Water Transport in Isolated Maize Roots

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

Water 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.     

Radial Water and Solute Flows in Roots of Zea mays: I. WATER FLOW

Water flows are described in root segments from which the centralstele had been removed (‘sleeves’). The water flowwas measured either as an exchange flow, using tracers, or asnet osmotic flow. Metabolic inhibitors inhibited both typesof flow. From the above measurements, the over-all hydraulicconductivity (L^p) and permeability coefficient () for waterwere calculated. By measuring the effect of water drag on theflow of sucrose and urea, reflection coefficients () were calculatedfor sucrose and urea. A model of water flow, which includescoupling to chemical reactions, is discussed.     

The Radial Exchange of Labelled Water in Isolated Steles of Maize Roots

The kinetics of the efflux of labelled water from isolated stelesof maize roots have been studied.Analysis of the efflux kineticsindicated that the rate-limiting step was possibly permeationacross cell ‘membranes‘. On this basis we have computedfrom the efflux curve that the ‘typical‘ membranepermeability of stelar cells to water is about 1.8x10^-5 cm s^-1;thus, the cells in the stele are about four times more permeableto water than the cortical cells. The water permeability ofthe stelar cells, however, may have been substantially alteredby the extraction of steles from the cortical sleeves. Similarefflux studies were performed on ‘dead‘ steles preparedby boiling the normal steles for a short time. These experimentsdemonstrated that diffusion within the tissue and external ‘unstirredlayer‘ was rate-controlling the initial stage of the effluxfrom ‘dead‘ steles.A common feature of both effluxcurves for normal and ‘dead‘ steles was that eachdeviated from theoretical cylindrical diffusion curves for largevalues of time. Certain experimental evidence suggested thatthis deviation resulted from the existence of a slowly exchangingunidentified compartment occupying at least 8 per cent of thestelar volume.     

Osmotic Water Permeability of Vacuolar and Plasma Membranes Isolated from Maize Roots

The method of stopped flow was used to follow the changes in light scattering by the vesicles of plasmalemma and tonoplast isolated from maize (Zea maysL.) roots and treated by osmotic pressure. In both membrane preparations, the rate of the process depended on the osmotic gradient and was described with the simple exponential function. The rate constants derived from these functions were the following: the coefficient of water permeability in the tonoplast (P= 165 ± 7 m/s) exceeded by an order of magnitude the corresponding index for plasmalemma (11 ± 2 m/s). The presence of HgCl₂(1.6 nmol/g membrane protein) decreased the tonoplast water permeability by 80%. Microviscosity studies of the hydrocarbon zone in the isolated membranes by using a fluorescent diphenylhexatriene probe demonstrated that the two membranes do not differ in the phase state of their lipid bilayer. The authors conclude that the observed difference in water permeability does not depend on the state of the lipid phase and probably reflects the dissimilar functional activity of plasmalemma and tonoplast aquaporins.     

水分亏缺下玉米根系ZmPIP1亚族基因的表达

在PEG-6000胁迫条件下,以微管蛋白基因为内参基因、水通道蛋白基因ZmPIP1-1和ZmPIP1-2为检测基因,采用半定量逆转录聚合酶链式反应（RT-PCR）体系检测它们在玉米根系中的表达情况。实验结果是：胁迫条件下,ZmPIP1-1的表达量在杂交F,代‘户单4号’（抗旱）和母本‘天四’（抗旱）根系中增多,它的表达量与品种的抗旱性呈正相关,并且胁迫不同时间段它的表达量有差异;而ZmPIP1-2在3个玉米品种的不同水分处理条件下,表达量均没有明显变化。这提示,水分胁迫条件下根系中某些种类的水通道蛋白基因的表达量增多,并且与品种的抗旱性有关;而另一些水通道蛋白基因的表达不受水分亏缺的影响。     

Effect of Water Stress and Mercuric Chloride on the Translational Diffusion of Water in Maize Seedling Roots

Water diffusion in maize roots (Zea mays L., cv. Donskaya 1) was investigated with a pulsed gradient NMR using mercuric chloride as an inhibitor of water channels in cell membranes. A novel operation program was applied that allowed selective evaluation of fractional amounts of water transported through various pathways—the apoplastic, symplasmic, and transmembrane routes. The blockage of water channels with HgCl₂ reduced the rates of water diffusion by a factor of 1.5–2. This effect was reversible and was removed by the addition of -mercaptoethanol. The coefficient of water diffusion changed with time elapsed after the HgCl₂ treatment. The effect of water stress on the rates of water diffusion was similar to that of HgCl₂. Remarkably, the water-stressed roots of maize seedlings were insensitive to the inhibitor of water channels. The results are interpreted in terms of redistribution of water flows among various routes in plant tissues. Water stress and mercuric chloride treatments decelerate the transmembrane water transport and promote water flow along the apoplastic pathway. These responses might arise from the reversible regulation of water movement along various transport pathways.     

Effect of Temperature on the Radial Exchange of Labelled Water in Maize Roots

The temperature dependence of the efflux kinetics of labelledwater in isolated maize roots has been studied. The purposeof these experiments was to obtain the energy of activation,E (kcal/mole), of the rate-limiting step in this radial exchangeprocess under various experimental conditions. Estimates ofE were obtained from linear relations between ln{D'_w} and thereciprocal of the absolute temperature; values of the apparentdiffusion coefficient, D'_w, of labelled water in the root werefound from an analytical treatment of the efflux data in termsof a cylindrical diffusion model. The energy of activation forlabelled-water exchange in normal roots was 14.9 kcal/mole.The corresponding value for ‘dead’ (boiled) rootswas 3.9 kcal/mole. These values of E substantiate the view thatin normal roots the penetration of water across the membranesof the root cells constitutes the rate-determining step in theefflux whereas in ‘dead’ roots extracellular diffusionof water is the source of rate-control. Similar temperature dependence studies were performed on theefflux kinetics from normal and ‘dead’ roots treatedwith 10^–5 M phenylmercuric acetate (PMA). The energiesof activation for labelled-water exchange in normal and ‘dead’roots under these conditions were respectively 15.5 and 5.3kcal/mole. Moreover, the results of the efflux experiments onPMA-treated roots were considered to indicate that this inhibitorproduces an alteration in some structural aspect of the rate-controlling‘membranes’.     

Regulation of K+ Channels in Maize Roots by Water Stress and Abscisic Acid

Root cortical and stelar protoplasts were isolated from maize (Zea mays L.) plants that were either well watered or water stressed, and the patch-clamp technique was used to investigate their plasma membrane K⁺ channel activity. In the root cortex water stress did not significantly affect inward- or outward-rectifying K⁺ conductances relative to those observed in well-watered plants. In contrast, water stress significantly reduced the magnitude of the outward-rectifying K⁺ current in the root stele but had little effect on the inward-rectifying K⁺ current. Pretreating well-watered plants with abscisic acid also significantly affected K⁺ currents in a way that was consistent with abscisic acid mediating, at least in part, the response of roots to water stress. It is proposed that the K⁺ channels underlying the K⁺ currents in the root stelar cells represent pathways that allow K⁺ exchange between the root symplasm and xylem apoplast. It is suggested that the regulation of K⁺ channel activity in the root in response to water stress could be part of an important adaptation of the plant to survive drying soils.     

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

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

Nickel Toxicity and Distribution in Maize Roots

A new histochemical method for Ni determination has been developed and employed to study the pattern of Ni distribution in plant tissues. Two-day-old seedlings of maize (Zea mays L.) were transferred onto 15, 20, 25, and 35 M Ni(NO₃)₂ solutions in the presence of 3 mM Ca(NO₃)₂, and Ni localization in shoot and root tissues was investigated at days 2 and 7 of the incubation. Following two days of incubation, Ni was found in all root tissues, and its content increased with the period of exposure and from the tip to the root base. Independent of root region and tissue, Ni content in the protoplasts exceeded that in the cell walls. Ni penetrated the endodermal barrier and accumulated in the endodermis and pericycle to the highest concentration. Ni accumulation in the pericycle restricted root branching. Ni did not affect the final cell length, and the inhibition of root growth resulted from suppressed cell division. In the shoots, Ni content was below the level discerned by the dimethylglyoximine method; we therefore conclude that maize belongs to excluder plants, with their root systems functioning as a barrier limiting heavy metal intake by aboveground organs. The pattern of Ni transport differs from that of Cd and Pb; this difference stands for specific toxic effects of Ni, including an arrest of root branching.     

Main-hydrolysis of White Birch Chips Wetted with Sulfuric Acid Solution of Medium Strength Using a Through Drying Operation

The purpose of this study is to find the relation between the saccharification ratio and the mixing ratio in a new wood saccharification process with strong sulfuric acid by employing a through drying operation. The saccharification yield in the main-hydrolysis of white birch chips with a sulfuric acid solution of a medium strength is expressed by the following empirical equations according to respective kind of samples of white birch chips used. With mixing ratio R ranging from 0.4 to 1.0.

when raw white birch chips (0.1°10°10 mm) are used and

when pre-hydrolyzed white birch chips (1.5×3.5~4.5×10~15 mm) are used.     

The development of structural changes in epidermal cells of Maize Roots During Water Stress

Ultrastructural changes following increasing periods of water stress induced by means of polyethylene glycol 4000 (from 5 min to 18 h) were investigated in young epidermal cells of the primary roots ofZea mays. The sensitivity of the individual cell components to water stress was considered according to the time sequence in which their alterations appeared. Endoplasmic reticulum (ER) and mitochondria proved to be the most sensitive cell components, their structure changing after 5 min of water stress. By 8 h of stress, the condensation of nuclear chromatin in some cells was apparent, preceding polyribosome degradation but not all the other changes of the stressed cells. Similar types of structural alterations appeared slightly earlier in the more differentiated epidermal cells. The rapid changes in the structure of ER and mitochondria coincided with the rapid decrease of water potential of the root tips and the immediate cessation of the root growth.     

Radial Water and Solute Flows in Roots of Zea mays: II. ION FLUXES ACEOSS ROOT CORTEX

Unidirectional fluxes of Na⁺, K⁺, Cl^–, and phosphate weremeasured across a preparation of hollow cylinder of a segmentof maize root. Cation fluxes are 2–3 orders of magnitudelarger than those of the anions. From the Ussing flux ratio,it was concluded that cations are moving passively while anionsare pumped actively across the root. The anion pumps are acceleratedby the uncoupler DNP and inhibited by cyanide. It is suggestedthat they are coupled directly to the electron transport system,rather than to ATP. From analysis of the diffusion coefficientsof the ions it was concluded that radial ion movement in theroot cortex must be through, rather than between, the cells.     

Light-Regulated Gravitropism in Seedling Roots of Maize

Red light-induced changes in the gravitropism of roots of Zea mays variety Merit is a very low fluence response with a threshold of 10⁻⁹ moles per square meter and is not reversible by far red light. Blue light also affects root gravitropism but the sensitivity of roots to blue is 50 to 100 times less than to an equal fluence of red. In Z. mays Merit we conclude that phytochrome is the sole pigment associated with light-induced changes in root gravitropism.     

Origins of the Electrical Potential Difference between the Xylem Sap of Maize Roots and the External Solution

The effects of 2,4-dinitrophenol, of changes in the temperatureand concentration of the ambient solution and of variationsin salt status on the electrical potential difference betweenthe xylem exudate of maize roots and the ambient solution havebeen examined. The results are discussed in the light of someof the factors which could give rise to a potential differencebetween the sap and the solution. The rapid response of thepotential difference to dinitrophenol and to changes in temperaturesuggest that, at least in part, it arises directly from metabolicprocesses. Rapid changes in the potential difference broughtabout by addition of salts may be attributed to differentialrates of movement of anions and cations in the initial uptakeprocess. Over longer periods the potential difference appearsto be dependent on the concentration, but not the compositionof the ambient solution, and on the salt status of the roots.The salt status influences the relative rates at which anionsand cations are transported to the xylem sap, and a correlationhas been found between the potential difference and the ratioof the rates of movement of chloride or sulphate to potassiumto the sap. The implications of these findings on the elucidationof the pathways whereby ions are transported to the sap arediscussed.     

Calcium and Rhizodermal Differentiation in Primary Maize Roots

Rhizodermal differentiation of maize (Zea mays L. cv. LG 11)roots cultured in humid air was influenced by a pretreatmentfor 2 h in CaCl₂ or CaSO₄ solutions. This increased the numberof hair-producing roots and the density of hairs. Ethylene glycol-bis-(ß-aminoethylether)N,N'-tetraacetic acid (EGTA) was inhibitory. Root hairsemerged in the part of the cell nearer to the tip. Trichoblastswere shorter and elongated more slowly than atrichoblasts. Theelongation of the lower part of the trichoblast was less thanthat of the upper part. Key words: Cell length, cell number, hair position