Gating of aquaporins by low temperature in roots of chilling-sensitive cucumber and chilling-tolerant figleaf gourd

Effects of low temperature (8 degrees C) on the hydraulic conductivity of young roots of a chilling-sensitive (cucumber, Cucumis sativus L.) and a chilling-resistant (figleaf gourd, Cucurbita ficifolia Bouche) crop have been measured at the levels of whole root systems (root hydraulic conductivity, Lp(r)) and of individual cortical cells (cell hydraulic conductivity, Lp). Exposure of roots to low temperature (LRT) for up to 6 d caused a stronger suberization of the endodermis in cucumber compared with figleaf gourd, but no development of exodermal Casparian bands in either species. Changes in anatomy after 6 d of LRT treatment corresponded with a reduction in hydrostatic root Lp(r) of cucumber roots by a factor of 24, and by a factor of 2 in figleaf gourd. In figleaf gourd, there was a reduction only in hydrostatic Lp(r) but not in osmotic Lp(r) suggesting that the activity of water channels was not much affected by LRT treatment in this species. Changes in cell Lp in response to chilling and recovery were similar to the root levels, although they were more intense at the root level. Activation energies (E(a)) and Q10 of water flow as measured at the cell level were high in cucumber (E(a)=109+/-13 kJ mol(-1); Q(10)=4.8+/-0.7; n=6-10 cells), but small in figleaf gourd (E(a)=11+/-2 kJ mol(-1); Q10=1.2+/-0.1; n=6-10 cells). Roots of figleaf gourd recovered better from LRT treatment than those of cucumber. In figleaf gourd, recovery (at both the root and cell level) often resulted in Lp and Lp(r) values which were even bigger than the original, i.e. there was an overshoot in hydraulic conductivity. These effects were larger for osmotic (representing the cell-to-cell passage of water) than for hydrostatic Lp(r). After a short-term (1 d) exposure to 8 degrees C followed by 1 d at 20 degrees C, hydrostatic Lp(r) of cucumber nearly recovered and that of figleaf gourd still remained higher due to the overshoot. By contrast, osmotic Lp(r) and cell Lp in both species remained high by a factor of 3 compared with the control, possibly due to an increased activity of water channels. After preconditioning of roots at LRT, increased hydraulic conductivity was completely inhibited by HgCl2 at both the root and cell levels. Different from figleaf gourd, recovery from chilling was not complete in cucumber after longer exposure to LRT. It is concluded that at LRT, both changes in the activity of aquaporins (AQPs) and alterations of root anatomy determine the water uptake in both species. The high temperature dependence of cell Lp in cucumber suggests conformational changes of AQPs during LRT treatment which result in channel closure and in a strong gating of AQP activity by low temperature. This mechanism is thought to be different from that in figleaf gourd where AQPs reacted in the conventional way, i.e. low temperature affected the mobility of water molecules in AQPs rather than their open/closed state, and Q(10) was low.     

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.     

Abscisic acid and hydraulic conductivity of maize roots: a study using cell- and root-pressure probes

Using root- and cell-pressure probes, the effects of the stress hormone abscisic acid (ABA) on the water-transport properties of maize roots (Zea mays L.) were examined in order to work out dose and time responses for root hydraulic conductivity. Abscisic acid applied at concentrations of 100–1,000 nM increased the hydraulic conductivity of excised maize roots both at the organ (root Lp_r: factor of 3–4) and the root cell level (cell Lp: factor of 7–27). Effects on the root cortical cells were more pronounced than at the organ level. From the results it was concluded that ABA acts at the plasmalemma, presumably by an interaction with water channels. Abscisic acid therefore facilitated the cell-to-cell component of transport of water across the root cylinder. Effects on cell Lp were transient and highly specific for the undissociated (+)-cis-trans-ABA. The stress hormone ABA facilitates water uptake into roots as soils start drying, especially under non-transpiring conditions, when the apoplastic path of water transport is largely excluded. Received: 26 February 2000 / Accepted: 17 August 2000     

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.     

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。     

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.     

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.     

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     

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.     

Effect of root anaerobiosis on the water relations of several Pyrus species

Solution culture experiments were designed to investigate the plant water relations of 3 Pyrus species subjected to root anaerobiosis. Root anaerobiosis induced partial stomatal closure prior to alterations in leaf water potential (ΨLW) or root osmotic potential (ΨRπ). In contrast, stomatal closure was accompanied by a decline in root hydraulic conductivity (Lp). Anoxia markedly reduced ΨLW for Pyrus communis L. and eventually led to wilting and defoliation. Pyrus betulaefolia Bunge and Pyrus calleryana Decne, however, were less affected by root anaerobiosis. To delineate if the increased root resistance was in the radial or longitudinal direction, 10⁻⁴ M cistrans abscisic acid (ABA) was added to detopped root systems of P. communis in solution culture after steady-state rates of Lp were established. A consistent 25 to 30% promotion of Lp was observed 1.5 h after the addition of ABA for aerobically treated plants. ABA did not influence Lp when applied to roots previously deprived of O₂ for 4 days. Additional evidence against the limiting resistance being in the radial direction was obtained when water fluxes were compared through intact P. communis roots, roots with all feeder roots detached, and stems without root systems. Severing feeder roots from anaerobically treated plants did not increase water flux to rates observed for aerobically treated plants. Resistance progressed basipetally to eventually encompass the stem itself. These results can only be explained by occlusion of the xylem vessels.     

Water Relations of Leaf Epidermal Cells of Tradescantia virginiana

Water-relation parameters (cell turgor pressure [P], volumetric elastic modulus [epsilon] and hydraulic conductivity [Lp]) of individual leaf epidermal cells of Tradescantia virginiana have been determined with the pressure-probe technique. Turgor was 4.5 +/- 2.1 [41] bar (mean +/- sd; in brackets the number of cells) and ranged from 0.9 to 9.6 bar. By vacuum infiltration with nutrient solution, it was raised to 7.5 +/- 1.5 [5] bar (range: 5.3-8.8 bar). There was a large variability in the absolute value of epsilon of individual cells. epsilon ranged from 40 to 360 bar; mean +/- sd: 135 +/- 83 bar; n = 50 cells. epsilon values of individual cells seemed to be rather independent of changes in cell turgor. A critical assessment of the errors incurred in determining epsilon by the technique is included. The half-times of water exchange of individual cells ranged from 1 to 35 seconds, which gave values of 0.2 to 11 x 10(-6) centimeters per second per bar for Lp (mean +/- sd: 3.1 +/- 2.3 x 10(-6) centimeters per second per bar; n = 39 cells). The large range in Lp and epsilon is believed to be due to the difficulties in determining the effective surface area of water exchange of the cells. Lp is not influenced by active salt pumping driven by respiration energy inasmuch as it was not altered by 0.1 millimolar KCN. The temperature dependence of Lp (T((1/2))) was measured for the first time in individual higher-plant cells. Lp increased by a factor of 2 to 4, when the temperature was increased by 10 C. The activation energy of water exchange was found to be between 50 and 186 kilojoules per mole. Within the large range of variation it was found that T((1/2)), Lp, and epsilon did not change under various experimental conditions (intact and excised tissue, water content and turgidity, age, etc.). Similar results were obtained for the epidermal cells of Tradescantia andersoniana. The measurements suggest that the entire epidermis would respond very rapidly (i.e. with a half-time of 1 to 30 s) to a demand for water from the stomata.     

Effect of abscisic acid on exudation of sunflower roots as affected by nutrient status, glucose level and aeration

Three-week-old sunflower plants ( Helianthus annuus L. cv. Halcón) grown in nutrient solution at two K⁺ levels (0.3 and 2.5 m M ) were used to study the effect of 4 μ M abscisic acid (ABA) on the transport of K⁺ (Rb⁺) and water to the exuding stream of decapitated plants. Other conditions of the bathing medium of the roots were also assayed, such as presence of 10 m M glucose, aeration and time of ABA application. In the first 2 or 3 h after ABA application, ABA always promoted water and ion fluxes, even under the most unfavorable conditions such as low K⁺ roots without glucose or under anaerobiosis. The ABA-promoting effect on ion and water flow was higher with glucose in the medium. Under anaerobiosis the ABA effect disappeared after 3 h. With glucose and aeration the ABA-promoting effect appeared early and continued for several hours, although the effect decreased with time. If ABA was applied 24 h before excision, the effect was small or even negative. We suggest that ABA acts directly on membranes of certain root cells (endodermal or both endodermal and cortical cells) by increasing their permeability and thus releasing ions. This will decrease cell turgor pressure and, indirectly, the hydraulic conductivity of the whole root. Under conditions of higher hydraulic conductivity, the presence of ions and glucose in the root stimulates the transport of ions into the xylem. and thus increases the osmotic water flow.     

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.     

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.     

Light and turgor affect the water permeability (aquaporins) of parenchyma cells in the midrib of leaves of Zea mays

In response to light, water relation parameters (turgor, half-time of water exchange, T(1/2), and hydraulic conductivity, Lp; T(1/2) proportional 1/Lp) of individual cells of parenchyma sitting in the midrib of leaves of intact corn (Zea mays L.) plants were investigated using a cell pressure probe. Parenchyma cells were used as model cells for the leaf mesophyll, because they are close to photosynthetically active cells at the abaxial surface, and there are stomata at both the adaxial and abaxial sides. Turgor ranged from 0.2 to 1.0 MPa under laboratory light condition (40 micromol m(-2) s(-1) at the tissue level), and individual cells could be measured for up to 6 h avoiding the variability between cells. In accordance with earlier findings, there was a big variability in T(1/2)s measured ranging from 0.5 s to 100 s, but the action of light on T(1/2)s could nevertheless be worked out for cells having T(1/2)s greater than 2 s. Increasing light intensity ranging from 100 micromol m(-2) s(-1) to 650 micromol m(-2) s(-1) decreased T(1/2) by a factor up to five within 10 min and increased Lp (and aquaporin activity) by the same factor. In the presence of light, turgor decreased due to an increase in transpiration, and this tended to compensate or even overcompensated for the effect of light on T(1/2). For example, during prolonged illumination, cell turgor dropped from 0.2 to 1.0 MPa to -0.03 to 0.4 MPa, and this drop caused an increase of T(1/2) and a reduction of cell Lp, i.e. there was an effect of turgor on cell Lp besides that of light. To separate the two effects, cell turgor (water potential) was kept constant while changing light intensity by applying gas pressure to the roots using a pressure chamber. At a light intensity of 160 micromol m(-2) s(-1), there was a reduction of T(1/2) by a factor of 2.5 after 10-30 min, when turgor was constant within +/-0.05 MPa. Overall, the effects of light on T(1/2) (Lp) were overriding those of turgor only when decreases in turgor were less than about 0.2 MPa. Otherwise, turgor became the dominant factor. The results indicate that the hydraulic conductivity increased with increasing light intensity tending to improve the water status of the shoot. However, when transpiration induced by light tends to cause a low turgidity of the tissue, cell Lp was reduced. It is concluded that, when measuring the overall hydraulic conductivity of leaves, both the effects of light and turgor should be considered. Although the mechanism(s) of how light and turgor influence the cell Lp is still missing, it most likely involves the gating of aquaporins by both parameters.     

Lateral ABA transport in maize roots (Zea mays): visualization by immunolocalization

The intensity of an ABA (abscisic acid) signal as a root-to-shoot signal, as well as its action on root hydraulic conductivity, strongly depends on the distribution of ABA during its radial transport across roots. Therefore ABA was visualized by immunolocalization with monoclonal ABA antibodies under conditions of lateral water flow induced by the application of a pressure gradient to the cut surface of the mesocotyl of maize seedlings. From the labelling of rhizodermis, hypodermis, cortical cells, and endodermis of roots of hydroponically (no exodermis) and aeroponically (with exodermis) grown seedlings it is concluded that the exodermis acts as a barrier to apoplastic transport that controls ABA uptake and efflux, but that the endodermis can easily be overcome via an apoplastic bypass. In longitudinal sections the strongest ABA signals originated from the root cap and the meristematic root tip, which is in agreement with the non-vacuolated cells of these tissues being an effective anion trap for ABA.     

Water uptake and hydraulic properties of elongating cells in hydrotropically bending roots of Pisum sativum L

The water potential and hydraulic conductivity (Lp) of elongating cells in hydrotropically bending roots of the ageotropic mutant ageotropum of pea (Pisum sativum L.) were measured in situ. When agar blocks with water potentials of -0.03 and -0.8 MPa were unilaterally applied directly to a root tip, cells in the most rapidly elongating zone, 3-4 mm from the tip, showed marked differential growth. The rate of water uptake by a cell on the side treated with an agar block with a lower water potential was significantly larger in the outer first and second layers of cortex than on the other side. There were no differences in the values of turgor pressure, osmotic potential and calculated water potential between the two sides either in elongating or in mature cells, indicating the absence of any difference in the growth-induced water potential on the two sides of the root. Lp was significantly larger on the side with the agar block with lower water potential. The results suggest that the difference in the rate of water uptake during the differential cell growth that occurs during root hydrotropism might be induced mainly by a change in Lp.     

Morphology of carrot somatic embryos formed in medium with abscisic acid

Somatic embryogenesis was induced from embryogenie cells derived from cotyledon expiants cultured on MS medium supplemented with 1 mg/L 2,4-D. In order to clarify the effect of abscisic acid (ABA) on the morphology of somatic embryos, embryogénie cell clumps or developing somatic embryos were treated continuously, or briefly, with ABA during culture. When embryogenie cells in MS medium without 2,4-D were treated with 0.04 mg/L ABA for the first week, normal embryos with two cotyledons increased slightly and embryos with anomalous cotyledons decreased. However when cell clumps in 2,4-D-free medium were treated with ABA in the second week normal embryos with two cotyledons decreased prominently and this decrease of normal embryos also occurred in the continuous ABA treatment during culture. Thus the morphological abnormalities in somatic embryogenesis occurred by exogenous ABA treatment beyond globular stage or by continuous treatment. The length of somatic embryos with anomalous cotyledons was larger than that of normal embryos with two cotyledons in control but both the normal and anomalous somatic embryos treated with ABA were almost similar in length. Somatic embryos formed in medium with ABA were larger in size than those in control due mainly to enlarged cotyledons. The enlarged cotyledons were composed of a greater number of cells than those of control. Therefore the enlargement of cotyledon by exogenous ABA seems to be not due to the enlargement of cells in cotyledons.     

Phloem transport of abscisic acid in Ricinus communis L. seedlings

Sieve tube sap exuded from the cut hypocotyl of castor bean seedlings (Ricinus communis L.) was found to contain 0.2–0.5 mmol m^?3abscisic acid (ABA). The ABA concentration in the sieve tube sap always exceeded that in root pressure exudate under a wide range of water supply. Exudation of sieve tube sap from the cut hypocotyls caused water loss, and this induced ‘water shortage’ in the cotyledons which resulted in the ABA concentration in the cotyledons increasing by 3-fold and that in the sieve tube sap increasing by up to 50-fold within 7h. The wounded surface of the cut hypocotyl was not responsible for the ABA increase. Incubation of the cotyledons of endosperm-free seedlings in various ABA concentrations (up to 100 mmol m^?3) increased the ABA concentration in sieve tube sap. The concomitant increase in ABA, both in cotyledons and in sieve tube sap, had no effect on the phloem loading of sucrose, K⁺ and Mg²⁺ within the experimental period, i.e. up to 10h. It can be concluded that (i) the phloem is an important transport path for ABA, (ii) water stress at the phloem loading sites elevates phloem-mobile ABA, which may then serve as a water stress signal for sinks, for example stem and roots (not only for stomata), and (iii) the ABA concentration of cells next to or in the phloem is more important than the average ABA content in the whole cotyledon for determining the ABA concentration in sieve tube sap.