Artificially Induced Water and Sulfate Transport through Sunflower Roots

A new experimental method is used to determine simultaneously the quantity and composition of the sap exuded by a detopped root system at the same time that a pressure deficit of desired magnitude can be applied to the stem stump. The technique was used in a study of the transport of radioactive sulfate through the roots of young sunflower plants placed on complete nutrient solutions labelled with ³⁵S. The complications by the time factor on the composition and rate of the sap stream in experiments of this type were observed and discussed. The time of detopping the roots was very critical as the conditions of sulfate transport were greatly changed some time after the excision. A rectilinear connection existed between the rate of sulfate transport in the sap and the water flow at sap flow velocities comparable with transpiration rates. When the transport of water was very slow, the rate of sulfate transport became constant and independent of the water stream. It was suggested that diffusion or water flow could act as motive force for the ion transport in some non-metabolic phase of transfer in the roots. The addition of 2,4-DNP to the test solution severely interfered with the water and sulfate transport conditions in the roots.     

Permeability and Self-Induction as Factors in Water Transport through Bean Roots

Water uptake of root systems of Phaseolus vulgaris was measured in a Scholander pressure cylinder at a constant pressure. Water uptake rises gradually till a steady state is reached after 15 to 60 minutes. This time course can be described as a transport process with the property of a self-induction. The latter was not affected by temperature. It is concluded that the property of self-induction is located in the cytoplasm or at the interface between cytoplasm and plasmalemma of the endodermal cells. It is suggested that cytoplasmic streaming controls the time course of water transport.     

Water Transport in Isolated Maize Roots

A simple model has been devised which predicts the concentration,C^x_s, of salt (e.g. KCl) in the exudate from isolated roots asa function of the salt concentration, C⁰_s, in the medium. Thechief assumption, made in deriving the relationship betweenC^x_s and C⁰₅, is that the exudation of water, J, from the rootsconsists of two components (one being osmotic, Ø, inorigin and the other, Ø⁰, flowing in the absence of anosmotic gradient). The exudation of salt, J_s, calculated asJ C^x_s, was found to be dependent on C⁰_s. Our investigationson maize roots were concerned with estimations of L_p and Ø^vand determinations of C^x_s as a function of C⁰_s. Satisfactoryagreement between prediction and experiment was found in thesepreliminary studies. It is considered that water movement inisolated roots can be explained by a simple osmotic model withthe additional possibility that a relatively small non-osmoticwater flow occurs.     

Water Movement Through Plant Roots

Mathematical analysis of the hydraulics of water movement throughplant roots, in terms of radial and axial resistances, has ledto equations which provide new insights into the effects ofthe component resistan ces on water uptake by and movement throughindividual roots and root systems. The ratio of axial to radialresistance determines the optimum length of a root and its totalresistance to water movement. The equations permit direct calculationsof the plant water potentials necessary, at the base of theplant, to sustain given flow rates through root systems withgiven characteristics. Lateral spacing and the resistance ofindividual laterals are the dominant factors determining totalflux per unit area into a root. When soil water potential increases with depth (surface layersdrier) root resistance tends to decrease with increasing flowrate; the reverse occurs when the surface is wetter than thelower layers. Calculated patterns of water movement into andthrough roots, in relation to soil water potential and flowrate through the root, indicate efflux from root to soil undercertain conditions. This is considered to reflect reality, althoughthe fluxes are probably transient or intermittent. The equations presented should be combined with equations describingwater movement through soil to define the behaviour of the wholeroot-soil system adequately.     

植物根系吸水机理的研究进展

近年来,植物根系吸水机理在细胞、组织和整体水平上的研究进展非常迅速,对阐明植物抗旱机制及其高效利用有限水资源途径的探讨具有重要意义.本文主要对植物根的复合结构和根系在土壤中的分布、根系中水流性质等方面的最新研究状况进行了概述,特别详细地论述了水通道蛋白的表达及功能与根系中水分运动的关系、以及根系输水的调节和根系吸水过程中的信号传导方面的研究动态,并且评价了根的复合运输模型和根系吸水的数学模型等,最后就其可能生理意义及其应用前景作了评述.     

植物根系吸水过程中根系水流阻力的变化特征

以植物根系吸水的人工模拟试验所测得的数据为依据,运用水流的电模拟原理,定理分析了不同土壤水分水平处理下植物根系吸水过程中根系水流阻力各主要分量的大小、变化规律及其相对重要性．结果表明,在同一水分水平处理中,植物根内木质部传导阻力（Ｒｃ）随生长时间的推移而减小,随土层深度的加深而增大,土根接触阻力（Ｒｓｒ）、植物根系吸收阻力（Ｒｒ）随生长时间表现出先下降后上升阶段的动态变化特征;在不同水分水平处理中,Ｒｃ、Ｒｓｒ、Ｒｒ均随土壤湿度减小而大幅度增大;在植物根系水流阻力各分量中,Ｒｒ占根系水流阻力的比例为５５％～９６％,Ｒｓｒ约占根系水流阻力的４％～４５％,而Ｒｃ仅占根系水流阻力的７×１０－６,故Ｒｒ是决定植物根系吸水速率的重要因素     

Pressure-Induced Water and Solute Flow Through Plant Roots

Water and salt flows through detopped sunflower, tomato andred kidney bean roots under applied pressure were studied usinga pressure chamber. Values of J_v for these root systems weremeasured applying variable pressure on the root medium, andL_p calculated. The K, Na and Cl fluxes under applied pressure were comparedwith those in intact plants at the same water flow rates. Tento 100 times higher Na and Cl fluxes were observed through detoppedroots under pressure as compared to those in the unpressurized,intact plants. It is suggested that the roots under pressureare not completely analogous to intact plant roots, and thatpressure-induced flow may not be a reliable method of determiningcharacteristics of ion flow in roots in relation to water flow. Key words: Volume flow, Hydraulic conductivity, K selectivity     

植物根源逆境信使及其产生和传输

植物不仅仅是恶劣环境的被动受害者 ,而且它也具有对环境变化快速感知和主动适应的能力 ,逆境信息传递就是其适应不良环境的重要策略。本文主要介绍了环境胁迫下 ,调控植物地上部的根源信使、信使的产生和传输以及气孔运动对根源信使的响应机制等方面的研究进展。     

Manipulation of Auxin Transport in Plant Roots during Rhizobium Symbiosis and Nematode Parasitism

The plant rhizosphere harbors many different microorganisms, ranging from plant growth–promoting bacteria to devastating plant parasites. Some of these microbes are able to induce de novo organ formation in infected roots. Certain soil bacteria, collectively called rhizobia, form a symbiotic interaction with legumes, leading to the formation of nitrogen-fixing root nodules. Sedentary endoparasitic nematodes, on the other hand, induce highly specialized feeding sites in infected plant roots from which they withdraw nutrients. In order to establish these new root structures, it is thought that these organisms use and manipulate the endogenous molecular and physiological pathways of their hosts. Over the years, evidence has accumulated reliably demonstrating the involvement of the plant hormone auxin. Moreover, the auxin responses during microbe-induced de novo organ formation seem to be dynamic, suggesting that plant-associated microbes can actively modify their host''s auxin transport. In this review, we focus on recent findings in auxin transport mechanisms during plant development and on how plant symbionts and parasites have evolved to manipulate these mechanisms for their own purposes.     

Interaction between Osmotic- and Pressure-induced Water Flow in Plant Roots

When the pressure gradient across a root alters, there is often an apparent change in the permeability of the root to water. Fiscus (Plant Physiol. 1975. 55: 917-922) has suggested that this can be explained by a simple two-compartment model which takes into account rates of solute uptake into the xylem. A method of testing actual data against the Fiscus model is proposed; this shows that in some cases the apparent changes in permeability cannot be explained by the model. The model is not adequate to predict the amounts of solute reaching the xylem by passive drag: a three-compartment model would be more realistic.     

Plant Roots Increase Bacterivorous Nematode Dispersion through Nonuniform Glass-bead Media

Dispersion of bacterivorous nematodes in soil is a crucial ecological process that permits settlement and exploitation of new bacterial-rich patches. Although plant roots, by modifying soil structure, are likely to influence this process, they have so far been neglected. In this study, using an original three-compartment microcosm experimental design and polyvinyl chloride (PVC) bars to mimic plant roots, we tested the ability of roots to improve the dispersion of bacterivorous nematode populations through two wet, nonuniform granular (glass bead) media imitating contrasting soil textures. We showed that artificial roots increased migration time of bacterivorous nematode populations in the small-bead medium, suggesting that plant roots may play an important role in nematode dispersion in fine-textured soils or when soil compaction is high.     

A Possible Role for Rhizobacteria in Water Treatment by Plant Roots

By accumulating Cd from solution, roots of hydroponically grown Indian mustard (Brassica juncea L.), a high biomass crop plant, were able to cause substantial reductions in the concentration of Cd in solution. The removal of Cd from solution was linearly correlated with Cd accumulation by roots. Screening of 300 different rhizobacterial isolates identified several that, when inoculated onto roots of Indian mustard, significantly enhanced the total amount of Cd removed from solution. Further investigations revealed that this enhancement was because of an overall increase in root biomass in the rhizobacterial-treated plants. Rhizobacteria were found to accumulate Cd from solution, and ultrastructural observations suggested that rhizobacteria promote the precipitation of Cd on the root surface. By precipitating Cd at the root surface, rhizobacteria reduce the amount of Cd taken up into roots, thereby protecting the plants, and in particular the roots, from the toxic effects of Cd. This reduced Cd toxicity allows for the increased proliferation of roots observed when plants are inoculated with certain rhizobacteria.     

The Interaction between Osmotic- and Pressure-induced Water Flow in Plant Roots

This paper presents a general model for coupled solute and water flow through plant roots based on the thermodynamics of irreversible processes. The model explains in a straight-forward manner such experimentally observed phenomena as changes in root resistance, increased solute flux, and apparent negative resistance, which have been reported for root systems under the influence of a hydrostatic pressure gradient. These apparent anomalies are explained on the basis of the interaction between the osmotic and hydrostatic driving forces and the well known “sweeping away” or dilution effect. We show that with a constant hydraulic conductivity the only features necessary to explain these phenomena are some type of membrane or membranelike structure and a mechanism for actively accumulating solutes.     

Focus Issue on Roots: Visualization of Root Water Uptake: Quantification of Deuterated Water Transport in Roots Using Neutron Radiography and Numerical Modeling

Our understanding of soil and plant water relations is limited by the lack of experimental methods to measure water fluxes in soil and plants. Here, we describe a new method to noninvasively quantify water fluxes in roots. To this end, neutron radiography was used to trace the transport of deuterated water (D₂O) into roots. The results showed that (1) the radial transport of D₂O from soil to the roots depended similarly on diffusive and convective transport and (2) the axial transport of D₂O along the root xylem was largely dominated by convection. To quantify the convective fluxes from the radiographs, we introduced a convection-diffusion model to simulate the D₂O transport in roots. The model takes into account different pathways of water across the root tissue, the endodermis as a layer with distinct transport properties, and the axial transport of D₂O in the xylem. The diffusion coefficients of the root tissues were inversely estimated by simulating the experiments at night under the assumption that the convective fluxes were negligible. Inverse modeling of the experiment at day gave the profile of water fluxes into the roots. For a 24-d-old lupine (Lupinus albus) grown in a soil with uniform water content, root water uptake was higher in the proximal parts of lateral roots and decreased toward the distal parts. The method allows the quantification of the root properties and the regions of root water uptake along the root systems.Understanding how and where plant roots extract water from soil remains an open question for both plant and soil scientists. One of the open questions concerns the locations of water uptake along the root system (Frensch and Steudle, 1989; Doussan et al., 1998; Steudle, 2000; Zwieniecki et al., 2003; Javaux et al., 2008). A motivation of these studies is that a better prediction of root water uptake may help to optimize irrigation and identify optimal traits to capture water. Despite its importance, there is little experimental information on the spatiotemporal distribution of the uptake zone along roots growing in soil. The lack of experimental data is largely due to the technical difficulties in measuring water fluxes in soils and roots.Quantitative information on the rate and location of root water uptake along roots growing in soil is needed to better understand the function of roots in extracting water from the soil and tolerating drought events. Such information may show which parts of roots are more involved in water extraction and how root hydraulic properties change during root growth and exposure to water-limiting conditions. For instance, it is not clear how root anatomy and the hydraulic conductivity of roots change as the soil becomes dry or the transpiration demand increases. Quantitative information of the location of root water uptake can be used to estimate the spatial distribution of hydraulic conductivities along roots. This information is needed to parameterize the most recent and advanced models of root water uptake, such as those of Doussan et al. (1998) and Javaux et al. (2008).Most of the experimental information on the spatial distribution of water uptake is limited to roots grown in hydroponic and aeroponic cultures (Frensch and Steudle, 1989; Varney and Canny, 1993; Zwieniecki et al., 2003; Knipfer and Fricke, 2010a). These investigations substantially improved our knowledge of the mechanism of water transport in roots. However, roots grown in hydroponic and aeroponic cultures may have different properties than those of roots grown in soils. As the soil dries, the hydraulic conductivity of roots and of the root-soil interface changes and likely affects the profile of root water uptake (Blizzard and Boyer, 1980; Nobel and Cui, 1992; Huang and Nobel, 1993; McCully, 1995; North and Nobel, 1997; Carminati et al., 2011; Knipfer et al., 2011; McLean et al., 2011; Carminati, 2012).New advances in imaging techniques are opening new avenues for noninvasively studying water uptake by roots in soils (Doussan et al., 1998; Garrigues et al., 2006; Javaux et al., 2008; Pohlmeier et al., 2008; Moradi et al., 2011). Imaging methods such as x-ray computed tomography, light transmission imaging, NMR, and computed neutron radiography allow quantifying the changes of water content in the root zone with different accuracy and spatial resolution. However, due to the concomitant soil water redistribution, the local changes in soil water content are not trivially related to root uptake. Consequently, the estimation of root water uptake requires coupling the imaging methods with the modeling of water flow in the soil, which, in turn, requires accurate information on the hydraulic properties of soil and roots. An additional complexity is represented by the peculiar and only partly understood hydraulic properties of the soil in the vicinity of the roots, the so-called rhizosphere.The hydraulic properties of the rhizosphere are influenced by root and microorganism activity, soil compaction due to root growth, and the formation of air-filled gaps between soil and roots when roots shrink (Nye, 1994; North and Nobel, 1997; Carminati et al., 2010; Aravena et al., 2011; Moradi et al., 2011; Carminati, 2013; Zarebanadkouki and Carminati, 2014). To date, it has been technically difficult to quantify the hydraulic properties of the rhizosphere. Carminati et al. (2011) showed that the hydraulic properties of the first 1 to 2 mm near the root affect the profile of water content and water potential toward the root.Recently, we introduced a novel method to noninvasively trace the flow of water in soil and roots (Zarebanadkouki et al., 2012, 2013). The method combines neutron radiography and the injection of deuterated water (D₂O). Neutron radiography is an imaging technique that allows one to quantify the water distribution in thin soil samples with high accuracy and spatial resolution (Moradi et al., 2008). D₂O is an isotope of normal water. Its chemical and physical properties are similar to those of water, but in contrast to water, it is almost transparent in neutron transmission imaging (Matsushima et al., 2012). This property makes D₂O an excellent tracer for neutron imaging of water flow.In our previous experiments (Zarebanadkouki et al., 2012, 2013), D₂O was injected next to selected roots and its transport was monitored using time-series neutron radiography with a spatial resolution of 150 μm and a temporal resolution of 10 s for a duration of 2 h. We grew lupine (Lupinus albus) in aluminum containers (width of 25 cm, height of 30 cm, and thickness of 1 cm) filled with a sandy soil. The soil was partitioned into different compartments with a 1-cm layer of coarse sand acting as a capillary barrier (three vertical and four horizontal layers placed at regular intervals). The capillary barriers limited the transport of D₂O into a given region of soil and facilitated the quantification of D₂O transport into the roots. Figure 1 shows selected neutron radiographs of D₂O injection during the day and night. This figure is modified from Zarebanadkouki et al. (2013). The radiographs show that (1) the radial transport of D₂O into the roots was faster during the day than during the night and (2) the axial transport of D₂O along the roots was visible only during the day, while it was negligible at night. The differences between nighttime and daytime measurements were caused by the net flow of water induced by transpiration.Open in a separate windowFigure 1.Neutron radiographs of two samples after injection of 4 mL of D₂O during the day (A and B) and during the night (C and D). D₂O was injected in one compartment during the nighttime and in two compartments during the daytime. The images show the differences between the actual radiographs at time t and the radiograph before injection (t = 0). Brighter colors indicate lower neutron attenuation and higher D₂O-water ratio. The images show that (1) the transport of D₂O was faster during the day than during the night and (2) D₂O moved axially beyond the capillary barrier toward the shoot only during the day. Images are closeups of the original field of view of 15.75 × 15.75 cm showing the distribution of D₂O in the soil and root after D₂O injection. Figures are extracted from Zarebanadkouki et al. (2013). (A neutron radiograph of the whole sample used for daytime measurement is given in Figure 9.) [See online article for color version of this figure.]The interpretation of tracing experiments with D₂O in which water and D₂O are mixed is not straightforward (Carminati and Zarebanadkouki, 2013; Warren et al., 2013a, 2013b). To determine the convective fluxes from the radiographs, Zarebanadkouki et al. (2012, 2013) introduced a diffusion-convection model of D₂O transport in roots. The model was solved analytically. The model described the increase of the average D₂O concentration in the root with a double-exponential equation, in which the rate constants of the first and second phases were related to the transport of D₂O into the cortex and the stele of the roots. Although the model included important details of the root structure, such as different pathways of water across the root tissue, the diffusion of D₂O across the root tissue was strongly simplified. In particular, our previous model assumed that as soon as the roots were immersed in D₂O, the apoplastic free space of the root cortex was instantaneously saturated with D₂O. In other words, we assumed that all cortical cells and the root endodermis were simultaneously immersed in an identical concentration of D₂O equal to that of the soil. Additionally, we assumed that D₂O concentration inside the cortical cell and the root stele was uniform (well-stirred compartment).Although the radiographs clearly showed a significant axial transport of D₂O beyond the capillary barrier during the daytime (Fig. 1B), the model of Zarebanadkouki et al. (2013) was not capable of simulating it appropriately. Indeed, our previous model could only simulate the changes in D₂O concentration in the root segments immersed in D₂O. Since the concentration of D₂O in the root segment beyond the capillary barrier carries additional information on the axial and radial fluxes along the roots, we decided to modify our model to include such information.Another approximation of the previous model was the assumption that the radial water flow to the root was uniform along the root segment immersed in D₂O. However, Zarebanadkouki et al. (2013) found significant variations in root water uptake along the roots and suggested that root water uptake should be measured with a better spatial resolution.The objective of this study was to provide an adequate model to interpret tracing experiments with D₂O. We developed two different models to describe the transport of D₂O into roots. (1) In the first model, we described the transport of D₂O into the roots by taking into account the different pathways of water across the root tissue (i.e. the apoplastic and the cell-to-cell pathways). Although this model captures the complexity of the root structure, it requires several parameters, such as the ratio of the water flow in the apoplast over the water flow in the cell-to-cell pathway. We refer to this model as the composite transport model. (2) In the second model, we simplified the root tissue into a homogenous flow domain comprising both pathways. The latter model is a simplification of the complex root anatomy, but it has the advantage of requiring fewer parameters. We refer to this model as the simplified model.In the next sections, we introduce the two modeling approaches and run a sensitivity analysis to test whether the transport of D₂O into roots is sensitive to the parameters of the composite transport model. The question was, do we need the composite transport model to accurately estimate the water flow into the roots based on the experiments with neutron radiography? Or alternatively, can we use the simplified model to estimate the fluxes without the need of introducing several parameters?Our final goal was to develop a numerical procedure to extract quantitative information on the water fluxes and the root hydraulic properties based on the tracing experiments with neutron radiography. Based on the results of the sensitivity analysis, we chose the simplified model to simulate the experiments. By fitting the observed D₂O transport into the roots, we calculated the profiles of water flux across the roots of a 24-d-old lupine as well as the diffusion permeability of its roots.     

Some Peculiarities of Water Transport through Plasticized Nonporous Membranes

“Liquid” and “plasticized” solvent membranes are of interest as possible analogues of biological systems. Semipermeable homogeneous films are prepared by plasticizing polyvinylchloride with organic phosphates. Water permeability of such films is relatively high. For a material containing 70% of 1.4-dihydroxyphenyl-bis(dibutylphosphate), the diffusion coefficient of water at room temperature was estimated to be about 1 × 10^-6 cm²/sec. Conditioning of a plasticized membrane, under the osmotic gradient of solution of sodium nitrate, leads to profound changes in its morphology and to a drastic increase of its water permeability. The induced changes are reversible to a large extent. Their reversibility in various solutions may be correlated with the respective differences in permselectivity. The structure of expanded membranes and the mechanism of changes taking place under the osmotic gradients are discussed.     

Control of Sodium Transport in Sunflower Roots

Electrochemical potential differences (driving forces) for sodiumdistributed between the outside solution and the exuding sapof water-culture-grown sunflower plants (Helianthus annuius)have been determined. The results indicated that sodium wasmoving from the outside solution to the xylem against the electrochemicalpotential gradient at external concentrations below approximately0.30 mM Na. At higher external concentrations sodium appearedto be actively excluded from the xylem. An electrical potential difference between the exuding sap andthe external solution of approximately 30 mV was observed. Itwas unaffected by the external sodium concentration. Use ofa short-circuiting technique indicated that the trans-root potentialresides at the plasmalemma of the cortical cells. Driving forces on sodium distributed between the external solutionand the root and between the xylem sap and the root were calculated.They indicated that the root is able to accumulate sodium activelyboth from the external solution and the xylem sap. It is concludedthat sodium transport to the xylem in this species is controlledby the balance of these two opposing forces.     

Water Transport Properties of Roots and Root Cortical Cells in Proton- and Al-Stressed Maize Varieties

Root and root cell pressure-probe techniques were used to investigate the possible relationship between Al- or H+-induced alterations of the hydraulic conductivity of root cells (LPc) and whole-root water conductivity (LPr) in maize (Zea mays L.) plants. To distinguish between H+ and Al effects two varieties that differ in H+ and Al tolerance were assayed. Based on root elongation rates after 24 h in nutrient solution of pH 6.0, pH 4.5, or pH 4.5 plus 50 [mu]M Al, the variety Adour 250 was found to be H+-sensitive and Al-tolerant, whereas the variety BR 201 F was found to be H+-tolerant but Al-sensitive. No Al-induced decrease of root pressure and root cell turgor was observed in Al-sensitive BR 201 F, indicating that Al toxicity did not cause a general breakdown of membrane integrity and that ion pumping to the stele was maintained. Al reduced LPc more than LPr in Al-sensitive BR 201 F. Proton toxicity in Adour 250 affected LPr more than LPc. In this Al-tolerant variety LPc was increased by Al. Nevertheless, this positive effect on LPc did not render higher LPr values. In conclusion, there were no direct relationships between Al- or H+-induced decreases of LPr and the effects on LPc. To our knowledge, this is the first time that the influence of H+ and Al on root and root cell water relations has been directly measured by pressure-probe techniques.