Journal of Experimental Botany, Vol. 51, No. 343, pp. 239-248,
January 2000
© 2000 Oxford University Press
The influence of supra-optimal root-zone temperatures on growth and stomatal conductance in Capsicum annuum L.
1 Department of Botany, The University of
Queensland, St Lucia 4072, Australia
2 School of Science, Nanyang Technological
University, 469 Bukit Timah Road, Singapore 259756
Received 4 May 1999; Accepted 24 September 1999
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Abstract |
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 Top Abstract IntroductionMaterials and methodsReferences |
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Pepper (Capsicum annuum L.) plants were grown aeroponically in a Singapore greenhouse under natural diurnally fluctuating ambient shoot temperatures, but at two different root-zone temperatures (RZTs): a constant 20±2 °C RZT and a diurnally fluctuating ambient (A) (25–40 °C) RZT. Plants grown at 20-RZT had more leaves, greater leaf area and dry weight than A-RZT plants. Reciprocal transfer experiments were conducted between RZTs to investigate the effect on plant growth, stomatal conductance (gs) and water relations. Transfer of plants from A-RZT to 20-RZT increased plant dry weight, leaf area, number of leaves, shoot water potential (
shoot), and gs; while
transfer of plants from 20-RZT to A-RZT decreased these parameters. Root hydraulic
conductivity was measured in the latter transfer and decreased by 80% after 23 d at
A-RZT. Transfer of plants from 20-RZT to A-RZT had no effect on xylem ABA concentration or
xylem nitrate concentration, but reduced xylem sap pH by 0.2 units. At both RZTs, gs measured in the youngest fully expanded leaves increased with plant development.
In plants with the same number of leaves, A-RZT plants had a higher gs
than 20-RZT plants, but only under high atmospheric vapour pressure deficit. The roles of
chemical signals and hydraulic factors in controlling gs of aeroponically
grown Capsicum plants at different RZTs are discussed. Key words: Capsicum, root temperature, growth, high-temperature stress, stomata, chemical signalling.
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Introduction |
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TopAbstractIntroductionMaterials and methodsReferences |
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Ambient tropical conditions of high temperatures and light intensities severely reduce growth of temperate vegetable crops if root-zone temperature (RZT) is not controlled (Lee and Cheong, 1996
; He and Lee, 1998a, b). Potential mechanisms of growth inhibition at ambient RZT include poor nutrient
uptake and/or perturbed water relations.
Although bulk tissue nutrient concentration can
indicate RZT-induced nutrient deficiency (Sheppard et al., 1986), methods which maximize the contribution of the xylem to the sample, such as petiolar (Olsen and Lyons, 1994) or stump (Barthes et al.,
1996) sap extracts, may be better predictors of nutrient status. Xylem sap nitrate
concentration ([X-NO-3]) may be an important indicator of perturbed
nutrient relations for several reasons. Nitrate is a major xylem sap solute (Bassirirad et al., 1991) and leaf growth responds sensitively and rapidly to any
perturbation in nitrate supply (Palmer et al., 1996). Changes in
[X-NO-3] will reflect changes in root uptake as there is minimal
recirculation in the phloem (Jeschke and Pate, 1992). Furthermore, nitrate
uptake may be particularly sensitive to elevated RZT in some species (Udagawa et al., 1991).
Supra-optimal RZT can induce shoot water deficit by
altering the balance between water uptake by the root system and water loss from the shoots.
Transfer of roots to a supra-optimal temperature transiently (up to 48 h; Newman and
Davies, 1988) increases root hydraulic conductivity (Lp). However, in the
long-term (e.g. 28 d; Graves et al., 1991) Lp is
decreased. When water loss exceeds water uptake, the resultant declines in leaf turgor can
directly restrict leaf growth (Bunce, 1977) and close stomata (Turner, 1974), resulting in loss of photosynthetic productivity.
However,
variability in stomatal responses and shoot water relations under supra-optimal RZT suggests that
altered shoot water relations may not entirely account for the observed stomatal behaviour.
Certainly, there are cases where a concurrent reduction of both shoot water potential (shoot) and stomatal conductance (gs) (Graves and Aiello,
1997) suggests that stomata are responding directly to reduced shoot or
reduced turgor. Reduced leaf without stomatal closure (Graves et al., 1991; Menzel et al., 1994) may result from
osmotic adjustment maintaining shoot turgor. Stomatal closure without reduced leaf (Graves et al., 1991; Behboudian et al., 1994) has been interpreted as evidence for root-derived chemical signals
moving via the xylem to the shoots to reduce gs. A principal
candidate for such a signal is the plant hormone abscisic acid (ABA) (reviewed in Dodd et al., 1996). Recently, it has been shown that alkalization of the
xylem sap, without increased xylem sap ABA concentration ([X-ABA]), can cause stomatal
closure (Wilkinson et al., 1998). However, as far as is known,
there are no reports measuring either [X-ABA] or xylem pH in plants grown at supra-optimal
RZTs.
Previous work has shown significant effects of manipulating RZT on lettuce
growth in the tropics (Lee and Cheong, 1996; He and Lee,
1998a, b). However, investigation of
chemical signalling in lettuce is impractical, due to the difficulty of obtaining a xylem sap sample
free of contamination from the latex which exudes from cut lettuce stumps. Xylem sap extraction
from pepper (Capsicum annuum L.) shoots using a pressure bomb presented no such
problems. Pepper growth at different RZTs has not been evaluated previously under tropical
aerial conditions. However, at shoot temperatures of 23/19 °C (day/night), pepper
showed increased growth with increasing root temperatures up to 30 °C, but then a
negative response of leaf area production and dry weight at 36 °C RZT (Gossellin and Trudel, 1986). It was hypothesized, therefore, that pepper would show
reduced growth and gs at ambient RZTs in Singapore. It was also
evaluated whether shoot water relations and/or xylem sap composition were altered at different
RZTs.
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Materials and methods |
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Plant material and growth conditions
Experiments 1 and 2:
Experiments were conducted using aeroponics units (described by Lee and Cheong, 1996
) maintained in a
greenhouse at Nanyang Technological University, Singapore (latitude 2° N) during April–May
1997. Pepper (Capsicum annuum L. cv. Jin Jiao No. 3) seeds were germinated in
seedling trays. Two batches of seedlings were germinated at a 2 week interval, and transferred to
water-soaked polyurethane cubes 20 d after sowing. Three days later, the foam cubes
were transferred to aeroponics units. Nutrient solution (full-strength Netherlands Standard
Composition; Douglas, 1982) was misted over the roots for one
20 s burst every 90 s. Solution conductivity was maintained at 2.2 mS. When a data logger (Data Hog, Skye, Wales) was available, temperature (thermistor) and relative humidity (Vaisala-type sensor) were recorded inside the greenhouse every 10 min. During periods of logger unavailability, greenhouse data were estimated from empirical relationships constructed between greenhouse conditions and those at a nearby Singapore Department of Environment meteorological station. The aerial parts of all plants were exposed to ambient greenhouse temperatures and vapour pressure deficits (VPDs). Average maximum greenhouse temperatures and VPDs during the experimental period were 37.0±0.7 °C and 3.9±0.2 kPa, respectively. The average minimum greenhouse temperature was 26.4±0.2 °C. Half the plants were maintained at a constant RZT of 20±2 °C while the remainder were exposed to a diurnally fluctuating ambient RZT (A-RZT). On sunny days, temperature of the nutrient solution at A-RZT remained at 40±2 °C between 11.00 h and 18.00 h. Incident quantum flux at the canopy surface reached maxima of c. 1700 µmol m-2 s-1.
Experiment 3:
An additional experiment was conducted in
September 1998 with Capsicum annuum cv. Indra F1 hybrid. Plant culture was as
previously described. Several batches of A-RZT and 20-RZT plants were transplanted into the
aeroponics units over several weeks to produce plants grown at different RZTs, but of a similar
developmental stage at the time of measurement. Average maximum greenhouse temperatures
and VPDs during the experimental period were 38.7±0.8 °C and 4.2±0.3 kPa,
respectively. The average minimum greenhouse temperature was 24.9±0.2 °C.
Physiological measurements
Root-zone temperature transfers
were conducted 55 d (Experiment 1) and 41 d (Experiment 2) after sowing, using
two separate batches of plants. Half the plants of each batch were maintained at their original
RZT (either 20- or A-RZT), while the remainder were transferred to the other RZT at
07.30 h under diffuse light (<100 µmol m-2 s-1). There were thus four RZT treatments: 20-RZT, A-RZT, 20
A-RZT, and
A20-RZT.
Every 3–5 d over a 25 d measurement period (5 d
before and 20 d after the temperature transfer), shoot and gs were determined between 09.00 h and 15.00 h. Stomatal
conductance of the three youngest fully expanded leaves (gs) was measured
using a porometer (AP4, Delta-T Devices, Burwell, UK). Small leaf size of A-RZT plants in
Experiment 2 prevented porometric determination of gs until day 4.
Xylem sap was sampled from the same plants used for gs and water
potential measurements. The water potential (shoot) of the entire shoot (decapitated
above the cotyledons) was determined using a pressure bomb (Plant Moisture Systems, Santa
Barbara, CA, USA) whose chamber was lined with moistened filter paper. Following
measurement of shoot in plants originally grown at 20-RZT, xylem sap was
collected at 0.5 MPa above the balancing pressure for 5 min for subsequent
analysis of xylem sap constituents. Small size of plants originally grown at A-RZT prevented the
collection of sufficient xylem sap for analysis. After these measurements, the number of leaves
(>10 mm long), total leaf area and total dry weight of each plant were determined.
On one occasion in Experiment 2, root hydraulic conductivity (Lp) was estimated
by adapting a method used previously (Ford and Harrison-Murray, 1997).
Plants were decapitated just below the cotyledons, and the root system plus stump was placed in
a beaker of nutrient solution in the pressure bomb. After the plant was sealed into the chamber,
the pressure was raised in 0.1 MPa increments (to a maximum of 0.6 MPa) and
held for 3 min at each pressure to allow collection of the exudate by a capillary tube. The
slope of the relationship between exudate flow rate (in mm3 s-1) and applied pressure over the linear part of the curve (0.2–0.6 MPa) gave the
root hydraulic conductivity (Lp). In all cases, correlation coefficients (r2) were >0.95. The temperature of the nutrient solution was determined before and after
pressure application, and did not deviate by more than 3 °C during this time. Removal of
the root system in preliminary tests confirmed that the major resistance to water flow was in the
roots, and not the stem.
Experiment 3 measured gs of leaves
of 20-RZT and A-RZT plants at multiple points of insertion on the main stem on sunny and
cloudy days. shoot was also measured over the course of a sunny day.
Measurement of xylem sap constituents
Xylem ABA concentration was
measured by gas chromatography-mass spectrometry (GC-MS). Twenty nanograms of [2H6] ABA were added to 40 mm3 of sap. Sap was
evaporated to dryness under vacuum, redissolved in 20 mm3 of methanol,
then diazomethane added to methylate ABA. Samples were then dried, redissolved in
20 mm3 of ethyl acetate, and 2 mm3 of sample
injected onto a fused silica DB 5 MS capillary column (25
mx0.2 mmx0.33 µm) with a 5% phenyl, 95% methyl silicone stationary phase
coating (J & W Scientific, Folsom CA, USA). The ion ratios 190:194 and 162:166 were used to
calculate the ABA concentration of duplicate samples.
Xylem nitrate
concentration was measured in 20 mm3 of sap by the nitrite colorimetric
reaction with cadmium as the reducing agent. After 3 min of reaction, 1000 mm3 of the reaction mixture was added to 500 mm3 of sulphanilic
acid and 500 mm3 of 
-naphthyl ethylene diamine dihydrochloride. This
mixture was incubated for 30 min before absorbance at 540 nm was determined
using a spectrophotometer (Model U-1100, Hitachi).
Xylem pH was measured with a microelectrode (Model 98-02 BN, Orion Instruments, Boston, MA, USA) interfaced with a pH meter (Model 900I3, TPS Ionode, Brisbane, Australia).
Statistics
Student's t-test was used to discriminate differences between plants which were
transferred to a new RZT and those that remained at their original RZT. The significance of
regressions was tested in JMP In (SAS Institute Inc., Cary, NC, USA).
Results
On the day prior to the RZT transfers, there were significant (P<0.05)
differences in the number of leaves per plant, total leaf area, and total dry weight of plants grown
at different RZTs (Table 1). Total leaf area and total dry weight of
A-RZT plants was c. 10% of 20-RZT plants. The effect on leaf number was much less,
presumably due to the initiation of leaves in the period between germination and transfer of the
plants to the growth units.
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Transfer of plants from A-RZT to 20-RZT generally increased total dry weight (Fig. 1a, b
), total leaf area (Fig. 1c, d) and number of
leaves (Fig. 1e, f). This response was quite slow to occur, presumably because the plants had yet
to reach their exponential growth phase. In contrast to this sluggish response, transfer of plants
from 20-RZT to A-RZT decreased total dry weight (Fig. 2a, b), total leaf
area (Fig. 2c, d) and number of leaves (Fig. 2e, f) within 8–13 d. In Experiment 1, this
resulted in a 33% reduction in total dry weight after 18 d (Fig. 2a), and a 29% reduction
in both total leaf area (Fig. 2c) and number of leaves (Fig. 2e) after 13 d. Reduced leaf
area expansion and initiation between 13 d and 18 d in 20-RZT plants was due to
fruiting of these plants. In Experiment 2, the RZT transfer resulted in a 38% reduction in total dry
weight after 13 d (Fig. 2b), a 41% reduction in total leaf area after 13 d (Fig. 2d),
and a 23% reduction in number of leaves after 8 d (Fig. 2f).
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) and those grown at
A-RZT but transferred to 20-RZT (
) at day 0. Average atmospheric VPD in the greenhouse
between 09.00 h and 15.00 h on the measurement occasions is given (g, h).
Stomatal conductance measurements are mean±SE of 12 leaves on four plants, while dry weight,
leaf area, number of leaves and
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) and
those grown at 20-RZT but transferred to A-RZT (•) at day 0. Average atmospheric VPD in the
greenhouse between 09.00 h and 15.00 h on the measurement occasions is given
(g, h). Stomatal conductance measurements are mean±SE of 12 leaves on four plants, while dry
weight, leaf area, number of leaves and In both experiments,
shoot of A-RZT and A20-RZT plants increased as the plants grew (Fig. 1i, j).
Transferred plants showed significantly (P<0.01) higher shoot on day 8 in
Experiment 1 (Fig. 1i), and day 18 in Experiment 2 (Fig. 1j). On both occasions, stomatal
conductance of transferred plants was also significantly (P<0.05) higher, and also on
day 17 in Experiment 1 (Fig. 1k), and day 13 in Experiment 2 (Fig. 1l).
shoot of 20-RZT plants and 20A-RZT plants did not statistically differ (P>0.05)
except on day 17 in Experiment 2 (Fig. 2j), when shoot of 20A-RZT plants was
0.18 MPa lower than 20-RZT plants. Regression analysis of data collected after the
transfer (i.e. after day 0) showed no significant (P>0.05) effect of time on shoot (Fig. 2i, j). There were four occasions when stomatal conductance of 20A-RZT plants
was lower than that of 20-RZT plants (Fig. 2k, l).
In 20-RZT and 20A-RZT plants, the
shoot was large enough to permit collection of sufficient xylem sap for analysis throughout the
transfer experiments. For all data collected after the transfer (i.e. after day 0) in both experiments,
regression analysis showed no significant (P>0.05) effect of time on xylem sap ABA
concentration, xylem nitrate concentration or xylem sap pH (data not shown). Accordingly, data
from all harvests were combined to give mean values at each RZT (Table 2). shoot, [X-ABA] and [X-NO-3] in both
experiments, and xylem pH in Experiment 1, did not significantly (P>0.05) differ
between 20-RZT and 20A-RZT plants. In Experiment 2, xylem pH of 20A-RZT plants was
significantly (P<0.05) less (0.2 units) than plants that remained at 20-RZT, a difference
of similar magnitude to that seen in Experiment 1.
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Root hydraulic conductivity (Lp) of 20-RZT and 20
A-RZT plants was measured 23 d after the RZT transfer in
plants remaining from Experiment 2. Measurements occurred at two nutrient solution
temperatures, 20 °C and 30 °C, to allow for temperature effects on the viscosity
of water. Irrespective of nutrient solution temperature at the time of Lp determination, Lp of 20A-RZT plants was only 20% of 20-RZT plants (Fig. 3). This difference remained when data were normalized for different root dry weights between
plants.
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To examine relationships between gs,
shoot and number of leaves, only data for sunny days (average VPD between 09.00 h
and 15.00 h >2.5 kPa) were considered (Figs 1g, h; 2g, h). Data were grouped according
to the RZT at which the plants were originally grown. The significance of regressions for various
data sets is given in Table 3. For both groups of plants, gs increased with an increasing number of leaves per plant. The slope of this
relationship differed according to the RZT at which the plants were originally grown (Fig. 4a). Between 10 and 15 leaves per plant, plants originally grown at A-RZT
maintained a much higher gs. shoot also increased
with the number of leaves, but only in plants that were originally grown at A-RZT (Fig. 4b).
However, analysis of the data sets from both groups of plants showed no relationship between shoot and the number of leaves when data for plants with <8 leaves were excluded.
Stomatal conductance was correlated with shoot, but only in plants that were
originally grown at A-RZT (Fig. 4c).
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Further experiments sought to confirm the effect of RZT on gs at two different VPDs, in plants of the same developmental stage. On the cloudy day (average VPD during measurement=1.5 kPa), gs of plants at both RZTs increased with leaf insertion level on the main stem. However, there was no difference in gs between A-RZT and 20-RZT plants for any leaf. On the sunny day (average VPD during measurement=3.0 kPa), gs of plants at both RZTs was lower than on the low VPD day. On this occasion, 20-RZT plants were wilted and had a gs only 30–45% of A-RZT plants. On another sunny day (2 d prior to the gs measurements),
shoot of 20-RZT plants (-1.08±0.04 MPa, n=8) was much less than shoot of A-RZT plants
(-0.78±0.02 MPa, n=7).
Discussion
Capsicum annuumis chilling sensitive and night temperatures less than 20 °C limit
leaf initiation and growth (Mercado et al., 1997). Although the
20-RZT treatment may therefore be suboptimal, in these experiments it still greatly increased
growth compared to A-RZT plants (Table 1). Thus Capsicumresponds similarly to
lettuce (Lee and Cheong, 1996; He and Lee, 1998a, b) although further experiments conducted
simultaneously under identical aerial conditions have shown that dry matter accumulation at
A-RZT is more impaired in lettuce than pepper (J He and SK Lee, unpublished observations).
Although the growth of pepper plants at 20-RZT was much greater than at A-RZT, an optimal
RZT for pepper under tropical aerial conditions is yet to be determined.
The RZT transfer experiments generally showed co-ordinate changes in total dry weight, leaf area and number of leaves (Figs 1, 2). Leaf area development is dependent on three component processes: individual leaf area expansion, the initiation of new leaves, and initiation of new axillary shoots. In Capsicum, the accumulation of leaf number after the appearance of a set number of main-stem leaves (12 in our experiments) is dependent on monopodial branching. This branching process was particularly responsive to RZT (Fig. 2e, f; TW Choong, J He and SK Lee, unpublished observations). Individual leaf area at specific nodes was not determined, which would have discriminated whether RZT affected leaf expansion per se, or simply the rate of plant development. Future studies should aim to determine the sensitivity of the component processes to high RZT stress.
Delayed leaf area development could be a
response to either water stress or suboptimal nitrogen nutrition. However, no differences were
found in xylem nitrate concentrations following transfer of plants from 20-RZT to A-RZT (Table
2). This contrasts with previous data which found that exposure of sorghum roots to
35 °C for only 4 h reduced [X-NO-3] by 30% (Bassirirad et al., 1991). Although [X-NO-3] was unaffected by the RZT transfer in these experiments, it is possible that NO-<rb<ei3<rb<ee flux to the shoots was decreased due to stomatal closure reducing transpirational
flow. Since large reductions in leaf area preceded stomatal closure (cf. Fig. 2c, d and Fig. 2k, l),
it seems unlikely that decreased nitrate flux was the primary cause of reduced leaf area
development.
Although reduced total leaf area may partially explain the reduction
in total dry weight of 20A-RZT plants, so may reductions in photosynthetic rate of specific
leaves. Photosynthesis at A-RZT may be reduced by both photoinhibition (He and
Lee, 1998a, b) and stomatal closure. In this
paper, the regulation of stomatal conductance was investigated.
Analysis of changes
in gs between specific RZT treatments in the transfer experiments
(Figs 1, 2) was complicated by varying environmental conditions (Fig. 1g, h) and plant ontogeny
(Fig. 4a). The effect of plant development on gs was especially
striking considering that the youngest fully expanded leaves were used in all measurements.
Other species do not show increases in maximum gs with increasing
node number (e.g. sunflower; Zhang and Davies, 1989), but in Capsicum, interpretation of differences in gs between
treatments must consider the effects of measuring leaves at different node numbers. In the
A-RZT to 20-RZT transfer, there were minimal effects of the new RZT on leaf number (Fig. 1e,
f), thus measurement of plants at different developmental stages is unlikely to account fully for
the increased gs of A20-RZT plants. However, the
decreased gs of 20A-RZT plants (relative to 20-RZT plants) was
first detected when these plants had fewer leaves (cf. Fig. 2e, f and Fig. 2k, l), and thus
measurement of plants at different developmental stages may have contributed to the measured
differences in gs. However, when leaf number in the 20-RZT and
20A-RZT plants was more similar at the end of the experiment, an alternative explanation was
needed for stomatal closure.
Consequently, it was necessary to use plants of the same
developmental stage to study the effects of atmospheric vapour pressure deficit (VPD) and RZT
on gs (Fig. 5). Stomatal response to either
variable was dependent on the other. At low VPDs, gs of A-RZT
and 20-RZT plants were similar. Under high VPDs, 20-RZT plants wilted, resulting in stomatal
closure. Wilting at chilling temperatures (of whole plants with both roots and shoots at the same
temperature) has previously been shown to depend on atmospheric VPD (McWilliam et al., 1982). Low temperature induced reductions in root hydraulic
conductivity (Lp) (Markhart et al., 1979) result in water
uptake being unable to keep pace with transpiration. Although decreased Lp of 20-RZT
plants would be expected simply due to the lower viscosity of water at this temperature,
differences in root morphology between roots grown at different temperatures may also have
contributed.
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Stomatal behaviour in the RZT transfer experiments may also have been affected by changes in Lp. Reduced Lp may lower
shoot, which
in turn can cause stomatal closure. Certainly, in both RZT transfer experiments there were
measurement occasions when altered shoot was temporally associated with altered
stomatal behaviour. Greater stomatal conductance of A20-RZT plants (relative to A-RZT
plants) was generally associated with increased shoot (cf. Fig. 1i, j and Fig. 1k, l),
implying a hydraulic limitation of gs in A-RZT plants. This
limitation is unlikely to result from high temperature per se (since Lp increases
with increasing temperature), but via an effect of high RZT on root development. During the
early stages of growth, Lp rapidly increases with development (Fiscus and
Markhart, 1979), which would explain the observation that shoot
increased with plant development in plants originally grown at A-RZT (Fig. 1i, j). Although Lp was not assessed in these experiments, the increased root growth of A20-RZT plants
(data not shown) may have increased Lp (even though root temperature was lower),
allowing improved shoot water status.
When 20-RZT plants were transferred to A-RZT,
a large reduction in Lp was measured after 23 d (Fig. 3). Roots of these plants
appeared swollen and brown, indicating that they were unable to acclimate to their new root
temperature. Despite this large reduction in Lp, the decrease in gs of 20A-RZT plants following the RZT transfer was associated with decreased shoot on only one occasion at the end of Experiment 2 (Fig. 2j). Since shoot
did not change with time after the transfer in both 20-RZT and 20A-RZT plants, it was possible
to average shoot data across all measurement occasions. However, this still failed
to yield significant (P<0.10) differences in shoot (Table 2). Thus it was
necessary to examine the possibility that altered xylem sap composition was responsible for
stomatal closure in 20A-RZT plants.
Although increased xylem ABA concentration is
often well correlated with stomatal closure (reviewed in Dodd et al., 1996), no change in [X-ABA] was detected after transfer of 20-RZT plants to A-RZT
(Table 2). Although increases in ABA flux of an order of magnitude may cause stomatal closure
independent of changes in ABA concentration (Trejo et al., 1995), the reduced gs of 20A-RZT plants implied that
ABA flux was also decreased. Recent studies have considered a role for stomatal sensitivity to
xylem-borne ABA in stomatal closure (Dodd et al., 1996), and a
possible effect of RZT on this sensitivity should be assessed before entirely dismissing a role for
ABA in stomatal closure in 20A-RZT plants.
Alkalization of the xylem sap is a
common response to many root stresses and can cause stomatal closure at [X-ABA]s typical of
those found in well-watered plants (Wilkinson et al., 1998).
High RZT stress did alter xylem sap pH (Table 2), but in the opposite direction from that
required to close the stomata. There are cases where droughted plants show reduced xylem sap
pH (Schurr and Schulze, 1996) thus acidification of xylem sap in
response to a root stress is not without precedent. Clearly the regulation of xylem sap pH as a
stress signal requires further investigation.
Reduced xylem sap cytokinin flux has also
been implicated in stomatal closure (Shashidhar et al., 1996) and
it is known that supra-optimal RZT can reduce root cytokinin concentrations (Tachibana et al., 1997). Preliminary xylem sap analyses showed that
20A-RZT plants had half the xylem cytokinin concentration of 20-RZT plants (data not
shown). However, variation of cytokinin concentrations and fluxes within a physiologically
relevant range had no effect on stomata in transpiration bioassays (IC Dodd, CA Beveridge, A
Fletcher, and RE Munns, unpublished observations).
Since no strong evidence was found
that alterations in chemical signalling were responsible for stomatal closure of 20A-RZT
plants, a role for hydraulic factors was reconsidered. One difficulty of working in greenhouses is
that plant water status can fluctuate rapidly with changes in incident light intensity due to
shadows caused by the greenhouse structure and the passing of clouds. Since the roots of
aeroponically grown plants are continuously supplied with water, changes in plant water status
may well be transient and limited to periods of high VPD. This may explain why significant (P<0.10) differences in shoot between 20-RZT and 20A-RZT plants were not
detected (Table 2).
Conclusions
The data from this study show that the factor(s) controlling leaf initiation and development
were independent of the factor(s) controlling stomatal conductance (gs) in
aeroponically grown Capsicum at different root temperatures. Plants grown at 20-RZT
developed faster than those at A-RZT, despite lower stomatal conductance under conditions of
high VPD (which are characteristic of sunny days in Singapore greenhouses). Although hydraulic
factors (low root hydraulic conductivity causing shoot wilting) are probably responsible for the
lower stomatal conductance of 20-RZT plants, more work is needed to measure shoot turgor
directly, and to understand the factor(s) (presumably chemical signals from the roots) that
regulate leaf initiation and development.
In the RZT transfer experiments, A20-RZT
plants showed accelerated development while 20A-RZT plants showed retarded development,
as expected from studies of plants grown at one RZT. However, stomatal responses of these
plants were the opposite of that expected, in that A20-RZT plants showed increased stomatal
conductance while 20A-RZT plants showed stomatal closure. Root temperature effects on plant
development and root morphology are likely to have modified root hydraulic conductivity (Lp) independently of any effect of temperature on water viscosity. Alterations in Lp have in turn affected shoot, which is hypothesized to have directly
affected the stomata. Confirmation of this hypothesis will require measurement of Lp in
similar RZT transfer experiments, and include manipulation of shoot turgor via root
pressurisation to determine whether shoot can directly affect the stomata.
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Acknowledgments |
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We thank the Singapore Department of Environment for provision of meteorological data, and Dr Brian Loveys, CSIRO Division of Plant Industry, Adelaide, for the kind gift of [2H6] ABA and independent assays of [X-ABA]. Mr Andrew Fletcher is thanked for assistance in optimizing the nitrate assay.
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Notes |
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3 To whom correspondence should be addressed. Fax: +61 7 3365 1699. E-mail: I.Dodd{at}botany.uq.edu.au
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Abbreviations |
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A-RZT, ambient root-zone temperature; A
20-RZT,
plants grown initially at A-RZT then transferred to 20-RZT; gs, stomatal conductance; Lp, root hydraulic conductivity; RZT, root-zone temperature; leaf, leaf water potential; shoot, shoot water potential; 20-RZT, root-zone temperature of
20±2 °C; 20A-RZT, plants grown initially at 20-RZT then transferred to
A-RZT; VPD, atmospheric vapour pressure deficit; [X-ABA], xylem
sap ABA concentration; [X-NO-3 xylem sap nitrate
concentration.. |
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