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First published online November 10, 2006; 10.1104/pp.106.089110 Plant Physiology 143:68-77 (2007) © 2007 American Society of Plant Biologists
Modification of Leaf Apoplastic pH in Relation to Stomatal Sensitivity to Root-Sourced Abscisic Acid Signals1State Key Laboratory of Plant Biochemistry, Department of Horticulture, College of Agronomy and Biotechnology, China Agricultural University, Beijing 100094, China; and Department of Biological Sciences, Lancaster Environment Centre, University of Lancaster, Lancaster LA1 4YQ, United Kingdom
The confocal microscope was used to determine the pH of the leaf apoplast and the pH of microvolumes of xylem sap. We quantified variation in leaf apoplast and sap pH in relation to changes in edaphic and atmospheric conditions that impacted on stomatal sensitivity to a root-sourced abscisic acid signal. Several plant species showed significant changes in the pH of both xylem sap and the apoplast of the shoot in response to environmental perturbation. Xylem sap leaving the root was generally more acidic than sap in the midrib and the apoplast of the leaf. Increasing the transpiration rate of both intact plants and detached plant parts resulted in more acidic leaf apoplast pHs. Experiments with inhibitors suggested that protons are removed from xylem sap as it moves up the plant, thereby alkalinizing the sap. The more rapid the transpiration rate and the shorter the time that the sap resided in the xylem/apoplastic pathway, the smaller the impact of proton removal on sap pH. Sap pH of sunflower (Helianthus annuus) and Commelina communis did not change significantly as soil dried, while pH of tomato (Lycopersicon esculentum) sap increased as water availability in the soil declined. Increasing the availability of nitrate to roots also significantly alkalinized the xylem sap of tomato plants. This nitrogen treatment had the effect of enhancing the sensitivity of the stomatal response to soil drying. These responses were interpreted as an effect of nitrate addition on sap pH and closure of stomata via an abscisic acid-based mechanism.
There has been much recent interest in a role for xylem sap pH in modulating responses of stomata and the leaf growth response to a variety of environmental variables (e.g. Wilkinson, 2004  ). Long-distance chemical signaling can be an important component of the plant's capacity to respond to a changing environment in such a way that gas exchange and growth and development are matched to resource availability. In some plants, alkalinization of xylem sap can be a response to soil drying (Wilkinson and Davies, 1997), while sap pH can also increase when soils are flooded (Jackson et al., 2003). Other authors have reported diurnal variation in sap pHs and also seasonal changes in sap pH that can be substantial (Schurr et al., 1992). Supplying detached leaves with neutral or alkaline buffers (pH 7) via the transpiration stream can restrict transpiration and growth, and Wilkinson and Davies (1997) and Wilkinson et al. (1998) have suggested that changes in sap pH may be a very early indication to shoots that water availability in the soil is starting to decline.
Leaf apoplastic pH can have a direct physiological impact on both guard cell functioning and cell expansion, but it will also impact significantly on the effect exerted by the hormone abscisic acid (ABA) on both of these processes. This is because ABA is a weak acid (pKa = 4.8) and will therefore be distributed within the cellular compartments of the plant according to the anion-trap concept and the Henderson-Hasselbalch equation (Slovik et al., 1995
Nutrient relations of plants will also impact significantly on xylem sap pHs. Plants that are well supplied with nitrate will show higher xylem and apoplastic pH (Mengel et al., 1994 Long-distance chemical signaling to shoots of perturbation of soil conditions requires that there should be a link between the chemical information content of the sap as it leaves the root and the information content of the chemical signal as it arrives at potential sites of regulation of gas exchange and growth in the shoot. There is very little information of this kind in the literature. This investigation of the nature of this link has necessitated the development of novel techniques to assess the pH of small samples of xylem sap and the pH of the leaf apoplast.
In vitro calibration using the pH indicator BCECF showed a linear relationship between sample pH and the ratio signal between pHs 5.5 and 7.5. In vivo calibrations using pH indicators SNARF and NERF showed linear relationships between pHs 4.8 and 6.3 (Fig. 1 ). Different dyes were used in different experiments as detailed below.
Xylem saps were collected by exudation from either stem bases or tips of leaf blades of Commelina communis, sunflower (Helianthus annuus), and tomato (Lycopersicon esculentum). In all cases, pHs of xylem sap exuding from the stem bases were more acidic than those exuding from the leaf tip (Fig. 2 ). Pseudo color ratio-images of the pH indicator BCECF loaded into sunflower stems and leaf vascular systems (Fig. 3 ) supported in vitro pH assessments and showed that stem base sap pH measured in vivo was significantly more acidic than sap in the midribs of leaves.
Pressurized whole plants yielded xylem sap at a range of pressures and flow rates (Fig. 4 ). The faster that sap flowed through the plant, the more acidic the sap pH. To assess the buffer capacity of xylem sap, different concentrations of MES buffer were fed to derooted sunflower plant sealed in a pressure chamber with a single leaf protruding. The pH of the MES buffer was 5.3, which corresponded to the value in xylem sap collected from well-watered sunflower plants. Leaf xylem sap was collected from the midrib at a rate equivalent to the transpiration rate, and pH was determined using the pH indicator BCECF. When buffer capacity was low, xylem sap collected from leaves was significantly more alkaline than the sap fed to the base of the plant (Fig. 5 ). These data suggest the removal of protons from the sap as the transpiration stream moves through the plant. This conclusion is supported by the application of the proton-ATPase inhibitor vanadate with xylem sap. These treatments reduced the pH of xylem sap collected from the leaf midrib. In other words, vanadate has prevented the alkalinization of the xylem sap as it moved through the plant (Fig. 6 ).
Our techniques detected significant variation in apoplastic pH of the leaves in response to variation in soil water status, nutrient supply, and evaporative demand (Fig. 7 ). In tomato, mild soil drying resulted in an alkalinization of the apoplast (pH 5.3–5.6), while the effect of feeding nitrate to plants rather than ammonium increases the leaf apoplast pH from around 5.0 to above 6.0 (Figs. 7 and 8 ). These results show clearly that xylem sap leaving roots with a pH that largely reflects the water and nutrient availability in the rooting substrate can in some plants impact on leaf apoplastic pH and therefore has the capacity to influence growth, gas exchange, and stomatal functioning, arguably to match these variables to resource availability. It is clear, however, from the data in Figure 7 that the rate at which the transpiration stream moves through the plant also has a significant effect on the pH of the sap arriving in the shoot and therefore the pH of the leaf apoplast. At high vapor pressure deficit, the transpiration stream moves rapidly, and the resulting apoplast pH is more acidic than that in the shoots of plants transpiring more slowly. These results are consistent with those data shown in Figures 5 and 6 and suggest that the longer the residence time of sap in the plant (slower transpiration rate), the more protons are removed, resulting in greater alkalinization of sap and leaf apoplast. These data suggest that the plant may have a means to measure and respond to its own transpiration rate.
It is clear from the data presented above that a number of different variables will influence leaf apoplastic pH and thereby potentially influence stomatal behavior. To test this hypothesis, detached leaves of Commelina were fed artificial xylem saps with and without low concentrations of ABA and with different ionic species in the saps. When ABA was fed with distilled water, a reduction in leaf conductance of around 40% to 50% was generated. Addition of increasing concentrations of nitrate to this sap increased the impact of ABA on stomata, while addition of ammonium ions decreased the ABA effect (Fig. 9 ). Increasing concentrations of sodium and potassium chloride had no effect on the sensitivity of Commelina stomata to ABA. The mechanistic basis of the modification in ABA sensitivity of stomata as a result of variation in concentration of different N species was tested via an experiment with isolated epidermis of Commelina (Fig. 10 ). Stomata in epidermis incubated on buffer opened to around 16 µm, and addition of potassium nitrate and ammonium chloride at 20 mM had no effect on stomatal opening whether or not 10 µM ABA was added to the incubation buffer. ABA alone reduced stomatal aperture in this assay to around 6 µm. The absence of an effect of nitrate and ammonium ions on stomata in isolated epidermis contrasts with the significant effects of these same concentrations of ions applied to whole leaves (Fig. 9). The contrast in effects argues for an indirect effect of nitrate and ammonium on stomata, rather than a direct effect on guard cells. It can be inferred that effect of nitrate or ammonium on apparent stomatal sensitivity to ABA is due to their effects on the modification of leaf apoplastic pH and the distribution of ABA in the leaf.
To investigate whether nitrate was able to modify stomatal sensitivity to root signals in intact plants, intact tomato plants rooted in compost were subjected to a mild soil-drying treatment with nitrate supplied in the soil (control with no nitrate added). Stomata closed as soil dried (Fig. 11 ) with no apparent change in shoot water status. Sensitivity of stomata to soil drying was significantly increased by addition of 20 mM potassium nitrate to the soil before the soil-drying treatment was initiated. While soil drying led to an increase in xylem sap pH, in this experiment, it led to a decrease in nitrate content of xylem sap (Fig. 12 ). Moreover, addition of extra nitrate to prevent the soil drying-induced decrease of nitrate content was not able to prevent the pH increase, suggesting that the soil drying-induced pH increase in this plant (tomato) was not related to the changes of nitrate content of xylem sap. Surprisingly, further experiments revealed that while soil drying decreased xylem nitrate content in tomato plants (Fig. 12), the same soil-drying treatment significantly increased the nitrate content of xylem sap in Commelina plants (Fig. 13C ), perhaps because of a greater restriction of water flux by drought in this plant.
We report here on a confocal microscope technique that allows an assessment of both the pH of small (2 µL) samples of extracted xylem sap and the pH of the leaf apoplast and the xylem contents (measured in planta). This technique allows for the first time, to our knowledge, in individual plants, a demonstration of the link between the pH of the sap leaving the root and the pH of the leaf apoplast, a key requirement for long-distance signaling of the impacts on shoot growth and functioning of perturbations in the soil environment.
In the well-watered, conventionally fertilized plant, a relatively acidic xylem sap and leaf apoplast will cause ABA arriving in the transpiration stream to partition into alkaline compartments in the leaf with consequent restricted access for the hormone to sites of action on the guard cells. These alkaline compartments are the symplast of the leaf cells and the phloem, and this kind of partitioning is referred to as alkaline trapping of ABA (e.g. Slovik et al., 1995
There is much information in the literature to show how the water status of the soil, the nutrient status of the soil, and a variety of other conditions can affect the pH of the sap in the xylem (for a summary, see Wilkinson and Davies, 1997
This article shows that as xylem sap leaves the root and moves through the plant to the shoot, it becomes significantly more alkaline (Figs. 2 and 3). Experiments with proton-ATPase inhibitors suggest that this is because protons are removed from the xylem stream, presumably by ATPases associated with the xylem parenchyma (Fig. 6). One of the implications of this is that if the transpiration stream is driven harder by, for example, high evaporative demand of the atmosphere, then xylem sap will reside for less time in the pathway between the leaf and the root, and the impact of ATPases on sap pH will therefore be less (Fig. 4). Rapidly transpiring plants will therefore have lower xylem and apoplastic pHs than the same plants subjected to lower evaporative demands (Fig. 7), and, via the moderation of apoplastic pH, stomata will therefore have the capacity to respond to/measure plant transpiration rate, as suggested by Monteith (1995) In functional terms, the importance of the response of the apoplastic sap pH to changes in soil water availability, nutrient status of the sap, radiant load, and transpiration rate can be viewed in the schematic shown in Figure 14 . At least in tomato, soil drying will alkalinize the pH of the xylem sap leaving the roots and also the apoplast of the shoots (Impact 1). This effect can be enhanced if the plants are well supplied with nitrate (Impact 2), and the effect of allowing well-fertilized soil to dry is that stomata are particularly sensitive to soil drying (Fig. 9) and to low concentrations of ABA (Fig. 11). These effects are consistent with repartitioning of ABA through the shoot according to pH gradients (note the lack of an interaction between the impact of N and pH on stomatal behavior in detached epidermis; Fig. 10). In addition, there is a very different impact of ammonium N on stomata (Fig. 9), a response that is to be predicted because of the acidifying effect of this ion on apoplastic pH (Fig. 7).
We have discussed elsewhere (Wilkinson and Davies, 2002 ) the alkalinization of apoplastic sap as a direct result of a high radiant load on shoots (Impact 3). This effect should allow the plant to integrate the impact on stomata of hot and drying climatic conditions with a reduction in soil water availability. Further, we have shown here how reduced transpiration fluxes as the stomata close will allow further alkalinization of xylem sap (Impact 4) due to the action of ATPases in the xylem parenchyma of the transpiration pathway. Reduced transpiration flux as a result of partial stomatal closure will feed back directly onto shoot water status (Fig. 14; Impact 5) with potentially highly beneficial effects on plant development (Tardieu and Davies, 1993).
While the schematic in Figure 14 allows us to hypothesize ways in which different soil conditions may interact to affect plant functioning, it does not throw light upon the mechanistic basis on which a change in soil water availability can impact on sap pH. One suggestion is that this occurs via a change in the nitrogen status of the sap (see e.g. Davies, 2006
We selected three different plant species for these studies: tomato (Lycopersicon esculentum), sunflower (Helianthus annuus), and Commelina communis. Commelina was used in studies of leaf apoplastic pH in relation to stomatal sensitivity to ABA, while tomato and sunflower were used to investigate root-to-shoot communication of the effects of changes in soil conditions. Seeds of C. communis were sown in John Innes Number 2 compost (Keith Singleton's Seaview Nurseries). After emergence, seedlings were transplanted into 90- x 90-mm pots and grown in a controlled environment with the following conditions: day/night temperature 27°C and 18°C, humidity 40%, photoperiod 15 h with a photosynthetic photon flux density (PPFD) of 350 µmol m–2 s–1. Plants were watered daily to the drip point. When plants were 3 weeks old, the third fully expanded leaf was used as experimental material. Unless noted otherwise (e.g. in partial root drying [PRD] experiments), tomato and sunflower seeds were sown and grown as described for Commelina. Four-week-old plants were used as experimental material.
BCECF, DM-NERF, and SNARF were sourced from Molecular Probes. The nitrate test kit was from Nitrate Elimination, and other chemicals were from Sigma.
Determination of pH of xylem sap samples generally requires that a relatively large volume of sap is collected (several tens or hundreds of microliters). Here, we developed a method (the micro-determination method) using a sap sample of less than 0.5 µL. The micro-determination of pH was based on ratiometric measurement using the pH indicator BCECF-dextran. A glass frame (60 mm x 26 mm with the frame width of 5 mm and thickness of 0.2 mm) was mounted on a microscope slide. With such a design, as many as 20 individual sap samples may be spotted inside the frame. For in vitro calibration or pH determination, 0.5-µL sap samples were mixed with 0.5 µL BCECF solution (500 µg/mL in distilled water). The mixture (1 µL) was placed on the slide and a coverslip was put in place, after which the pH of samples was directly assayed using a confocal microscope system (Leica SP). The excitation wavelengths were respectively set to 476 and 488 nm at power intensities of 5%, and the fluorescence emission window was set to 525 to 545 nm. Fluorescence images were acquired using a 10x objective, and the mean fluorescence intensities were calculated based on whole images. For in vitro calibration, a pH calibration standard was usually made with a buffer system such as phosphate or MES. In our preliminary experiment, we found that these buffers produced a calibration curve, which was rather different from that produced with xylem sap-adjusted pH standards. For this reason, to make a pH calibration standard, we adopted xylem sap-adjusted pHs instead of buffers. To do this, about 2 mL of xylem sap was collected and adjusted, with 10 mM KOH, to a pH series as follows: pH 5.0, 5.5, 6.0, 6.5, 7.0, and 7.5. Our preliminary study confirmed that the pH value determined by the method described above was the same as determined by a pH meter.
The pH indicator NERF could be loaded through the intact root system and the dye accumulated very largely in the leaf apoplast. In some experiments, the pH indicator SNARF was used instead of NERF because NERF production was discontinued during the study. When the second leaf of Commelina was fully expanded, the plant was carefully washed out from soil with plenty of water. The plants were transferred into water and incubated for 1 h before transfer to NERF solution (100 µg/mL) in a 20-mL tube and allowed to transpire for 1.5 h under growth conditions. To investigate the leaf apoplastic pH in relation to different transpiration rates, after the loading of dye was completed, the plants were taken out of the dye solution, and immediately roots were put into water and plants were held at 25°C at each of two different atmospheric humidities, i.e. 35% and 90%, respectively. The plants were allowed to transpire for a further 1 h, then the whole plant was fixed on the platform of the confocal microscope and the apoplastic pH was determined immediately (see below). For calibration and some other experiments, a specific concentration of ions or chemicals was loaded into detached leaves. To load the dye relatively rapidly, which is important for investigations of variation in apoplastic pH in relation to variation in xylem sap pH, a pressure-facilitated loading method was developed. Briefly, when the third leaf of Commelina was fully expanded, the second leaf with 5 cm of stem attached was cut under degassed water. NERF or SNARF solution (200 µg/mL) was contained in a specially designed 2-mL vial. A rubber pad was mounted inside the cap of the vials, and a central hole was made in the cap and pad (5 mm in diameter), through which the stem of a detached leaf was inserted. When the cap was screwed tight, the stem was tightly sealed by the pad. A syringe needle connected with a pressure chamber supplying a pressure of about 0.5 bar was injected inside the vial, and care was taken that the needle did not touch the stem and the dye solution. With such a vial, the detached leaves were allowed to transpire in the growth conditions. No water soaking of plant tissue was detected during dye loading using this method. After the dye had been loaded for 1.5 h, the leaf apoplastic pH was determined using the confocal microscope (see below). For fluorescence imaging and ratiometric determination of leaf apoplastic pH, the pH indicator NERF was excited, respectively, with 514 nm and 488 nm at a power intensity of 20%, and the fluorescence window was set to 530 to 550 nm. For apoplastic pH measurement with SNARF, a single excitation line was set to 488 nm, and two fluorescence windows were set to 570 to approximately 590 nm and 630 to approximately 650 nm, respectively. Fluorescence images were acquired with a 20x objective. In vivo calibration was carried out with detached leaves fed with calibration buffers containing: 100 mM MES, pH 5.0, 5.5, 6.0, or HEPES, pH 6.5 and 7.0, 120 mM KCL, 25 µM nigericin, and 1 mM KCN. For each pH determination, three leaves and two areas (about 400 µm2) on either side of the midrib were examined. Leaf apoplastic pH was calculated based on the calibration curve.
Collection of Leaf Xylem Sap To assess the modification of the sap pH by the vascular system, sunflower plants were used. When the plants were 4 weeks old, all the leaves were detached except for the youngest fully expanded leaf. Whole or derooted plants were placed in a pressure chamber with the petiole sealed by a silicone rubber bung so that only the leaf blade was left outside. To collect leaf xylem sap, the upper part of the leaf blade was cut off, and xylem sap could be collected from the excision surface of the midrib. To study the effect of sap flow rate on leaf xylem sap pH, the whole plant was sealed in a pressure chamber and an increasing pressure was applied in steps with pressure increased between steps at around 0.03 MPa/min. Xylem sap was collected at each pressure plateau. When the sap flow rate was about 1.5-fold higher than the estimated maximum transpiration rate, gradually decreased pressure of 0.03 MPa/min was applied so that xylem sap could be collected at reducing flow rates. The transpiration of sunflower is very fast, and as pressure increased or decreased, up to 40 µL sap was expressed at each pressure step. To maximize the chance of achieving a steady state with respect to sap flow and pH at each pressure plateau, only the last few microliters were collected for pH determination. To study the effect of sap buffer capacity or H+-ATPase inhibitor on leaf xylem sap pH, derooted plants were placed in 10-mL vials containing either different concentrations of MES buffer or xylem sap collected from well-watered plants with vanadate added. The plants together with the vials were then sealed into a pressure chamber as described above and allowed to transpire. Xylem sap was collected at a pressure 0.05 MPa above the balancing pressure.
Fluorescence Imaging in Xylem Vessels
Feeding Experiment
Experiment with C. communis Epidermis
Measurement of Nitrate Concentration
When tomato plants were 10 d old, the roots of individual plants were separated into two equal parts and placed into separate pots (90 mm in diameter) containing John Innes Number 1 compost. Preliminary tests showed that the size of the pots would allow soil drying in single pots for 2 weeks, by which time soil water potential decreased to about –1.5 MPa. Plants were watered daily to the drip point in both pots and grown on for another 3 weeks, following which PRD was performed. Before PRD was performed, plants were randomly grouped for three treatments: controls, where plants were watered well on both halves of the roots; a treatment where one-half of the roots were well watered and water withheld from the other roots; and a third treatment where on the day before PRD was performed, plants were supplied with 20 mM nitrate poured onto both halves of a separated root system (to the drip point), after which PRD was performed as described above. Preliminary tests showed that soil drying could lead to a significant decrease in xylem nitrate concentration in tomato and that supply of 20 mM nitrate solution just counteracted water deficit-induced reduction in nitrate concentration in xylem sap. Abaxial stomatal conductance (gs) was measured using a porometer (AP4; Delta-T Devices) at various times during the process of PRD. Leaf water potential was measured using a pressure chamber (Soil Moisture Equipment).
To investigate soil drying-induced changes in nitrate content in C. communis and tomato, 4-week-old plants were used, and water was withheld from treatment plants while control plants remained well watered. On different days after soil drying, pH, xylem sap nitrate content, leaf conductance, leaf water potential, and volumetric soil water content were determined with a whole plant pressure chamber, as described above. In addition, 20 mM KNO3 was poured onto soil (to the drip point) around some tomato plants on the day before soil drying. Initial tests showed that supplying 20 mM KNO3 was able to prevent the decrease in xylem sap nitrate content brought about by soil drying. Received August 31, 2006; accepted November 5, 2006; published November 10, 2006.
1 This work was supported by the National Basic Research Program of China (grant no. 2003CB114300), by the National Natural Science Foundation of China (grant no. 30470160), by the Department of Environment, Food and Rural Affairs, UK, and by the Royal Society of London.  The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: William J. Davies (w.davies{at}lancaster.ac.uk). www.plantphysiol.org/cgi/doi/10.1104/pp.106.089110 * Corresponding author; e-mail w.davies{at}lancaster.ac.uk; fax 44–1524–510217.
Bacon MA, Wilkinson S, Davies WJ (1998) pH-regulated leaf cell expansion in droughted plants is abscisic acid dependent. Plant Physiol 118: 1507–1515 Davies WJ (2006) Responses of plant growth and functioning to changes in water supply in a changing climate. In JIL Morison, M Morecroft, eds, Plant Growth and Climate Change. Blackwell, Oxford, pp 96–117 Gollan T, Schurr U, Schulze ED (1992) Stomatal response to drying soil in relation to changes in xylem sap composition of Helianthus annuus. I. The concentration of cations, anions, amino acids in, and pH of, the xylem sap. Plant Cell Environ 15: 551–559[CrossRef] Goodger JQD, Sharp RE, Marsh EL, Schachtman DP (2005) Relationships between xylem sap constituents and leaf conductance of well-watered and water-stressed maize across three xylem sap sampling techniques. J Exp Bot 56: 2389–2400 Hartung W (1983) The site of action of abscisic acid at the guard cell plasmalemma of Valerianella locusta. Plant Cell Environ 6: 427–428[CrossRef] Hartung W, Radin JW (1989) Abscisic acid in the mesophyll apoplast and in the root xylem sap of water-stressed plants: the significance of pH gradients. Curr Top Plant Biochem Physiol 8: 110–124 Hartung W, Sauter A, Hose E (2001) Abscisic acid in the xylem: Where does it come from and where does it go to? J Exp Bot 53: 27–32[CrossRef][Web of Science] Hoffmann B, Kosegarten H (1995) FTIC-dextran for measuring apoplastic pH gradients between various cell types in sunflower leaves. Physiol Plant 95: 327–335[CrossRef] Jackson MB, Saker LR, Crisp CM, Else MA, Janowiak F (2003) Ionic signaling from roots to shoots of flooded tomato plants in relation to stomatal closure. Plant Soil 253: 103–113[CrossRef] Lips SH (1997) The role of inorganic nitrogen ions in plant adaptation processes. Russ J Plant Physiol 44: 421–431 Mengel K, Planker R, Hoffmann B (1994) Relationship between leaf apoplast pH and iron chlorosis of sunflower (Helianthus annuus L.). J Plant Nutr 25: 1053–1065 Monteith JL (1995) A reinterpretation of stomatal responses to humidity. Plant Cell Environ 22: 357–364[CrossRef] Mühling KH, Lauchli A (2001) Influence of chemical form and concentration of nitrogen on apoplastic pH of leaves. J Plant Nutr 25: 399–411[CrossRef] Schurr U, Gollan T, Schulze ED (1992) Stomatal response to drying soil in relation to changes in the xylem sap composition of Helianthus annuus. 2. Stomatal sensitivity to abscisic acid imported from the xylem sap. Plant Cell Environ 15: 561–567[CrossRef] Shaner DL, Boyer JS (1976) Nitrate reductase activity in maize (Zea mays L.) leaves. II. Regulation by nitrate flux at low water potential. Plant Physiol 58: 505–509 Slovik S, Daeter W, Hartung W (1995) Compartmental redistribution and long-distance transport of abscisic acid (ABA) in plants as influenced by environmental changes in the rhizosphere: a biomathematical model. J Exp Bot 46: 881–894 Tardieu F, Davies WJ (1993) Root-shoot communication and whole-plant regulation of water flux. In JAC Smith, H Griffiths, eds, Water Deficits: Plant Responses from Cell to Community. BIOS Scientific, Oxford, pp 147–162 Wilkinson S (2004) Water use efficiency and chemical signalling. In MA Bacon, ed, Water Use Efficiency in Plant Biology. Blackwell Press, Oxford, pp 75–112 Wilkinson S, Corlett JE, Oger L, Davies WJ (1998) Effects of xylem pH on transpiration from wild-type and flacca tomato leaves: a vital role for abscisic acid in preventing excessive water loss even from well-watered plants. Plant Physiol 117: 703–709 Wilkinson S, Davies WJ (1997) Xylem sap pH increase: a drought signal received at the apoplastic face of the guard cell which involves the suppression of saturable ABA uptake by the epidermal symplast. Plant Physiol 113: 559–573[Abstract] Wilkinson S, Davies WJ (2002) ABA-based chemical signalling: the co-ordination of responses to stress in plants. Plant Cell Environ 25: 195–210[CrossRef][Medline] This article has been cited by other articles:
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), excitation was sequentially set to 514 and 488 nm at power intensities of 5%, and emission fluorescence at 530 to approximately 550 nm was collected. Fluorescence intensity respectively determined at 514 and 488 nm was used to calculate the ratio (514 nm/488 nm). For SNARF (
), excitation was set to 488 nm, and emission fluorescence was collected respectively at 570 to approximately 590 nm and 630 to approximately 650 nm. Fluorescence intensity respectively determined at 570 to approximately 590 nm and 630 to approximately 650 nm was used to calculate the ratio (630 to approximately 650 nm/570 to approximately 590 nm).
, water withheld from one pot; 
, watering withheld from one pot supplied with extra nitrate. Values are means ± SE of five samples. Symbols above the plot (C) indicate statistical significance between treatments according to t test with a 95% confidence.
