Xylem Sap pH Increase: A Drought Signal Received at the Apoplastic Face of the Guard Cell That Involves the Suppression of Saturable Abscisic Acid Uptake by the Epidermal Symplast

Drought increased the pH of Commelina communis xylem sap from 6.1 to 6.7. Conductances of transpiring leaves were 50% lower in pH 7.0 than in pH 6.0 buffers, but bulk leaf abscisic acid (ABA) concentration and shoot water status were unaffected by pH. Stomatal apertures of isolated abaxial epidermis incubated on simple buffers increased with external pH, so in vivo this must be overridden by alternative pH effects. Reductions in leaf transpiration rate at pH 7.0 were dependent on the presence of 10-8 mol dm-3 ABA in the xylem stream. We inferred that at pH 7.0 leaf apoplastic ABA concentrations increased: pH did not affect distributions of ABA among leaf tissues, but isolated epidermis and mesophyll tissue took up more 3H-ABA from pH 6.0 than from pH 7.0 buffers. The apoplastic ABA increase at pH 7.0 may result from reduced symplastic sequestration. A portion of 3H-ABA uptake by the epidermis was saturable at pH 6.0 but not at pH 7.0. An ABA uptake carrier may contribute to ABA sequestration by the leaf symplast of well-watered plants, and its inactivity at pH 7.0 may favor apoplastic ABA accumulation in draughted plants. Effects of external pH on stomatal apertures in the isolated epidermis indicate that published data supporting a role for internal guard cell ABA receptors should be reassessed.     

Apoplastic pH and Fe3+ Reduction in Intact Sunflower Leaves

It has been hypothesized that under NO₃⁻ nutrition a high apoplastic pH in leaves depresses Fe³⁺ reductase activity and thus the subsequent Fe²⁺ transport across the plasmalemma, inducing Fe chlorosis. The apoplastic pH in young green leaves of sunflower (Helianthus annuus L.) was measured by fluorescence ratio after xylem sap infiltration. It was shown that NO₃⁻ nutrition significantly increased apoplastic pH at distinct interveinal sites (pH ≥ 6.3) and was confined to about 10% of the whole interveinal leaf apoplast. These apoplastic pH increases presumably derive from NO₃⁻/proton cotransport and are supposed to be related to growing cells of a young leaf; they were not found in the case of sole NH₄⁺ or NH₄NO₃ nutrition. Complementary to pH measurements, the formation of Fe²⁺-ferrozine from Fe³⁺-citrate was monitored in the xylem apoplast of intact leaves in the presence of buffers at different xylem apoplastic pH by means of image analysis. This analysis revealed that Fe³⁺ reduction increased with decreasing apoplastic pH, with the highest rates at around pH 5.0. In analogy to the monitoring of Fe³⁺ reduction in the leaf xylem, we suggest that under alkaline nutritional conditions at interveinal microsites of increased apoplastic pH, Fe³⁺ reduction is depressed, inducing leaf chlorosis. The apoplastic pH in the xylem vessels remained low in the still-green veins of leaves with intercostal chlorosis.     

Effects of iron deficiency on the composition of the leaf apoplastic fluid and xylem sap in sugar beet. Implications for iron and carbon transport

The effects of iron deficiency on the composition of the xylem sap and leaf apoplastic fluid have been characterized in sugar beet (Beta vulgaris Monohil hybrid). pH was estimated from direct measurements in apoplastic fluid and xylem sap obtained by centrifugation and by fluorescence of leaves incubated with 5-carboxyfluorescein and fluorescein isothiocyanate-dextran. Iron deficiency caused a slight decrease in the pH of the leaf apoplast (from 6.3 down to 5.9) and xylem sap (from 6.0 down to 5.7) of sugar beet. Major organic acids found in leaf apoplastic fluid and xylem sap were malate and citrate. Total organic acid concentration in control plants was 4.3 mM in apoplastic fluid and 9.4 mM in xylem sap and increased to 12.2 and 50.4 mM, respectively, in iron-deficient plants. Inorganic cation and anion concentrations also changed with iron deficiency both in apoplastic fluid and xylem sap. Iron decreased with iron deficiency from 5.5 to 2.5 microM in apoplastic fluid and xylem sap. Major predicted iron species in both compartments were [FeCitOH](-1) in the controls and [FeCit(2)](-3) in the iron-deficient plants. Data suggest the existence of an influx of organic acids from the roots to the leaves via xylem, probably associated to an anaplerotic carbon dioxide fixation by roots.     

Light-induced pH and K+ changes in the apoplast of intact leaves

The K⁺-sensitive fluorescent dye benzofuran isophthalate (PBFI) and the pH-sensitive fluorescein isothiocyanate dextran (FITC-Dextran) were used to investigate the influence of light/dark transitions on apoplastic pH and K⁺ concentration in intact leaves of Vicia faba L. with fluorescence ratio imaging microscopy. Illumination by red light led to an acidification in the leaf apoplast due to light-induced H⁺ extrusion. Similar apoplastic pH responses were found on adaxial and abaxial sides of leaves after light/dark transition. Stomatal opening resulted only in a slight pH decrease (0.2 units) in the leaf apoplast. Gradients of apoplastic pH exist in the leaf apoplast, being about 0.5–1.0 units lower in the center of the xylem veins as compared with surrounding cells. The apoplastic K⁺ concentration in intact leaves declined during the light period. A steeper light-induced decrease in apoplastic K⁺, possibly caused by higher apoplastic K⁺, was found on the abaxial side of leaves concentration. Simultaneous measurements of apoplastic pH and K⁺ demonstrated that a light-induced decline in apoplastic K⁺ concentration indicative of net K⁺ uptake into leaf cells occurs independent of apoplastic pH changes. It is suggested that the driving force that is generated by H⁺ extrusion into the leaf apoplast due to H⁺-ATPase activity is sufficient for passive K⁺ influx into the leaf cells. Received: 7 March 2000 / Accepted: 12 May 2000     

Apoplastic pH and Fe(3+) reduction in intact sunflower leaves

It has been hypothesized that under NO(3)(-) nutrition a high apoplastic pH in leaves depresses Fe(3+) reductase activity and thus the subsequent Fe(2+) transport across the plasmalemma, inducing Fe chlorosis. The apoplastic pH in young green leaves of sunflower (Helianthus annuus L.) was measured by fluorescence ratio after xylem sap infiltration. It was shown that NO(3)(-) nutrition significantly increased apoplastic pH at distinct interveinal sites (pH >/= 6.3) and was confined to about 10% of the whole interveinal leaf apoplast. These apoplastic pH increases presumably derive from NO(3)(-)/proton cotransport and are supposed to be related to growing cells of a young leaf; they were not found in the case of sole NH(4)(+) or NH(4)NO(3) nutrition. Complementary to pH measurements, the formation of Fe(2+)-ferrozine from Fe(3+)-citrate was monitored in the xylem apoplast of intact leaves in the presence of buffers at different xylem apoplastic pH by means of image analysis. This analysis revealed that Fe(3+) reduction increased with decreasing apoplastic pH, with the highest rates at around pH 5. 0. In analogy to the monitoring of Fe(3+) reduction in the leaf xylem, we suggest that under alkaline nutritional conditions at interveinal microsites of increased apoplastic pH, Fe(3+) reduction is depressed, inducing leaf chlorosis. The apoplastic pH in the xylem vessels remained low in the still-green veins of leaves with intercostal chlorosis.     

Stress-dependent redistribution of abscisic acid (ABA) in Hordeum vulgare L leaves: the role of epidermal ABA metabolism, tonoplastic transport and the cuticle

When ¹⁴C-labelled abscisic acid ([¹⁴C]ABA) was supplied to isolated protoplasts of the barley leaf at pH 6, initial rates of metabolism were about five times higher in epidermal cell protoplasts than in mesophyll cell protoplasts if equal cytosolic volumes were considered. In spite of the fact that epidermal cells make up only about 35% of the total water space in barley leaves, and despite the small cytosolic volume of these cells, in intact leaves all epidermal cells would thus metabolize half as much ABA per unit time as the mesophyll cells (0–27 and 0–51 mmol h^?1 m^?3 leaf water). Therefore, under these conditions epidermal cells seem to be a stronger sink than mesophyll cells for ABA that arrives via the transpiration stream. However, at an apoplastic pH of 7–25, which occurs in stressed leaves, the proportion of total metabolized ABA would be much smaller in epidermal than in mesophyll cells (0–029 and 0–204 mmolh^?l m^?3 leaf water). Our results indicate that under conditions of slightly alkaline apoplastic pH the epidermis may serve as the main source for fast stress-dependent ABA redistribution into the guard cell apoplast. This is partly the result of ABA transport across the epidermal tonoplast, which is dependent on the apoplastic pH and possibly on the cytosolic calcium concentration. The cuticle seems to be of no particular importance in stress-induced apoplastic ABA shifts and cannot be regarded as a significant sink for high ABA concentrations under stress.     

Apoplastic Sugar Concentration and pH in Barley Leaves Infected with Brown Rust

Soluble sugars were extracted by low speed centrifugation fromthe apoplast of leaves of barley (Hordeum distichum L.) infiltratedwith water. Infection of the leaf with the brown rust fungus(Puccinia hordeii) resulted in a reduction in the concentrationof sucrose, glucose and fructose in the apoplast. Sugars werepresent in an apoplastic space occupying 12 and 17 cm³ m^–2of leaf area in healthy and infected tissue, respectively. Uptakeof hexoses by intercellular hyphae is suggested as a cause ofthis reduction. The pH of apoplastic sap extracted from rust-infectedleaves was increased to pH 7·3 from pH 6·6 incontrols. The effect of a reduced apoplastic sugar pool andincreased pH on export from infected leaves is discussed. Key words: Apoplast, barley (Hordeum distichum L.), brown rust (Puccinia hordeii Otth.), pH, sucrose, hexose     

Apoplastic and symplastic pathways of atrazine and glyphosate transport in shoots of seedling sunflower

[¹⁴C]Atrazine (2-chloro-4-[ethylamino]-6-[isopropylamino]-s-triazine) and [¹⁴C]glyphosate (N-[phosphonomethyl]glycine) were xylem fed to sunflower shoots at 100 micromolar for 1 hour in the light, then placed in the dark at 100% relative humidity for 1, 4, 7, or 10 hours. The distribution of atrazine and glyphosate between shoot parts, in the leaves, and between the aoplast and symplast of the leaf was determined. The apoplastic concentrations and distribution patterns of atrazine and glyphosate in the leaves were evaluated using a pressure dehydration technique, our results were compared to the previously reported distribution patterns of the naturally occurring apoplastic leaf solutes, and the apoplastic dye PTS (trisodium 3-hydroxy-5,8,10-pyrenetrisulfonate). The pattern of atrazine and glyphosate distribution in the shoot, and between the leaf apoplast and symplast, was found to reflect the potential of these herbicides to enter the shoot symplast. The results of this study are discussed with respect to current theories of xenobiotic transport in plants, and have been found to be consistent with the intermediate permeability hypothesis for xenobiotic transport.     

Root and bacterial secretions regulate the interaction between plants and PGPR leading to distinct plant growth promotion effects

Background and aims

Long distance signals in xylem from roots to leaves are important in plant response to drought stress. Abscisic acid (ABA) plays a key role in drought signaling in plants but apoplastic pH may modulate its effect by distributing ABA into various compartments in leaves. We aimed to reveal the dynamics of changes in sap pH and its relationships with the transport of inorganic and organic ions in eight herbaceous plant species under continuously declining soil water content. We tested several hypotheses related to the mechanism of pH changes in xylem.

Methods

We used a pressure chamber to collect xylem sap and to measure of leaf/stem water potential at various stages of soil drying. We measured pH and concentrations of the most abundant inorganic (NO₃ ^?, SO₄ ^2?, PO₄ ^3? and Cl^?) and organic (malate and citrate) anions in xylem sap.

Results

Species differed considerably in the dynamics of pH changes in xylem in drying soil. Changes in xylem sap pH during drying did not relate to the nitrogen assimilation strategy but may be affected by sap flow rate. Simultaneous changes in the concentrations of inorganic and organic anions were highly species-specific.

Conclusions

High variability among species in the observed relationships in response to drought indicates that comparisons among different studies and the generalization of results should be made with caution.

     

Apoplastic pH and growth in expanding leaves of Vicia faba under salinity

Salinity affects water availability in the soil and subsequently the plant uptake capacity. Upon exposure to salt stress, leaf growth in monocot plants has been shown to be reduced instantaneously, followed by a gradual acclimation. The growth reactions are caused by an initial water deficit and an accompanied osmotic effect, followed by an IAA-induced sequestration of protons into the apoplast that increases leaf growth again as explained by the acid growth theory. In this study, we investigated the dynamics of growth reactions and apoplastic pH in leaves of the dicot Vicia faba in the presence of NaCl during the initiation of salt stress. Concurrent changes in apoplastic pH were detected by ratiometric fluorescence microscopy using the fluorescent dye fluorescein tetramethylrhodamine dextran. To elucidate the possible relation between the dynamics of leaf growth and apoplastic pH, results of the ratio imaging technique were combined with an in vivo growth analysis imaging approach. Leaf growth rate of V. faba was highest in the dusk and the early night phase; at this time a concomitant decrease of the apoplastic pH was observed. Under salinity, the apoplastic pH in leaves of V. faba increased with a simultaneous decrease of leaf growth towards increasing developmental stages, but with complex aberrations in the 24-h-leaf-growth pattern compared to control leaves. In conclusion, these results show that salt stress leads to an increase in apoplastic pH and to a declined leaf growth activity with complex 24-h-interactions of growth and pH in V. faba.     

Abscisic acid in apoplastic sap can account for the restriction in leaf conductance of white lupins during moderate soil drying and after rewatering

We studied the effects of drought on leaf conductance (g) and on the concentration of abscisic acid (ABA) in the apoplastic sap of Lupinus albus L. leaves. Withholding watering for 5d resulted in complete stomatal closure and in severe leaf water deficit. Leaf water potential fully recovered immediately after rewatering, but the aftereffect of drought on stomata persisted for 2d. ABA and sucrose were quantified in pressurized leaf xylem extrudates. We assumed that the xylem sucrose concentration is negligible and hence that the presence of sucrose in leaf extrudates indicated that they were contaminated by phloem. To eliminate this interference, the concentration of ABA in leaf apoplast was estimated by extrapolation to zero sucrose concentration, using the regression between ABA and sucrose concentrations. The estimated apoplastic ABA concentration increased by 100-fold with soil drying and did not return to pre-stress values immediately following rewatering. g was closely related to the concentration of ABA in leaf apoplast. Furthermore, the feeding of exogenous ABA to leaves detached from well-watered plants brought about the same degree of depression in g as resulted from the drought-induced increase in ABA concentration. We therefore conclude that the observed changes in the concentration of ABA in leaf apoplast were quantitatively adequate to explain drought-induced stomatal closure and the delay in stomatal reopening following rewatering.     

Apoplastic pH of intact leaves of Vicia faba as influenced by light

The fluorochrome FITC-dextran was used to measure the effectof light on the apoplastic pH of intact Vicia faba leaves withthe ratio imaging technique. In darkadapted leaves the apoplasticpH varied depending on the leaf between 5.2 and 5.9. Red light(660 nm, 4–12 W m^–2) leads to multiphasic responses:in the first seconds an alkalinization ({small tilde}0.3 pHunits), and thereafter an acidification of the leaf apoplast({small tilde}0.4 pH units) were observed. Both effects couldbe inhibited by DCMU. While variation of CO₂ concentration revealedno effect on light-induced apoplastic pH changes, a decreasein O₂ concentration decreased the effect. On the basis of ourdata it is suggested that the influence of photosynthesis onplasmalemma H⁺ ATPase is responsible for the observed effects,rather than altered CO₂ uptake. Key words: Leaf apoplast, apoplastic pH, light, ratio imaging, pH-sensitive fluorescent dye, Vicia fab     

Regulation of Apoplastic pH in Source Leaves of Vicia faba by Gibberellic Acid

Leaf discs of broad bean (Vicia faba L.), peeled on the spongy mesophyll side, rapidly altered the pH of the surrounding medium (apoplast). Using pH indicator paper appressed against the leaf, immediately after peeling, initial apoplastic pH was estimated to be 4.5. Changes in the apoplastic pH were measured with a microelectrode placed into a 100-microliter drop of an unbuffered solution (2 millimolar KCl, 0.5 millimolar CaCl₂, and 200 millimolar mannitol) on the peeled surface. Discs acidified the medium until the pH stabilized at about 5.0 (about 10 minutes). Acidification was inhibited by 50 micromolar sodium vanadate, an inhibitor of the plasmalemma H⁺-ATPase and attenuated by omitting the osmoticum or potassium ions from the medium. Fusicoccin (10 micromolar) greatly enhanced the rate of acidification. The presence of 0.1 to 1 micromolar gibberellic acid resulted in a slower rate of medium acidification. Gibberellic acid appeared to modulate the activity of the H⁺-translocating ATPase located at the plasma membrane of the mesophyll cells.     

Apoplastic Solute Concentrations of Organic Acids and Mineral Nutrients in the Leaves of Several Fagaceae

Ion chromatographic methods determined organic acids and mainnutrient minerals in the apoplastic solution from leaves ofseveral Fagaceae (Quercus ilex L., Quercus cerris L., Quercusvirgiliana (Ten.) Ten, and Fagus sylvatica L.). The anions oforganic acids found in high amounts (250 to 650 µM) werequinate, malate, and oxalate. Lactate, pyruvate, formate andacetate were detected in relatively low amounts with concentrationsbetween 20 and 200 µM. The total concentration of organicacids in the apoplastic sap ranged between 1.5 and 2 mM. Thetotal concentration of inorganic cations (K⁺, Mg²⁺, NH₄⁺, Ca²⁺,Na⁺) and anions (C1^–, NO₃^–, SO^2–₄ and PO^3–₄)in the apoplastic sap varied between 5 and 10 mM, and 0.35 and1.8 mM, respectively. We conclude that the concentration oforganic acid ions in the leaf apoplast depends mainly on theexchange with the leaf cells and is influenced by the electrochemicalgradient between the symplast and the apoplast in relation tothe water potential of the leaf. The determination of formateand acetate in the apoplastic compartment of leaves lend weightto the argument that the production of these acids by treesis a important emission source to the atmosphere. (Received June 9, 1998; Accepted April 8, 1999)     

A critical experimental evaluation of methods for determination of NH4+ in plant tissue, xylem sap and apoplastic fluid

Ammonium (NH₄⁺) is a central intermediate in the N metabolism of plants, but the quantitative importance of NH₄⁺ in transporting N from root to shoot and the capability of plants to store NH₄⁺ in leaves are still matters of substantial controversy. This paper shows that some of these controversies have to be related to the use of inadequate analytical procedures used for extraction and quantification of NH₄⁺ in plants. The most frequently used methods for determination of NH₄⁺, viz. colorimetric methods based on the classical Berthelot reaction, suffered severely from interference caused by amino acids, amines, amides and proteins. For some of these metabolites the interference was positive, while for others it was negative, making correction impossible. Consequently, colorimetric analysis is inapplicable for determination of NH₄⁺ in plants. Results obtained by ion chromatography may overestimate the NH₄⁺ concentration due to co‐elution of NH₄⁺ with amines like methylamine, ethylamine, ethanolamine and the non‐protein amino acid Γ‐aminobutyric acid. Derivatization of NH₄⁺ with o‐phthalaldehyde at alkaline pH and subsequent quantification of NH₄⁺ by fluorescence spectroscopy was also associated with interference. However, when pH was lowered to 6.8 during derivatization and 2‐mercaptoethanol was used as reductant, NH₄⁺ could be determined with a high selectivity and sensitivity down to a detection limit of 3.3 μM in a 10‐μl sample volume. Derivatization was performed on‐line using a column‐less HPLC system, enabling rapid quantification of NH₄⁺ in a few minutes. Flow injection analysis with on‐line gas dialysis was, likewise, free from interference, except when applied on highly senescent plant material containing volatile amines. Labile N metabolites in leaf tissue extract, xylem sap and apoplastic fluid were degraded to NH₄⁺ during extraction and subsequent instrumental analysis if the samples were not stabilised. A simple and efficient stabilisation could be obtained by addition of 10 mM ice‐cold HCOOH to the plant extraction medium or to the samples of apoplastic fluid or xylem sap. We conclude that significant concentrations of NH₄⁺, exceeding 1 mM, may occur in xylem sap, leaf apoplastic fluid and leaf tissue water of nitrate‐grown tomato and oilseed rape plants. The measured NH₄⁺ concentrations were not a result of excessive N supplies, as even plants grown under mildly N‐deficient conditions contained NH₄⁺.     

Abscisic acid introduced into the transpiration stream accumulates in the guard-cell apoplast and causes stomatal closure

Petioles of water‐sufficient intact Vicia faba L. plants were infused with 1 µm abscisic acid (ABA) to simulate the import of root‐source ABA. This protocol permitted quantitative ABA delivery, up to 300 pmol ABA over 60 min, to the leaf without ambiguities associated with perturbations in plant–water status. The ABA concentrations in whole‐leaf samples and in apoplastic sap increased with the amount infused; ABA degradation was not detected. The ABA concentration in apoplastic sap was consistent with uptake of imported ABA into the leaf symplast, but this interpretation is qualified. Our focus was quantitative cellular compartmentation of imported ABA in guard cells. Unlike when leaves are stressed, the guard‐cell symplast ABA content did not increase because of ABA infusion (P = 0·48; 3·0 ± 0·5 versus 4·0 ± 1·2 fg guard‐cell‐pair^?1). However, the guard‐cell apoplast ABA content increased linearly (R² = 0·98) from ?0·2 ± 0·5 to 3·1 ± 1·3 fg guard‐cell‐pair^?1 (≈ 3·1 µm ) and was inversely related to leaf conductance (R² = 0·82). Apparently, xylem ABA accumulates in the guard‐cell wall as a result of evaporation of the apoplast solution. This mechanism provides for integrating transpiration rate and ABA concentration in the xylem solution.     

Movement of Abscisic Acid into the Apoplast in Response to Water Stress in Xanthium strumarium L

The effect of water stress on the redistribution of abcisic acid (ABA) in mature leaves of Xanthium strumarium L. was investigated using a pressure dehydration technique. In both turgid and stressed leaves, the ABA in the xylem exudate, the `apoplastic' ABA, increased before `bulk leaf' stress-induced ABA accumulation began. In the initially turgid leaves, the ABA level remained constant in both the apoplast and the leaf as a whole until wilting symptoms appeared. Following turgor loss, sufficient quantities of ABA moved into the apoplast to stimulate stomatal closure. Thus, the initial increase of apoplastic ABA may be relevant to the rapid stomatal closure seen in stressed leaves before their bulk leaf ABA levels rise.

Following recovery from water stress, elevated levels of ABA remained in the apoplast after the bulk leaf contents had returned to their prestress values. This apoplastic ABA may retard stomatal reopening during the initial recovery period.

     

Regulation of Water Deficit-Induced Abscisic Acid Accumulation by Apoplastic Ascorbic Acid in Maize Seedlings

Water deficit-induced abscisic acid （ABA） accumulation is one of the most important stress signaling pathways in plant cells. Redox regulation of cellular signaling has currently attracted particular attention, but much less is known about its roles and mechanisms in plant signaling. Herein, we report that water deficit-induced ABA accumulation could be regulated by ascorbic acid （AA）-controlled redox status in leave apoplast. The AA content in non-stressed leaves was approximately 3 umol/g FW, corresponding to a mean concentration of 3 mmol/L in a whole cell. Because AA is mainly localized in the cytosol and chloroplasts, the volume of which is much smaller than that of the whole cell, AA content in cytosolic and chloroplast compartments should be much higher than 3 mmol/L. Water deficit-induced ABA accumulation in both leaf and root tissues of maize seedlings was significantly inhibited by AA and reduced glutathione （GSH） at concentrations of 500 umol/L and was completely blocked by 50 mmol/L AA and GSH. These results suggest that the AA-induced inhibition of ABA accumulation should not occur at sites where AA exists in high concentrations. Although water deficit led to a small increase in the dehydroascorbic acid （DHA） content, no significant changes in AA content were observed in either leaf or root tissues. When compared with the whole leaf cell, the AA content in the apoplastic compartment was much lower （i.e. approximately 70 nmol/g FW, corresponding to 0.7 mmol/L）. Water deficit induced a significant decrease （approximately 2.5-fold） in the AA content and an increase （approximately 3.4-fold） in the DHA content in the apoplastic compartment, thus leading to a considerably decreased redox status there, which may have contributed to the relief of AA-induced inhibition of ABA accumulation, alternatively, promoting water deficit-induced ABA accumulation. Reactive oxygen species （ROS） could not mimic water deficit in inducing ABA accumulation, suggesting that the inhibition of ABA accumulation by AA or GSH was not related to their ROS-scavenging ability. The results of the present study suggest that the redox status in the apoplastic compartment, as determined by AA and DHA, may play a vital role in the regulation of the signaling process for water deficit-induced ABA accumulation.     

Relative importance of osmotic-stress and ion-specific effects on ABA-mediated inhibition of leaf expansion growth in Phaseolus vulgaris

The osmotic and ion-specific components of salt-induced inhibition of leaf expansion growth were investigated in beans grown from 12 h to several days in either NaCl-containing solution cultures, an isosmotic concentrated macronutrient solution, or a vermiculite–compost mixture with low Na⁺ but high Cl^– availability. Inhibition of leaf expansion and leaf ABA increase was more intense in the NaCl than in the isosmotic macronutrient treatment. Root Na⁺ was highly correlated to inhibition of leaf expansion and leaf or xylem sap ABA. When Na⁺ was sequestered in soil, salinized plants showed no reduction in leaf expansion or ABA increase, regardless of the presence of high leaf Cl^– concentrations. Stomatal conductance exhibited an exponential relationship with the reciprocal value of xylem sap ABA. Our results indicate that an ion-specific effect caused by Na⁺ in roots may account for an ABA-mediated reponse of both stomatal closure and leaf expansion inhibition.     

Ion Relations of Symplastic and Apoplastic Space in Leaves from Spinacia oleracea L. and Pisum sativum L. under Salinity

Salt tolerant spinach (Spinacia oleracea) and salt sensitive pea (Pisum sativum) plants were exposed to mild salinity under identical growth conditions. In order to compare the ability of the two species for extra- and intracellular solute compartmentation in leaves, various solutes were determined in intercellular washing fluids and in aqueously isolated intact chloroplasts. In pea plants exposed to 100 millimolar NaCl for 14 days, apoplastic salt concentrations in leaflets increased continuously with time up to 204 (Cl⁻) and 87 millimolar (Na⁺), whereas the two ions reached a steady concentration of only 13 and 7 millimolar, respectively, in spinach leaves. In isolated intact chloroplasts from both species, sodium concentrations were not much different, but chloride concentrations were significantly higher in pea than in spinach. Together with data from whole leaf extracts, these measurements permitted an estimation of apoplastic, cytoplasmic, and vacuolar solute concentrations. Sodium and chloride concentration gradients across the tonoplast were rather similar in both species, but spinach was able to maintain much steeper sodium gradients across the plasmamembrane compared with peas. Between day 12 and day 17, concentrations of other inorganic ions in the pea leaf apoplast increased abruptly, indicating the onset of cell disintegration. It is concluded that the differential salt sensitivity of pea and spinach cannot be traced back to a single plant performance. Major differences appear to be the inability of pea to control salt accumulation in the shoot, to maintain steep ion gradients across the leaf cell plasmalemma, and to synthesize compatible solutes. Perhaps less important is a lower selectivity of pea for K⁺/Na⁺ and NO₃⁻/Cl⁻ uptake by roots.