Relative Water Research Articles

Reduced soil moisture drives leaf anatomical shifts more than chronically elevated temperatures in European temperate trees.

Chronic reductions in soil moisture combined with high air temperatures can modify tree carbon and water relations. However, little is known about how trees acclimate their foliar structure to the individual and combined effects of these two climate drivers. We used open-top chambers to determine the multi-year effects of chronic air warming (+5 °C) and soil moisture reduction (-50%) alone and in combination on the foliar anatomy of two European tree species. We further investigated how these climate drivers affected the relationship between foliar anatomy and physiology/chemistry in young downy oak and European beech trees. After 4 years, reduced soil moisture led to development of thinner leaves with a narrower epidermis and lower gas exchange for oak and beech, but to a lesser extent in the latter. In contrast, prolonged warming did not affect the anatomical and physiological/chemical traits in either species. Warming also did not exacerbate the impacts of dry soils, highlighting soil moisture as the key driver in leaf anatomical shifts. While soil moisture altered oak foliar anatomy, and the physiology and chemistry of both species, our work revealed a limited acclimation potential towards more drought- and heat-tolerant leaves as conditions become drier and warmer, suggesting potentially high vulnerability of both species to future climate predictions.

Advancing Chickpea Breeding: Omics Insights for Targeted Abiotic Stress Mitigation and Genetic Enhancement.

Chickpea is a major source of proteins and is considered the most economically vital food legume. Chickpea production is threatened by several abiotic and biotic factors worldwide. The main constraints limiting worldwide chickpea production are abiotic conditions such as drought, heat, salinity, and cold. It is clear that chickpea is treasured for its nutritive value, in particular its high protein content, and hence study of problems like drought, cold and salinity stresses are very important concerning chickpeas. In this regard, several physiological, biochemical, and molecular mechanisms are reviewed to confer tolerance to abiotic stress. The most crippling economic losses in agriculture occur due to these abiotic stressors, which affect plants in many ways. All these abiotic stresses affect the water relations of the plant, both at the cellular level as well as the whole-plant level, causing both specific and non-specific reactions, damage and adaptation reactions. These stresses share common features. Breeding programs use a huge collection of over 100,000 chickpea accessions as their foundation. Significant advancements in conventional breeding, including mutagenesis, gene/allele introgression, and germplasm introduction, have been made through this method. Abiotic tolerance and yield component selection are made easier by creating unique DNA markers for the genus Cicer, which has been made possible by developments in high-throughput sequencing and molecular biology. Transcriptomics, proteomics, and metabolomics have also made it possible to identify particular genes, proteins, and metabolites linked to chickpea tolerance to abiotic stress. Chickpea abiotic stress tolerance has been directly and potentially improved by biotechnological applications, which are covered by all 'Omics' approaches. It requires information on the abiotic stress response at the different molecular levels, which comprises gene expression analysis for metabolites or proteins and its impact on phenotype. Studies on chickpea genome-wide expression profiling have been conducted to determine important candidate genes and their regulatory networks for abiotic stress response. This study aimed to offer a detailed overview of the diverse 'Omics' approaches for resilience's to abiotic stresses on chickpea plants.

Sea Water Turbidity Variability and Relation to Tides and Environmental Factors in the Korean Coastal Region of the Yellow Sea

Sea Water Turbidity Variability and Relation to Tides and Environmental Factors in the Korean Coastal Region of the Yellow Sea

Least absolute shrinkage and selection operator regression used to select important features when predicting wheat yield from various genotype groups

Abstract Bread wheat and durum wheat genotypes were grown in field experiments at two locations in New South Wales, Australia across several years and using two sowing times (‘early’ v. ‘late’). Genotypes were grouped based on genetic similarity. Grain yield, grain size, soil characteristics and daily weather data were collected. The weather data were used to calculate water and heat stress indices for four key growth periods around flowering. Least absolute shrinkage and selection operator (LASSO) was used to predict grain yield and to identify the most influential features (a combination of index and growth period). A novel approach involving the crop water supply–demand ratio effectively summarized water relations during growth. LASSO predicted grain yield quite well (adjusted R2 from 0.57 to 0.98), especially in a set of durum genotypes. However, the addition of other important variables such as lodging score, disease incidence, weed incidence and insect damage could have improved modelling results. Growth period 2 (30 days pre-flowering up to flowering) was the most sensitive for yield loss from heat stress and water stress for most features. Although one group of bread wheat genotypes was more sensitive to water stress (drought) in period 3 (20 days pre-flowering to 10 days post-flowering). Evapotranspiration was a significant positive feature but only in the vegetative phase (pre-flowering, period 1). This study confirms the usefulness of LASSO modelling as a technique to make predictions that could be used to identify genotypes that are suitable candidates for further investigation by breeders for their stress-tolerance ability.

Superabsorbent hydrogels: A new tool for vineyard water management?

New superabsorbent hydrogels (SH) could be groundbreaking technologies for the adaptation of agriculture to climate change, but knowledge about their physio-chemical traits, their effects on soil hydrology, and their efficacy on tree crop physiology is very limited. In this work, two potassium polyacrylate SH (SH1 and SH2) and an organic material derived SH (SH3) were tested for their water holding capacity and the capability to improve soil hydrology according to the water demands of cultivated grapevine, one of the most relevant rainfed tree crops. All SH absorbed a significant amount of water as compared to their relative weight (from 9.19 g H2O/g polymer of SH3, to 369.64 g H2O/g polymer of SH2) and made it available at water potential (Ψ) levels compatible with grapevine root system uptake. When incorporated to the soil, all SH successfully increased soil field capacity, affecting also wilting point, and increasing maximum plant available water (from +43 % of SH2 to +84 % of SH1). All SH increased stem Ψ of potted vines subjected to a progressive water deficit, as compared to controls (+0.25 MPa at four days from irrigation suspension, +0.2 MPa at eight days from water suspension, when Control vines showed a stem Ψ of −1.63 MPa).Our work demonstrated for the first time the potentialities of three different SH and their positive effects on soil water relations. Both synthetic and organic-derived SH could improve vine water status under limited water supply. While other studies are needed to clarify effects according to different rootstocks and soils, the results pave the way for the integration of SH in vineyard water management.

Influence of environmental parameters on marine plankton diversity in the southern coastal waters of Korea: Emphasis on thermal stratification

Plankton are fundamental to aquatic ecosystems, forming the base of the marine food web and playing a crucial role in the global carbon cycle. While the impact of environmental factors on marine plankton ecosystems has been widely studied, the seasonal variability in plankton diversity has received comparatively less attention. Here, we investigated the complexity and distribution of plankton species in Korean coastal waters in relation to seasonal environmental changes, particularly those linked to water mass characteristics and the presence of a thermocline, using alpha and beta diversity indices. A thermocline was observed at all stations in July, coinciding with the lowest diversity and richness, but was absent in September, when diversity indices peaked. In the upper layer above the thermocline, diatom species such as Pseudo-nitzschia delicatissima and Skeletonema costatum were dominant. Analysis of the Shannon index suggested that planktonic ecosystems in the South Sea of Korea from May to September 2021 were well-balanced. These findings enhance our understanding of the potential effects of environmental variability and thermal stratification on plankton biodiversity and community dynamics in Korean coastal waters.

Differential root and cell regulation of maize aquaporins by the arbuscular mycorrhizal symbiosis highlights its role in plant water relations.

This study aims to elucidate if the regulation of plant aquaporins by the arbuscular mycorrhizal (AM) symbiosis occurs only in roots or cells colonized by the fungus or at whole root system. Maize plants were cultivated in a split-root system, with half of the root system inoculated with the AM fungus and the other half uninoculated. Plant growth and hydraulic parameters were measured and aquaporin gene expression was determined in each root fraction and in microdissected cells. Under well-watered conditions, the non-colonized root fractions of AM plants grew more than the colonized root fraction. Total osmotic and hydrostatic root hydraulic conductivities (Lo and Lpr) were higher in AM plants than in non-mycorrhizal plants. The expression of most maize aquaporin genes analysed was different in the mycorrhizal root fraction than in the non-mycorrhizal root fraction of AM plants. At the cellular level, differential aquaporin expression in AM-colonized cells and in uncolonized cells was also observed. Results indicate the existence of both, local and systemic regulation of plant aquaporins by the AM symbiosis and suggest that such regulation is related to the availability of water taken up by fungal hyphae in each root fraction and to the plant need of water mobilization.

A Theoretical Framework to Quantify Ecosystem Pressure-Volume Relationships.

'Water potential' is the biophysically relevant measure of water status in vegetation relating to stomatal, canopy and hydraulic conductance, as well as mortality thresholds; yet, this cannot be directly related to measured and modelled fluxes of water at plot- to landscape-scale without understanding its relationship with 'water content'. The capacity for detecting vegetation water content via microwave remote sensing further increases the need to understand the link between water content and ecosystem function. In this review, we explore how the fundamental measures of water status, water potential and water content are linked at ecosystem-scale drawing on the existing theory of pressure-volume (PV) relationships. We define and evaluate the concept and limitations of applying PV relationships to ecosystems where the quantity of water can vary on short timescales with respect to plant water status, and over longer timescales and over larger areas due to structural changes in vegetation. As a proof of concept, plot-scale aboveground vegetation PV curves were generated from equilibrium (e.g., predawn) water potentials and water content of the above ground biomass of nine plots, including tropical rainforest, savanna, temperate forest, and a long-term Amazonian rainforest drought experiment. Initial findings suggest that the stored water and ecosystem capacitance scale linearly with biomass across diverse systems, while the relative values of ecosystem hydraulic capacitance and physiologically accessible water storage do not vary systematically with biomass. The bottom-up scaling approach to ecosystem water relations identified the need to characterise the distribution of water potentials within a community and also revealed the relevance of community-level plant tissue fractions to ecosystem water relations. We believe that this theory will be instrumental in linking our detailed understanding of biophysical processes at tissue-scale to the scale at which land surface models operate and at which tower-based, airborne and satellite remote sensing can provide information.

Widespread and systematic effects of fire on plant–soil water relations

Wildfire activity and the hydrological cycle are strongly interlinked. While it is well known that wildfire occurrence and intensity are controlled by water availability, less is known about the effects of wildfire on plant and soil water cycling, especially at large scales. Here we investigate this by analysing fire impacts on the coupling between plant and soil water content, at the global scale, using remote sensing of soil moisture, vegetation water content and burned area. We find a strong effect of fire on plant–soil water relations, accelerating soil moisture loss by 17% and leading to faster gains in vegetation water content by 62%, both of which are positively related to fire severity and largest in forests. This effect is spatially extensive, with accelerated soil moisture loss found in 67%, and increased vegetation water content gain found in 67% of all analysed burned areas. After fire, plants also tended to have less control on their water content (that is, were more anisohydric). In summary, fire changes ecosystem functioning by increasing ecosystem water losses and shifting the relationship between soil and vegetation water budgets. With climate change, wildfire is likely to play an increasingly important role in ecosystem water cycling and subsequent ecosystem recovery.

Carbon isotope composition of biomass and water use efficiency in young plants of Jatropha curcas L. under irrigated or water deficit conditions

Jatropha curcas L. is a non-food crop quoted as a promising natural feedstock for biodiesel production in tropical and subtropical regions. Although an efficient mechanism of drought avoidance through stomatal control of transpiration has been demonstrated in this species, low carbon assimilation and growth rates preclude any advantage of such strategy under water limitation. Two greenhouse experiments were conducted with the objective of investigating varietal differences in water use as the trade-off between carbon isotope composition (δ13C) in different tissues and water use efficiency, in young plants of J. curcas. The first experiment was a survey with seven provenances of J. curcas under non-limiting water availability. There were no significant differences among provenances for leaf gas exchange rates, growth and whole-plant transpiration (T). However, significant effects of provenance and tissue (stem bark or leaf) in δ13C were demonstrated. The provenances CNPAE183 and CNPAE222 were selected for the second experiment, as variations in water use traits and δ13C between the two provenances were observed. Soil water deficit, imposed for 18 days, led to significant physiological, biochemical, and morphological changes. An early and sharp response to water deficit in CNPAE183 as compared to CNPAE222 was observed, as indicated by a 44% higher rate of decrease of T in the former. In addition, water deficit led to increase of δ13C, which was more pronounced in CNPAE222 (13% in young leaves) when compared to CNPAE183 (11% in young leaves). In both provenances, δ13C was less negative in young as compared to mature tissues. Significant and direct correlations between stem bark and leaf δ13C and leaf-level intrinsic water use efficiency were observed. Contrasting results, particularly on T and δ13C, suggest the existence of genetic diversity for water relations and metabolic traits linked to drought tolerance in J. curcas. Using stem bark or leaf δ13C measurements as the basis for large-scale screening of water-use efficient genotypes should be considered.

Drought and heat stress interactions: Unveiling the molecular and physiological responses of Persian walnut

While numerous studies explored the response of walnut plants to drought stress (DS), there remains a significant gap in the knowledge regarding the impact of heat stress (HS) and the combined effects of DS and HS on the recovery capacity of walnut trees. This study aimed to investigate the mechanism of Persian walnut (cv. Chandler) response to the combined DS and HS, focusing on various aspects including photosynthesis, water relations, and osmotic regulation. The treatments involved subjecting plants to DS (through a withholding method for 24 d), HS (gradually up to 40 °C for 8 d), and a combined DS and HS, which were compared to a control group (no stress) during the stress and recovery phases. The results showed that DS had significantly more negative effects on chlorophyll content, relative water content (RWC), leaf water potential (WP), osmotic potential (OP) compared to HS. Involvement of osmoregulation mechanism was detected more in DS and HS plants through the accumulation of proline, glycine Betaine and total soluble carbohydrates. The functionality of photosynthesis was significantly impacted by both HS and DS, respectively. While the HS accelerated the change of the abovementioned physiological processes in drought-stressed seedlings. Consistently, more pronounced damage was found in leaves under the combined stress, alongside the decrease RWC, chlorophyll content and fluorescence ratios. Based on the analysis of the linear mixed-effect model, the effects of combined stress and HS on photosynthesis parameters were detected in the early stages of stress compared to DS. Within a range of stresses, the abovementioned physiological processes of individual and combined-stressed plants recovered to levels comparable to those of the control. Our results also showed a substantial reduction in the expression of the photosynthetic genes (Fd, Cyt b6f, and PsbB) in Persian walnut saplings under abiotic stress conditions indicating significant damage to their photosynthetic apparatus. This study highlights that, under scenarios of aggravating drought occurring with heat, walnut seedlings could face a high risk of damage to physiological structures in relation to the synergistically increased hydraulic and thermal impairments.

Mitigating the Adverse Effects of Salt Stress on Pepper Plants Through Arbuscular Mycorrhizal Fungi (AMF) and Beneficial Bacterial (PGPR) Inoculation

This study investigates arbuscular mycorrhizal fungi (AMF), plant growth-promoting rhizobacteria (PGPR), and their combined application under salt stress (200 mM NaCl), emphasizing their synergistic potential to enhance plant resilience. Conducted in a controlled climate chamber, key parameters such as plant height, biomass, SPAD values, ion leakage, relative water content (RWC), osmotic potential, and mineral uptake were assessed. Salt stress significantly reduced plant growth, chlorophyll content, and nutrient absorption. However, AMF and PGPR improved plant performance, with co-inoculation showing the highest efficacy in increasing RWC, nutrient uptake, and maintaining membrane stability. AMF and PGPR treatments enhanced potassium retention and reduced sodium and chloride accumulation, mitigating ionic imbalances. The improved chlorophyll content and water relations under co-inoculation demonstrate the potential of these biostimulants to boost photosynthesis and plant resilience. These findings highlight AMF and PGPR as eco-friendly solutions for sustainable agriculture, promoting crop productivity and stress tolerance under saline conditions.

The impact of water relations on the vase life of contrasting Anthurium andraeanum (Hort.) cut flowers

ABSTRACT Genotypic differences in the moderation of water relations have been shown to play a key role in explaining differences in vase life in Anthurium andraeanum (Hort.). In this study, we compared the impact of a range of holding solutions on water relations in two cultivars with contrasting vase lives: ‘Spirit’ and ‘Honduras’ (designated as short vase life – ‘Spirit-S’ and long vase life – ‘Honduras-L’). The application of a biocide (8-hydroxyquinoline sulphate; 8-HQS), ‘stem’ re-cutting and a combination of both were tested for their potential to improve water uptake and vase life in the short-lived cultivar Spirit-S. The effects of osmotic conditions were compared for both cultivars, by placing cut flowers in a Polyethylene Glycol (PEG) solution. Re-cutting and biocide altered the pattern of water uptake, but neither treatment improved vase life, suggesting that occlusion of the peduncle/microbial obstruction were not important factors in determining vase life. PEG significantly reduced vase life in both cultivars. Honduras-L responded to PEG by significantly reducing its spathe osmotic potential, while no change was observed in Spirit-S. Thus, PEG-induced drought resulted in osmotic adjustment only in the long vase life cultivar, suggesting that a genotype’s capacity for osmotic adjustment influences vase life.

Characterizing the hysteretic effects of water and salinity stresses on root-water-uptake

Characterizing the effects of previous water and salinity stresses is critical for the evaluation of plant water status, which, in turn, is essential for understanding soil-plant water relations and optimizing irrigation schemes. Recent research has found that hysteresis of plant response following water stress alone can be described by an exponential function of the stress degree on the previous day. To explore and quantify the effects of hysteresis concerning salinity stress and combined water-salinity stress, a hydroponic experiment and a soil column experiment on winter wheat, and a field experiment on cotton were conducted. Like water stress, previous salinity stress and combined water-salinity stress also resulted in hysteretic effects on root-water-uptake. Leaf stomatal conductance and plant transpiration rate of stressed crops could only recover gradually from a previous stressed status after re-watering. When stress was mild, compensatory recovery was found, while incomplete recovery occurred when stress was severe. Although the recovery process was closely related to stress history and type, a recovery coefficient was quantified universally with an exponential function of the stress extent on the previous day (with a coefficient of determination R2 ≥ 0.60). Consideration of hysteresis for water and salinity stresses with a mathematical model led to significant improvement in the simulation of both relative transpiration rate (R2 = 0.94, root mean squared error RMSE = 0.04, maximal absolute error MAE = 0.12) and soil water content (R2 = 0.90, RMSE = 0.01 cm3 cm–3, MAE = 0.03 cm3 cm–3), especially during the recovery periods severely affected by historical stress. Consideration of hysteresis is expected to benefit regulation of soil water and salinity and thus enhance water use efficiency. However, the mechanisms underlying hysteresis, especially the compensatory recovery mechanisms, still need to be further investigated.

Water Relations and Physiological Traits Associated with the Yield Components of Winter Wheat (Triticum aestivum L.)

Water stress in agricultural systems may occur slowly or abruptly. Plant reactions to stress differ with regard to its level and duration. The level of plant susceptibility to water deprivation primarily depends on the management of the water content and metabolism adjustments. The aim of this study was to determine the correlation between water-based plant parameters and the yield components of 90 genotypes of winter wheat (Triticum aestivum L.). Since the loss of water is frequently used as a selection criterion to assess drought tolerance, the relationships between the yield and leaf water content, osmotic potential, and gas exchange characteristics were examined. Genotypes 1, 25, 34, 36, 42, 43, 46, 57, 66, 73, and 90 showed 33–45% larger numbers of grains/plant, 19–25% higher weights of grains/plant, and 4% higher thousand grain weights compared to other genotypes. The higher values of the yield components were accompanied by 20–30% lower leaf water content, 39–52% lower osmotic potential, and 4–39% lower water use efficiency. The principal component analysis revealed that the wheat genotypes had noticeable differences in a few physiological parameters that depended on the sowing date. Electrolyte leakage showed a substantial correlation with the sowing date, suggesting that it may not be a suitable factor for the prediction of drought tolerance. The factors that distinguished the examined genotypes the most were the leaf water content, osmotic potential, and water use efficiency. In addition, a significant correlation was observed between the mentioned parameters and yield components. As a result, these parameters may be helpful in genotype characterization in relation to water stress susceptibility, offering a trustworthy plant selection test.

Effect of Plant Growth Regulators On Water Relations, Proximate Composition, and Ascorbate/glutathione Cycle of Late-sown Wheat Under Saline Conditions

Effect of Plant Growth Regulators On Water Relations, Proximate Composition, and Ascorbate/glutathione Cycle of Late-sown Wheat Under Saline Conditions

Effects of silicon application on <i>Betula pendula</i> seedlings

Silicon (Si) is a beneficial element for many plant species, conferring resistance to drought and herbivory, but its effects on trees are less known. We studied responses of silver birch (<i>Betula pendula</i>), grown in peat, to liquid Si supplementation (Si concentration 0.65 mM) on 1) growth, 2) water economy and 3) element accumulation plus 4) feeding preference of an insect, <i>Epirrita autumnata</i> and a mammalian herbivore, <i>Microtus agrestis</i>. Plant growth was not affected but control (Si–) plants shed their old leaves earlier. Detached Si+ leaves lost water 11%-units less than Si–, and the integrated water-use efficiency based on <sup>13</sup>C analysis was higher in Si+. Foliar Se was higher and Mn and S lower in Si+. Root Mg concentrations were higher and Pb lower in Si+. <i>Epirrita autumnata</i> did not prefer either treatment, but <i>M. agrestis</i> preferred Si– stems. Silicon improved birch water relations as indicated by the leaf drying resistance and increased water-use efficiency. The changes in metal accumulation were probably beneficial, but the lower S/Se ratio requires attention. Furthermore, Si decreased palatability to a mammalian herbivore. Using Si as fertilizer in nurseries could be possible to increase birch tolerance to water stress and herbivory.

Evaluating the effects of rice husk derived nanosilica on growth, photosynthesis, and antioxidant activity in hybrid maize

Drought stress was exacerbated by changing climatic conditions and impact the plant water relations from the cellular to whole plant level, thereby reducing the crop productivity and causing economic losses in agriculture. The present study was aimed to explore the potentials of nanosilica synthesized from rice husk in improving growth and drought stress tolerance in maize. Synthesized nanosilica from rice husk has spherical morphology, amorphous structure, siloxane bonding and higher purity (99.1 %). Soil application of varied levels of synthesized nanosilica (0, 5, 10, 15, 20, 30 and 40 mg kg−1) was tested with hybrid maize plants grown under irrigated and drought stress (100 and 50 % field capacity, respectively). The results of the study revealed that, drought stress has adverse effect on plant growth, biomass production and physiological parameters. However soil application of nanosilica has significantly improved growth, physiological and biochemical attributes of both irrigated and drought stressed plants. Nanosilica application at 20 mg kg−1 for irrigated and 30 mg kg−1 for drought stressed hybrid maize plants has recorded higher plant height, biomass, chlorophyll a, chlorophyll b, superoxide dismutase, peroxidase, phenols, soluble proteins and considerably decreased the proline, electrolyte leakage and melondialdehyde content in hybrid maize. In contrary, higher doses of nanosilica (>30 mg kg−1) caused a slight reduction in plant growth and antioxidant activity under both the water regimes. Hence, we conclude that, soil application of 20 and 30 mg kg−1 nanosilica synthesized from rice husk had a positive effect on plant growth and development besides aided in mitigating drought stress.

Drought tolerance and recovery capacity of two ornamental shrubs: Combining physiological and biochemical analyses with online leaf water status monitoring for the application in urban settings

When plants are transferred from nursery to urban environments, they often face drought stress due to inadequate maintenance, such as insufficient irrigation. Using drought tolerant species may help mitigate the adverse impact of drought stress in urban settings. Additionally, utilizing novel technologies for water status monitoring may help optimize irrigation schedules to prevent transplanting failures. This study investigated the physiological and biochemical responses of two ornamental shrubs, Photinia x fraseri and Viburnum tinus, subjected to water stress of increasing severity and rewatering. Water relations, gas exchanges, chlorophyll fluorescence and biochemical analyses were conducted alongside real-time monitoring of water status using leaf-water-meter sensors (LWM).The progression of water stress had a notable negative impact on leaf gas exchanges and water relations in both species. Notably, P. fraseri avoided photoinhibition by reducing chlorophyll content and actual efficiency of PSII. Adjustments in leaf phenolic compounds played a significant role in enhancing drought tolerance of both species due to their antioxidant and photoprotective properties.Upon rewatering, both species exhibited complete recovery in their physiological functions, underscoring their remarkable tolerance and resilience to drought stress. Additionally, LWM sensors efficiently tracked the dehydration levels, exhibiting a rising trend during the water stress progression and a subsequent decline after rewatering for both species. These findings confirm the reliability of LWM sensors in monitoring physiological status of plants in outdoor contexts, making them a suitable tool for use in urban settings.

Drought response strategies of vascular epiphytes in isolated pasture trees in a Costa Rican tropical montane landscape.

Vascular epiphytes of tropical montane cloud forests are vulnerable to climate change, particularly as cloud bases elevate and reduce atmospheric inputs to the system. However, studies have generally focused on epiphytes in contiguous forests, with little research being done on epiphytes on isolated pasture trees. We investigated water relations of pasture-tree epiphytes at three sites located below and above the elevation of the average cloud base in Monteverde, Costa Rica. We measured sap velocity and four microclimate variables in both the dry and wet season of 2018. We also measured functional traits, including pressure volume (PV) curves, predawn/midday water potential, and various lab-based water relations traits. We used linear mixed models to assess the correlation between microclimate and sap velocity in both seasons and ANOVA to assess the variation in PV curve and water potential variables. The turgor loss point generally increased from the wettest to driest site. However, this trend was driven primarily by the increasing prevalence of leaf succulence at drier sites. Microclimatic variables correlated strongly with sap velocity in the wet season, but low soil moisture availability caused this correlation to break down during the dry season. Our results emphasize the vulnerability of cloud forest epiphytes to rising cloud bases. This vulnerability may be more severe in pasture trees that lack the potential buffer of surrounding forest, but additional research that directly compares the canopy microclimate conditions between forest and pasture trees is needed to confirm this possibility.