A model for water transport in the stele of plant roots

Summary The mechanism of water movement across roots is, as yet, not well understood. Some workable black box theories have already been proposed. They, however, assumed unrealistic cell membranes with low values of , or were based on a poor anatomical knowledge of roots. The role of root stele in solute and water transport seems to be especially uncertain. An attempted explanation of the nature of root exudation and root pressure by applying the apoplast canal theory (Katou andFurumoto 1986 a, b) to transport in the root stele is given. The canal equations are solved for boundary conditions based on anatomical and physiological knowledge of the root stele. It is found that the symplast cell membrane, cell wall and net solute transport into the wall apoplast are the essential constituents of the canal system. Numerical analysis shows that the canal system enables the coupled transport of solutes and water into a xylem vessel, and the development of root pressure beyond the level predicted by the osmotic potential difference between the ambient medium and the exudate. Observations on root exudation and root pressure previously reported seem to be explained quite well. It is concluded that the movement of water in the root stele although apparently active is essentially osmotic.Abbreviations J _v ^ex volume exudation per root surface - J₀ non-osmotic exudation - L^r overall radial hydraulic conductivity of an excised root - reflection coefficient - C_s difference in the osmotic concentration between the bathing medium and the exudate - R gas constant - T absolute temperature - C_K molar concentration of K⁺ - C_Cl molar concentration of Cl^– - C_j molar concentration of ion species j - P_j membrane permeability of ion j - z_j valence of ion j - F Faraday constant - V_ix intracellular electric potential with reference to the canal     

A model for radial water transport across plant roots

Summary A model based on the canal theory (Katou andFurumoto 1986 a, b) is proposed for the absorption of solute and water at the root periphery. The present canal model in the periphery and the model which was previously proposed for the exudation in the stele (Katou et al. 1987), are organized into a model for radial transport across excised plant roots, in the light of anatomical and physiological knowledge of maize roots. The canal equations for both canals are numerically solved to give quite a good explanation for the observed exudation of maize roots. It is found that the regulation of solute transport has a primary importance in the regulation of water transport across excised roots. The internal cell pressure of the symplast adjusts the water absorption at the root periphery to the water secretion into the vessels. There seems no need for this explanation of the radial water transport across roots to assume cell membranes with low reflection coefficient or variable water permeability. It would seem that the apoplast wall layers play a crucial role in metabolic control of water transport in roots as well as in hypocotyls.Abbreviations J _s ^ex* the theoretically estimated rate of solute exudation per unit surface area of model maize roots - J that of volume exudation per unit surface area of model maize roots - the reflection coefficient of the cell membrane against solutes     

A new concept of root exudation1

After discussing numerous models for exudation from the xylem of roots, we present a new biphasic exudation model based on osmoregulation of the root symplast by stretch-activated ion channels (SA channels). We tested some features of the model in maize roots. (1) Using a microdrop recorder we showed that bathing the roots in 50 mmol m^?3 gadolinium ions, known to inhibit some SA channels, inhibited xylem exudation by over 80% after 24h application. (2) Measuring xylem exudation from single roots into an attached micropipette revealed the capacity of the roots to perform strong autonomous exudation pulses. (3) In partially encased roots, the rhizodermis exuded water concurrently to xylem exudation. These results were regarded as supporting our model. An interesting observation with the microdrop recorder, which does not address the theory, is that addition of a variety of inorganic ions to distilled water as the roots' bathing medium instantaneously and reversibly increases xylem exudation, evidently nonosmotically.     

Experimental evaluation of an efflux-influx model of C exudation by individual apical root segments

The aim of this study was to evaluate if a model describing the efflux and the influx of C through the root surface could be fitted to experimental short-term kinetics of carbon (C) exudation by individual apical root segments in maize (Zea mays L.). The efflux of C was set constant or modelled by a power function of the distance from the apex to simulate the greater release of C around the root tip commonly reported in the literature. The influx was proportional to the C concentration in the external solution to simulate the active re-uptake of exudates by the root. Plants were exposed to full light or to shade to manipulate C allocation to roots. The model with a constant efflux gave satisfactory fits to the kinetics of exudation (average R(2)=0.66). The average gross efflux was then 2.1 mug C cm(-2) root surface h(-1). The model was improved if exudation was set more intense towards the root apex (average R(2)=0.74). The estimated gross efflux decreased then from 5.2 mug C cm(-2) h(-1) at the apex to 1.8 mug C cm(-2) h(-1) for the region located 5-25 cm from the root tip. The decrease in net exudation of individual roots due to the shading of plants was weak, which may indicate that the import of C by the primary roots studied was not reduced significantly. By describing the exudation of an apical root segment of variable length and diameter, the model is a first step in linking exudation to root system architecture models and to whole plant functioning.     

A novel method of quantifying root exudation in the presence of soil microflora

A microcosm is described in which root exudation may be estimated in the presence of microorganisms. Ryegrass seedlings are grown in microcosms in which roots were spatially separated from a microbial inoculant by a Millipore membrane. Seedlings grown in the microcosms were labelled with [¹⁴C]-CO₂, and the fate of the label within the plant and rhizosphere was determined. Inoculation of the microcosms with Cladosporium resinae increased net fixation of the [¹⁴C] label compared to plants grown under sterile conditions. Inoculation also increased root exudation. The use of the microcosm was illustrated and its applications discussed.     

QTL mapping of root hair and acid exudation traits and their relationship to phosphorus uptake in common bean

The relationship between root-hair growth, acid exudation and phosphorus (P) uptake as well as the quantitative trait loci (QTLs) associated with these traits were determined for a recombinant inbred line (RIL) population derived from the cross of two contrasting common bean (Phaseolus vulgaris L.) genotypes, DOR364 and G19833, which were grown in solution culture and under field conditions with low-P availability. In the solution-culture study, root-hair density, root-hair length, H⁺ exudation and total acid exudation were measured. Substantial genotypic variability was observed for these traits and their response to P availability. The P-efficient parent G19833 had greater root-hair density, longer root-hair length, and greater exudation of H⁺ and total acid than the P-inefficient genotype DOR364. These traits segregated continuously in the RIL population, with obvious tendency of trait transgression. Genetic analysis revealed that the root traits measured had various heritabilities, with h _b ² ranging from 43.24 to 86.70%. Using an integrated genetic map developed for the population, a total of 19 QTLs associated with root hair, acid exudation and P-uptake traits were detected on 8 linkage groups. P uptake in the field was positively correlated with total acid exudation, basal root-hair length, and basal root-hair density. Acid-exudation traits were intercorrelated, as were root-hair traits. Total acid exudation was positively correlated with basal root-hair density and length. Linkage analysis revealed that some of the root-trait QTLs were closely linked with QTLs for P uptake in the field. We propose that marker-assisted selection (MAS) might be a feasible alternative to conventional screening of phenotypic root traits.     

Differences between calcifuge and acidifuge plants in root exudation of low-molecular organic acids

The nature and quantity of low-molecular organic acids (LOAs) exuded by the roots of nine species of calcifuge and nine species of acidifuge wild plants from northern Europe were determined by ion chromatography. Particular attention was paid to differences between the calcifuge and the acidifuge species in the proportions of different LOAs in their root exudates. Great differences in mol% root exudation between the calcifuge and the acidifuge species were found in some acids. The calcifuge species exuded more acetic acid, the acidifuge species more oxalic acid and much more citric acid. In three calcifuge species, however, root exudation of oxalic acid was appreciable, whereas acetic acid exudation was low in these species. The phosphate- and Fe-solubilizing ability of eight LOAs in a rhizosphere limestone soil was also tested. Oxalic acid was the most efficient phosphate solubilizer and citric acid, by far, the most efficient Fe-solubilizer at the concentration (10 mM) tested. It might be hypothesized that acidifuge species use oxalate to solubilize phosphate and citrate to solubilize Fe, in limestone soil. The inability of calcifuge species to grow in limestone soil might, therefore, be due to low root exudation of these acids and, as a result, inability to solubilize phosphate and Fe in limestone soil.     

Defoliation alters the relative contributions of recent and non-recent assimilate to root exudation from Festuca rubra

The deposition of organic compounds from plant roots is a key determinant of rhizosphere microbial activity and community structure. Consequently, C-flow from roots to soil is an important process in coupling plant and microbial productivity, via impacts on microbial nutrient cycling in soil. Experimentally, isotopic tracers (¹³C or ¹⁴C) are used to track C inputs to soil and microbial communities. However, in many such studies the relationship between labelled C-flows and total C-flows are not established, limiting the interpretative value of the results. In this study, we applied steady-state near natural abundance ¹³CO₂ labelling to determine the impact of partial defoliation of Festuca rubra on root exudation. This approach in axenic culture facilitated determination of the contribution of pre- and post-defoliation assimilates both to root C-flow and plant tissues. The results demonstrated that total root exudation was increased in the two days following defoliation. This was concurrent with reduced net CO₂ assimilation and reduced allocation of post-defoliation assimilates below-ground and to active root meristems. Through determination of the δ¹³C of root exudates, it was established that the source of the increased root exudation was pre-defoliation assimilate. However, this response was transient, with reduced deposition of pre- and post-defoliation assimilates from roots during the period 2–4 d following defoliation. The results highlight the limitations of pulse-labelling approaches as a means of quantifying impacts of treatments on root exudation, particularly where the treatment is likely to affect plant C-partitioning or the balance between deposition to, and re-mobilization from, C-storage pools.     

Phosphorus deficiency-induced modifications in citrate catabolism and in cytosolic pH as related to citrate exudation in cluster roots of white lupin

A possible contribution of alterations in metabolic sequences involved in citrate catabolism, to intracellular accumulation and subsequent release of citrate was investigated in cluster roots of phosphorus (P)-deficient white lupin (Lupinus albus L.). Citrate accumulation during maturation of root clusters was associated with decreased levels of intracellular soluble P_i and ATP, and with reduced rates of respiration. Inhibitor studies with KCN and salicylhydroxamic acid (SHAM) suggest a reduced capacity of both the cytochrome pathway and of the alternative respiration with a concomitant decrease of immunochemically detectable protein levels of the alternative oxidase. Reduced respiration seems to be related to a general impairment of the respiratory system, rather than to limitation of respiratory substrates such as P_i and adenylates, as indicated by the absence of stimulatory effects of the uncoupler CCCP. The citrate/malate ratio in juvenile root clusters with high rates of respiration and low inherent levels of citrate accumulation was increased by short-term application (4–8 h) of azide and SHAM as respiration inhibitors. During maturation of root clusters, a shift from intracellular malic acid to citric acid accumulation was associated also with down-regulation of ATP citrate lyase (ACL), which catalyzes cleavage of citrate into acetyl-CoA and oxaloacetate with a putative function as anapleurotic source for the production of acetyl-CoA under P-deficient conditions. Inhibition of nitrate uptake and assimilation is a general response to P limitation in many plant species including white lupin. Reduced consumption of the amino acceptor 2-oxoglutaric acid as a product of citrate turnover may therefore contribute to increased citrate accumulation. Accordingly, artificial inhibition of nitrate reduction by localized application of tungstate significantly increased the citrate/malate ratio in juvenile root clusters. Lowering the cytosolic pH by external application of propionate stimulated citrate and malate exudation in non-cluster lateral roots and in developing root clusters. This effect was reverted by preincubation with phosphonate to buffer the cytosol. The results suggest that acidification of the cytosol may be an important factor, triggering the transient release of citrate and protons from mature root clusters in P-deficient white lupin.     

The Root Cap Cuticle: A Cell Wall Structure for Seedling Establishment and Lateral Root Formation

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Effects of HgCl2 on principal parameters of exudation from maize detached root system

To elucidate the role of aquaporins in the control of the root pressure, we tested the effects of HgCl₂ (aquaporin blocker) at concentrations from 10^?8 to 10^?2 M on the exudation rate (J _w). Experiments were performed with detached roots of 5–7-day-old etiolated maize (Zea mays L.) seedlings. HgCl₂ suppressed exudation by 50–70% at the concentration of 2 × 10^?5 M. At the concentration of 2.5 × 10^?4 M, HgCl₂ reduced J _w during first 20–40 min, but in 2 h, it activated exudation by ten and more times. In this case, the root and osmotic pressures of the exudates increased by 1.5 times. The hydraulic conductance reduced approximately by 30% during first 30 min and increased severalfold in 2 h. The temperature coefficient (Q₁₀) of J _w attained 14 in 2 h. At the concentration of 10^?2 M, HgCl₂-induced acceleration of exudation was replaced by its inhibition essentially immediately. We suggested that a driving force for HgCl₂-induced stimulation of the J _w might be an increase in the osmotic component of the root pressure or the involvement of its nonosmotic (so-called metabolic) component. The results allow a supposition that aquaporins are involved in the control of water transport in the root.     

根系分泌物生态学研究

在植物生长过程中 ,由根系的不同部位分泌或溢泌一些无机离子及有机化合物 ,这些物质统称为根系分泌物。植物在其生长过程中不断地分泌无机离子及有机化合物 ,这是植物长期适应外界环境而形成的一种适应机制。早在 2 0世纪 5 0年代就有人对植物根系分泌物进行了研究 ,Ｒｏｖｉｒａ等[4 3] 和Ｖａｎｃｕｒａ等[4 7] 对根土界面根系分泌物进行了系统的研究 ,切尔诺布里维卡[2 6 ] 研究了植物根系分泌物的生物学作用 ,揭示了其在间作中的作用 ,直到 70年代对根系分泌物的研究才出现了蓬勃发展的趋势。近年来的研究表明 ,根系分泌物是保持根际微…     

Root exudation and rhizoplane bacterial abundance of barley (Hordeum vulgare L.) in relation to nitrogen fertilization and root growth

The abundance of bacteria in the rhizoplane of barley varieties was investigated at different soil nitrogen levels. Increased amendments of nitrogen resulted in higher bacterial numbers in the rhizoplane of barley seedlings of different varieties. A negative correlation was found between nitrogen level in the soil and the growth rate of the seedling roots. The effect of nitrogen on the bacterial abundances could be indirect through changed root growth and thereby changed exudation. The exudation of soluble organic carbon componds from barley seedling roots were measured in hydroponic culture. The effect of natural variation in root growth rate and of different concentrations of nitrogen in the nutrient solution was investigated. The amount of exudates consituted 2–66% of the dry weight increase in root biomass, depending on the root growth. Slower growing roots released considerably more organic carbon per unit root weight than faster growing roots. The variation in root exudation appeared to be mainly explained by differences in root growth, rather than of the nitrogen concentration in the nutrient solution. A significantly higher exudation rate was found during day time compared to night.     

Impact of phosphorus supply on root exudation, aerenchyma formation and methane emission of rice plants

This study evaluated the impact of P supply on rice plant development and the methane budget of rice fields by 2 different approaches: (1) root growth, exudation and aerenchyma formation were recorded in an experiment with hydroponic solution; (2) dissolved CH₄ concentration and CH₄ emission were investigated in a pot experiment. In both approaches, we used three different cultivars and three levels of P supply. In the experiment with solution culture (0.5 ppm, 5 ppm, and 10 ppm P), root exudation ranged between 0.5 to 36.7 mol C plant^–1 h^–1 and increased steadily with plant growth at given P level. Low P supply resulted in

Pathways and processes of water and nutrient movement in roots

Recent work in our laboratory provides evidence for a revised view of the functioning of roots of maize, and probably of all the grasses. The development of coherent soil sheaths on the distal 30-cm of these roots, and the loss of the sheaths further back, led us to investigate the differences in surface structure, anatomy, carbon exudation and microflora of the sheathed and bare zones. The significant differences are summarized. But the fact which underlies all these differences is the maturation of the late metaxylem (LMX). In the sheathed zones the LMX elements are still alive and non-conducting; only the early metaxylem (EMX) and protoxylem are open. In the bare zones they are open vessels. This leads directly to the dryness of bare zones and the wetness of sheathed zones, and indirectly to the other differences noted. Branch root junctions are shown to be structures of great significance. Besides connecting the branches to the axile systems, they serve also to connect the EMX and LMX vessels, and contain a tracheid barrier which prevents air embolisms entering the main vessels. These discoveries force us to revise the traditional view of water uptake by the root hair zone, and to suggest that much water must also enter bare roots, possibly via the laterals. There is some published evidence for this. The living LMX elements of the sheathed zone accumulate large concentrations of potassium which must joint the transpiration water at the transition to the bare zone. Calculations suggest that this may be only a tenth of the requirement of a mature plant, and that the balance may enter the bare zones with the transpiration water.     

Root exudation rate as functional trait involved in plant nutrient‐use strategy classification

Plants adopt a variety of life history strategies to succeed in the Earth's diverse environments. Using functional traits which are defined as “morphological, biochemical, physiological, or phonological” characteristics measurable at the individual level, plants are classified according to their species’ adaptative strategies, more than their taxonomy, from fast growing plant species to slower‐growing conservative species. These different strategies probably influence the input and output of carbon (C)‐resources, from the assimilation of carbon by photosynthesis to its release in the rhizosphere soil via root exudation. However, while root exudation was known to mediate plant‐microbe interactions in the rhizosphere, it was not used as functional trait until recently. Here, we assess whether root exudate levels are useful plant functional traits in the classification of plant nutrient‐use strategies and classical trait syndromes? For this purpose, we conducted an experiment with six grass species representing along a gradient of plant resource‐use strategies, from conservative species, characterized by low biomass nitrogen (N) concentrations and a long lifespans, to exploitative species, characterized by high rates of photosynthesis and rapid rates of N acquisition. Leaf and root traits were measured for each grass and root exudate rate for each planted soil sample. Classical trait syndromes in plant ecology were found for leaf and root traits, with negative relationships observed between specific leaf area and leaf dry matter content or between specific root length and root dry matter content. However, a new root trait syndrome was also found with root exudation levels correlating with plant resource‐use strategy patterns, specifically, between root exudation rate and root dry matter content. We therefore propose root exudation rate can be used as a key functional trait in plant ecology studies and plant strategy classification.     

Aluminum resistance in two cultivars of Zea mays L.: Root exudation of organic acids and influence of phosphorus nutrition

Root exudation of organic acids as Al-chelating compounds and P nutrition have been suggested to play a major role in Al-resistance in higher plants. Effects of Al exposure on maize plant growth, and organic acid root content and root exudation under various levels of P nutrition were examined. Sikuani, a Colombian maize cultivar tolerant to acid soils with high Al saturation, and Corso, a Swiss cultivar, were grown in sterile hydroponic conditions for 21 days. Al-caused inhibition of root growth was lower in Sikuani than in Corso. Al effect on plant growth was decreased with increasing P content in roots. Al content in roots increased with increasing P content and was higher in Sikuani than in Corso. When exposed to Al, the contents in root apices as well as the root exudation of citric and malic acids in Corso and citric, malic and succinic acids in Sikuani increased, and were higher in Sikuani than in Corso. Increased PEP carboxylase (PEPC) activity in root apices after Al exposure partially explained the variations of organic acid content in the roots. These Al-induced changes in PEPC activity, organic acid content and exudation were reduced in plants supplied with higher P concentrations during the 21 days prior to treatment. Increased secretion of organic acids after exposure to Al appeared to be specific to Al and was not totally explained by increased root content in organic acids.     

Response of root respiration and root exudation to alterations in root C supply and demand in wheat

Rising atmospheric CO₂ concentrations have highlighted the importance of being able to understand and predict C fluxes in plant-soil systems. We investigated the responses of the two fluxes contributing to below-ground efflux of plant root-dependent CO₂, root respiration and rhizomicrobial respiration of root exudates. Wheat (Triticum aestivum L., var. Consort) plants were grown in hydroponics at 20°C, pulse-labelled with ¹⁴CO₂ and subjected to two regimes of temperature and light (12 h photoperiod or darkness at either 15°C or 25°C), to alter plant C supply and demand. Root respiration was increased by temperature with a Q ₁₀ of 1.6. Root exudation was, in itself, unaltered by temperature, however, it was reduced when C supply to the roots was reduced and demand for C for respiration was increased by elevated temperature. The rate of exudation responded much more rapidly to the restriction of C input than did respiration and was approximately four times more sensitive to the decline in C supply than respiration. Although temporal responses of exudation and respiration were treatment dependent, at the end of the experimental period (2 days) the relative proportion of C lost by the two processes was conserved despite differences in the magnitude of total root C loss. Approximately 77% of total C and 67% of ¹⁴C lost from roots was accounted for by root respiration. The ratio of exudate specific activity to CO₂ specific activity converged to a common value for all treatments of 2, suggesting that exudates and respired CO₂were not composed of C of the same age. The results suggest that the contributions of root and rhizomicrobial respiration to root-dependent below-ground respiration are conserved and highlight the dangers in estimating short-term respiration and exudation only from measurements of labelled C. The differences in responses over time and in the age of C lost may ultimately prove useful in improving estimates of root and rhizomicrobial respiration.