Low temperature and mechanical stresses differently gate aquaporins of root cortical cells of chilling-sensitive cucumber and -resistant figleaf gourd

The inhibition of the hydraulic conductivity of individual cortical cells (Lp) of young roots of cucumber and figleaf gourd by low root temperature (8 °C, LRT) was measured using a cell pressure probe. When LRT was imposed, the Lp of the two species responded differently. Water permeability of cortical cells of chilling-sensitive cucumber decreased by a factor of 10, but there was only a small effect in the chilling-resistant figleaf gourd. Mechanical stresses (pulses of cell turgor pressure) resulted in a similar inhibition for both species by a factor of 6.5. When applied at LRT, abscisic acid (ABA) partially or even completely reversed the effects of chilling and mechanical stresses of both species. At the control temperature of 22 °C, 50 µm of the aquaporin (AQP) inhibitor HgCl₂ acted on root cells of both species, although the effect on root cells of figleaf gourd was small. There was no effect of HgCl₂, when AQPs were already closed either by LRT or by mechanical stress. The effect of mechanical stress (pressure pulses) was substantially bigger than that of HgCl₂. When AQPs were closed by big pulses in the presence of 50 µm HgCl₂, they could be partially re-opened in the presence of the inhibitor by applying small pulses, suggesting that there are at least two different types of channels present, which respond differently to mechanical stress or to the heavy metal. The presence of 1 µm ABA in the root medium prevented the effects of LRT and mechanical stress, namely an increase in the half-times of water exchange (T^w_1/2 ∝ 1/Lp). In the absence of stresses at short T^w_1/2, there was no effect of ABA. It is concluded that the responsiveness of AQPs of the two species differs in the presence of LRT but not under conditions of mechanical stress. In both cases, however, ABA has an ameliorative effect. The results suggest that the presence of ABA reduces the activation energy of changes of the conformation of AQPs, when switching between open and closed states. Mechanisms of the gating of AQP activity by LRT and mechanical stresses and the possible role of the stress hormone ABA are discussed.     

Light-induced transpiration alters cell water relations in figleaf gourd (Cucurbita ficifolia) seedlings exposed to low root temperatures

Water relation parameters including elastic modulus (epsilon), half-times of water exchange (T(w)(1/2)), hydraulic conductivity and turgor pressure (P) were measured in individual root cortical and cotyledon midrib cells in intact figleaf gourd (Cucurbita ficifolia) seedlings, using a cell pressure probe. Transpiration rates (E) of cotyledons were also measured using a steady-state porometer. The seedlings were exposed to low ambient (approximately 10 micromol m(-2) s(-1)) or high supplemental irradiance (approximately 300 micromol m(-2) s(-1) PPF density) at low (8 degrees C) or warm (22 degrees C) root temperatures. When exposed to low irradiance, all the water relation parameters of cortical cells remained similar at both root temperatures. The exposure of cotyledons to supplemental light at warm root temperatures, however, resulted in a two- to three-fold increase in T(w)(1/2) values accompanied with the reduced hydraulic conductivity in both root cortical (Lp) and cotyledon midrib cells (Lp(c)). Low root temperature (LRT) further reduced Lp(c) and E, whether it was measured under low or high irradiance levels. The reductions of Lp as the result of respective light and LRT treatments were prevented by the application of 1 microM ABA. Midrib cells required higher concentrations of ABA (2 microM) in order to prevent the reduction in Lp(c). When the exposure of cotyledons to light was accompanied by LRT, however, ABA proved ineffective in reversing the inhibition of Lp. LRT combined with high irradiance triggered a drastic 10-fold reduction in water permeability of cortical and midrib cells and increased epsilon and T(w)(1/2) values. Measurement of E indicated that the increased water demand by the transpiring plants was fulfilled by an increase in the apoplastic pathway as principal water flow route. The importance of water transport regulation by transpiration affecting the hydraulic conductivity of the roots is discussed.     

Physiological basis of different allelopathic reactions of cucumber and figleaf gourd plants to cinnamic acid

To provide an insight into the mechanism of interspecific interactions mediated by allelochemicals, cucumber and figleaf gourd seedlings were compared on their response to cinnamic acid, an autotoxin from root exudates of cucumber. Reactive oxygen species metabolism and plasma membrane H(+)-ATPase activity were examined in roots upon exposure to cinnamic acid. This exposure resulted in significant increases in activities of NADPH oxidase, superoxide dismutase, guaiacol peroxidase, and catalase, as well as in O(2)(.-) production and H(2)O(2) content, in cucumber roots but not in figleaf gourd roots. Notably, the cucumber roots produced significant amount of reactive oxygen species (ROS) immediately after cinnamic acid treatment, consequently increasing membrane peroxidation, decreasing membrane H(+)-ATPase activity, and losing root viability. By contrast, no such changes were observed in figleaf gourd roots. All these results indicated that there was an interspecies difference in the recognition of allelochemicals, which induced oxidative stress accompanied by root cell death in cucumber, an autotoxic plant, but not in figleaf gourd, a cucumber relative.     

Differential impact of low temperature on fatty acid unsaturation and lipoxygenase activity in figleaf gourd and cucumber roots

Previous studies show that low temperature strongly induces suberin layers in the roots of chilling-sensitive cucumber plants, while in contrast, low temperature produces a much weaker induction of suberin layers in the roots of the chilling-tolerant figleaf gourd [S.H. Lee, G.C. Chung, S. Steudle, Gating of aquaporins by low temperature in roots of chilling-sensitive cucumber and -tolerant figleaf gourd, J. Exp. Bot. 56 (2005) 985-995; S.H. Lee, G.C. Chung, E. Steudle, Low temperature and mechanical stresses differently gate aquaporins of root cortical cells of chilling-sensitive cucumber and figleaf gourd, Plant Cell Environ. (2005) in press; S.J. Ahn, Y.J. Im, G.C. Chung, B.H. Cho, S.R. Suh, Physiological responses of grafted-cucumber leaves and rootstock roots affected by low root temperature, Scientia Hort. 81 (1999) 397-408]. Here, the effect of low temperature on fatty acid unsaturation and lipoxygenase activity was examined in cucumber and figleaf gourd. The double bond index demonstrated that membrane lipid unsaturation shows hyperbolic saturation curve in figleaf gourd roots while a biphasic response in cucumber roots to low temperature. In figleaf gourd, the hyperbolic response in the double bond index was primarily due to accumulation of linolenic acid. Chilling stress also significantly induced lipoxygenase activity in figleaf gourd roots. These results suggest that the degree of unsaturation of root plasma membrane lipids correlates positively with chilling-tolerance. Therefore, studies that compare the effects of chilling on cucumber and figleaf gourd may provide broad insight into stress response mechanisms in chilling-sensitive and chilling-tolerant plants. Furthermore, these studies may provide important information regarding the relationship between lipid unsaturation and lipoxygenase function/activity, and between lipoxygenase activity and water channeling during the response to chilling stress. The possible roles of these processes in chilling tolerance are discussed.     

Cold Stress-Induced Acclimation in Rice is Mediated by Root-Specific Aquaporins

Cold acclimation process plays a vital role in the survival of chilling- and freezing-tolerant plants subjected to cold temperature stress. However, it remains elusive whether a cold acclimation process enhances root water uptake (a component of chilling tolerance) in chilling-sensitive crops such as rice. By analyzing the root hydraulic conductivity under cold stress for a prolonged time, we found that cold stress induced a gradual increase in root osmotic hydraulic conductivity [Lp(r(os))]. Compared with the control treatment (roots and shoots at 25°C), low root temperature (LRT) treatment (roots at 10°C; shoots at 25°C) dramatically reduced Lp(r(os)) within 1 h. However, Lp(r(os)) gradually increased during prolonged LRT treatment and it reached 10-fold higher values at day 5. Moreover, a coordinated up-regulation of root aquaporin gene expression, particularly OsPIP2;5, was observed during LRT treatment. Further, comparison of aquaporin gene expression under root-only chilling (LRT) and whole-plant chilling conditions, and in the roots of intact plants vs. shootless plants, suggests that a shoot to root signal is necessary for inducing the expression of aquaporin genes in the root. Collectively, these results demonstrate that a cold acclimation process for root water uptake functions in rice and is possibly regulated through aquaporins.     

Oxidative gating of water channels (aquaporins) in corn roots

An oxidative gating of water channels (aquaporins: AQPs) was observed in roots of corn seedlings as already found for the green alga Chara corallina. In the presence of 35 mM hydrogen peroxide (H2O2)--a precursor of hydroxyl radicals (*OH)--half times of water flow (as measured with the aid of pressure probes) increased at the level of both entire roots and individual cortical cells by factors of three and nine, respectively. This indicated decreases in the hydrostatic hydraulic conductivity of roots (Lp(hr)) and of cells (Lp(h)) by the same factors. Unlike other stresses, the plant hormone abscisic acid (ABA) had no ameliorative effect either on root LP(hr) or on cell Lp(h) when AQPs were inhibited by oxidative stress. Closure of AQPs reduced the permeability of acetone by factors of two in roots and 1.5 in cells. This indicated that AQPs were not ideally selective for water but allowed the passage of the organic solute acetone. In the presence of H2O2, channel closure caused anomalous (negative) osmosis at both the root and the cell level. This was interpreted by the fact that in the case of the rapidly permeating solute acetone, channel closure caused the solute to move faster than the water and the reflection coefficient (sigma s) reversed its sign. When H2O2 was removed from the medium, the effects were reversible, again at both the root and the cell level. The results provide evidence of oxidative gating of AQPs, which leads on to inhibition of water uptake by the roots. Possible mechanisms of the oxidative gating of AQPs induced by H2O2 (*OH) are discussed.     

Rapid accumulation of hydrogen peroxide in cucumber roots due to exposure to low temperature appears to mediate decreases in water transport

Water transport across root systems of young cucumber (Cucumis sativus L.) seedlings was measured following exposure to low temperature (LT, 8-13 degrees C) for varying periods of time. In addition, the amount of water transported through the stems was evaluated using a heat-balance sap-flow gauge. Following LT treatment, hydrogen peroxide was localized cytochemically in root tissue by the oxidation of cerium (III) chloride. The effects of hydrogen peroxide on the hydraulic conductivity of single cells (Lp) in root tissues, and on the H+-ATPase activity of isolated root plasma membrane, have been worked out. Cytochemical evidence suggested that exposure of roots to LT stress caused a release of hydrogen peroxide in the millimolar range in the vicinity of plasma membranes. In response to a low root temperature (8 degrees C), the hydraulic conductivity of the root (Lp(r)) decreased by a factor of 4, and the half-times of water exchange increased by a factor of 5-6. Decreasing root temperatures from 25-13 degrees C increased the half-times of water exchange in a cell by a factor of 6-9. The measurement of axial water transport with a heat-balance sap-flow gauge showed that only a small amount of water was transported when 8 degrees C was imposed on cucumber roots. Lp and the H+-ATPase activity of the isolated root plasma membrane were very sensitive to externally applied hydrogen peroxide at a concentration of 1-16 mM. These observations suggest that the accumulation of hydrogen peroxide appears to mediate decreases in water transport in cucumber roots under low temperature.     

Temperature effects on the reactive oxygen species formation and antioxidant defence in roots of two cucurbit species with contrasting root zone temperature optima

Suboptimal root zone temperature (14°C) was imposed on chilling-sensitive cucumber (Cucumis sativus L.) and chilling-tolerant figleaf gourd (Cucurbita ficifolia Bouché) plants. Exposure of roots to low temperature for up to 10 days caused a strong growth inhibition in cucumber compared with figleaf gourd. Physiological analysis showed that generation of reactive oxygen species (ROS) such as hydrogen peroxide and superoxide anion was significantly induced in cucumber plants as fast as 1 day after low root zone temperature treatment. In addition to the significant induction of antioxidant superoxide dismutase activity, low root zone temperature also increased the mitochondrial electron transport allocated to alternative pathway while decreased cytochrome pathway salicylhydroxamic acid-resistant respiration. However, these defense responses could not compensate for the ROS production, resulting in membrane lipid peroxidation and loss of root cell viability in the low root zone temperature treated cucumber roots. In contrast, 14°C root zone temperature had no significant effects on figleaf gourd plant growth, antioxidant enzymes, ROS levels and alternative respiratory pathway. Hence, difference in ROS metabolism would be associated with the remarkable difference in adaptability of cucumber and figleaf gourd plants in response to suboptimal root zone temperature condition.     

Hydraulic conductivity of rice roots

A pressure chamber and a root pressure probe technique have been used to measure hydraulic conductivities of rice roots (root Lp(r) per m(2) of root surface area). Young plants of two rice (Oryza sativa L.) varieties (an upland variety, cv. Azucena and a lowland variety, cv. IR64) were grown for 31-40 d in 12 h days with 500 micromol m(-2) s(-1) PAR and day/night temperatures of 27 degrees C and 22 degrees C. Root Lp(r) was measured under conditions of steady-state and transient water flow. Different growth conditions (hydroponic and aeroponic culture) did not cause visible differences in root anatomy in either variety. Values of root Lp(r) obtained from hydraulic (hydrostatic) and osmotic water flow were of the order of 10(-8) m s(-1) MPa(-1) and were similar when using the different techniques. In comparison with other herbaceous species, rice roots tended to have a higher hydraulic resistance of the roots per unit root surface area. The data suggest that the low overall hydraulic conductivity of rice roots is caused by the existence of apoplastic barriers in the outer root parts (exodermis and sclerenchymatous (fibre) tissue) and by a strongly developed endodermis rather than by the existence of aerenchyma. According to the composite transport model of the root, the ability to adapt to higher transpirational demands from the shoot should be limited for rice because there were minimal changes in root Lp(r) depending on whether hydrostatic or osmotic forces were acting. It is concluded that this may be one of the reasons why rice suffers from water shortage in the shoot even in flooded fields.     

Water uptake by roots: effects of water deficit

The variable hydraulic conductivity of roots (Lp(r)) is explained in terms of a composite transport model. It is shown how the complex, composite anatomical structure of roots results in a composite transport of both water and solutes. In the model, the parallel apoplastic and cell-to-cell (symplastic and transcellular) pathways play an important role as well as the different tissues and structures arranged in series within the root cylinder (epidermis, exodermis, cortex, endodermis, stelar parenchyma). The roles of Casparian bands and suberin lamellae in the root's endo- and exodermis are discussed. Depending on the developmental state of these apoplastic barriers, the overall hydraulic resistance of roots is either more evenly distributed across the root cylinder (young unstressed roots) or is concentrated in certain layers (exo- and endodermis in older stressed roots). The reason for the variability of root Lp(r), is that hydraulic forces cause a dominating apoplastic flow of water around protoplasts, even in the endodermis and exodermis. In the absence of transpiration, water flow is osmotic in nature which causes a high resistance as water passes across many membranes on its passage across the root cylinder. The model allows for a high capability of roots to take up water in the presence of high rates of transpiration (high demands for water from the shoot). By contrast, the hydraulic conductance is low, when transpiration is switched off. Overall, this results in a non-linear relationship between water flow and forces (gradients of hydrostatic and osmotic pressure) which is otherwise hard to explain. The model allows for special root characteristics such as a high hydraulic conductivity (water permeability) in the presence of a low permeability of nutrient ions once taken up into the stele by active processes. Low root reflection coefficients are in line with the idea of some apoplastic bypasses for water within the root cylinder. According to the composite transport model, the switch from the hydraulic to the osmotic mode is purely physical. In the presence of heavily suberized roots, the apoplastic component of water flow may be too small. Under these conditions, a regulation of radial water flow by water channels dominates. Since water channels are under metabolic control, this component represents an 'active' element of regulation. Composite transport allows for an optimization of the water balance of the shoot in addition to the well-known phenomena involved in the regulation of water flow (gas exchange) across stomata. The model is employed to explain the responses of plants to water deficit and other stresses. During water deficit, the cohesion-tension mechanism of the ascent of sap in the xylem plays an important role. Results are summarized which prove the validity of the coehesion/tension theory. Effects of the stress hormone abscisic acid (ABA) are presented. They show that there is an apoplastic component of the flow of ABA in the root which contributes to the ABA signal in the xylem. On the other hand, (+)-cis-trans-ABA specifically affects both the cell level (water channel activity) and water flow driven by gradients in osmotic pressure at the root level which is in agreement with the composite transport model. Hydraulic water flow in the presence of gradients in hydrostatic pressure remains unchanged. The results agree with the composite transport model and resemble earlier findings of high salinity obtained for the cell (Lp) and root (Lp(r)) level. They are in line with known effects of nutrient deprivation on root Lp(r )and the diurnal rhythm of root Lp(r )recently found in roots of LOTUS.     

Chemical composition of apoplastic transport barriers in relation to radial hydraulic conductivity of corn roots (Zea mays L.)

The hydraulic conductivity of roots (Lp_r) of 6- to 8-d-old maize seedlings has been related to the chemical composition of apoplastic transport barriers in the endodermis and hypodermis (exodermis), and to the hydraulic conductivity of root cortical cells. Roots were cultivated in two different ways. When grown in aeroponic culture, they developed an exodermis (Casparian band in the hypodermal layer), which was missing in roots from hydroponics. The development of Casparian bands and suberin lamellae was observed by staining with berberin-aniline-blue and Sudan-III. The compositions of suberin and lignin were analyzed quantitatively and qualitatively after depolymerization (BF₃/methanol-transesterification, thioacidolysis) using gas chromatography/mass spectrometry. Root Lp_r was measured using the root pressure probe, and the hydraulic conductivity of cortical cells (Lp) using the cell pressure probe. Roots from the two cultivation methods differed significantly in (i) the Lp_r evaluated from hydrostatic relaxations (factor of 1.5), and (ii) the amounts of lignin and aliphatic suberin in the hypodermal layer of the apical root zone. Aliphatic suberin is thought to be the major reason for the hydrophobic properties of apoplastic barriers and for their relatively low permeability to water. No differences were found in the amounts of suberin in the hypodermal layers of basal root zones and in the endodermal layer. In order to verify that changes in root Lp_r were not caused by changes in hydraulic conductivity at the membrane level, cell Lp was measured as well. No differences were found in the Lp values of cells from roots cultivated by the two different methods. It was concluded that changes in the hydraulic conductivity of the apoplastic rather than of the cell-to-cell path were causing the observed changes in root Lp_r. Received: 17 March 1999 / Accepted: 22 June 1999     

Exposure of roots of cucumber (Cucumis sativus) to low temperature severely reduces root pressure, hydraulic conductivity and active transport of nutrients

When root temperature dropped below 25°C, there was a sharp drop in the root pressure ( P _r) and hydraulic conductivity of excised roots ( Lp _r) of young cucumber ( Cucumis sativus L.) seedlings as measured with the root pressure probe. A detailed analysis of root hydraulics provided evidence for a larger reduction in the osmotic component of Lp _r (77%) in comparison with the hydrostatic component (34%) in response to the exposure of the root system to 13°C. The activity of the plasma membrane H⁺-ATPase (EC 3.6.1.35) was reduced from 30 to 16 µmol Pi mg⁻¹ protein h⁻¹ upon exposure to 8°C for 1 day. Ultrastructural observations showed no evidence of loosening of the microstructure of endodermal cell walls in low temperature (LT)-treated roots. It is concluded that the rapid drop in the P _r in response to LT is largely caused by a reduction in the activity of the plasma membrane H⁺-ATPase rather than by loosening of the endodermal wall which would cause substantial solute losses. On the other hand, water permeability of root cell membrane at LT was related to changes in the activity (open/closed state) of water channels.     

氮磷亏缺对玉米根系水流导度的影响

在人工气候室水培条件下,从单根和整株根系两个层次研究了N、P营养与玉米(Zea mays L.)根系水流导度(root hydraulic conductivity,Lpr)间的关系。结果表明：表型抗旱的杂交种F1代户单4号和母本天四的单根水导和整株根系水导均高于不抗旱的父本478,其中天四的单根水导最高,而户单4号的整株根系水导最高。N、P亏缺均使玉米单根水导和整株根系水导降低,但与N亏块相比,P亏缺的植株具有较高的整株根系水导和较低的单根水导。整株根系的水导更能反映植物根系的输水性能。     

Comparison between gradient-dependent hydraulic conductivities of roots using the root pressure probe: the role of pressure propagations and implications for the relative roles of parallel radial pathways

Hydrostatic pressure relaxations with the root pressure probe are commonly used for measuring the hydraulic conductivity (Lp(r)) of roots. We compared the Lp(r) of roots from species with different root hydraulic properties (Lupinus angustifolius L. 'Merrit', Lupinus luteus L. 'Wodjil', Triticum aestivum L. 'Kulin' and Zea mays L. 'Pacific DK 477') using pressure relaxations, a pressure clamp and osmotic gradients to induce water flow across the root. Only the pressure clamp measures water flow under steady-state conditions. Lp(r) determined by pressure relaxations was two- to threefold greater than Lp(r) from pressure clamps and was independent of the direction of water flow. Lp(r) (pressure clamp) was two- to fourfold higher than for Lp(r) (osmotic) for all species except Triticum aestivum where Lp(r) (pressure clamp) and Lp(r) (osmotic) were not significantly different. A novel technique was developed to measure the propagation of pressure through roots to investigate the cause of the differences in Lp(r). Root segments were connected between two pressure probes so that when root pressure (P(r)) was manipulated by one probe, the other probe recorded changes in P(r). Pressure relaxations did not induce the expected kinetics in pressure in the probe at the other end of the root when axial hydraulic conductance, and probe and root capacitances were accounted for. An electric circuit model of the root was constructed that included an additional capacitance in the root loaded by a series of resistances. This accounted for the double exponential kinetics for intact roots in pressure relaxation experiments as well as the reduced response observed with the double probe experiments. Although there were potential errors with all the techniques, we considered that the measurement of Lp(r) using the pressure clamp was the most unambiguous for small pressure changes, and provided that sufficient time was allowed for pressure propagation through the root. The differences in Lp(r) from different methods of measurement have implications for the models describing water transport through roots and the potential role of aquaporins.     

Apoplastic barriers,aquaporin gene expression and root and cell hydraulic conductivity in phosphate-limited sheepgrass plants

Mineral nutrient supply can affect the hydraulic property of roots. The aim of the present work on sheepgrass (Leymus chinensis L.) plants was to test whether any changes in root hydraulic conductivity (Lp; exudation analyses) in response to a growth-limiting supply of phosphate (P) are accompanied by changes in (1) cell Lp via measuring the cell pressure, (2) the aquaporin (AQP) gene expression by performing qPCR and (3) the formation of apoplastic barriers, by analyzing suberin lamella and Casparian bands via cross-sectional analyses in roots. Plants were grown hydroponically on complete nutrient solution, containing 250 µM P, until they were 31–36 days old, and then kept for 2–3 weeks on either complete solution, or transferred on solution containing 2.5 µM (low-P) or no added P (no-P). Phosphate treatments caused significant decreases in root and cell-Lp and AQP gene expression, while the formation of apoplastic barriers increased, particularly in lateral roots. Experiments using the AQP inhibitor mercury (Hg) suggested that a significant portion of radial root water uptake in sheepgrass occurs along a path involving AQPs, and that the Lp of this path is reduced under low- and no-P. It is concluded that a growth-limiting supply of phosphate causes parallel changes in (1) cell Lp and aquaporin gene expression (decrease) and (2) apoplastic barrier formation (increase), and that the two may combine to reduce root Lp. The reduction in root Lp, in turn, facilitates an increased root-to-shoot surface area ratio, which allocates resources to the root, sourcing the limiting nutrient.     

Control of water uptake by rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.): role of the outer part of the root

A new pressure-perfusion technique was used to measure hydraulic and osmotic properties of the outer part of roots (OPR) of 30-day-old rice plants (lowland cultivar: IR64, and upland cultivar: Azucena). The OPR comprised rhizodermis, exodermis, sclerenchyma and one cortical cell layer. The technique involved perfusion of aerenchyma of segments from two different root zones (20-50 mm and 50-100 mm from the tip) at precise rates using aerated nutrient solution. The hydraulic conductivity of the OPR (Lp(OPR)=1.2x10(-6) m s(-1) MPa(-1)) was larger by a factor of 30 than the overall hydraulic conductivity (Lp(r)=4x10(-8) m s(-1) MPa(-1)) as measured by pressure chamber and root pressure probe. Low reflection coefficients were obtained for mannitol and NaCl for the OPR (sigma(sOPR)=0.14 and 0.09, respectively). The diffusional water permeability ( P(dOPR)) estimated from isobaric flow of heavy water was smaller by three orders of magnitude than the hydraulic conductivity (Lp(OPR)/ P(fOPR)). Although detailed root anatomy showed well-defined Casparian bands and suberin lamellae in the exodermis, the findings strongly indicate a predominantly apoplastic water flow in the OPR. The Lp(OPR) of heat-killed root segments increased by a factor of only 2, which is in line with the conclusion of a dominating apoplastic water flow. The hydraulic resistance of the OPR was not limiting the passage of water across the root cylinder. Estimations of the hydraulic properties of aerenchyma suggested that the endodermis was rate-limiting the water flow, although the aerenchyma may contribute to the overall resistance. The resistance of the aerenchyma was relatively low, because mono-layered cortical septa crossing the aerenchyma ('spokes') short-circuited the air space between the stele and the OPR. Spokes form hydraulic bridges that act like wicks. Low diffusional water permeabilities of the OPR suggest that radial oxygen losses from aerenchyma to medium are also low. It is concluded that in rice roots, water uptake and oxygen retention are optimized in such a way that hydraulic water flow can be kept high in the presence of a low efflux of oxygen which is diffusional in nature.     

Detoxification of hydrogen peroxide maintains the water transport activity in figleaf gourd (Cucurbita ficifolia) root system exposed to low temperature

The figleaf gourd ( Cucurbita ficifolia Bouché) root system has the ability to take up water and nutrients at low soil temperatures, and in the present paper, we attempt to reveal some of the molecular mechanisms behind this low-temperature tolerance. Exposure of figleaf gourd root system to low temperature induced accumulation of H₂O₂ along the plasma membrane but not in the cytoplasm. H⁺-ATPase (EC 3.6.1.35) activity of isolated root plasma membranes and root hydraulic conductivity ( Lp_r ) were largely insensitive to externally applied H₂O₂. However, using bromocresol purple, it was shown that the acidification of the medium surrounding the root was strongly inhibited with low temperature- and H₂O₂-treated roots. Addition of catalase (EC 1.11.1.6) to the root medium during low-temperature exposure led to a recovery of H⁺-efflux along the root surface and increased Lp_r , demonstrating the importance of an H₂O₂ detoxification system in the root cells. Additional evidence for an increased Lp_r was obtained by the Fenton reaction wherein a warming of the solution increased the activity of the detoxification system. All available evidence suggests that the ability of figleaf gourd root system to maintain a low level of H₂O₂ in the cytoplasm and to detoxify reactive oxygen species is related to the maintenance of water transport activity at low temperatures.     

A re-examination of the minor role of unstirred layers during the measurement of transport coefficients of Chara corallina internodes with the cell pressure probe

The impact of unstirred layers (USLs) during cell pressure probe experiments with Chara corallina internodes has been quantified. The results show that the hydraulic conductivity (Lp) measured in hydrostatic relaxations was not significantly affected by USLs even in the presence of high water flow intensities ('sweep-away effect'). During pressure clamp, there was a reversible reduction in Lp by 20%, which was explained by the constriction of water to aquaporins (AQPs) in the C. corallina membrane and a rapid diffusional equilibration of solutes in arrays where water protruded across AQPs. In osmotic experiments, Lp, and permeability (Ps) and reflection (sigma s) coefficients increased as external flow rate of medium increased, indicating some effects of external USLs. However, the effect was levelling off at 'usual' flow rates of 0.20-0.30 m s(-1) and in the presence of vigorous stirring by air bubbles, suggesting a maximum thickness of external USLs of around 30 microm including the cell wall. Because the diameters of internodes were around 1 mm, internal USLs could have played a significant or even a dominating role, at least in the presence of the rapidly permeating solutes used [acetone, 2-propanol and dimethylformamide (DMF)]. A comparison of calculated (diffusion kinetics) and of measured permeabilities indicated an upper limit of the contribution of USLs for the rapidly moving solute acetone of 29%, and of 15% for the less rapidly permeating DME The results throw some doubt on recent claims that in C. corallina, USLs rather than the cell membrane dominate solute uptake, at least for the most rapidly moving solute acetone.     

Zinc treatment of hydroponically grown barley plants causes a reduction in root and cell hydraulic conductivity and isoform‐dependent decrease in aquaporin gene expression

The cellular and molecular basis of a reduction in root water uptake in plants exposed to heavy metals such as zinc (Zn) is poorly studied. The aim of the present study on hydroponically grown barley (Hordeum vulgare) was to test whether any reduction in root hydraulic conductivity (Lp) in response to Zn treatment is accompanied by a reduction in cell Lp and gene expression level of aquaporin (AQP) isoforms. Plants were grown in the presence of 0.25 μM, (control), 0.1 and 1 mM Zn in the root medium and analysed when they were 16–18 days old. Root and cell Lp was determined through exudation and cell pressure probe analyses, respectively, and gene expression of five candidate AQPs was analysed [real time quantitative polymerase chain reaction (PCR)]. Zinc treatments caused significant reductions (25–83%) in transpiration rate, root and shoot fresh weight, surface area and stomatal conductance. Zinc concentrations in tissues increased more than 100‐fold. Root Lp decreased by 24% (0.1 mM Zn) and 58% (1 mM Zn), while cell Lp decreased by 45 and 71%, respectively. Gene expression of AQPs decreased by 14–80%; decreases were statistically significant for HvPIP1;3, HvPIP2;4 and HvPIP2;5. Turgor in root cortex cells was not reduced by Zn treatments. It is concluded that reductions in plant water flow in response to Zn treatment are facilitated through decreases in root (Lp) and shoot (stomata) hydraulics. The decrease in root Lp is facilitated through reductions in cell Lp and AQP gene expression and may also reflect increased suberization in the endodermis.     

环境条件对黄瓜硅吸收分配和果面蜡粉形成的影响

为探明环境条件影响黄瓜果面蜡粉形成的机制,以‘山农5号’黄瓜为接穗,‘黄诚根2号’南瓜(去蜡粉能力强)和‘云南黒籽南瓜’(去蜡粉能力弱)为砧木进行嫁接,在日光温室不同栽培茬口(冬春茬和秋冬茬)下研究了硅吸收分配和果面蜡粉量差异,并在人工气候室内模拟不同季节环境条件[T₁:温度28 ℃/18 ℃(昼/夜),相对湿度55%/65%,光照强度600 μmol·m^-2·s^-1;T₂:温度22 ℃/12 ℃(昼/夜),相对湿度85%/95%,光照强度300 μmol·m^-2·s^-1],研究其对硅吸收分配和硅转运蛋白基因表达的影响。结果表明:日光温室栽培条件下,与秋冬茬相比,冬春茬黄瓜商品成熟果实表面蜡粉量显著增加,其中以‘云南黑籽南瓜’嫁接黄瓜受影响较大,自根黄瓜和‘黄诚根2号’嫁接黄瓜受影响较小;相同栽培季节,均以‘云南黑籽南瓜’嫁接黄瓜果面蜡粉量和器官硅含量最多,自根黄瓜次之,‘黄诚根2号’嫁接黄瓜最少。人工气候室内,T₁环境下黄瓜各器官硅含量、叶片和根系硅转运蛋白基因表达量均高于T₂;相同环境条件下,黄瓜各器官硅含量和叶片硅转运蛋白基因表达量均为‘云南黑籽南瓜’嫁接黄瓜＞自根黄瓜＞‘黄诚根2号’嫁接黄瓜。综上,环境条件改变了黄瓜植株对硅的吸收分配,进而影响果面蜡粉形成,适宜的环境条件有利于减少果面蜡粉量;高温、强光、低湿环境导致黄瓜果面蜡粉量增多,砧木对嫁接黄瓜硅吸收和果面蜡粉形成有显著影响。