clustering at the phylogenetic level. Therefore, the inhabitation and assembly of the phyllosphere bacterial community are related to leaf microtopography, which provides deeper insight into the interaction between plants and bacteria.

Acid or base? How do plants regulate the ecology of their phylloplane?

Plants interface with and modify the external environment across their surfaces, and in so doing, can control or mitigate the impacts of abiotic stresses and also mediate their interactions with other organisms. Botanically, it is known that plant roots have a multi-faceted ability to modify rhizosphere conditions like pH, a factor with a large effect on a plant's biotic interactions with microbes. But plants can also modify pH levels on the surfaces of their leaves. Plants can neutralize acid rain inputs in a period of hours, and either acidify or alkalinize the pH of neutral water droplets in minutes. The pH of the phylloplane-that is, the outermost surface of the leaf-varies across species, from incredibly acidic (carnivorous plants: as low as pH 1) to exceptionally alkaline (species in the plant family, Malvaceae, up to pH 11). However, most species mildly acidify droplets on the phylloplane by 1.5 orders of magnitude in pH. Just as rhizosphere pH helps shape the plant microbiome and is known to influence belowground interactions, so too can phylloplane pH influence aboveground interactions in plant canopies. In this review, we discuss phylloplane pH regulation from the physiological, molecular, evolutionary, and ecological perspectives and address knowledge gaps and identify future research directions.

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.

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.

Similar effects as shade tolerance induced by dust accumulation and size penetration of particulates on cotton leaves

BACKGROUND

Dust accumulation covers the leaf's surface and influences foliar physiological activity. Two independent experiments were carried out to instigate the foliar responses to dust accumulation and the penetration limitation of small dust particles (< 1 μm) on the foliar surface, respectively. In experiment I, three dust accumulation intensities were achieved by a dust spraying treatment. Photosynthesis CO₂ exchange and fast chlorophyll fluorescence transient were measured, as well as chlorophyll contents and leaf thickness. In experiment II, the penetration limits of small particulates on the leaf surface were examined by feeding nano-fluorescent microspheres.

RESULTS

Dust accumulation alleviated the photoinhibition of Photosystem II and decreased photosynthesis, as represented by net photosynthetic rates (P_N) and stomatal conductance to water vapor (g_s). Photosynthetic response curves between net photosynthetic rate (P_N) and photosynthetically active radiation (PAR) showed that heavy dust accumulation (34.98 ± 2.6 mg cm^- 2) increased the light compensation point (LCP) and light saturation point (LSP) and decreased photosynthesis rates under saturating light (P_Nmax). Leaves became thin due to the lack of a palisade layer while chlorophyll content increased under dust accumulation. Confocal laser scanning microscopy (CLSM) images showed that the larger particles (1 μm) distributed in the regions below the stomata and the smaller ones (0.1 μm) were detected in the wider areas below stomata.

CONCLUSIONS

These results suggested that dust accumulation induced similar effects as shade tolerance in cotton leaves but did not trigger more photochemical acclimation to low light. Dust particles (< 1 μm) penetrated leaf surface through stomata.

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.

Sporangiospore Viability and Oospore Production in the Spinach Downy Mildew Pathogen, Peronospora effusa

Downy mildew of spinach, caused by the obligate pathogen Peronospora effusa, remains the most important constraint in the major spinach production areas in the United States. This disease can potentially be initiated by asexual sporangiospores via "green bridges", sexually derived oospores from seed or soil, or dormant mycelium. However, the relative importance of the various types of primary inoculum is not well known. The ability of P. effusa sporangiospores to withstand abiotic stress, such as desiccation, and remain viable during short- and long-distance dispersal and the ability of oospores to germinate and infect seedlings remain unclear. Thus, the primary objectives of this research were to evaluate the impact of desiccation on sporangiospore survival and infection efficiency and examine occurrence, production, and germination of oospores. Results indicate that desiccation significantly reduces sporangiospore viability as well as infection potential. Leaf wetness duration of 4 h was needed for disease establishment by spinach downy mildew sporangiospores. Oospores were observed in leaves of numerous commercial spinach cultivars grown in California in 2018 and Arizona in 2019. Frequency of occurrence varied between the two states-years. The presence of opposite mating types in spinach production areas in the United States was demonstrated by pairing isolates in controlled crosses and producing oospores on detached leaves as well as intact plants. Information from the study of variables that affect sporangiospore viability and oospore production will help in improving our understanding of the epidemiology of this important pathogen, which has implications for management of spinach downy mildew.

Heterogeneity of Stomatal Pore Area Is Suppressed by Ambient Aerosol in the Homobaric Species, Vicia faba

Stomatal pore area is heterogeneous across leaf surfaces. This has been considered as "patchy stomatal conductance," and may have substantial implications for photosynthetic efficiency. Aerosols have always been important elements of plant environments, but their effects on stomatal control of plant water relations, and stomatal heterogeneity specifically, have not been considered. Here we evaluate the spatial coordination of pore area in the glabrous and homobaric leaves of Vicia faba grown under two aerosol treatments and measured at four levels of VPD. We construct a large dataset (n > 88,000 discrete comparisons) of paired pore areas and distances between the pores. Plants were grown in ambient urban air and in filtered air (FA) to determine the effect of ambient aerosol on stomatal properties. Pore area exhibited spatial organization, as well as considerable variability among closely co-located pores. The difference between pore areas was positively correlated with the distance between the pores, in both aerosol treatments and at all VPDs. However, aerosol deposition reduced both the magnitude of variability between pores and the rate at which this variability increased with pore separation distance. These data support previous conclusions that deposition of hygroscopic aerosol may create a thin aqueous film across the leaf surface that connects neighboring stomata to each other and to the leaf interior. Aerosol impacts on stomatal heterogeneity and gas exchange are not adequately considered in current assessments of stomatal control.

Dry Deposition of Ozone over Land: Processes, Measurement, and Modeling

Dry deposition of ozone is an important sink of ozone in near surface air. When dry deposition occurs through plant stomata, ozone can injure the plant, altering water and carbon cycling and reducing crop yields. Quantifying both stomatal and nonstomatal uptake accurately is relevant for understanding ozone's impact on human health as an air pollutant and on climate as a potent short-lived greenhouse gas and primary control on the removal of several reactive greenhouse gases and air pollutants. Robust ozone dry deposition estimates require knowledge of the relative importance of individual deposition pathways, but spatiotemporal variability in nonstomatal deposition is poorly understood. Here we integrate understanding of ozone deposition processes by synthesizing research from fields such as atmospheric chemistry, ecology, and meteorology. We critically review methods for measurements and modeling, highlighting the empiricism that underpins modeling and thus the interpretation of observations. Our unprecedented synthesis of knowledge on deposition pathways, particularly soil and leaf cuticles, reveals process understanding not yet included in widely-used models. If coordinated with short-term field intensives, laboratory studies, and mechanistic modeling, measurements from a few long-term sites would bridge the molecular to ecosystem scales necessary to establish the relative importance of individual deposition pathways and the extent to which they vary in space and time. Our recommended approaches seek to close knowledge gaps that currently limit quantifying the impact of ozone dry deposition on air quality, ecosystems, and climate.

A review of measurements of air-surface exchange of reactive nitrogen in natural ecosystems across North America

This review summarizes the state of the science of measurements of dry deposition of reactive nitrogen (Nr) compounds in North America, beginning with current understanding of the importance of dry deposition at the U.S. continental scale followed by a review of micrometeorological flux measurement methods. Measurements of Nr air-surface exchange in natural ecosystems of North America are then summarized, focusing on the U.S. and Canada. Drawing on this synthesis, research needed to address the incompleteness of dry deposition budgets, more fully characterize temporal and geographical variability of fluxes, and better understand air-surface exchange processes is identified. Our assessment points to several data and knowledge gaps that must be addressed to advance dry deposition budgets and air-surface exchange modeling for North American ecosystems. For example, recent studies of particulate (NO3-) and gaseous (NOx, HONO, peroxy nitrates) oxidized N fluxes challenge the fundamental framework of unidirectional flux from the atmosphere to the surface employed in most deposition models. Measurements in forest ecosystems document the importance of in-canopy chemical processes in regulating the net flux between the atmosphere and biosphere, which can result in net loss from the canopy. These results emphasize the need for studies to quantify within- and near-canopy sources and sinks of the full suite of components of the Nr chemical system under study (e.g., NOy or HNO3-NH3-NH4NO3). With respect to specific ecosystems and geographical locations, additional flux measurements are needed particularly in agricultural regions (NH3), coastal zones (NO3- and organic N), and arid ecosystems and along urban to rural gradients (NO2). Measurements that investigate non-stomatal exchange processes (e.g., deposition to wet surfaces) and the biogeochemical drivers of bidirectional exchange (e.g., NH3) are considered high priority. Establishment of long-term sites for process level measurements of reactive chemical fluxes should be viewed as a high priority long-term endeavor of the atmospheric chemistry and ecological communities.

Toward the improvement of total nitrogen deposition budgets in the United States

Frameworks for limiting ecosystem exposure to excess nutrients and acidity require accurate and complete deposition budgets of reactive nitrogen (Nr). While much progress has been made in developing total Nr deposition budgets for the U.S., current budgets remain limited by key data and knowledge gaps. Analysis of National Atmospheric Deposition Program Total Deposition (NADP/TDep) data illustrates several aspects of current Nr deposition that motivate additional research. Averaged across the continental U.S., dry deposition contributes slightly more (55%) to total deposition than wet deposition and is the dominant process (>90%) over broad areas of the Southwest and other arid regions of the West. Lack of dry deposition measurements imposes a reliance on models, resulting in a much higher degree of uncertainty relative to wet deposition which is routinely measured. As nitrogen oxide (NOx) emissions continue to decline, reduced forms of inorganic nitrogen (NHx = NH3 + NH4+) now contribute >50% of total Nr deposition over large areas of the U.S. Expanded monitoring and additional process-level research are needed to better understand NHx deposition, its contribution to total Nr deposition budgets, and the processes by which reduced N deposits to ecosystems. Urban and suburban areas are hotspots where routine monitoring of oxidized and reduced Nr deposition is needed. Finally, deposition budgets have incomplete information about the speciation of atmospheric nitrogen; monitoring networks do not capture important forms of Nr such as organic nitrogen. Building on these themes, we detail the state of the science of Nr deposition budgets in the U.S. and highlight research priorities to improve deposition budgets in terms of monitoring and flux measurements, leaf- to regional-scale modeling, source apportionment, and characterization of deposition trends and patterns.

Life on a leaf

Microscopic water films allow bacteria to survive the seemingly dry surface of plant leaves.

Bacterial survival in microscopic surface wetness

Plant leaves constitute a huge microbial habitat of global importance. How microorganisms survive the dry daytime on leaves and avoid desiccation is not well understood. There is evidence that microscopic surface wetness in the form of thin films and micrometer-sized droplets, invisible to the naked eye, persists on leaves during daytime due to deliquescence - the absorption of water until dissolution - of hygroscopic aerosols. Here, we study how such microscopic wetness affects cell survival. We show that, on surfaces drying under moderate humidity, stable microdroplets form around bacterial aggregates due to capillary pinning and deliquescence. Notably, droplet-size increases with aggregate-size, and cell survival is higher the larger the droplet. This phenomenon was observed for 13 bacterial species, two of which - Pseudomonas fluorescens and P. putida - were studied in depth. Microdroplet formation around aggregates is likely key to bacterial survival in a variety of unsaturated microbial habitats, including leaf surfaces.

Pseudomonas syringae Increases Water Availability in Leaf Microenvironments via Production of Hygroscopic Syringafactin

The epiphytic bacterium Pseudomonas syringae strain B728a produces the biosurfactant syringafactin, which is hygroscopic. The water-absorbing potential of syringafactin is high. Syringafactin attracts 250% of its weight in water at high relative humidities but is less hygroscopic at lower relative humidities. This finding suggests that the benefit of syringafactin to the producing cells is strongly context dependent. The contribution of syringafactin to the water availability around cells on different matrices was assessed by examining the water stress exhibited by biosensor strains expressing gfp via the water-stress-activated proU promoter. Wild-type cells exhibited significantly less green fluorescent protein (GFP) fluorescence than a syringafactin-deficient strain on dry filters in atmospheres of high water saturation, as well as on leaf surfaces, indicating greater water availability. When infiltrated into the leaf apoplast, wild-type cells also subsequently exhibited less GFP fluorescence than the syringafactin-deficient strain. These results suggest that the apoplast is a dry but humid environment and that, just as on dry but humid leaf surfaces, syringafactin increases liquid water availability and reduces the water stress experienced by P. syringaeIMPORTANCE Many microorganisms, including the plant pathogen Pseudomonas syringae, produce amphiphilic compounds known as biosurfactants. While biosurfactants are known to disperse hydrophobic compounds and to reduce water tension, they have other properties that can benefit the cells that produce them. Leaf-colonizing bacteria experience frequent water stress, since liquid water is present only transiently on or in leaf sites that they colonize. The demonstration that syringafactin, a biosurfactant produced by P. syringae, is sufficiently hygroscopic to increase water availability to cells, thus relieving water stress, reveals that P. syringae can modify its local habitat both on leaf surfaces and in the leaf apoplast. Such habitat modification may be a common role for biosurfactants produced by other bacterial species that colonize habitats (such as soil) that are not always water saturated.

Hydraulic redistribution of foliar absorbed water causes turgor-driven growth in mangrove seedlings

Although foliar water uptake (FWU) has been shown in mature Avicennia marina trees, the importance for its seedlings remains largely unknown. A series of experiments were therefore performed using artificial rainfall events in a greenhouse environment to assess the ecological implications of FWU in A. marina seedlings. One-hour artificial rainfall events resulted in an increased leaf water potential, a reversed sap flow, and a rapid diameter increment signifying a turgor-driven growth of up to 30.1 ± 5.4 μm. Furthermore, the application of an artificial rainfall event with deuterated water showed that the amount of water absorbed by the leaves and transported to the stem was directly and univocally correlated to the observed growth spurts. The observations in this process-based study show that FWU is an important water acquisition mechanism under certain circumstances and might be of ecological importance for the establishment of A. marina seedlings. Distribution of mangrove trees might hence be more significantly disturbed by climate change-driven changes in rainfall patterns than previously assumed.

Influence of Drought on Foliar Water Uptake Capacity of Temperate Tree Species

Foliar water uptake (FWU) has been investigated in an increasing number of species from a variety of areas but has remained largely understudied in deciduous, temperate tree species from non-foggy regions. As leaf wetting events frequently occur in temperate regions, FWU might be more important than previously thought and should be investigated. As climate change progresses, the number of drought events is expected to increase, basically resulting in a decreasing number of leaf wetting events, which might make FWU a seemingly less important mechanism. However, the impact of drought on FWU might not be that unidirectional because drought will also cause a more negative tree water potential, which is expected to result in more FWU. It yet remains unclear whether drought results in a general increase or decrease in the amount of water absorbed by leaves. The main objectives of this study are, therefore: (i) to assess FWU-capacity in nine widely distributed key tree species from temperate regions, and (ii) to investigate the effect of drought on FWU in these species. Based on measurements of leaf and soil water potential and FWU-capacity, the effect of drought on FWU in temperate tree species was assessed. Eight out of nine temperate tree species were able to absorb water via their leaves. The amount of water absorbed by leaves and the response of this plant trait to drought were species-dependent, with a general increase in the amount of water absorbed as leaf water potential decreased. This relationship was less pronounced when using soil water potential as an independent variable. We were able to classify species according to their response in FWU to drought at the leaf level, but this classification changed when using drought at the soil level, and was driven by iso- and anisohydric behavior. FWU hence occurred in several key tree species from temperate regions, be it with some variability, which potentially allows these species to partly reduce the effects of drought stress. We recommend including this mechanism in future research regarding plant–water relations and to investigate the impact of different pathways used for FWU.

Mathematical Modeling of Diffusion of a Hydrophilic Ionic Fertilizer in Plant Cuticles: Surfactant and Hygroscopic Effects

The agricultural industry requires improved efficacy of sprays being applied to crops and weeds to reduce their environmental impact and increase financial returns. One way to improve efficacy is by enhancing foliar penetration. The plant leaf cuticle is the most significant barrier to agrochemical diffusion within the leaf. The importance of a mechanistic mathematical model has been noted previously in the literature, as each penetration experiment is dictated by its specific parameters, namely plant species, environmental conditions such as relative humidity and spray formulation including adjuvant addition. A mechanistic mathematical model has been previously developed by the authors, focusing on plant cuticle diffusion of calcium chloride through tomato fruit cuticles including pore swelling, ion binding and evaporation, along with the ability to vary the active ingredient concentration and type, relative humidity and plant species. Here we further develop this model to include adjuvant effects as well as the hygroscopic nature of deliquescent ionic solutions with evaporation on the cuticle surface. These modifications to a penetration and evaporation model provide a novel addition to the literature and allow the model to be applied to many types of evaporating ionic hygroscopic solutions on many types of substrates, not just plant cuticles. We validate our theoretical model results against appropriate experimental data, discuss key sensitivities and relate theoretical predictions to physical mechanisms. The important governing mechanisms influencing surfactant enhanced penetration of ionic active through plant cuticles were found to be aqueous pore radius, pore density, cuticle thickness and initial contact angle of the applied droplet; ion binding, relative humidity and evaporation including hygroscopic water absorption parameters for point of deliquescence. The sensitivity analysis indicated surfactants increase penetration by changing the point of deliquescence of a solution, which alters the water absorption and the initial contact angle, which alters the number of pores under the droplet. The results of the validation and sensitivity analysis imply that this model accounts for many of the mechanisms governing penetration in plant cuticles.

The value of wet leaves

Contents Summary 1156 I. Introduction 1156 II. How often are leaves wet? 1157 III. The costs of leaf wetting 1157 IV. The real and potential benefits of leaf wetting 1161 V. Wet leaves: costs, benefits and tradeoffs in a changing world 1165 Acknowledgements 1166 References 1166 SUMMARY: An often-overlooked feature of all plants is that their leaf surfaces are wet for significant periods over their lifetimes. Leaf wetting has a number of direct and indirect effects on plant function from the scale of the leaf to that of the ecosystem. The costs of leaf wetting for plant function, such as the growth of pathogens and the leaching of nutrients, have long been recognized. However, an emerging body of research has also begun to demonstrate some very clear benefits. For instance, leaf wetting can improve plant-water relations and lead to increased photosynthesis. Leaf wetting may also lead to synergistic effects on plant function, such as when leaf water potential improvements lead to enhanced growth that does not occur when plant leaves are dry. We identify important reasons why leaf wetting can be critical for plant sciences to not only acknowledge, but also directly address, in future research. To do so, we provide a framework for the consideration of the relative balance of the various costs and benefits resulting from leaf wetting, as well as how this balance may be expected to change given projected scenarios of global climate change in the future.

Ambient aerosol increases minimum leaf conductance and alters the aperture-flux relationship as stomata respond to vapor pressure deficit (VPD)

Aerosols are important components of the global plant environment, with beneficial and deleterious impacts. The direct effects of aerosol deposition on plant-water relationships remain poorly characterized but potentially important. Vicia faba was grown in ambient urban air and in the same air with aerosol excluded, in a moderately polluted environment using two exposure protocols. Simultaneous measurement of gas exchange and stomatal pore aperture was combined with leaf dehydration kinetics and microscopic evaluation of leaf wetness formation and aerosol deposition patterns. The ambient aerosol was shown to be hygroscopic. Aerosol exposure increased minimum leaf conductance, shown by dehydration kinetics, and nocturnal water vapor flux, shown by dark-adapted gas exchange. Aerosol exposure decreased stomatal apertures at each level of vapor pressure deficit (VPD) and increased stomatal conductance at comparable levels of aperture. Overall, these effects were modest, and largest when stomata were wide open. The uncoupling of conductance (flux-based) from aperture (directly measured microscopically) implies that aerosol-induced water loss is not fully under stomatal control. This reduces drought tolerance and may provide a mechanism by which deposited aerosol plays a direct role in stomatal response to VPD.

Spatio-temporal variations in PM leaf deposition: A meta-analysis

Particulate matter (PM) pollution in urban cities is of great concern for public health due to its global and adverse effect of human health while ecosystems function and vegetation control is an effective and eco-friendly way to alleviate PM pollution. We reviewed 150 studies conducted in 15 countries that were published between 1960 and 2016 and used a meta-analysis to examine the time trends and regional differences in leaf deposited PM of urban greening plants. The results suggested that the weekly PM leaf deposition varied markedly with both plant species and space-time and the average value was 1.71 ± 0.05 g m^-2·wk^-1, and the variations occurred because of vegetation factors, characteristics of the PM source and meteorological factors. Moreover, fine particulate matter accounts for the minimum proportion of the total PM mass but its number ratio is maximum, more than 90% of the total number of particles. This meta-analysis illustrated the spatio-temporal trends and variations in PM leaf deposition and the influencing factors, which provides a scientific basis for the mechanism of PM deposition on leaf surface as well as plant selection and configuration in urban greening.

Novel Protein-Protein Inhibitor Based Approach to Control Plant Ethylene Responses: Synthetic Peptides for Ripening Control

Ethylene signaling is decisive for many plant developmental processes. Among these, control of senescence, abscission and fruit ripening are of fundamental relevance for global agriculture. Consequently, detailed knowledge of the signaling network along with the molecular processes of signal perception and transfer are expected to have high impact on future food production and agriculture. Recent advances in ethylene research have demonstrated that signaling of the plant hormone critically depends on the interaction of the ethylene receptor family with the NRAMP-like membrane protein ETHYLENE INSENSITIVE 2 (EIN2) at the ER membrane, phosphorylation-dependent proteolytic processing of ER-localized EIN2 and subsequent translocation of the cleaved EIN2 C-terminal polypeptide (EIN2-CEND) to the nucleus. EIN2 nuclear transport, but also interaction with the receptors sensing the ethylene signal, both, depend on a nuclear localization signal (NLS) located at the EIN2 C-terminus. Loss of the tight interaction between receptors and EIN2 affects ethylene signaling and impairs plant ethylene responses. Synthetic peptides derived from the NLS sequence interfere with the EIN2-receptor interaction and have utility in controlling plant ethylene responses such as ripening. Here, we report that a synthetic peptide (NOP-1) corresponding to the NLS motif of Arabidopsis EIN2 (aa 1262-1269) efficiently binds to tomato ethylene receptors LeETR4 and NR and delays ripening in the post-harvest phase when applied to the surface of sampled green fruits pre-harvest. In particular, degradation of chlorophylls was delayed by several days, as monitored by optical sensors and confirmed by analytical methods. Similarly, accumulation of β-carotene and lycopene in the fruit pulp after NOP-1 application was delayed, without having impact on the total pigment concentration in the completely ripe fruits. Likewise, the peptide had no negative effects on fruit quality. Our molecular and phenotypic studies reveal that peptide biologicals could contribute to the development of a novel family of ripening inhibitors and innovative ripening control in climacteric fruit.

How does the VPD response of isohydric and anisohydric plants depend on leaf surface particles?

Atmospheric vapour pressure deficit (VPD) is the driving force for plant transpiration. Plants have different strategies to respond to this 'atmospheric drought'. Deposited aerosols on leaf surfaces can interact with plant water relations and may influence VPD response. We studied transpiration and water use efficiency of pine, beech and sunflower by measuring sap flow, gas exchange and carbon isotopes, thereby addressing different time scales of plant/atmosphere interaction. Plants were grown (i) outdoors under rainfall exclusion (OD) and in ventilated greenhouses with (ii) ambient air (AA) or (iii) filtered air (FA), the latter containing <1% ambient aerosol concentrations. In addition, some AA plants were sprayed once with 25 mM salt solution of (NH4 )2 SO4 or NaNO3 . Carbon isotope values (δ(13) C) became more negative in the presence of more particles; more negative for AA compared to FA sunflower and more negative for OD Scots pine compared to other growth environments. FA beech had less negative δ(13) C than AA, OD and NaNO3 -treated beech. Anisohydric beech showed linearly increasing sap flow with increasing VPD. The slopes doubled for (NH4 )2 SO4 - and tripled for NaNO3 -sprayed beech compared to control seedlings, indicating decreased ability to resist atmospheric demand. In contrast, isohydric pine showed constant transpiration rates with increasing VPD, independent of growth environment and spray, likely caused by decreasing gs with increasing VPD. Generally, NaNO3 spray had stronger effects on water relations than (NH4 )2 SO4 spray. The results strongly support the role of leaf surface particles as an environmental factor affecting plant water use. Hygroscopic and chaotropic properties of leaf surface particles determine their ability to form wicks across stomata. Such wicks enhance unproductive water loss of anisohydric plant species and decrease CO2 uptake of isohydric plants. They become more relevant with increasing number of fine particles and increasing VPD and are thus related to air pollution and climate change. Wicks cause a deviation from the analogy between CO2 and water pathways through stomata, bringing some principal assumptions of gas exchange theory into question.

Evaluation of the surface free energy of plant surfaces: toward standardizing the procedure

Plant surfaces have been found to have a major chemical and physical heterogeneity and play a key protecting role against multiple stress factors. During the last decade, there is a raising interest in examining plant surface properties for the development of biomimetic materials. Contact angle measurement of different liquids is a common tool for characterizing synthetic materials, which is just beginning to be applied to plant surfaces. However, some studies performed with polymers and other materials showed that for the same surface, different surface free energy values may be obtained depending on the number and nature of the test liquids analyzed, materials' properties, and surface free energy calculation methods employed. For 3 rough and 3 rather smooth plant materials, we calculated their surface free energy using 2 or 3 test liquids and 3 different calculation methods. Regardless of the degree of surface roughness, the methods based on 2 test liquids often led to the under- or over-estimation of surface free energies as compared to the results derived from the 3-Liquids method. Given the major chemical and structural diversity of plant surfaces, it is concluded that 3 different liquids must be considered for characterizing materials of unknown physico-chemical properties, which may significantly differ in terms of polar and dispersive interactions. Since there are just few surface free energy data of plant surfaces with the aim of standardizing the calculation procedure and interpretation of the results among for instance, different species, organs, or phenological states, we suggest the use of 3 liquids and the mean surface tension values provided in this study.

Wettability, polarity, and water absorption of holm oak leaves: effect of leaf side and age

Plant trichomes play important protective functions and may have a major influence on leaf surface wettability. With the aim of gaining insight into trichome structure, composition, and function in relation to water-plant surface interactions, we analyzed the adaxial and abaxial leaf surface of holm oak (Quercus ilex) as a model. By measuring the leaf water potential 24 h after the deposition of water drops onto abaxial and adaxial surfaces, evidence for water penetration through the upper leaf side was gained in young and mature leaves. The structure and chemical composition of the abaxial (always present) and adaxial (occurring only in young leaves) trichomes were analyzed by various microscopic and analytical procedures. The adaxial surfaces were wettable and had a high degree of water drop adhesion in contrast to the highly unwettable and water-repellent abaxial holm oak leaf sides. The surface free energy and solubility parameter decreased with leaf age, with higher values determined for the adaxial sides. All holm oak leaf trichomes were covered with a cuticle. The abaxial trichomes were composed of 8% soluble waxes, 49% cutin, and 43% polysaccharides. For the adaxial side, it is concluded that trichomes and the scars after trichome shedding contribute to water uptake, while the abaxial leaf side is highly hydrophobic due to its high degree of pubescence and different trichome structure, composition, and density. Results are interpreted in terms of water-plant surface interactions, plant surface physical chemistry, and plant ecophysiology.

Surface morphology of electrospun PLA fibers: mechanisms of pore formation

Interaction of relative humidity with solvent(s) is relatively more important in obtaining surface pores than solvent volatility and phase separation.