The Experts below are selected from a list of 11571 Experts worldwide ranked by ideXlab platform
Umeo Takahama - One of the best experts on this subject based on the ideXlab platform.
-
age dependent changes in levels of ascorbic acid and chlorogenic acid and activities of peroxidase and superoxide dismutase in the Apoplast of tobacco leaves mechanism of the oxidation of chlorogenic acid in the Apoplast
Plant and Cell Physiology, 1999Co-Authors: Umeo Takahama, Masayo Hirotsu, Takayuki OnikiAbstract:In order to understand browning in tobacco plants during aging, age-dependent changes in the levels of ascorbic acid (AA) and chlorogenic acid (CGA) and its isomers were investigated in the Apoplast and the symplast of the leaves. Also activities of peroxidase (POX) and superoxide dismutase (SOD) were determined. AA decreased during aging until it was no longer detectable in the Apoplast, while symplastic AA remained although the level decreased on aging. In contrast, levels of CGA and its isomers and activity of POX in the Apoplast increased on aging, while those in the symplast remained nearly constant in mature and old leaves. The activity of SOD in the Apoplast increased during aging, while that in the symplast decreased. Oxidation of CGA by the Apoplastic solution was observed in the absence of externally added H:O2, and the oxidation was inhibited by SOD and catalase. Brown components, which contained caffeic acid moieties, accumulated in the Apoplast on aging and the components produced Of and H2O2 by autooxidation. From these results, we conclude (i) that brown components are formed in the Apoplast by the CGA/POX system, (ii) that the H2O2 required for the reaction can be provided by the CGA/POX system itself and by autooxidation of the brown components, and (iii) that Apoplastic SOD functions to generate H2O2 from Apoplastically formed Of.
-
ascorbic acid dependent regulation of redox levels of chlorogenic acid and its isomers in the Apoplast of leaves of nicotiana tabacum l
Plant and Cell Physiology, 1998Co-Authors: Umeo TakahamaAbstract:There is a question whether ascorbic acid (AA) can control redox levels of phenolics in the Apoplast. The present study was designed to answer this question. AA, dehydroascorbic acid (DHA), chlorogenic acid (CGA) and its two structural isomers were present in the Apoplast of leaves of tobacco (Nicotiana tabacum L. cv. BelW3). The levels of A A plus DHA (AA + DHA) and the ratios of AA to (AA + DHA) decreased while the levels of CGA plus its isomers increased during leaf aging. o-Quinones of CGA plus its isomers were found in the Apoplast only in aged leaves of which Apoplastic level of A A was nearly zero. In addition,'activity of Apoplastic peroxidase that could oxidize CGA and its isomers increased during leaf aging. From the observations, it is concluded that AA can regulate the accumulation of the o-quinones of CGA and its isomers in the Apoplast. Based on the conclusion, it is proposed that soluble peroxidase in the Apoplast has two functions, namely, (i) scavenging of H2O2 and/or regulation of the level of Apoplastic H2O2 in the presence of AA, and (ii) accumulation of oxidation products of the phenolics in the absence of AA.
-
redox state of ascorbic acid in the Apoplast of stems of kalanchoe daigremontiana
Physiologia Plantarum, 1993Co-Authors: Umeo TakahamaAbstract:The aqueous phase of cell walls in stems of Kalanchoe daigremontiana Hamet et Perrier de la Bâthie (Apoplast) contained ascorbic acid (AA) and dehydroascorbic acid (DHA). Ratios of AA/(AA + DHA) were 0.31 ± 0.12 (SD, n = 4), whereas those of whole stems (tissues plus Apoplast) were >0.9. The amounts of (AA + DHA) in the stems were 1970 ± 190 (SD, n = 4) nmol g−1 fresh weight and those in the Apoplast were 14 ± 2 (SD, n = 4) nmol g−1 fresh weight of stems. Ratios of AA/(AA + DHA) differed in different tissues of the stems. The ratios of AA/(AA + DHA) of Apoplast plus symplast were in the following order: pith ⋍ epidermis plus cortex > vascular bundle system, and those of Apoplast were: pith > epidermis plus cortex > vascular bundle system. Ratios of AA/(AA + DHA) in the Apoplast of the different tissues decreased to about 1/3 of the original values after wounding, while the amounts of (AA + DHA) remained largely unaffected. In contrast, soluble Apoplastic peroxidase activities increased 30- to 70-fold on wounding. Hydrogen peroxide infiltrated into stems caused a rapid oxidation of AA. Coniferyl alcohol was oxidized by peroxidase in intercellular washing fluid and by cell wall-bound peroxidase. The oxidation of coniferyl alcohol by peroxidase in intercellular washing fluid was completely inhibited as long as AA was present in reaction mixtures. The oxidation of the coniferyl alcohol by cell wall-bound peroxidase was partially inihibited by AA and the degree of inhibition was dependent upon the concentration of AA. The possible functions of AA in the Apoplast are discussed in relation to the control of peroxidase-dependent oxidation of phenolics.
Christophmartin Geilfus - One of the best experts on this subject based on the ideXlab platform.
-
the ph of the leaf Apoplast is critical for the formation of pseudomonas syringae induced lesions on leaves of the common bean phaseolus vulgaris
Plant Science, 2020Co-Authors: Christophmartin Geilfus, Li Wang, Jiawen WuAbstract:Abstract Inoculating a virulent strain of Pseudomonas syringae pv. phaseolicola (Pph) into the leaf of common bean (Phaseolus vulgaris) causes the leaf Apoplast to alkalinize. Whether or not this Apoplastic pH event facilitates virulence of Pph in interaction with common bean is unclear. For elucidating this topic, (i) Pph colonization of the common bean leaf Apoplast, (ii) the formation of bacterial lesions, and (iii) Apoplastic sucrose concentration were investigated in relation to the Apoplastic leaf pH. For this, the Pph-induced leaf Apoplastic alkalinization was attenuated by spray application of either a synthetic auxin or an acidic pH buffer. Apoplastic pH was quantified in planta via microscopy-based pH imaging. Apoplastic washing fluids were extracted to quantify both colonization of bacteria in leaf Apoplast and the concentration of Apoplastic sucrose. Results reveal that the Apoplastic alkalinization facilitated bacterial colonization of the Apoplast. Number of colony forming units and area of bacterial lesions were reduced when Pph-induced Apoplastic alkalinization was attenuated by foliar application of a synthetic auxin or acidic pH buffer. Application of both agents attenuated the Pph-induced increase of sucrose in the leaf Apoplast, which is nutrient for bacteria. Data demonstrate that the Pph-mediated leaf Apoplastic alkalinisation favours bacterial colonization.
-
the ph of the Apoplast dynamic factor with functional impact under stress
Molecular Plant, 2017Co-Authors: Christophmartin GeilfusAbstract:Abstract The Apoplast is an interconnected compartment with a thin water-film that alkalinizes under stress. This systemic pH increase may be a secondary effect without functional implications, arising from ion movements or proton-pump regulations. On the other hand, there are increasing indications that it is part of a mechanism to withstand stress. Regardless of this controversy, alkalinization of the Apoplast has received little attention. The Apoplastic pH (pH apo ) increases not only during plant–pathogen interactions but also in response to salinity or drought. Not much is known about the mechanisms that cause the leaf Apoplast to alkalinize, nor whether, and if so, how functional impact is conveyed. Controversial explanations have been given, and the unusual complexity of pH apo regulation is considered as the primary reason behind this lack of knowledge. A gathering of scattered information revealed that changes in pH apo convey functionality by regulating stomatal aperture via the effects exerted on abscisic acid. Moreover, Apoplastic alkalinization may regulate growth under stress, whereas this needs to be verified. In this review, a comprehensive survey about several physiological mechanisms that alkalize the Apoplast under stress is given, and the suitability of Apoplastic alkalinization as transducing element for the transmission of sensory information is discussed.
-
early changes in Apoplast composition associated with defence and disease in interactions between phaseolus vulgaris and the halo blight pathogen pseudomonas syringae pv phaseolicola
Plant Cell and Environment, 2016Co-Authors: Brendan M Oleary, Christophmartin Geilfus, Helen C Neale, Robert W Jackson, Dawn L Arnold, Gail M PrestonAbstract:The Apoplastic compartment is the arena in which endophytic pathogens such as Pseudomonas syringae grow and interact with plant cells. Using a combination of metabolomic and ion analysis techniques, this study shows how the composition of Phaseolus vulgaris leaf Apoplastic fluid changes during the first six hours of compatible and incompatible interactions with two strains of Pseudomonas syringae pv. phaseolicola (Pph) that differ in the presence of the genomic island PPHGI-1. Leaf inoculation with the avirulent island-carrying strain Pph 1302A elicited effector-triggered immunity (ETI) and resulted in specific changes in Apoplast composition, including increases in Apoplastic conductivity, pH, citrate, γ-aminobutyrate (GABA) and K+, that are linked to the onset of plant defence responses. Other Apoplastic changes, including increases in Ca2+, Fe2/3+ Mg2+, sucrose, β- cyanoalanine and several amino acids, occurred to a relatively similar extent in interactions with both Pph 1302A and the virulent, island-less strain Pph RJ3. Metabolic footprinting experiments established that Pph preferentially metabolizes malate, glucose and glutamate, but excludes certain other abundant Apoplastic metabolites, including citrate and GABA, until preferred metabolites are depleted. These results demonstrate that Pph is well-adapted to the leaf Apoplast metabolic environment and that loss of PPHGI-1 enables Pph to avoid or suppress changes in Apoplast composition linked to plant defences.
-
chloride inducible transient Apoplastic alkalinizations induce stomata closure by controlling abscisic acid distribution between leaf Apoplast and guard cells in salt stressed vicia faba
New Phytologist, 2015Co-Authors: Christophmartin Geilfus, Axel Mithofer, Jutta Ludwigmuller, Christian Zorb, Karl H MuehlingAbstract:Summary Chloride stress causes the leaf Apoplast transiently to alkalize, an event that is presumed to contribute to the ability of plants to adapt to saline conditions. However, the initiation of coordinated processes downstream of the alkalinization is unknown. We hypothesize that chloride-inducible pH dynamics are a key chemical feature modulating the compartmental distribution of abscisic acid (ABA) and, as a consequence, affecting stomata aperture. Apoplastic pH and stomata aperture dynamics in intact Vicia faba leaves were monitored by microscopy-based ratio imaging and porometric measurements of stomatal conductance. ABA concentrations in leaf Apoplast and guard cells were compared with pH dynamics by gas-chromatography-mass-spectrometry (GC-MS) and liquid-chromatography–tandem-mass spectrometry (LC-MS/MS). Results demonstrate that, upon chloride addition to roots, an alkalizing factor that initiates the pH dynamic propagates from root to leaf in a way similar to xylem-distributed water. In leaves, it induces a systemic transient Apoplastic alkalinization that causes Apoplastic ABA concentration to increase, followed by an elevation of endogenous guard cell ABA. We conclude that the transient alkalinization, which is a remote effect of chloride stress, modulates the compartmental distribution of ABA between the leaf Apoplast and the guard cells and, in this way, is instrumental in inducing stomata closure during the beginning of salinity.
Bernd Hoffmann - One of the best experts on this subject based on the ideXlab platform.
-
FITC‐dextran for measuring Apoplast pH and Apoplastic pH gradients between various cell types in sunflower leaves
Physiologia Plantarum, 1995Co-Authors: Bernd Hoffmann, Harald KosegartenAbstract:The liquid in the free space of leaf cell walls, the Apoplast, is in direct contact with the plasma membrane and its nutrient uptake systems. Therefore, the pH of the Apoplast is of utmost interest. We have elaborated a non-destructive method by which excised sunflower leaves (Helianthus annuus cv. Erika) were perfused with fluorescein isothiocyanate-dextran (FITC-dextran) (4 000 Da) via the transpiration stream. We showed that leaf Apoplast pH can be measured by using the fluorescence ratio technique together in conjunction with this dye. Evidence is provided that FITC-dextran does not penetrate the plasma membrane over a period of ca 17 h from the beginning of dye perfusion. Dye enrichment in the leaf Apoplast did not cause an ‘inner filter effect’ and thus the fluorescence ratio was only dependent on pH. In vivo calibration yielded a pKa of 5.92, which was virtually identical to the pKa of 5.93 calculated for dye solutions. Hence, FITC-dextran can be detected in complex environments and covers a pH range prevailing in the leaf Apoplast. Based on this method we developed a microscope image technique visualizing pH gradients between various cell types. The pH in the lumen of the xylem vessel was ca 0.3–0.5 units lower than that of the Apoplast of surrounding cells. Nitrate present in the leaf Apoplast caused an increase in pH, especially in the dark. Under these conditions, in the intercostal area, the Apoplast pH around the stomata was ca 0.5–1.0 units higher than that of the surrounding epidermal cells.
-
fitc dextran for measuring Apoplast ph and Apoplastic ph gradients between various cell types in sunflower leaves
Physiologia Plantarum, 1995Co-Authors: Bernd Hoffmann, Harald KosegartenAbstract:The liquid in the free space of leaf cell walls, the Apoplast, is in direct contact with the plasma membrane and its nutrient uptake systems. Therefore, the pH of the Apoplast is of utmost interest. We have elaborated a non-destructive method by which excised sunflower leaves (Helianthus annuus cv. Erika) were perfused with fluorescein isothiocyanate-dextran (FITC-dextran) (4 000 Da) via the transpiration stream. We showed that leaf Apoplast pH can be measured by using the fluorescence ratio technique together in conjunction with this dye. Evidence is provided that FITC-dextran does not penetrate the plasma membrane over a period of ca 17 h from the beginning of dye perfusion. Dye enrichment in the leaf Apoplast did not cause an ‘inner filter effect’ and thus the fluorescence ratio was only dependent on pH. In vivo calibration yielded a pKa of 5.92, which was virtually identical to the pKa of 5.93 calculated for dye solutions. Hence, FITC-dextran can be detected in complex environments and covers a pH range prevailing in the leaf Apoplast. Based on this method we developed a microscope image technique visualizing pH gradients between various cell types. The pH in the lumen of the xylem vessel was ca 0.3–0.5 units lower than that of the Apoplast of surrounding cells. Nitrate present in the leaf Apoplast caused an increase in pH, especially in the dark. Under these conditions, in the intercostal area, the Apoplast pH around the stomata was ca 0.5–1.0 units higher than that of the surrounding epidermal cells.
-
measurements of ph in the Apoplast of sunflower leaves by means of fluorescence
Physiologia Plantarum, 1992Co-Authors: Bernd Hoffmann, Rudiger Flanker, Konad MengelAbstract:A method was elaborated by which the pH in leaf Apoplast can be measured. The technique is based on the pH dependent fluorescence of 5-carboxyfluorescein (5-CF) or fluorescein isothiocyanate (FITC). The fluorescein isothiocyanate is coupled with a macromolecular dextran molecule (FITC-dextran). For eliminating the effect of the absolute dye concentration the dual excitation technique was applied. It was shown that the ratio of fluorescence excited by light of 491 nm and 463 nm was virtually independent of the concentration of 5-CF and that this fluorescence ratio was related to the pH. The plasmalemma is practically impermeable to FITC-dextran and in the test we carried out over a period of 6 h not the slightest indication was found that it may penetrate the plasma membrane. For 5-CF this cannot be ruled out completely. It is possible that at pH values below 4.5 it may penetrate biological membranes at low rates. Experiments with leaves of sunflower (Helianthus animus cv. Erika) perfused with 5-carboxyfluorescein and supplied with different nitrogen forms showed that NH+4 application resulted in a decrease and NO+3 application in an increase of the leaf Apoplast pH. Leaf spraying with fasicoccin was followed by a pH decrease, while leaf spraying with the protonophores p-trifluoromethoxy carbonytcyanide phenylhydra-zon (FCCP) or nigericin resulted in neutral Apoplastic pH. These results provide evidence that the method is well suited for measuring the response of the leaf Apoplast pH to changing physiological conditions.
William H. Outlaw - One of the best experts on this subject based on the ideXlab platform.
-
Guard-cell Apoplastic sucrose concentration - A link between leaf photosynthesis and stomatal aperture size in the Apoplastic phloem loader Vicia faba L.
Plant Cell and Environment, 2007Co-Authors: Yun Kang, William H. Outlaw, Peter C Andersen, G. B. FioreAbstract:In broad bean (Vicia faba L.), an Apoplastic phloem loader, the sucrose concentration increases up to approximately 2 mM in the leaf Apoplast and up to approximately 150 mM in the guard-cell Apoplast during the photoperiod. This high concentration in the guard-cell Apoplast results from transpiration and is sufficient osmotically to reduce stomatal aperture size by up to 3 microm or approximately 25% of the maximum aperture size. In this paper, we investigated a parallel and required role for high bulk-leaf Apoplastic sucrose concentration, which correlates with high photosynthesis rate. An empirically determined combination of lowered light intensity and lowered CO(2) concentration reduced the photosynthesis rate to nominally one-fifth of the control value without a significant change in transpiration. This reduction in photosynthesis caused the sucrose concentration in the leaf Apoplast--the immediate source pool for guard cells--to decrease by 70% (to 0.4 mM). In turn, sucrose concentration in the guard-cell Apoplast decreased by approximately 80% (to approximately 40 mM). These results complete the required evidence for a non-exclusive, transpiration-linked, photosynthesis-dependent passive mechanism for the modulation of stomatal aperture size. In an ancillary investigation, hexoses in the bulk-leaf Apoplast decreased when photosynthesis was lowered, but their concentrations in the guard-cell Apoplast of control plants indicated that their osmotic contribution was negligible.
-
transpiration rate an important factor controlling the sucrose content of the guard cell Apoplast of broad bean
Plant Physiology, 2001Co-Authors: William H. Outlaw, Xiaoyi De VlieghereheAbstract:Evaporation of water from the guard cell wall concentrates Apoplastic solutes. We hypothesize that this phenomenon provides two mechanisms for responding to high transpiration rates. First, Apoplastic abscisic acid is concentrated in the guard cell wall. Second, by accumulating in the guard cell wall, Apoplastic sucrose (Suc) provides a direct osmotic feedback to guard cells. As a means of testing this second hypothesized mechanism, the guard cell Suc contents at a higher transpiration rate (60% relative humidity [RH]) were compared with those at a lower transpiration rate (90% RH) in broad bean (Vicia faba), an Apoplastic phloem loader. In control plants (constant 60% RH), the guard cell Apoplast Suc content increased from 97 ± 81 femtomol (fmol) guard cell pair−1 to 701 ± 142 fmol guard cell pair−1 between daybreak and midday. This increase is equivalent to approximately 150 mm external, which is sufficient to decrease stomatal aperture size. In plants that were shifted to 90% RH before daybreak, the guard cell Apoplast Suc content did not increase during the day. In accordance, in plants that were shifted to 90% RH at midday, the guard cell Apoplast Suc content declined to the daybreak value. Under all conditions, the guard cell symplast Suc content increased during the photoperiod, but the guard cell symplast Suc content was higher (836 ± 33 fmol guard cell pair−1) in plants that were shifted to 90% RH. These results indicate that a high transpiration rate may result in a high guard cell Apoplast Suc concentration, which diminishes stomatal aperture size.
-
the guard cell Apoplast as a site of abscisic acid accumulation in vicia faba l
Plant Cell and Environment, 2001Co-Authors: S Q Zhang, William H. OutlawAbstract:Abscisic acid (ABA) integrates the water status of a plant and causes stomatal closure. Physiological mechanisms remain poorly understood, however, because guard cells flanking stomata are small and contain only attomol quantities of ABA. Here, pooled extracts of dissected guard cells of Vicia faba L. were immunoassayed for ABA at sub-fmol sensitivity. A pulse of water stress was imposed by submerging the roots in a solution of PEG. The water potentials of root and leaf declined during 20 min of water stress but recovered after stress relief. During stress, the ABA concentration in the root Apoplast increased, but that in the leaf Apoplast remained low. The ABA concentration in the guard-cell Apoplast increased during stress, providing evidence for intra-leaf ABA redistribution and leaf Apoplastic heterogeneity. Subsequently, the ABA concentration of the leaf Apoplast increased, consistent with ABA import via the xylem. Throughout, the ABA contents of the guard-cell Apoplast, but not the guard-cell symplast, were convincingly correlated with stomatal aperture size, identifying an external locus for ABA perception under these conditions. Apparently, ABA accumulates in the guard-cell Apoplast by evaporation from the guard-cell wall, so the ABA signal in the xylem is amplified maximally at high transpiration rates. Thus, stomata will display apparently higher sensitivity to leaf Apoplastic ABA if stomata are widely open in a relatively dry atmosphere.
-
The guard‐cell Apoplast as a site of abscisic acid accumulation in Vicia faba L.
Plant Cell and Environment, 2001Co-Authors: S Q Zhang, William H. OutlawAbstract:Abscisic acid (ABA) integrates the water status of a plant and causes stomatal closure. Physiological mechanisms remain poorly understood, however, because guard cells flanking stomata are small and contain only attomol quantities of ABA. Here, pooled extracts of dissected guard cells of Vicia faba L. were immunoassayed for ABA at sub-fmol sensitivity. A pulse of water stress was imposed by submerging the roots in a solution of PEG. The water potentials of root and leaf declined during 20 min of water stress but recovered after stress relief. During stress, the ABA concentration in the root Apoplast increased, but that in the leaf Apoplast remained low. The ABA concentration in the guard-cell Apoplast increased during stress, providing evidence for intra-leaf ABA redistribution and leaf Apoplastic heterogeneity. Subsequently, the ABA concentration of the leaf Apoplast increased, consistent with ABA import via the xylem. Throughout, the ABA contents of the guard-cell Apoplast, but not the guard-cell symplast, were convincingly correlated with stomatal aperture size, identifying an external locus for ABA perception under these conditions. Apparently, ABA accumulates in the guard-cell Apoplast by evaporation from the guard-cell wall, so the ABA signal in the xylem is amplified maximally at high transpiration rates. Thus, stomata will display apparently higher sensitivity to leaf Apoplastic ABA if stomata are widely open in a relatively dry atmosphere.
Harald Kosegarten - One of the best experts on this subject based on the ideXlab platform.
-
FITC‐dextran for measuring Apoplast pH and Apoplastic pH gradients between various cell types in sunflower leaves
Physiologia Plantarum, 1995Co-Authors: Bernd Hoffmann, Harald KosegartenAbstract:The liquid in the free space of leaf cell walls, the Apoplast, is in direct contact with the plasma membrane and its nutrient uptake systems. Therefore, the pH of the Apoplast is of utmost interest. We have elaborated a non-destructive method by which excised sunflower leaves (Helianthus annuus cv. Erika) were perfused with fluorescein isothiocyanate-dextran (FITC-dextran) (4 000 Da) via the transpiration stream. We showed that leaf Apoplast pH can be measured by using the fluorescence ratio technique together in conjunction with this dye. Evidence is provided that FITC-dextran does not penetrate the plasma membrane over a period of ca 17 h from the beginning of dye perfusion. Dye enrichment in the leaf Apoplast did not cause an ‘inner filter effect’ and thus the fluorescence ratio was only dependent on pH. In vivo calibration yielded a pKa of 5.92, which was virtually identical to the pKa of 5.93 calculated for dye solutions. Hence, FITC-dextran can be detected in complex environments and covers a pH range prevailing in the leaf Apoplast. Based on this method we developed a microscope image technique visualizing pH gradients between various cell types. The pH in the lumen of the xylem vessel was ca 0.3–0.5 units lower than that of the Apoplast of surrounding cells. Nitrate present in the leaf Apoplast caused an increase in pH, especially in the dark. Under these conditions, in the intercostal area, the Apoplast pH around the stomata was ca 0.5–1.0 units higher than that of the surrounding epidermal cells.
-
fitc dextran for measuring Apoplast ph and Apoplastic ph gradients between various cell types in sunflower leaves
Physiologia Plantarum, 1995Co-Authors: Bernd Hoffmann, Harald KosegartenAbstract:The liquid in the free space of leaf cell walls, the Apoplast, is in direct contact with the plasma membrane and its nutrient uptake systems. Therefore, the pH of the Apoplast is of utmost interest. We have elaborated a non-destructive method by which excised sunflower leaves (Helianthus annuus cv. Erika) were perfused with fluorescein isothiocyanate-dextran (FITC-dextran) (4 000 Da) via the transpiration stream. We showed that leaf Apoplast pH can be measured by using the fluorescence ratio technique together in conjunction with this dye. Evidence is provided that FITC-dextran does not penetrate the plasma membrane over a period of ca 17 h from the beginning of dye perfusion. Dye enrichment in the leaf Apoplast did not cause an ‘inner filter effect’ and thus the fluorescence ratio was only dependent on pH. In vivo calibration yielded a pKa of 5.92, which was virtually identical to the pKa of 5.93 calculated for dye solutions. Hence, FITC-dextran can be detected in complex environments and covers a pH range prevailing in the leaf Apoplast. Based on this method we developed a microscope image technique visualizing pH gradients between various cell types. The pH in the lumen of the xylem vessel was ca 0.3–0.5 units lower than that of the Apoplast of surrounding cells. Nitrate present in the leaf Apoplast caused an increase in pH, especially in the dark. Under these conditions, in the intercostal area, the Apoplast pH around the stomata was ca 0.5–1.0 units higher than that of the surrounding epidermal cells.
