Foliar water uptake: Processes, pathways, and integration into plant water budgets

Imaging canopy temperature: shedding (thermal) light on ecosystem processes

Canopy temperature T_can is a key driver of plant function that emerges as a result of interacting biotic and abiotic processes and properties. However, understanding controls on T_can and forecasting canopy responses to weather extremes and climate change are difficult due to sparse measurements of T_can at appropriate spatial and temporal scales. Burgeoning observations of T_can from thermal cameras enable evaluation of energy budget theory and better understanding of how environmental controls, leaf traits and canopy structure influence temperature patterns. The canopy scale is relevant for connecting to remote sensing and testing biosphere model predictions. We anticipate that future breakthroughs in understanding of ecosystem responses to climate change will result from multiscale observations of T_can across a range of ecosystems.

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

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

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.

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

PREMISE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Positive pressure in xylem and its role in hydraulic function

Although transpiration-driven transport of xylem sap is well known to operate under absolute negative pressure, many terrestrial, vascular plants show positive xylem pressure above atmospheric pressure on a seasonal or daily basis, or during early developmental stages. The actual location and mechanisms behind positive xylem pressure remain largely unknown, both in plants that show seasonal xylem pressure before leaf flushing, and those that show a diurnal periodicity of bleeding and guttation. Available evidence shows that positive xylem pressure can be driven based on purely physical forces, osmotic exudation into xylem conduits, or hydraulic pressure in parenchyma cells associated with conduits. The latter two mechanisms may not be mutually exclusive and can be understood based on a similar modelling scenario. Given the renewed interest in positive xylem pressure, this review aims to provide a constructive way forward by discussing similarities and differences of mechanistic models, evaluating available evidence for hydraulic functions, such as rehydration of tissues, refilling of water stores, and embolism repair under positive pressure, and providing recommendations for future research, including methods that avoid or minimise cutting artefacts.

Tracing plant–environment interactions from organismal to planetary scales using stable isotopes: a mini review

Natural isotope variation forms a mosaic of isotopically distinct pools across the biosphere and flows between pools integrate plant ecology with global biogeochemical cycling. Carbon, nitrogen, and water isotopic ratios (among others) can be measured in plant tissues, at root and foliar interfaces, and in adjacent atmospheric, water, and soil environments. Natural abundance isotopes provide ecological insight to complement and enhance biogeochemical research, such as understanding the physiological conditions during photosynthetic assimilation (e.g. water stress) or the contribution of unusual plant water or nutrient sources (e.g. fog, foliar deposition). While foundational concepts and methods have endured through four decades of research, technological improvements that enable measurement at fine spatiotemporal scales, of multiple isotopes, and of isotopomers, are advancing the field of stable isotope ecology. For example, isotope studies now benefit from the maturation of field-portable infrared spectroscopy, which allows the exploration of plant–environment sensitivity at physiological timescales. Isotope ecology is also benefiting from, and contributing to, new understanding of the plant–soil–atmosphere system, such as improving the representation of soil carbon pools and turnover in land surface models. At larger Earth-system scales, a maturing global coverage of isotope data and new data from site networks offer exciting synthesis opportunities to merge the insights of single-or multi-isotope analysis with ecosystem and remote sensing data in a data-driven modeling framework, to create geospatial isotope products essential for studies of global environmental change.

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.

Zoomingin on quartzitic outcrops: micro-habitat influences on flora and vegetation

Abstract Rock outcrop vegetation is recognized worldwide by its singular and biodiverse flora. Campo Rupestre forms hyperdiverse mosaics in rocky environments across a wide latitudinal and altitudinal gradient, with high species turnover at macro- and micro-scales. The surrounding biomes, climate, and geological formations are the main drivers of species turnover on a macro-scale while micro-habitat seems to be the main one determining the peculiarities of the Campo Rupestre on a micro-scale. In a quartzitic Campo Rupestre area we evaluate how the outcrop micro-habitats influence floristic composition and functional traits. The study area is located in the municipality of Ouro Preto, Minas Gerais state, southeastern Brazil. Two main outcrop habitats were considered: top surfaces, with bare rock, shallow depressions and ephemeral ponds; and lateral surfaces, with clefts and crevices. We recorded the vascular species, their respective life-forms (according to Raunkiaer’s system) as well as their coverage in 18 plots. We identified 71 species in 31 families. The floristic spectra and species composition were similar between top and lateral surfaces. There was no significant difference among the vegetational spectra. However, hemicryptophytes were slightly dominant on top, while on the sides chamaephytes were the dominant life-form. Understanding plant adaptations to these environments provides insights into the mechanisms underlying geomorphological heterogeneity in plant communities.

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

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

Modified Hiltner Dew Balance to Re-Estimate Dewfall Accumulation as a Reliable Water Source in the Negev Desert

Dew formation is an essential component of the water balance in dry ecosystems, but measuring dew is challenging due, in part, to its dependency on the surface on which it forms. We detail the use of a modified Hiltner dew balance to illustrate how more accurate measurements of dewfall may be obtained. Using a modified Hiltner dew balance, we measured dewfall in the Negev Desert continuously for 3 years (2013–2015). Data analyses examined the relationship between dew formation, rain events and other environmental parameters in order to re-evaluate the importance of dew in the water budget. In line with previous research, our findings demonstrate that dewfall is a substantial and stable input of water in the Negev desert, providing inputs in the dry summer and the wet winter. Our results show that while dewfall was larger and more prevalent in proximity to rain events, a notable portion of dewfall took place on days distant from any rain event. The Hiltner dew balance modifications proved to be reliable and increased the efficacy of measuring the quantity and timing of dew formation. This study demonstrates the importance of integrating dewfall data into decision-making models for dryland ecosystems and agriculture, as well as into climate models.

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.

Effect of irradiation and canopy position on anatomical and physiological features of Fagus sylvatica and Quercus petraea leaves

Growing conditions at different tree canopy positions may significantly vary and lead to foliar changes even within the same tree. An assessment of foliar anatomy, including also epidermal features, can help us understand how plants respond to environmental factors. Working with two model tree species (i.e., Quercus petraea and Fagus sylvatica) grown at their southernmost European distribution area in Central Spain, the influence of irradiation and canopy height was examined by sampling lower canopy leaves and comparing them with fully irradiated, top canopy leaves and shaded top canopy leaves grown for months within a bag made of shade netting fabric before they sprouted. At the end of the summer, samples were collected, and several parameters were analysed. The results indicate that SLA (specific leaf area) differences are significant both between species and groups. Leaf and cuticle thickness differed significantly between groups while stomatal densities only between species. Regarding mineral concentrations, differences between species were significant for K, Mn, N and N: P ratios. It is concluded that leaf responses to environmental conditions may be variable both within the same tree and between species.

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

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

Foliar Water Uptake in Trees: Negligible or Necessary?

Foliar water uptake (FWU) has been identified as a mechanism commonly used by trees and other plants originating from various biomes. However, many questions regarding the pathways and the implications of FWU remain, including its ability to mitigate climate change-driven drought. Therefore, answering these questions is of primary importance to adequately address and comprehend drought stress responses and associated growth. In this review, we discuss the occurrence, pathways, and consequences of FWU, with a focus predominantly on tree species. Subsequently, we highlight the tight coupling between FWU and foliar fertilizer applications, discuss FWU in a changing climate, and conclude with the importance of including FWU in mechanistic vegetation models.

Foliar water uptake in arid ecosystems: seasonal variability and ecophysiological consequences

Foliar water uptake (FWU) has been reported for different species across several ecosystems types. However, little attention has been given to arid ecosystems, where FWU during dew formation or small rain events could ameliorate water deficits. FWU and their effects on leaf water potential (Ψ_Leaf) were evaluated in grasses and shrubs exploring different soil water sources in a Patagonian steppe. Also, seasonal variability in FWU and the role of cell wall elasticity in determining the effects on Ψ_Leaf were assessed. Eleven small rain events (< 8 mm) and 45 days with dew formation were recorded during the study period. All species exhibited FWU after experimental wetting. There was a large variability in FWU across species, from 0.04 mmol m^-2 s^-1 in species with deep roots to 0.75 mmol m^-2 s^-1 in species with shallow roots. Species-specific mean FWU rates were positively correlated with mean transpiration rates. The increase in Ψ_Leaf after leaf wetting varied between 0.65 MPa and 1.67 MPa across species and seasons. The effects of FWU on Ψ_Leaf were inversely correlated with cell wall elasticity. FWU integrated over both seasons varied between 28 mol m^-2 in species with deep roots to 361 mol m^-2 in species with shallow roots. Taking into account the percentage of coverage of each species, accumulated FWU represented 1.6% of the total annual transpiration of grasses and shrubs in this ecosystem. Despite this low FWU integrated over time compared to transpiration, wetting leaves surfaces can help to avoid larger water deficit during the dry season.

Die hard: timberline conifers survive annual winter embolism

During winter, timberline trees are exposed to drought and frost, factors known to induce embolism. Studies indicated that conifers cope with winter embolism by xylem refilling. We analysed the loss of hydraulic conductivity (LC) in Picea abies branch xylem over 10 years, and correlated winter embolism to climate parameters. LC was investigated by direct X-ray micro-computer tomography (micro-CT) observations and potential cavitation fatigue by Cavitron measurements. Trees showed up to 100% winter embolism, whereby LC was highest, when climate variables indicated frost drought and likely freeze-thaw stress further increased LC. During summer, LC never exceeded 16%, due to hydraulic recovery. Micro-CT revealed homogenous embolism during winter and that recovery was based on xylem refilling. Summer samples exhibited lower LC in outermost compared to older tree rings, although no cavitation fatigue was detected. Long-term data and micro-CT observations demonstrate that timberline trees can survive annual cycles of pronounced winter-embolism followed by xylem refilling. Only a small portion of the xylem conductivity cannot be restored during the first year, while remaining conduits are refilled without fatigue during consecutive years. We identify important research topics to better understand the complex induction and repair of embolism at the timberline and its relevance to general plant hydraulics.

Vertical gradients in foliar physiology of tall Picea sitchensis trees

In tall conifers, leaf structure can vary dramatically with height due to decreasing water potential (Ψ) and increasing light availability. This variation in leaf structure can have physiological consequences such as increased respiratory costs, reduced internal carbon dioxide conductance rates and ultimately reduced maximum photosynthetic rates (Amax). In Picea sitchensis (Bong.) Carrière, the leaf structure varies along the vertical gradient in ways that suggest compensatory changes to enhance photosynthesis, and this variation seems to be driven largely by light availability rather than by Ψ. These trends in leaf structure coupled with remarkably fast growth rates and dependence on moist environments inspire two important questions about P. sitchensis: (i) does foliar water uptake minimize the adverse effects of decreasing Ψ with height on leaf structure, and (ii) do trends in leaf structure increase photosynthetic rates despite increasing height? To answer these questions, we measured foliar water uptake capacity, predawn (Ψpd) and midday water potential and gas-exchange rates as they varied between 25- and 89-m heights in 300-year-old P. sitchensis trees in northwestern California. Our major findings for P. sitchensis include the following: (i) foliar water uptake capacity was quite high relative to published values for other woody species; (ii) foliar water uptake capacity increased between the crown base and treetop; (iii) wet season Ψpd was higher than predicted by the gravitational potential gradient, indicating foliar water uptake; and (iv) the maximum photosynthetic rate increased with height, presumably due to shifts in leaf structure between the crown base and treetop, mitigating height-related decreases in Amax. These findings suggest that together, the use of fog, dew and rain deposits on leaves and shifts in the leaf structure to conserve and possibly enhance photosynthetic capacity likely contribute to the rapid growth rates measured in this species.

The 18 O-signal transfer from water vapour to leaf water and assimilates varies among plant species and growth forms

The ¹⁸ O signature of atmospheric water vapour (δ¹⁸ O_V ) is known to be transferred via leaf water to assimilates. It remains, however, unclear how the ¹⁸ O-signal transfer differs among plant species and growth forms. We performed a 9-hr greenhouse fog experiment (relative humidity ≥ 98%) with ¹⁸ O-depleted water vapour (-106.7‰) on 140 plant species of eight different growth forms during daytime. We quantified the ¹⁸ O-signal transfer by calculating the mean residence time of O in leaf water (MRT_LW ) and sugars (MRT_Sugars ) and related it to leaf traits and physiological drivers. MRT_LW increased with leaf succulence and thickness, varying between 1.4 and 10.8 hr. MRT_Sugars was shorter in C₃ and C₄ plants than in crassulacean acid metabolism (CAM) plants and highly variable among species and growth forms; MRT_Sugars was shortest for grasses and aquatic plants, intermediate for broadleaf trees, shrubs, and herbs, and longest for conifers, epiphytes, and succulents. Sucrose was more sensitive to δ¹⁸ O_V variations than other assimilates. Our comprehensive study shows that plant species and growth forms vary strongly in their sensitivity to δ¹⁸ O_V variations, which is important for the interpretation of δ¹⁸ O values in plant organic material and compounds and thus for the reconstruction of climatic conditions and plant functional responses.

Linking water relations and hydraulics with photosynthesis

For land plants, water is the principal governor of growth. Photosynthetic performance is highly dependent on the stable and suitable water status of leaves, which is balanced by the water transport capacity, the water loss rate as well as the water capacitance of the plant. This review discusses the links between leaf water status and photosynthesis, specifically focussing on the coordination of CO₂ and water transport within leaves, and the potential role of leaf capacitance and elasticity on CO₂ and water transport.

Diffuse light and wetting differentially affect tropical tree leaf photosynthesis

Most ecosystems experience frequent cloud cover resulting in light that is predominantly diffuse rather than direct. Moreover, these cloudy conditions are often accompanied by rain that results in wet leaf surfaces. Despite this, our understanding of photosynthesis is built upon measurements made on dry leaves experiencing direct light. Using a modified gas exchange setup, we measured the effects of diffuse light and leaf wetting on photosynthesis in canopy species from a tropical montane cloud forest. We demonstrate significant variation in species-level response to light quality independent of light intensity. Some species demonstrated 100% higher rates of photosynthesis in diffuse light, and others had 15% greater photosynthesis in direct light. Even at lower light intensities, diffuse light photosynthesis was equal to that under direct light conditions. Leaf wetting generally led to decreased photosynthesis, particularly when the leaf surface with stomata became wet; however, there was significant variation across species. Ultimately, we demonstrate that ecosystem photosynthesis is significantly altered in response to environmental conditions that are ubiquitous. Our results help to explain the observation that net ecosystem exchange can increase in cloudy conditions and can improve the representation of these processes in Earth systems models under projected scenarios of global climate change.

Shoot surface water uptake enables leaf hydraulic recovery in Avicennia marina

The significance of shoot surface water uptake (SSWU) has been debated, and it would depend on the range of conditions under which it occurs. We hypothesized that the decline of leaf hydraulic conductance (K_leaf ) in response to dehydration may be recovered through SSWU, and that the hydraulic conductance to SSWU (K_surf ) declines with dehydration. We quantified effects of leaf dehydration on K_surf and effects of SSWU on recovery of K_leaf in dehydrated leaves of Avicennia marina. SSWU led to overnight recovery of K_leaf , with recovery retracing the same path as loss of K_leaf in response to dehydration. SSWU declined with dehydration. By contrast, K_surf declined with rehydration time but not with dehydration. Our results showed a role of SSWU in the recovery of leaf hydraulic conductance and revealed that SSWU is sensitive to leaf hydration status. The prevalence of SSWU in vegetation suggests an important role for atmospheric water sources in maintenance of leaf hydraulic function, with implications for plant responses to changing environments.

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

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

Water-use strategies in flowers from a neotropical savanna under contrasting environmental conditions during flowering

Flowers require high amounts of water, which ultimately may compromise pollinator attractiveness under water limitation. Water-use and -conservation strategies in leaves from hot and dry ecosystems are well documented, yet little is known about mechanisms of water allocation in flowers, particularly in tropical savanna ecosystems. We evaluated traits related to corolla water status in two Kielmeyera species that differ in flowering phenology and flower size: larger-flowered K. regalis blooms during the rainy summer and smaller-flowered K. coriacea blooms during the dry winter. To test the hypothesis that water demand in corollas increases with increasing vapor pressure deficit (VPD), we analyzed interspecific differences in corolla stomatal conductance and density, water content, and fresh and dry mass per unit area. We also performed hand-pollination and pollinator-exclusion experiments to determine variation in floral longevity. Corolla transpiration rates were higher in K. coriacea (157 vs 95 g·H₂O·m^-2·h^-1 for K. coriacea and K. regalis, respectively), and increased with VPD in both species. Stomatal density was 25-fold higher in K. coriacea, and corolla fresh and dry mass per unit of area were 47% and 21% higher, respectively, in K. coriacea, due to thick pectin-rich cell walls. The high pectin content increases water content in corollas of K. coriacea. Regardless of pollination, flowers lasted one day in K. coriacea and three in K. regalis. Our study suggests structure-function relationships of floral traits with flowering season, and that K. coriacea displays small and short-lived corollas with high water content to buffer the high evaporative demand during the dry period.

Arboreal Epiphytes in the Soil-Atmosphere Interface: How Often Are the Biggest “Buckets” in the Canopy Empty?

Arboreal epiphytes (plants residing in forest canopies) are present across all major climate zones and play important roles in forest biogeochemistry. The substantial water storage capacity per unit area of the epiphyte “bucket” is a key attribute underlying their capability to influence forest hydrological processes and their related mass and energy flows. It is commonly assumed that the epiphyte bucket remains saturated, or near-saturated, most of the time; thus, epiphytes (particularly vascular epiphytes) can store little precipitation, limiting their impact on the forest canopy water budget. We present evidence that contradicts this common assumption from (i) an examination of past research; (ii) new datasets on vascular epiphyte and epi-soil water relations at a tropical montane cloud forest (Monteverde, Costa Rica); and (iii) a global evaluation of non-vascular epiphyte saturation state using a process-based vegetation model, LiBry. All analyses found that the external and internal water storage capacity of epiphyte communities is highly dynamic and frequently available to intercept precipitation. Globally, non-vascular epiphytes spend <20% of their time near saturation and regionally, including the humid tropics, model results found that non-vascular epiphytes spend ~1/3 of their time in the dry state (0–10% of water storage capacity). Even data from Costa Rican cloud forest sites found the epiphyte community was saturated only 1/3 of the time and that internal leaf water storage was temporally dynamic enough to aid in precipitation interception. Analysis of the epi-soils associated with epiphytes further revealed the extent to which the epiphyte bucket emptied—as even the canopy soils were often <50% saturated (29–53% of all days observed). Results clearly show that the epiphyte bucket is more dynamic than currently assumed, meriting further research on epiphyte roles in precipitation interception, redistribution to the surface and chemical composition of “net” precipitation waters reaching the surface.

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.

Foliar water-uptake strategies are related to leaf water status and gas exchange in plants from a ferruginous rupestrian field

PREMISE

Fog is a frequent event in Brazilian rupestrian field and plays an important role in the physiology of several plant species. Foliar water uptake (FWU) of fog may be fast or slow depending on the species. However, fog water may negatively affect CO₂ assimilation. Thus, the interference in the water and carbon balance as a result of different strategies of FWU was evaluated to verify whether fog may mitigate possible water deficit in leaves.

METHODS

Four plant species with different FWU strategies were studied in a ferruginous rupestrian field with frequent fog. Gas exchange and water potential were measured before dawn and at midday during the dry and rainy seasons, separating foggy from non-foggy days during the dry season.

RESULTS

The FWU speed negatively influences CO₂ assimilation in the dry season, possibly because of its negative relationship with stomatal conductance, since reduced stomatal aperture impairs carbon entrance. Fog presence increased leaf water potential both in early morning and midday during the dry season. However, during the rainy season, the values of leaf water potential were lower at midday, than during the dry season with fog at midday, which favors leaf gas exchanges.

CONCLUSIONS

FWU interferes negatively, but briefly with CO₂ assimilation. Nevertheless, FWU prevents water loss through transpiration and increases the water status of plants in the dry season. That is, FWU results in a compensation between CO₂ assimilation and foliar hydration, which, in fact, is beneficial to the plants of this ecosystem.

Terahertz spectral imaging based quantitative determination of spatial distribution of plant leaf constituents

BACKGROUND

Plant leaves have heterogeneous structures composed of spatially variable distribution of liquid, solid, and gaseous matter. Such contents and distribution characteristics correlate with the leaf vigor and phylogenic traits. Recently, terahertz (THz) techniques have been proved to access leaf water content and spatial heterogeneity distribution information, but the solid matter content and gas network information were usually ignored, even though they also affect the THz dielectric function of the leaf.

RESULTS

A particle swarm optimization algorithm is employed for a one-off quantitative assay of spatial variability distribution of the leaf compositions from THz data, based on an extended Landau-Lifshitz-Looyenga model, and experimentally verified using Bougainvillea spectabilis leaves. A good agreement is demonstrated for water and solid matter contents between the THz-based method and the gravimetric analysis. In particular, the THz-based method shows good sensitivity to fine-grained differences of leaf growth and development stages. Furthermore, such subtle features as damages and wounds in leaf could be discovered through THz detection and comparison regarding spatial heterogeneity of component contents.

CONCLUSIONS

This THz imaging method provides quantitative assay of the leaf constituent contents with the spatial distribution feature, which has the potential for applications in crop disease diagnosis and farmland cultivation management.