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.
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
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).
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.
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.
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.
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.
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).
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.
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 (Forsythia—Davies 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, 7—Forsythia) 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., 1999—Fraxinus excelsior; Hedrich et al., 2001—Vicia 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.
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.
leaf elongation rate
photon flux density
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.