Abstract

The apoplastic pH of intact Forsythia×intermedia (cv. Lynwood) and tomato (Solanum lycopersicum) plants has been manipulated using buffered foliar sprays, and thereby stomatal conductance (gs), leaf growth rate, and plant water loss have been controlled. The more alkaline the pH of the foliar spray, the lower the gs and/or leaf growth rate subsequently measured. The most alkaline pH that was applied corresponds to that measured in sap extracted from shoots of tomato and Forsythia plants experiencing, respectively, soil drying or a relatively high photon flux density (PFD), vapour pressure deficit (VPD), and temperature in the leaf microclimate. The negative correlation between PFD/VPD/temperature and gs determined in well-watered Forsythia plants exposed to a naturally varying summer microclimate was eliminated by spraying the plants with relatively alkaline but not acidic buffers, providing evidence for a novel pH-based signalling mechanism linking the aerial microclimate with stomatal aperture. Increasing the pH of the foliar spray only reduced gs in plants of the abscisic acid (ABA)-deficient flacca mutant of tomato when ABA was simultaneously sprayed onto leaves or injected into stems. In well-watered Forsythia plants exposed to a naturally varying summer microclimate (variable PFD, VPD, and temperature), xylem pH and leaf ABA concentration fluctuated but were positively correlated. Manipulation of foliar apoplastic pH also affected the response of gs and leaf growth to ABA injected into stems of intact Forsythia plants. The techniques used here to control physiology and water use in intact growing plants could easily be applied in a horticultural context.

Introduction

Plants in drying soil and/or air must limit water loss to sustain a positive water balance in shoots and roots. Stomata are induced to close, and leaf growth is reduced in order to limit the surface area from which water can be lost, and these changes occur very sensitively in response to changes in rhizospheric (Sobeih et al., 2004) and aerial microclimatic (Tardieu and Davies, 1992, 1993) conditions. An increase in xylem and/or bulk leaf abscisic acid (ABA) concentration is often associated with the drying of the soil around the root (Zhang and Davies, 1989, 1990), or of the air around the shoot [measured as an increase in vapour pressure deficit (VPD), Trejo et al., 1995; Nejad and Van Meeteren, 2007]. The synthesis of ABA is stimulated by the dehydration of root and/or leaf cells (Zhang and Tardieu, 1996; Nambara and Marion-Poll, 2005), and/or ABA can be translocated around the plant in response to environmental perturbation (see below). Roots sensing a loss of soil moisture often transport more ABA to the shoot via the xylem vessels before the water status of the shoot becomes reduced (Zhang and Davies, 1989). In the shoot, ABA sourced from roots, stems, and/or leaves interacts with guard cells to close stomata (Israelsson et al., 2006), and with the growing cells of the leaf to reduce expansion (Bacon, 1999), although there is still some controversy over the growth-regulatory role of ABA (Sharp, 2002).

However, it is important to note the variability in the apparent sensitivity of stomatal conductance (gs) and growth to a given concentration of ABA in the xylem stream (Trejo and Davies, 1991; Gollan et al., 1992; Schurr et al., 1992; Tardieu and Davies, 1992, 1993). Zhang and Outlaw (2001a) determined that changes in the ABA concentration of the apoplastic fluid immediately adjacent to a single guard cell pair were correlated with the stomatal response to mild stress in Vicia faba L. in the absence of more widespread changes in ABA concentration. Trejo et al. (1993) detected a linear relationship between the ABA concentration in the leaf epidermal subcompartment and the stomatal response in Commelina communis L., whereas only a very poor relationship existed between the bulk leaf ABA concentration and stomatal aperture. Such sensitive and localized changes in ABA concentration, to which stomata respond, arise partly as a result of environment-induced changes in the ability of the cells of the stem and of the different leaf tissues to filter out and remove ABA from, or to release ABA into the apoplastic stream as it travels from the root to the leaf, or as it traverses a single leaf (Slovik and Hartung, 1992a, b; Slovik et al., 1995; Wilkinson and Davies, 1997).

The amount of ABA that is removed by the symplast (i.e. that enters the cells to become stored or metabolized) from the xylem and the leaf apoplast, before the transpiration stream reaches its target cells, depends in part on the pH of these compartments (Kaiser and Hartung, 1981; Hartung and Radin, 1989), which has also been shown to be sensitive to the environment in some species (see below). Apoplastic pH can thereby determine the concentration of ABA that finally arrives at the guard cells or growing cells (Slovik and Hartung, 1992a, b; and see Wilkinson and Davies, 2002; Wilkinson, 2004). In general, a more acidic xylem/apoplastic pH (usually between 5.0 and 6.0) exists in the sap of unstressed plants and allows the greatest removal of ABA from the xylem and leaf apoplast, such that less reaches the guard cells. More alkaline sap pH values have been detected when plants are exposed to ‘stress’ (see below), usually between 6.4 and 7.2. Under these circumstances, the pH gradient over the cell membrane that normally drives ABA removal from the apoplast is reduced, and ABA can accumulate to concentrations high enough to affect stomatal guard cells by the time that the transpiration stream reaches their distant locality, even in the absence of de novo synthesis. Thus pH changes generated by the root that are propagated along the xylem vessels to penetrate to the leaf apoplast (Jia and Davies, 2007), or that are generated within the leaf (see below), can function as chemical messengers that alert the shoot of the need to conserve water, by adjusting the amount of ABA that finally reaches the guard cells or the growing cells of the leaf.

Increases in root and/or shoot xylem sap pH have most commonly been shown to occur in response to soil drying (Gollan et al., 1992; Wilkinson and Davies, 1997; Bacon et al., 1998; Wilkinson et al., 1998; and see Wilkinson, 2004), although this is not a universal phenomenon (Thomas and Eamus, 2002). Alterations in pH can be one of the first chemical changes measurable in xylem sap from plants exposed to drying soil (Bahrun et al., 2002; Sobeih et al., 2004), even when moisture tensions are low enough that a supply of water is still freely available. Sap pH also increases in plants with roots that are exposed to soil flooding (Jackson et al., 2003; Else et al., 2006), to changes in soil nutrient status (Kirkby and Armstrong, 1980; Dodd et al., 2003; Jia and Davies, 2007; and see Wilkinson et al., 2007), and in response to salt stress (Gao et al., 2004). These changes can occur before the shoot water status of the plant is affected by perturbation at the roots (Schurr et al., 1992; Dodd et al., 2003), within 1–2 d of the onset of perturbation (Mingo et al., 2003), or even within a few hours (Jackson et al., 2003; Gao et al., 2004; Else et al., 2006), often prior to, or coincident with, the associated environmentally induced change in plant physiology (gs or growth).

That xylem pH can alter shoot physiology via an ABA-mediated mechanism has been experimentally demonstrated by Wilkinson and Davies (1997), Wilkinson et al. (1998), and Bacon et al. (1998). Artificial buffers adjusted to relatively alkaline pH values, equivalent to those found in the xylem of plants experiencing soil drying, did not reduce transpiration or growth when supplied to the xylem stream of detached shoots or leaves of ABA-deficient mutants or ABA-depleted wild-type plants, unless ABA was also supplied via the transpiration stream, even at a concentration which was not sufficient alone to affect physiology.

More recently, changes in shoot or leaf xylem/apoplastic sap pH have also been detected in response to natural or imposed changes in the aerial environment (Savchenko et al., 2000; Hedrich et al., 2001—CO2; Felle and Hanstein, 2002—light, CO2; Mühling and Lauchli, 2000; Stahlberg et al., 2001—light; Davies et al., 2002; Wilkinson and Davies, 2002—light, VPD, and/or temperature; Jia and Davies, 2007—VPD). Photon flux density (PFD—a measure of light intensity), VPD, and air temperature are positively correlated under most ambient conditions, and as stated above stomatal closure is often associated with high VPD. High PFD/VPD/temperature was associated with increased xylem pH and reduced gs in Forsythia×intermedia (cv. Lynwood) and Hydrangea macrophylla (cv. Bluewave) when intact plants were exposed to natural fluctuations in the summer microclimate over the course of several weeks (Davies et al., 2002; Wilkinson and Davies, 2002). In related work, Tardieu and Davies (1992, 1993) observed that stomata and growing leaves became more responsive to the ABA concentration in the xylem as the VPD increased around leaves of field-grown maize. Since it is known that ABA can be involved in the stomatal closure response to low humidity (Xie et al., 2006; but see Assmann et al., 2000), it is tempting to suggest a role for ABA-based pH signalling in stomatal regulation when the aerial microclimate becomes potentially stressful. As well as mediating changes in foliar compartmentalization of incoming root-sourced xylem-borne ABA, microclimate-induced changes in pH may also regulate ABA release from symplastic stores in stems and/or leaves, and/or ABA removal from the leaf via the phloem, and recirculation via the root to the shoot (Jia and Zhang, 1997; Sauter and Hartung, 2002; Else et al., 2006; see Wilkinson and Davies 2002).

Here it is demonstrated that changes in foliar apoplastic pH can function as ABA-mediated signals of perturbations in the rhizospheric and/or the aerial environment that can be adaptive in the face of stress in the intact plant. Evidence is provided for a novel pH-based link between the aerial environment and stomatal aperture. Foliar sprays of phosphate buffer iso-osmotically adjusted to a range of pH values are used to manipulate leaf apoplastic pH and plant physiology artificially in intact unstressed Forsythia and tomato plants, and in intact plants of the ABA-deficient flacca mutant of tomato (Imber and Tal, 1970).

Materials and methods

Plant material

Seeds of tomato (Solanum lycopersicum L. cv. Ailsa Craig), of both the wild type and its ABA-deficient flacca mutant (Imber and Tal, 1970), were sown in Levington F2 compost (Fisons, Ipswich, UK). Seedlings were transplanted to 1.0 l pots in a greenhouse with a variable day/night temperature, with supplemental lighting (provided by 600 W sodium Plantastar lamps, Osram, Germany) giving a photoperiod of 16 h. They were watered daily with tap water to the drip point, and weekly with full-strength Hoagland nutrient solution. Flacca plants were sprayed twice weekly with 0.2 mmol m−3 ABA (Sigma, UK) to maintain water relations. This treatment was withdrawn 1 week prior to experimentation. Plants of 4–8 weeks of age were used in the experiments, which were conducted in the greenhouse (conditions as above).

Pruned, 1-year-old Forsythia×intermedia (cv. Lynwood) plants were potted in the spring into a medium comprising 100% sphagnum peat with 6.0 g l−1 Osmocote Plus 12–14 month controlled-release fertilizer and 1.5 g l−1 MgCO3. Plants were grown to ∼100 cm in height and experimentally manipulated in polythene tunnels exposed to a naturally varying summer microclimate throughout, in 3.0 or 5.0 l pots supplied with ∼300–500 cm3 water daily (depending on pot size and prevailing conditions) by hand or via drip irrigation, to maintain them in a well-watered state.

Manipulation of apoplastic pH in intact plants using foliar sprays

Forsythia and tomato (wild-type or flacca mutant) plants were sprayed daily for up to 8 d between 09.00 h and 10.00 h over the entire foliated region, with water or potassium phosphate buffer (10 mol m−3 KH2PO4/K2HPO4). Buffers were adjusted to a range of pH values (5.0–6.7) by altering the ratio of the two salts, such that different treatments were iso-osmotic. In some experiments using the flacca tomato mutant, ABA (0.06–0.15 mmol m−3) was added to the buffer spray. It is assumed that the foliar spray penetrates to the interior of the leaf via ingress through stomatal pores (Kosegarten et al., 2001). In some cases, stems of Forsythia plants and flacca mutant tomato plants were injected between 10.00 h and 11.00 h daily with water or ABA (0.3 cm3 water or 0.06 mmol m−3 ABA for flacca; 1–10 mmol m−3 ABA for Forsythia). The injection point was immediately sealed with lanolin. It is assumed that at least a portion of the injected solution penetrated to the xylem vessels of the stem, and was thereby transported to the leaves above this point. gs, PFD, and leaf temperature were measured daily at ∼14.00–15.00 h with a porometer (AP-4, Delta-T Devices Ltd, UK) in 4–6 replicates of Forsythia and tomato wild-type and flacca mutant plants that had been sprayed with buffers adjusted to a range of pH values, and injected with water or ABA where stated. The gs was measured in the most recently fully expanded leaf/leaflet, ensuring that the measurement leaf was above the ABA/water injection point (in the same branch) where appropriate. These positions were ∼10–20 cm apart. Forsythia leaf lengths were measured daily with a ruler at ∼16.00 h to establish a rate of elongation in expanding leaves, again 10–20 cm above the point of injection where appropriate.

Measuring environmental and physiological variables in intact Forsythia plants exposed to a naturally varying microclimate

In a separate experiment, gs of intact plants was monitored every 2–3 d over the course of several weeks exposure to natural summer variations in the local microclimate (with respect to PFD, VPD, and temperature) using the porometer. Measurements were taken at the same time of day (∼14.00 h) from the first fully expanded leaf (usually the fourth from the apex), and the ambient PFD incident on each leaf, and its surface temperature were also measured using the porometer. In addition, equivalent leaves were also harvested from 4–5 equivalent plants on six occasions throughout the experiment (every 4–5 d), to determine bulk leaf ABA concentration (see below). Xylem sap was also collected from stems cut from these plants using a Scholander pressure bomb (SKPM 1400; Skye Instruments Ltd, Powys, UK). Sap was expressed from the apical 10–20 cm of detached shoots at overpressures of –0.4 to –0.8 MPa. The pH was determined in each extract within 10 min (Micro Combination pH electrode, Lazar Research Laboratories Inc., CA, USA). Bulk extraction of a mixture of xylem and leaf apoplastic sap from a portion of the shoot apex, as opposed to micro-sampling of the apoplast local to the guard cell, allows measurement of larger scale more widespread changes in sap pH and ABA concentration (see Discussion).

Radioimmunoassay for measuring leaf ABA concentration

Upon harvesting leaf tissue this was immediately frozen in liquid nitrogen. Frozen leaf tissue was freeze-dried for 48 h, finely ground, and extracted overnight at 5.0 °C with distilled deionized water using an extraction ratio of 1:50 (g DW:cm3 water). The ABA concentration of the extract was determined using a radioimmunoassay (RIA) following the protocol of Quarrie et al. (1988), using [G-3H](±)-ABA at a specific activity of 2.0 TBq mmol−1 (Amersham International, Bucks, UK), and the monoclonal antibody AFRC MAC 252 (generously provided by Dr SA Quarrie; Institute of Plant Science Research, Norwich, UK) which is specific for (+)-ABA. Any ABA present in the tissue extract inhibits the binding of the tritiated ABA to the antibody. This concentration is quantified using a series of standards of known non-radioactive ABA concentration in the assay such that sample counts can be calibrated from the resultant standard curve. However, the possibility exists in any species that compounds with a similar structure to ABA but without ABA activity may contaminate the tissue extract. Every species must be tested for these contaminants using a spike dilution test. The standard curve is tested in the assay in the presence and absence of an increasing dilution of tissue extract. If the resulting standard curves at each dilution remain parallel, then the only compound present in the tissue extract that affects the relationship between the binding of the added non-radioactive ABA and the tritiated ABA to the antibody is endogenous ABA itself. Spike tests on leaf tissue collected from well-watered Forsythia plants were carried out, and no contaminants were found (not shown).

Transpiration bioassays

The effects of exogenous ABA concentration were tested on the rate of transpirational water loss through stomata (a measure of stomatal openness) from detached leaves of greenhouse-cultivated Forsythia (growth conditions as for tomato). Leaves (third to seventh below growing apices) were detached from plants which had been kept for 1.0 h in the dark, and petioles were re-cut under water before immediate transfer to treatment solutions, in order to prevent embolism (blockage of the xylem vessels with air). Treatment solutions were 5.0 cm3 water±ABA at the appropriate concentration, in 6.0 cm3 plastic vials covered in aluminium foil to prevent evaporation from the solution surface. Leaves were introduced through slits in the foil so that petioles were submerged. The vials containing the leaves were placed under lights (PFD 400 μmol m−2 s−1) at 24 °C before 11.00 h. They were weighed approximately every 30 min for up to 5 h, after which time leaf area was measured in a leaf area meter (Li-3000A; Li-Cor Inc., Lincoln, NE, USA). Water loss was converted from weight to mmol, and expressed on a per unit leaf area per second basis. Means and standard errors from five replicates were determined.

Results

Effects of foliar sprays adjusted to a range of pH values on gs and growth in intact Forsythia and tomato plants

Foliar sprays adjusted to relatively alkaline ‘stressful’ pH values (6.4–6.7) reduced the gs of intact well-watered Forsythia and wild-type tomato plants, and reduced leaf elongation rates (LERs) in Forsythia plants, in comparison with controls sprayed with water (Fig. 1A–C). These pH values are equivalent to those detected in sap expressed from well-watered Forsythia plants experiencing a high PFD (Davies et al., 2002; and see below), and from tomato plants exposed to drying soil (Wilkinson et al., 1998). Foliar sprays adjusted to more acidic pH values, equivalent to those detected in sap from plants experiencing lower PFDs (Forsythia) or well-watered soil (tomato), did not reduce gs or LERs. The effect on gs was induced after 2.0 d (one 09.00 h foliar spray application per day), and the effect on LER was induced within 5.0 d of the start of treatment.

Fig. 1.

The effect of pH on mean gs (n=6 ±SE; A, C) and leaf elongation rate (LER; n=5 ± SE; B) when intact pot-grown Forsythia (A, B) or tomato (C) plants were sprayed once daily over the foliated region with water (controls) or phosphate buffers (10 mol m−3 KH2PO4/K2HPO4) iso-osmotically adjusted to a range of pH values.

Fig. 1.

The effect of pH on mean gs (n=6 ±SE; A, C) and leaf elongation rate (LER; n=5 ± SE; B) when intact pot-grown Forsythia (A, B) or tomato (C) plants were sprayed once daily over the foliated region with water (controls) or phosphate buffers (10 mol m−3 KH2PO4/K2HPO4) iso-osmotically adjusted to a range of pH values.

Effects of foliar sprays adjusted to a range of pH values on the relationship between PFD and gs in intact Forsythia plants

There was a negative correlation between current PFD and morning gs sampled between 09.30 h and 10.30 h approximately every 1–2 d in intact Forsythia plants that were exposed to natural summer variations in the aerial microclimate over the course of 8 d (Fig. 2A). It is assumed that the correlation between gs and VPD, and between gs and leaf temperature would be the same, given that these climatic variables are positively correlated under most ambient conditions. The value of the r2 coefficient of the second order regression between PFD and gs (Sigmaplot 2001) was reduced as the pH of the foliar spray applied to the plants increased from 5.0 to 6.7 (Fig. 2A–D). Controls (Fig. 2A) were sprayed daily with water. Manipulating an increase in the foliar apoplastic pH effectively removed the correlation between PFD (and, by inference, VPD and leaf temperature) and gs.

Fig. 2.

Ambient PFD incident at ∼10.00 h on the first fully expanded leaf of intact pot-grown Forsythia plants in polythene tunnels, plotted against the gs of the same leaf, in plants which were sprayed daily over the foliated region with water or phosphate buffers (10 mol m−3 KH2PO4/K2HPO4) iso-osmotically adjusted to a range of pH values. A bar indicating maximum and minimum leaf surface temperature is also shown. Points represent measurements taken every 1–2 d over 8 d in each of four plants per pH treatment. Second-order regressions are shown at each pH, with 95% confidence intervals (dotted lines) and r2 curve coefficients, as calculated on Sigmaplot 2001.

Fig. 2.

Ambient PFD incident at ∼10.00 h on the first fully expanded leaf of intact pot-grown Forsythia plants in polythene tunnels, plotted against the gs of the same leaf, in plants which were sprayed daily over the foliated region with water or phosphate buffers (10 mol m−3 KH2PO4/K2HPO4) iso-osmotically adjusted to a range of pH values. A bar indicating maximum and minimum leaf surface temperature is also shown. Points represent measurements taken every 1–2 d over 8 d in each of four plants per pH treatment. Second-order regressions are shown at each pH, with 95% confidence intervals (dotted lines) and r2 curve coefficients, as calculated on Sigmaplot 2001.

Effects of foliar sprays adjusted to a range of pH values on gs and its response to ABA in intact plants of the ABA-deficient flacca mutant of tomato

Foliar sprays adjusted to relatively alkaline pH values, to which gs was responsive in the wild type (Fig. 1C), did not reduce gs in leaves of flacca plants compared with water-sprayed controls (Fig. 3). However, the wild-type response to alkaline pH was restored in the flacca plants when ABA was simultaneously sprayed onto the leaves or injected into the stems (Fig. 3A, B) at a concentration approximating that measured in sap from well-watered wild-type plants that alone did not reduce gs. The effect of ABA in combination with pH 6.8 to reduce gs could be detected 2 d after the start of treatment (one 09.00 h application per day). More acidic foliar sprays did not reduce gs in either the presence or absence of ABA. The different rates of control gs in the two experiments (Fig. 3A, B) were a result of differing ambient conditions in the greenhouse (temperatures differed over a range of 5 °C and PFDs over a range of 150 μmol m−2 s−1), and of the differing stages of the measurement leaf (immature—Fig. 3A; mature—Fig. 3B). In some experiments foliar sprays adjusted to pH 6.0 increased gs in comparison with water-sprayed controls, especially in the absence of ABA (not shown), presumably as the endogenous apoplastic pH of the control plants was >6.0.

Fig. 3.

The effect of pH±ABA on mean immature (A, n=6 ±SE) or mature (B, n=6 ±SE) leaf gs in intact flacca tomato mutant plants. The foliated region was sprayed with water (controls) or phosphate buffers (10 mol m−3 KH2PO4/K2HPO4) iso-osmotically adjusted to pH 6.0 or pH 6.8. (A) The effect of ABA on the response to pH when the ABA (0.06 mmol m−3) was supplied to the leaf surface in the water/buffer spray. (B) The effect of ABA on the response to pH when plants were injected with water or 0.15 mmol m−3 ABA (0.30 cm3) ∼5–10 cm below the measurement leaf. Spraying and injecting took place 4–6 h prior to measurement of gs.

Fig. 3.

The effect of pH±ABA on mean immature (A, n=6 ±SE) or mature (B, n=6 ±SE) leaf gs in intact flacca tomato mutant plants. The foliated region was sprayed with water (controls) or phosphate buffers (10 mol m−3 KH2PO4/K2HPO4) iso-osmotically adjusted to pH 6.0 or pH 6.8. (A) The effect of ABA on the response to pH when the ABA (0.06 mmol m−3) was supplied to the leaf surface in the water/buffer spray. (B) The effect of ABA on the response to pH when plants were injected with water or 0.15 mmol m−3 ABA (0.30 cm3) ∼5–10 cm below the measurement leaf. Spraying and injecting took place 4–6 h prior to measurement of gs.

Forsythia plants were able to generate and respond to ABA

When the soil around the roots of intact Forsythia plants was allowed to dry, increases in bulk leaf ABA could be detected within 3 d (not shown). When leaves were harvested and shoot xylem/apoplastic sap was expressed approximately every 4–5 d from intact Forsythia plants exposed to a naturally varying summer aerial microclimate (with respect to PFD, VPD, and temperature), sap pH and bulk leaf ABA concentration varied but were positively correlated (Fig. 4). Stomatal closure was associated with more alkaline xylem pH at relatively high PFD/VPD/temperatures (Davies et al., 2002; Wilkinson and Davies, 2002), and with higher bulk leaf ABA concentrations (not shown). Stomata of detached Forsythia leaves were competent to close in response to ABA in a concentration-dependent manner when this was supplied via the xylem at the cut petiole, and subsequent rates of transpiration were measured (Fig. 5).

Fig. 4.

The pH of xylem/apoplastic sap expressed from the apical 10–20 cm of shoots severed from pot-grown Forsythia plants, plotted against the bulk leaf ABA concentration of leaves detached from the same plant at the same time (∼15.00–16.00 h). Measurements were taken every 4–5 d over the course of several weeks from 4–5 plants experiencing variable mid-summer ambient conditions in polythene tunnels with respect to PFD/VPD/temperature (PFD ranging from ∼130 μmol m−2 s−1 to 1100 μmol m−2 s−1). A second-order regression is shown, with a 95% confidence interval (dotted line) and an r2 coefficient, calculated using Sigmaplot 2001.

Fig. 4.

The pH of xylem/apoplastic sap expressed from the apical 10–20 cm of shoots severed from pot-grown Forsythia plants, plotted against the bulk leaf ABA concentration of leaves detached from the same plant at the same time (∼15.00–16.00 h). Measurements were taken every 4–5 d over the course of several weeks from 4–5 plants experiencing variable mid-summer ambient conditions in polythene tunnels with respect to PFD/VPD/temperature (PFD ranging from ∼130 μmol m−2 s−1 to 1100 μmol m−2 s−1). A second-order regression is shown, with a 95% confidence interval (dotted line) and an r2 coefficient, calculated using Sigmaplot 2001.

Fig. 5.

The effect of water or ABA concentration on the mean rate of water loss via transpiration from mature, detached Forsythia leaves (n=5, means with standard error bars shown) severed from greenhouse-raised plants. Water or ABA solutions were fed to the xylem by submerging the petiole in the treatment solution.

Fig. 5.

The effect of water or ABA concentration on the mean rate of water loss via transpiration from mature, detached Forsythia leaves (n=5, means with standard error bars shown) severed from greenhouse-raised plants. Water or ABA solutions were fed to the xylem by submerging the petiole in the treatment solution.

Effects of foliar sprays adjusted to a range of pH values on the response of gs and leaf growth to ABA injected into the stems of intact Forsythia plants

ABA injected into the stems of intact Forsythia plants reduced gs and LER in a concentration-dependent manner 4–6 h later when leaves were sprayed with buffers adjusted to pH 5.0 and pH 5.8, but not when these were sprayed with a more alkaline buffer (pH 6.7; Figs 6, 7). gs and leaf growth were most sensitive to ABA at pH 5.8. At pH 6.7, gs and leaf growth rate were already reduced even in water-injected controls, such that ABA could not induce any further reductions in these variables.

Fig. 6.

The effect of pH on the response of gs to water or ABA in intact Forsythia plants, when the foliated region was sprayed once daily (09.00 h) with phosphate buffers (10 mol m−3 KH2PO4/K2HPO4) iso-osmotically adjusted to a range of pH values, and the stems were injected once daily (10.00 h) with water or ABA (0.30 cm3) ∼6–10 cm below the measurement leaf. gs was measured in the most recently mature leaf in each of six plants, and means with standard errors are shown.

Fig. 6.

The effect of pH on the response of gs to water or ABA in intact Forsythia plants, when the foliated region was sprayed once daily (09.00 h) with phosphate buffers (10 mol m−3 KH2PO4/K2HPO4) iso-osmotically adjusted to a range of pH values, and the stems were injected once daily (10.00 h) with water or ABA (0.30 cm3) ∼6–10 cm below the measurement leaf. gs was measured in the most recently mature leaf in each of six plants, and means with standard errors are shown.

Fig. 7.

The effect of pH on the mean response of leaf elongation rate (LER; n=6 ±SE) to water or ABA in intact Forsythia plants, when the foliated region was sprayed daily (09.00 h) with phosphate buffers (10 mol m−3 KH2PO4/K2HPO4) iso-osmotically adjusted to a range of pH values, and the stems were injected once daily (10.00 h) with water or ABA (0.30 cm3) ∼6–10 cm below the measurement leaf.

Fig. 7.

The effect of pH on the mean response of leaf elongation rate (LER; n=6 ±SE) to water or ABA in intact Forsythia plants, when the foliated region was sprayed daily (09.00 h) with phosphate buffers (10 mol m−3 KH2PO4/K2HPO4) iso-osmotically adjusted to a range of pH values, and the stems were injected once daily (10.00 h) with water or ABA (0.30 cm3) ∼6–10 cm below the measurement leaf.

Discussion

A technique is reported here that allows manipulation of foliar apoplastic pH in intact plants, such that it has been possible to assess the effects of this variable on shoot physiology in vivo. Intact plants were sprayed with iso-osmotic phosphate buffers adjusted to a range of pH values. Kosegarten et al. (2001) demonstrated that a similar technique (spraying sunflower leaves on intact plants with diluted citric and sulphuric acids) indeed resulted in changes in apoplastic pH, as measured with a fluorescent dye loaded into the apoplast of the leaves.

For the first time it has been demonstrated that applications of buffers adjusted to a ‘stressful’ pH of between 6.4 and 7.0 can close stomata and reduce leaf growth in the intact plant (Fig. 1). These changes could be induced in the absence of the environmental perturbation that normally generates an equivalent endogenous pH change, in this case soil drying (tomato—Wilkinson et al., 1998) or a stressful aerial leaf microclimate (ForsythiaDavies et al., 2002). Implementation of this technique in a horticultural/agricultural context could be of great benefit with regard to water conservation (see below). That ABA was either necessary for the induction of stomatal closure by relatively alkaline buffers (in ABA-deficient flacca mutants of tomato—Fig. 3) or that responses of gs and leaf growth to injected ABA were modulated by the pH application (Figs 6, 7Forsythia) indicated that the effect of the foliar buffer was to adjust the compartmentation of ABA within the leaf, thereby altering the concentration that finally penetrated to the apoplastic micro-compartment around the stomatal guard cells or the growing cells of the leaf (see Introduction; Wilkinson and Davies, 2002; Wilkinson, 2004). When the foliar spray was more alkaline, more of the ABA in the transpiration stream (whether this was sourced from the root or the shoot, see below) was presumably allowed to by-pass the cells of the stem and of the leaf without being sequestered, such that more finally penetrated to the guard cells and the growing cells in the leaf (Fig. 4; Slovik and Hartung, 1992a, b; Wilkinson and Davies, 1997; Bacon et al., 1998; Wilkinson et al., 1998). In addition, ABA loading into the xylem lumen from existing stores in stem and/or petiole parenchyma can occur when the pH of sap perfused through the lumen is more alkaline (Sauter and Hartung, 2002; Else et al., 2006; and see Fig. 4). ABA concentrations in sap collected from leaf petioles were higher than those in sap collected from stem bases of flooded tomato plants exhibiting increases in sap alkalinity (Else et al., 2006). Another pH-based effect on shoot ABA redistribution has been demonstrated by Jia and Zhang (1997). Movement of ABA out of maize leaves via the phloem was reduced when the leaf apoplast was adjusted to a relatively high pH, such that ABA accumulated in the leaf. Thus there is also a contribution to the alkalinity-induced enrichment of xylem and leaf apoplastic sap with ABA that is sourced from the stem and the leaves.

In previous work (Davies et al., 2002; Wilkinson and Davies, 2002) it was observed that a stressful aerial microclimate (high PFD, VPD, temperature) correlates with xylem alkalization (Forsythia, Hydrangea) and with stomatal closure (Forsythia). It was suggested that a change in apoplastic pH could be generated within the leaf/shoot by some aspect of the aerial microclimate (VPD, PFD, and/or temperature; see also Wilkinson, 2004). However, this is the first time that a causal link between aerial perturbation, apoplastic pH, and stomatal response has been demonstrated. That applying relatively alkaline buffers to the leaves removed the correlation between decreasing stress and stomatal opening (Fig. 2) indicates that an acidic milieu contributes to stomatal opening as the environment becomes less stressful. When this is considered alongside the findings that (i) the aerial environment can change the pH and the stomatal response in parallel (Davies et al., 2002; Wilkinson and Davies, 2002) and (ii) xylem pH and leaf ABA concentration are positively correlated (Fig. 4), it is evident that the environment can affect stomata through a change in pH and thereby via a change in the amount of root- and/or leaf-sourced ABA that is able to penetrate to the guard cells. These pH changes do not act directly on stomata, and require ABA to do so in intact leaves (Fig. 3). Opposing, and presumably direct effects of pH have been observed on stomatal behaviour when stomatal apertures were measured after incubation of epidermal peels on solutions buffered to a range of pH values (Wilkinson and Davies, 1997; Wilkinson, 1999). The more potent effect of a propagated pH change in the intact leaf is on the distribution of ABA between leaf compartments, and this must over-ride any direct effects of pH on guard cell biochemistry.

It is important to distinguish here between (at least) three different groups of environmentally induced pH change that seem to occur. First, rapid (inducible within minutes), often transient changes in pH are apparently localized to the immediate guard cell apoplast. These may be induced by environment-led changes in stomatal aperture and the accompanying ionic fluxes in and out of the guard cells themselves (Bowling and Edwards, 1984; Edwards et al., 1988; Hedrich et al., 2001; Felle and Hanstein, 2002). Stomatal closure, induced by various imposed environmental changes, correlated with short-term localized alkalization. Secondly, light/dark transitions and changes in atmospheric CO2 concentration can induce rapid short-term oscillations in apoplastic pH which seem to result from changes in photosynthetic leaf cell H+-ATPase activity and/or changes in photosynthetically derived malate concentration, which can propagate a short distance within the leaf apoplast (Mühling and Lauchli, 2000; Hedrich et al., 2001; Stahlberg et al., 2001; Felle and Hanstein, 2002). Thirdly, and more relevant here, are those which arise more slowly after perturbation (∼4–48 h, Sobeih et al., 2004; Else et al., 2006) at a point distant from the guard cells, which are of a longer duration (several days, Bahrun et al., 2002; Mingo et al., 2003), and which can be transported over longer distances, often between organs (Hoffmann and Kosegarten, 1995; Else et al., 2006; Jia and Davies, 2007). The latter are more likely to be those involved in long-distance ABA-based signalling, whilst the rapid transient pH changes localized to the apoplast around guard cells and photosynthetic cells are likely to be non-message-carrying by-products of a prior stomatal or photosynthetic cell response, and/or to be involved in localized changes in ion transport activity required to generate the biochemical driving force for nutrient uptake or stomatal movement in response to light/CO2 (Zeiger and Zhu, 1998; Assmann and Shimazaki, 1999).

There has been previous speculation about the mechanism behind the more widespread xylem sap pH change that can occur in response to variations in the leaf microclimate, with respect to individual effects of PFD, VPD, and/or temperature (Wilkinson and Davies, 2002; Wilkinson, 2004). Mechanisms whereby soil drying, flooding, and variations in nutrient availability can lead to an increase in xylem sap pH have been discussed elsewhere (Wilkinson, 1999, 2004; Wilkinson and Davies, 2002; Else et al., 2006; Wilkinson et al., 2007). It was suggested that VPD may underlie the aerial environment-led alterations in xylem sap pH (Wilkinson and Davies, 2002), in order to explain how stomata can become more responsive to a given dose of ABA in the xylem stream as VPD increases (Tardieu and Davies, 1992, 1993). However, Jia and Davies (2007) increased VPD at a fixed temperature and PFD around C. communis leaves and found that, alone, this actually acidified the apoplast, which would tend to increase ABA retention by the symplast. The authors suggested that fewer protons were removed from the transpiration stream when it by-passed the symplast more rapidly at high VPD. Thus it would seem that increasing VPD acts more directly on guard cell biochemistry (Assmann et al., 2000; Bunce, 2006) to increase stomatal and/or growth sensitivity to ABA (Grantz, 1990; Tardieu and Davies, 1992, 1993; Bunce, 1996; Xie et al., 2006), and that PFD and/or air temperature (see below) remain the most likely effectors of the pH changes described above. Nevertheless, it must be noted that evidence still exists to show that high VPD can limit stomatal aperture by increasing the apoplastic ABA concentration in the vicinity of the guard cells (Zhang and Outlaw, 2001b).

Is PFD the driver for the changes in pH measured in apoplastic sap expressed from plants experiencing a range of microclimatic conditions (Davies et al., 2002; Wilkinson and Davies, 2002)? Bunce (2006) found that at a fixed temperature and fixed high VPD, increasing the PFD from 300 to 1500 μmol m−2 s−1 re-opened stomata in four different species. Kaiser and Hartung (1981) provided evidence that saturating light can induce chloroplastic alkalization, causing ABA to be retained inside these organelles and reducing stomatal sensitivity to externally supplied ABA. Heckenberger et al. (1996) found that gs was less sensitive to ABA fed to sunflower leaves at a PFD of 450 μmol m−2 s−1 as opposed to 200 μmol m−2 s−1 when the temperature was fixed at 21 °C. It would seem, therefore, that pH, leaf ABA concentration, and stomatal aperture are most likely to be responding to the high temperature that occurs when Forsythia plants are exposed to a stressful aerial microclimate, rather than to the associated high PFD or VPD.

High temperature may increase CO2 removal from the apoplast for photosynthesis, which will tend to alkalize apoplastic sap by virtue of its low buffering capacity (Savchenko et al., 2000). High temperature will also increase nitrate transport into the leaf (Aslam et al., 2001; Castle et al., 2006), via effects either on the transpiration rate or on the activation state of uptake and transport proteins. Increases in xylem nitrate concentration have been shown to increase apoplastic pH in several species (see Wilkinson et al., 2007). Co-transport of nitrate into the symplast with two protons depletes the apoplast of positive charge (Ullrich, 1992). The reduction of nitrate to ammonium in the cells of the leaf (which increases with substrate availability), and the subsequent synthesis of ammonium to organic material generates OH anions (Raven and Smith, 1976). Both effects will tend to increase xylem sap pH. Changes in xylem sap nitrate concentration above the deficient range have recently been shown to be powerful modulators of stomatal aperture and leaf growth (Jia and Davies, 2007; Wilkinson et al., 2007). Relatively high xylem nitrate concentrations, within the physiological range, closed stomata and/or reduced leaf growth synergistically with ABA and/or alkalinity, in maize, tomato, and Commelina. The effect could be removed by supplying the nitrate to the xylem of detached leaves in a more acidic buffer. Nitrate may only be a factor in apoplastic pH alkalization in species which metabolize this anion in the shoot as opposed to the root (Wilkinson, 1999; Wilkinson et al., 2007). Alternatively, high rates of photosynthesis induced by high temperature (to an optimum) may influence leaf apoplastic pH by inducing an increase in sugar uptake into the phloem, which can occur via proton co-transport across the plasma membrane of the sieve tube cells, again depleting the apoplast of positive charge (Wilkinson, 1999). Finally it has been shown that an increase in the apoplastic malate concentration coincides with alkalization and stomatal closure (Patonnier et al., 1999Fraxinus excelsior; Hedrich et al., 2001Vicia faba and Solanum tuberosum). Increased photosynthesis at high temperature may lead to greater malate synthesis in leaf cells, and its subsequent release to the apoplast. Further research is required to establish whether temperature can alter apoplastic pH via any of the mechanisms proposed above. It must also be noted here that temperature can have other more direct effects on stomatal guard cells (Ilan et al., 1995) which may be overridden by the accumulation of ABA. Stomatal sensitivity to ABA also increases with temperature up to 25 °C (Wilkinson et al., 2001), and this effect may act in conjunction with the increase in apoplastic alkalinity.

Whatever the cause of the pH change, it appears to be important in modulating physiological responses to the aerial environment in intact plants (Fig. 2) via an ABA-dependent mechanism (Figs 3, 4, 6, 7). It is proposed that environmental factors that affect aerial parts of the plant (e.g. PFD, temperature, VPD, diurnal/seasonal change, and fungal infection) can interact with factors that affect underground parts (drought, flooding, salt stress, nutrient availability, and nodulation) and/or the whole plant (water/nutrient availability and temperature) by impinging on the ABA concentration finally perceived by the guard cells and the growing cells of the leaf. The pH of distinct regions of the root, stem, and shoot, each perhaps responding locally to differing levels and types of stimuli/perturbation, will govern the amount of ABA that becomes ‘locked away’ in these localities, or that is free to travel on to the responsive cells at the culmination of the transpiration stream in the leaf apoplast. Perturbations that increase the pH of a particular region will amplify the ABA signal as the transpiration stream traverses it. This effect is depicted in Fig. 8.

Fig. 8.

Diagrammatic representation of the effects of the rhizosphere and the aerial microclimate on xylem and apoplastic pH in the different regions of the plant, and on modulation by pH of ABA translocation/sequestration within and between the different plant tissues/organs. The ABA concentration finally perceived by the stomata is thus representative of the entire plant environment via effects of the environment on pH. Stomata respond to this ABA concentration by adjusting their aperture and controlling water loss.

Fig. 8.

Diagrammatic representation of the effects of the rhizosphere and the aerial microclimate on xylem and apoplastic pH in the different regions of the plant, and on modulation by pH of ABA translocation/sequestration within and between the different plant tissues/organs. The ABA concentration finally perceived by the stomata is thus representative of the entire plant environment via effects of the environment on pH. Stomata respond to this ABA concentration by adjusting their aperture and controlling water loss.

It is important to note here that the present data provide new evidence for the concept that the shoot may generate and respond to ABA-based signals independently of those sourced from the root (Wilkinson and Davies, 2002). However, given the dynamic nature of both the rhizospheric and the aerial environment, ABA-based signals from both sources, and interactions between them, are likely to be important. Whilst recently described data have been interpreted to rule out a function for root-sourced ABA in communicating water deficit from the root to the shoot (Christmann et al., 2007), recirculation of ABA originally synthesized in the shoot back into the transpiration stream via the root was not accounted for by the authors. This will represent a substantial proportion of the root-sourced signal (Wolf et al., 1990; Neales and Mcleod, 1991). Stores of ABA within roots can also be mobilized when soil begins to dry (Slovik et al., 1995). Thus de novo biosynthesis of ABA in roots is not necessarily a requirement for long-distance root-sourced ABA signalling under all circumstances, although a wealth of evidence exists in the literature to show that ABA biosynthesis in roots and transport of this to the shoot does occur in drying soil (see references in Davies and Zhang, 1991; Wilkinson, 1999; Davies et al., 2002; Wilkinson and Davies, 2002). Furthermore, use of the root pressure vessel to restore shoot water relations has demonstrated the potential for root-sourced chemical signals to act independently of hydraulic signals (Gollan et al., 1986). It is proposed that interactions between ABA and pH will allow the shoot to modify its response to an ABA-based root signal as a function of local climatic conditions, in the face of the potential from both environments for dehydration. This may be especially important in tall woody species where leaves are much further away from the root. Preliminary results from our laboratory show that stomata of several woody species are competent to respond to an increase in apoplastic pH induced by a foliar spray to the intact plant, even when soil drying does not alkalize xylem sap from the same species (Sharp RG and WJ Davies, unpublished results).

This research could potentially benefit the horticultural and/or agricultural industry. Buffered foliar sprays could be used to reduce plant water loss during cultivation, and/or to induce plants to grow more slowly or to specified proportions, without employing more traditional deficit irrigation techniques. This is particularly pertinent with regard to current global warming.

Abbreviations

    Abbreviations
  • ABA

    abscisic acid

  • cv

    cultivar

  • gs

    stomatal conductance

  • LER

    leaf elongation rate

  • PFD

    photon flux density

  • RIA

    radioimmunoassay

  • VPD

    vapour pressure deficit

The authors would like to thank Dr DLR De Silva and Dr J Theobald for conducting the RIA experiments, and Maureen Harrison, Anne Keates, and Philip Nott for technical support. The work was financially supported by DEFRA Horticulture Link project HLO132LHN/HDC HNS 97.

References

Aslam
M
Travis
RL
Rains
DW
Diurnal fluctuations of nitrate uptake and in vivo nitrate reductase activity in Pima and Acala cotton
Crop Science
 , 
2001
, vol. 
41
 (pg. 
372
-
378
)
Assmann
SM
Shimazaki
K-L
The multisensory guard cell, stomatal responses to blue light and abscisic acid
Plant Physiology
 , 
1999
, vol. 
119
 (pg. 
809
-
816
)
Assmann
SM
Snyder
JA
Lee
YRJ
ABA-deficient (aba1) and ABA-insensitive (abi1-1, abi2-1) mutants of Arabidopsis have a wild-type stomatal response to humidity
Plant, Cell and Environment
 , 
2000
, vol. 
23
 (pg. 
387
-
395
)
Bacon
MA
The biochemical control of leaf expansion during drought
Plant Growth Regulation
 , 
1999
, vol. 
29
 (pg. 
101
-
112
)
Bacon
MA
Wilkinson
S
Davies
WJ
pH-regulated leaf cell expansion in droughted plants is abscisic acid dependent
Plant Physiology
 , 
1998
, vol. 
118
 (pg. 
1507
-
1515
)
Bahrun
A
Jensen
CR
Asch
F
Mogensen
VO
Drought-induced changes in xylem pH, ionic composition, and ABA concentration act as early signals in field-grown maize (Zea mays L)
Journal of Experimental Botany
 , 
2002
, vol. 
53
 (pg. 
251
-
263
)
Bowling
DJF
Edwards
A
pH gradients in the stomatal complex of Tradescantia virginiana
Journal of Experimental Botany
 , 
1984
, vol. 
35
 (pg. 
1641
-
1645
)
Bunce
JA
Does transpiration control stomatal responses to water vapour pressure deficit?
Plant, Cell and Environment
 , 
1996
, vol. 
19
 (pg. 
131
-
135
)
Bunce
JA
How do leaf hydraulics limit stomatal conductance at high water vapour pressure deficits
Plant, Cell and Environment
 , 
2006
, vol. 
29
 (pg. 
1644
-
1650
)
Castle
ML
Crush
JR
Rowarth
JS
The effect of root and shoot temperature of 8 °C or 24 °C on the uptake and distribution of nitrogen in white clover (Trifolium repens L.)
Australian Journal of Agricultural Research
 , 
2006
, vol. 
57
 (pg. 
577
-
581
)
Christmann
A
Weiler
EW
Steudle
E
Grill
E
A hydraulic signal in root-to-shoot signalling of water shortage
The Plant Journal
 , 
2007
, vol. 
52
 (pg. 
167
-
174
)
Davies
WJ
Wilkinson
S
Loveys
BR
Stomatal control by chemical signalling and the exploitation of this mechanism to increase water use efficiency in agriculture
New Phytologist
 , 
2002
, vol. 
153
 (pg. 
449
-
460
)
Davies
WJ
Zhang
JH
Root signals and the regulation of growth and development of plants in drying soil
Annual Review of Plant Physiology and Plant Molecular Biology
 , 
1991
, vol. 
42
 (pg. 
55
-
76
)
Dodd
IC
Tan
LP
He
J
Do increases in xylem sap pH and/or ABA concentration mediate stomatal closure following nitrate deprivation?
Journal of Experimental Botany
 , 
2003
, vol. 
54
 (pg. 
1281
-
1288
)
Edwards
MC
Smith
GN
Bowling
DJF
Guard-cells extrude protons prior to stomatal opening—a study using fluorescence microscopy and pH micro-electrodes
Journal of Experimental Botany
 , 
1988
, vol. 
39
 (pg. 
1541
-
1547
)
Else
MA
Taylor
JM
Atkinson
CJ
Anti-transpirant activity in xylem sap from flooded tomato (Lycopersicon esculentum Mill.) plants is not due to pH-mediated redistributions of root- or shoot-sourced ABA
Journal of Experimental Botany
 , 
2006
, vol. 
57
 (pg. 
3349
-
3357
)
Felle
HH
Hanstein
S
The apoplastic pH of the substomatal cavity of Vicia faba leaves and its regulation responding to different stress factors
Journal of Experimental Botany
 , 
2002
, vol. 
53
 (pg. 
73
-
82
)
Gao
D
Knight
MR
Trewavas
AJ
Sattelmacher
B
Plieth
C
Self-reporting Arabidopsis expressing pH and [Ca2+] indicators unveil ion dynamics in the cytoplasm and in the apoplast under abiotic stress
Plant Physiology
 , 
2004
, vol. 
134
 (pg. 
898
-
908
)
Gollan
T
Passioura
JB
Munns
R
Soil water status affects the stomatal conductance of fully turgid wheat and sunflower leaves
Australian Journal of Plant Physiology
 , 
1986
, vol. 
13
 (pg. 
459
-
464
)
Gollan
T
Schurr
U
Schulze
ED
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 and Environment
 , 
1992
, vol. 
15
 (pg. 
551
-
559
)
Grantz
DA
Plant response to atmospheric humidity
Plant, Cell and Environment
 , 
1990
, vol. 
13
 (pg. 
667
-
679
)
Hartung
W
Radin
JW
Abscisic acid in the mesophyll apoplast and in the root xylem sap of water-stressed plants: the significance of pH gradients
Current Topics in Plant Biochemistry and Physiology
 , 
1989
, vol. 
8
 (pg. 
110
-
124
)
Heckenberger
U
Schurr
U
Schulze
E-D
Stomatal response to abscisic acid fed into the xylem of intact Helianthus annuus (L.) plants
Journal of Experimental Botany
 , 
1996
, vol. 
47
 (pg. 
1405
-
1412
)
Hedrich
R
Neimanis
S
Savchenko
G
Felle
H
Kaiser
WM
Heber
U
Changes in apoplastic pH and membrane potential in leaves in relation to stomatal responses to CO2, malate, abscisic acid or interruption of water supply
Planta
 , 
2001
, vol. 
213
 (pg. 
594
-
601
)
Hoffmann
B
Kosegarten
H
FITC-dextran for measuring apoplast pH and apoplastic pH gradients between various cell types in sunflower leaves
Physiologia Plantarum
 , 
1995
, vol. 
95
 (pg. 
327
-
335
)
Ilan
N
Moran
N
Schwartz
A
The role of potassium channels in the temperature control of stomatal aperture
Plant Physiology
 , 
1995
, vol. 
108
 (pg. 
1161
-
1170
)
Imber
D
Tal
M
Phenotypic reversion of flacca, a wilty mutant of tomato, by abscisic acid
Science
 , 
1970
, vol. 
169
 (pg. 
592
-
593
)
Israelsson
M
Siegel
RS
Young
J
Hashimoto
M
Iba
K
Schroeder
JI
Guard cell ABA and CO2 signaling network updates and Ca2+ sensor priming hypothesis
Current Opinion in Plant Biology
 , 
2006
, vol. 
9
 (pg. 
654
-
663
)
Jackson
MB
Saker
LR
Crisp
CM
Else
MA
Janowiak
F
Ionic signalling from roots to shoots of flooded tomato plants in relation to stomatal closure
Plant and Soil
 , 
2003
, vol. 
253
 (pg. 
103
-
113
)
Jia
WS
Davies
WJ
Modification of leaf apoplastic pH in relation to stomatal sensitivity to root-sourced ABA signals
Plant Physiology
 , 
2007
, vol. 
143
 (pg. 
68
-
77
)
Jia
WS
Zhang
JH
Comparison of exportation and metabolism of xylem-delivered ABA in maize leaves at different water status and xylem sap pH
Plant Growth Regulation
 , 
1997
, vol. 
21
 (pg. 
43
-
49
)
Kaiser
WM
Hartung
W
Uptake and release of abscisic acid by photoautotrophic mesophyll cells, depending on pH gradients
Plant Physiology
 , 
1981
, vol. 
68
 (pg. 
202
-
206
)
Kirkby
EA
Armstrong
MJ
Nitrate uptake by roots as regulated by nitrate assimilation in the shoot of castor oil plants
Plant Physiology
 , 
1980
, vol. 
65
 (pg. 
286
-
290
)
Kosegarten
H
Hoffmann
B
Mengel
K
The paramount influence of nitrate in increasing apoplastic pH of young sunflower leaves to induce Fe deficiency chlorosis, and the re-greening effect brought about by acidic foliar sprays
Journal of Plant Nutrition and Soil Science
 , 
2001
, vol. 
164
 (pg. 
155
-
163
)
Mingo
DM
Bacon
MA
Davies
WJ
Non-hydraulic regulation of fruit growth in tomato plants (Lycopersicon esculentum cv. Solairo) growing in drying soil
Journal of Experimental Botany
 , 
2003
, vol. 
54
 (pg. 
1205
-
1212
)
Mühling
KH
Lauchli
A
Light-induced pH and K+ changes in the apoplast of intact leaves
Planta
 , 
2000
, vol. 
212
 (pg. 
9
-
15
)
Nambara
E
Marion-Poll
A
Abscisic acid biosynthesis and catabolism
Annual Review of Plant Biology
 , 
2005
, vol. 
56
 (pg. 
165
-
185
)
Neales
TF
Mcleod
AL
Do leaves contribute to the abscisic acid present in the xylem sap of ‘droughted’ sunflower plants?
Plant, Cell and Environment
 , 
1991
, vol. 
14
 (pg. 
979
-
986
)
Nejad
AR
van Meeteren
U
The role of abscisic acid in disturbed stomatal response characteristics of Tradescantia virginiana during growth at high relative air humidity
Journal of Experimental Botany
 , 
2007
, vol. 
58
 (pg. 
627
-
636
)
Patonnier
MP
Peltier
JP
Marigo
G
Drought-induced increase in xylem malate and mannitol concentrations and closure of Fraxinus excelsior L. stomata
Journal of Experimental Botany
 , 
1999
, vol. 
50
 (pg. 
1223
-
1229
)
Quarrie
SA
Whitford
PN
Appleford
NEJ
Wang
TL
Cook
SK
Henson
IE
Loveys
BR
A monoclonal antibody to (S)-abscisic acid: its characterization and use in radioimmunoassay for measuring abscisic acid in crude extracts of cereals and lupin leaves
Planta
 , 
1988
, vol. 
173
 (pg. 
330
-
339
)
Raven
JA
Smith
FA
Nitrogen assimilation and transport in vascular land plants in relation to intercellular pH regulation
New Phytologist
 , 
1976
, vol. 
76
 (pg. 
415
-
431
)
Sauter
A
Hartung
W
The contribution of internode and mesocotyl tissues to root to shoot signalling of abscisic acid
Journal of Experimental Botany
 , 
2002
, vol. 
53
 (pg. 
297
-
302
)
Savchenko
G
Wiese
C
Neimanis
S
Hedrich
R
Heber
U
pH regulation in apoplastic and cytoplasmic cell compartments of leaves
Planta
 , 
2000
, vol. 
211
 (pg. 
246
-
255
)
Schurr
U
Gollan
T
Schulze
ED
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 and Environment
 , 
1992
, vol. 
15
 (pg. 
561
-
567
)
Sharp
RE
Interaction with ethylene: changing views on the role of abscisic acid in root and shoot growth responses to water stress
Plant, Cell and Environment
 , 
2002
, vol. 
25
 (pg. 
211
-
222
)
Slovik
S
Daeter
W
Hartung
W
Compartmental redistribution and long-distance transport of abscisic acid (ABA) in plants as influenced by environmental changes in the rhizosphere. A biomathematical model
Journal of Experimental Botany
 , 
1995
, vol. 
46
 (pg. 
881
-
894
)
Slovik
S
Hartung
W
Compartmental distribution and redistribution of abscisic acid in intact leaves. II. Model analysis
Planta
 , 
1992
, vol. 
187
 (pg. 
26
-
36
)
Slovik
S
Hartung
W
Compartmental distribution and redistribution of abscisic acid in intact leaves. III. Analysis of the stress signal chain
Planta
 , 
1992
, vol. 
187
 (pg. 
37
-
47
)
Sobeih
WY
Dodd
IC
Bacon
MA
Grierson
D
Davies
WJ
Long-distance signals regulating stomatal conductance and leaf growth in tomato (Lycopersicon esculentum) plants subjected to partial root-zone drying
Journal of Experimental Botany
 , 
2004
, vol. 
55
 (pg. 
2353
-
2363
)
Stahlberg
R
Van Volkenburgh
E
Cleland
RE
Long-distance signalling with Coleus×hybridus leaves; mediated by changes in intra-leaf CO2?
Planta
 , 
2001
, vol. 
213
 (pg. 
342
-
351
)
Tardieu
F
Davies
WJ
Stomatal response to abscisic acid is a function of current plant water status
Plant Physiology
 , 
1992
, vol. 
98
 (pg. 
540
-
545
)
Tardieu
F
Davies
WJ
Integration of hydraulic and chemical signalling in the control of stomatal conductance and water status of droughted plants
Plant, Cell and Environment
 , 
1993
, vol. 
16
 (pg. 
341
-
349
)
Thomas
DS
Eamus
D
Seasonal patterns of xylem sap pH, xylem [ABA], leaf water potential and stomatal conductance of 6 evergreen and deciduous Australian savanna tree species
Australian Journal of Botany
 , 
2002
, vol. 
50
 (pg. 
229
-
236
)
Trejo
CL
Clephan
AL
Davies
WJ
How do stomata read abscisic acid signals?
Plant Physiology
 , 
1995
, vol. 
109
 (pg. 
803
-
811
)
Trejo
CL
Davies
WJ
Drought-induced closure of Phaseolus vulgaris L. stomata precedes leaf water deficit and any increase in xylem ABA concentration
Journal of Experimental Botany
 , 
1991
, vol. 
42
 (pg. 
1507
-
1515
)
Trejo
CL
Davies
WJ
Ruiz
LDP
Sensitivity of stomata to abscisic acid—an effect of the mesophyll
Plant Physiology
 , 
1993
, vol. 
102
 (pg. 
497
-
502
)
Ullrich
WR
Mengel
K
Pilbeam
DJ
Transport of nitrate and ammonium through plant membranes
Nitrogen metabolism in plants
 , 
1992
Oxford
Oxford University Press
(pg. 
121
-
137
)
Wilkinson
S
pH as a stress signal
Plant Growth Regulation
 , 
1999
, vol. 
29
 (pg. 
87
-
99
)
Wilkinson
S
Bacon
MA
Water use efficiency and chemical signalling
Water use efficiency in plant biology
 , 
2004
Oxford
Blackwell Publishing Ltd
(pg. 
75
-
112
)
Wilkinson
S
Bacon
MA
Davies
WJ
Nitrate signalling to stomata and growing leaves: interactions with soil drying, ABA and xylem sap pH in maize
Journal of Experimental Botany
 , 
2007
, vol. 
58
 (pg. 
1705
-
1716
)
Wilkinson
S
Clephan
AL
Davies
WJ
Rapid low temperature-induced stomatal closure occurs in cold-tolerant Commelina communis L. leaves but not in cold-sensitive Nicotiana rustica L. leaves, via a mechanism that involves apoplastic calcium but not abscisic acid
Plant Physiology
 , 
2001
, vol. 
126
 (pg. 
1566
-
1578
)
Wilkinson
S
Corlett
JE
Oger
L
Davies
WJ
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 Physiology
 , 
1998
, vol. 
117
 (pg. 
703
-
709
)
Wilkinson
S
Davies
WJ
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
Plant Physiology
 , 
1997
, vol. 
113
 (pg. 
559
-
573
)
Wilkinson
S
Davies
WJ
ABA-based chemical signalling: the co-ordination of responses to stress in plants
Plant, Cell and Environment
 , 
2002
, vol. 
25
 (pg. 
195
-
210
)
Wolf
O
Jeschke
WD
Hartung
W
Long-distance transport of abscisic acid in NaCl-treated intact plants of Lupinus albus
Journal of Experimental Botany
 , 
1990
, vol. 
226
 (pg. 
593
-
600
)
Xie
X
Wang
Y
Williamson
L
, et al.  . 
The identification of genes involved in the stomatal response to reduced atmospheric relative humidity
Current Biology
 , 
2006
, vol. 
16
 (pg. 
882
-
887
)
Zeiger
E
Zhu
J
Role of zeaxanthin in blue light photoreception and the modulation of light–CO2 interactions in guard cells
Journal of Experimental Botany
 , 
1998
, vol. 
49
 (pg. 
433
-
442
)
Zhang
J
Davies
WJ
Abscisic acid produced in dehydrating roots may enable the plant to measure the water status of the soil
Plant, Cell and Environment
 , 
1989
, vol. 
12
 (pg. 
73
-
81
)
Zhang
J
Davies
WJ
Does ABA in the xylem control the rate of leaf growth in soil dried maize and sunflower plants?
Journal of Experimental Botany
 , 
1990
, vol. 
41
 (pg. 
1125
-
1132
)
Zhang
J
Tardieu
F
Relative contributions of apices and mature tissues to ABA synthesis in droughted maize root systems
Plant and Cell Physiology
 , 
1996
, vol. 
37
 (pg. 
598
-
605
)
Zhang
SQ
Outlaw
WH
Jr
The guard-cell apoplast as a site of abscisic acid redistribution in Vicia faba L
Plant, Cell and Environment
 , 
2001
, vol. 
24
 (pg. 
347
-
356
)
Zhang
SQ
Outlaw
WH
Jr
Abscisic acid introduced into the transpiration stream accumulates in the guard-cell apoplast and causes stomatal closure
Plant, Cell and Environment
 , 
2001
, vol. 
24
 (pg. 
1045
-
1054
)
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