Directory UMM :Data Elmu:jurnal:T:Tree Physiology:Vol15.1995:
Tree Physiology 15, 129--133
© 1995 Heron Publishing----Victoria, Canada
Mechanisms of drought response in Thuja occidentalis L. II.
Post-conditioning water stress and stress relief
D. R. EDWARDS and M. A. DIXON
Department of Horticultural Science, University of Guelph, Guelph, Ontario, N1G 2W1, Canada
Received October 25, 1993
Summary We examined the extent of osmotic adjustment
and the changes in relative water content (RWC) and transpiration rate (i.e., relative stomatal function) that occur in waterdeficit-conditioned 6-year-old Thuja occidentalis L. (eastern
white cedar) trees in response to a severe drought. Trees conditioned by successive cycles of mild or moderate nonlethal
water stress (conditioning) and nonconditioned trees were
exposed to drought (i.e., −2.0 MPa predawn water potential) to
determine if water deficit conditioning enhanced tolerance to
further drought stress. Following drought, all trees were well
watered for 11 days to evaluate how quickly osmotic potential,
RWC and transpiration rate returned to preconditioning values.
Both nonconditioned trees and mildly conditioned trees
exhibited similar responses to drought, whereas moderately
conditioned trees maintained higher water potentials and transpiration rates were 38% lower. Both conditioned and nonconditioned trees exhibited a similar degree of osmotic adjustment
(−0.39 MPa) in response to drought relative to the well-watered control trees. The well-watered control trees, nonconditioned trees and mildly conditioned trees had similar leaf
RWCs that were about 3% lower than those of the moderately
conditioned trees.
Following the 11-day stress relief, there were no significant
differences in osmotic potential between the well-watered control trees and any of the drought-treated trees. Daily transpiration rates and water potential integrals (WPI) of all
drought-treated trees approached those of the well-watered
control trees during the stress relief period. However, the
relationship between cumulative transpiration and WPI
showed that previous exposure to drought stress reduced transpiration rates. Leaf RWC of the moderately conditioned trees
remained slightly higher than that of the nonconditioned and
mildly conditioned trees.
Keywords: drought stress, osmotic adjustment, transpiration,
water potential integral, water relations.
Introduction
Thuja occidentalis L. is found in habitats representing a broad
range of soil water contents. Recent studies by Matthes-Sears
and Larson (1991) have indicated that T. occidentalis does not
exhibit ecotypic differentiation (Habeck 1958, Musselman et
al. 1975). This has led to the suggestion that physiological
(biotic variables) rather than genetic influences might explain
this species’ broad tolerance to water availability.
There is evidence that T. occidentalis exhibits osmotic adjustment in response to water deficits (Collier and Boyer 1989,
Edwards and Dixon 1995), but available information on the
stomatal responses to water deficits is contradictory. MatthesSears and Larson (1990) concluded that T. occidentalis may
not possess a sensitive stomatal mechanism to water stress,
whereas we have found that the stomatal mechanism of T. occidentalis is sensitive to even relatively mild water stress
(water potentials greater than −1 MPa).
According to Kramer (1983) and Turner (1986), plants respond to drought either by postponing dehydration by maintaining a high plant water potential or by tolerating
dehydration by surviving at a low plant water potential. Plants
that rely on a mechanism of dehydration postponement, such
as increased stomatal resistance, exhibit reduced transpiration
rates and maintain a relatively high water potential in response
to drought stress. Plants that rely on dehydration tolerance may
exhibit osmotic adjustment, which is a mechanism of turgor
maintenance that sustains transpiration at low leaf water potentials.
The objectives of our investigation were (1) to determine the
extent to which conditioning enhanced tolerance to subsequent
water stress, and (2) to determine if and how quickly plant
responses (i.e., transpiration, RWC and osmotic potential) returned to preconditioning values when water stress was relieved.
Materials and methods
Plant material, location, and fertilizer and watering procedures
have been described (Edwards and Dixon 1995). Twenty, 6year-old Thuja occidentalis L. cv. Emerald trees were divided
into three groups. The control group, consisting of 10 trees,
was watered every 2 to 3 days. The other two groups, each
consisting of five trees, were subjected to either five cycles of
water deficit stress to a predawn water potential threshold of
− 0.9 MPa (mild conditioning) or four cycles of water deficit
stress to a predawn water potential threshold of −1.4 MPa
(moderate conditioning).
The drought treatment began immediately after the trees in
130
EDWARDS AND DIXON
the moderate conditioning treatment had received their fourth
and final conditioning cycle (Day 256). The control group was
divided into two groups of five trees; one group was well
watered every 2 or 3 days (well-watered control) and the other
group (nonconditioned control) along with the mildly and
moderately conditioned trees were exposed to a severe drought
for 9 days. The drought episode was ended when an average
predawn water potential (Ψpd) of approximately −2.0 MPa was
reached by the nonconditioned group of trees (the first group
to reach this threshold). Immediately following drought, all
trees were watered every 2 or 3 days for 11 days (stress relief
phase). Environmental conditions for the study were similar to
those detailed in the accompanying paper (Edwards and Dixon
1995). Leaf osmotic potential and RWC were sampled at the
end of the drought and stress relief phases. Transpiration and
water potential measurements were made continuously. The
measurement procedures have been described in detail in the
accompanying paper (Edwards and Dixon 1995).
Osmotic potential and RWC
Osmotic potential at full turgor was measured on freeze disrupted leaf tissue using in situ stem psychrometers in a calibration configuration (Edwards and Dixon 1995). Osmotic
potential (OP) data were expressed as osmotic potential difference (OPD = OP of stressed trees − OP of control).
Relative water content was measured on the same leaf tissue
as osmotic potential as described in the accompanying paper
(Edwards and Dixon 1995).
Gravimetric determination of transpiration
Transpiration rates were determined by weighing the potted
trees as described in the accompanying paper (Edwards and
Dixon 1995). Transpiration for either a drought episode or
stress relief phase was calculated by subtracting the weight at
the end of the cycle (at 1800 h) from the free-drainage weight
determined at the beginning of the cycle (2100 h). Daily
transpiration was calculated by determining the amount of
water transpired per hour and summing these values for a day.
Cumulative transpiration (CT) was calculated by determining
the amount of water transpired per hour and summing these
values for the drought episode or stress relief phase.
because transpiration displayed an asymptotic response to
WPI. In the stress relief phase, all groups were fitted with
linear equations, CT = βo + β1(WPI) (952 data points per
equation, R2 = 0.98). Statistical LSD tests were performed on
the resulting coefficient (β1) among the groups when their
equations were compatible (i.e., β1 from linear and logarithm
equations could not be tested together).
Results
Postconditioning drought stress
All trees subjected to drought exhibited significantly lower
WPIs and transpiration rates than the well-watered controls.
There were no significant differences in WPI (−224 to −230
MPa h) among any of the drought-treated trees. The effects of
the severe drought on mean transpiration rates of nonconditioned trees (14.78 ± 0.76 g g −1) and mildly conditioned trees
(14.32 ± 1.17 g g−1) were not significantly different. However,
trees in both of these treatments exhibited significantly greater
transpiration rates (P = 0.0001) in response to the severe
drought than moderately conditioned trees (11.5 ± 0.27 g g−1).
The transpiration rate and WPI of well-watered control trees
were 25.6 ± 1.4 g g −1 and −109.8 ± 2.3 MPa h, respectively.
Plots of daily WPI indicated that toward the end of the severe
drought episode, moderately conditioned trees exhibited significantly less water stress than nonconditioned and mildly
conditioned trees (Figure 1). Daily transpiration rates generally declined in conditioned trees with duration of the severe
drought episode (Figure 2). For the first 2 days of the severe
drought, moderately conditioned trees had significantly lower
transpiration rates relative to nonconditioned and mildly conditioned trees. Thereafter, transpiration rates among nonconditioned and conditioned trees were similar (Figure 2). The low
transpiration rate of the well-watered controls on Day 262 was
due to the influence of ambient environmental conditions (va-
Data analysis
The trees were arranged on two benches in a completely
randomized design. Statistical analysis was carried out with
SAS statistical software (Cary, North Carolina). Protected
LSD (least significant differences) was used to determine
treatment differences for OPD, RWC, daily transpiration and
WPI.
Cumulative transpiration was regressed on WPI for the
drought episode and stress relief phase. The regression models
used for the drought episode data were: CT = βo + β1(WPI)
(where β0 is the intercept and β1 is the coefficient) for the
control (864 data points per equation, R2 = 0.99) and CT = βo
+ β1 ln(WPI) for all stressed groups (864 data points per
equation, R2 = 0.95--0.97). A logarithmic equation was chosen
Figure 1. Hourly water potential integral totaled daily for trees in the
postconditioning drought episode. Each symbol is the mean of five
trees. Symbols within dates with no letters are not significantly different at α = 0.05. Standard error values ranged from 0.53 to 0.85.
DROUGHT RESPONSE IN THUJA OCCIDENTALIS
131
Stress relief
Figure 2. Daily transpiration for trees in the postconditioning drought
episode. Each symbol is the mean of five trees. Symbols within dates
with no letters are not significantly different at α = 0.05. Standard error
values ranged from 0.1 to 0.4.
por pressure of 0.25 kPa versus an average of 0.98 kPa for the
severe drought episode).
The relationship between CT and WPI was linear for the
well-watered controls and curvilinear for all trees subjected to
severe drought (Figure 3). There was a tendency for moderately conditioned trees to exhibit a lower transpiration rate as
the severe drought progressed (i.e., lower transpiration rate per
unit of WPI).
There were no significant differences (P = 0.70) in OPD
among the trees subjected to the severe drought. Leaf RWC of
well-watered control trees and moderately conditioned trees
were similar (0.933 ± 0.001 and 0.926 ± 0.004, respectively).
These values were significantly higher (P = 0.0001) than those
of nonconditioned trees (0.892 ± 0.008) and mildly conditioned trees (0.903 ± 0.005).
Figure 3. The relationship between cumulative transpiration (grams of
water per gram of leaf dry weight) and hourly water potential integral
(MPa h) during the postconditioning drought episode. Each line is the
result of regression, performed on 2592 data points, and for all regression equations, β0 and β1 are significant at P = 0.0001. Equations are
as follows: Control (no stress), CT = 0.778 − 0.2347(WPI), R2 = 0.92;
Nonconditioned, CT = −7.5194 + 4.2401 ln(WPI), R2 = 0.78; Low
stress, CT = −6.4335 + 4.1740 ln(WPI), R2 = 0.87; Moderate stress,
CT = −12.4293 + 4.6141 ln(WPI), R2 = 0.93.
During the 11-day period of stress relief, WPI and transpiration
were significantly lower (P = 0.0006) in trees that had been
subjected to severe drought than in well-watered control trees.
There were no significant differences in WPI and transpiration
rates among the drought-stressed trees during the stress relief
period. Values for WPI ranged from −160 to −176 MPa h and
transpiration rates ranged from 25.6 to 27.7 g g −1 for the
drought-stressed trees compared with values of −133.2 ± 4.5
MPa h and 35.24 ± 1.55 g g−1 for the well-watered controls for
the same period.
In all drought-stressed trees, WPI and transpiration gradually recovered during the stress relief period (Figures 4 and 5),
but the relationship between CT and WPI changed (Figure 6).
Nonconditioned and conditioned trees transpired significantly
(β1, P = 0.001) less water per unit of plant water stress than the
well-watered control trees. The mean β1 coefficient for the
Figure 4. Hourly water potential integral totaled daily for trees during
the stress relief phase. Each symbol is the mean of five trees. Symbols
within dates with no letters are not significantly different at α = 0.05.
Standard error values ranged from 0.24 to 0.78.
Figure 5. Daily transpiration for trees during the stress relief phase.
Each line represents a regression of CT on WPI performed on 2592
data points. Each symbol is the mean of five trees. Symbols within
dates with no letters are not significantly different at α = 0.05. Standard error values ranged from 0.1 to 0.6.
132
EDWARDS AND DIXON
Figure 6. The relationship between cumulative transpiration (CT,
grams of water per gram of leaf dry weight) and hourly water potential
integral (MPa h) for the stress relief period. Each line represents a
regression of CT on WPI performed on 2856 data points, and for all
regression equations, β0 and β1 are significant at P = 0.001 except were
noted. Equations are as follows: Control (no stress), CT = −0.4268 −
0.2827 (WPI), R2 = 0.91 (β0 not significant); Low stress, CT = −2.8790
− 0.1909 (WPI), R2 = 0.95; Nonconditioned, CT = −2.8149 − 0.1631
(WPI), R2 = 0.96; Moderate stress, CT = −2.0814 − 0.1411 (WPI),
R2 = 0.97.
control was − 0.30, whereas the corresponding values for the
nonconditioned trees and the mildly and moderately conditioned trees were −0.17, − 0.19 and −0.14, respectively (LSD
= 0.055).
There were no significant differences (P = 0.15) in osmotic
potential among any of the drought-treated trees during the
stress relief period. Values for OPD were −0.03 ± 0.01, −0.06
± 0.03 and −0.03 ± 0.02 MPa for the nonconditioned trees and
the mildly and moderately conditioned trees, respectively (OP
of −1.28 to −1.34 MPa). Leaf RWC of the well-watered control
trees, nonconditioned trees and mildly conditioned trees were
similar (0.919 ± 0.002, 0.929 ± 0.009 and 0.920 ± 0.007,
respectively), whereas leaf RWC of moderately conditioned
trees was significantly higher (0.937 ± 0.004; P = 0.04) than
those of the well-watered control trees and nonconditioned
trees.
Discussion
Postconditioning drought episode
The degree of osmotic adjustment was similar in nonconditioned and conditioned trees. Therefore, repeated conditioning
by either mild or moderate nonlethal drought did not influence
the capacity of T. occidentalis to adjust osmotically in response
to subsequent drought. These findings contrast with those of
Zwiazek and Blake (1989) who found that conditioned Picea
mariana (Mill) BSP. had lower osmotic potentials than nonconditioned trees. This difference may reflect species-dependent adaptation.
Moderate conditioning (i.e., Ψpd = −1.4 MPa) caused an
increase in tolerance or resistance to subsequent drought,
whereas the responses of RWC, WPI and transpiration to
drought in mildly conditioned trees were similar to those in
nonconditioned trees. Moderately conditioned trees had higher
leaf RWC and often higher daily WPI and lower transpiration
rates during the severe drought period than nonconditioned
and mildly conditioned trees.
Water potential integral and transpiration totaled for the
entire drought episode were less informative than the daily
changes in these parameters. For example, early in the drought
episode, transpiration of the moderately conditioned trees was
about 40% less than that of the nonconditioned trees and
mildly conditioned trees, but in the later stages of the drought
(Day 262), the transpiration rate was similar among the three
treatments (Figure 2). The early decrease in transpiration rate
can be attributed to recovery from the conditioning phase of
the experiment. Water potential integral totaled for the drought
episode was similar among all drought-treated trees. However,
the daily plots of WPI indicated that the moderately conditioned trees exhibited significantly less stress during the latter
stages of the drought than the nonconditioned and mildly
conditioned trees, possibly indicating enhanced drought tolerance.
The higher WPI of the moderately conditioned trees contributed to less severe internal water deficits and thus a higher leaf
RWC compared with the nonconditioned trees and mildly
conditioned trees. Zwiazek and Blake (1989) reported that
stomata of conditioned P. mariana seedlings closed earlier and
more completely when exposed to drought stress than the
stomata of nonconditioned seedlings, and the early stomatal
closure resulted in conditioned seedlings having lower transpiration rates and higher water potentials (cf. Seiler and Johnson
1985, 1988).
Responses to a period of stress relief
Both conditioned and nonconditioned trees had lower WPI and
transpiration rates than the well-watered control trees when
totaled for the entire stress relief period. However, analyses of
daily WPI and transpiration rates indicated that recovery occurred in about 5 days and, although often not significant,
drought-stressed trees always had lower WPI and transpiration
rates than the well-watered control trees for the remainder of
the drought episode. Decreased transpiration rates in conditioned trees after a stress relief phase has also been observed
in Pinus taeda L. (Bongarten and Teskey 1986, Teskey et al.
1987). Buxton et al. (1985) reported that evaporative losses by
Picea glauca (Moench) Voss., P. mariana and Pinus banksiana
Lamb. conditioned in −0.4 and −0.8 MPa polyethylene glycol
(PEG) solutions returned to their original rates when returned
to control solutions. However, transpiration rates of trees exposed to −1.2 and −1.6 MPa PEG solutions showed no signs of
recovery 72 h after stress relief (Buxton et al. 1985). Nonconditioned trees (exposed to only one episode of water stress)
exhibited similar responses in terms of transpiration rates and
WPI to the conditioned trees, indicating that even a single
exposure to stress caused a change in stomatal functioning.
Evaluation of the relationship between CT and WPI during the
stress relief phase also indicated that there was a carryover
response to drought stress.
DROUGHT RESPONSE IN THUJA OCCIDENTALIS
Following stress relief, the osmotic potential of all droughttreated trees was similar to that of the well-watered control
trees. Others have also noted that accumulated solutes dissipate quickly when drought stress is relieved, sometimes within
3 days (Kramer 1983 and Morgan 1984, Blake et al. 1991).
Plants that rely on osmotic adjustment as a drought tolerance
mechanism must maintain solutes during periods of high water
for the next drought (Morgan 1984).
The moderately conditioned trees exhibited a higher leaf
RWC in response to the drought stress than the nonconditioned
and mildly conditioned trees. The higher leaf RWC of the
moderately conditioned trees could be a result of decreased
transpiration rates during the drought episode.
Conclusions
The mechanisms of response to drought in T. occidentalis were
manifest in both short-term and long-term components that
were influenced by the severity and duration of the water
stress. In the short term, a relatively modest osmotic adjustment occurred as a result of a combination of active solute
accumulation and passive solute concentration, which did not
appear to influence transpiration significantly via turgor maintenance. Osmotic adjustment was initiated by as little as a
single drought episode and was completely eliminated following relief of water stress. We conclude that the role of osmotic
adjustment in this species is to help maintain turgor during
transient midday water deficits in the early stages of drought
(Ranney et al. 1991). In the long term, stomatal function was
influenced by drought stress resulting in reduced transpiration
and hence maintenance of leaf tissue hydration. That is, T. occidentalis is a species that tolerates drought by postponing
water stress. Drought-induced inhibition of transpiration persisted following relief of water stress, suggesting that mechanisms associated with hormone (ABA) accumulation
(Zwiazek and Blake 1989) or modification of leaf cell wall
properties (e.g., elastic modulus) (Levitt 1986, Blake et al.
1991) are involved in the drought response of this species.
Acknowledgments
The authors gratefully acknowledge the technical assistance of Mr.
Jamie Lawson and funding from the Natural Sciences and Engineering
Research Council of Canada.
References
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stress on turgor pressure and cell elasticity changes in black spruce
seedlings. Can. J. For. Res. 21:1329--1333.
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Bongarten, B.C. and R.O. Teskey. 1986. Water relations of loblolly
pine seedlings from diverse geographic origins. Tree Physiol.
1:265--276.
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induction of moisture stress. Can. J. Bot. 63:1171--1176.
Collier, D.E. and M.G. Boyer. 1989. The water relations of Thuja
occidentalis L. from two sites of contrasting moisture availability.
Bot. Gaz. 150:445--448.
Edwards, D.R. and M.A. Dixon. 1995. Investigating mechanisms of
response to drought in Thuja occidentalis L. I. Water stress conditioning and osmotic adjustment. Tree Physiol. 15:121--127.
Habeck, J.R. 1958. White cedar ecotypes in Wisconsin. Ecology
39:457--463.
Kramer, P.J. 1983. Water relations of plants. Academic Press Inc., New
York, 489 p.
Levitt, J. 1986. Recovery of turgor by wilted, excised cabbage leaves
in the absence of water uptake. Plant Physiol. 82:147--153.
Matthes-Sears, U. and D.W. Larson. 1990. Environmental controls of
carbon uptake in two woody species with contrasting distributions
at the edge of cliffs. Can. J. Bot. 68:2371--2380.
Matthes-Sears, U. and D.W. Larson. 1991. Growth and physiology of
Thuja occidentalis L. from cliffs and swamps: is variation habitat or
site specific? Bot. Gaz. 152:500--508.
Morgan, J.M. 1984. Osmoregulation and water stress in higher plants.
Annu. Rev. Plant Physiol. 35:299--319.
Musselman, R.C., D.T. Lester and M.S. Abrams. 1975. Localized
ecotypes of Thuja occidentalis L. in Wisconsin. Ecology 56:647-655.
Ranney, T.G., R.E. Bir and W.A. Skroch. 1991. Comparative drought
resistance among six species of birch (Betula): influence of mild
water stress on water relations and leaf gas exchange. Tree Physiol.
8:351--360.
Seiler, J.R. and J.D. Johnson. 1985. Photosynthesis and transpiration
of loblolly pine seedlings as influenced by moisture-stress conditioning. For. Sci. 31:742--749.
Seiler, J.R. and J.D. Johnson. 1988. Physiological and morphological
responses of three half-sib families of loblolly pine to water stress
conditioning. For. Sci. 34:487--495.
Teskey, R.O., B.C. Bongarten, B.M. Cregg, P.M. Dougherty and T.C.
Hennessey. 1987. Physiology and genetics of tree growth response
to moisture and temperature stress: an examination of the characteristics of loblolly pine (Pinus taeda L.). Tree Physiol. 3:41--61.
Turner, N.C. 1986. Adaptation to water deficits: a changing perspective. Aust. J. Plant Physiol. 10:175--190.
Zwiazek, J.J. and T.J. Blake. 1989. Effects of preconditioning on
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© 1995 Heron Publishing----Victoria, Canada
Mechanisms of drought response in Thuja occidentalis L. II.
Post-conditioning water stress and stress relief
D. R. EDWARDS and M. A. DIXON
Department of Horticultural Science, University of Guelph, Guelph, Ontario, N1G 2W1, Canada
Received October 25, 1993
Summary We examined the extent of osmotic adjustment
and the changes in relative water content (RWC) and transpiration rate (i.e., relative stomatal function) that occur in waterdeficit-conditioned 6-year-old Thuja occidentalis L. (eastern
white cedar) trees in response to a severe drought. Trees conditioned by successive cycles of mild or moderate nonlethal
water stress (conditioning) and nonconditioned trees were
exposed to drought (i.e., −2.0 MPa predawn water potential) to
determine if water deficit conditioning enhanced tolerance to
further drought stress. Following drought, all trees were well
watered for 11 days to evaluate how quickly osmotic potential,
RWC and transpiration rate returned to preconditioning values.
Both nonconditioned trees and mildly conditioned trees
exhibited similar responses to drought, whereas moderately
conditioned trees maintained higher water potentials and transpiration rates were 38% lower. Both conditioned and nonconditioned trees exhibited a similar degree of osmotic adjustment
(−0.39 MPa) in response to drought relative to the well-watered control trees. The well-watered control trees, nonconditioned trees and mildly conditioned trees had similar leaf
RWCs that were about 3% lower than those of the moderately
conditioned trees.
Following the 11-day stress relief, there were no significant
differences in osmotic potential between the well-watered control trees and any of the drought-treated trees. Daily transpiration rates and water potential integrals (WPI) of all
drought-treated trees approached those of the well-watered
control trees during the stress relief period. However, the
relationship between cumulative transpiration and WPI
showed that previous exposure to drought stress reduced transpiration rates. Leaf RWC of the moderately conditioned trees
remained slightly higher than that of the nonconditioned and
mildly conditioned trees.
Keywords: drought stress, osmotic adjustment, transpiration,
water potential integral, water relations.
Introduction
Thuja occidentalis L. is found in habitats representing a broad
range of soil water contents. Recent studies by Matthes-Sears
and Larson (1991) have indicated that T. occidentalis does not
exhibit ecotypic differentiation (Habeck 1958, Musselman et
al. 1975). This has led to the suggestion that physiological
(biotic variables) rather than genetic influences might explain
this species’ broad tolerance to water availability.
There is evidence that T. occidentalis exhibits osmotic adjustment in response to water deficits (Collier and Boyer 1989,
Edwards and Dixon 1995), but available information on the
stomatal responses to water deficits is contradictory. MatthesSears and Larson (1990) concluded that T. occidentalis may
not possess a sensitive stomatal mechanism to water stress,
whereas we have found that the stomatal mechanism of T. occidentalis is sensitive to even relatively mild water stress
(water potentials greater than −1 MPa).
According to Kramer (1983) and Turner (1986), plants respond to drought either by postponing dehydration by maintaining a high plant water potential or by tolerating
dehydration by surviving at a low plant water potential. Plants
that rely on a mechanism of dehydration postponement, such
as increased stomatal resistance, exhibit reduced transpiration
rates and maintain a relatively high water potential in response
to drought stress. Plants that rely on dehydration tolerance may
exhibit osmotic adjustment, which is a mechanism of turgor
maintenance that sustains transpiration at low leaf water potentials.
The objectives of our investigation were (1) to determine the
extent to which conditioning enhanced tolerance to subsequent
water stress, and (2) to determine if and how quickly plant
responses (i.e., transpiration, RWC and osmotic potential) returned to preconditioning values when water stress was relieved.
Materials and methods
Plant material, location, and fertilizer and watering procedures
have been described (Edwards and Dixon 1995). Twenty, 6year-old Thuja occidentalis L. cv. Emerald trees were divided
into three groups. The control group, consisting of 10 trees,
was watered every 2 to 3 days. The other two groups, each
consisting of five trees, were subjected to either five cycles of
water deficit stress to a predawn water potential threshold of
− 0.9 MPa (mild conditioning) or four cycles of water deficit
stress to a predawn water potential threshold of −1.4 MPa
(moderate conditioning).
The drought treatment began immediately after the trees in
130
EDWARDS AND DIXON
the moderate conditioning treatment had received their fourth
and final conditioning cycle (Day 256). The control group was
divided into two groups of five trees; one group was well
watered every 2 or 3 days (well-watered control) and the other
group (nonconditioned control) along with the mildly and
moderately conditioned trees were exposed to a severe drought
for 9 days. The drought episode was ended when an average
predawn water potential (Ψpd) of approximately −2.0 MPa was
reached by the nonconditioned group of trees (the first group
to reach this threshold). Immediately following drought, all
trees were watered every 2 or 3 days for 11 days (stress relief
phase). Environmental conditions for the study were similar to
those detailed in the accompanying paper (Edwards and Dixon
1995). Leaf osmotic potential and RWC were sampled at the
end of the drought and stress relief phases. Transpiration and
water potential measurements were made continuously. The
measurement procedures have been described in detail in the
accompanying paper (Edwards and Dixon 1995).
Osmotic potential and RWC
Osmotic potential at full turgor was measured on freeze disrupted leaf tissue using in situ stem psychrometers in a calibration configuration (Edwards and Dixon 1995). Osmotic
potential (OP) data were expressed as osmotic potential difference (OPD = OP of stressed trees − OP of control).
Relative water content was measured on the same leaf tissue
as osmotic potential as described in the accompanying paper
(Edwards and Dixon 1995).
Gravimetric determination of transpiration
Transpiration rates were determined by weighing the potted
trees as described in the accompanying paper (Edwards and
Dixon 1995). Transpiration for either a drought episode or
stress relief phase was calculated by subtracting the weight at
the end of the cycle (at 1800 h) from the free-drainage weight
determined at the beginning of the cycle (2100 h). Daily
transpiration was calculated by determining the amount of
water transpired per hour and summing these values for a day.
Cumulative transpiration (CT) was calculated by determining
the amount of water transpired per hour and summing these
values for the drought episode or stress relief phase.
because transpiration displayed an asymptotic response to
WPI. In the stress relief phase, all groups were fitted with
linear equations, CT = βo + β1(WPI) (952 data points per
equation, R2 = 0.98). Statistical LSD tests were performed on
the resulting coefficient (β1) among the groups when their
equations were compatible (i.e., β1 from linear and logarithm
equations could not be tested together).
Results
Postconditioning drought stress
All trees subjected to drought exhibited significantly lower
WPIs and transpiration rates than the well-watered controls.
There were no significant differences in WPI (−224 to −230
MPa h) among any of the drought-treated trees. The effects of
the severe drought on mean transpiration rates of nonconditioned trees (14.78 ± 0.76 g g −1) and mildly conditioned trees
(14.32 ± 1.17 g g−1) were not significantly different. However,
trees in both of these treatments exhibited significantly greater
transpiration rates (P = 0.0001) in response to the severe
drought than moderately conditioned trees (11.5 ± 0.27 g g−1).
The transpiration rate and WPI of well-watered control trees
were 25.6 ± 1.4 g g −1 and −109.8 ± 2.3 MPa h, respectively.
Plots of daily WPI indicated that toward the end of the severe
drought episode, moderately conditioned trees exhibited significantly less water stress than nonconditioned and mildly
conditioned trees (Figure 1). Daily transpiration rates generally declined in conditioned trees with duration of the severe
drought episode (Figure 2). For the first 2 days of the severe
drought, moderately conditioned trees had significantly lower
transpiration rates relative to nonconditioned and mildly conditioned trees. Thereafter, transpiration rates among nonconditioned and conditioned trees were similar (Figure 2). The low
transpiration rate of the well-watered controls on Day 262 was
due to the influence of ambient environmental conditions (va-
Data analysis
The trees were arranged on two benches in a completely
randomized design. Statistical analysis was carried out with
SAS statistical software (Cary, North Carolina). Protected
LSD (least significant differences) was used to determine
treatment differences for OPD, RWC, daily transpiration and
WPI.
Cumulative transpiration was regressed on WPI for the
drought episode and stress relief phase. The regression models
used for the drought episode data were: CT = βo + β1(WPI)
(where β0 is the intercept and β1 is the coefficient) for the
control (864 data points per equation, R2 = 0.99) and CT = βo
+ β1 ln(WPI) for all stressed groups (864 data points per
equation, R2 = 0.95--0.97). A logarithmic equation was chosen
Figure 1. Hourly water potential integral totaled daily for trees in the
postconditioning drought episode. Each symbol is the mean of five
trees. Symbols within dates with no letters are not significantly different at α = 0.05. Standard error values ranged from 0.53 to 0.85.
DROUGHT RESPONSE IN THUJA OCCIDENTALIS
131
Stress relief
Figure 2. Daily transpiration for trees in the postconditioning drought
episode. Each symbol is the mean of five trees. Symbols within dates
with no letters are not significantly different at α = 0.05. Standard error
values ranged from 0.1 to 0.4.
por pressure of 0.25 kPa versus an average of 0.98 kPa for the
severe drought episode).
The relationship between CT and WPI was linear for the
well-watered controls and curvilinear for all trees subjected to
severe drought (Figure 3). There was a tendency for moderately conditioned trees to exhibit a lower transpiration rate as
the severe drought progressed (i.e., lower transpiration rate per
unit of WPI).
There were no significant differences (P = 0.70) in OPD
among the trees subjected to the severe drought. Leaf RWC of
well-watered control trees and moderately conditioned trees
were similar (0.933 ± 0.001 and 0.926 ± 0.004, respectively).
These values were significantly higher (P = 0.0001) than those
of nonconditioned trees (0.892 ± 0.008) and mildly conditioned trees (0.903 ± 0.005).
Figure 3. The relationship between cumulative transpiration (grams of
water per gram of leaf dry weight) and hourly water potential integral
(MPa h) during the postconditioning drought episode. Each line is the
result of regression, performed on 2592 data points, and for all regression equations, β0 and β1 are significant at P = 0.0001. Equations are
as follows: Control (no stress), CT = 0.778 − 0.2347(WPI), R2 = 0.92;
Nonconditioned, CT = −7.5194 + 4.2401 ln(WPI), R2 = 0.78; Low
stress, CT = −6.4335 + 4.1740 ln(WPI), R2 = 0.87; Moderate stress,
CT = −12.4293 + 4.6141 ln(WPI), R2 = 0.93.
During the 11-day period of stress relief, WPI and transpiration
were significantly lower (P = 0.0006) in trees that had been
subjected to severe drought than in well-watered control trees.
There were no significant differences in WPI and transpiration
rates among the drought-stressed trees during the stress relief
period. Values for WPI ranged from −160 to −176 MPa h and
transpiration rates ranged from 25.6 to 27.7 g g −1 for the
drought-stressed trees compared with values of −133.2 ± 4.5
MPa h and 35.24 ± 1.55 g g−1 for the well-watered controls for
the same period.
In all drought-stressed trees, WPI and transpiration gradually recovered during the stress relief period (Figures 4 and 5),
but the relationship between CT and WPI changed (Figure 6).
Nonconditioned and conditioned trees transpired significantly
(β1, P = 0.001) less water per unit of plant water stress than the
well-watered control trees. The mean β1 coefficient for the
Figure 4. Hourly water potential integral totaled daily for trees during
the stress relief phase. Each symbol is the mean of five trees. Symbols
within dates with no letters are not significantly different at α = 0.05.
Standard error values ranged from 0.24 to 0.78.
Figure 5. Daily transpiration for trees during the stress relief phase.
Each line represents a regression of CT on WPI performed on 2592
data points. Each symbol is the mean of five trees. Symbols within
dates with no letters are not significantly different at α = 0.05. Standard error values ranged from 0.1 to 0.6.
132
EDWARDS AND DIXON
Figure 6. The relationship between cumulative transpiration (CT,
grams of water per gram of leaf dry weight) and hourly water potential
integral (MPa h) for the stress relief period. Each line represents a
regression of CT on WPI performed on 2856 data points, and for all
regression equations, β0 and β1 are significant at P = 0.001 except were
noted. Equations are as follows: Control (no stress), CT = −0.4268 −
0.2827 (WPI), R2 = 0.91 (β0 not significant); Low stress, CT = −2.8790
− 0.1909 (WPI), R2 = 0.95; Nonconditioned, CT = −2.8149 − 0.1631
(WPI), R2 = 0.96; Moderate stress, CT = −2.0814 − 0.1411 (WPI),
R2 = 0.97.
control was − 0.30, whereas the corresponding values for the
nonconditioned trees and the mildly and moderately conditioned trees were −0.17, − 0.19 and −0.14, respectively (LSD
= 0.055).
There were no significant differences (P = 0.15) in osmotic
potential among any of the drought-treated trees during the
stress relief period. Values for OPD were −0.03 ± 0.01, −0.06
± 0.03 and −0.03 ± 0.02 MPa for the nonconditioned trees and
the mildly and moderately conditioned trees, respectively (OP
of −1.28 to −1.34 MPa). Leaf RWC of the well-watered control
trees, nonconditioned trees and mildly conditioned trees were
similar (0.919 ± 0.002, 0.929 ± 0.009 and 0.920 ± 0.007,
respectively), whereas leaf RWC of moderately conditioned
trees was significantly higher (0.937 ± 0.004; P = 0.04) than
those of the well-watered control trees and nonconditioned
trees.
Discussion
Postconditioning drought episode
The degree of osmotic adjustment was similar in nonconditioned and conditioned trees. Therefore, repeated conditioning
by either mild or moderate nonlethal drought did not influence
the capacity of T. occidentalis to adjust osmotically in response
to subsequent drought. These findings contrast with those of
Zwiazek and Blake (1989) who found that conditioned Picea
mariana (Mill) BSP. had lower osmotic potentials than nonconditioned trees. This difference may reflect species-dependent adaptation.
Moderate conditioning (i.e., Ψpd = −1.4 MPa) caused an
increase in tolerance or resistance to subsequent drought,
whereas the responses of RWC, WPI and transpiration to
drought in mildly conditioned trees were similar to those in
nonconditioned trees. Moderately conditioned trees had higher
leaf RWC and often higher daily WPI and lower transpiration
rates during the severe drought period than nonconditioned
and mildly conditioned trees.
Water potential integral and transpiration totaled for the
entire drought episode were less informative than the daily
changes in these parameters. For example, early in the drought
episode, transpiration of the moderately conditioned trees was
about 40% less than that of the nonconditioned trees and
mildly conditioned trees, but in the later stages of the drought
(Day 262), the transpiration rate was similar among the three
treatments (Figure 2). The early decrease in transpiration rate
can be attributed to recovery from the conditioning phase of
the experiment. Water potential integral totaled for the drought
episode was similar among all drought-treated trees. However,
the daily plots of WPI indicated that the moderately conditioned trees exhibited significantly less stress during the latter
stages of the drought than the nonconditioned and mildly
conditioned trees, possibly indicating enhanced drought tolerance.
The higher WPI of the moderately conditioned trees contributed to less severe internal water deficits and thus a higher leaf
RWC compared with the nonconditioned trees and mildly
conditioned trees. Zwiazek and Blake (1989) reported that
stomata of conditioned P. mariana seedlings closed earlier and
more completely when exposed to drought stress than the
stomata of nonconditioned seedlings, and the early stomatal
closure resulted in conditioned seedlings having lower transpiration rates and higher water potentials (cf. Seiler and Johnson
1985, 1988).
Responses to a period of stress relief
Both conditioned and nonconditioned trees had lower WPI and
transpiration rates than the well-watered control trees when
totaled for the entire stress relief period. However, analyses of
daily WPI and transpiration rates indicated that recovery occurred in about 5 days and, although often not significant,
drought-stressed trees always had lower WPI and transpiration
rates than the well-watered control trees for the remainder of
the drought episode. Decreased transpiration rates in conditioned trees after a stress relief phase has also been observed
in Pinus taeda L. (Bongarten and Teskey 1986, Teskey et al.
1987). Buxton et al. (1985) reported that evaporative losses by
Picea glauca (Moench) Voss., P. mariana and Pinus banksiana
Lamb. conditioned in −0.4 and −0.8 MPa polyethylene glycol
(PEG) solutions returned to their original rates when returned
to control solutions. However, transpiration rates of trees exposed to −1.2 and −1.6 MPa PEG solutions showed no signs of
recovery 72 h after stress relief (Buxton et al. 1985). Nonconditioned trees (exposed to only one episode of water stress)
exhibited similar responses in terms of transpiration rates and
WPI to the conditioned trees, indicating that even a single
exposure to stress caused a change in stomatal functioning.
Evaluation of the relationship between CT and WPI during the
stress relief phase also indicated that there was a carryover
response to drought stress.
DROUGHT RESPONSE IN THUJA OCCIDENTALIS
Following stress relief, the osmotic potential of all droughttreated trees was similar to that of the well-watered control
trees. Others have also noted that accumulated solutes dissipate quickly when drought stress is relieved, sometimes within
3 days (Kramer 1983 and Morgan 1984, Blake et al. 1991).
Plants that rely on osmotic adjustment as a drought tolerance
mechanism must maintain solutes during periods of high water
for the next drought (Morgan 1984).
The moderately conditioned trees exhibited a higher leaf
RWC in response to the drought stress than the nonconditioned
and mildly conditioned trees. The higher leaf RWC of the
moderately conditioned trees could be a result of decreased
transpiration rates during the drought episode.
Conclusions
The mechanisms of response to drought in T. occidentalis were
manifest in both short-term and long-term components that
were influenced by the severity and duration of the water
stress. In the short term, a relatively modest osmotic adjustment occurred as a result of a combination of active solute
accumulation and passive solute concentration, which did not
appear to influence transpiration significantly via turgor maintenance. Osmotic adjustment was initiated by as little as a
single drought episode and was completely eliminated following relief of water stress. We conclude that the role of osmotic
adjustment in this species is to help maintain turgor during
transient midday water deficits in the early stages of drought
(Ranney et al. 1991). In the long term, stomatal function was
influenced by drought stress resulting in reduced transpiration
and hence maintenance of leaf tissue hydration. That is, T. occidentalis is a species that tolerates drought by postponing
water stress. Drought-induced inhibition of transpiration persisted following relief of water stress, suggesting that mechanisms associated with hormone (ABA) accumulation
(Zwiazek and Blake 1989) or modification of leaf cell wall
properties (e.g., elastic modulus) (Levitt 1986, Blake et al.
1991) are involved in the drought response of this species.
Acknowledgments
The authors gratefully acknowledge the technical assistance of Mr.
Jamie Lawson and funding from the Natural Sciences and Engineering
Research Council of Canada.
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