Water Potential Gradient Research Articles

Foliar water uptake does not contribute to embolism repair in beech (Fagus sylvatica L.).

Foliar water uptake has recently been suggested as a possible mechanism for the restoration of hydraulically dysfunctional xylem vessels. In this paper we used a combination of ecophysiological measurements, X-ray microcomputed tomography and cryo-scanning electron microscopy during a drought treatment to fully evaluate this hypothesis. Based on an assessment of these methods in beech (Fagus sylvatica L.) seedlings we were able to (1) confirm an increase in the amount of hydraulically redistributed water absorbed by leaves when the soil water potential decreased, and (2) locate this redistributed water in hydraulically active vessels in the stem. However, (3) no embolism repair was observed irrespective of the organ under investigation (i.e. stem, petiole or leaf) or the intensity of drought. Our data provide evidence for a hydraulic pathway from the leaf surface to the stem xylem following a water potential gradient, but this pathway exists only in functional vessels and does not play a role in embolism repair for beech.

Drying banana seeds for ex situ conservation.

The ability of seeds to withstand drying is fundamental to ex situ seed conservation but drying responses are not well known for most wild species including crop wild relatives. We look at drying responses of seeds of Musa acuminata and Musa balbisiana, the two primary wild relatives of bananas and plantains, using the following four experimental approaches: (i) We equilibrated seeds to a range of relative humidity (RH) levels using non-saturated lithium chloride solutions and subsequently measured moisture content (MC) and viability. At each humidity level we tested viability using embryo rescue (ER), tetrazolium chloride staining and germination in an incubator. We found that seed viability was not reduced when seeds were dried to 4% equilibrium relative humidity (eRH; equating to 2.5% MC). (ii) We assessed viability of mature and less mature seeds using ER and germination in the soil and tested responses to drying. Findings showed that seeds must be fully mature to germinate and immature seeds had negligible viability. (iii) We dried seeds extracted from ripe/unripe fruit to 35-40% eRH at different rates and tested viability with germination tests in the soil. Seeds from unripe fruit lost viability when dried and especially when dried faster; seeds from ripe fruit only lost viability when fast dried. (iv) Finally, we dried and re-imbibed mature and less mature seeds and measured embryo shrinkage and volume change using X-ray computer tomography. Embryos of less mature seeds shrank significantly when dried to 15% eRH from 0.468 to 0.262mm3, but embryos of mature seeds did not. Based on our results, mature seeds from ripe fruit are desiccation tolerant to moisture levels required for seed genebanking but embryos from immature seeds are mechanistically less able to withstand desiccation, especially when water potential gradients are high.

Hydrotropism: Analysis of the Root Response to a Moisture Gradient.

Plant adaptation to environmental stress generated by low water availability requires continuous search for moisture niches in the soil. Thus, roots have evolved a hydrotropic response to sense differences in water potential of the soil, and through asymmetric growth, roots can bend to avoid lower water potential sites. Different experimental systems have been devised for hydrotropism assays, which usually rely on air moisture or split agar assays. This latter system uses plates containing an osmolyte only in a region of the plate in order to generate a water potential gradient. Seedlings are placed on the agar plate containing normal medium (NM) so that their root tips are near the junction between NM and the region supplemented with the osmolyte. As a result,a hydrotropic responseis elicited to avoid the low water potential medium,whichis reflected inthe root curvature angle.

Abscisic acid is required for root elongation associated with Ca2+ influx in response to water stress

Abscisic acid (ABA) is a critical hormone for plant survival upon water stress. In this study, a large-scale mutants of Arabidopsis ecotype Columbia-0 (Col-0) by ethyl methanesulfonate (EMS)-mutagenesis were generated, and an improved root elongation under water-stress 1 (irew1) mutant showing significantly enhanced root growth was isolated upon a water potential gradient assay. Then, irew1 and ABA-related mutants in Arabidopsis or tomato plants were observed under water potential gradient assay or water-deficient condition. ABA pathway, Ca2+ response and primary root (PR) elongation rate were monitored in addition to DNA- and RNA-Seq analyses. We found that based on phenotyping and transcriptional analyses, irew1 exhibited the enhanced PR growth, ABA and Ca2+ responses compared to wild-type subjected to water stress. Interestingly, exogenous Ca2+ application enhanced PR growth of irew1, ABA-biosynthesis deficient mutants in Arabidopsis and tomato plants in response to water potential gradients or water-deficient condition. In combination with other ABA-related mutants and pharmacological study, our results suggest that ABA is required for root elongation associated with Ca2+ influx in response to water stress.

Water use efficiency of castor bean under semi-arid conditions of Brazil

Castor bean is one of the main and promising agricultural crops for the production of biodiesel, castor oil and castor bean cake, which have high added value, especially the castor oil. The objective of the present study was to evaluate the water use efficiency (WUE) of castor bean cultivars by assessing the soil water balance components (storage and water storage variation; internal drainage; capillary rise; irrigation depth; rainfall and actual evapotranspiration), according to the phenological stages. The experimental design was in randomized blocks, in 8 × 6 factorial scheme (eight cultivars: BRS Paraguaçu; EBDA MPA11; EBDA MPB01; IAC 2028; IAC 226; BRS Nordestina; IAC Guarani, AG IMA 110–204 and BRS Energia and six periods of evaluation), with three repetitions and 15 plants in each experimental plot. Daily monitoring of soil water content was performed with a capacitance probe (Frequency Domain Reflectometry - FDR, PR2/6 model), at depths of 0.1, 0.2, 0.3, 0.4 and 0.6 m, with 0.6 m being the lower limit of the soil control volume for castor bean. The total soil water potential gradient was determined with tensiometers installed at depths of 0.5 and 0.7 m, and the flux densities were estimated using the Darcy-Buckingham equation. Supplemental irrigation was efficient in maintaining water availability during the crop cycle. Soil water storage was lower for the most productive castor bean cultivar (EBDA MPA 11), with an inverse relationship between yield and water storage, due to its water requirement. The highest and lowest values of internal drainage and capillary rise were found for BRS Nordestina and IAC 226, respectively. EBDA MPA 11 had the best water use efficiency among the castor bean cultivars, differing only from IAC Guarani and EBDA MPB 01.

Coordination between liquid and gas phase conductance enables survival during water stress in an isohydrodynamic Eucalyptus cultivar

The regulation on transpiration and water status is critical for tree survival during drought. Eucalyptus is introduced and widely planted in southern China for timber production. Eucalyptus plantations are expanding into rugged terrains with shallow soils and soil water capacitance. With the ongoing decline in precipitation and the typical monsoon climate in this region, the trees suffer regular seasonal drought, yet their response mechanisms to drought remain obscure. We examined leaf water status, stomatal and hydraulic behavior, and whole-tree transpiration of a wildly planted eucalyptus cultivar, Eucalyptus urophylla × Eucalyptus grandis, across seasons with different water availability in order to identify its response strategy to drought. During the study period, both predawn (ΨPD) and midday (ΨMD) leaf water potential varied considerably, but the water potential gradient between the roots and leaves (ΔΨS-L), measured as the difference between ΨPD and ΨMD, remained constant, indicating an isohydrodynamic regulation on water status under drought. Despite strong stomatal down regulation on transpiration in response to increasing evaporative demand, leaf water potential declined to levels approaching -2 MPa at midday in the driest months, inducing more than 60% loss of xylem hydraulic conductance. Both stomatal and whole-tree hydraulic conductance linearly declined with increasing drought intensity. Whole-tree hydraulic conductance was exponentially correlated with the sensitivity of stomatal conductance to VPD. These results demonstrated that stomatal and hydraulic regulation on transpiration of eucalyptus was coupled through tight correlation between stomatal response to evaporative demand and whole-tree hydraulic conductance. This coordination between liquid and gas phase conductance found in this study was helpful to keep a balance between allowable maximum transpiration and hydraulic safety of eucalyptus under varying soil and atmosphere moisture conditions.

Isohydricity of Two Different Citrus Species under Deficit Irrigation and Reclaimed Water Conditions.

Citrus species are frequently subjected to water and saline stresses worldwide. We evaluated the effects of diurnal changes in the evaporative demands and soil water contents on the plant physiology of grapefruit and mandarin crops under saline reclaimed (RW) and transfer (TW) water conditions, combined with two irrigation strategies, fully irrigated (fI) and non-irrigated (nI). The physiological responses were different depending on the species. Grapefruit showed an isohydric pattern, which restricted the use of the leaf water potential (Ψl) as a plant water status indicator. Its water status was affected by salinity (RW) and water stress (nI), mainly as the combination of both stresses (RW-nI); however, mandarin turned out to be relatively more tolerant to salinity and more sensitive to water stress, mainly because of its low hydraulic conductance (K) levels, showing a critical drop in Ψl that led to severe losses of root–stem (Kroot–stem) and canopy (Kcanopy) hydraulic conductance in TW-nI. This behavior was not observed in RW-nI because a reduction in canopy volume as an adaptive characteristic was observed; thus, mandarin exhibited more anisohydric behavior compared to grapefruit, but isohydrodynamic since its hydrodynamic water potential gradient from roots to shoots (ΔΨplant) was relatively constant across variations in stomatal conductance (gs) and soil water potential. The gs was considered a good plant water status indicator for irrigation scheduling purposes in both species, and its responses to diurnal VPD rise and soil drought were strongly correlated with Kroot–stem. ABA did not show any effect on stomatal regulation, highlighting the fundamental role of plant hydraulics in driving stomatal closure.

Direct evidence for dynamics of cell heterogeneity in watercored apples: turgor-associated metabolic modifications and within-fruit water potential gradient unveiled by single-cell analyses

Watercore is a physiological disorder in apple (Malus × domestica Borkh.) fruits that appears as water-soaked tissues adjacent to the vascular core, although there is little information on what exactly occurs at cell level in the watercored apples, particularly from the viewpoint of cell water relations. By combining picolitre pressure-probe electrospray-ionization mass spectrometry (picoPPESI-MS) with freezing point osmometry and vapor pressure osmometry, changes in cell water status and metabolisms were spatially assayed in the same fruit. In the watercored fruit, total soluble solid was lower in the watercore region than the normal outer parenchyma region, but there was no spatial difference in the osmotic potentials determined with freezing point osmometry. Importantly, a disagreement between the osmotic potentials determined with two methods has been observed in the watercore region, indicating the presence of significant volatile compounds in the cellular fluids collected. In the watercored fruit, cell turgor varied across flesh, and a steeper water potential gradient has been established from the normal outer parenchyma region to the watercore region, retaining the potential to transport water to the watercore region. Site-specific analysis using picoPPESI-MS revealed that together with a reduction in turgor, remarkable metabolic modifications through fermentation have occurred at the border, inducing greater production of watercore-related volatile compounds, such as alcohols and esters, compared with other regions. Because alcohols including ethanol have low reflection coefficients, it is very likely that these molecules would have rapidly penetrated membranes to accumulate in apoplast to fill. In addition to the water potential gradient detected here, this would physically contribute to the appearance with high tissue transparency and changes in colour differences. Therefore, it is concluded that these spatial changes in cell water relations are closely associated with watercore symptoms as well as with metabolic alterations.

Turgor-limited predictions of tree growth, height and metabolic scaling over tree lifespans.

Increasing evidence suggests that tree growth is sink-limited by environmental and internal controls rather than by carbon availability. However, the mechanisms underlying sink-limitations are not fully understood and thus not represented in large-scale vegetation models. We develop a simple, analytically solved, mechanistic, turgor-driven growth model (TDGM) and a phloem transport model (PTM) to explore the mechanics of phloem transport and evaluate three hypotheses. First, phloem transport must be explicitly considered to accurately predict turgor distributions and thus growth. Second, turgor-limitations can explain growth-scaling with size (metabolic scaling). Third, turgor can explain realistic growth rates and increments. We show that mechanistic, sink-limited growth schemes based on plant turgor limitations are feasible for large-scale model implementations with minimal computational demands. Our PTM predicted nearly uniform sugar concentrations along the phloem transport path regardless of phloem conductance, stem water potential gradients and the strength of sink-demands contrary to our first hypothesis, suggesting that phloem transport is not limited generally by phloem transport capacity per se but rather by carbon demand for growth and respiration. These results enabled TDGM implementation without explicit coupling to the PTM, further simplifying computation. We test the TDGM by comparing predictions of whole-tree growth rate to well-established observations (site indices) and allometric theory. Our simple TDGM predicts realistic tree heights, growth rates and metabolic scaling over decadal to centurial timescales, suggesting that tree growth is generally sink and turgor limited. Like observed trees, our TDGM captures tree-size- and resource-based deviations from the classical ¾ power-law metabolic scaling for which turgor is responsible.

Evidence of foliar water uptake in a conifer species

Foliar water uptake (FWU) is the main mechanism used by herbaceous plants in arid areas to absorb small amounts of precipitation. It has an important ecological and hydrological effect for alleviating plant drought stress. However, it is unknown whether woody plants can absorb water through the leaves when water absorption via roots is insufficient. This study explored whether FWU exists and the FWU conditions in semi-humid rock mountainous areas. Isotopes were labeled to detect FWU under different soil water content (SWC) gradients (3.9–6.5%, 6.5–9.1%, 9.1–15.6%, 15.6–20.8%, and 20.8–26.0%) and different precipitation gradients (1 mm/h, 5 mm/h, 10 mm/h, and 15 mm/h) in simulated precipitation experiments with indoor potted plants. The results showed that FWU occurred in each treatment if the SWC ≤ 21.9% no matter the precipitation amount. Water absorption via FWU increased with the increase of precipitation intensity but decreased with the increase of SWC. The greatest ratios of FWU were 2.77% and 9.52% of precipitation intensity of 1 mm/h and 15 mm/h, respectively, in the 3.9–6.5% treatment. The precipitation absorbed by the leaves can be transported to the xylem or root system along the water potential gradient of leaves–branches–roots. The precipitation with reverse migration in branches and roots increased with the increase of the water potential gradient of leaves–branches–roots. These findings suggest that Platycladus orientalis can uptake water through roots and leaves, effectively alleviating drought stress. This study provides new insights into water use patterns and water migration in trees.

A minimally disruptive method for measuring water potential in planta using hydrogel nanoreporters

Leaf water potential is a critical indicator of plant water status, integrating soil moisture status, plant physiology, and environmental conditions. There are few tools for measuring plant water status (water potential) in situ, presenting a critical barrier for developing appropriate phenotyping (measurement) methods for crop development and modeling efforts aimed at understanding water transport in plants. Here, we present the development of an in situ, minimally disruptive hydrogel nanoreporter (AquaDust) for measuring leaf water potential. The gel matrix responds to changes in water potential in its local environment by swelling; the distance between covalently linked dyes changes with the reconfiguration of the polymer, leading to changes in the emission spectrum via Förster Resonance Energy Transfer (FRET). Upon infiltration into leaves, the nanoparticles localize within the apoplastic space in the mesophyll; they do not enter the cytoplasm or the xylem. We characterize the physical basis for AquaDust's response and demonstrate its function in intact maize (Zea mays L.) leaves as a reporter of leaf water potential. We use AquaDust to measure gradients of water potential along intact, actively transpiring leaves as a function of water status; the localized nature of the reporters allows us to define a hydraulic model that distinguishes resistances inside and outside the xylem. We also present field measurements with AquaDust through a full diurnal cycle to confirm the robustness of the technique and of our model. We conclude that AquaDust offers potential opportunities for high-throughput field measurements and spatially resolved studies of water relations within plant tissues.

Germination of an invasive fern responds better than native ferns to water and light stress in a Mexican cloud forest

Some fern species are among the world’s worst weeds, but little is known about the establishment of alien ferns in relatively conserved ecosystems. Exotic species may outperform native ones during germination and facilitate invasion into new environments. Macrothelypteris torresiana is an Asian fern species that was introduced and spread throughout multiple areas of tropical America. We tested the hypothesis that germination of the exotic fern will be higher at high resource levels and will have wider ecological amplitude compared to native fern species in a cloud forest in central Mexico. Spore germination was evaluated through a gradient of water potential (0 to − 1.0 MPa), photon flux density (0 to 200 µmol m−2 s−1), and light quality (0 to 7.2 red:far-red ratio) in the laboratory, whereas plant relative frequency was determinate in the forest to describe the distribution of the ferns under the light environment. Comparing to three native ferns (sun-loving Hypolepis blepharochlaena and shade-loving Blechnum wardiae and Polystichum ordinatum), M. torresiana was more tolerant to water deficit, germinated better under low photon flux but responded similarly to light quality gradient, and occurred over a far wider range of field light conditions. Our results support the hypothesis that the germination requirements of the invasive fern species are less specific and thus facilitate the colonization of sites with diverse light conditions and water availability.

Experimental Study on the Lateral Seepage Characteristics in the Tension Saturated Zone.

To study the lateral seepage field in the tension saturated zone (TSZ), an experiment with no evaporation and precipitation infiltration was carried out in a self-made seepage tank filled up with fine sand. Based on the data and plots obtained, the lateral seepage field distribution features in the TSZ can be divided into three area for discussion: ascending area, descending area, and the nearly horizontal flow area. In the ascending and descending area, the total water potential gradient diminished from the recharge area to the discharge area and the seepage velocity was faster. In the nearly horizontal flow area, the total water potential gradient was lower and the seepage velocity was slower. The pressure potential gradually decreased horizontally from the recharge area to the discharge area, while in the vertical profile, it gradually decreased from the bottom to the top in the whole seepage area. In the absence of evaporation, the vertical water exchange among the saturated zone, TSZ, and unsaturated zone in nearly horizontal flow area is weak. Contrarily, in the ascending area and descending area, vertical water flows through both the phreatic surface and the upper interface of the TSZ. When there is lateral seepage in the TSZ, the thickness of the TSZ generally increases from the ascending area to the nearly horizontal area and then to the descending area. It should be pointed out that in the nearly horizontal area, the TSZ thickness is approximately equal to the height of the water column. Overall, the lateral seepage in the TSZ can be regarded as a stable siphon process, hence the siphon tube model can be further used to depict this lateral seepage.

Hydraulic segmentation does not protect stems from acute water loss during fire.

The heat plume associated with fire has been hypothesized to cause sufficient water loss from trees to induce embolism and hydraulic failure. However, it is unclear whether the water transport path remains sufficiently intact during scorching or burning of foliage to sustain high water loss. We measured water uptake by branches of Magnolia grandiflora while exposing them to a range of fire intensities and examined factors influencing continued water uptake after fire. Burning caused a 22-fold mean increase in water uptake, with greatest rates of water loss observed at burn intensities that caused complete consumption of leaves. Such rapid uptake is possible only with steep gradients in water potential, which would likely result in substantial cavitation of xylem and loss of conductivity in intact stems. Water uptake continued after burning was complete and was greatest following burn intensities that killed leaves but did not consume them. This post-fire uptake was mostly driven by rehydration of the remaining tissues, rather than evaporation from the tissues. Our results indicate that the fire plume hypothesis can be expanded to include a wide range of burning conditions experienced by plants. High rates of water loss are sustained during burning, even when leaves are killed or completely consumed.

Root lateral interactions drive water uptake patterns under water limitation

The plasticity of root water uptake determines the maintenance of transpiration during periods of water limitation and drought. However, the mechanistic basis of plant water uptake, as well as the implications of water uptake strategies at the individual and ecosystem scale remain elusive. We model three-dimensional root water uptake under variably saturated conditions for a one-hectare temperate forest plot for a growing season with a pronounced mid-season dry period. Variations in root architecture, hydraulic properties, and degree of lateral interaction between root systems produce divergent local responses to water limitation and provide insights on individual and community response to meteorological conditions. Results demonstrate the plasticity of ecosystem-scale responses to surface drying, where interacting roots shift regions of active uptake to deeper soil layers with less abundant root biomass. These shifts, a product of both root system and soil hydraulic properties, illustrate intimate links between root and soil hydraulics in determining plant water sourcing. We further demonstrate that root lateral interactions are beneficial at the ecosystem-scale, even when trees compete for water. Specifically, a more spatially extensive root system facilitates access to a larger soil water reservoir, often ameliorating water limitation and reducing sharp water potential gradients. While the reduction of water stress is a benefit, it can be offset by increased root construction and maintenance costs associated with the larger rooting system. A plausible “viability” region of root communal co-existence is therefore implied where competitive pressures and root production costs are balanced by a potential water benefit.

Bio-irrigation: A Drought Alleviation Strategy through Induced Hydro-parasitization under Bi-cropping Practices of Rainfed Agro-ecosystem: A Review

Exploring ecosystem services for environment sustainability is the trending area of research in the field of natural resource management (NRM). Water is an important entity of agro-ecosystem, dryland agriculture greatly suffers due to want of moisture. Bi-cropping is one practice where different crops are grown in proximity to realize various benefits under uncertainties of dryland agriculture. Literacy among multifarious benefits of bi-cropping over monoculture is fairly rich among the researchers as well as growers. However, bio-irrigation is one such co-benefits which address about drought alleviating strategies under bi-cropping practice. In this technique, deep rooted plants suck up water from deep moist sub-soil and deposit part of that sucked water in the upper dry soil layers due to water potential gradient, during this hydraulic lift and redistribution shallow rooted neighboring crops in close proximity gets due benefits of this lifted water in alleviating drought. This is high time to device cropping systems of water limited environment to unlock the potentiality of dryland production units. Based on the published studies Piliostigma reticulatum, Guiera senegalensis, Panicum maximum, Festuca arundinacea and Cajanus cajan were identified as potential bio-irrigator arid agro-ecosystem.

Evidence for root adaptation to a spatially discontinuous water availability in the absence of external water potential gradients

We hereby show that root systems adapt to a spatially discontinuous pattern of water availability even when the gradients of water potential across them are vanishingly small. A paper microfluidic approach allowed us to expose the entire root system of Brassica rapa plants to a square array of water sources, separated by dry areas. Gradients in the concentration of water vapor across the root system were as small as 10-4⋅mM⋅m-1 (∼4 orders of magnitude smaller than in conventional hydrotropism assays). Despite such minuscule gradients (which greatly limit the possible influence of the well-understood gradient-driven hydrotropic response), our results show that 1) individual roots as well as the root system as a whole adapt to the pattern of water availability to maximize access to water, and that 2) this adaptation increases as water sources become more rare. These results suggest that either plant roots are more sensitive to water gradients than humanmade water sensors by 3-5 orders of magnitude, or they might have developed, like other organisms, mechanisms for water foraging that allow them to find water in the absence of an external gradient in water potential.

Leaf elongation response to blue light is mediated by stomatal-induced variations in transpiration in Festuca arundinacea.

Reduced blue light irradiance is known to enhance leaf elongation rate (LER) in grasses, but the mechanisms involved have not yet been elucidated. We investigated whether leaf elongation response to reduced blue light could be mediated by stomata-induced variations of plant transpiration. Two experiments were carried out on tall fescue in order to monitor LER and transpiration under reduced blue light irradiance. Additionally, LER dynamics were compared with those observed in the response to vapour pressure deficit (VPD)-induced variations of transpiration. Finally, we developed a model of water flow within a tiller to simulate the observed short-term response of LER to various transpiration regimes. LER dramatically increased in response to blue light reduction and then reached new steady states, which remained higher than the control. Reduced blue light triggered a simultaneous stomatal closure which induced an immediate decrease of leaf transpiration. The hydraulic model of leaf elongation accurately predicted the LER response to blue light and VPD, resulting from an increase in the growth-induced water potential gradient in the leaf growth zone. Our results suggest that the blue light signal is sensed by stomata of expanded leaves and transduced to the leaf growth zone through the hydraulic architecture of the tiller.

Root water uptake under heterogeneous soil moisture conditions: an experimental study for unraveling compensatory root water uptake and hydraulic redistribution

Plant roots often encounter heterogeneity in soil water content and respond by compensating water uptake from wet zones to cope with the transpiration demand. Simultaneously, plants may also exhibit root-mediated hydraulic redistribution from wet to dry zones. Experiments were conducted to simultaneously monitor compensated root water uptake and hydraulic redistribution in the vadose zone. Vertical and horizontal split-root lysimeters were used to hydraulically isolate maize roots under altering soil water conditions. Compensated root water uptake and root-mediated hydraulic redistribution were monitored by continuous measurement of water content in the lysimeter compartments. Soil water heterogeneity and limited soil water availability were found to accelerate the root water uptake from moist region to compensate the reduced water availability in dry zone. However, no measurable root mediated hydraulic redistribution was observed in short term basis, despite high water potential gradient in the root zone of both lysimeters. The night-time transpiration and xylem refilling processes seem to override the hydraulic redistribution on a diurnal basis in our experiment. Our study shows that compensated root water uptakeplays a major role in meeting the transpiration demand. In contrast, the role of root mediated hydraulic redistribution is found to benegligible in maize plant.

Comparative analysis reveals gravity is involved in the MIZ1-regulated root hydrotropism.

Hydrotropism is the directed growth of roots toward the water found in the soil. However, mechanisms governing interactions between hydrotropism and gravitropism remain largely unclear. In this study, we found that an air system and an agar-sorbitol system induced only oblique water-potential gradients; an agar-glycerol system induced only vertical water-potential gradients; and a sand system established both oblique and vertical water-potential gradients. We employed obliquely oriented and vertically oriented experimental systems to study hydrotropism in Arabidopsis and tomato plants. Comparative analyses using different hydrotropic systems showed that gravity hindered the ability of roots to search for obliquely oriented water, whilst facilitating roots' search for vertically oriented water. We found that the gravitropism-deficient mutant aux1 showed enhanced hydrotropism in the oblique orientation but impaired root elongation towards water in the vertical orientation. The miz1 mutant exhibited deficient hydrotropism in the oblique orientation but normal root elongation towards water in the vertical orientation. Importantly, in contrast to miz1, the miz1/aux1 double mutant exhibited hydrotropic bending in the oblique orientation and attenuated root elongation towards water in the vertical orientation. Our results suggest that gravitropism is required for MIZ1-regulated root hydrotropism in both the oblique orientation and the vertical orientation, providing further insight into the role of gravity in root hydrotropism.