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Get In-Stock Alert. Delivery not available. Pickup not available. Methods Development of apoplastic barriers along barley seminal roots was detected using various staining methods, and the suberin amounts in the apical and basal zones were analysed using gas chromatography—mass spectometry GC-MS. The hydraulic conductivity of roots Lp r and of cortical cells Lp c was measured using root and cell pressure probes. Key Results When grown in hydroponics, barley roots did not form an exodermis, even at their basal zones.
However, they developed an endodermis. The endodermal suberin accounted for the total suberin of the roots. The absolute amount in the basal zone was significantly higher than in the apical zone, which was inversely proportional to the Lp r. Comparison of Lp r and Lp c suggested that cell to cell pathways dominate for water transport in roots.
Conclusions Suberized endodermis significantly reduces Lp r of seminal roots. The water and solute transport across barley roots is composite in nature and they do not behave like ideal osmometers. The composite transport model should be extended by adding components arranged in series cortex, endodermis in addition to the currently included components arranged in parallel apoplastic, cell to cell pathways.
In recent years, there has been an increasing amount of interest in modelling root hydraulics. This interest is due to the fact that within the soil—plant—air continuum SPAC , the water taken up by plant roots either can be used for plant growth and development or can be lost by transpiration Kramer, The discovery of aquaporins AQPs in the early s suggested that they were a major regulatory component for water transport across cell membranes within the SPAC Maurel, ; Tyerman et al. However, this picture has changed in light of further quantitative data from pressure probes concerning the hydraulic properties of individual root cells and the overall hydraulic conductivities of entire roots Zhu and Steudle, ; Steudle and Peterson, In many of these studies, the results have indicated that the apoplastic path contributes to water transport, even across the endodermis.
There are exceptions to these results, however, as reported in young roots of barley Steudle and Jeschke, , bean Steudle and Brinckmann, and Arabidopsis thaliana Ranathunge and Schreiber, In these plants, cell to cell water transport dominated in roots, suggesting that AQPs were the major influence on water transport.
The existence of two pathways, along with composite transport, would provide some explanation for the observed variance in root hydraulic conductivity Lp r besides the action of AQPs Brouwer, ; Weatherley, ; Kramer and Boyer, Regulation of Lp r has been discussed in terms of the variable contributions of different pathways to the overall water flow in response to stresses such as drought, which affect root anatomy as well as water channel activity Vandeleur et al.
This discussion is focused on the interaction between the two parallel pathways: the cell to cell and the apoplastic. This factor may be important when considering transient effects and thick roots Meyer et al.
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More importantly, the model usually considers only parallel pathways. In reality, roots contain additional transport components, such as the cortex and the stele, which are arranged in series.
The endodermis, in particular, is known to be a substantial barrier for both water and solutes. On the other hand, the axial hydraulic resistance is usually a component of minor importance Frensch and Steudle, Recently, Knipfer and Fricke used root pressure probes to repeat some of the osmotic experiments conducted by Steudle and Jeschke , using NaCl as the osmotic solute. These results were most probably due to an endodermis that was impermeable to the solute.
This observation differed from the earlier results of Steudle and Jeschke for barley and for a number of other plant species Steudle and Peterson, , and references therein. Knipfer and Fricke concluded from their results that the cell to cell component of water transport, rather than the apoplastic component, was dominant in barley, confirming the previous data of Steudle and Jeschke that compared the cellular and overall root Lp r by pressure probe measurements.
In the present study, we critically investigate the proposed cell to cell transport model of barley roots Steudle and Jeschke, by combining anatomical, biochemical and physiological studies at the cellular and root level. We also determine how suberized barriers in the cell walls affect water transport, in addition to the extended measurements of the permeability patterns of these roots using several electrolytes and non-electrolytes as test solutes.
Caryopses of barley Hordeum vulgare L. Six days later, the seedlings were transferred into a hydroponic system containing modified half-strength Hoagland solution in a climatic chamber Fricke and Peters, The plants used in the experiments were grown for 16—20 d, including the germination period. At this stage, the plants had 3—4 developed leaves and 6—7 seminal roots.
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The aliphatic suberin in cell walls was detected by yellow fluorescence under ultraviolet light filter set: exciter filter, G ; chromatic beam splitter, FT ; barrier filter, LP The seminal roots were divided into two zones. Zone-I was the younger part of the root, without laterals, which included the growing root tip. Zone-II was the mature half of the root, towards the base, and included lateral roots. Root segments were enzymatically digested to remove cellulose and pectins from the cell walls Zeier and Schreiber, , and the steles were isolated along with the suberized endodermis.
The isolated cell wall samples were then purified, dried and subjected to transesterification to release suberin monomers according to the procedures of Kolattukudy and Agrawal Gas chromatographic analysis and mass spectrometric identification of the derivatized degradation products were performed as described by Zeier and Schreiber , The amounts were calculated for the unit surface area of the roots. Four replicates were used for each root zone. This part had dead and functional conductive xylem vessels with lignified cell walls. The non-conductive root tip part hydraulically isolated zone , which was approx.
This part did not have mature and functional xylem vessels, which were filled with the cytoplasm. Ten replicates were used for each measurement. In the osmotic experiments, the nutrient solution in the external medium was rapidly exchanged by 30 m m approx. The osmotic Lp r was calculated from the rate constant of the water phase of the biphasic osmotic root pressure relaxations half-time of the first phase; Fig.
Both experiments were conducted in two ways. In the first, the root medium was rapidly stirred to minimize the external unstirred layers, while in the second, the medium was kept stagnant without stirring to determine the effect of external unstirred layers on water permeability. Stirring was achieved by rapidly bubbling the medium with air Ye and Steudle, During stagnant unstirred conditions, the bubbling was stopped.
Cell volume V and cell surface area A was calculated from the length and the diameter of cortical cells, assuming they are cylindrical in shape. The total root volume was calculated using the conductive length and the diameter of the root. The osmotic concentrations of the tested solutes were measured using a freezing point depression osmometer Osmomat ; Gonotec, Berlin, Germany.
After each measurement, the proper function of the seal was checked by cutting off the root at the seal. When the xylem of the root remained open, there was a drastic decrease in the half-times by at least one order of magnitude and an increase in hydraulic conductance after the cut. If these results were not observed, the experiment was discarded. Appearance of Casparian bands CBs and suberin lamellae SL in the endodermis, and presence of suberin in the rhizodermis of barley roots.
Freehand cross-sections of seminal roots of to day-old barley plants stained with either berberine—aniline blue A-D or lipophilic fluorochrome, Fluorol yellow E, F and H. Section stained with Toluidine blue O, showing only four cortical cell layers in the cortex I. The suberin lamellae SL were detected by an intense, bright yellow fluorescence in the cell walls after staining cross-sections with fluorol yellow Fig. No exodermis was detected in the hydroponically grown barley roots.
However, clear autofluorescence Fig. The cross-section, stained with Toludine blue O, confirmed that there were four cortical cell layers present in the cortex Fig. Total amounts A , substance classes B and monomer compositions C of aliphatic and aromatic suberin in the stele of to day-old barley seminal roots.
Analyses were done for two root zones: Zone-I was the younger zone that included the root apex, and Zone-II was the mature zone, including laterals, towards the base. Aromatic suberin was mainly composed of ferulic and coumaric acids Fig. The chain length distribution of the aliphatic monomers varied from C 16 to C 30 Fig. Very short chains, such as C 16 and C 18 , were prominent in all substance classes. Time course vs.
A hydroponically grown, day-old barley seminal root without laterals was attached to the root pressure probe. B Responses of root pressure P r in relation to the change in osmotic pressure in the external medium either by adding 59 mOsmol kg —1 NaCl to the medium or by removing it from the medium. Biphasic responses consist of rapid water efflux or influx followed by slow solute influx or efflux.
The addition of NaCl to the medium resulted in a drop of P r but it failed to reach the original pressure due to the inhibition of other electrogenic pumps in the plasma membrane. Hydraulic conductivity Lp r of end-segments of seminal roots of barley grown in aerated hydroponics for 14—20 d, measured with a root pressure probe. Hydrostatic and osmotic Lp r were measured either rapidly stirring well-stirred or without stirring unstirred the root medium.
Osmotic Lp r was measured by replacing the external nutrient solution by 59 mOsmol kg —1 NaCl in the nutrient solution. The Lp r of the apical zone Zone-I and whole roots of to day-old barley seminal roots, grown in aerated hydroponics, and measured with a root pressure probe under well-stirred conditions. The Lp r of the whole root is approx. The actual volume calculations were not possible for the exactly measured cells and instead average cell diameter and length obtained from the root cross-sections of the same place were used for the calculations.
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A similar phenomenon was observed in corn root cortical cells Wan et al. Their results were interpreted as being caused by the tremendous water flux during the puncturing procedure, which acted as a mechanical stress and resulted in the closure of AQPs. Wan et al. The equation assumes that the hydraulic resistances of individual cell layers arranged in series are additive and that cell membranes must be crossed twice per layer.
For geometric reasons, the inner rings contribute more to the overall resistance than the outer rings.
Both the rhizodermis and cortex, including the endodermis, were considered in these calculations. However, in this calculation for barley, it was assumed that all the endodermal cells contributed to the water flow through membranes. However, the calculated Lp r value should be considered the upper limit due to the exclusion of the endodermal resistance.
Cutting experiment on an excised maize root attached to the root pressure probe. When the root was successively cut with a razor blade, starting at the apex, the root pressure dropped immediately when developed xylem was cut. Between cuts, hydrostatic pressure relaxations were performed in order to measure changes in the hydraulic resistance of the root.
The cutting experiments also allowed the longitudinal hydraulic resistance in the root to be estimated and provided information about the tightness of the silicone seal. In the pressure probe measurement, it is assumed that the radial resistance is significantly greater than the axial resistance to overall water flow in roots. The cutting experiment demonstrated that this is true. However, as soon as the fully matured and functional early metaxylem vessels starts at approx. The addition of ethanol, KCl and mannitol to the medium resulted in biphasic responses due to rapid efflux of water, followed by slow influx of solutes.
In contrast, addition of sucrose and K 4 [Fe CN 6 ] to the external medium gave monophasic responses in which the second phase or solute influx solute permeation into the root is missing.
Molecular Biology and Physiology of Water and Solute Transport
The results indicated a marked difference between the P sr values of barley roots for different solutes Table 3. Most interesting was the fact that K 4 [Fe CN 6 ] did not permeate at all, similar to previous observations by Ranathunge et al. The P sr values for sucrose which has a larger molecular weight than mannitol and K 4 [Fe CN 6 ] were not measurable because the solute phase was missing for these compounds second phase in the osmotic experiments; Fig.
For the first time, we have investigated the role of suberized barriers, a part of the apoplast, for water and solute transport of barley seminal roots.