Directory UMM :Data Elmu:jurnal:T:Tree Physiology:vol17.1997:
Tree Physiology 17, 797--803
© 1997 Heron Publishing----Victoria, Canada
Genotypic variation in drought tolerance of poplar in relation to
abscisic acid
SHAOLIANG CHEN,1 SHASHENG WANG,1,2 ARIE ALTMAN3 and ALOYS HÜTTERMANN4
1
2
3
4
Experimental Center of Forest Biology, Beijing Forestry University, Beijing, 100083, China
Author to whom correspondence should be addressed
The Otto Warburg Center For Biotechnology in Agriculture, The Hebrew University of Jerusalem, Rehovot 76-100, Israel
Forest Botanical Institute, Göttingen University, Busgenweg 2, 37077, Göttingen, Germany
Received October 23, 1996
Summary We investigated effects of water stress and external abscisic acid (ABA) supply on shoot growth, stomatal
conductance and water status in 1-year-old cuttings of a
drought-sensitive poplar genotype Populus × euramericana
cv. I-214 (Italica) and a drought-tolerant genotype P. ‘popularis 35-44’ (popularis). Populus popularis was more productive and maintained higher leaf water potentials throughout the
drought treatment than cv. Italica. Supply of ABA to the xylem
sap caused a greater decline in growth and more leaf abscission
in shoots of cv. Italica than in shoots of P. popularis. Immediately after initiation of the drought treatment in P. popularis,
the ABA concentration ([ABA]) of the xylem increased rapidly
and stomatal conductance declined; however, stomatal conductance had returned to control values by the third day of the
drought treatment, coincident with a gradual decline in xylem
[ABA]. In contrast, xylem [ABA] of cv. Italica initially increased more slowly than that of P. popularis in response to
the drought treatment, but the increase continued for 3 days at
which time a tenfold increase in xylem [ABA] was observed
that was followed by abscission of more than 40% of the leaves.
We conclude that sensitivity of poplar roots to variation in soil
water content varies by clone and that a rapid short-term
accumulation of ABA in shoots in response to water stress may
contribute to drought tolerance.
Keywords: ABA, plant water status, Populus popularis, shoot
growth, stomatal conductance, water stress.
Introduction
There is much variation in drought tolerance among genotypes
of the genus Populus (Ceulemans et al. 1978, Tschaplinski and
Blake 1989, Rhodenbaugh and Pallardy 1993, Chen et al.
1996). The physiological basis of the genetic variation in
drought tolerance of Populus has not been established. In
general, a reduction in leaf water content or potential results in
a decline in growth and stomatal conductance. However, recent
studies have shown that shoot growth and stomatal conductance can also be suppressed in the absence of a coincident
change in plant water status (Blackman and Davies 1985,
Gowing et al. 1990). Based on observations that the abscisic
acid concentration ([ABA]) of xylem is linked to soil water
reserves (Tardieu et al. 1992), and that stomatal conductance
and leaf growth are negatively correlated with xylem [ABA]
(Zhang and Davies 1990), it has been suggested that roots
sense soil water deficiency and communicate this information
to shoots by a chemical signal, possibly ABA (Coutts 1981,
Blackman and Davies 1984, Gollan et al. 1986, 1992, Smit
et al. 1990, Davies and Zhang 1991, Scurr et al. 1992).
Although the role of ABA in stress physiology has received
much attention, attempts to correlate ABA production and
drought tolerance in plants have yielded conflicting results.
Drought-tolerant plants of several maize cultivars accumulate
more ABA or increase production of ABA more rapidly than
drought-sensitive cultivars (Larque-Saavedra and Wain 1974).
Spring wheat cultivars with high accumulations of ABA produce higher yields than cultivars with low accumulations of
ABA (Innes et al. 1984). Conversely, sorghum, which is more
drought-resistant than maize, accumulates less ABA than
maize (Beardsell and Cohen 1975) and maximal ABA contents
are negatively correlated with drought resistance (Ilahi and
Dorffling 1982). These conflicting studies on crop species
indicate different mechanisms of drought tolerance in different
species.
Stomatal response to drought differs among poplar genotypes (Schulte and Hinckley 1987a, 1987b) and clonal differences in leaf ABA accumulation have been observed under soil
drying conditions (Liu and Dickmann 1992). However, the
relationship between drought-induced ABA accumulation and
drought tolerance in poplar has not been elucidated.
For more than 30 years, P. popularis has been used in
preference to other fast growing but more drought-sensitive
genotypes for afforestation in northern China. In an attempt to
determine the physiological mechanism underlying the
drought tolerance of P. popularis, we compared the endogenous [ABA] of the xylem of P. popularis with that of the
drought-sensitive genoptye P. × euramericana cv. Italica. We
also examined how these two genotypes respond to drought
and exogenous ABA.
798
CHEN ET AL.
ABA Treatment
Materials and methods
Plant materials
In April 1995, 20-cm cuttings of two poplar genotypes, Populus × euramericana cv. I-214 (cv. Italica) and P. ‘popularis
35-44’ (P. popularis) were planted in 25-liter pots containing
loam soil. The cuttings were placed in a greenhouse, kept well
watered and fertilized with 1 liter of Hoagland’s nutrient solution every 2 weeks. The experiments started in July when the
cuttings were 1.4--1.8 m high and had 50--70 leaves.
Water stress treatment
Water stress was induced by withholding water until soil water
content fell to between 30 and 40% of field capacity (FC) (soil
water potential at 30% FC was --2.108 MPa, Figure 1) for 3
weeks. Cuttings in the control treatment were kept well watered by maintaining soil in the pots at 70% FC. To keep soil
water content close to the target value, pots were weighed
twice a day and watered as required. Height growth of four
replicate trees per treatment was measured weekly for 3 weeks.
Predawn water potential of upper mature leaves was obtained
at 0300--0400 h with a pressure chamber. Gas exchange and
stomatal conductance were measured at 0830--0930 h with an
Li-6200 portable photosynthesis system (Li-Cor, Inc., Lincoln,
NE) under natural conditions. Photosynthetically active radiation (PAR) was approximately 1200 µmol m −2 s −1 and air
temperature (Tair) was 32--34 °C. Following gas exchange
measurements, the concentration of ABA ([ABA]) in the xylem was determined.
On clear days, one 30-cm plastic tube (5 mm diameter) was
attached with glue to the stem of each plant. After the glue had
dried, holes (1 mm diameter) were drilled through the waterfilled tubes to the middle of the xylem, and the holes in the
tubes were immediately plugged to prevent leaking. Plants
were monitored for water uptake over a 30-min period and
plants with similar uptake rates were selected for the experiment. Ten ml of 200 µM ABA was fed into the xylem of each
plant by way of the tube. When the feeding was completed,
distilled water was added continuously to avoid air entering the
xylem. Control plants were fed with distilled water only. The
pHs of the ABA solution and distilled water were adjusted to
that of the xylem prior to feeding (pH 6.8). Four well-watered
plants and four mildly water-stressed (40% FC) plants of each
genotype were selected for the ABA treatment.
Plant measurements
For 10 days following ABA application, gas exchange of upper
mature leaves was measured at 1030--1130 h with a Li-Cor
Li-6200 portable photosynthesis system under natural conditions where PAR was approximately 1200--1600 µmol m −2 s −1
and Tair was 34--36 °C. Following the gas exchange measurements, midday water potential was measured with a pressure
chamber on the leaves directly above and below the leaf sampled for gas exchange measurement. Height growth was determined every day and relative height growth rate was computed
using the formula:
Relative growth rate = (lnH2 − lnH 1) / (t2 − t1),
Determination of xylem [ABA]
Fifteen- to 20-cm apical growing tips (with 7 or 8 leaves each)
were excised from water-stressed (30% FC) and well-watered
(70% FC) plants, and the xylem sap was immediately expelled
with a pressure chamber using a pressure of approximately
1.0 MPa over the balancing pressure. One to 2 ml of extruded
xylem sap was collected from each tip and used for analysis of
ABA by HPLC (Chen and Wang 1992).
where H 2, H 1 are the height growth variables at the beginning
and the end of the time interval, and t2 − t1 is the time interval
of interest.
Data analysis
The data were subjected to ANOVA and significant differences
between means were determined by Duncan’s multiple-range
test. Unless otherwise stated, differences were considered statistically significant when P < 0.05.
Results
Growth
Figure 1. Relationship between soil water potential and soil water
content (% of field capacity, FC).
Under conditions of optimum soil water content (70% FC),
cuttings of cv. Italica maintained significantly higher relative
height growth rates than cuttings of P. popularis, whereas
P. popularis cuttings maintained significantly higher relative
height growth rates than cv. Italica cuttings during the 3-week
severe drought (30% FC) treatment (Figure 2).
After ABA was fed into the xylem of well-watered plants of
both genotypes (70% FC), the relative height growth rate of
cv. Italica plants was greatly reduced, whereas there was no
observed decline in relative height growth rate of P. popularis
plants over the sampling period, even though the relative
height growth rate of control P. popularis plants was typically
TREE PHYSIOLOGY VOLUME 17, 1997
XYLEM [ABA] AND DROUGHT TOLERANCE IN POPLAR
799
less than that of the control cv. Italica plants (Figure 3). Four
days after ABA treatment, the relative height growth rate of
cv. Italica plants began to recover, and by the sixth day after
ABA treatment there was no significant difference in relative
height growth rates between ABA-treated and control cv. Italica
plants.
Table 1. Effects of soil water content and the introduction of 10 ml of
200 µM ABA into the xylem sap on leaf abscission of P. popularis and
cv. Italica poplar genotypes. Values are means (± SE) of four measurements.
Leaf abscission
70% FC
cv. Italica
P. popularis
Compared with well-watered control plants (70% FC), ABA
treatment of well-watered plants (70% FC + ABA) caused leaf
abscission in both genotypes (Table 1). The supply of 10 ml of
200 µM ABA to mildly water-stressed plants (40% FC) increased the amount of leaves shed by 20--40% and cv. Italica
plants exhibited greater leaf loss than P. popularis plants.
Treatment
70% FC + ABA
cv. Italica
P. popularis
40% FC
cv. Italica
P. popularis
No. of leaves
abscised per plant
0
0
12 ± 2
6±2
0
0
% of leaves
abscised
0
0
17.5
10.2
0
0
30% FC
cv. Italica
P. popularis
26 ± 2
0
43.3
0
40% FC + ABA
cv. Italica
P. popularis
24 ± 3
13 ± 2
36.9
24.7
Figure 2. Effects of soil water content on relative height growth rates
of P. popularis and cv. Italica poplar genotypes. Each point is the mean
of four measurements. Bars represent the standard error of the mean.
Figure 3. Effects of the introduction of 10 ml of 200 µM ABA to the
xylem sap on relative height growth rates of well-watered P. popularis
and cv. Italica poplar genotypes. Each point is the mean of four
measurements. Bars represent the standard error of the mean.
Figure 4. Effects of soil water content on predawn leaf water potentials
of P. popularis and cv. Italica poplar genotypes. Each point is the mean
of four measurements. Bars represent the standard error of the mean.
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800
CHEN ET AL.
The severe drought treatment (30% FC) caused 43.3% of
leaves to abscise in cv. Italica plants, which was similar to the
magnitude of leaf loss induced in the mildly water-stressed
plants treated with ABA (40% FC + ABA). In contrast, no leaf
abscission occurred in severely water-stressed (30% FC)
plants of P. popularis over the 3-week study.
Plant water status
Predawn leaf water potential (Ψ p) of cv. Italica plants decreased to --0.6 MPa on the first day of exposure to severe
water stress (30% FC). The Ψ p gradually recovered to control
values following abscission of old leaves on the fourth day of
the drought treatment. The magnitude of drought-induced decline in Ψ p was much less in P. popularis plants than in
cv. Italica plants (Figure 4). In general, water-stressed plants of
P. popularis had a Ψ p that was about 0.1 MPa lower than that
of well-watered control plants throughout the drought period.
Gas exchange and plant water status after ABA treatment
Introduction of ABA to the xylem sap greatly reduced stomatal
conductance (C s), net photosynthetic rates (Pn) and transpiration rates (E) of well-watered plants of both genotypes for the
first 3 days after treatment (Figure 5). The ABA-induced decrease in E resulted in a 0.3--0.6 MPa increase in midday water
potential (Ψm ) compared with that of the control plants (Figure 5). However, the effects of ABA on gas exchange and plant
water status disappeared within 5 days of application, after
Figure 5. Effects of the introduction of 10 ml of 200 µM
ABA to the xylem sap on stomatal conductance (C s), transpiration rate (E), net
photosynthetic rate (Pn), and
midday leaf water potential of
well-watered P. popularis and
cv. Italica poplar genotypes.
Each point is the mean of four
measurements. Bars represent
the standard error of the mean.
TREE PHYSIOLOGY VOLUME 17, 1997
XYLEM [ABA] AND DROUGHT TOLERANCE IN POPLAR
which time there were no significant differences in Cs, Pn, E or
Ψm betweeen control and ABA-treated plants.
Xylem ABA concentrations and gas exchange
Severe water stress (30% FC) caused significant increases in
xylem [ABA] in both genotypes (Figure 6), but there were
genotypic differences in the timing of the ABA response to
water stress. On the first day of exposure to severe drought,
P. popularis exhibited a 3.5-fold increase in xylem [ABA],
801
whereas cv. Italica exhibited only a minor increase (0.7-fold)
in xylem [ABA]. Thereafter, the xylem [ABA] in P. popularis
decreased gradually, whereas the xylem [ABA] in cv. Italica
continued to increase to reach a maximum on the third day of
the drought treatment that was 10 times higher than that of the
well-watered control plants.
In both genotypes, C s, Pn and E were significantly decreased
in response to the 30% FC treatment. However, coincident
with the gradual decline in xylem [ABA] in P. popularis after
the onset of water stress, Cs and Pn recovered to a large degree.
Figure 6. Effects of soil water
content on xylem [ABA], stomatal conductance (C s), transpiration rate (E), and net
photosynthetic rate (Pn) of
P. popularis and cv. Italica
poplar genotypes. Each point
is the mean of four measurements. Bars represent the
standard error of the mean.
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802
CHEN ET AL.
There was no evidence of recovery of either C s or Pn in the
cv. Italica plants.
Discussion
In the 30% FC water-stress treatment, P. popularis plants
maintained a higher relative height growth rate than cv. Italica
plants, mainly because plants of the P. popularis genotype had
a high photosynthetic rate (Figure 6), no leaf loss (Table 1) and
a high leaf water potential (Figure 4). The maintenance of a
high water balance in P. popularis plants appeared to be a
consequence of the initial rapid increase in xylem [ABA]
contributing to water conservation (Figure 5). Compared with
cv. Italica plants, xylem [ABA] increased more rapidly in
P. popularis plants in response to water stress (Figure 6).
Similarly, O’Regan et al. (1993) found that a slow growing
maize cultivar, which was drought resistant, accumulated more
ABA during the onset of water stress than a faster growing
cultivar. In contrast, cv. Italica plants accumulated ABA more
slowly and over a longer period in response to the drought
treatment, and plant water balance was reestablished only after
a 43.3% leaf loss. The drought-induced loss of leaves was
probably associated with the endogenous accumulation of
ABA.
The difference in the timing and pattern of ABA production
in response to the onset of water stress implies that the sensitivity of the root system to soil drying varies with genotype.
The drought-induced increase in xylem ABA is thought to
originate mainly from roots (Zhang and Davies 1987, Robertson 1990). This suggests that root tips of P. popularis provide
an early indicator of soil drying to the shoot by synthesizing
ABA, which triggers adjustments in the shoots that confer
drought resistance. Compared with P. popularis, cv. Italica
root tips appear to be less sensitive to soil drying and so the
water balance of the plant is more severely affected by drought.
The direct introduction of ABA into the xylem sap may
imitate the drought-induced increase in xylem [ABA]. Exogenously applied ABA exerted a significant influence on stomatal opening of both genotypes; however, shoots of
P. popularis were more tolerant of exogenous ABA than
shoots of cv. Italica, as shown by their lower leaf loss (Table 1)
and a smaller ABA-induced decrease in height growth (Figure 3). Tolerance to ABA may explain the absence of leaf
abscission in P. popularis during the period corresponding to
the increase in xylem [ABA] at the onset of water stress. We
conclude that the sensitivity of root tips to soil drying and the
sensitivity of shoots to water stress signals, especially xylem
[ABA], differ between drought-tolerant and drought-sensitive
genotypes.
Acknowledgments
This research was funded by the Ministry for Economic Cooperation
of the Federal Republic of Germany under the German-Israeli Agricultural Research Agreement for the Benefit of the Third World
(GIARA). We thank Dr. Lindsay Fung for data analysis and critical
reading of the manuscript.
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XYLEM [ABA] AND DROUGHT TOLERANCE IN POPLAR
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TREE PHYSIOLOGY ON-LINE at http://www.heronpublishing.com
© 1997 Heron Publishing----Victoria, Canada
Genotypic variation in drought tolerance of poplar in relation to
abscisic acid
SHAOLIANG CHEN,1 SHASHENG WANG,1,2 ARIE ALTMAN3 and ALOYS HÜTTERMANN4
1
2
3
4
Experimental Center of Forest Biology, Beijing Forestry University, Beijing, 100083, China
Author to whom correspondence should be addressed
The Otto Warburg Center For Biotechnology in Agriculture, The Hebrew University of Jerusalem, Rehovot 76-100, Israel
Forest Botanical Institute, Göttingen University, Busgenweg 2, 37077, Göttingen, Germany
Received October 23, 1996
Summary We investigated effects of water stress and external abscisic acid (ABA) supply on shoot growth, stomatal
conductance and water status in 1-year-old cuttings of a
drought-sensitive poplar genotype Populus × euramericana
cv. I-214 (Italica) and a drought-tolerant genotype P. ‘popularis 35-44’ (popularis). Populus popularis was more productive and maintained higher leaf water potentials throughout the
drought treatment than cv. Italica. Supply of ABA to the xylem
sap caused a greater decline in growth and more leaf abscission
in shoots of cv. Italica than in shoots of P. popularis. Immediately after initiation of the drought treatment in P. popularis,
the ABA concentration ([ABA]) of the xylem increased rapidly
and stomatal conductance declined; however, stomatal conductance had returned to control values by the third day of the
drought treatment, coincident with a gradual decline in xylem
[ABA]. In contrast, xylem [ABA] of cv. Italica initially increased more slowly than that of P. popularis in response to
the drought treatment, but the increase continued for 3 days at
which time a tenfold increase in xylem [ABA] was observed
that was followed by abscission of more than 40% of the leaves.
We conclude that sensitivity of poplar roots to variation in soil
water content varies by clone and that a rapid short-term
accumulation of ABA in shoots in response to water stress may
contribute to drought tolerance.
Keywords: ABA, plant water status, Populus popularis, shoot
growth, stomatal conductance, water stress.
Introduction
There is much variation in drought tolerance among genotypes
of the genus Populus (Ceulemans et al. 1978, Tschaplinski and
Blake 1989, Rhodenbaugh and Pallardy 1993, Chen et al.
1996). The physiological basis of the genetic variation in
drought tolerance of Populus has not been established. In
general, a reduction in leaf water content or potential results in
a decline in growth and stomatal conductance. However, recent
studies have shown that shoot growth and stomatal conductance can also be suppressed in the absence of a coincident
change in plant water status (Blackman and Davies 1985,
Gowing et al. 1990). Based on observations that the abscisic
acid concentration ([ABA]) of xylem is linked to soil water
reserves (Tardieu et al. 1992), and that stomatal conductance
and leaf growth are negatively correlated with xylem [ABA]
(Zhang and Davies 1990), it has been suggested that roots
sense soil water deficiency and communicate this information
to shoots by a chemical signal, possibly ABA (Coutts 1981,
Blackman and Davies 1984, Gollan et al. 1986, 1992, Smit
et al. 1990, Davies and Zhang 1991, Scurr et al. 1992).
Although the role of ABA in stress physiology has received
much attention, attempts to correlate ABA production and
drought tolerance in plants have yielded conflicting results.
Drought-tolerant plants of several maize cultivars accumulate
more ABA or increase production of ABA more rapidly than
drought-sensitive cultivars (Larque-Saavedra and Wain 1974).
Spring wheat cultivars with high accumulations of ABA produce higher yields than cultivars with low accumulations of
ABA (Innes et al. 1984). Conversely, sorghum, which is more
drought-resistant than maize, accumulates less ABA than
maize (Beardsell and Cohen 1975) and maximal ABA contents
are negatively correlated with drought resistance (Ilahi and
Dorffling 1982). These conflicting studies on crop species
indicate different mechanisms of drought tolerance in different
species.
Stomatal response to drought differs among poplar genotypes (Schulte and Hinckley 1987a, 1987b) and clonal differences in leaf ABA accumulation have been observed under soil
drying conditions (Liu and Dickmann 1992). However, the
relationship between drought-induced ABA accumulation and
drought tolerance in poplar has not been elucidated.
For more than 30 years, P. popularis has been used in
preference to other fast growing but more drought-sensitive
genotypes for afforestation in northern China. In an attempt to
determine the physiological mechanism underlying the
drought tolerance of P. popularis, we compared the endogenous [ABA] of the xylem of P. popularis with that of the
drought-sensitive genoptye P. × euramericana cv. Italica. We
also examined how these two genotypes respond to drought
and exogenous ABA.
798
CHEN ET AL.
ABA Treatment
Materials and methods
Plant materials
In April 1995, 20-cm cuttings of two poplar genotypes, Populus × euramericana cv. I-214 (cv. Italica) and P. ‘popularis
35-44’ (P. popularis) were planted in 25-liter pots containing
loam soil. The cuttings were placed in a greenhouse, kept well
watered and fertilized with 1 liter of Hoagland’s nutrient solution every 2 weeks. The experiments started in July when the
cuttings were 1.4--1.8 m high and had 50--70 leaves.
Water stress treatment
Water stress was induced by withholding water until soil water
content fell to between 30 and 40% of field capacity (FC) (soil
water potential at 30% FC was --2.108 MPa, Figure 1) for 3
weeks. Cuttings in the control treatment were kept well watered by maintaining soil in the pots at 70% FC. To keep soil
water content close to the target value, pots were weighed
twice a day and watered as required. Height growth of four
replicate trees per treatment was measured weekly for 3 weeks.
Predawn water potential of upper mature leaves was obtained
at 0300--0400 h with a pressure chamber. Gas exchange and
stomatal conductance were measured at 0830--0930 h with an
Li-6200 portable photosynthesis system (Li-Cor, Inc., Lincoln,
NE) under natural conditions. Photosynthetically active radiation (PAR) was approximately 1200 µmol m −2 s −1 and air
temperature (Tair) was 32--34 °C. Following gas exchange
measurements, the concentration of ABA ([ABA]) in the xylem was determined.
On clear days, one 30-cm plastic tube (5 mm diameter) was
attached with glue to the stem of each plant. After the glue had
dried, holes (1 mm diameter) were drilled through the waterfilled tubes to the middle of the xylem, and the holes in the
tubes were immediately plugged to prevent leaking. Plants
were monitored for water uptake over a 30-min period and
plants with similar uptake rates were selected for the experiment. Ten ml of 200 µM ABA was fed into the xylem of each
plant by way of the tube. When the feeding was completed,
distilled water was added continuously to avoid air entering the
xylem. Control plants were fed with distilled water only. The
pHs of the ABA solution and distilled water were adjusted to
that of the xylem prior to feeding (pH 6.8). Four well-watered
plants and four mildly water-stressed (40% FC) plants of each
genotype were selected for the ABA treatment.
Plant measurements
For 10 days following ABA application, gas exchange of upper
mature leaves was measured at 1030--1130 h with a Li-Cor
Li-6200 portable photosynthesis system under natural conditions where PAR was approximately 1200--1600 µmol m −2 s −1
and Tair was 34--36 °C. Following the gas exchange measurements, midday water potential was measured with a pressure
chamber on the leaves directly above and below the leaf sampled for gas exchange measurement. Height growth was determined every day and relative height growth rate was computed
using the formula:
Relative growth rate = (lnH2 − lnH 1) / (t2 − t1),
Determination of xylem [ABA]
Fifteen- to 20-cm apical growing tips (with 7 or 8 leaves each)
were excised from water-stressed (30% FC) and well-watered
(70% FC) plants, and the xylem sap was immediately expelled
with a pressure chamber using a pressure of approximately
1.0 MPa over the balancing pressure. One to 2 ml of extruded
xylem sap was collected from each tip and used for analysis of
ABA by HPLC (Chen and Wang 1992).
where H 2, H 1 are the height growth variables at the beginning
and the end of the time interval, and t2 − t1 is the time interval
of interest.
Data analysis
The data were subjected to ANOVA and significant differences
between means were determined by Duncan’s multiple-range
test. Unless otherwise stated, differences were considered statistically significant when P < 0.05.
Results
Growth
Figure 1. Relationship between soil water potential and soil water
content (% of field capacity, FC).
Under conditions of optimum soil water content (70% FC),
cuttings of cv. Italica maintained significantly higher relative
height growth rates than cuttings of P. popularis, whereas
P. popularis cuttings maintained significantly higher relative
height growth rates than cv. Italica cuttings during the 3-week
severe drought (30% FC) treatment (Figure 2).
After ABA was fed into the xylem of well-watered plants of
both genotypes (70% FC), the relative height growth rate of
cv. Italica plants was greatly reduced, whereas there was no
observed decline in relative height growth rate of P. popularis
plants over the sampling period, even though the relative
height growth rate of control P. popularis plants was typically
TREE PHYSIOLOGY VOLUME 17, 1997
XYLEM [ABA] AND DROUGHT TOLERANCE IN POPLAR
799
less than that of the control cv. Italica plants (Figure 3). Four
days after ABA treatment, the relative height growth rate of
cv. Italica plants began to recover, and by the sixth day after
ABA treatment there was no significant difference in relative
height growth rates between ABA-treated and control cv. Italica
plants.
Table 1. Effects of soil water content and the introduction of 10 ml of
200 µM ABA into the xylem sap on leaf abscission of P. popularis and
cv. Italica poplar genotypes. Values are means (± SE) of four measurements.
Leaf abscission
70% FC
cv. Italica
P. popularis
Compared with well-watered control plants (70% FC), ABA
treatment of well-watered plants (70% FC + ABA) caused leaf
abscission in both genotypes (Table 1). The supply of 10 ml of
200 µM ABA to mildly water-stressed plants (40% FC) increased the amount of leaves shed by 20--40% and cv. Italica
plants exhibited greater leaf loss than P. popularis plants.
Treatment
70% FC + ABA
cv. Italica
P. popularis
40% FC
cv. Italica
P. popularis
No. of leaves
abscised per plant
0
0
12 ± 2
6±2
0
0
% of leaves
abscised
0
0
17.5
10.2
0
0
30% FC
cv. Italica
P. popularis
26 ± 2
0
43.3
0
40% FC + ABA
cv. Italica
P. popularis
24 ± 3
13 ± 2
36.9
24.7
Figure 2. Effects of soil water content on relative height growth rates
of P. popularis and cv. Italica poplar genotypes. Each point is the mean
of four measurements. Bars represent the standard error of the mean.
Figure 3. Effects of the introduction of 10 ml of 200 µM ABA to the
xylem sap on relative height growth rates of well-watered P. popularis
and cv. Italica poplar genotypes. Each point is the mean of four
measurements. Bars represent the standard error of the mean.
Figure 4. Effects of soil water content on predawn leaf water potentials
of P. popularis and cv. Italica poplar genotypes. Each point is the mean
of four measurements. Bars represent the standard error of the mean.
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CHEN ET AL.
The severe drought treatment (30% FC) caused 43.3% of
leaves to abscise in cv. Italica plants, which was similar to the
magnitude of leaf loss induced in the mildly water-stressed
plants treated with ABA (40% FC + ABA). In contrast, no leaf
abscission occurred in severely water-stressed (30% FC)
plants of P. popularis over the 3-week study.
Plant water status
Predawn leaf water potential (Ψ p) of cv. Italica plants decreased to --0.6 MPa on the first day of exposure to severe
water stress (30% FC). The Ψ p gradually recovered to control
values following abscission of old leaves on the fourth day of
the drought treatment. The magnitude of drought-induced decline in Ψ p was much less in P. popularis plants than in
cv. Italica plants (Figure 4). In general, water-stressed plants of
P. popularis had a Ψ p that was about 0.1 MPa lower than that
of well-watered control plants throughout the drought period.
Gas exchange and plant water status after ABA treatment
Introduction of ABA to the xylem sap greatly reduced stomatal
conductance (C s), net photosynthetic rates (Pn) and transpiration rates (E) of well-watered plants of both genotypes for the
first 3 days after treatment (Figure 5). The ABA-induced decrease in E resulted in a 0.3--0.6 MPa increase in midday water
potential (Ψm ) compared with that of the control plants (Figure 5). However, the effects of ABA on gas exchange and plant
water status disappeared within 5 days of application, after
Figure 5. Effects of the introduction of 10 ml of 200 µM
ABA to the xylem sap on stomatal conductance (C s), transpiration rate (E), net
photosynthetic rate (Pn), and
midday leaf water potential of
well-watered P. popularis and
cv. Italica poplar genotypes.
Each point is the mean of four
measurements. Bars represent
the standard error of the mean.
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XYLEM [ABA] AND DROUGHT TOLERANCE IN POPLAR
which time there were no significant differences in Cs, Pn, E or
Ψm betweeen control and ABA-treated plants.
Xylem ABA concentrations and gas exchange
Severe water stress (30% FC) caused significant increases in
xylem [ABA] in both genotypes (Figure 6), but there were
genotypic differences in the timing of the ABA response to
water stress. On the first day of exposure to severe drought,
P. popularis exhibited a 3.5-fold increase in xylem [ABA],
801
whereas cv. Italica exhibited only a minor increase (0.7-fold)
in xylem [ABA]. Thereafter, the xylem [ABA] in P. popularis
decreased gradually, whereas the xylem [ABA] in cv. Italica
continued to increase to reach a maximum on the third day of
the drought treatment that was 10 times higher than that of the
well-watered control plants.
In both genotypes, C s, Pn and E were significantly decreased
in response to the 30% FC treatment. However, coincident
with the gradual decline in xylem [ABA] in P. popularis after
the onset of water stress, Cs and Pn recovered to a large degree.
Figure 6. Effects of soil water
content on xylem [ABA], stomatal conductance (C s), transpiration rate (E), and net
photosynthetic rate (Pn) of
P. popularis and cv. Italica
poplar genotypes. Each point
is the mean of four measurements. Bars represent the
standard error of the mean.
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CHEN ET AL.
There was no evidence of recovery of either C s or Pn in the
cv. Italica plants.
Discussion
In the 30% FC water-stress treatment, P. popularis plants
maintained a higher relative height growth rate than cv. Italica
plants, mainly because plants of the P. popularis genotype had
a high photosynthetic rate (Figure 6), no leaf loss (Table 1) and
a high leaf water potential (Figure 4). The maintenance of a
high water balance in P. popularis plants appeared to be a
consequence of the initial rapid increase in xylem [ABA]
contributing to water conservation (Figure 5). Compared with
cv. Italica plants, xylem [ABA] increased more rapidly in
P. popularis plants in response to water stress (Figure 6).
Similarly, O’Regan et al. (1993) found that a slow growing
maize cultivar, which was drought resistant, accumulated more
ABA during the onset of water stress than a faster growing
cultivar. In contrast, cv. Italica plants accumulated ABA more
slowly and over a longer period in response to the drought
treatment, and plant water balance was reestablished only after
a 43.3% leaf loss. The drought-induced loss of leaves was
probably associated with the endogenous accumulation of
ABA.
The difference in the timing and pattern of ABA production
in response to the onset of water stress implies that the sensitivity of the root system to soil drying varies with genotype.
The drought-induced increase in xylem ABA is thought to
originate mainly from roots (Zhang and Davies 1987, Robertson 1990). This suggests that root tips of P. popularis provide
an early indicator of soil drying to the shoot by synthesizing
ABA, which triggers adjustments in the shoots that confer
drought resistance. Compared with P. popularis, cv. Italica
root tips appear to be less sensitive to soil drying and so the
water balance of the plant is more severely affected by drought.
The direct introduction of ABA into the xylem sap may
imitate the drought-induced increase in xylem [ABA]. Exogenously applied ABA exerted a significant influence on stomatal opening of both genotypes; however, shoots of
P. popularis were more tolerant of exogenous ABA than
shoots of cv. Italica, as shown by their lower leaf loss (Table 1)
and a smaller ABA-induced decrease in height growth (Figure 3). Tolerance to ABA may explain the absence of leaf
abscission in P. popularis during the period corresponding to
the increase in xylem [ABA] at the onset of water stress. We
conclude that the sensitivity of root tips to soil drying and the
sensitivity of shoots to water stress signals, especially xylem
[ABA], differ between drought-tolerant and drought-sensitive
genotypes.
Acknowledgments
This research was funded by the Ministry for Economic Cooperation
of the Federal Republic of Germany under the German-Israeli Agricultural Research Agreement for the Benefit of the Third World
(GIARA). We thank Dr. Lindsay Fung for data analysis and critical
reading of the manuscript.
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