The integration of whole-root and cellular hydraulic conductivities in cereal roots

The hydraulic conductivities of excised whole root systems of wheat (Triticum aestivum L. cv. Atou) and of single excised roots of wheat and maize (Zea mays L. cv. Passat) were measured using an osmotically induced back-flow technique. Ninety minutes after excision the values for single excised roots ranged from 1.6·10^-8 to 5.5·10^-8 m·s^-1·MPa^-1 in wheat and from 0.9·10^-8 to 4.8·10^-8 m·s^-1·MPa^-1 in maize. The main source of variation was a decrease in the value as root length increased. The hydraulic conductivities of whole root systems, but not of single excised roots, were smaller 15 h after excision. This was not caused by occlusion of the xylem at the cut end of the coleoptile. The hydraulic conductivities of epidermal, cortical and endodermal cells were measured using a pressure probe. Epidermal and cortical cells of both wheat and maize roots gave mean values of 1.2·10^-7 m·s^-1·MPa^-1 but in endodermal cells (measured only in wheat) the mean value was 0.5·10^-7 m·s^-1·MPa^-1. The cellular hydraulic conductivities were used to calculate the root hydraulic conductivities expected if water flow across the root was via transcellular (vacuole-to-vacuole), apoplasmic or symplasmic pathways. The results indicate that, in freshly excised roots, the bulk of water flow is unlikely to be via the transcellular pathway. This is in contrast to our previous conclusion (H. Jones, A.D. Tomos, R.A. Leigh and R.G. Wyn Jones 1983, Planta 158, 230–236) which was based on results obtained with whole root systems of wheat measured 14–15 h after excision and which probably gave artefactually low values for root hydraulic conductivity. It is now concluded that, near the root tip, water flow could be through a symplasmic pathway in which the only substantial resistances to water flow are provided by the outer epidermal and the inner endodermal plasma membranes. Further from the tip, the measured hydraulic conductivities of the roots are consistent with flow either through the symplasmic or apoplasmic pathways.Symbols L _{p, cell} cell hydraulic conductivity - L _{p, root} root hydraulic conductivity - L _{p, root} calculated root hydraulic conductivity - root reflection coefficient     

Radial transport of water across cortical sleeves of excised roots ofZea mays L.

The stationary radial volume flows across maize (Zea mays L.) root segments without steles (sleeves) were measured under isobaric conditions. The driving force of the volume flow is an osmotic difference between the internal and external compartment of the root preparations. It is generated by differences in the concentrations of sucrose, raffinose or polyethylene glycol. The flows are linear functions of the corresponding osmotic differences ( ) up to osmotic values which cause plasmolysis. The straight lines obtained pass through the origin. No asymmetry of the osmotic barrier could be detected within the range of driving forces applied ( =±0.5 MPa), corresponding to volume-flow densities of j_{v, s}=±7·10^–8 m·s^–1. Using the literature values for the reflection coefficients of sucrose and polyethylene glycol in intact roots (E. Steudle et al. (1987) Plant Physiol.84, 1220–1234), values for the sleeve hydraulic conductivity of about 1·10^–7 m·s^–1 MPa^–1 were calculated. They are of the same order of magnitude as those reported in the literature for the hydraulic conductivity of intact root segments when hydrostatic pressure is applied.Abbreviations and symbols a _s outer surface of sleeve segment - c concentration of osmotically active solute - j _{v, s} radial volume flow density across sleeve segment - Lp_s hydraulic conductivity of sleeves - Lp_r hydraulic conductivity of intact roots - _N thickness of Nernst diffusion layer - reflection coefficient of root for solute - osmotic value of bulk phase - osmotic coefficient     

Hydraulic resistance to radial water flow in growing hypocotyl of soybean measured by a new pressure-perfusion technique

Hydraulic resistances to water flow have been determined in the cortex of hypocotyls of growing seedlings of soybean (Glycine max L. Merr. cv. Wayne). Data at the cell level (hydraulic conductivity, Lp; half-time of water exchange, T _1/2; elastic modulus, ; diffusivity for the cell-to-cell pathway, D _c) were obtained by the pressure probe, diffusivities for the tissue (D _t) by sorption experiments and the hydraulic conductivity of the entire cortex (Lp_r) by a new pressure-perfusion technique. For cortical cells in the elongating and mature regions of the hypocotyls T _1/2=0.4–15.1 s, Lp=0.2·10^-5–10.0·10^-5 cm s^-1 bar^-1 and D _c=0.1·10^-6–5.5·10^-6 cm² s^-1. Sorption kinetics yielded a tissue diffusivity D _t=0.2·10^-6–0.8·10^-6 cm² s^-1. The sorption kinetics include both cell-wall and cell-to-cell pathways for water transport. By comparing D _c and D _t, it was concluded that during swelling or shrinking of the tissue and during growth a substantial amount of water moves from cell to cell. The pressure-perfusion technique imposed hydrostatic gradients across the cortex either by manipulating the hydrostatic pressure in the xylem of hypocotyl segments or by forcing water from outside into the xylem. In segments with intact cuticle, the hydraulic conductance of the radial path (Lp_r) was a function of the rate of water flow and also of flow direction. In segments without cuticle, Lp_r was large (Lp_r=2·10^-5–20·10^-5 cm s^-1 bar^-1) and exceeded the corticla cell Lp. The results of the pressure-perfusion experiments are not compatible with a cell-to-cell transport and can only the explained by a preferred apoplasmic water movement. A tentative explanation for the differences found in the different types of experiments is that during hydrostatic perfusion the apoplasmic path dominates because of the high hydraulic conductivity of the cell wall or a preferred water movement by film flow in the intercellular space system. For shrinking and swelling experiments and during growth, the films are small and the cell-to-cell path dominates. This could lead to larger gradients in water potential in the tissue than expected from Lp_r. It is suggested that the reason for the preference of the cell-to-cell path during swelling and growth is that the solute contribution to the driving force in the apoplast is small, and tensions normally present in the wall prevent sufficiently thick water films from forming. The solute contribution is not very effective because the reflection coefficient of the cell-wall material should be very small for small solutes. The results demonstrate that in plant tissues the relative magnitude of cell-wall versus cell-to-cell transport could dependent on the physical nature of the driving forces (hydrostatic, osmotic) involved.Abbreviations and symbols D _c diffusivity of the cell-to-cell pathway - D _t diffusivity of the tissue - radial flow rate per cm² of segment surface - Lp hydraulic conductivity of plasma-membrane - Lp_r radial hydraulic conductance of the cortex - T _1/2 half-time of water exchange between cell and surroundings - volumetric elastic modulus     

The Osmometer Model of the Root: Water and Solute Relations of Roots of Phaseolus coccineus

Water and solute relations of young roots of Phaseolus coccineus have been measured using the root pressure probe. Biphasic root pressure relaxations were obtained when roots were treated with solutions containing different osmotic test solutes. From the relaxations, the hydraulic conductivity (Lp_r), the permeability coefficient (P_sr), and the reflection coefficient (σ_sr) of the roots could be evaluated. Lp_r was 1.8 to 8.4 . 10^?8 m . s^?1 . MPa^?1 and P_sr (in 10^?10 m . s^?1): methanol, 27–62; ethanol, 44–73; urea, 5–11; mannitol, 1.5; KCl, 7.1–9.2; NaCl, 2.1; NaNO₃, 3.7. The hydraulic conductivity was similar when using osmotic and hydrostatic pressure gradients as driving forces. The hydraulic conductivity of individual root cortex cells (Lp) was by two orders of magnitude larger than Lp_r (Lp = 0.3 to 4.7 . 10^?6 m . s^?1 . MPa^?1) which indicated a predominant cell-to-cell rather than an apoplasmic transport of water in the Phaseolus root. Except for distances shorter than 20 mm from the root apex, the hydraulic resistance of the roots was limited by the radial movement of water across the root cylinder and not by the hydraulic resistance within the xylem. Reflection coefficients were low: methanol: 0.16–0.34; ethanol: 0.15–0.47; urea: 0.41–0.51; mannitol: 0.68; KCl: 0.43–0.54; NaCl: 0.59; NaNO₃: 0.54. The transport coefficients (Lp_r, P_sr, σ_sr) have been critically examined for influences of unstirred layers and active transport. The low σ_sr suggests that the common treatment of the root as a rather perfect osmometer (σ_sr = 1) analogous to plant cells should be treated cautiously. The reasons for the low σ_sr and the possible implications of the absolute values of the transport parameters for the absorption of water and nutrients are discussed.     

Effects of anaerobic conditions on water and solute relations,and on active transport in roots of maize (Zea mays L.)

The effects of anoxia on water and solute transport across excised roots of young maize plants (Zea mays L. cv. Tanker) grown hydroponically have been studied. With the aid of the root pressure probe, root pressure (P_r), root hydraulic conductivity (Lp_r), and root permeability (P_sr), and reflection ( _sr) coefficients were measured using potassium nitrate (a typical nutrient salt) and sodium nitrate (an atypical nutrient salt) as solutes. During a period of 10–15 h, anaerobic treatment (0.0–0.2 g O₂·m^-3 in root medium) caused a decrease of root pressure by 0.01–0.28 MPa (by 10–80% of original root pressure) after a short transient increase. For a time period of 5 h, the decrease in the stationary root pressure was not reversible. Under anaerobic conditions, roots still behaved like osmometers and were not leaky. The root hydraulic conductivity measured in osmotic experiments (osmotic solute: NaNO₃) was smaller by one to two orders of magnitude than that measured in the presence of hydrostatic gradients. Both the osmotic and hydrostatic hydraulic conductivity decreased during anaerobic treatment by 28 and 44%, respectively, at a constant reflection coefficient of the solutes ( _sr=0.3–1.0). As with root pressure, changes in root permeability to water and solutes were not reversible within 5 h. Under aerobic conditions and at low external concentrations (31–59 mOsmol·kg^-1), osmotic response curves were monophasic for KNO₃, i.e. there was no passive uptake of solutes. Response curves became biphasic at higher concentrations (100–150 mOsmol·kg^-1)- For NaNO₃, response curves were biphasic at all concentrations. Presumably, this pattern was a consequence of the fact that potassium had already accumulated in the xylem. During anoxia, accumulation of potassium in the xylem was reduced, and biphasic responses were also obtained at lower potassium concentrations applied to the medium. The results are discussed in terms of a pump/leak model of the root in which anoxia affects both the active ion pumping and the permeability of the root to nutrient salts (leakage). The effects of anaerobiosis on the passive transport properties of the root (Lp_r, P_sr, _sr) are in line with the recently proposed composite transport model of the root.Abbreviations and Symbols A_r root surface area - Lp_r root hydraulic conductivity - Lp_rh hydrostatic hydraulic conductivity of root - Lp_ro osmotic hydraulic conductivity of root - P_r root pressure - P_sr permeability coefficient of root - _sr reflection coefficient of root The authors thank Mr. Walter Melchior for the curve-fitting program used to work out Lp_rh values from root pressure relaxations and Mr. Mohammad Hajirezai (Lehrstuhl für Pflanzenphysiologie, Universität Bayreuth) for making the ATP measurements. The assistance of Mrs. Libuse Badewitz in making the drawings and the technical help of Mr. Burkhard Stumpf are also gratefully acknowledged.     

Measurement of apoplasmic and cell-to-cell components of root hydraulic conductance by a pressure-clamp technique

A pressure-clamp technique was devised for the direct measurement of cell-to-cell and apoplasmic components of root hydraulic conductance; the experimental results were analyzed in terms of a theoretical model of water and solute flow, based on a composite membrane model of the root. When water is forced under a constant pressure into a cut root system, an exponential decay of flow is observed, until a constant value is attained; when pressure is released, a reverse water flow out of the root system is observed which shows a similar exponential behavour. The model assumes that the transient flow occurs through a cell-to-cell pathway and the observed decrease is the result of accumulation of solutes in front of the root semi-permeable membrane, whilst the steady-state component results from the movement of water through the parallel apoplasmic pathway. Root conductance components are estimated by fitting the model to experimental data. The technique was applied to the root systems of potted cherry (Prunus avium L.) seedlings; average apoplasmic conductance was 15.5 × 10^–9m³· s^–1· MPa^–1, with values ranging from 12.0 × 10^–9 to 18.5 × 10^–9m³· s^–1· MPa^–1; average cell-to-cell conductance was 11.7 × 10⁹ m³· s^–1· MPa^–1, with values ranging from 8.5 × 10^–9 to 15.3 × 10^–9 m³ · s^–1·MPa^–1. Cell-to-cell conductance amounted on average to 43% of total root conductance, with values between 41 and 45%. Leaf specific conductance (conductance per unit of leaf area supported) of the root systems ranged from 2.7 × 10^–8 to 5.6 × 10^–8 m· s^–1·MPa^–1, with an average of 3.7 × 10^–8 m · s^–1·MPa^–1. The newly developed technique allows the interaction of mass flow of water and of solutes to be explored in the roots of soil-grown plants.Abbreviations and Symbols A L_p root hydraulic conductance - A^aL _p ^a root apoplasmic conductance - A^ccL _p ^cc root cell-to-cell conductance - C_s(t) concentration of solutes in apical root compartment at time t - J_v flow of water through the root - J _v ^a apoplasmic flow of water - J_v/cc cell-to-cell flow of water - LSC leaf specific conductance of the root system - P root hydrostatic pressure - P_appl applied pressure - _s(t) root osmotic pressure at time t - _m osmotic pressure of rooting medium - reflection coefficient of root membrane - time constant of cell-to-cell flow decay This research was funded within the EC Project Long-term effects of CO₂-increase and climate change on European forests (LTEEF) (EV5V-CT94-0468); F.M. was supported by a Ministero dell' Universitá e della Ricerca Scientifica e Tecnologica — British Council agreement (Project The ecological significance of cavitation in woody plants); M.C. was supported by a Consiglio Nazionale delle Ricerche — British Council agreement. We gratefully thank Prof. P.G. Jarvis (University of Edinburgh, UK) for revising an earlier version of this paper and Prof. E. Steudle (University of Bayreuth, Germany) for helpful comments.     

Water Transport across Maize Roots : Simultaneous Measurement of Flows at the Cell and Root Level by Double Pressure Probe Technique

A double pressure probe technique was used to measure simultaneously water flows and hydraulic parameters of individual cells and of excised roots of young seedlings of maize (Zea mays L.) in osmotic experiments. By following initial flows of water at the cell and root level and by estimating the profiles of driving forces (water potentials) across the root, the hydraulic conductivity of individual cell layers was evaluated. Since the hydraulic conductivity of the cell-to-cell path was determined separately, the hydraulic conductivity of the cell wall material could be evaluated as well (Lp_cw = 0.3 to 6.10⁻⁹ per meter per second per megapascal). Although, for radial water flow across the cortex and rhizodermis, the apoplasmic path was predominant, the contribution of the hydraulic conductance of the cell-to-cell path to the overall conductance increased significantly from the first layer of the cortex toward the inner layers from 2% to 23%. This change was mainly due to an increase of the hydraulic conductivity of the cell membranes which was Lp = 1.9.10⁻⁷ per meter per second per megapascal in the first layer and Lp = 14 to 9.10⁻⁷ per meter per second per megapascal in the inner layers of the cortex. The hydraulic conductivity of entire roots depended on whether hydrostatic or osmotic forces were used to induce water flows. Hydrostatic Lp_r was 1.2 to 2.3.10⁻⁷ per meter per second per megapascal and osmotic Lp_r = 1.6 to 2.8.10⁻⁸ per meter per second per megapascal. The apparent reflection coefficients of root cells (σ_s) of nonpermeating solutes (KCI, PEG 6000) decreased from values close to unity in the rhizodermis to about 0.7 to 0.8 in the cortex. In all cases, however, σ_s was significantly larger than the reflection coefficient of entire roots (σ_sr). For KCI and PEG 6000, σ_sr was 0.53 and 0.64, respectively. The results are discussed in terms of a composite membrane model of the root.     

Compartment analysis of plant cells by means of turgor pressure relaxation: II. Experimental results onChara corallina

Summary The three-compartment model (paper I) described turgor pressure relaxations as a sum of two exponential functions. The predicted shape of the curves could be confirmed inChara corallina by improving the recording and processing of data. An evaluation on the basis of the three-compartment model provided values for the hydraulic conductivity of the plasmalemma ofLp _p=2×10^–5 to 4×10^–5 cm sec^–1 bar^–1 and ofLp _i=3×10^–5 to 1×10^–4 cm sec^–1 bar^–1 for the tonoplast (assuming the area to be 90% of the plasmalemma area). The mean proportion of the total volume occupied by the cytoplasm was calculated to be 9%. This value is consistent with the concept of a highly vacuolated cell. Other explanations for the biphasic relaxation curves are discussed.     

Hydraulic conductivities of competing root systems of Grevillea robustaand maize in agroforestry

Models of water uptake in mixed stands of vegetation commonly assume that water is partitioned among competing root systems in proportion to relative root length densities. Such an approach assumes implicitly that roots of different species have equivalent hydraulic properties. This was tested for root systems of Grevillea robustaA. Cunn. and maize (Zea maysL.) at a semi-arid site in Kenya. The hydraulic conductances for roots of both species were measured in situat the scale of the whole root or root system using a high pressure flow meter (HPFM). Hydraulic conductivities (_r) were expressed per unit root length. Root lengths were estimated for maize plants by soil coring and for G. robustausing a fractal branching model calibrated against soil coring. Mean _r was 1.88×10^–7 ±0.28×10^–7kg s^–1 MPa^–1 m^–1 for G. robustaand 1.25×10^–7 ±0.13×10^–7kg s^–1 MPa^–1 m^–1 for maize. Values of _r were not significantly different (P<0.05), suggesting that the assumption of hydraulic equivalence for root systems of the two species may be valid, at least when hydrostatic gradients are the major driving force for water uptake. Differences in conductivities between these species could arise, however, because of variation in the hydraulic properties of roots not accounted for here, for example because of root age, phenology or responses to the soil environment.     

Mercurial-sensitive water transport in barley roots

An isolated barley root was partitioned into the apical and basal part across the partition wall of the double-chamber osmometer. Transroot water movement was induced by subjecting the apical part to a sorbitol solution, while the basal part with the cut end was in artificial pond water. The rate of transroot osmosis was first low but enhanced by two means, infilitration of roots by pressurization and repetition of osmosis. Both effects acted additively. The radial hydraulic conductivity (Lp_r) was calculated by dividing the initial flow rate with the surface area of the apical part of the root, to which sorbitol was applied, and the osmotic gradient between the apical and basal part of the root. Lp_r which was first 0.02–0.04 pm s⁻¹ Pa⁻¹ increased up to 0.25–0.4 pm s⁻¹ Pa⁻¹ after enhancement. Enhancement is assumed to be caused by an increase of the area of the plasma membrane which is avallable to osmotic water movement. The increased Lp_r is in the same order of magnitude as the hydraulic conductivity (Lp) of epidermal and cortical cells of barley roots obtained by Steudie and Jeschke (1983). HgCl₂, a potent inhibitor of water channels, suppressed Lp_r of non-infiltrated and infiltrated roots down to 17% and 8% of control values, respectively. A high sensitivity of Lp_r to HgCl₂ suggests that water channels constitute the most conductive pathway for osmotic radial water movement in barley roots.     

Bottlenecks to water transport in Quercus robur L.: the abscission zone and its physiological consequences

Cladoptosis, the abscission of twigs, is the main mechanism of changes in crown structure in senescing pedunculate oak (Quercus robur L.). We tested the hypotheses that abscission zones in nodes of old pedunculate oak trees reduce leaf-specific hydraulic conductance of shoots and thereby limit the stomatal conductance and assimilation.Hydraulic conductance and leaf-specific hydraulic conductance, measured with a high pressure flowmeter in 0.5–1.5 m long shoots, were significantly lower in shoots of low vigour compared to vigorous growing shoots in a 165-years-old stand in the southeast of Germany. Two types of bottlenecks to water transport could be identified in shoots of old oak trees, namely nodes and abscission zones. In young twigs, vessel diameter and vessel density in nodes with abscission zones were significantly reduced compared with internodes. In nodes without abscission zones, vessel density was significantly reduced. The reduction of hydraulic conductance was especially severe in the smallest and youngest shoots with diameters less than 2 mm. Internodes of 1–5 mm sapwood diameter had an average hydraulic conductance of 7.13×10⁻⁶±0.2×10⁻⁶ kg s⁻¹ m⁻¹ MPa⁻¹, compared to 4.54×10⁻⁶±0.3×10⁻⁶ kg s⁻¹ m⁻¹ MPa⁻¹ in those with nodes.Maximum stomatal conductance and maximum net assimilation rate increased significantly with hydraulic conductance and leaf-specific hydraulic conductance. Maximum rate of net photosynthesis A_max of the most vigorous shoots (VC0) (7.34±0.55 μmol m⁻² s⁻¹) was significantly higher (P<0.001) than in shoots of other vigour classes (5.97±0.28 μmol m⁻² s⁻¹). Our data support the hypothesis that the changes in shoot and consequently crown architecture that are observed in ageing and declining trees can limit photosynthesis by reducing shoot hydraulic conductance. Abscission zones increase the hydraulic disadvantage of less vigorous compared to vigorously growing twigs. Cladoptosis might serve as a mechanism of selection between twigs of different efficiency.     

Water transport in maize roots : measurement of hydraulic conductivity, solute permeability, and of reflection coefficients of excised roots using the root pressure probe

A root pressure probe has been used to measure the root pressure (P_r) exerted by excised main roots of young maize plants (Zea Mays L.). Defined gradients of hydrostatic and osmotic pressure could be set up between root xylem and medium to induce radial water flows across the root cylinder in both directions. The hydraulic conductivity of the root (Lp_r) was evaluated from root pressure relaxations. When permeating solutes were added to the medium, biphasic root pressure relaxations were observed with water and solute phases and root pressure minima (maxima) which allowed the estimation of permeability (P_Sr) and reflection coefficients (σ_sr) of roots. Reflection coefficients were: ethanol, 0.27; mannitol, 0.74; sucrose, 0.54; PEG 1000, 0.82; NaCl, 0.64; KNO₃, 0.67, and permeability coefficients (in 10⁻⁸ meters per second): ethanol, 4.7; sucrose, 1.6; and NaCl, 5.7. Lp_r was very different for osmotic and hydrostatic gradients. For hydrostatic gradients Lp_r was 1·10⁻⁷ meters per second per megapascal, whereas in osmotic experiments the hydraulic conductivity was found to be an order of magnitude lower. For hydrostatic gradients, the exosmotic Lp_r was about 15% larger than the endosmotic, whereas in osmotic experiments the polarity in the water movement was reversed. These results either suggest effects of unstirred layers at the osmotic barrier in the root, an asymmetrical barrier, and/or mechanical effects. Measurements of the hydraulic conductivity of individual root cortex cells revealed an Lp similar to Lp_r (hydrostatic). It is concluded that, in the presence of external hydrostatic gradients, water moves primarily in the apoplast, whereas in the presence of osmotic gradients this component is much smaller in relation to the cell-to-cell component (symplasmic plus transcellular transport).     

Water and solute transport along developing maize roots

Hydraulic and osmotic properties were measured along developing maize (Zea mays L.) roots at distances between 15 and 465 mm from the root tip to quantify the effects of changes in root structure on the radial and longitudinal movement of water and solutes (ions). Root development generated regions of different hydraulic and osmotic properties. Close to the root tip, passive solute permeability (root permeability coefficient, P_sr) was high and selectivity (root reflection coefficient, _sr) low, indicative of an imperfect semipermeable root structure. Within the apical 100–150 mm, P_sr decreased by an order of magnitude and _sr increased significantly. Root hydraulic conductivity (Lp_r) depended on the nature of the force (hydrostatic and osmotic). Osmotic Lp_r was smaller by an order of magnitude than hydrostatic Lp_r and decreased with increasing distance from the root tip. Throughout the root, responses in turgor of cortical cells and late metaxylem to step changes in xylem pressure applied to the base of excised roots were measured at high spatial resolution. The resulting profiles of radial and longitudinal propagation of pressure showed that the endodermis had become the major hydraulic barrier in older parts of the root, i.e. at distances from the apex ä 150 mm. Other than at the endodermis, no significant radial hydraulic resistance could be detected. The results permit a detailed analysis of the root's composite structure which is important for its function in collecting and translocating water and nutrients.Abbreviations and Symbols CPP cell pressure probe - IT root segments with intact tips; - Lp_r root hydraulic conductivity - Lp_rh hydrostatic hydraulic conductivity of root - Lp_ro osmotic hydraulic conductivity of root - P_app hydrostatic pressure applied to cut end of root - P_c cell turgor - P_{c, cor} turgor of cortical cell - P_c,xyl turgor of late metaxylem vessel - P_ro stationary root pressure - P_r0,seal stationary root pressure of sealed root segment - P_sr solute permeability coefficient of root - RPP root pressure probe - TR root segments with tip removed - _sr reflection coefficient of root Dedicated to Professor Andreas Sievers on the occasion of his retirement     

The effect of endogenous and externally supplied nitrate on nitrate uptake and reduction in sugarbeet seedlings

The pericarp of the dormant sugarbeet fruit acts as a storage reservoir for nitrate, ammonium and -amino-N. These N-reserves enable an autonomous development of the seedling for 8–10 d after imbibition. The nitrate content of the seed (1% of the whole fruit) probably induces nitrate-reductase activity in the embryo enclosed in the pericarp. Nitrate that leaks out of the pericarp is reabsorbed by the emerging radicle. Seedlings germinated from seeds (pericarp was removed) without external N-supply are able to take up nitrate immediately upon exposure via a low-capacity uptake system (v_max = 0.8 mol NO ₃ ^- ·(g root FW)^–1·h^–1; K_s = 0.12 mM). We assume that this uptake system is induced by the seed nitrate (10 nmol/seed) during germination. Induction of a high-capacity nitrate-uptake system (v_max = 3.4 mol NO ₃ ^- ·(g root FW)^–1·h^–1; K_s = 0.08 mM) by externally supplied nitrate occurs after a 20-min lag and requires protein synthesis. Seedlings germinated from whole fruits absorb nitrate via a highcapacity uptake mechanism induced by the pericarp nitrate (748 nmol/pericarp) during germination. The uptake rates of the high-capacity system depend only on the actual nitrate concentration of the uptake medium and not on prior nitrate pretreatments. Nitrate deprivation results in a decline of the nitrate-uptake capacity (t_1/2 of v_max = 5 d) probably caused by the decay of carrier molecules. Small differences in K_s but significant differences in v_max indicate that the low- and high-capacity nitrate-uptake systems differ only in the number of identical carrier molecules.Abbreviations NR nitrate reductase - pFPA para-fluorophenylalanine This work was supported by a grant from Bundesministerium für Forschung und Technologie and by Kleinwanzlebener Saatzucht AG, Einbeck.     

Abscisic acid and water transport in sunflowers

The role of abscisic acid (ABA) in the transport of water and ions from the root to the shoot of sunflower plants (Helianthus annuus) was investigated by application of ABA either to the root medium or to the apical bud. The exudation at the hypocotyl stump of decapitated seedlings was measured with and without hydrostatic pressure (0–0.3 MPa) applied to the root. All ABA concentrations tested (10^-10–10^-4 mol·l^-1) promoted exudation. Maximal amounts of exudate (200% of control) were obtained with ABA at 10^-6·mol·l^-1 and an externally applied pressure of 0.1 MPa. The effect was rapid and long-lasting, and involved promotion of ion release to the xylem (during the first hours) as well as an increase in hydraulic conductivity. Abscisic acid applied to the apical bud had effects similar to those of the rootapplied hormone. Increased rates of exudation were also obtained after osmotic stress was applied to the root; this treatment increased the endogenous level of ABA in the root as well as in the shoot. Water potentials of the hypocotyls of intact plants increased when the roots were treated with ABA at 5°C, whereas stomatal resistances were lowered. The results are consistent with the view that ABA controls the water status of the plant not only by regulating stomatal transpiration, but also by regulating the hydraulic conductivity of the root.Abbreviations and symbols ABA abscisic acid - Tv volume flow - Lp hydraulic conductivity - PEG polyethyleneglycol - water potential - osmotic potential - osmotic value - P hydrostatic pressure     

Hydraulic and osmotic properties of spruce roots

Hydraulic and osmotic properties of roots of 2-year-old Norwayspruce seedlings (Plcea abiea (L.) Karst) were investigatedusing different techniques (steady flow, pressure probe, andstop flow technique). Root pressures were measured using theroot pressure probe. Compared to roots of herbaceous plantsor deciduous trees, excised root systems of spruce did not developappreciable root pressure (-0.001 to 0.004 MPa or -10 to 40cm of water column). When hydrostatic pressure gradients wereused to drive water flows across the roots, hydraulic conductivities(Lp_r) were determined in two types of experiments: (i) rootpressure relaxations (using the root pressure probe) and (ii)steady flow experiments (pneumatic pressures applied to theroot system or xylem or partial vacuum applied to the xylem).Root Lp_r ranged between 0.2 and 810^–8m s^–1 MPa^–1(on average) depending on the conditions. In steady flow experiments,Lpr depended on the pressure applied (or on the flow acrossthe roots) and equalled (0.190.12) to (1.21.7)10^–8m s^–1 MPa^–1 at pressures between 0.2 and 0.4 MPaand (1.51.3)10^–8 m s^–1 MPa^–1 at appliedpressures between 0.8 and 1.0 MPa. When pressures or vacuumwere applied to the xylem, Lp_r values were similar. The hydraulicconductivity measured during pressure relaxations (transientwater flows) was similar to that obtained at high pressures(and water flows). Although there was a considerable scatterin the data, there was a tendency of the hydraulic conductivityof the roots to decrease with increasing size of the root system.When osmotic gradients were used to drive water flows, Lp_r valuesobtained with the root pressure probe were much smaller thanthose measured in the presence of hydrostatic gradients. Onaverage, a root Lp_r=0.01710^–8 was found for osmotic andLp_r=6.410^–8 m s^–1 MPa^–1 in correspondinghydrostatic experiments, i.e. the two values differed by a factorwhich was as large as 380. The same hydraulic conductivity asthat obtained in osmotic experiments using the pressure probewas obtained by the 'stop flow techniquel. In this technique,the suction created by an osmoticum applied to the root wasbalanced by a vacuum applied to the xylem. Lp_r values of rootsystems did not change significantly when measured for up to5 d. In osmotic experiments with different solutes (Na₂S0₄,K₂S0₄, Ca(NO₃)₂, mannitol), no passive uptake of solutes couldbe detected, i.e. the solute permeability was very low whichwas different from earlier findings on roots of herbs. Reflectioncoefficients of spruce roots (O were low for solutes for whichplant cell membranes exhibit values of virtually unity (     

Osmotic responses of maize roots

Water and solute relations of excised seminal roots of young maize (Zea mays L) plants, have been measured using the root pressure probe. Upon addition of osmotic solutes to the root medium, biphasic root pressure relaxations were obtained as theoretically expected. The relaxations yielded the hydraulic conductivity Lp _r) the permeability coefficient (P _sr), and the reflection coefficient (σ _sr) of the root. Values of Lp _r in these experiments were by nearly an order of magnitude smaller than Lp _r values obtained from experiments where hydrostatic pressure gradients were used to induce water flows. The value of P _sr was determined for nine different osmotica (electrolytes and nonelectrolytes) which resulted in rather variable values (0.1·10^-8–1.7·10^-8m·s^-1). The reflection coefficient σ _sr of the same solutes ranged between 0.3 and 0.6, i.e. σ _sr was low even for solutes for which cell membranes exhibit a σ _s≈1. Deviations from the theoretically expected biphasic responses occured which may have reflected changes of either P _sr or of active pumping induced by the osmotic change. The absolute values of Lp _r, P _sr, and σ _sr have been critically examined for an underestimation by unstirred layer effecs. The data indicate a considerable apoplasmic component for radial movement of water in the presence of hydrostatic gradients and also some solute flow byppassing root protoplasts. In the presence of osmotic gradients, however, there was a substantial cell-to-cell transport of water. Cutting experiments demonstrated that the hydraulic resistance for the longitudinal movement of water was much smaller than for radial transport except for the apical ends of the segments (length=5 to 20 mm). The differences in Lp _r as well as the low σ _sr values suggest that the simple osmometer model of the root with a single osmotic barrier exhibiting nearly semipermeable properties should be extended for a composite membrane model with hydraulic and osmotic barriers arranged in series and in parallel.     

Effects of Salinity on Water Transport of Excised Maize (Zea mays L.) Roots

The root pressure probe was used to determine the effects of salinity on the hydraulic properties of primary roots of maize (Zea mays L. cv Halamish). Maize seedlings were grown in nutrient solutions modified by additions of NaCl and/or extra CaCl₂ so that the seedlings received one of four treatments: Control, plus 100 millimolar NaCl, plus 10 millimolar CaCl₂, plus 100 millimolar NaCl plus 10 millimolar CaCl₂. The hydraulic conductivities (Lp_r) of primary root segments were determined by applying gradients of hydrostatic and osmotic pressure across the root cylinder. Exosmotic hydrostatic Lp_r for the different treatments were 2.8, 1.7, 2.8, and 3.4·10⁻⁷ meters per second per megapascals and the endosmotic hydrostatic Lp_r were 2.4, 1.5, 2.7, and 2.3·10⁻⁷ meters per second per megapascals, respectively. Exosmotic Lp_r of the osmotic experiments were 0.55, 0.38, 0.68, and 0.60·10⁻⁷ meters per second per megapascals and the endosmotic Lp_r were 0.53, 0.21, 0.56, and 0.54·10⁻⁷ meters per second per megapascals, respectively. The osmotic Lp_r was significantly smaller (4-5 times) than hydrostatic Lp_r. However, both hydrostatic and osmotic Lp_r experiments showed that salinization of the growth media at regular (0.5 millimolar) calcium levels decreased the Lp_r significantly (30-60%). Addition of extra calcium (10 millimolar) to the salinized media caused ameliorative effects on Lp_r. The low Lp_r values may partially explain the reduction in root growth rates caused by salinity. High calcium levels in the salinized media increased the relative availability of water needed for growth. The mean reflection coefficients of the roots using NaCl were between 0.64 and 0.73 and were not significantly different for the different treatments. The mean values of the root permeability coefficients to NaCl of the different treatments were between 2.2 and 3.5·10⁻⁹ meters per second and were significantly different only in one of four treatments. Cutting the roots successively from the tip and measuring the changes in the hydraulic resistance of the root as well as staining of root cross-sections obtained at various distances from the root tip revealed that salinized roots had mature xylem elements closer to the tip (5-10 millimeters) compared with the controls (30 millimeters). Our results demonstrate that salinity has adverse effects on water transport and that extra calcium can, in part, compensate for these effects.     

Do root hydraulic properties change during the early vegetative stage of plant development in barley (Hordeum vulgare)?

Background and Aims

As annual crops develop, transpirational water loss increases substantially. This increase has to be matched by an increase in water uptake through the root system. The aim of this study was to assess the contributions of changes in intrinsic root hydraulic conductivity (Lp, water uptake per unit root surface area, driving force and time), driving force and root surface area to developmental increases in root water uptake.

Methods

Hydroponically grown barley plants were analysed during four windows of their vegetative stage of development, when they were 9–13, 14–18, 19–23 and 24–28 d old. Hydraulic conductivity was determined for individual roots (Lp) and for entire root systems (Lp_r). Osmotic Lp of individual seminal and adventitious roots and osmotic Lp_r of the root system were determined in exudation experiments. Hydrostatic Lp of individual roots was determined by root pressure probe analyses, and hydrostatic Lp_r of the root system was derived from analyses of transpiring plants.

Key Results

Although osmotic and hydrostatic Lp and Lp_r values increased initially during development and were correlated positively with plant transpiration rate, their overall developmental increases (about 2-fold) were small compared with increases in transpirational water loss and root surface area (about 10- to 40-fold). The water potential gradient driving water uptake in transpiring plants more than doubled during development, and potentially contributed to the increases in plant water flow. Osmotic Lp_r of entire root systems and hydrostatic Lp_r of transpiring plants were similar, suggesting that the main radial transport path in roots was the cell-to-cell path at all developmental stages.

Conclusions

Increase in the surface area of root system, and not changes in intrinsic root hydraulic properties, is the main means through which barley plants grown hydroponically sustain an increase in transpirational water loss during their vegetative development.     

Location of the major barriers to water and ion movement in young roots of Zea mays L.

The main barriers to the movement of water and ions in young roots of Zea mays were located by observing the effects of wounding various cell layers of the cortex on the roots' hydraulic conductivities and root pressures. These parameters were measured with a root pressure probe. Injury to the epidermis and cortex caused no significant change in hydraulic conductivity and either no change or a slight decline in root pressure. Injury to a small area of the endodermis did not change the hydraulic conductivity but caused an immediate and substantial drop in root pressure. When large areas of epidermis and cortex were removed (15–38% of total root mass), the endodermis was always injured and root pressure fell. The hydraulic conductance of the root increased but only by a factor of 1.2–2.7. The results indicate that the endodermis is the main barrier to the radial movement of ions but not water. The major barrier to water is the membranes and apoplast of all the living tissue. These conclusions were drawn from experiments in which hydrostatic-pressure differences were used to induce water flows across young maize roots which had an immature exodermis and an endodermis with Casparian bands but no suberin lamellae or secondary walls. The different reactions of water and ions to the endodermis can be explained by the huge difference in the permeability of membranes to these substances. A hydrophobic wall barrier such as the Casparian band should have little effect on the movement of water, which permeates membranes and, perhaps, also the Casparian bands easily. However, hydrophobic wall depositions largely prevent the movement of ions. Several hours after wounding the endodermis, root pressure recovered to some extent in most of the experiments, indicating that the wound in the endodermis had been partially healed.Abbreviations Lp_r hydraulic conductivity of root; T_1/2 = half-time of water exchange between root xylem and external medium This research was supported by a grant from EUROSILVA (project no. 39473C) to E.S., and by a Bilateral Exchange Grant jointly funded by the Deutsche Forschungsgemeinschaft and the Natural Sciences and Engineering Research Council of Canada to C.A.P. We thank Mr. Burkhard Stumpf for his excellent technicial assistance.