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Plant Physiol. (1998) 117: 703-709 RAPID COMMUNICATION
Effects of Xylem pH on Transpiration from Wild-Type and
flacca Tomato Leaves1
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ABSTRACT |
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Top
Abstract
Introduction
Methods
Results & Discussion
References
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The pH of xylem sap from tomato (Lycopersicon esculentum) plants increased from pH 5.0 to 8.0 as the soil dried. Detached wild-type but not flacca leaves exhibited reduced transpiration rates when the artificial xylem sap (AS) pH was increased. When a well-watered concentration of abscisic acid (0.03 µM) was provided in the AS, the wild-type transpirational response to pH was restored to flacca leaves. Transpiration from flacca but not from wild-type leaves actually increased in some cases when the pH of the AS was increased from 6.75 to 7.75, demonstrating an absolute requirement for abscisic acid in preventing stomatal opening and excessive water loss from plants growing in many different environments.
Jones (1980) Another chemical change related to soil drying in the absence of a
reduction in shoot water status is an increase in the pH of the xylem
sap flowing from the roots (Schurr et al., 1992
It was originally suggested that an increase in xylem sap pH could
putatively enhance stomatal closure by changing the distribution of the
ABA that is present in all nonstressed plants at a low "background"
concentration, without requiring de novo ABA synthesis (Schurr et al.,
1992 In contrast to the indirect ABA-mediated effect of pH on stomata, it
was also demonstrated that increasing the pH of the external solution
(from 5.0 to 7.0) bathing isolated abaxial epidermis tissue peeled from
well-watered C. communis leaves actually increased stomatal aperture (Wilkinson and Davies, 1997 Seeds of tomato (Lycopersicon esculentum L. cv Ailsa
Craig) (wild-type) or flacca were sown in Levington F2
compost (Fisons, Ipswich, UK). Seedlings were transplanted to 0.125- × 0.125-m pots in a greenhouse with a day/night temperature of 22/15°C. They were watered daily with tap water and weekly with full-strength Hoagland nutrient solution (Epstein, 1972 Chemicals
INTRODUCTION
Top
Abstract
Introduction
Methods
Results & Discussion
References
and Cowan (1982) were the first to suggest that
plants can "measure" soil water status independently of shoot water
status via the transfer of chemical information from roots to shoots.
Dehydrating roots in drying soil synthesize ABA more rapidly than fully
turgid tissue, and resultant increases in the ABA concentration of
xylem sap flowing toward the still-turgid shoot constitutes a chemical
signal to the leaves (for review, see Davies and Zhang, 1991): the
xylem vessels give up their contents to the leaf apoplast, thereby
increasing the ABA concentration in this compartment. ABA receptors on
the external surface of stomatal guard cells respond to the apoplastic
ABA concentration (Hartung, 1983; Anderson et al., 1994; but see
Schwartz et al., 1994). When bound, the receptors transduce a reduction
in guard cell turgor, which leads to stomatal closure (Assmann, 1993). This maintains shoot water potential despite the reduction in soil
water availability.
). The pH of the xylem
and/or apoplastic sap of plants can also change dramatically in
response to soil flooding, diurnal or annual rhythms, and mineral
nutrient supply (Table I) in the absence
of concomitant changes in either root or shoot water status. We already
know that, like the increase in xylem ABA concentration described
above, an increase in xylem pH can also act as a signal to leaves to close their stomata (Wilkinson and Davies, 1997). Since the conditions that affect xylem/apoplastic pH can also affect transpiration (light
intensity [Cowan et al., 1982]; soil drying [Davies and Zhang,
1991]; nitrate supply [Clarkson and Touraine, 1994]; soil flooding
[Else, 1996]), the possibility exists that the pH change that they
induce could be the means by which they alter stomatal aperture.
View this table:
Table I.
pH changes that occur in plant xylem or apoplastic
sap under various conditions
; Slovik and Hartung, 1992a, 1992b). This hypotheses is built on
the well-known fact that weak acids such as ABA accumulate in more
alkaline compartments (Kaiser and Hartung, 1981). More recently,
Wilkinson and Davies (1997) and Thompson et al. (1997) directly
demonstrated that increases in xylem sap pH reduced rates of water loss
from Commelina communis and tomato (Lycopersicon
esculentum) leaves detached from well-watered plants. This was
found to be mediated by the relatively low endogenous concentration of
ABA (about 0.01 mmol m
3) contained in the xylem vessels
and apoplast of these leaves, a concentration of ABA that did not
itself affect transpiration at a well-watered sap pH of 6.0. The
mechanism by which the combination of high sap pH and such a low
concentration of ABA was able to increase the apoplastic ABA
concentration sufficiently to close stomata was also elucidated: the
mesophyll and epidermis cells of these leaves had a greatly reduced
ability to sequester ABA away from the apoplast when the pH of the
latter was increased by the incoming xylem sap (Wilkinson and Davies,
1997).
). Mechanisms for this
direct effect of pH on guard cells have been speculated on by Thompson
et al. (1997). If this process were to occur in vivo, environments that
increase xylem sap pH could potentially induce excessive water loss
from the plants experiencing them, over and above rates of
transpiration occurring in unstressed plants. The latter may contain
stomata with apertures smaller than the maximum that is possible, even
under favorable local conditions. It was assumed that high-pH-induced
apoplastic ABA accumulation in C. communis in vivo was
sufficient to override the direct stomatal opening effect seen in the
isolated tissue (Wilkinson and Davies, 1997). To test these
possibilities, effects of pH on transpiration rates from leaves of the
flacca mutant of tomato were investigated. flacca does not synthesize ABA as efficiently as
wild-type tomato (Parry et al., 1988; Taylor et al., 1988). It contains
a very low endogenous ABA concentration (Tal and Nevo, 1973), although it retains the ability to respond to an application of this hormone (Imber and Tal, 1970). The results demonstrate not only that ABA mediates high xylem sap pH-induced stomatal closure but also that it is
necessary to prevent high xylem sap pH-induced stomatal opening and
dangerously excessive water loss.
MATERIALS AND METHODS
Top
Abstract
Introduction
Methods
Results & Discussion
References
). flacca plants
were sprayed twice weekly with 0.2 mmol m3
(+)-ABA to keep growth conditions between the two cultivars as similar
as possible. Plants 4 to 5 weeks of age were watered twice daily. At
this stage flacca plants were no longer sprayed with ABA to
prevent contamination of subsequent experiments. Although humidity was
not controlled, the increased water supply prevented leaf wilt, but
leaf surface area eventually became smaller than that of wild-type
leaves. When plants were 6 to 9 weeks old, single leaflets from the two
or three youngest fully expanded wild-type leaves were used as a source
of experimental material. In the case of flacca, the three
most apical leaflets from appropriate leaves were detached together and
used as a single experimental unit to standardize leaf surface area
between the two cultivars.
Measurement of Xylem Sap pH, Shoot Water Potential, and Soil Water Content
Fifty-two wild-type plants were watered as described above, whereas another group of 52 plants was left unwatered for 1 to 2 d. Several measurements were taken from these plants during daylight during the 48-h period, and correlations between them were investigated. We found no evidence for a diurnal effect on xylem sap pH when soil moisture content was kept high (results not shown).]); however, the reduction in flow rate is proportional between
stressed and unstressed plants (Else, 1996). To artificially increase
sap flow rates by pressurizing the root system can dehydrate the tissue
(Hartung and Radin, 1989), and this may be disproportionate between
stressed and unstressed plants.
Transpiration Bioassay
Each experiment described below was carried out on a separate batch of plants of an equivalent age. Leaflets were removed from plants that had been kept in the dark for 1 h, and the petiole was recut under degassed distilled water to avoid embolism. They were immediately transferred to plastic vials of 7.0 × 106
m3 volume that were covered with aluminum foil
secured with Parafilm (American National Can, Greenwich, CT) to reduce
evaporation. A "V"-shaped slit was cut into the foil so that the
leaf could have access to the contents of the vial. Each of these
contained 5.0 × 106
m3 of AS (unless otherwise stated): 1.0 mol
m3
KH2PO4, 1.0 mol
m3
K2HPO4, 1.0 mol
m3 CaCl2, 0.1 mol
m3 MgSO4, 3.0 mol
m3 KNO3, and 0.1 mol
m3 MnSO4 buffered to
specified pHs with 1.0 mol L1 HCl or KOH, with
or without 0.03 mmol m3 (+)-ABA (the
approximate concentration present in the xylem sap of well-watered,
well-drained tomato plants [Else, 1996]). The vials were randomized
in a growth cabinet (PPFD 500 µmol m2
s1) and weighed every 30 min for 3 to 4 h,
after which time the leaf area above the foil was measured in a
planimeter (Paton Industries, Pty. Ltd., South Australia). The
transpirational rate of water loss was calculated per unit leaf area
(single surface) every 30 min.
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RESULTS AND DISCUSSION |
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Top
Abstract
Introduction
Methods
Results & Discussion
References
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Correlations between Xylem Sap pH, Soil Water Content, and Shoot Water Potential
The pH of xylem sap from wild-type tomato plants grown as described above was negatively correlated with gravimetric soil water content (Fig. 1A), which only affected shoot water potential within a range of 0.05 MPa (Fig. 1B). Sap pH increased over a range of 3 units as soil water content decreased, although most readings were between pH 5.5 and 7.5. These results are comparable to those in which xylem sap pH was measured over a drying period in other species (Table I) except in castor bean, in which sap became more acidic with drought (Schurr and Schulze, 1996), and the
range of pH covered here is similar to that detected in B. pendula shoot xylem sap by Sauter and Ambrosius (1986). As
expected (Davies and Zhang, 1991), these changes were detected in
plants that exhibited only very small variations in shoot water
potential, indicating that they must be signals arising from an effect
of decreased soil water on the root.
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). More recently, Fromard et
al. (1995) found that xylem vessel-associated cells from Robinia pseudoacacia wood, which contain high concentrations of plasma membrane proton-pumping ATPases, were able to influence xylem sap pH.
Presumably this also occurs in roots. There is some evidence that
drought-related and PAR-related pH changes in xylem/apoplastic sap
might result from differential proton-pumping activity in the cells of
these plants (Hartung and Radin, 1989; Marré et al., 1989;
Hoffmann and Kosegarten, 1995). It is not known, however, whether
proton-pumping rates are directly affected by tissue dehydration, i.e.
whether the pH changes are confined to the root or whether they are
indirectly influenced by alterations in ion concentrations in the sap
flowing from roots adjacent to drying soil. If the effect of drought on
ATPase activity is indirect, reduced proton pumping from shoot xylem
parenchyma cells could enhance increases in the pH of the sap flowing
upward from the root. Some evidence exists for the latter, since
increases in stress-induced sap pH tend to become more marked when
measured farther up the plant stem (Hartung and Radin, 1989; Hoffmann
and Kosegarten, 1995). These findings could also be explained by the
possibility that xylem/apoplastic pH might be controlled by ionic
exchange between sap constituents and the walls of adjacent cells (Ryan
et al., 1992). Since the ionic composition of sap flowing from the
roots is changed by the proximity of drying soil (Gollan et al., 1992), the pattern of exchange with wall-bound ions along the length of the
transpiration stream could be altered, thereby changing its pH.
; Tetlow
and Farrar, 1993). Since sugar uptake by the phloem involves ATPase
activity (Delrot and Bonnemain, 1981), apoplastic sugar content may
also influence apoplastic pH and could explain some of the changes in
sap pH with time of day or year.
; Allen et al., 1988). Increasing
the nitrate concentration supplied to roots of sunflower plants has
recently been demonstrated to increase the apoplastic pH of leaves when
sap was extracted by centrifugation (Dannel et al., 1995) or
investigated with fluorescent dyes (Mengel et al., 1994; Hoffmann and
Kosegarten, 1995). It is thought that proton-nitrate cotransport over
the plasma membrane of leaf cells and localized production of
OH after nitrate reduction in the cytoplasm are
responsible for increased apoplastic pH (Ullrich, 1992). However,
increased nitrate levels in xylem sap are not always associated with
increases in pH, and in fact, the reverse can be true (Gollan et al.,
1992; Jackson et al., 1996). The fact that some plant species reduce nitrate in the roots and others perform this vital function in shoots
(Andrews, 1986) could explain some of the variability in xylem/apoplastic pH between species and in the effects of environmental conditions on their physiology.
Effects of pH on Transpiration from Wild-Type Leaves
The transpiration rates of freshly detached wild-type tomato leaflets taking up AS buffered to a well-watered pH of 6.0 were comparable to those of leaves taking up distilled water only, indicating that AS itself had no effect on stomatal aperture (Fig. 2A). Transpiration was reduced from control (well-watered) levels within 90 min when the pH of the AS supplied to the leaves was adjusted to the pH of xylem sap extracted from plants growing in drying soils (Fig. 2A, pH 6.75; Fig. 2B, pH 7.75). These findings support the hypothesis that increased sap pH closes stomata, but they do not provide evidence for the involvement of ABA in this process. For C. communis such evidence was provided (Wilkinson and Davies, 1997) by preincubating leaves overnight
in distilled water with or without a low concentration of ABA,
equivalent to that found in xylem sap from well-watered plants (0.01 mmol m3). Only when ABA had been present were
the leaves able to respond to subsequent treatments with high- pH
buffers. It was assumed that the solution in which the leaves were
preincubated replaced the contents of the xylem vessels and the
apoplast of the fresh leaf. High-pH-induced ABA accumulation in the
apoplast adjacent to the guard cells was no longer possible in leaves
treated in water alone and, consequently, stomata remained open.
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High pH Only Reduces Transpiration from flacca Leaves in Combination with a Low Concentration of ABA
Figure 3 illustrates an example of several experiments that showed that flacca leaf transpiration rates were not reduced by pH of up to 7.75 (within the range detected in droughted tomato plants), although the stomata were able to close in response to decreased PAR. This is in direct contrast to the situation in wild-type leaves (Fig. 2). However, when a low, well-watered concentration of (+)-ABA (0.03 mmol m3) was supplied in the AS, which did not
reduce transpiration at pH 6.25, the high pH mimicked its normal effect
in wild-type leaves, i.e. it reduced the rate of transpirational water
loss (Fig. 4). This is direct evidence
that ABA is an absolute requirement for the reduction of
transpirational water loss from detached tomato leaves by increased
xylem sap pH.
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High pH Can Directly Increase Transpiration from flacca Leaves
Figure 4A also shows that in the absence of ABA the transpiration rate of flacca leaves at pH 7.75 was initially higher than at pH 6.25. This effect was present to different degrees in different experiments and was sometimes completely absent (Fig. 3). Figure 4B illustrates an example of a group of experiments in which, in the absence of exogenous ABA, the transpiration rate at pH 7.75 was steadily much higher than at pH 6.75. This is a novel finding in whole leaves, although as described above there is evidence that increasing the external pH opens stomata in isolated abaxial epidermal peels of C. communis (Wilkinson and Davies, 1997; see also Schwartz
et al., 1994).
) revealed that flacca leaves picked on
different days contained between 0.1 and 1.5 nmol
g1 dry weight ABA, whereas wild-type tomato
leaves contained between 2.0 and 6.0 nmol g1
dry weight ABA. Bulk leaf ABA is not always a good measure of xylem
ABA, and cross-reactive substances in tomato that interfere with the
radioimmunoassay can result in considerable overestimations of the ABA
concentration actually present (Else et al., 1996). However, these
results demonstrate the potential for the endogenous ABA concentration
of an individual flacca plant to vary on a daily basis. The
plants used in the experiment illustrated in Figure 3 may have
contained closer to 1.5 nmol g1 dry weight bulk
leaf ABA, whereas those used in Figure 4B may have had a 15-fold lower
concentration, with those in Figure 4A containing an intermediate
amount.
3) were all that was required to inhibit the
pH-induced stomatal opening effect seen in some experiments (Fig. 4B)
demonstrates the potency of this hormone and its great capacity
for plant protection, at least in tomato.
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CONCLUSIONS |
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These findings show that the increases in xylem sap pH described in Table I have the potential to widen the stomata in the leaves of these plants, were it not for the ubiquitous presence of a low background ("well-watered") concentration of ABA. The pH signal is potentially harmful, and since it seems to be such a common response to changes in the external environment that are not necessarily otherwise stressful, plants require ABA to function normally, even when they are not experiencing a water deficit in any of their tissues. The plant has used a potentially harmful chemical change induced by a plethora of different factors to actually improve its water use efficiency and therefore its chances of survival.
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FOOTNOTES |
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Received December 9, 1997;
accepted March 10, 1998.
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ABBREVIATIONS |
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Abbreviation: AS, artificial sap.
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ACKNOWLEDGMENT |
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The authors wish to thank Mark Bacon (Lancaster University) for help with data presentation.
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LITERATURE CITED |
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Top
Abstract
Introduction
Methods
Results & Discussion
References
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 M. M. CHAVES, J. S. PEREIRA, J. MAROCO, M. L. RODRIGUES, C. P. P. RICARDO, M. L. OSORIO, I. CARVALHO, T. FARIA, and C. PINHEIRO How Plants Cope with Water Stress in the Field? Photosynthesis and Growth Ann. Bot., June 15, 2002; 89(7): 907 - 916. [Abstract] [Full Text] [PDF]
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N. M. Holbrook, V.R. Shashidhar, R. A. James, and R. Munns Stomatal control in tomato with ABA-deficient roots: response of grafted plants to soil drying J. Exp. Bot., June 1, 2002; 53(373): 1503 - 1514. [Abstract] [Full Text] [PDF]
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V. Stiller and J. S. Sperry Cavitation fatigue and its reversal in sunflower (Helianthus annuus L.) J. Exp. Bot., May 1, 2002; 53(371): 1155 - 1161. [Abstract] [Full Text] [PDF]
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A. Bahrun, C. R. Jensen, F. Asch, and V. O. Mogensen Drought-induced changes in xylem pH, ionic composition, and ABA concentration act as early signals in field-grown maize (Zea mays L.) J. Exp. Bot., February 1, 2002; 53(367): 251 - 263. [Abstract] [Full Text] [PDF]
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W. Hartung, A. Sauter, and E. Hose Abscisic acid in the xylem: where does it come from, where does it go to? J. Exp. Bot., January 1, 2002; 53(366): 27 - 32. [Abstract] [Full Text] [PDF]
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A. Sauter, W.J. Davies, and W. Hartung The long-distance abscisic acid signal in the droughted plant: the fate of the hormone on its way from root to shoot J. Exp. Bot., October 1, 2001; 52(363): 1991 - 1997. [Abstract] [Full Text] [PDF]
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C. G. Mata and L. Lamattina Nitric Oxide Induces Stomatal Closure and Enhances the Adaptive Plant Responses against Drought Stress Plant Physiology, July 1, 2001; 126(3): 1196 - 1204. [Abstract] [Full Text] [PDF]
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A.G. Netting pH, abscisic acid and the integration of metabolism in plants under stressed and non-stressed conditions: cellular responses to stress and their implication for plant water relations J. Exp. Bot., February 2, 2000; 51(343): 147 - 158. [Abstract] [Full Text] [PDF]
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I.C. Dodd, J. He, C.G.N. Turnbull, S.K. Lee, and C. Critchley The influence of supra-optimal root-zone temperatures on growth and stomatal conductance in Capsicum annuum L. J. Exp. Bot., February 2, 2000; 51(343): 239 - 248. [Abstract] [Full Text] [PDF]
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M. A. Bacon, S. Wilkinson, and W. J. Davies pH-Regulated Leaf Cell Expansion in Droughted Plants Is Abscisic Acid Dependent Plant Physiology, December 1, 1998; 118(4): 1507 - 1515. [Abstract] [Full Text]
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