Plant Water Potential Research Articles

Enhanced Satellite Monitoring of Dryland Vegetation Water Potential Through Multi‐Source Sensor Fusion

AbstractDrylands are critical in regulating global carbon sequestration, but the resiliency of these semi‐arid shrub, grassland and forest systems is under threat from global warming and intensifying water stress. We used synergistic satellite optical‐Infrared (IR) and microwave remote sensing observations to quantify plant‐to‐stand level vegetation water potentials and seasonal changes in dryland water stress in the southwestern U.S. Machine‐learning was employed to re‐construct global satellite microwave vegetation optical depth (VOD) retrievals to 500‐m resolution. The re‐constructed results were able to delineate diverse vegetation conditions undetectable from the original 25‐km VOD record, and showed overall favorable correspondence with in situ plant water potential measurements (R from 0.60 to 0.78). The VOD water potential estimates effectively tracked plant water storage changes from hydro‐climate variability over diverse sub‐regions. The re‐constructed VOD record improves satellite capabilities for monitoring the storage and movement of water across the soil‐vegetation‐atmosphere continuum in heterogeneous drylands.

PSInet: a new global water potential network.

Given the pressing challenges posed by climate change, it is crucial to develop a deeper understanding of the impacts of escalating drought and heat stress on terrestrial ecosystems and the vital services they offer. Soil and plant water potential play a pivotal role in governing the dynamics of water within ecosystems and exert direct control over plant function and mortality risk during periods of ecological stress. However, existing observations of water potential suffer from significant limitations, including their sporadic and discontinuous nature, inconsistent representation of relevant spatio-temporal scales and numerous methodological challenges. These limitations hinder the comprehensive and synthetic research needed to enhance our conceptual understanding and predictive models of plant function and survival under limited moisture availability. In this article, we present PSInet (PSI-for the Greek letter Ψ used to denote water potential), a novel collaborative network of researchers and data, designed to bridge the current critical information gap in water potential data. The primary objectives of PSInet are as follows. (i) Establishing the first openly accessible global database for time series of plant and soil water potential measurements, while providing important linkages with other relevant observation networks. (ii) Fostering an inclusive and diverse collaborative environment for all scientists studying water potential in various stages of their careers. (iii) Standardizing methodologies, processing and interpretation of water potential data through the engagement of a global community of scientists, facilitated by the dissemination of standardized protocols, best practices and early career training opportunities. (iv) Facilitating the use of the PSInet database for synthesizing knowledge and addressing prominent gaps in our understanding of plants' physiological responses to various environmental stressors. The PSInet initiative is integral to meeting the fundamental research challenge of discerning which plant species will thrive and which will be vulnerable in a world undergoing rapid warming and increasing aridification.

Hyperspectral signals in the soil: Plant-soil hydraulic connection and disequilibrium as mechanisms of drought tolerance and rapid recovery.

Predicting soil water status remotely is appealing due to its low cost and large-scale application. During drought, plants can disconnect from the soil, causing disequilibrium between soil and plant water potentials at pre-dawn. The impact of this disequilibrium on plant drought response and recovery is not well understood, potentially complicating soil water status predictions from plant spectral reflectance. This study aimed to quantify drought-induced disequilibrium, evaluate plant responses and recovery, and determine the potential for predicting soil water status from plant spectral reflectance. Two species were tested: sweet corn (Zea mays), which disconnected from the soil during intense drought, and peanut (Arachis hypogaea), which did not. Sweet corn's hydraulic disconnection led to an extended 'hydrated' phase, but its recovery was slower than peanut's, which remained connected to the soil even at lower water potentials (-5 MPa). Leaf hyperspectral reflectance successfully predicted the soil water status of peanut consistently, but only until disequilibrium occurred in sweet corn. Our results reveal different hydraulic strategies for plants coping with extreme drought and provide the first example of using spectral reflectance to quantify rhizosphere water status, emphasizing the need for species-specific considerations in soil water status predictions from canopy reflectance.

Multi-year aboveground data of minirhizotron facilities in Selhausen

Improved understanding of crops’ response to soil water stress is important to advance soil-plant system models and to support crop breeding, crop and varietal selection, and management decisions to minimize negative impacts. Studies on eco-physiological crop characteristics from leaf to canopy for different soil water conditions and crops are often carried out at controlled conditions. In-field measurements under realistic field conditions and data of plant water potential, its links with CO2 and H2O gas fluxes, and crop growth processes are rare. Here, we presented a comprehensive data set collected from leaf to canopy using sophisticated and comprehensive sensing techniques (leaf chlorophyll, stomatal conductance and photosynthesis, canopy CO2 exchange, sap flow, and canopy temperature) including detailed crop growth characteristics based on destructive methods (crop height, leaf area index, aboveground biomass, and yield). Data were acquired under field conditions with contrasting soil types, water treatments, and different cultivars of wheat and maize. The data from 2016 up to now will be made available for studying soil/water-plant relations and improving soil-plant-atmospheric continuum models.

Dynamic regulation of water potential in Juniperus osteosperma mediates ecosystem carbon fluxes.

Some plants exhibit dynamic hydraulic regulation, in which the strictness of hydraulic regulation (i.e. iso/anisohydry) changes in response to environmental conditions. However, the environmental controls over iso/anisohydry and the implications of flexible hydraulic regulation for plant productivity remain unknown. In Juniperus osteosperma, a drought-resistant dryland conifer, we collected a 5-month growing season time series of in situ, high temporal-resolution plant water potential ( ) and stand gross primary productivity (GPP). We quantified the stringency of hydraulic regulation associated with environmental covariates and evaluated how predawn water potential contributes to empirically predicting carbon uptake. Juniperus osteosperma showed less stringent hydraulic regulation (more anisohydric) after monsoon precipitation pulses, when soil moisture and atmospheric demand were high, and corresponded with GPP pulses. Predawn water potential matched the timing of GPP fluxes and improved estimates of GPP more strongly than soil and/or atmospheric moisture, notably resolving GPP underestimation before vegetation green-up. Flexible hydraulic regulation appears to allow J. osteosperma to prolong soil water extraction and, therefore, the period of high carbon uptake following monsoon precipitation pulses. Water potential and its dynamic regulation may account for why process-based and empirical models commonly underestimate the magnitude and temporal variability of dryland GPP.

Xylem safety in relation to the stringency of plant water potential regulation of European beech, Norway spruce, and Douglas-fir trees during severe drought

Key messageNorway spruce operates with larger hydraulic safety margins (HSM) than beech and Douglas-fir despite the known drought sensitivity of spruce, questioning a pivotal role of HSM in drought tolerance.The exceptional 2018/2019 drought exposed Central Europe’s forests to severe stress, highlighting the need to better understand stomatal regulation strategies and their relationship to xylem safety under extreme drought. We studied diurnal, seasonal, and inter-annual variation in stomatal conductance (gs) and leaf water potential (ΨLeaf) in co-occurring European beech (F. sylvatica), Norway spruce (P. abies), and Douglas-fir (P. menziesii) trees in the two summers and related them to hydraulic traits characterizing drought resistance. In 2018, F. sylvatica exhibited a continuous ΨLeaf decline from June to September, as is characteristic for an anisohydric strategy, while P. abies closed stomata early and reached the least negative ΨLeaf-values at the end of summer. P. menziesii showed low ΨLeaf-values close to P12 (the xylem pressure at onset of embolism) already in July. Both conifers closed stomata when approaching P12 and maintained low gs-levels throughout summer, indicative for isohydric regulation. In 2019, all three species showed a linear decline in ΨLeaf, but F. sylvatica crossed P12 in contrast to the conifers. The three species exhibited similar water potentials at turgor loss point (− 2.44 to − 2.51 MPa) and branch P50 (xylem pressure at 50% loss of hydraulic conductance; − 3.3 to − 3.8 MPa). Yet, F. sylvatica and P. menziesii operated with smaller hydraulic safety margins (HSM means: 0.79 and 0.77 MPa) than P. abies (1.28 MPa). F. sylvatica reduced leaf size and specific leaf area in 2019 and increased Huber value. Our species comparison during extreme drought contradicts the general assumption that conifers operate with larger HSMs than angiosperm trees. Contrary to expectation, P. abies appeared as hydraulically less vulnerable than Douglas-fir.

Influence of Terrestrial Parameters on the Ecophysiological Composition of Young Plants Kahya Senegalensis in Saline Environment

This study aims at analyzing the behavior of young African mahogany plants in a saline environment, focusing on perceptions related to transpiration, photosynthetically active radiation and water potential. The experiment was conducted in drainage lysimeters, using different salinity levels of irrigation water. Environmental data, such as photosynthetically active radiation, water potential and electrical conductivity, were collected to investigate their influence on the transpiration of African mahogany plants. The research was carried out in an experimental field in the northeast region of Brazil, over a period of four months, with young African mahogany plants (Khaya senegalensis), using water with an electrical conductivity of 0.5 dS.m -1 (control treatment); 1.25 dS.m -1; 2 dS.m -1; 2.75 dS.m -1; 3.5 dS.m -1; 4.25 dS.m -1 and 5 dS.m -1 in a total of seven treatments, with three replications and 21 experimental units (plants). Plant transpiration was measured using a steady-state diffusion porometer. The electrical conductivity of the irrigation water was monitored using a portable conductivity meter. The results presented in the analyzes indicate a complex and interdependent relationship between leaf transpiration, photosynthetically active radiation and plant water potential. Leaf transpiration is influenced by variations in solar radiation, which plays a crucial role in regulating this process. It was observed that leaf transpiration is lower in the morning, from 7 to 9 am and in the late afternoon, from 5 to 6 pm, when incident solar radiation is at lower values.

The legacy of past droughts induces water-sparingly behaviour in Grüner Veltliner grapevines.

Drought is becoming more frequent and severe in numerous wine-growing regions. Nevertheless, limited research has examined the legacy of recurrent droughts, focusing on leaf physiology and anatomy over consecutive seasons. We investigated drought legacies (after 2 years of drought exposure) in potted grapevines, focusing on stomatal behaviour under well-watered conditions during the third year. Vines were subjected for two consecutive years to short- (SD) or long-term (LD) seasonal droughts, or well-watered conditions (WW). In the third year, all plants were grown without water limitation. Water potential and gas exchange were monitored throughout the three seasons, while leaf morpho-anatomical traits were measured at the end of the third year. During droughts (1st and 2nd year), stem water potential of SD and LD plants fell below -1.1 MPa, with a consequent 75% reduction in stomatal conductance (gs ) compared to WW. In the 3rd year, when all vines were daily irrigated to soil capacity (midday stem water potential ~ -0.3 MPa), 45% lower values of gs were observed in the ex-LD group compared to both ex-SD and ex-WW. Reduced midrib vessel diameter, lower leaf theoretical hydraulic conductivity, and smaller stomata were measured in ex-LD leaves compared to ex-SD and ex-WW, likely contributing to the reduced gas exchange. Our findings suggest that grapevines exposed to drought may adopt a more water-conserving strategy in subsequent seasons, irrespective of current soil water availability, with the degree of change influenced by the intensity and duration of past drought events.

Characterization and utilization of multi-trait plant growth promoting rhizobacteria from arid soils of western Rajasthan for enhancing drought resilience in an arid legume

In the study, plant growth promoting rhizobacteria (PGPR) indigenous to arid western Rajasthan were isolated and screened for plant growth promoting (PGP) capabilities. Ten isolates showing multiple PGP activities were identified as Pseudomonas sp. G6-3, Staphylococcus sp. G6-4, Bacillus badius G8-6, Agrobacterium sp. G8-7, Bacillus pseudomycoides G10-1, Staphylococcus capitis subsp. capitis G11-4, Bacillus paralicheniformis G11-5, Bacillus subtilis G12-3, Stenotrophomonas pavanii G12-4, and Solibacillus isronensis G21-9. Physiological profiling of these isolates demonstrated them to be tolerant to drought (25% PEG-6000), high temperature (40 °C), alkalinity (pH 10.0), and salinity (4.5% NaCl). These PGPR significantly improved clusterbean seedling characteristics in moisture stress-induced hydroponic flask cultures and pots. PGPR inoculation of clusterbean in drought-induced pots significantly enhanced the plant dry weight by 15–33%, plant water potential by 5–8%, and the relative water content of leaves by 4–16%. There was also a significant reduction in the levels of anti-oxidant enzymes and proline in plant tissues under drought conditions, which indicated lower levels of stress experienced by the treated plants. These were translated into yield enhancements by 13–15% under rainfed conditions. The study demonstrates the ability of stress tolerant PGPR from arid western Rajasthan in enhancing drought resilience of arid legumes besides promoting their growth, and their potential utility in sustainable arid agriculture systems.

Photosynthesis, chlorophyll content and water potential of a mistletoe-host pair in a semi-arid savanna

Mistletoes can have detrimental effects on host trees. Mistletoe-infected trees may suffer from reduced growth and loss of vigor thereby contributing to low productivity of host trees. However, hosts may tolerate low levels of infection. This study aimed to determine photosynthetic rate, chlorophyll content and transpiration rate of a mistletoe-host pair (Viscum rotundifolium-Boscia albitrunca) and the water potential of two mistletoe host pairs (V. rotundifolium-B. albitrunca and Tapinanthus oleifolius-Senegalia mellifera) in a semi-arid savanna at Windhoek, Namibia. Photosynthetic rates were measured using a LI-6400XT photosynthesis system, while photosynthetic pigments were extracted in acetone and quantified spectrophotometrically. Plant water potential was measured using a Scholander pressure chamber. Results revealed greater photosynthetic rates and amounts of photosynthetic pigments in B. albitrunca than in V. rotundifolium while transpiration rates showed opposite trends to photosynthetic rates. Non-host B. albitrunca trees recorded significantly higher photosynthetic rates than the hosts, which suggests a parasite-induced reduction of photosynthetic rate in host plants. Water potential was greater in host trees than in mistletoes. These findings show negative effects of mistletoes on host trees, which may ultimately affect their reproduction and survival. Further research may be required to determine effects of parasite density on host trees.

Melatonin improves plant water status, photosynthetic performance, and antioxidant defense system in highbush blueberry (Vaccinium corymbosum L.) plants subjected to drought stress

Drought stress causes a decrease in agricultural yields. Recently, it has been reported that the phytohormone melatonin (MT), participates in different plant growth and developmental processes. Highbush blueberry (Vaccinium corymbosum L.) is quickly affected by drought, decreasing plant growth, yield, and quality. Thus, the aim of this study was to evaluate the role of melatonin on physiological performance, water status, chlorophyll concentration, and antioxidant metabolism in V. corymbosum plants exposed to drought stress. One-year-old V. corymbosum plants were subjected to four treatments: 1) 100% field capacity (FC) without melatonin; 2) 100% FC with 0.1 mM MT; 3) 50% FC without MT; and 4) 50% FC with MT. Plant water content, leaf area, photosynthetic performance, antioxidants compounds, and chlorophyll concentration were determined at different times post MT application (0, 3, 7, and 10 days). In our study, leaf area, plant water status, the photosynthetic performance and photosynthetic pigments decreased in the treatments with irrigation at 50% FC compared to the treatment whit irrigation at 100% FC. Interestingly, MT application improved plant water potential, CO2 assimilation, chlorophyll a, and total phenols by 35, 65, 22, and 27% respectively, at 50% FC treated with MT compared to plants without MT at 10 days of the experiment. Our results demonstrate that MT enhanced physiological and biochemical responses of V. corymbosum plants to cope with drought stress.

Cessation of berry growth coincides with leaf complete stomatal closure at pre-veraison for grapevine (Vitis vinifera) subjected to progressive drought stress.

Drought events have devasting impacts on grape berry production. The aim of this study was to investigate berry growth in the context of leaf stomatal closure under progressive drought stress. Potted grapevine plants (varieties 'Syrah' and 'Cabernet Sauvignon') were evaluated at pre-verasion (30-45 d after anthesis, DAA) and post-veraison (90-107 DAA). Berry diameter, berry absolute growth rate (AGR), leaf stomatal conductance (Gs) at midday, plant water potential at predawn and midday (ΨPD and ΨMD, respectively), and soil relative water content were measured repeatedly. The ΨPD-threshold of 90 % loss in stomatal conductance (Gs10, i.e. complete stomatal closure) was determined. Data were related to plant dehydration phases I, II and III with corresponding boundaries Θ1 and Θ2, using the water potential curve method. At pre-veraison, berry AGR declined together with leaf Gs in response to soil drying in both varieties. Berry AGR transitioned from positive to negative (shrinkage) values when leaf Gs approached zero. The Gs10-threshold was -0.81 MPa in 'Syrah' and -0.74 MPa in 'Cabernet Sauvignon' and was linked to boundary Θ1. At post-veraison, berry AGR was negligible and negative AGR values were not intensified by increasing drought stress in either variety. Leaf complete stomatal closure under progressive drought stress coincides with cessation of berry growth followed by shrinkage at pre-veraison (growth stage 1).

Vapour pressure deficit was not a primary limiting factor for gas exchange in an irrigated, mature dryland Aleppo pine forest.

Climate change is often associated with increasing vapour pressure deficit (VPD) and changes in soil moisture (SM). While atmospheric and soil drying often co-occur, their differential effects on plant functioning and productivity remain uncertain. We investigated the divergent effects and underlying mechanisms of soil and atmospheric drought based on continuous, in situ measurements of branch gas exchange with automated chambers in a mature semiarid Aleppo pine forest. We investigated the response of control trees exposed to combined soil-atmospheric drought (low SM, high VPD) during the rainless Mediterranean summer and that of trees experimentally unconstrained by soil dryness (high SM; using supplementary dry season water supply) but subjected to atmospheric drought (high VPD). During the seasonal dry period, branch conductance (gbr ), transpiration rate (E) and net photosynthesis (Anet ) decreased in low-SM trees but greatly increased in high-SM trees. The response of E and gbr to the massive rise in VPD (to 7 kPa) was negative in low-SM trees and positive in high-SM trees. These observations were consistent with predictions based on a simple plant hydraulic model showing the importance of plant water potential in the gbr and E response to VPD. These results demonstrate that avoiding drought on the supply side (SM) and relying on plant hydraulic regulation constrains the effects of atmospheric drought (VPD) as a stressor on canopy gas exchange in mature pine trees under field conditions.

Distinct early transcriptional regulations by turgor and osmotic potential in the roots of Arabidopsis.

In a context of climate change, deciphering signaling pathways driving plant adaptation to drought, changes in water availability, and salt is key. A crossing point of these plant stresses is their impact on plant water potential (Ψ), a composite physico-chemical variable reflecting the availability of water for biological processes such as plant growth and stomatal aperture. The Ψ of plant cells is mainly driven by their turgor and osmotic pressures. Here we investigated the effect of a variety of osmotic treatments on the roots of Arabidopsis plants grown in hydroponics. We used, among others, a permeating solute as a way to differentiate variations on turgor from variations in osmotic pressure. Measurement of cortical cell turgor pressure with a cell pressure probe allowed us to monitor the intensity of the treatments and thereby preserve the cortex from plasmolysis. Transcriptome analyses at an early time point (15 min) showed specific and quantitative transcriptomic responses to both osmotic and turgor pressure variations. Our results highlight how water-related biophysical parameters can shape the transcriptome of roots under stress and provide putative candidates to explore further the early perception of water stress in plants.

Proteomic profiles and leaf antioxidative metabolism in cacao juvenile plants subjected to Pb and Pb+Fe doses and to soil flooding

Under natural conditions, plants can be affected by several types of abiotic stresses, such as flooding and the accumulation of toxic metals in the soil. Pb is a non-essential metallic element for plants and is highly toxic for living beings. Fe, as an essential micronutrient, is involved in fundamental plant processes, such as photosynthesis. The main objective of this work was to evaluate the proteomic profiles and the activity of key enzymes of leaf antioxidant metabolism in juvenile plants of the CCN 51 cacao genotype, grown in flooding soil and subjected to a dose of Pb and to different equimolar doses of Pb+Fe. Doses of Pb and Pb+Fe did not interfere with the predawn leaf water potential (ΨWL) of plants, but soil flooding was a key factor for the decline of ΨWL. There was a differential accumulation of Pb and Fe in the roots and leaves, both in flooded soil and non-flooded. Most of Fe and Pb uptake by the root system predominantly accumulated in the roots and only a small fraction was translocated to the shoot, regardless of the water content in the soil. Flooding and the variation of Pb+Fe doses in the soil did not change the transport of Pb to the plant shoot since there were no major alterations in the levels of Pb in the leaves. Leaf enzymatic antioxidant metabolism was altered, mainly by flooding, in relation to Pb or Pb+Fe doses applied in soil. The time of exposure of plants to stressor factors was important for the changes in the activity of key enzymes of the leaf antioxidant metabolism. Soil flooding, with doses of 2 mmol Pb kg−1 soil + 0.5 mmol Fe kg−1 soil, at 6 and 24 h after treatment application, had a greater influence on ΨWL, antioxidant enzymatic metabolism and leaf proteome, and affected proteins related to photosynthesis, DNA, antioxidative system and carbohydrate metabolism, whose abundance was overexpressed or repressed compared to the control.

Synergistic effect of β-sitosterol and biochar application for improving plant growth of Thymus vulgaris under heat stress

Climate change has become the global concern due to its drastic effects on the environment. Agriculture sector is the backbone of food security which remains at the disposal of climate change. Heat stress is the is the most concerning effect of climate change which negatively affect the plant growth and potential yields. The present experiment was conducted to assess the effects of exogenously applied β-sitosterol (Bs at 100 mg/L) and eucalyptus biochar (Eb at 5%) on the antioxidants and nutritional status in Thymus vulgaris under heat stressed conditions. The pot experiment was conducted in completely randomize design in which thymus plants were exposed to heat stress (33 °C) and as a result, plants showed a substantial decline in morpho-physiological and biochemical parameters e.g., a reduction of 59.46, 75.51, 100.00, 34.61, 22.65, and 38.65% was found in plant height, shoot fresh weight, root fresh weight, dry shoot weight, dry root weight and leaf area while in Bs + Eb + heat stress showed 21.16, 56.81, 67.63, 23.09, 12.84, and 35.89% respectively as compared to control. In the same way photosynthetic pigments, transpiration rate, plant nutritional values and water potential increased in plants when treated with Bs and Eb in synergy. Application of Bs and Eb significantly decreased the electrolytic leakage of cells in heat stressed thymus plants. The production of reactive oxygen species was significantly decreased while the synthesis of antioxidants increased with the application of Bs and Eb. Moreover, the application Bs and Eb increased the concentration of minerals nutrients in the plant body under heat stress. Our results suggested that application of Bs along with Eb decreased the effect of heat stress by maintaining nutrient supply and enhanced tolerance by increasing the production of photosynthetic pigments and antioxidant activity.

Combining root and soil hydraulics in macroscopic representations of root water uptake

AbstractPlant water uptake and plant and soil water status are important for the soil water balance and plant growth. They depend on atmospheric water demand and the accessibility of soil water to plant roots, which is in turn related to the hydraulic properties of the root system and the soil around root segments. We present a simulation model that describes water flow in the soil–plant system mechanistically considering both root and soil hydraulic properties. We developed an approach to upscale three‐dimensional (3D) flow in the soil toward root segments of a 3D root architecture to a model that considers one‐dimensional flow between horizontal soil layers and radial flow to root segments in that layer. The upscaled model couples upscaled linear flow equations in the root system with an analytical solution of the nonlinear radial flow equation between the soil and roots. The upscaled model avoids simplifying assumptions about root hydraulic properties and water potential drops near roots made in, respectively, soil‐ and root‐centered models. Xylem water potentials and soil–root interface potentials are explicitly simulated and show, respectively, large variations with depth and large deviations from bulk soil water potentials under dry soil conditions. Accounting for hydraulic gradients in the soil around root segments led to an earlier but slower reduction of transpiration during a drought period and a better plant water status with higher nighttime plant water potentials.

Variation in agro-physiological parameters of Vigna radiata (L. R. Wilczek) under different supplemental irrigation frequencies

The study was aimed at investigating the effect of supplemental irrigations on physiological and agronomic responses of mungbean through evaluation of transpiration, leaf water potential, chlorophyll content and seeds yield during drought pockets. A completely randomized block device with three replications was used to conduct the rainy season study. Four watering regimes were applied from the 26th day after seed sowing and during drought pockets: rain only (0J), tap water after 5 days without rain (5J), after 10 days without rain (10J) and after 15 days without rain (15J). Supplemental irrigation had a significant effect on transpiration, leaf water potential and chlorophyll content of plant leaves. Leaf water potential was the lowest and the highest transpiration at 70 DAS under the 5J supplemental irrigation regime, with higher chlorophyll content. The study also revealed that the frequency of irrigation every 5 days without rain significantly improved the harvest index (p?0.004), the number and mass of seeds per plant (p?0.001), justifying the choice of this water treatment for a better production of well-filled seeds and tops.

Key hydraulic traits control the dynamics of plant dehydration in four contrasting tree species during drought.

Trees are at risk of mortality during extreme drought, yet our understanding of the traits that govern the timing of drought-induced hydraulic failure remains limited. To address this, we tested SurEau, a trait-based soil-plant-atmosphere model designed to predict the dynamics of plant dehydration as represented by changes in water potential, against those observed in potted trees of four contrasting species (Pinus halepensis, Populus nigra, Quercus ilex, and Cedrus atlantica) exposed to drought. SurEau was parameterised with a range of plant hydraulic and allometric traits, soil, and climatic variables. We found close correspondence between predicted and observed plant water potential (MPa) dynamics during early phase drought leading to stomatal closure, as well as during latter phase drought leading to hydraulic failure in all four species. A global model's sensitivity analysis revealed that for a common plant size (leaf area) and soil volume, dehydration times from full hydration to stomatal closure (Tclose) was most strongly controlled by leaf osmotic potential (Pi0) and its influence on stomatal closure, in all four species, while maximum stomatal conductance (gsmax) also contributed to Tclose in Q. ilex and C. atlantica. Dehydration times from stomatal closure to hydraulic failure (Tcav) was most strongly controlled by Pi0, the branch residual conductance (gres), and Q10a sensitivity of gres in the three evergreen species, while xylem embolism resistance (P50) was most influential in the deciduous species Populus nigra. Our findings point to SurEau as a highly useful model for predicting changes in plant water status during drought and suggest adjustments made in key hydraulic traits are potentially beneficial to delaying the onset of drought-induced hydraulic failure in trees.

Deep roots mitigate drought impacts on tropical trees despite limited quantitative contribution to transpiration

Deep rooting is considered a central drought-mitigation trait with vast impact on ecosystem water cycling. Despite its importance, little is known about the overall quantitative water use via deep roots and dynamic shifts of water uptake depths with changing ambient conditions. Knowledge is especially sparse for tropical trees. Therefore, we conducted a drought, deep soil water labeling and re-wetting experiment at Biosphere 2 Tropical Rainforest. We used in situ methods to determine water stable isotope values in soil and tree water in high temporal resolution. Complemented by soil and stem water content and sap flow measurements we determined percentages and quantities of deep-water in total root water uptake dynamics of different tree species. All canopy trees had access to deep-water (max. uptake depth 3.3 m), with contributions to transpiration ranging between 21 % and 90 % during drought, when surface soil water availability was limited. Our results suggest that deep soil is an essential water source for tropical trees that delays potentially detrimental drops in plant water potentials and stem water content when surface soil water is limited and could hence mitigate the impacts of increasing drought occurrence and intensity as a consequence of climate change. Quantitatively, however, the amount of deep-water uptake was low due to the trees' reduction of sap flow during drought. Total water uptake largely followed surface soil water availability and trees switched back their uptake depth dynamically, from deep to shallow soils, following rainfall. Total transpiration fluxes were hence largely driven by precipitation input.