The plant water pump: why water flows uphill of water potential gradients in a root hydraulic anatomy model

Guttation is the exudation of xylem sap from vascular plant leaves. This process is particularly interesting because in its configuration root water uptake occurs against the hydrostatic pressure driving force. Hence, it emphasizes the contribution of another driving force that lifts water in plants: the osmotic potential gradient.The current paradigm of root water uptake explains that, due to the endodermal apoplastic barrier, water flows across root radius from the same principles as through selective membranes: driven by the total water potential gradient. This theory relies on the idea that during guttation, osmolites loaded in xylem vessels decrease xylem total water potential, making it more negative than the total soil water potential, and generating water inflow by osmosis as in an osmometer.However, this theory fails at explaining experiments in which guttation occurs without sufficient solute loading in root xylem of maize (Enns et al., 1998; Enns et al., 2000) and arrowleaf saltbush (Bai et al., 2007) among others; studies concluding that experimental observations &#8220;could not be explained with the current theories in plant physiology&#8221;. Such flow rates towards combined increasing pressure potentials and increasing osmotic potentials between separate apoplastic compartments would necessitate an effective root radial conductivity that is negative; a mind bender.What piece of hydraulic network would make it possible for water to flow against the total water potential driving force?We implemented Steudle&#8217;s composite water transport model in the explicit root cross-section anatomical hydraulic network MECHA (Couvreur et al., 2018). All apoplastic, transmembrane and symplastic pathways are interconnected in the network. The results show that while root radial conductivity is particularly sensitive to cell membrane permeability, the combination of conductive plasmodesmata and increased dilution of protoplast osmotic potentials inwards is a key to explain root water flow towards increasing total potentials. A triple cell theory is suggested as new paradigm of root radial flow.ReferencesBai X-F, Zhu J-J, Zhang P, Wang Y-H, Yang L-Q, Zhang L (2007) Na+ and Water Uptake in Relation to the Radial Reflection Coefficient of Root in Arrowleaf Saltbush Under Salt Stress. Journal of Integrative Plant Biology 49: 1334-1340Couvreur V, Faget M, Lobet G, Javaux M, Chaumont F, Draye X (2018) Going with the Flow: Multiscale Insights into the Composite Nature of Water Transport in Roots. Plant Physiology 178: 1689-1703Enns LC, Canny MJ, McCully ME (2000) An investigation of the role of solutes in the xylem sap and in the xylem parenchyma as the source of root pressure. Protoplasma 211: 183-197Enns LC, McCully ME, Canny MJ (1998) Solute concentrations in xylem sap along vessels of maize primary roots at high root pressure. J. Exp. Bot. 49: 1539-1544

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Mucilage exudation facilitates root water uptake in dry soils

Functional Plant Biology ◽

10.1071/fp13330 ◽

2014 ◽

Vol 41 (11) ◽

pp. 1129 ◽

Cited By ~ 61

Author(s):

Mutez A. Ahmed ◽

Eva Kroener ◽

Maire Holz ◽

Mohsen Zarebanadkouki ◽

Andrea Carminati

Keyword(s):

Hydraulic Conductivity ◽

Water Content ◽

Water Uptake ◽

Root Water Uptake ◽

Root Surface ◽

Pressure Probe ◽

Root Pressure ◽

Plant Trait ◽

Suction Cup ◽

Relaxation Curves

As plant roots take up water and the soil dries, water depletion is expected to occur in the rhizosphere. However, recent experiments showed that the rhizosphere was wetter than the bulk soil during root water uptake. We hypothesise that the increased water content in the rhizosphere was caused by mucilage exuded by roots. It is probably that the higher water content in the rhizosphere results in higher hydraulic conductivity of the root–soil interface. In this case, mucilage exudation would favour the uptake of water in dry soils. To test this hypothesis, we covered a suction cup, referred to as an artificial root, with mucilage. We placed it in soil with a water content of 0.03 cm3 cm–3, and used the root pressure probe technique to measure the hydraulic conductivity of the root–soil continuum. The results were compared with measurements with roots not covered with mucilage. The root pressure relaxation curves were fitted with a model of root water uptake including rhizosphere dynamics. The results demonstrated that when mucilage is added to the root surface, it keeps the soil near the roots wet and hydraulically well conductive, facilitating the water flow from dry soils towards the root surface. Mucilage exudation seems to be an optimal plant trait that favours the capture of water when water is scarce.

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Leaf Gas Exchange and Water Relations of Lupins and Wheat. II. Root and Shoot Water Relations of Lupin During Drought-Induced Stomatal Closure

Functional Plant Biology ◽

10.1071/pp9890415 ◽

1989 ◽

Vol 16 (5) ◽

pp. 415 ◽

Cited By ~ 13

Author(s):

CR Jensen ◽

IE Henson ◽

NC Turner

Keyword(s):

Water Potential ◽

Soil Water ◽

Water Transport ◽

Water Uptake ◽

Water Relations ◽

Leaf Gas Exchange ◽

Stomatal Closure ◽

Soil Water Potential ◽

Root Water Uptake ◽

Leaf Conductance

Plants of Lupinus cosentinii Guss. cv. Eregulla were grown in a sandy soil in large containers in a glasshouse and exposed to drought by withholding water. Under these conditions stomatal closure had previously been shown to be initiated before a significant reduction in leaf water potential was detected. In the experiments reported here, no significant changes were found in water potential or turgor pressure of roots or leaves when a small reduction in soil water potential was induced which led to a 60% reduction in leaf conductance. The decrease in leaf conductance and root water uptake closely paralleled the fraction of roots in wet soil. By applying observed data of soil water and root characteristics, and root water uptake for whole pots in a single-root model, the average water potential at the root surface was calculated. Potential differences for water transport in the soil-plant system, and the resistances to water flow were estimated using the 'Ohm's Law' analogy for water transport. Soil resistance was negligible or minor, whereas the root resistance accounted for 61-72% and the shoot resistance accounted for about 30% of the total resistance. The validity of the measurements and calculations is discussed and the possible role of root- to-shoot communication raised.

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Estimates of tree root water uptake from soil moisture profile dynamics

10.5194/bg-2019-466 ◽

2019 ◽

Author(s):

Conrad Jackisch ◽

Samuel Knoblauch ◽

Theresa Blume ◽

Erwin Zehe ◽

Sibylle K. Hassler

Keyword(s):

Time Series ◽

Soil Moisture ◽

Water Uptake ◽

Sap Flow ◽

Water Cycle ◽

Xylem Sap ◽

Root Water Uptake ◽

Soil Conditions ◽

Diurnal Changes ◽

Moisture Profile

Abstract. Root water uptake (RWU) as one important process in the terrestrial water cycle can help to better understand the interactions in the soil water plant system. We conducted a field study monitoring soil moisture profiles in the rhizosphere of beech trees at two sites with different soil conditions. We infer RWU from step-shaped, diurnal changes in soil moisture. While this approach is a feasible, easily implemented method during wet and moderate conditions, limitations were identified during drier states and for more heterogeneous soil settings. A comparison with time series of xylem sap velocity reveals that RWU and sap flow are complementary measures of the transpiration process. The high correlation between the sap flow time series of the two sites, but lower correlation between the RWU time series, suggests that the trees adapt RWU to soil heterogeneity and site differences.

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Impact of soil water potential pattern on root water uptake distribution and leaf water potential

10.5194/egusphere-egu21-15095 ◽

2021 ◽

Author(s):

Ali Mehmandoost Kotlar ◽

Mathieu Javaux

Keyword(s):

Water Potential ◽

Root System ◽

Soil Water ◽

Water Flow ◽

Water Uptake ◽

Leaf Water Potential ◽

Root Water Uptake ◽

Leaf Water ◽

Hydraulic Lift ◽

Water Potentials

Root water uptake is a major process controlling water balance and accounts for about 60% of global terrestrial evapotranspiration. The root system employs different strategies to better exploit available soil water, however, the regulation of water uptake under the spatiotemporal heterogeneous and uneven distribution of soil water is still a major question. To tackle this question, we need to understand how plants cope with this heterogeneity by adjustment of above ground responses to partial rhizosphere drying. Therefore, we use R-SWMS simulating soil water flow, flow towards the roots, and radial and the axial flow inside the root system to perform numerical experiments on a 9-cell gridded rhizotrone (50 cm&#215;50 cm). The water potentials in each cell can be varied and fixed for the period of simulation and no water flow is allowed between cells while roots can pass over the boundaries. Then a static mature maize root architecture to different extents invaded in all cells is subjected to the various arrangements of cells' soil water potentials. R-SWMS allows determining possible hydraulic lift in drier areas. With these simulations, the variation of root water and leaf water potential will be determined and the role of root length density in each cell and corresponding average soil-root water potential will be statistically discussed.

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Quantifying how plants with different species-specific water-use strategies cope with the same drought-prone hydro-ecological conditions

10.5194/egusphere-egu2020-9364 ◽

2020 ◽

Author(s):

Deepanshu Khare ◽

Gernot Bodner ◽

Mathieu Javaux ◽

Jan Vanderborght ◽

Daniel Leitner ◽

...

Keyword(s):

Soil Moisture ◽

Drought Stress ◽

Water Potential ◽

Water Uptake ◽

Hydraulic Properties ◽

Root Water Uptake ◽

Leaf Water ◽

Plant Water ◽

Soil Hydraulic Properties ◽

Stomatal Control

Plant transpiration and root water uptake are dependent on multiple traits that interact with site soil characteristics and environmental factors such as radiation, atmospheric temperature, relative humidity, and soil-moisture content. Models of root architecture and functions are increasingly employed to simulate root-soil interactions. Root water uptake is thereby affected by the root hydraulic architecture, soil moisture conditions, soil hydraulic properties, and the transpiration demand as controlled by atmospheric conditions. Stomatal conductance plays a vital role in regulating transpiration in plants. We performed simulations of plant water uptake for plants having different mechanisms to control transpiration, spanned by isohydric/anisohydric spectrum. Isohydric plants follow the strategy to close their stomata in order to maintain the leaf water potential at a constant level, while anisohydric plants leave their stomata open when leaf water potentials fall due to drought stress. Modelling the stomatal regulation effectively will result in a more reliable model that will regulate the excessive loss of water. We implemented hydraulic and chemical stomatal control of root water uptake following the current approach where stomatal control is regulated by simulated water potential and/or chemical signal concentration. In order to maintain water uptake from dry soil, low plant water potentials are required, which may lead to reversible or permanent cavitation. We parameterise our model with field data, including climate data and soil hydraulic properties under different tillage conditions. This helps us to understand the behaviour of different crops under drought conditions and predict at which growing stage the stress hits the plant. We conducted the simulations for different scenarios to study the effect of hydraulic and chemical regulation on root system performance under drought stress.

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Measurements and simulation of leaf xylem water potential and root water uptake in heterogeneous soil water contents

Advances in Water Resources ◽

10.1016/j.advwatres.2018.12.009 ◽

2019 ◽

Vol 124 ◽

pp. 96-105 ◽

Cited By ~ 4

Author(s):

Faisal Hayat ◽

Mutez Ali Ahmed ◽

Mohsen Zarebanadkouki ◽

Gaochao Cai ◽

Andrea Carminati

Keyword(s):

Water Potential ◽

Soil Water ◽

Water Uptake ◽

Root Water Uptake ◽

Xylem Water Potential ◽

Heterogeneous Soil ◽

Water Contents ◽

Soil Water Contents

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Corrigendum to ‘‘Measurements and simulation of leaf xylem water potential and root water uptake in heterogeneous soil water contents’’

Advances in Water Resources ◽

10.1016/j.advwatres.2020.103566 ◽

2020 ◽

Vol 138 ◽

pp. 103566

Author(s):

Faisal Hayat ◽

Mutez Ali Ahmed ◽

Mohsen Zarebanadkouki ◽

Gaochao Cai ◽

Andrea Carminati

Keyword(s):

Water Resources ◽

Water Potential ◽

Soil Water ◽

Water Uptake ◽

Root Water Uptake ◽

Xylem Water Potential ◽

Heterogeneous Soil ◽

Water Contents ◽

Soil Water Contents

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Water relations of winter wheat: 3. Components of leaf water potential and the soil-plant water potential gradient

The Journal of Agricultural Science ◽

10.1017/s0021859600035334 ◽

1983 ◽

Vol 100 (3) ◽

pp. 581-589 ◽

Cited By ~ 9

Author(s):

J. S. Wallace ◽

J. A. Clark ◽

M. McGowan

Keyword(s):

Water Stress ◽

Winter Wheat ◽

Water Potential ◽

Soil Water ◽

Soil Water Potential ◽

Potential Gradient ◽

Potential Drop ◽

Root Surface ◽

Total Water ◽

Leaf Osmotic Potential

SUMMARYDiurnal and seasonal changes in the total, osmotic and turgor potentials of winter wheat leaves are compared in two seasons of mild and severe soil water stress. Gradients of total water potential in the soil-plant system are also presented. In both seasons the total water potential of the leaves decreased in parallel with the soil water potential, concurrently leaf osmotic potential also decreased sufficiently to maintain positive leaf turgor potential. Eventually, under severe water stress, soil water potential approached –1·5 MPa and leaf turgor potential tended to zero during the middle of the day.The potential drop across the soil-root system was twice that along the stem. Estimates of the water potential at the root surface varied diurnally and were often lower than the bulk soil water potential. In dry soil plants were unable to equilibrate with the soil water potential overnight. These results are consistent with the existence of significant resistance to water flow across the rhizosphere.

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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

Horticulture Research ◽

10.1038/s41438-021-00603-1 ◽

2021 ◽

Vol 8 (1) ◽

Author(s):

Hiroshi Wada ◽

Keisuke Nakata ◽

Hiroshi Nonami ◽

Rosa Erra-Balsells ◽

Miho Tatsuki ◽

...

Keyword(s):

Water Potential ◽

Volatile Compounds ◽

Water Relations ◽

Freezing Point ◽

Potential Gradient ◽

Pressure Probe ◽

Ionization Mass ◽

Cell Water ◽

Water Potential Gradient ◽

Osmotic Potentials

AbstractWatercore 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.

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