Tree Physiology 24, 1173–1180
© 2004 Heron Publishing—Victoria, Canada
Effects of root medium pH on water transport in paper birch (Betula
papyrifera) seedlings in relation to root temperature and abscisic
acid treatments
M. KAMALUDDIN1 and JANUSZ J. ZWIAZEK1,2
1
2
Department of Renewable Resources, 4-42 Earth Sciences Bldg., University of Alberta, Edmonton, AB T6G 2E3, Canada
Corresponding author ([email protected])
Received November 28, 2003; accepted February 28, 2004; published online August 2, 2004
Summary We investigated the effects of root medium pH on
water transport in whole-plant and detached roots of paper
birch (Betula papyrifera Marsh.). Exposure of seedling roots to
pH 4 and 8 significantly decreased root hydraulic conductivity
(L p) and stomatal conductance (gs), compared with pH 6.
When roots of solution-culture-grown (pH 6) seedlings were
transferred to pH 4 or 8, their steady-state water flow (Qv) declined within minutes, followed by a decline in gs. The root oxygen uptake rates were not significantly affected by the pH
treatments. Treatment of roots with mercuric chloride resulted
in a large decrease in Qv at pH 6; the extent of this decrease was
similar to that brought about by pH 4 and 8. Lowering root temperature from 21 to 4 °C decreased Qv irrespective of medium
pH. Low root temperatures did not offset the effects of medium
pH 4 on Q v and the roots in this treatment had a high activation
energy for water flow. Conversely, roots exposed to pH 8 had a
low activation energy, similar to that at pH 6. When 2 µM abscisic acid, (±)-cis-trans-ABA, was added to the root medium,
Qv increased in roots that were incubated at pH 6. It also increased slightly in roots incubated at pH 4, but not at pH 8. The
increase at pH 4 and 6 was temperature-dependent, occurring
at 21 °C, but not 4 °C. We suggest that the pH treatments are responsible for altering root water flow properties through their
effects on the activity of water channels. These results support
the concept that ABA effects on water channels are modulated
by other, possibly metabolic- and pH-dependent factors.
Keywords: activation energy, root hydraulic conductivity, stomatal conductance.
Introduction
Transport of water across cell membranes is a fundamental requirement for regulating plant water status. Many stress factors including salinity (Azaizeh and Steudle 1991), drought
(Martre et al. 2001, Siemens and Zwiazek 2003), root temperature (Wan and Zwiazek 1999) and oxygen deprivation (Dell’Amico et al. 2001, Kamaluddin and Zwiazek 2002, TournaireRoux et al. 2003) affect root water transport by altering hy-
draulic conductivity of roots (L p). Although many environmental factors affect pH in the rhizosphere and within root
cells (Tyerman et al. 2002), effects of pH on root water transport have not been thoroughly examined. Plant roots can alter
the pH at the root–soil interface (Bledsoe and Zasoski 1983,
Durand and Bellon 1994). Root medium pH, in turn, can
change cellular pH (Felle 2002) and affect water transport
(Gunsé et al. 1997). However, little is known about the mechanisms of pH action on root water flow and the resulting effects
on plant water status.
Shoot water status in plants is profoundly affected by the tissue water flow resistance, most of which is attributable to resistance in the path between root surface and the xylem
(Steudle and Peterson 1998). This radial water flow depends
on apoplastic, symplastic and transcellular pathways (Steudle
and Frensch 1996, Steudle and Peterson 1998). In the transcellular pathway, water is transported across cell membranes,
predominantly through water channels (Daniels et al. 1994,
Maurel 1997, Tyerman et al. 1999) and its rate is regulated by
changes in the density of water channel proteins and the activity of water channels (Chrispeels and Maurel 1994, Steudle
and Henzler 1995, Johansson et al. 1998). There is evidence
that phosphorylation of some plant water channel proteins increases water channel activity (Maurel et al. 1995, Johansson
et al. 1998) and that this process can be affected by apoplastic
water potential (Johansson et al. 1996).
Although the limited amount of work that has been done on
the effects of pH on water permeability of plant cells and roots
has produced variable results (Tyerman and Steudle 1984,
Gunsé et al. 1997, Ktitorova et al. 1998), it has been shown that
some mammalian water channel proteins are sensitive to pH
(Yasui et al. 1999, Zeuthen and Klaerke 1999, NémethCahalan and Hall 2000). Cellular pH is sensitive to external pH
(Gunsé et al. 1997, Katsuhara et al. 1997, Gerendas and Ratcliffe 2000) and other environmental factors, including low
temperature (Yoshida 1995). Therefore, it is plausible that
changes in the root medium pH affect plant water uptake
through effects on water channels.
There is growing evidence that abscisic acid (ABA) affects
root water flow (Glinka 1973, Quintero et al. 1998, Hose et al.
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KAMALUDDIN AND ZWIAZEK
2000) and that the effect of ABA on water flow is pH dependent (Bürner et al. 1993, Freundl et al. 1998, Hose et al. 2000). It
is possible that ABA acts as a chemical signal, triggering
changes in root hydraulic conductivity. However, because increased root hydraulic conductivity may be detrimental to severely stressed plants (Siemens and Zwiazek 2003), ABA action must be controlled by other internal factors. This may explain why some studies found no effect of ABA on root water
flow properties (Markhart et al. 1979a, Wan et al. 2001). It is
plausible that, through its effects on enzyme activity and cell
metabolism, the regulation of pH may provide a rapid and effective mechanism for controlling water channel activity.
We examined root water transport properties in birch (Betula papyrifera Marsh.) exposed to different root medium pH
values. We examined the hypothesis that root hydraulic conductivity is sensitive to pH and that pH and root temperature
affect root hydraulic responses to ABA. We also hypothesized
that changes in root water properties have an immediate impact on shoot hydration and stomatal conductance.
Materials and methods
Plant materials and growth room conditions
Betula papyrifera seeds were germinated and seedlings grown
for 6 weeks in plastic pots (130-mm diameter) filled with a
peat and sand mixture (1:1, v/v). A quarter-strength modified
Hoagland’s solution (Epstein 1972) was regularly applied to
the potted seedlings. At six weeks, 54 seedlings were randomly
assigned to pH treatments, as outlined below, and placed in a
growth room with a 16-h photoperiod, a photosynthetic photon
flux (PPF) of 350 µmol m – 2 s –1, day/night temperatures of
22 °C/18 °C and a constant relative humidity of approximately
65%.
In another experiment, birch seedlings were grown in styrofoam containers filled with a peat and sand mixture (1:1, v/v)
for 3 weeks before being transferred to solution culture. The
roots of seedlings were gently washed free of soil in cold tap
water and the seedlings were transferred to 10-l containers
with quarter-strength modified Hoagland’s solution (Epstein
1972). There were either 8 or 10 seedlings in each of the
16 containers. The containers with seedlings were placed in a
controlled-environment growth room under the conditions described above.
Treatments
Three pH treatments were established with equimolar concentrations of H2SO4, NaOH or Na2SO4 dissolved in distilled water. Low pH (3.25), high pH (8.97) and medium (pH 6.01) pH
were achieved by adding 24.5 mg H2SO4 l –1, 20.0 mg NaOH
l –1 and 35.5 mg Na2SO4 l –1 to distilled water, respectively.
These treatments resulted in 24.0 mg SO4 l –1 at low pH,
11.5 mg Na l –1 at high pH and 24 mg SO4 + 11.5 mg Na l –1 at
medium pH. For the short-term experiments, when excised
roots of plants grown in quarter-strength Hoagland’s solution
were immersed in the pH solution, there were changes in pH of
the bulk solution as expected (Bledsoe and Zasoski 1983,
Durand and Bellon 1994). After 2.5 h, pH of the bulk solutions
was pH 4 (4.0 – 4.5), pH 8 (7.6–8.0) and pH 6 (5.7–6.3) as
measured for five samples in each pH treatment.
In the long-term experiment, the pH solutions were added to
peat-moss media four times a day, following the application of
the Hoagland’s solution. The leachates had pH values similar
to those stated above.
Measurements of root hydraulic conductance and
conductivity
Measurements of root hydraulic conductance (Kr ) were carried out in excised root systems of 6-week-old potted seedlings subjected to the respective pH treatments (n = 6) on days
1, 4 and 6. The pH solutions were applied to the soil four times
a day following the application of quarter-strength Hoagland’s
solution twice a day in the morning and afternoon. A highpressure flow meter (HPFM, Dynamax Inc., Houston, TX)
measured Kr as previously described (Kamaluddin and
Zwiazek 2002). During the measurements, each root system
was kept intact in the pot and subjected to pressures increasing
from 0 to 0.35 MPa. The slope of the regression between pressure and K r gives the rate of change in water flow with increasing pressure (kg MPa –1 s –1) (Tyree et al. 1995). The volume of
each root system was determined from the volume of displaced water (Kamaluddin and Zwiazek 2002) and root hydraulic conductivity (L p) was obtained by dividing Kr by the
root volume, expressed in kg MPa –1 s –1 cm –3 root volume.
Measurements of leaf stomatal conductance
Leaf stomatal conductance (gs) was measured on days 1, 4 and
6, following the commencement of pH treatments. The same
seedlings measured for gs were used for recording L p. The
measurements were carried out in a growth chamber, under the
conditions described above, with a steady-state porometer
(LI-1600, Li-Cor, Lincoln, NE). Photosynthetic photon flux
during the measurements was about 350 µmol m –2 s –1. A fully
expanded leaf was measured for each of the six seedlings per
treatment (n = 6).
Measurements of steady-state root water flow
The steady-state root flow rate (Qv) was measured by the hydrostatic pressure method (Wan and Zwiazek 1999, Kamaluddin and Zwiazek 2002) with solution-culture-grown seedlings.
A 0.25-l glass cuvette containing 230 ml of distilled water was
inserted into a pressure chamber (PMS Instruments, Corvallis,
OR). The solution was continuously stirred with a magnetic
stirrer during the measurements. For the measurements, the
stem was severed above the collar region and the roots sealed
in the pressure chamber. The entire root system was immersed
in the solution with the debarked part of the stem protruding
through a rubber gasket secured to the lid of the pressure
chamber. Chamber pressure was gradually increased to
0.3 MPa and held constant during the measurements. The protruding stem was fitted to a graduated pipette by a short piece
of rubber tubing and the water expressed through the stem was
collected in the pipette. Root Qv of the whole root system was
monitored over time by recording the volume of sap every
TREE PHYSIOLOGY VOLUME 24, 2004
EFFECTS OF ROOT MEDIUM pH ON WATER TRANSPORT IN SEEDLINGS
5 min and the results were expressed in µl H2O min –1 per root
system.
The steady-state root flow rate of each root system was recorded for 30 min under a constant pressure of 0.3 MPa before
treatment with pH solutions in the following order: (1) pressure was released and the appropriate amount of pH stock-solution was added to the bathing solution; (2) pressure was
restored to 0.3 MPa and the flow was monitored for 60 min; (3)
pressure was released and 50 µM HgCl2 was added to the bathing solution; (4) pressure was restored to 0.3 MPa and the flow
was monitored for another 60 min; (5) pressure was released
and 20 mM 2-mercaptoethanol was added to the bathing solution; and (6) pressure was restored to 0.3 MPa and the flow was
monitored for 30 min. In this way, Qv of six root systems was
monitored for each pH treatment (n = 6). The mean Qv value
obtained over the initial 30 min normalized the data for each
root system.
Root respiration measurements
Root respiration was measured as oxygen uptake with a Clarktype electrode (Yellow Springs Instruments, Yellow Springs,
OH). Oxygen uptake rates were determined by placing each
excised root system in an airtight cylinder containing distilled
water before and after the treatment with pH solutions. An appropriate amount of pH stock solution was added to the bathing solution to achieve the predetermined pH and the roots
were incubated at room temperature for 2 h. The bathing solution was kept continuously stirred with a stirring bar during
measurements. Oxygen uptake was monitored for 30 min by
recording data every 3 min in six root systems for each pH
treatment (n = 6). Respiration rate was the average of oxygen
uptake over time and expressed in mg min –1 per root system.
Mean oxygen uptake rate recorded before treatment was used
to normalize the data for each root system.
Measurements of gs in intact seedlings and excised shoots
Whole seedlings and excised shoots grown in solution culture
treated with the respective pH solutions were measured for gs.
The measurements of gs were carried out as described above at
a PPF of 350 µmol m –2 s –1 in the same growth chamber where
the seedlings were growing. Thirty-six seedlings from the
large containers were transferred to 0.5-l plastic containers
filled with distilled water mixed with appropriate amounts of
the pH stock-solutions. The shoots of 18 seedlings were severed under water above the root collar and the cut ends of the
stems kept immersed in the pH solution. The bathing solution
was continuously aerated. A fully expanded leaf was marked
on each seedling or excised shoot and measured for 26 h in six
seedlings or excised shoots for each pH treatment (n = 6). The
pH treatments were initiated immediately after shoot excision,
and the first gs measurements were carried out 2 h after the
commencement of pH treatments.
1175
tive pH solution and sealed in a pressure chamber. The whole
root system was surrounded by a copper coil connected to a
circulating cooler system (F3, HAAKE, Berlin) to maintain
the desired root temperature (± 0.1 °C). Steady-state root flow
rate was measured following the procedure as described above
with root temperatures decreasing from 21 to 16, 10 and 4 °C.
The temperatures were monitored with a microprocessor thermometer with a fine-wire type J-K-T thermocouple sealed into
the pressure chamber through the rubber stopper. The chamber
was pressurized to a constant pressure of 0.3 MPa with the
compressed air from the gas cylinder, and the solution was
continuously stirred with a magnetic stirrer. The steady-state
root flow rate was monitored every 3 min, for 21 min at each
temperature, in six root systems at each pH (n = 6). The data
were normalized to separate the temperature and pH effects on
root water flow. The mean Qv obtained at 21 °C normalized the
data for each root system.
The Arrhenius plots were obtained by plotting the logarithm
of Q v against the reciprocal of the absolute temperatures. Activation energy for root water flow was calculated from the
slope of the curve. Activation energy for root water flow was
determined for individual root systems and the mean E a and
standard deviation calculated.
Measurements of Qv in response to ABA
Excised roots from the seedlings grown in solution culture
were used for the measurements of Qv following the procedure
described above. The excised root system was immersed in the
respective pH solution, sealed into the pressure chamber and
pressurized to a constant pressure of 0.3 MPa. The steady-state
root flow rate was measured at a root temperature of 21 or
4 °C. The temperature was controlled by the circulating cooling system connected to the pressure chamber, as described
above. When the desired root temperature was reached and
root Qv became stable at a given temperature and pH, the Qv of
each root system was recorded every 3 min for 30 min. Roots
were then treated with (±)-cis-trans-ABA isomer (Sigma, St.
Louis, MO), which has been reported to affect root water flow
(Hose et al. 2000). For the ABA treatment, pressure was released from the pressure chamber and 2 µM ABA was added to
the bathing solution. The pressure of 0.3 MPa was restored and
the flow was monitored every 3 min for 90 min. The steadystate root flow rate was measured for six root systems at each
pH (n = 6). The mean Qv obtained over the initial 30 min normalized the data for each root system.
Data analysis
The data presented in the figures are the means of at least six
replicates. The analysis of variance and Duncan’s Multiple
Range Test were employed to determine statistically significant differences between the treatments.
Root water flow: rate and activation energy
Results
The Qv and activation engery for root water flow (E a ) of excised roots of the seedlings grown in solution culture were determined. Excised root systems were immersed in the respec-
Root hydraulic conductivity and gs
Roots exposed to pH 4 and 8 showed significantly lower L p
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KAMALUDDIN AND ZWIAZEK
compared with those exposed to pH 6 (Figure 1A) within one
day of pH treatments. Mean L p for pH 4 was about 26% lower
than that for pH 6 on Day 1 and the differences were similar
during the next 5 days (Figure 1A).
As with L p, leaf gs was significantly lower in plants subjected to pH 4 and 8 treatments compared with pH 6 (Figure 1B). The difference in mean gs between pH 6 and pH 4 and
8 was ~12% on Day 1 and ~46% on Day 4 (Figure 1B).
Steady-state root water flow and root respiration
Pressure-induced root Qv decreased within several minutes of
exposure to pH 4 and 8, whereas the Qv values at pH 6 remained almost constant over the measurement period (Figure 2A). Mean Q v before pH treatments were 67, 76, 66 and
72 µl min –1 for pH 4, 8, 6 and 6 (control), respectively. After
1 h of the pH treatments, the mean Q v decreased to 50 µl min –1
at pH 4, 58 µl min –1 at pH 8, 65 µl min –1 at pH 6 and 70 µl
min –1 at pH 6 (control). After 1 h of the pH treatments, mean
Q v at pH 4 decreased to 75% of the pretreatment Qv and the decline was similar at pH 8. A sharp decline in Qv over time was
observed when pH 6-treated roots were subjected to 50 µM
Figure 2. Short-term effects of medium pH on (A) root water flow
(Q v) and (B) oxygen uptake rates. Data are means ± SE (n = 6). Arrows in (A) indicate the time of treatment with pH solutions, HgCl2 or
2-mercaptoethanol (ME). The Q v values were normalized to the mean
rate over the initial 30 min before the initiation of pH treatments.
There were two sets of plants at pH 6.0. One set was treated with mercury, whereas the other set was maintained as the control.
HgCl2 (Figure 2A). One hour following the commencement of
HgCl2 treatment, mean Qv declined by 37% at pH 6 compared
with the Q v before Hg treatment versus 18 and 16% at pH 4
and 8, respectively. The HgCl2-induced declines in Q v were
partially reversed with the addition of 20 mM
2-mercaptoethanol (ME) to the bathing solution (Figure 2A).
The addition of ME increased mean Q v by 25, 13 and 15% of
the rates before ME treatment at pH 6, 8 and 4, respectively.
The solution pH did not significantly affect oxygen uptake
rates of roots (Figure 2B). After 2 h of incubation, root oxygen
uptake rates did not significantly (P = 0.85) differ among pH 4,
6 and 8 from the respective pretreatment rates.
Short-term responses of gs
Figure 1. Effects of medium pH on (A) root hydraulic conductivity
(L p ), and (B) stomatal conductance (gs ). Data are means ± SE (n = 6).
Measurements were taken over a period of 6 days after the initiation of
pH treatments. First data points indicate measurements taken 24 h after the initiation of the treatments.
Both pH 4 and 8 treatments significantly reduced gs in intact
seedlings (Figure 3A). The decreases in gs were observed 18 h
after the initiation of pH treatments and after 26 h mean gs declined to approximately 45–50% of the corresponding value in
TREE PHYSIOLOGY VOLUME 24, 2004
EFFECTS OF ROOT MEDIUM pH ON WATER TRANSPORT IN SEEDLINGS
1177
Figure 3. Short-term effects of medium pH on stomatal conductance
(gs ) of (A) whole plants and (B) excised shoots. Data are means ± SE
(n = 6). First data points indicate measurements taken 2 h after the initiation of pH treatments.
pH 6-treated seedlings. The effect was similar at pH 4 and 8
throughout the measurement period (Figure 3A).
The gs values declined over time in excised shoots irrespective of the pH treatment and there were no significant differences between the treatments (Figure 3B).
Steady-state root water flow responses to root pH and
temperature
Steady-state root water flow gradually declined with decreasing temperature, irrespective of pH (Figure 4A). Factorial
analysis of the data revealed significant effects of both temperature (P < 0.01) and pH (P < 0.01) on Qv. Interaction of root
temperatures and medium pH was not significant (P = 0.62).
At pH 4, mean Qv decreased from 65 µl min –1 at 21 °C to 22 µl
min –1 at 4 °C, at pH 8 from 54 µl min –1 at 21 °C to 25 µl min –1
at 4 °C, and at pH 6 from 47 µl min –1 at 21 °C to 24 µl min –1 at
4 °C. Over the descending temperature range, mean Qv gradually decreased to 34, 46 and 51% of the corresponding mean
Qv at 21 °C at pH 4, 6 and 8, respectively (Figure 4A). The effect of pH in lowering Qv was more pronounced at pH 4 and,
over the temperature range, this pH treatment gave rise to significantly lower Qv values than the remaining pH treatments.
Across the temperature range, mean Qv between pH 6 and pH
8 did not differ at P = 0.05.
The roots showed linear Arrhenius plots for Qv (Figure 4B).
At 21 °C, mean Qv values were 65, 54 and 47 µl min –1 at pH 4,
8 and 6, respectively. Difference in root size among pH treatments was the likely cause of the differences in the flow rates.
Figure 4. Short-term effects of root temperatures on (A) root water
flow (Qv), and (B) Arrhenius plots for Qv. Roots were incubated at different medium pHs during measurement. The Qv was continuously
measured at 0.3 MPa by changing root temperatures (T ) from 21 to
4 °C. In (A) Qv was normalized to the mean rate at 21 °C for each root
system. Data in (A) are means ± SE (n = 6).
The differences in Qv, however, should not have affected the
magnitude of Ea because it depends on the slope of the regression line. In the present experiment, Ea was significantly (P <
0.01) affected by pH treatments. Values of Ea were 10.62 (±
0.55), 8.19 (± 0.48) and 6.57 (± 0.42) kcal mol –1 for pH 4, 6
and 8, respectively, and the differences among treatment
means were statistically significant (P = 0.04).
Steady-state root water flow responses to ABA
After 30 min following pH treatments, mean Qv decreased
from 68 to 60 µl min –1 at pH 4, 63 to 56 µl min –1 at pH 8 and 66
to 64 µl min –1 at pH 6 for the roots measured at 4 °C. For the
other set of roots measured at 21 °C, within 30 min after pH
treatment, mean Qv decreased from 65 to 60 µl min –1 and 67 to
62 µl min –1 at pH 4 and 8, respectively, with no decrease in Qv
(68 µl min –1 at the start and 70 µl min –1 after 30 min) at pH 6.
The decreases in Qv diminished in Figure 5 because the data
were normalized with the mean Qv obtained over the initial
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30 min. Small, but significant increases in mean Qv in response
to root-applied ABA were observed at pH 4 and 6 at the 21 °C
root temperature (Figure 5A). In roots incubated at pH 6, Qv
gradually increased within 15–20 min following the ABA
treatment up to about 20% of the initial value measured after
70 min of treatment. Individual plant responses to ABA were
highly variable at pH 4. Out of the six roots measured at pH 4,
three showed slight increases in Qv, whereas the others showed
no response, resulting in high standard errors (Figure 5A). At a
root medium temperature of 4 °C, ABA had no effect on Qv in
any of the pH treatments (Figure 5B).
Discussion
When roots were exposed for up to 6 days to different pHs, L p
and gs were substantially lower at pH 4 and 8 compared with
pH 6. The L p values reported here are comparable with the
published data for other tree species (Kamaluddin and
Zwiazek 2002). Therefore, we also investigated stomatal responses and root water flow properties within the initial several hours following pH treatments and we examined the effects of root medium pH on gs in the presence and absence of
roots. Unlike values at pH 6, Qv at pH 4 and 8 decreased within
Figure 5. Short-term effects of 2 µM (±)-cis-trans-ABA on Qv measured at (A) 21 °C and (B) 4 °C. Roots were incubated at different medium pHs during measurement. Data are means ± SE (n = 6).
minutes, followed by a decline in gs. An inhibition of root water flow by high (Ktitorova et al. 1998) and low (Ord et al
1977, Gunsé et al. 1997) pH has been observed in several plant
species. Our results indicate that root water channels may be
involved in this response. This is evidenced by the patterns of
mercury-induced inhibition of water flow for the different pH
treatments. At pH 6, root Qv was rapidly reduced by HgCl2, but
was little affected when the roots were treated with HgCl2 at
pH 4 or 8. Mercury compounds inhibit membrane water transport by blocking water channels (Chrispeels and Maurel 1994,
Wan and Zwiazek 1999). In our experiment, the inhibition in
Qv by medium pH was unaccompanied by a significant decline
in oxygen uptake rates (Figure 2B), suggesting that the inhibition of Qv by medium pH was not mediated through the general energy generating system (Kamaluddin and Zwiazek
2001, 2002).
Lowering root temperatures significantly (P < 0.01) decreased Qv irrespective of medium pH (Figure 4A). Low root
temperature decreases water flow through roots (Babalola et
al. 1968, McWilliam et al. 1982, Wan and Zwiazek 1999) by
decreasing the fluidity of cell membranes (Markhart et al.
1979b) and inhibiting water channel activity (Wan et al. 2001).
When we separated Qv responses to low temperatures by factorial analysis, medium pH effects were highly significant (P <
0.0001). Roots had higher E a values for water flow at pH 4
than at pH 8. At the cell level, a low E a indicates water flow
through water channels (Tyerman et al. 1999). When water
channels are blocked by Hg, E a increases substantially (Hertel
and Steudle 1997, Schütz and Tyerman 1997) because water
moving through the lipid bilayer has to overcome a high-energy barrier (Tyerman et al. 1999). However, at the whole-root
level, a change in E a, could also indicate a shift between the
apoplastic and cell-to-cell pathways. Apoplastic flow resistance may increase at low pH through shrinkage of cell walls
and a decrease in cell wall volume (Meychik and Yermakov
2001).
In our study, the Arrhenius plots of water flow appeared to
be linear functions of the temperature changes from 21 to 4 °C
(Figure 4B). Both linear and nonlinear Arrhenius plots for root
water flow have been reported in various plant species (Markhart et al. 1979b, Wan and Zwiazek 1999). Nonlinear Arrhenius plots can also be produced by repeated exposure of roots
to 4 °C during the measurements (Wan et al. 2001).
Short-term exposure of roots to pH 4 or 8 induced a significant decline in gs (Figure 3A). This is further evidence of a
strong link between leaf gs and root hydraulic conductance that
has been observed after inhibition of cell-to-cell transport in
roots (Wan and Zwiazek 1999, Kamaluddin and Zwiazek
2001, 2003, Siemens and Zwiazek 2003). Abscisic acid, it has
been proposed, acts as a stress signal between roots and shoots
(Blackman and Davies 1985, Zhang et al. 1987). In our study,
root-applied ABA increased Qv, mostly in the pH 6 treatment,
but only at 21 °C and not at 4 °C. Similarly, in maize roots exposed to an alkaline medium (pH 8), ABA did not affect root
and cell L p (Hose et al. 2000). At high pH, ABA converts into
its anionic form (Hose et al. 2000) and cannot readily permeate
cell membranes (Bürner et al. 1993).
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EFFECTS OF ROOT MEDIUM pH ON WATER TRANSPORT IN SEEDLINGS
Under acidic conditions, a fraction of ABA remains in its
undissociated form and it is suggested that the undissociated
ABA rather than its anionic form causes an increase in water
flow through roots (Hose et al. 2000). In our study, the maximum response of Q v to ABA was observed in roots exposed to
pH 6. At pH 4, ABA caused a slight or no increase in Q v. An
ABA concentration lower or higher than that used in our experiment has been shown to affect L p in maize (Hose et al.
2000) and sunflower roots (Quintero et al. 1998).
In several plant species, exogenous ABA enhanced L p (Quintero et al. 1998, Hose et al. 2000). However, applied ABA had
no effect on Qv in aspen roots (Wan and Zwiazek 2001),
whereas it decreased Q v in soybean roots (Markhart et al.
1979a). The observed increases in Q v in response to ABA may
be attributed to the flow increases through both apoplastic and
cell- to-cell paths. However, the increases in apoplastic flow as
a result of ABA action may be negligible (Quintero et al.
1998). The mechanism by which ABA increases water flow
through the cell-to-cell pathway is unknown, but water channels may be involved (Tyerman et al. 1999, Hose et al. 2000).
Abscisic acid may either trigger existing water channels to
open or affect the density of water channels (Steudle and
Henzler 1995). In our study, the pattern and extent of ABA effects suggest that ABA increased Qv by stimulating water
channel activity of root cell membranes.
In summary, our results demonstrated a large and rapid decrease of root water flow rates and root hydraulic conductivity
when birch roots were exposed to either low or high medium
pH. We suggest that water channel function could be affected
below or above an optimum pH range and result in a decrease
of water flow through roots and leading to a loss of leaf hydration and stomatal closure. Root-applied ABA increased
water flow through roots in a pH- and temperature-dependent
manner.
Acknowledgments
Funding for this study was provided by the Natural Sciences and Engineering Research Council of Canada research grant to J.J.Z.
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