A Correlation between Structure and Function in the Root of Zea mays

The exudation rates of fluid and potassium ions from isolatedmaize roots were determined before and after excision of certainlengths of root tip. The results of this study suggest thatexcised maize roots possess the ability to absorb potassium(and presumably chloride) ions and concomitant amounts of waterover a considerable distance (10 cm) from the tip. Moreover,the observed power of absorption of ions and water into thetranslocatory pathway decreases in passing from the tip towardsthe base of the root. Both light and electron microscope techniques were used to examinethe anatomy of primary roots similar to those used in the physiologicalexperiments. The principal observation was that the xylem vesselsnear the root tip contain membrane-bounded cytoplasm with organelles.The number of mature xylem vessels, i.e. without cytoplasm,progressively increased in transverse sections cut from 1 to10 cm from the root tip; above 10 cm from the root tip all ofthe xylem vessels were found to be completely mature. It isevident that prima facie a connexion exists between this singleaspect of root anatomy and fluid exudation from excised roots. The uptake of tritiated water by roots and its transport intoexudates was examined. These data were analysed on the assumptionthat the exchange of external labelled water with the exudatewas achieved by the fluid exudation itself; this analysis indicatedthat an operational volume, similar to that of the total xylemvolume within the root, must become labelled during the formationof the exudate.     

Growth Medium and Phosphorus Supply Affect Cluster Root Formation and Citrate Exudation by Lupinus albus Grown in a Sand/Solution Split-Root System

We examined cluster root formation and root exudation by white lupin (Lupinus albus L. cv. Kiev Mutant) in response to growth medium and phosphorus supply in a sand/solution split-root system. The split-root system consisted of a nutrient solution compartment and a siliceous sand compartment. Phosphorus was applied at 1 (low-P plants) or 50 (high-P plants) μM as KH₂PO₄ to the solution compartment and at 10, 50 or 250 mg P kg⁻¹ as hydroxyapatite (Ca-P) to the sand compartment. In contrast to the high-P plants, P concentration and P uptake in the low-P plants increased with increasing P supply to the sand compartment. The NaHCO₃-extractable P was lower in the rhizosphere of the low-P plants than the high-P ones. The proton extrusion rate by the solution-grown roots of the low-P plants was higher than that of the high-P plants at the early growth stage. For the low-P plants, the proportion of dry root biomass allocated to cluster roots was higher in the solution compartment than that in the sand compartment. The citrate exudation increased in the sand compartment and decreased in the solution compartment with time, showing a lack of synchronization in citrate exudation by two root halves grown in different media. The cluster root proportion and citrate exudation in both compartments decreased with increasing shoot P concentration. An additional experiment with no P added to either root compartment showed that the proportion of cluster roots was about 9% lower in the sand than solution compartments. The results suggest that cluster root formation and citrate exudation can be significantly affected by the root growth medium in addition to being regulated by shoot P status. More P can be exploited from sparingly available Ca-P by the low-P plants than the high-P ones due to greater citrate exudation under P deficiency.     

Root exudation from Hordeum vulgare in response to localized nitrate supply

Root proliferation as a response to exploit zones of nutrient enrichment in soil has been demonstrated for a wide range of plant species. However, the effectiveness of this as a strategy to acquire nutrients is also dependent on interactions with the soil microbial community. Specifically, C-flow from roots modifies microbial activity and probably the balance between nutrient mineralization and immobilization processes in the rhizosphere. In this study, near-natural abundance 13C-labelling and gene-reporter methods were applied to determine the effects of uneven nitrate supply to roots of Hordeum vulgare on assimilate partitioning and root exudation. Plants were initially grown in uniform nitrate supply in split-root, sand microcosms after which one treatment continued to receive uniform supply, and the other received nitrate to one root compartment only. At the time of imposing the treatments, the CO2 supplied to the plants was switched to a cylinder source, providing a distinct delta13C-signature and allowing the fate of new assimilate within the plants to be determined. The labelling approach allowed quantification of the expected preferential allocation of new C-assimilate to roots in enriched nitrate, prior to any measurable effect on whole biomass or root architecture. Biosensor (lux-marked Pseudomonas fluorescens 10586 pUCD607) bioluminescence, quantified spatially by CCD imaging, demonstrated that root exudation was significantly increased for roots in enriched nitrate. This response of root exudation, being primarily associated with root apices and concurrent with enhanced assimilate supply, strongly suggests that C-flow from roots is an integral component of the proliferation response to nitrate.     

Inert gas diffusion in heterogeneous tissue I: Without perfusion

A mathematical model is proposed to describe transient gas diffusion into a block of heterogenous tissue placed on an impermeable base. The corresponding asymptotic sultion of mass uptake of the gas is derived on the assumption that the diffusion constant is very much smaller in the cellular phase. It is expected that this will be useful in evaluating the diffusion constant in cellular material, and the volume fraction of extracellular fluid, providing the partition coefficient is known. The phenomenon of mutual interaction and multiple feedback between cellular and extracellular fluid is clearly seen in the overall response of the tissue. In this regard it is shown that the extraction of the two least dominant time constants, by backward projection of the experimental data curve of gas uptake, is likely to confuse the numerical evaluation of the physical parameters of the system. In an appendix, the problem of diffusion straight through a tissue slice is solved at the asymptotic stage, before steady state is reached. The resulting expression predicts the by-passing of cells by the diffusing gas and shows how the parameters cannot reliably be, determined.     

Effect of head-down tilt on brain water distribution

Vascular and tissue fluid dynamics in the microgravity of space environments is commonly simulated by head-down tilt (HDT). Previous reports have indicated that intracranial pressure and extracranial vascular pressures increase during acute HDT and may cause cerebral edema. Tissue water changes within the cranium are detectable by T2 magnetic resonance imaging. We obtained T2 images of sagittal slices from five subjects while they were supine and during -13 degrees HDT using a 1.5-Tesla whole-body magnet. The analysis of difference images demonstrated that HDT leads to a 21% reduction of T2 in the subarachnoid cerebrospinal fluid (CSF) compartment and a 11% reduction in the eyes, which implies a reduction of water content; no increase in T2 was observed in other brain regions that have been associated with cerebral edema. These findings suggest that water leaves the CSF and ocular compartments by exudation as a result of increased transmural pressure causing water to leave the cranium via the spinal CSF compartment or the venous circulation.     

Novel rhizobox design to assess rhizosphere characteristics at high spatial resolution

Available tools to study rhizosphere characteristics at a sub-mm spatial resolution suffer from a number of shortfalls, including geometrically and physiologically ill-defined root layers containing soil or other growth medium. Such designs may result in over- or underestimation of root-induced changes in the rhizosphere. We present a novel rhizobox design that overcomes these shortfalls. Plants are pre-grown in a soil–root compartment with an opening slit at the bottom. As plants reach the targeted physiological stage, this compartment is transferred on top of a rhizosphere soil compartment attached to a vertical root-only compartment. The latter is made up of a membrane (pore size 7 m to restrict root hair growth into the rhizosphere compartment or 30 m to restrict only root growth) and a transparent acrylic window which is gently pressed against the membrane and rhizosphere soil compartment using an adjustable screw. This design allows roots to penetrate from the upper soil–root compartment through the slit into the root-only compartment. Root growth and distribution can be monitored through the acrylic window using digital camera equipment. Upon termination of the experiment, the rhizosphere compartment is removed and frozen prior to separation of sub-mm soil layers using microtome techniques. In a test experiment, canola (Brassica napus L. cv. Sprinter) developed a fairly dense root monolayer within 8 days. Using measurement of soil characteristics at 0.5–1-mm increments across the rhizosphere we demonstrate that the proposed rhizobox design is yielding reproducible data. Due to exudation of LMWOC, we found a statistically significant increase of DOC towards the root plane, whereas more stable soil characteristics were not affected by root activity. Limitations and further extensions of this rhizobox design, including the use of micro suction cups and microsensors for pH and redox potential to measure spatial and temporal changes in a non-destructive manner are discussed along with potential applications such as validation of rhizosphere models.     

l-Malate as an Essential Component of the Xylem Fluid of Corn Seedling Roots

Corn seedling xylem exudate has a pH of 5.30 +/- 0.05 due to the presence of 10 millimolar malate which has a pK(a) of 5.13. This concentration of malate tends to buffer the xylem fluid at this pH. Exogenous treatment of corn seedling roots with CaCl(2) caused a concentration-dependent decrease in the pH of the xylem fluid as well as a decrease in the volume of fluid secreted into the xylem. Exogenous 50 millimolar CaCl(2) decreased exudate volume to 8% of control within 0.5 hour. Nitrate and malate deposition into the xylem was prevented by pretreatment in 5 millimolar CaCl(2); nitrate deposition was resumed shortly after resumption of malate deposition. Fifty millimolar l-tartrate, an inhibitor of the Cl(-), malate-activated ATPase (in vitro) of the tonoplast, also decreased exudate volume as well as slightly lowering exudate pH. The osmolality of the exudate was found to be constant at 70 +/- 11 milliosmomoles per kilogram in all treatments. Dixon plots (log of exudation rate versus pH of exudate) indicated a pK(a) of 5.11 for the exudation process which is very close to the pK(a) of l-malate (5.13). In addition, a Dixon plot of the l-glutamine deposition (l-glutamine being the major form of reduced nitrogen in the exudate) versus pH also indicated a pK(a) near 5.15.The pH optimum for glutamine transport into the xylem was 5.5. Deposition of glutamine into the xylem may be regulated by the xylem pH (5.30 +/- 0.05) which in turn may be regulated by the presence of 10 millimolar malate. It is proposed that the transport of glutamine into the xylem may provide the driving force for the exudation process.     

Quantitative evaluation of the role of organic acid exudation in the mobilization of rock phosphate by rape

Phosphorus-deficient rape plants appear to acidify part of their rhizosphere by exuding malic and citric acid. A simulation model was used to evaluate the effect of measured exudation rates on phosphate uptake from Mali rock phosphate. The model used was one on nutrient uptake, extended to include both the effect of ion uptake and exudation on rhizosphere pH and the effect of rhizosphere pH on the solubilization of rock phosphate. Only the youngest zones of the root system were assumed to exude organic acids. The transport of protons released by organic acids was described by mass flow and diffusion. An experimentally determined relation was used describing pH and phosphate concentration in the soil solution as a function of total soil acid concentration. Model parameters were determined in experiments on organic acid exudation and on the uptake of phosphate by rape from a mixture of quartz sand and rock phosphate. Results based on simulation calculations indicated that the exudation rates measured in rape plants deficient in phosphorus can provide the roots with more phosphate than is actually taken up. Presence of root hairs enhanced the effect of organic acid exudation on calculated phosphate uptake. However, increase of root hair length without exudation as an alternative strategy to increase phosphate uptake from rock phosphate appeared to be less effective than exudation of organic acids. It was concluded that organic acid exudation is a highly effective strategy to increase phosphate uptake from rock phosphate, and that it unlikely that other rhizosphere processes play an important role in rock phosphate mobilization by rape.     

The acute-phase response in endometriosis of women

Peritoneal fluid volume was determined and concentrations of C-reactive protein, alpha 1-antitrypsin, acid-alpha 1-glycoprotein, alpha 2-macroglobulin, haptoglobin, complement factors C3 and C4, IgG, IgA and IgM were measured in the supernatant of the peritoneal fluid and in serum by means of a radial-immunodiffusion technique in 25 patients with and in 45 patients without endometriosis. Peritoneal fluid volume was not different between the two groups. The peritoneal fluid:serum ratios for the proteins determined showed a significant inverse correlation with their molecular weight in both groups, indicating that their presence in peritoneal fluid is governed by exudation according to their molecular weight, rather than by active production in, or selective release into, the peritoneal cavity. In control patients only, the ratios of most of the individual proteins studied were significantly higher in the luteal than in the follicular phase. We suggest that the high values of peritoneal fluid:serum ratios in endometriotic tissue and peritoneal macrophages. In the luteal phase, the cycle-dependent increase of protein exudation obscures this additional contribution. We conclude that endometriosis does not cause marked intra-abdominal inflammatory changes. If the presence of endometriosis lowers fecundity, the mechanism probably does not involve acute-phase protein synthesis.     

Effect of transporters on the secretion of phytochemicals by the roots of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis>

Root exudation, the process by which plants secrete compounds into the soil, is becoming accepted as a communicative process that determines organismal interactions in the rhizosphere. However, the mechanistic processes involved in the root exudation of phytochemicals have not been elucidated; traditionally, exudation has been regarded as a passive process. There is evidence that transporters in plants (and other organisms) have been involved in the movement of chemicals across different membranes. Here, we describe the involvement of different transporters in root exudation of phytochemicals by employing a pharmacological approach. We used a range of concentrations of several compounds known to inhibit different transporters, including potassium cyanide, orthovanadate, quinidine, glibenclamide, nifedipine and verapamil, to examine the effects of transporter inhibition on root exudation profiles in Arabidopsis. Generally, the exudation profile of phenolic compounds in 18-day-old plants shows more than 15 major phytochemicals. In contrast, the inhibitors listed above caused differences in the secretion of specific compounds. For instance, nifedipine and verapamil completely inhibited the exudation of the phytochemicals with molecular masses of 142 and 294, respectively. These results highlight that root exudation of phytochemicals is an active process controlled at the biochemical level and that different transporters may be involved in this root-specific mechanism. Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at .     

Cortical Cell Breakdown and Lateral Root Primordium Development in Vicia faba L

The effect of the formation of a cavity in the cortex of theprimary root of Vicia faba adjacent to lateral root primordiaon root development has been investigated. Premature exposureof such primordia to the external medium by removing the overlyingtissues of the primary root has no effect on primordium developmentif that primordium was within 48 h of emerging as a lateralroot. Similar exposure of primordia which were at an earlierstage of development and consisted of between 3400 and 7000cells resulted in the generation of a stationary phase, withmost of the nuclei arrested in G1 (presynthetic interphase),48–72 h after exposure began, followed by nuclear degenerationby 96 h. Since no mature vascular tissue was found in theseprimordia until after they emerged as secondary roots, all ofthe nutrients necessary for the maintenance of cell proliferationin these meristems must reach them by simple diffusion fromthe surrounding medium. A preliminary analysis of the liquidcontents of the cavity next to developing primordia demonstratesit to be rich in carbohydrates and it is clear, from the resultsreported in this paper, that cell proliferation in primordia,consisting of a mean number of 5400 cells, is largely dependenton the substances present in the cavity fluid, although somematerials reach the primordium by diffusion from the cells ofthe primary root to which the primordium remains attached.     

Genetic control of root exudation

The literature on genetics of root exudation and on genotypic differences in qualitative and quantitative composition of root exudates in crop and native plant species was critically assessed. Differences in exudation have been reported for genotypes that differ in tolerance to nutrient deficiencies, ion toxicities, and pathogen attack. The exudation profile of a limited number of genotypes (frequently only two genotypes with the contrasting response to the environmental stress) have been reported to date. Little is known about the variability in larger samples of the germplasm or about actual genetics behind differential qualitative and quantitative composition of root exudates. Changing the exudation profile of a given genotype may be achieved by manipulating the biosynthetic capacity and by increasing the capacity of the plasma membrane to transport the specific compound out into the rhizosphere. Overexpression of the bacterial citrate synthase gene in the cytoplasm of tobacco plants resulted in exudation of large quantities of citrate into the rhizosphere and partial alleviation of the aluminium (Al) toxicity stress. A similar strategy of transforming plants with citrate synthase gene is being tried as a way of improving plant capacity to extract phosphorus (P) from soils with notoriously low P availability.More research into the genetic basis of qualitative and quantitative differences in root exudation is warranted. Understanding the genetic control of root exudation, followed by manipulation of qualitative and quantitative composition of root exudates, will result in better adaptation of plants to environmental conditions and a greater yield of crops.     

Temporal dynamics of carbon partitioning and rhizodeposition in wheat

The temporal dynamics of partitioning and rhizodeposition of recent photosynthate in wheat (Triticum aestivum) roots were quantified in situ in solution culture. After a 30-min pulse of (14)CO(2) to a single intact leaf, (14)C activities of individual carbon fluxes in the root, including exudation, respiration, and root content, were measured continuously over the next 20 h concurrently with (14)C efflux from the leaf. Immediately after the end of the (14)CO(2) pulse, (14)C activity was detected in the root, the hydroponic solution, and in root respiration. The rate of (14)C exudation from the root was maximal after 2 to 3 h, and declined to one-third of maximum after a further 5 h. Completion of the rapid phase of (14)C efflux from the leaf coincided with peak (14)C exudation rate. Thus, exudation flux is much more rapidly and dynamically coupled to current photosynthesis than has been appreciated. Careful cross-calibration of (14)C counting methods allowed a dynamic (14)C budget to be constructed for the root. Cumulative (14)C exudation after 20 h was around 3% of (14)C fixed in photosynthesis. Partitioning of photosynthate between shoot and root was manipulated by partial defoliation before applying the (14)CO(2) pulse to the remaining intact leaf. Although the rate of photosynthesis was largely unaffected by partial defoliation, the proportion of new photosynthate subsequently partitioned to and exuded from the root was substantially reduced. This clearly indicates that exudation depends more on the rate of carbon import into the root than on the rate of photosynthesis.     

The Effects of Temperature on the Exudation Process of Excised Primary Roots of Zea mays

The fluid exudation rates and the ionic compositions of theexudates of excised maize roots have been determined in bathingmedia of 0.1 mM, 1.0 mM, 10.0 mM, and 50.0 mM KCl, each containing0.1 mM CaCl₂, at temperature intervals between 10 °C and30 °C.Analysis of these data in terms of an osmotic modelfor excised root exudation shows that the observed temperaturevariation in fluid exudation rate is accounted for by the observedtemperature variation in the osmotic driving force, the saltconcentration difference from xylem fluid to bathing medium.Temperature variation in the osmotic permeability of the rootand of the non-osmotic water flow are not significantly differentfrom zero.     

Probing short-term root exudation in Zea mays

Exudation of maize roots was studied using a microdrop recorder. The high-resolution measurements of relatively short-term changes in exudation seems to be one of the most useful and unproblematic applications of the microdrop recorder. When mannitol, polyethylene glycol (PEG) and kinetin were supplied to the medium bathing, the surfaces of excised maize roots, a marked decrease in root exudation was observed. The action of fusicoccin and that of abscisic acid (ABA) showed a sharp and then a slower decline on root exudation, though, enhanced exudation was sustained over a much longer period, in comparison to that recorded for mannitol and polyethylene glycol. A decline in the volume of exudates is related to an increase in the water deficit, in coincidence to changes in the osmotic gradient between root cells and the bathing medium generated by expelling exudates.     

Short-term responses of Brassica rapa plants to low root temperature: Effects on nitrate uptake and its translocation to the shoot

Brassica rapa L. plants were grown hydroponically for 5 or 6 weeks at 20°C and then half batches of plants were transferred to tanks in which the root temperature was lowered decrementally over 1 h to 7°C. Changes in nitrate uptake rate (NUR) and nitrate transfer from roots were studied in relation to transpiration and root pressure xylem exudation flow rates over a 48- or 72-h period. The response of plants following the root temperature decrease was biphasic. During phase 1, NUR and water and solute flow rates through the root decreased sharply. Coping mechanisms came into operation during phase 2, and tended to offset the effects of low temperature. The 3-h cold-treated roots exhibited a very low NUR but 48-h cold-treated roots partly recovered their ability to absorb nitrate. Transpiration rate decreased more slowly (during 24 h) than both root xylem exudation and parameters of root conductivity (during 6 h). Beyond these respective times, transpiration rate was balanced while root xylem exudation clearly increased, but without returning to the level of control plants. Nitrate transfer to the root xylem was strongly and rapidly affected by low root temperature, but the subsequent readjustment was such that no or little difference compared with the control was apparent after 48 h. Water and solute flows were strongly decreased when nitrate was replaced by chloride in the culture solution during exudation sampling. The major role of nitrate in root hydraulic conductivity and root xylem exudation is discussed.     

Re-sorption of organic compounds by roots of Zea mays L. and its consequences in the rhizosphere

The exudation of soluble carbon compounds from Zea mays roots was investigated over a 10 day growth period under sterile and non-sterile solution culture conditions. The results showed that plants grown in sterile static solution culture, where C was allowed to accumulate, released 8 times less C than plants grown under culture conditions in which the solutions were replaced daily. The increased C loss from plant cultures in which exudates were removed daily was attributable to, (a) the reduced potential for root re-sorption of previously lost C, and (b), increasing diffusion gradients between the root and the surrounding bathing solution increasing passive leakage of exudates from the roots. In treatments where C was removed daily from the root-bathing solution, 86% of the total C lost was of a soluble low molecular weight nature, whereas, in sterile and non-sterile static cultures, allowing the accumulation of C over 10 days, this was reduced to 67.5 and 48% respectively. The main C fluxes operating in a solution culture system (efflux and influx of C by both roots and microorganisms) were examined using a computer simulation model to describe movement of soluble sugar-C in both sterile and non-sterile conditions. In sterile static cultures where C was allowed to accumulate in solution over a 10 day growth period, 98% of the C exuded was re-absorbed by the plant. Where C was removed daily from the root-bathing solution this was reduced to 86%. The predicted patterns of C accumulation were similar to those found in the experiments. Simulations showed that the pattern of accumulation and final equilibrium concentrations were dependent on the rate of exudation, the spatial characteristics of exudation, solution volume, root growth rate and the presence of a microbial population. Simulations under non-sterile conditions showed that roots can compete with microorganisms for exudates in solution indicating the possible importance of re-sorption in a soil environment. The results clearly indicate that roots are capable of regulating the net amount of C released into a solution culture with the amount of C collected being highly dependent on the experimental conditions employed. The possible implications of soluble C influx on processes operating within the rhizosphere and in experimental systems is discussed.     

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 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.