Effects of root medium pH on water transport in paper birch (Betula papyrifera) seedlings in relation to root temperature and abscisic acid treatments.

Abstract

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 (Lp) 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 degrees C decreased Qv irrespective of medium pH. Low root temperatures did not offset the effects of medium pH 4 on Qv 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 micro M 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 degrees C, but not 4 degrees 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.

Full text

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Summary
water transport in whole-plant and detached roots of paper
birch(BetulapapyriferaMarsh.).Exposureofseedlingrootsto
pH4and8significantlydecreasedroot hydraulicconductivity
(Lp) 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) de-
clinedwithinminutes,followedbyadeclineings.Therootox-
ygen uptake rates were not significantly affected by the pH
treatments.Treatment of roots with mercuric chloride resulted
inalargedecreaseinQvatpH6;theextentofthisdecreasewas
similartothatbroughtaboutbypH4and8.Loweringroottem-
perature from 21 to 4 °C decreased Qvirrespective of medium
pH.Lowroottemperaturesdidnotoffsettheeffectsofmedium
pH4onQvandtherootsinthistreatmenthadahighactivation
energyforwaterflow.Conversely,rootsexposedtopH8hada
low activation energy, similar to that at pH 6. When 2 µM abs-
cisic acid, (±)-cis-trans-ABA, was added to the root medium,
Qvincreased in roots that were incubated at pH 6. It also in-
creasedslightlyinrootsincubatedatpH4,butnotatpH8.The
increase at pH 4 and 6 was temperature-dependent, occurring
at21°C,butnot4°C.WesuggestthatthepHtreatmentsarere-
sponsible 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 channelsare modulated
by other, possibly metabolic- and pH-dependent factors.
WeinvestigatedtheeffectsofrootmediumpHon
Keywords: activation energy, root hydraulic conductivity, sto-
matal conductance.
Introduction
Transport of water across cell membranes is a fundamental re-
quirement for regulating plant water status. Many stress fac-
tors including salinity (Azaizeh and Steudle 1991), drought
(Martre et al. 2001, Siemens and Zwiazek 2003), root temper-
ature(WanandZwiazek1999)andoxygendeprivation(Dell’-
Amicoetal.2001,KamaluddinandZwiazek2002,Tournaire-
Roux et al. 2003) affect root water transport by altering hy-
draulic conductivity of roots (Lp). Although many environ-
mental factors affect pH in the rhizosphere and within root
cells (Tyerman et al. 2002), effects of pH on root water trans-
port 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
(Gunséetal.1997).However,littleisknownaboutthemecha-
nisms of pH action on root water flow and the resulting effects
on plant water status.
Shootwaterstatusinplantsisprofoundlyaffectedbythetis-
sue water flow resistance, most of which is attributable to re-
sistance 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 trans-
cellular 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 activ-
ity 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 in-
creases 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 cellsand roots
has produced variable results (Tyerman and Steudle 1984,
Gunséetal.1997,Ktitorovaetal.1998),ithasbeenshownthat
some mammalian water channel proteins are sensitive to pH
(Yasui et al. 1999, Zeuthen and Klaerke 1999, Németh-
CahalanandHall2000).CellularpHissensitivetoexternalpH
(Gunsé et al. 1997, Katsuhara et al. 1997, Gerendas and Rat-
cliffe 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.
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. KAMALUDDIN1and JANUSZ J. ZWIAZEK1,2
1Department of Renewable Resources, 4-42 Earth Sciences Bldg., University of Alberta, Edmonton, AB T6G 2E3, Canada
2Corresponding author (janusz.zwiazek@ualberta.ca)
Received November 28, 2003; accepted February 28, 2004; published online August 2, 2004
at University of Portland on May 24, 2011
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2000) and that the effect of ABA on water flow is pH depend-
ent(Bürneretal.1993,Freundletal.1998,Hoseetal.2000).It
is possible that ABA acts as a chemical signal, triggering
changes in root hydraulic conductivity. However, because in-
creased root hydraulic conductivity may be detrimental to se-
verely stressed plants (Siemens and Zwiazek 2003), ABA ac-
tion must be controlled by other internal factors. This may ex-
plain 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 ef-
fective mechanism for controlling water channel activity.
We examined root water transport properties in birch (Betu-
la papyrifera Marsh.) exposed to different root medium pH
values. We examined the hypothesis that root hydraulic con-
ductivity 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 im-
pact on shoot hydration and stomatal conductance.
Materials and methods
Plant materials and growth room conditions
Betula papyrifera seedswere germinated and seedlingsgrown
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
thepottedseedlings.Atsixweeks,54seedlingswererandomly
assigned to pH treatments, as outlined below, and placed in a
growthroomwitha16-hphotoperiod,aphotosyntheticphoton
flux (PPF) of 350 µmol m–2s–1, day/night temperatures of
22°C/18°Candaconstantrelativehumidityofapproximately
65%.
In another experiment, birch seedlingswere grown in styro-
foam 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 de-
scribed above.
Treatments
Three pH treatments were establishedwith equimolar concen-
trations of H2SO4, NaOH or Na2SO4dissolved in distilled wa-
ter. Low pH (3.25), high pH (8.97) and medium (pH 6.01) pH
were achieved by adding 24.5 mg H2SO4l–1, 20.0 mg NaOH
l–1and 35.5 mg Na2SO4l–1to distilled water, respectively.
These treatments resulted in 24.0 mg SO4 l–1at low pH,
11.5 mg Na l–1at high pH and 24 mg SO4+ 11.5 mg Na l–1at
medium pH. For the short-term experiments, when excised
roots of plants grown in quarter-strength Hoagland’s solution
wereimmersedinthepHsolution,therewerechangesinpHof
the bulk solution as expected (Bledsoe and Zasoski 1983,
DurandandBellon1994).After2.5h,pHofthebulksolutions
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.
Inthelong-termexperiment,thepHsolutionswereaddedto
peat-mossmediafour timesaday,followingtheapplicationof
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 car-
ried out in excised root systems of 6-week-old potted seed-
lings subjected to the respective pH treatments (n = 6) on days
1,4and6.ThepHsolutionswereappliedtothesoilfour times
adayfollowingtheapplicationofquarter-strengthHoagland’s
solution twice a day in the morning and afternoon. A high-
pressure flow meter (HPFM, Dynamax Inc., Houston, TX)
measured Kr as previously described (Kamaluddin and
Zwiazek 2002). During the measurements, each root system
waskeptintactinthepotandsubjectedtopressuresincreasing
from 0 to 0.35 MPa. The slope of the regression between pres-
sureandKrgivestherateofchangeinwaterflowwithincreas-
ing pressure(kg MPa–1s–1) (Tyree et al.1995). The volume of
each root system was determined from the volume of dis-
placed water (Kamaluddin and Zwiazek 2002) and root hy-
draulic conductivity (Lp) was obtained by dividing Krby the
root volume, expressed in kg MPa–1s–1cm–3root volume.
Measurements of leaf stomatal conductance
Leaf stomatalconductance(gs) wasmeasuredon days1,4 and
6, following the commencement of pH treatments. The same
seedlings measured for gswere used for recording Lp. The
measurementswerecarriedoutinagrowthchamber,underthe
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–2s–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 hy-
drostatic pressure method (Wan and Zwiazek 1999, Kamalud-
dinandZwiazek2002)withsolution-culture-grownseedlings.
A0.25-lglasscuvettecontaining230ml of distilledwater was
insertedinto apressurechamber (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 pro-
truding stem was fitted to a graduated pipette by a short piece
ofrubber tubingandthewaterexpressedthroughthestemwas
collected in the pipette. Root Qvof the whole root system was
monitored over time by recording the volume of sap every
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5 min and the results were expressed in µl H2O min–1per root
system.
The steady-state root flow rate of each root system was re-
cordedfor 30minunder aconstantpressureof 0.3MPabefore
treatment with pH solutions in the following order: (1) pres-
sure was released and the appropriate amount of pH stock-so-
lution was added to the bathing solution; (2) pressure was
restoredto0.3MPaandtheflowwasmonitoredfor60min;(3)
pressurewasreleasedand50µMHgCl2wasaddedtothebath-
ingsolution;(4)pressurewasrestoredto0.3MPaandtheflow
was monitored for another 60 min; (5) pressure was released
and 20 mM 2-mercaptoethanol was added to the bathing solu-
tion;and(6)pressurewasrestoredto0.3MPaandtheflowwas
monitored for 30 min. In this way, Qvof six root systems was
monitored for each pH treatment (n = 6). The mean Qvvalue
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 Clark-
type 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 ap-
propriate amount of pH stock solution was added to the bath-
ing solution to achieve the predetermined pH and the roots
were incubated at room temperature for 2 h. The bathing solu-
tion 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–1per root system.
Mean oxygen uptake rate recorded before treatment was used
to normalize the data for each root system.
Measurements of gsin intact seedlings and excised shoots
Whole seedlings and excised shoots grown in solution culture
treated with the respective pH solutions were measured for gs.
Themeasurementsofgswerecarriedoutasdescribedaboveat
a PPF of 350 µmol m–2s–1in 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 sev-
ered 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
pHtreatmentswereinitiatedimmediatelyaftershootexcision,
and the first gsmeasurements were carried out 2 h after the
commencement of pH treatments.
Root water flow: rate and activation energy
The Qvand activation engery for root water flow (Ea) of ex-
cised roots of the seedlings grown in solution culture were de-
termined. Excised root systems were immersed in the respec-
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
ratewasmeasuredfollowingtheprocedureasdescribedabove
with root temperatures decreasing from 21 to 16, 10 and 4 °C.
The temperatures were monitored with a microprocessor ther-
mometerwithafine-wiretypeJ-K-Tthermocouplesealedinto
thepressurechamberthroughtherubberstopper.Thechamber
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
werenormalizedtoseparatethetemperatureandpHeffectson
rootwaterflow.ThemeanQvobtainedat21°Cnormalizedthe
data for each root system.
TheArrheniusplotswereobtainedbyplottingthelogarithm
ofQvagainstthereciprocaloftheabsolutetemperatures.Acti-
vation 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 Eaand
standard deviation calculated.
Measurements of Qvin response to ABA
Excised roots from the seedlings grown in solution culture
wereusedforthemeasurementsofQvfollowingtheprocedure
describedabove.Theexcisedrootsystemwasimmersedinthe
respective pH solution, sealed into the pressure chamber and
pressurizedtoaconstantpressureof0.3MPa.Thesteady-state
root flow rate was measured at a root temperature of 21 or
4 °C. The temperature was controlled by the circulating cool-
ing system connected to the pressure chamber, as described
above. When the desired root temperature was reached and
rootQvbecamestableatagiventemperatureandpH,theQvof
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 re-
leasedfromthepressurechamberand2µMABAwasaddedto
thebathingsolution.Thepressureof0.3MPawasrestoredand
the flow was monitored every 3 min for 90 min. The steady-
state root flow rate was measured for six root systems at each
pH (n = 6). The mean Qvobtained over the initial 30 min nor-
malized 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 signifi-
cant differences between the treatments.
Results
Root hydraulic conductivity and gs
Roots exposed to pH 4 and 8 showed significantly lower Lp
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EFFECTS OF ROOT MEDIUM pH ON WATER TRANSPORT IN SEEDLINGS1175
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compared with those exposed to pH 6 (Figure 1A) within one
day of pH treatments. Mean Lpfor 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 Lp, leaf gswas significantly lower in plants sub-
jected to pH 4 and 8 treatments compared with pH 6 (Fig-
ure1B).ThedifferenceinmeangsbetweenpH6andpH4and
8 was ~12% on Day 1 and ~46% on Day 4 (Figure 1B).
Steady-state root water flow and root respiration
Pressure-induced root Qvdecreased within several minutes of
exposure to pH 4 and 8, whereas the Qvvalues at pH 6 re-
mained almost constant over the measurement period (Fig-
ure 2A). Mean Qvbefore pH treatments were 67, 76, 66 and
72 µl min–1for pH 4, 8, 6 and 6 (control), respectively. After
1hofthepHtreatments,themeanQvdecreasedto50µlmin–1
at pH 4, 58 µl min–1at pH 8, 65 µl min–1at pH 6 and 70 µl
min–1at pH 6 (control). After 1 h of the pH treatments, mean
QvatpH4decreasedto75%ofthepretreatmentQvandthede-
cline was similar at pH 8. A sharp decline in Qvover time was
observed when pH 6-treated roots were subjected to 50 µM
HgCl2(Figure2A).Onehourfollowingthecommencementof
HgCl2treatment, mean Qvdeclined by 37% at pH 6 compared
with the Qvbefore Hg treatment versus 18 and 16% at pH 4
and 8, respectively. The HgCl2-induced declines in Qvwere
partially reversed withthe
2-mercaptoethanol (ME) to the bathing solution (Figure 2A).
The addition of ME increased mean Qvby 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
ratesofroots(Figure2B).After2hofincubation,rootoxygen
uptakeratesdidnotsignificantly(P=0.85)differamongpH4,
6 and 8 from the respective pretreatment rates.
additionof 20mM
Short-term responses of gs
Both pH 4 and 8 treatments significantly reduced gsin intact
seedlings (Figure 3A). The decreases in gswere observed 18 h
after the initiation of pH treatments and after 26 h mean gsde-
clinedtoapproximately45–50%ofthecorrespondingvaluein
1176 KAMALUDDIN AND ZWIAZEK
TREE PHYSIOLOGY VOLUME 24, 2004
Figure 1. Effects of medium pH on (A) root hydraulic conductivity
(Lp),and(B)stomatalconductance(gs).Dataaremeans ±SE(n=6).
Measurementsweretakenoveraperiodof6daysaftertheinitiationof
pH treatments. First data points indicate measurements taken 24 h af-
ter the initiation of the treatments.
Figure 2. Short-term effects of medium pH on (A) root water flow
(Qv) and (B) oxygen uptake rates. Data are means ± SE (n = 6). Ar-
rows in(A)indicatethetimeoftreatmentwithpH solutions,HgCl2or
2-mercaptoethanol(ME).TheQvvalueswerenormalizedtothemean
rate over the initial 30 min before the initiation of pH treatments.
Therewere two setsofplantsatpH 6.0. One setwastreatedwithmer-
cury, whereas the other set was maintained as the control.
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pH 6-treated seedlings. The effect was similar at pH 4 and 8
throughout the measurement period (Figure 3A).
The gsvalues declined over time in excised shoots irrespec-
tive of the pH treatment and there were no significant differ-
encesbetween the treatments (Figure 3B).
Steady-state root water flow responses to root pH and
temperature
Steady-state root water flow gradually declined with decreas-
ing temperature, irrespective of pH (Figure 4A). Factorial
analysisofthedatarevealedsignificanteffectsofbothtemper-
ature (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 Qvdecreased from 65 µl min–1at 21 °C to 22 µl
min–1at 4 °C, at pH 8 from 54 µl min–1at 21 °C to 25 µl min–1
at 4 °C,and at pH 6 from 47 µl min–1at 21 °C to 24 µl min–1at
4 °C. Over the descending temperature range, mean Qvgradu-
ally decreased to 34, 46 and 51% of the corresponding mean
Qvat 21 °C at pH 4, 6 and 8, respectively (Figure 4A). The ef-
fect of pH in lowering Qvwas more pronounced at pH 4 and,
over the temperature range, this pH treatment gave rise to sig-
nificantly lower Qvvalues than the remaining pH treatments.
Across the temperature range, mean Qvbetween pH 6 and pH
8 did not differ at P = 0.05.
The roots showed linear Arrhenius plots for Qv(Figure 4B).
At21°C,meanQvvalueswere65,54and47µlmin–1atpH4,
8 and 6, respectively. Difference in root size among pH treat-
ments was the likely cause of the differences in the flow rates.
The differences in Qv, however, should not have affected the
magnitude of Eabecause it depends on the slope of the regres-
sion line. In the present experiment, Eawas significantly (P <
0.01) affected by pH treatments. Values of Eawere 10.62 (±
0.55), 8.19 (± 0.48) and 6.57 (± 0.42) kcal mol–1for 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 Qvdecreased
from68to60µlmin–1atpH4,63to56µlmin–1atpH8and66
to 64 µl min–1at 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,meanQvdecreasedfrom 65to60µlmin–1and67to
62 µl min–1at pH 4 and 8, respectively, with no decrease in Qv
(68 µl min–1at the start and 70 µl min–1after 30 min) at pH 6.
The decreases in Qvdiminished in Figure 5 because the data
were normalized with the mean Qvobtained over the initial
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EFFECTS OF ROOT MEDIUM pH ON WATER TRANSPORT IN SEEDLINGS1177
Figure 3. Short-term effects of medium pH on stomatal conductance
(gs) of (A) whole plantsand (B) excisedshoots. Data are means ± SE
(n=6).Firstdatapointsindicatemeasurementstaken2haftertheini-
tiation of pH treatments.
Figure 4. Short-term effects of root temperatures on (A) root water
flow(Qv),and(B)ArrheniusplotsforQv.Rootswereincubatedatdif-
ferent medium pHs during measurement. The Qvwas continuously
measured at 0.3 MPa by changing root temperatures (T) from 21 to
4 °C. In(A) Qvwasnormalizedtothemeanrateat21 °C foreach root
system. Data in (A) are means ± SE (n = 6).
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