Water Potential Measurements Research Articles

Disturbances in European beech water relation during an extreme drought

In the context of a probable increase in intensity and frequency of extreme summer drought events, a better understanding of the key processes involved in water relations is needed to improve the theoretical foundations of predictive process-based models. This paper aims to analyse how temperate deciduous trees cope with water shortage. The exceptional summer drought of 2003 in Europe provided an opportunity to monitor stomatal conductance and twig water potential in European beech (Fagus sylvatica L.) at predawn and midday and to analyse variations with respect to leaf height within the canopy. By comparing our field measurements of twig water potential to values found in the literature, we confirmed the strong impact of soil water shortage on crown water relations. This paper shows that (1) the vertical gradient of stomatal conductance within the crown disappeared under extreme soil water depletion; (2) at maximum drought intensity, predawn twig water potential (ψ pd) reached −2.3 MPa at a height of 14 m in the crown and −2.0 MPa at a height of 10 m. The significant differences in ψ pd between the two measurement heights in the canopy may be due to night transpiration; (3) there was a close relationship between predawn twig water potential and relative extractable soil water; (4) as drought conditions intensified, there was a close relationship between canopy radiation interception and predawn water potential, as estimated daily from relative extractable soil water.

Digital microfluidic chip technology for water permeability measurements on single isolated plant protoplasts

Dynamic follow up and analysis of isolated single cells that display non-adhering behavior is hindered by the fact that they float in the suspension medium and thus requires the implementation of systems that physically entrap the cells for successful analysis. We describe the potential of digital microfluidic (DMF) chip technology for conducting analysis of cells in suspension at the single cell resolution. More specifically, we demonstrate the use of DMF technology for the analysis of a group of single individual protoplasts from Arabidopsis thaliana plants that are labeled with magnetic particles and are immobilized on the DMF chip by magnetic forces. By transporting droplets with different osmotic conditions to the site where the cells are trapped, we challenge the cells and monitor their responses dynamically with a camera. The use of DMF technology for performing water potential measurements has the following advantages: (i) solid particles such as cells and magnetic beads are manipulated on the chip without the risk of clogging channels, (ii) low shear stress during droplet unit operations like mixing and transport that characterize DMF are particularly suited for analysis of delicate cells types that lack cell walls such as protoplasts, (iii) the throughput of the analysis is strongly increased as multiple protoplasts are analyzed simultaneously, in contrast with the traditional methods that can handle and challenge only one cell at a time. We show that the DMF analysis platform is effective in creating the steep osmotic gradients required for calculation of water permeability coefficients (P) as the values found are comparable with previously reported ones thus validating the suitability of the technology for such studies. This work illustrates a proof of concept for the applicability of DMF as an unparalleled and promising system for implementing single cells studies on non-adhering cells in an automated way.

Physics of the soil medium organization part 2: pedostructure characterization through measurement and modeling of the soil moisture characteristic curves

Accurate measurement of the two soil moisture characteristic curves, namely, water retention curve (WRC) and soil shrinkage curve (SSC) is fundamental for the physical modeling of hydrostructural processes in vadose zone. This paper is the application part following the theory presented in part I about physics of soil medium organization. Two native Aridisols in the state of Qatar named locally Rodah raod´ə soil and Sabkha sab′kə soil were studied. The paper concluded two main results: the first one is about the importance of having continuous and simultaneous measurement of soil water content, water potential and volume change. Such measurement is imperative for accurate and consistent characterization of each of the two moisture characteristic curves, and consequently the hydrostructural properties of the soil medium. The second is about the simplicity, reliability, strength and uniqueness of identifying the characteristic parameters of the two curves. The results also confirmed the validity of the thermodynamic-based equations of the two characteristic curves presented in part I.

Response of Short Duration Tropical Legumes and Maize to Water Stress: A Glasshouse Study

The study was conducted as a pot experiment in the tropical glasshouse to evaluate the response of grain legumes (Phaseolus vulgaris, Vigna unguiculata, andLablab purpureus) in comparison to maize (Zea mays) and estimate their potential and performance. Two experiments were established using completely randomized design. Physiological measurements (stomatal conductance, photosynthetic activities, and transpiration rates) were measured using LCpro instrument. Scholander bomb was used for the measurement of plant cell water potential. Significant difference was observed in different plant species with increase of different water regimes. Among the legumes,L. purpureusshowed better response in water stressed conditions. At the beginning, in dry watered treatment the photosynthetic rate was below 0 µmol m−2 s−1and in fully watered condition it was 48 µmol m−2 s−1. In dry treatment, total dry weight was 10 g/pot and in fully watered condition it was near to 20 g/pot inP. vulgaris. The study concludes that water stress condition should be taken into consideration for such type of crop cultivation in arid and semiarid regions.

Water relations of an encroaching vine and two dominant C4grasses in the serpentine barrens of southeastern Pennsylvania1

Abstract Globally rare serpentine barren ecosystems support unique communities, but in the Mid-Atlantic region of the United States, these systems are threatened by forest encroachment in the absence of disturbance. Serpentine barrens are commonly high-stress environments with low water and nutrient availability, as well as high heavy metal content. Forest encroachment into barrens is often preceded by prolific growth of the vine, Smilax rotundifolia L. To better understand how S. rotundifolia is able to colonize these inhospitable environments, the water relations of this species were examined along a gradient from serpentine barren to encroaching forest. These data were compared to the water relations of dominant C4 grasses in the barrens. Diurnal measurements of stomatal conductance and leaf water potential were made in June, July, and August 2012 at three study sites in southeastern Pennsylvania. Predawn water potential was often highest for S. rotundifolia growing in serpentine barrens, indicating hi...

Improved Protocol for the in Situ Measurement of Water Potential of Plants with a Thermocouple Psychrometer

Improved Protocol for the in Situ Measurement of Water Potential of Plants with a Thermocouple Psychrometer

Leaf hydraulics II: Vascularized tissues

Current models of leaf hydration employ an Ohm's law analogy of the leaf as an ideal capacitor, neglecting the resistance to flow between cells, or treat the leaf as a plane sheet with a source of water at fixed potential filling the mid-plane, neglecting the discrete placement of veins as well as their resistance. We develop a model of leaf hydration that considers the average conductance of the vascular network to a representative areole (region bounded by the vascular network), and represent the volume of tissue within the areole as a poroelastic composite of cells and air spaces. Solutions to the 3D flow problem are found by numerical simulation, and these results are then compared to 1D models with exact solutions for a range of leaf geometries, based on a survey of temperate woody plants. We then show that the hydration times given by these solutions are well approximated by a sum of the ideal capacitor and plane sheet times, representing the time for transport through the vasculature and tissue respectively. We then develop scaling factors relating this approximate solution to the 3D model, and examine the dependence of these scaling factors on leaf geometry. Finally, we apply a similar strategy to reduce the dimensions of the steady state problem, in the context of peristomatal transpiration, and consider the relation of transpirational gradients to equilibrium leaf water potential measurements.

A PRI-based water stress index combining structural and chlorophyll effects: Assessment using diurnal narrow-band airborne imagery and the CWSI thermal index

This work advances the evaluation and interpretation of the Photochemical Reflectance Index (PRI) as an indicator of water stress, over a range of canopy structures and pigment content levels. Very high resolution (VHR) narrow-band multispectral (10cm) and thermal (20cm) imagery was acquired diurnally, in four airborne campaigns conducted over an experimental vineyard site undergoing three different irrigation treatments. Field measurements of leaf stomatal conductance (Gs) and leaf water potential (Ψleaf) were acquired concurrently with the airborne campaigns and compared against the Crop Water Stress Index (CWSI), a widely accepted, thermal-based indicator of water stress, and against narrow-band multispectral indices calculated from pure-vegetation pixels. The study proposes a new formulation, a normalized PRI (PRInorm), in which the standard PRI index is normalized by an index that is sensitive to canopy structure (Renormalized Difference Vegetation Index, RDVI) and by a red edge index that is sensitive to chlorophyll content (R700/R670). The hypothesis investigated is that the new index, calculated as PRInorm=PRI/[RDVI·R700/R670], not only detects xanthophyll pigment changes as a function of water stress, but also normalizes for the chlorophyll content level and canopy leaf area reduction induced by stress. Results demonstrated that when comparing PRInorm against stomatal conductance (r2=0.79; p<0.001) and leaf water potential (r2=0.77; p<0.001) measured at midday, the new index performed better than the standard PRI (r2=0.52 and 0.49, respectively). Further, when using the four flights conducted during the diurnal experiment, the relationships with stomatal conductance also showed the superior performance of PRInorm (r2=0.68) as opposed to PRI (r2=0.4). The proposed normalized PRI was highly related (r2=0.75; p<0.001) to the thermal indicator of water stress, CWSI, which was used here as a benchmark. In comparison, the standard PRI index was found to be significantly related to CWSI (p<0.001), although the relationship was weaker (r2=0.58) than that obtained for PRInorm. In summary, this study demonstrates that PRInorm isolated better than PRI the physiological changes against a changing background of altered pigments and structure, tracking more precisely the diurnal dynamics of the stomatal aperture. Simulations conducted, using leaf and canopy radiative transfer models to elucidate these results, showed that PRInorm is more linearly related to canopy pigment content than the standard PRI, and was more capable of differentiating between stress levels, providing better insight into the results of this diurnal study.

Dehydration and osmotic adjustment in apple stem tissue during winter as it relates to the frost resistance of buds

In deciduous trees, measurement of stem water potential can be difficult during the leafless period in winter. By using thermocouple psychrometry, osmotic water potentials (Ψo; actual Ψo: Ψo(act); Ψo at full saturation: Ψo(sat)) of expressed sap of bark and bud tissue were measured in order to test if the severity of winter desiccation in apple stems could be sufficiently assessed with Ψo. Water potentials were related to frost resistance and freezing behaviour of buds. The determination of Ψo reliably allowed winter desiccation and osmotic adjustments in apple stem tissue to be assessed. In winter in bark tissue, a pronounced decrease in Ψo(act) and Ψo(sat) was found. Decreased Ψo(sat) indicates active osmotic adjustment in the bark as observed earlier in the leaves of evergreen woody plants. In terminal bud meristems, no significant osmotic adjustments occurred and dehydration during winter was much less. Osmotic water potentials, Ψo(act) and Ψo(sat), of bud tissue were always less negative than in the bark. To prevent water movement and dehydration of the bud tissue via this osmotic gradient, it must be compensated for either by a sufficiently high turgor pressure (Ψp) in bark tissue or by the isolation of the bud tissue from the bark during midwinter. During freezing of apple buds, freeze dehydration and extra-organ freezing could be demonstrated by significantly reduced Ψo(act) values of bud meristems that had been excised in the frozen state. Infrared video thermography was used to monitor freezing patterns in apple twigs. During extracellular freezing of intact and longitudinally dissected stems, infrared differential thermal analysis (IDTA) images showed that the bud meristem remains ice free. Even if cooled to temperatures below the frost-killing temperature, no freezing event could be detected in bud meristems during winter. In contrast, after bud break, terminal buds showed a second freezing at the frost-killing temperature that indicates deep supercooling. Our results demonstrate the applicability of thermocouple psychrometry for the assessment of winter desiccation in stem tissues of deciduous trees and corroborate the finding that dormant apple buds survive by extra-organ freezing and do not deep supercool. In addition, they indicate that significant changes of the frost-survival mechanism can occur during the apple bud development in spring.

Management of Postharvest Deficit Irrigation of Peach Trees Using Infrared Canopy Temperature

Remotely sensed canopy temperature from infrared thermometer (IRT) sensors is an effective tool for detecting plant water stress. Deficit irrigation, where the plants receive less than full irrigations and come under water stress at certain growth stages, can be used to help alleviate water shortage in the San Joaquin Valley of California. Thus, new irrigation scheduling procedures need to be developed. A field study was conducted from 2009 to 2011 to evaluate the performance of using midday infrared canopy to air temperature difference (ΔT) to manage postharvest deficit irrigation of early season peach trees. Threshold values of ΔT were selected, based on previous years' stem water potential and ΔT measurements, for four irrigation treatments: Furrow and surface drip irrigation with or without postharvest water stress. A wired network of 12 IRT sensors was installed above the orchard for canopy temperature measurement. Soil water content and stem water potential were also monitored weekly for each treatment. In both growing seasons (2009–2010 and 2010–2011), the measured ΔT values showed consistent differences among treatments, which correlated to both soil water content (R2 ≈ 0.47–0.53) and stem water potential readings (R2 ≈ 0.46–0.65). The relationship between fruit mass and postharvest irrigation amount indicated that up to 53% water savings could be achieved without impacting fruit size. The study demonstrated that infrared canopy temperature measured from above the tree top can potentially be used for managing deficit irrigation in peach and possibly other tree crops. Additional research is needed to further validate this approach.

Assessment of discretely measured indicators and maximum daily trunk shrinkage for detecting water stress in pomegranate trees

Measurements obtained by the continuous monitoring of trunk diameter fluctuations were compared with discrete measurements of midday stem water potential (Ψstem) and midday leaf conductance (gl) in adult pomegranate trees (Punica granatum L. cv. Mollar de Elche). Control plants (T0) were irrigated daily above their crop water requirements in order to attain non-limiting soil water conditions in 2009 and 2010, while T1 plants were subjected to water stress by depriving them of irrigation water for 34 days in 2010, after which time irrigation was restored and plant recovery was studied for 7 days. T1 plants showed a substantial degree of water stress, which developed slowly. Maximum daily trunk shrinkage (MDS) was identified to be the most suitable plant-based indicator for irrigation scheduling in adult pomegranate trees, because its signal:noise ((T1/T0):coefficient of variation) ratio was higher than that for Ψstem ((T1/T0):coefficient of variation) and gl ((T0/T1):coefficient of variation). MDS increased in response to water stress, but when the Ψstem fell below −1.67MPa, the MDS values decreased. For non-limiting water conditions, MDS could be predicted from mean daily air temperature (Tm) through exponential equations fitted to pooled data across several seasons. First-order equations were also obtained by pooling data across several seasons to predict MDS from crop reference evapotranspiration (ETo), mean daily air vapour pressure deficit (VPDm), Tm and solar radiation (Rs), but these should be used only within a certain range of values (ETo, 2.1–7.4mm; VPDm, 0.64–2.96kPa; Tm, 12.1–28.3°C; Rs, 119.4–331.3Wm−2). Hence, automated MDS measurements have the potential to be used in irrigation scheduling of pomegranate, and these values can be normalized to non-limiting water conditions by locally derived empirical relationships with meteorological variables.

Shoot hydraulic characteristics, plant water status and stomatal response in olive trees under different soil water conditions

To evaluate the impact of the amount and distribution of soil water on xylem anatomy and xylem hydraulics of current-year shoots, plant water status and stomatal conductance of mature ‘Manzanilla’ olive trees. Measurements of water potential, stomatal conductance, hydraulic conductivity, vulnerability to embolism, vessel diameter distribution and vessel density were made in trees under full irrigation with non-limiting soil water conditions, localized irrigation, and rain-fed conditions. All trees showed lower stomatal conductance values in the afternoon than in the morning. The irrigated trees showed water potential values around −1.4 and −1.6 MPa whereas the rain-fed trees reached lower values. All trees showed similar specific hydraulic conductivity (K s) and loss of conductivity values during the morning. In the afternoon, K s of rain-fed trees tended to be lower than of irrigated trees. No differences in vulnerability to embolism, vessel-diameter distribution and vessel density were observed between treatments. A tight control of stomatal conductance was observed in olive which allowed irrigated trees to avoid critical water potential values and keep them in a safe range to avoid embolism. The applied water treatments did not influence the xylem anatomy and vulnerability to embolism of current-year shoots of mature olive trees.

Feasibility of using trunk diameter fluctuation and stem water potential reference lines for irrigation scheduling of early nectarine trees

A three-year experiment on early nectarine (Prunus persica L. Batsch cv. Flanoba) trees was carried out with the aim of studying the impact of environmental variables on the trunk diameter fluctuation parameters and the usefulness of the reference-lines derived from the relationship between different meteorological variables and plant water status indicators for sustainable irrigation scheduling. Plants were drip-irrigated and submitted to non-limiting soil water conditions. Measurements of stem water potential at midday (Ψstem), trunk daily growth rate (TGR) and maximum daily trunk shrinkage (MDS) were related with meteorological variables. MDS presented the best fitting line-regressions with both the average air temperature and vapour pressure deficit during the period 11.00–15.00h solar time, since the most important fraction of trunk shrinkage occurred during that period of time, reaching 74% of MDS (on average over the three years studied), and sometimes 90%. The cause of the poor correlation of MDS with ET0 is discussed. Despite the continuous trunk growth of the trees during the experimental period and slight difference in crop load, inter-annual differences were not observed for the three reference lines obtained in any given year (fruit growth, early and late postharvest period).

New insight into cavitation mechanisms in high-capacity tensiometers based on high-speed photography

The high-capacity tensiometer developed by Ridley and Burland in 1993 is a milestone in experimental unsaturated soil mechanics. This device, which relies on development of tension in an enclosed water body, permits direct measurement of negative water potential. Many tensiometers have been built since 1993, all being based on the same principle although the design may differ slightly from the original. In particular, the common characteristic is a very small water reservoir that is believed to be the location of bubble nucleation, a phenomenon referred to as cavitation that impedes the sensor from functioning properly. Many have studied cavitation and different explanations or cavitation mechanisms have been proposed. However, all the considerations put forward were derived without being able to capture what happened inside the water reservoir at cavitation. This is now achieved with the new tensiometer specifically designed to “see” inside the water reservoir during suction measurement. For the first time, cavitation has been captured via high-speed photography and the mechanisms of cavitation can be explained using physical evidence. The first outcome is that it is possible for a high-capacity tensiometer to function to its full range with a large water reservoir. Then, the analysis of high-speed photographs reveals that the bubbles triggering cavitation are nucleated in the ceramic and make their way to the water reservoir. Cavitation occurs only when the air phase reaches the water reservoir.

Assessment of an empirical spatial prediction model of vine water status for irrigation management in a grapevine field

This study proposes and evaluates an empirically derived spatial model to extrapolate midday stem water potential (MSWP) measurements over a furrow irrigated grapevine field from a single reference site. The methodology used to build the model has previously been used successfully under non-irrigated conditions in France with pre-dawn leaf water potential. It has not previously been applied on irrigated vineyards with moderate water restriction and using MSWP. The precision of the model was calibrated and validated using a database of MSWP measurements collected from a commercial Cabernet Sauvignon (Vitis vinifera L.) vineyard located in the Maule Region, Chile, at various times during the 2009–2010, 2010–2011 and 2011–2012 growing seasons. The proposed spatial model was able to predict the spatial variability of MSWP with an RMSE<0.12MPa. Also, the model significantly improved the prediction of MSWP (r2=0.76) compared to the conventional monitoring carried out by winegrowers (r2≤0.48) under conditions of absent to severe water restriction (>−0.5 to −1.3MPa). The choice of the reference site for vine water status monitoring is important regardless of the method used. Results also showed that irrigation practices may impose a specific soil moisture regime in parts of the field; thus the selection of a reference site that is representative of the field conditions is very important for good model performance under irrigated conditions. Variability in soil and ground cover properties, rather than vine vigour, appeared to be the best information for assisting in the correct location of reference sites.

Limitations to adopting regulated deficit irrigation in stone fruit orchards: a case study

Fruit production development is resulting in large commercial orchards with improved water management standards. While the agronomic and economic benefits of regulated deficit irrigation (RDI) have long been established, the local variability in soils and climate and the irrigation system design limits its practical applications. This paper uses a case study approach (a 225 ha stone fruit orchard) to unveil limitations derived from environmental spatial variability and irrigation performance. The spatial variability of soil physical parameters and meteorology in the orchard was characterized, and its implication on crop water requirements was established. Irrigation depths applied during 2004-2009 were analysed and compared with crop water requirements under standard and RDI strategies. Plant water status was also measured during two irrigation seasons using stem water potential measurements. On-farm wind speed variability amounted to 55%, representing differences of 17% in reference evapotranspiration. During the study seasons, irrigation scheduling evolved towards deficit irrigation; however, the specific traits of RDI in stone fruits were not implemented. RDI implementation was limited by: 1) poor correspondence between environmental variability and irrigation system design; 2) insufficient information on RDI crop water requirements and its on-farm spatial variability within the farm; and 3) low control of the water distribution network.

Use of the Dixon-Tyree stem hygrometer in a rubber tree plantation

The Dixon-Tyree stem hygrometer has hitherto been used primarily indoors in controlled conditions. We used it, and the leaf pressure chamber, to measure water potentials in plantation rubber trees outside in the Côte d’Ivoire (May and September 2004). Measurements of stem and root xylem water potentials were considered to be reliable, provided they complied with technical constraints that we specify. The stem hygrometer is non-destructive if the sensor is well installed, and provides measurements for two to three weeks without intervention, although the initial measurements are sometimes erroneous.

Expression of ABA synthesis and metabolism genes under different irrigation strategies and atmospheric VPDs is associated with stomatal conductance in grapevine (Vitis vinifera L. cv Cabernet Sauvignon).

The influence of different levels of irrigation and of variation in atmospheric vapour pressure deficit (VPD) on the synthesis, metabolism, and transport of abscisic acid (ABA) and the effects on stomatal conductance were examined in field-grown Cabernet Sauvignon grapevines. Xylem sap, leaf tissue, and root tissue were collected at regular intervals during two seasons in conjunction with measurements of leaf water potential (Ψleaf) and stomatal conductance (gs). The different irrigation levels significantly altered the Ψleaf and gs of the vines across both seasons. ABA abundance in the xylem sap was correlated with gs. The expression of genes associated with ABA synthesis, NCED1 and NCED2, was higher in the roots than in the leaves throughout and highest in the roots in mid January, a time when soil moisture declined and VPD was at its highest. Their expression in roots was also inversely related to the levels of irrigation and correlated with ABA abundance in the roots, xylem sap, and leaves. Three genes encoding ABA 8’-hydroxylases were isolated and their identities confirmed by expression in yeast cells. The expression of one of these, Hyd1, was elevated in leaves when VPD was below 2.0–2.5 kPa and minimal at higher VPD levels. The results provide evidence that ABA plays an important role in linking stomatal response to soil moisture status and that changes in ABA catabolism at or near its site of action allows optimization of gas exchange to current environmental conditions.

Diurnal variations in water relations of deficit irrigated lemon trees during fruit growth period

Field-grown lemon trees (Citrus limon (L.) Burm. fil. cv. Fino) were subjected to different drip irrigation treatments: a control treatment, irrigated daily above crop water requirements in order to obtain non-limiting soil water conditions and two deficit irrigation treatments, reducing the water applied according to the maximum daily trunk shrinkage (MDS) signal intensity (actual MDS/control treatment MDS) threshold values of 1.25 (T1 treatment) and 1.35 (T2 treatment), which induced two different drought stress levels. Daily variations in leaf (Yleaf) and stem (Ystem) water potentials, leaf conductance, net photosynthesis, sap flow (SF) and trunk diameter fluctuations were studied on four occasions during the lemon fruit growth period. Ystem and Yleaf revealed a diurnal pattern in response to changes in evaporative demand of the atmosphere. Both water potentials decreased in response to water deficits, which were more pronounced in the T2 treatment. Ystem was seen to be a better plant water status indicator than Yleaf. The difference between the two values of Y (Ystem - Yleaf = DY) was closely correlated with sap flow, making it a suitable measure of leaf transpiration. Using the slope of this relationship, the canopy hydraulic conductance (KC) was estimated. When other continuously recorded plant-based indicators are not accessible, the concurrent measurement of leaf and stem water potentials at midday, which are relatively inexpensive to measure and user-friendly, act as sufficiently good indicators of the plant water status in field grown Fino lemon trees.

Parameter Identification for Root Growth based on Soil Water Potential Measurements – An Inverse Modeling Approach

In this study a strategy to calibrate process-based agro-hydrological models for water dynamics in the soil-crop system with soil sensor measurements was developed. An inverse problem was formulated to infer the root parameters based on the measured soil water potential at depths during crop growth. The root penetration down the soil profile is assumed to be driven by the accumulative daily mean air temperature, and root length density distribution in the soil profile is exponential. The forward agro-hydrological model for water dynamics in the soil- crop system was proposed by Yang et al. (J. Hydrol. 2009; 370:177-190). A micro-Genetic Algorithm was employed to infer the parameters. Results show that the predictions of soil water potential using the inferred values of root parameters agree fairly well with the measurements throughout the entire growing period, indicating that the deduced root parameters are credible and appropriate for the studied case. It follows that the strategy presented in the study enables accurate estimates of root parameters to be obtained from soil sensor measurements at various depths, and thus provides an effective way to calibrate models.