Changes In Water Potential Research Articles

Diurnal variations in the thickness of the inner bark of tree trunks in relation to xylem water potential and phloem turgor.

The inner bark plays important roles in tree stems, including radial exchange of water with the xylem and translocation of carbohydrates. Both processes affect the water content and the thickness of the inner bark on a diurnal basis. For the first time, we simultaneously measured the diurnal variations in the inner bark thickness of hinoki cypress (Chamaecyparis obtusa) by using point dendrometers and those of local xylem potential by using stem psychrometers located next to the dendrometers to determine how these variations were related to each other, to phloem turgor and carbohydrate transport. We also estimated the axial hydrostatic pressure gradient by measuring the osmolality of the sap extracted from the inner bark. The inner bark shrunk during the day and swelled during the night with an amplitude related to day-to-day and seasonal variations in climate. The relationship between changes in xylem water potential and inner bark thickness exhibited a hysteresis loop during the day with a median lag of 2h. A phloem turgor-related signal can be retrieved from the diurnal variations in the inner bark thickness, which was higher at the upper than at the lower position along the trunk. However, a downward hydrostatic pressure gradient was only observed at dawn, suggesting diurnal variations in the phloem sap flow velocity.

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

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

Mycorrhizal response strategies of trifoliate orange under well-watered, salt stress, and waterlogging stress by regulating leaf aquaporin expression

Aquaporins (AQPs) involved in water and small molecule transport respond to environmental stress, while it is not clear how arbuscular mycorrhizal fungi (AMF) regulate AQP expression. Here, we investigated the change in leaf water potential and expression level of four tonoplast intrinsic proteins (TIPs), six plasma membrane intrinsic proteins (PIPs), and four nodin-26 like intrinsic proteins (NIPs) genes in trifoliate orange (Poncirus trifoliata) inoculated with Funneliformis mosseae under well-watered (WW), salt stress (SS), and waterlogging stress (WS). Root AMF colonization and soil hyphal length collectively were reduced by SS and WS. Under WW, inoculation with AMF gave diverse responses of AQPs: six AQPs up-regulated, three AQPs down-regulated, and five AQPs did not change. Such up-regulation of more AQPs under mycorrhization and WW partly accelerated water absorption, thereby, maintaining higher leaf water potential. However, under SS, all the fourteen AQPs were dramatically induced by AMF inoculation, which improved water permeability of membranes and stimulated water transport of the host. Under WS, AMF colonization almost did not induce or even down-regulated these AQPs expressions with three exceptions (PtTIP2;2, PtPIP1;1, and PtNIP1;2), thus, no change in leaf water potential. As a result, mycorrhizal plants under flooding may have an escape mechanism to reduce water absorption. It is concluded that AMF had different strategies in response to environmental stresses (e.g. SS and WS) by regulating leaf AQP expression in the host (e.g. trifoliate orange).

Proximal Hyperspectral Imaging Detects Diurnal and Drought-Induced Changes in Maize Physiology.

Hyperspectral imaging is a promising tool for non-destructive phenotyping of plant physiological traits, which has been transferred from remote to proximal sensing applications, and from manual laboratory setups to automated plant phenotyping platforms. Due to the higher resolution in proximal sensing, illumination variation and plant geometry result in increased non-biological variation in plant spectra that may mask subtle biological differences. Here, a better understanding of spectral measurements for proximal sensing and their application to study drought, developmental and diurnal responses was acquired in a drought case study of maize grown in a greenhouse phenotyping platform with a hyperspectral imaging setup. The use of brightness classification to reduce the illumination-induced non-biological variation is demonstrated, and allowed the detection of diurnal, developmental and early drought-induced changes in maize reflectance and physiology. Diurnal changes in transpiration rate and vapor pressure deficit were significantly correlated with red and red-edge reflectance. Drought-induced changes in effective quantum yield and water potential were accurately predicted using partial least squares regression and the newly developed Water Potential Index 2, respectively. The prediction accuracy of hyperspectral indices and partial least squares regression were similar, as long as a strong relationship between the physiological trait and reflectance was present. This demonstrates that current hyperspectral processing approaches can be used in automated plant phenotyping platforms to monitor physiological traits with a high temporal resolution.

Nonisothermal effects on water potential measurement in a simple geometry

In nature, unsaturated porous media, like soils or plant tissues, are often submitted to temperature gradients which can trigger water transport. A nanofluidic tool is employed here to measure the changes in water potential in response to temperature variations in a model geometry. Variations of -7.9 MPa/K are observed in agreement with previous measurements but which differ from a simple modeling, pointing at subtle couplings between natural convection and the Soret effect at play in the setup.

Effect of sea water substitution on growth, physiological and biochemical processes of coconut (Cocos nucifera L.) seedlings—A hydroponic study

Coconut is grown along the coasts and islands that are vulnerable to climate change-induced sea level rise. Though coconut is considered moderately salt tolerant, our understanding on the growth and physiological response to sea water, either inundation or subsurface water contamination, is very limited. This understanding will enable to effectively manage coconut in coastal systems under future climatic scenarios. In this study, ten month old hydroponically grown coconut seedlings were subjected to 0, 10, 25, 50, 75 and 100 % of sea water substitution (SWS), equivalent to 2.17, 8.32, 16.32, 30.03, 42.14 and 53.69 dS m−1 EC, respectively. Substituting Hoagland solution in hydroponic system by sea water of increasing concentration (>50 % SWS) significantly changed physiological processes; Fv/Fm decreased and rs increased as early as 7 and 18 days after treatment imposition (DAT), respectively which led to significant decline in leaf area and root length expansion as early as 24 DAT. At 25 % SWS, root system (root length and root biomass) was stable but the aerial part biomass was declined by 47 %. On the other hand plant height, leaf area, collar girth and biomass accumulation of seedlings under 10 % SWS (8.32 EC) was on par with the control plants suggesting coconut seedlings could tolerate 10 % SWS. Though, PN declined by 19 % and 43 % at 10 % and 25 % SWS, respectively and a similar decline in gs without a concomitant change in leaf water potential suggested that root-generated signals regulated the stomatal movement in coconut under salinity. Still the biomass accumulation at 10 % SWS was not affected by decline in PN. Under increasing sea water treatments, most of the Na+ absorbed was compartmentalized in root and shoot, while leaf had more accumulation of K+, that ensured high K+/Na+ ratio in the leaves which is an important salinity tolerant mechanism observed in coconut. The leaf Cl− content also had strong negative correlation with [PN] (r=-0.873) and biomass (r=-0.833), therefore in addition to K+ and Na+ homeostasis, the level of tolerance to the increased Cl− content in the leaves may also play an important role in salinity tolerance of coconut. This understanding will help in making appropriate strategies for managing coconut grown at coastal systems in the face of sea level rise under climate change.

ОЦІНКА БАГАТОРІЧНОЇ МІНЛИВОСТІ СЕРЕДНЬОГО РІЧНОГО СТОКУ ВОДИ РІЧОК БАСЕЙНУ ПРИП’ЯТІ В МЕЖАХ УКРАЇНИ ТА ЙОГО РОЗРАХУНКОВІ ХАРАКТЕРИСТИКИ У ФАЗИ ВОДНОСТІ

The aim of the study was to analyze the long-term variability of the average annual water runoff of the rivers of the Pripyat basin within Ukraine and to assess its calculated characteristics in the high-water and low-water phases of water content.The research of the cyclical nature of long-term fluctuations of river water flow, which contributes to the establishment of long-term dynamics of water content and potential (forecast) changes – is a topical issue of modern hydrological research. To describe the long-term variability of river water flow and its structure (cycles and phases of water content) the most effective method is stochastic, based on mathematical statistics, the theory of random variables and functions, probability theory. Autocorrelation analysis, various statistical criteria (homogeneity, series, series lengths), total and difference integral curves, probability theory, correlations, statistical estimation of probable errors, etc. were used to identify stochastic regularities of long-term variability. According to the results of the study of long-term variability of the average annual water runoff of the rivers of the Pripyat basin within Ukraine, it is established that cycles with periods of 29±2 years have high reliability and indicate stability of periods of low (10±2 years) and high water content. 17±2 years). According to the identified stochastic patterns, it is assumed that by 2025-26 it is necessary to expect the continuation of the low-water phase of water, then with the duration of 16-17 years the high-water phase will begin and from 2044-45 there will be low water again until 2055-56. According to the proposed regression equations between the average annual water discharge for a long-term period and their average values during the high-water and low-water phases of water content (with very significant approximation coefficients) and the obtained transition coefficients, it is possible to establish the calculated characteristics of the average annual flow of water of various availability in the high-water and low-water phases of water content, thereby giving their forecast estimates.

Water Relations of Two Adjacently Growing Tree Species Shorea Robusta Gaertn-and Pinus Roxburbhii Sarg-in the Lower Himalayan Region

Water potential (predawn and mid day), water potential components (osmotic potential at full and zero turgor, relative water content), soil water potential and leaf conductance were measured for two adjacently growing tree species Shorea robusta Gaertn and Pinus roxburghii Sarg. at an elevation of 1370m. The stands were open and the density of S. robusta was 212 trees/ha and of P. roxburghii was 141trees/ha. Presence of high number of saplings indicates both the species were regenerating well in the site despite human disturbance. S. robusta maintained relatively high predawn water potential even in summers (above -0.50MPa) and P. roxburghii showed low predawn water potential in early summer and summer season (above-1.4 MPa). P. roxburghii maintained a relatively small daily change in water potential during early summer and summer season (0.33MPa and 0.27MPa) indicating greater ability of the species to close its stomata as drought intensifies. The values of osmotic potential at full and zero turgor remained more or less constant for S. robusta from monsoon to winter and then declined during early summer. P. roxburghii showed a gradual decline in osmotic potential values from monsoon to winter season. Chir-pine has the ability of invade and grow on sites that are water stressed which can be related to its capacity to show high osmotic adjustment.. The most negative values of soil water potential were in early summer in both years. The morning and afternoon conductance was lowest during early summer and highest in autumn season.

Miniaturized, Field-Deployable, Continuous Soil Water Potential Sensor

Soil water potential is a significant factor in determining the dynamics of water in the soil. However, few soil water potential sensors are available to conduct long-term, continuous measurements with full automation, due to cavitation formation inside the sensors while interacting with the soil. This paper presents a miniature, field-deployable soil water potential sensor capable of real-time measurement with a wide dynamic range from 0 to −800 kPa, a minimum detectable change of water potential of ~40 Pa, and a high sensitivity of $\sim 0.248~\mu \text{A}$ /kPa. The sensor consists of a shallow water reservoir sandwiched between a nanoporous ceramic plate and a thin silicon diaphragm with thermal oxide. The nanoscale pores of the ceramic plate allow for the increase of air entry tension, while the smooth and hydrophilic interior surfaces of the water reservoir help to minimize the trapping of air bubbles in the reservoir. When the sensor is embedded in unsaturated soils, the pre-filled water in the reservoir tends to leave the reservoir through the nanopores of the ceramic plate, until an equilibrium in water potential is achieved between the reservoir and the soil. The loss of water leads to bending of the silicon-based diaphragm toward the reservoir. The displacement of the diaphragm is quantified by a miniature optical displacement detector assembled with the sensor, which corresponds to the soil water potential. The presented soil sensor has been validated through both greenhouse and field experiments to monitor dynamic changes in soil water potential in real-time over multiple days.

Jasmonic Acid at the Crossroads of Plant Immunity and Pseudomonas syringae Virulence.

Sensing of pathogen infection by plants elicits early signals that are transduced to affect defense mechanisms, such as effective blockage of pathogen entry by regulation of stomatal closure, cuticle, or callose deposition, change in water potential, and resource acquisition among many others. Pathogens, on the other hand, interfere with plant physiology and protein functioning to counteract plant defense responses. In plants, hormonal homeostasis and signaling are tightly regulated; thus, the phytohormones are qualified as a major group of signaling molecules controlling the most widely tinkered regulatory networks of defense and counter-defense strategies. Notably, the phytohormone jasmonic acid mediates plant defense responses to a wide array of pathogens. In this review, we present the synopsis on the jasmonic acid metabolism and signaling, and the regulatory roles of this hormone in plant defense against the hemibiotrophic bacterial pathogen Pseudomonas syringae. We also elaborate on how this pathogen releases virulence factors and effectors to gain control over plant jasmonic acid signaling to effectively cause disease. The findings discussed in this review may lead to ideas for the development of crop cultivars with enhanced disease resistance by genetic manipulation.

A Development-Associated Decrease in Osmotic Potential Contributes to Fruit Ripening Initiation in Strawberry (Fragaria ananassa).

Fruit development and ripening are accompanied by a large increase in cellular soluble solid contents, which results in a significant decrease in osmotic potential (DOP). Here, we report that this development-associated DOP contributes to the initiation of ripening in strawberry (Fragaria ananassa Duch., Benihoppe) fruit. We show that fruit water potential significantly decreases at the onset of ripening as a result of the DOP. Further analysis using nuclear magnetic resonance spectroscopy (NMR) indicated that the change in fruit water potential was likely caused by catabolism of large molecules in receptacle cells, and bioinformatic analysis identified a family of osmotin-like proteins (OLP) that have a potential role in osmolyte accommodation. The gene expression of more than half of the OLP members increased substantially at the onset of fruit ripening, and specifically responded to DOP treatment, consistent with a close relationship between DOP and fruit ripening. We report that the DOP induced either by mannitol or water loss, triggered fruit ripening, as indicated by the elevated expression of multiple ripening genes and diverse ripening-associated physiological parameters. Collectively, these results suggest that the DOP contributes to strawberry fruit ripening initiation.

Blooms of a key grazer in the Southern Ocean – An individual-based model of Salpa thompsoni

The Southern Ocean near the Western Antarctic Peninsula (WAP) is strongly affected by climate change resulting in warmer air temperature, accompanied with reduced sea ice coverage, increased sea water temperature and potential changes in the abundances of two key grazer species Salpa thompsoni (salp) and Euphausia superba (Antarctic krill). While salp abundance is hypothesized to increase, krill abundance is hypothesized to decline with dramatic consequences for the entire food web of the Southern Ocean. A better understanding of the biotic interaction between krill and salps and their population dynamics is thus crucial. However, the life cycle of salps is complicated and barely understood. Therefore, we have developed an individual-based model describing the whole life cycle to better understand the population dynamics of salps and the conditions for blooms. The model has been used to explore if and under what conditions the empirical pattern of large variability in observed salp abundances at the WAP, generated by the long-term data of the US Antarctic Marine Living Resources Program (AMLR) can emerge from a small seeding population. The model reproduced this empirical pattern if daily growth rates of oozoids were higher than previously reported for the WAP (mean growth rate for oozoids ~ 1 mm d−1) and if growth rates of blastozooids were lower (mean growth rate ~ 0.2 mm d−1). The model suggests that a prerequisite for local salp blooms requires a small founding population in early spring. With climate change it has been suggested that more frequent and earlier transport of salps into the WAP or winter survival will occur. Hence, the risk of salp blooms in the WAP is likely to substantially increase. These findings highlight the importance for an improved quantitative understanding of how primary production and the southward advection of salps will be impacted by climate change.

A Dynamic Model for Strategies and Dynamics of Plant Water-Potential Regulation Under Drought Conditions.

Vegetation responds to drought through a complex interplay of plant hydraulic mechanisms, posing challenges for model development and parameterization. We present a mathematical model that describes the dynamics of leaf water-potential over time while considering different strategies by which plant species regulate their water-potentials. The model has two parameters: the parameter λ describing the adjustment of the leaf water potential to changes in soil water potential, and the parameter Δψww describing the typical ‘well-watered’ leaf water potentials at non-stressed (near-zero) levels of soil water potential. Our model was tested and calibrated on 110 time-series datasets containing the leaf- and soil water potentials of 66 species under drought and non-drought conditions. Our model successfully reproduces the measured leaf water potentials over time based on three different regulation strategies under drought. We found that three parameter sets derived from the measurement data reproduced the dynamics of 53% of an drought dataset, and 52% of a control dataset [root mean square error (RMSE) < 0.5 MPa)]. We conclude that, instead of quantifying water-potential-regulation of different plant species by complex modeling approaches, a small set of parameters may be sufficient to describe the water potential regulation behavior for large-scale modeling. Thus, our approach paves the way for a parsimonious representation of the full spectrum of plant hydraulic responses to drought in dynamic vegetation models.

A Cultivar-Sensitive Approach for the Continuous Monitoring of Olive (Olea europaea L.) Tree Water Status by Fruit and Leaf Sensing

Sustainable irrigation is crucial to reduce water use and management costs in modern orchard systems. Continuous plant-based sensing is an innovative approach for the continuous monitoring of plant water status. Olive (Olea europaea L.) genotypes can respond to drought using different leaf and fruit physiological and morphological mechanisms. This study aimed to identify whether fruit and leaf water dynamics of two different olive cultivars were differently affected by water deficit and their response to changes of midday stem water potential (Ψstem), the most common indicator of plant water status. Plant water status indicators such as leaf stomatal conductance (gs) and Ψstem were measured in the Sicilian olive cultivars Nocellara del Belice (NB) and Olivo di Mandanici (MN), in stage II and III of fruit development. Fruit gauges and leaf patch clamp pressure probes were mounted on trees and their raw data were converted in relative rates of fruit diameter change (RRfruit) and leaf pressure change (RRleaf), sensitive indicators of tissue water exchanges. The analysis of diel, diurnal and nocturnal fluctuations of RRfruit and RRleaf highlighted differences, often opposite, between the two cultivars under water deficit. A combination of statistical parameters extrapolated from RRfruit and RRleaf diurnal and nocturnal curves were successfully used to obtain significant multiple linear models for the estimation of midday Ψstem. Fruit and leaf water exchanges suggest that olive cultivar can either privilege fruit or leaf water status, with MN likely preserving leaf water status and NB increasing fruit tissue elasticity under severe water deficit. The results highlight the advantages of the integration of fruit and leaf water dynamics to estimate plant water status and the need for genotype-specific models in olive.

Water relations of two Sicilian grapevine cultivars in response to potassium availability and drought stress

We investigated the response of two Sicilian grapevine cultivars, Catarratto and Nero d'Avola, to potassium deficiency and drought stress. Two-year-old plants grafted on 1103 Paulsen were grown in agriperlite, with or without potassium in the fertigation solution for six weeks, and subjected to moderate drought stress by suspending irrigation for one week. Potassium content of leaves, roots and xylem sap were measured with an ion-selective electrode. Changes in stomatal conductance, stem and leaf water potential and hydraulic conductance were compared between genotypes and treatments. Potassium deficiency led to significant decreases in leaf potassium content in both cultivars and under both well-watered and drought stress conditions. Potassium content in xylem sap showed no significant differences between cultivars and was correlated with stem hydraulic conductance, particularly in the drought stress treatments. Under drought stress conditions, potassium availability led to an increase in stomatal conductance, particularly in Nero d’Avola. Both cultivars showed a rather isohydric behavior under these experimental conditions, and the level of isohydry varied with potassium availability. These results can be useful for the development of optimal fertigation practices and the selection of drought tolerant varieties.

Course of fruit cracking in ‘Whansan’ pears

This study was evaluated on time of occurrence and tissue characteristics in fruit cracking of mid-season cultivar Whansan Asian pears (Pyrus pyrifolia Nakai) with high amount of precipitation from 60 to 120 days after full bloom (DAFB). Fruit cracking was observed at different topographic ‘Whansan’ pear orchards as a pre-experiment. Pear trees at mountainous orchards received low amount of precipitation in June, between 50 and 80 DAFB, and had a high defoliation and low average of fruit weight and fruit cracking (2.5%) compared to those of pears grown at plain and middle-mountainous orchards. In a main experiment, cracked fruit contained lower concentrations of Ca and higher water soluble pectin in flesh or peel than those of values observed on the intact fruit. Fruit diameter rapidly increased at 40 DAFB and mostly 70–80 DAFB under repeated fluctuation of soil water potential between − 28 and − 67 kPa. Fruit were early completed their cell division and converted into cell enlargement phase between 30 and 50 DAFB, advancing formation of cork cells, while the number of large stone clusters significantly increased at 50 DAFB. Besides of less Ca uptake, both formation of cork and stone cells reduced cell flexibility in the flesh, causing initiation of internal micro-cracking under repetition of rapid change of soil water potential. Fruit cracking was visually progressed at 75 DAFB when fruit were mostly enlarged.

Frost fatigue and its spring recovery of xylem conduits in ring-porous, diffuse-porous, and coniferous species in situ

Frost-induced embolism and frost fatigue are two major aspects of frost damage to xylem water transport in trees. In this study, three species of each ring-porous, diffuse-porous, and coniferous trees growing in situ were used to explore their differences in winter embolism and frost fatigue. Changes in predawn water potential, predawn native embolism, maximal specific conductivity (Kmax), and cavitation resistance (P50, xylem water potential at 50% loss of conductivity) of current-year branches were measured from autumn to spring. Maximum native embolism of late winter was near 100% for ring-porous species, approximately 80% for diffuse-porous species, and below 50% for conifers. In early spring, there was no significant reduction of native embolism until formation of new vessels in ring-porous trees, while diffuse-porous trees and conifers exhibited a reduction in native embolism before development of new xylem. There was a significant decrease in P50 of ring- and diffuse-porous species over winter; however, in May P50 was markedly reduced along with formation of new vessels. Kmax of ring- and diffuse-porous species significantly decreased from autumn to late winter. The results revealed that vulnerability to cavitation and frost fatigue was related to conduit diameter. The strategies for coping with winter embolism differed among the three wood types: in ring-porous species there was no active embolism refilling; in diffuse-porous species there was refilling associated with positive xylem pressure; and in conifers there was refilling without positive xylem pressure. New vessels could completely restore stem hydraulic conductivity but only partially restore xylem cavitation resistance in spring.

The role of leaf water potential in the temperature response of mesophyll conductance.

Variation in temperature (T) is usually accompanied by changes in leaf water potential (Ψleaf ), which may influence mesophyll conductance (gm ). However, the effects of Ψleaf on gm have not yet been considered in models of the gm response to temperature. Temperature responses of gm and Ψleaf and the response of gm to Ψleaf were studied in rice (Oryza sativa) and wheat (Triticum aestivum), and then an empirical model of Ψleaf was incorporated into an existing gm -T model. In wheat, Ψleaf was dramatically decreased with increasing T, whereas in rice Ψleaf was less sensitive or insensitive to T. Without taking Ψleaf into account, gm for wheat showed no response to T. However, at a given Ψleaf , gm was significantly higher at high temperature compared with low. After incorporating the function of Ψleaf into the gm -T model, we suggest that the gm -T relationship can be influenced by the activation and deactivation energy for membrane permeability, Ψleaf gradient between temperatures, and the sensitivity of gm to Ψleaf , below a threshold (Ψleaf,0 ). The data presented here suggest that Ψleaf plays an important role in the gm -T relationship and should be considered in future studies related to the temperature response of gm and photosynthesis.

Can vegetation optical depth reflect changes in leaf water potential during soil moisture dry-down events?

Plant water use strategy is one of the key factors to predict drought impact on vegetation and land-atmosphere fluxes. Vegetation optical depth (VOD) based on microwave radiative transfer inversion has recently been used to assess plant water use strategy. However, VOD is sensitive to both total aboveground biomass (AGB) and leaf water content, with only the latter being a proxy of leaf water potential whose diurnal variation can be used to characterize vegetation iso/anisohydricity. In this study, by using a network of soil water measurements (used as a proxy for predawn leaf water potential), satellite retrieved normalized difference vegetation index (NDVI, as a proxy for AGB), and two satellite VOD products from the Advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E) sensor, we compare three linear models and one machine learning model to investigate to what extent can VOD be used to represent leaf water potential changes during soil moisture dry-down periods. Linear models with both NDVI and leaf water potential, on average, can explain 33% and 51% of VOD variations of each product respectively. Models using only NDVI explain 27% and 46% of the VOD variance, compared to less than 10% by models using leaf water potential only. With the NDVI and leaf water potential (full) model, leaf water potential contributes around 17% of the VOD variance, which is smaller than NDVI (33%). The machine learning model has overall better performance than the linear models, and also highlight the dominant contribution of AGB to VOD signals. After the AGB contribution to VOD is eliminated by normalizing daytime VOD with nighttime VOD, the residuals carry the information of diurnal variations of leaf water potential and calculations from both VOD datasets are consistent with each other (r = 0.42±0.17, P < 0.01 for 88 out of 94 sites). The response of VODdaytimeVODnighttime to soil water potential can also be used as a new metric for ecosystem iso/anisohydricity. Our study demonstrates that a large proportion of variations in VOD are caused by AGB for temperate ecosystems, and higher accuracy VOD products with additional root-zone soil water potential are needed for ecosystem iso/anisohydricity estimations.

Stem cycle analyses help decipher the nonlinear response of trees to concurrent warming and drought

High-resolution analysis of stem radius variation can quantify the impact of warming and drought on stem water balance and stem growth in black spruce [ Picea mariana (Mill.) B.S.P.)]. Drought affected plant water status and stem growth. However, warming affects the components of the circadian stem cycle differently if the impacts occur in the daytime or nighttime. The interactive effect of abiotic stresses had less impact on the circadian stem cycle than when the stresses occurred independently. Warming and recent droughts in boreal regions reflect the multiple dimensions of climate change. How these climate-related stresses will affect the stem growth of trees remains to be described. Plant water relations can detect the dynamics of stem depletion and replenishment under conditions of climate-forced stress. This study aimed to verify the impacts of a combination of asynchronous warming (nighttime versus daytime warming) and drought on stem water balance and stem growth in black spruce [Picea mariana (Mill.) B.S.P.)]. We investigated the water status and variations in stem radius of black spruce saplings growing in a controlled environment from May through August. We grew four-year-old saplings in warmer conditions either during the day (DW) or night (NW) at temperatures ca. 6 °C warmer than the ambient air temperature (CT). We then simulated a one-month drought in June. Automatic point dendrometers provided a high-resolution analysis of variations in stem radius, and we also monitored leaf water potentials and volumetric soil water content during the entire experimental period. We detected significant reductions in stem radius variation under water deficit conditions. In the daytime warming scenario, we observed a significant increase in the duration of contraction and a decrease in expansion of the stems. The amplitude of this contraction and expansion was reduced under the nighttime warming conditions. The main effect of warming was to enhance drought stress by accelerating soil water depletion. Changes in predawn water potential drove the duration of stem circadian cycles under conditions of daytime warming, whereas irreversible growth dynamics drove these cycles under nighttime warming conditions due to the midday water potential. The interaction of night/daytime asynchronous warming and drought reduced the amplitude rather than the duration of stem contraction and expansion. Water deficit decreased stem growth during the growing season. Asymmetric warming (as a single independent treatment) affected the timing and magnitude of stem circadian cycles. Under daytime warming scenarios, the duration of contraction and expansion were regulated mainly by predawn water potential, inducing longer (shorter) durations of contraction (expansion). Under nighttime warming, the smaller amplitudes of stem contraction and expansion were associated with midday water potential. Therefore, the interaction of abiotic stresses had less of an impact on the circadian stem cycle components than when these stresses were applied independently.