Tree Physiology 21, 599–607
© 2001 Heron Publishing—Victoria, Canada
Growth, leaf morphology, water use and tissue water relations of
Eucalyptus globulus clones in response to water deficit
PILAR PITA and JOSÉ A. PARDOS
Unidad de Anatomía, Fisiología y Genética Forestal, Escuela Técnica Superior de Ingenieros de Montes, Ciudad Universitaria, 28040 Madrid, Spain
Received May 15, 2000
Summary Changes in leaf size, specific leaf area (SLA),
transpiration and tissue water relations were studied in leaves
of rooted cuttings of selected clones of Eucalyptus globulus
Labill. subjected to well-watered or drought conditions in a
greenhouse. Significant differences between clones were
found in leaf expansion and transpiration. There was a significant clone × treatment interaction on SLA. Water stress significantly reduced osmotic potential at the turgor loss point (Π0)
and at full turgor (Π100), and significantly increased relative
water content at the turgor loss point and maximum bulk elastic
modulus. Differences in tissue water relations between clones
were significant only in the mild drought treatment. Among
clones in the drought treatments, the highest leaf expansion
and the highest increase in transpiration during the experiment
were measured in those clones that showed an early and large
decrease in Π0 and Π100.
Keywords: bulk elastic modulus, drought, leaf size, osmotic
potential, productivity, relative water content, specific leaf
area, transpiration, turgor maintenance.
Introduction
Eucalyptus globulus Labill. plantations play an important role
in cellulose production in Spain. About 500,000 ha are currently planted with E. globulus (1% of the country’s land surface), yielding 22% of Spain's total wood production (López
Arias 1992). Eucalyptus globulus is planted in two areas:
Galicia (northwest of the Iberian Peninsula), which has a mild
Atlantic climate and a mean rainfall of 1000 mm or more, and
the southwest of the Iberian Peninsula, which has a xeric Mediterranean climate, an annual rainfall of 500 mm at the driest
sites and a mean summer (June–September) rainfall of 32 mm.
Although E. globulus is not well adapted to the harsh summer climate of the southwest, it is preferred to other eucalypt
species because of its high fiber yield and rapid growth. For
these reasons a breeding program for E. globulus has been established by Empresa Nacional de Celulosas (ENCE), Spain,
to find highly productive clones that can withstand summer
droughts and high temperatures.
Forest productivity is strongly dependent on water availability (Pereira and Pallardy 1989, Pallardy et al. 1991). How-
ever, because traits related to growth often differ from those
related to survival (Pereira and Pallardy 1989, Jones 1993),
breeding for wood production in drought-prone areas is a
trade-off between traits that enhance tree survival and traits related to growth. Although growth reduction under drought has
been considered the main cause of yield reduction (Pereira and
Pallardy 1989), survival after planting is important for eucalypt plantations productivity in the Mediterranean area (Chambers and Borralho 1997). Hence, both survival and growth
traits need to be considered in breeding programs for E. globulus destined for plantations in southwestern Spain.
Trees have developed various mechanisms to withstand
drought and the cost of these mechanisms may differ in terms
of productivity (Ludlow 1989). For example, stomatal control
or a reduction in leaf area will almost certainly lead to a significant reduction in productivity (Turner and Jones 1980). Selection against drought avoidance mechanisms, e.g., droughtinduced leaf shedding, has been advocated as a means to increase productivity in drought-prone areas (Pereira and
Pallardy 1989). Mechanisms of turgor maintenance are common in drought-tolerant trees. Turgor maintenance provides
the potential for maintaining metabolic processes and increasing growth (Turner and Jones 1980). Changes in turgor pressure, as a result of osmotic adjustment (Turner and Jones
1980), or decreases in cell wall elasticity (Tyree and Hammel
1972) can contribute to turgor maintenance, allowing plants to
take up water at low soil water potentials (Pereira and Pallardy
1989).
Intraspecific morphophysiological differences in growth
(Osorio and Pereira 1994), early leaf expansion (Osorio et al.
1998), leaf morphology (Potts and Jordan 1994, Teklehaimanot et al. 1998) and extent of osmotic adjustment (Turner and
Jones 1980, Pereira and Pallardy 1989, Tuomela 1997) have
been identified as adaptations to drought in several tree species. The aim of this study was to investigate intraspecific
variation in leaf physiology in E. globulus clones and relate it
to survival and growth of the clones under drought conditions.
For this purpose, transpiration, growth, leaf expansion and
leaf water relations of seven selected clones were studied under semi-controlled, well-watered and drought conditions.
600
PITA AND PARDOS
Materials and methods
not survive soil water potentials less than –1.5 MPa for
2 months (Serrano 1992).
Plant material and experimental design
Rooted cuttings of seven Eucalyptus globulus clones were obtained from Empresa Nacional de Celulosas (ENCE), Spain.
These clones exhibit enhanced rooting ability and are currently used in forest plantations in south-western Spain. Preliminary results show that survival and growth are high under
a wide range of field conditions in Clone 131.3, moderate to
high in Clones 115.3 and 413.7 and moderate to low in Clones
115.2 and 334.1. Clone 131.2 is the most susceptible to attack
by the eucalypt borer Phoracanta semipunctata (F.), the main
eucalypt pest in southwestern Spain. Clone 161.3 has recently
been planted commercially because of its enhanced growth in
a plantation established as a clonal bank (I. Cañas, ENCE, unpublished data). Thirty replicate cuttings of each clone, with
an initial height of 280 mm, were planted in 3-l plastic pots
filled with the same weight of a 3:1 (w/w) sand:peat mixture
and placed in a greenhouse. The surface of the containers was
covered with inert beads to limit evaporation. The pots were
randomly arranged on a bench and periodically rotated to minimize effects of environmental heterogeneity.
The experiment was carried out from November to January.
The daily photoperiod was increased to 14 h with artificial
light from 400-W xenon lamps. Maximum photosynthetic
photon flux density was approximately 600 µmol m –2 s –1.
Daily temperature was kept between 10 and 30 °C and relative
humidity ranged from 40 to 70%. Seven ml of liquid fertilizer
containing 9.3 g l –1 N (with proportions of P, K, Ca and Mg as
described by Ingestad (1980)) plus micronutrients was applied
to each plant every 5 weeks with watering. For the first 25 days
in the greenhouse, the plants were periodically watered to field
capacity. Thereafter, three watering regimes were imposed for
95 days.
Watering regimes
The three treatments were: Control (C) in which soil water
content was maintained around 30% (dry weight); Stress 1
(S1) in which soil water content was kept around 9.5% (dry
weight); and Stress 2 (S2) in which soil water content was kept
around 8.6% (dry weight). Each plant and its container was
weighed every other day to estimate soil water content and the
amount of water transpired. The amount of water added to
each plant equaled the amount necessary to reach the target
soil water content plus half the water lost by transpiration between two consecutive irrigations. Plants were watered only
when the soil water content fell below target values. Because
mean transpiration was estimated only at the beginning of the
treatments, irrigation at the end of the experiment was insufficient to maintain the soil water content at the target value in
the control treatment, consequently, soil water content fell to
25% during the last 10 days.
Soil water content values were selected based on preliminary studies with similar plant material and soil. The S1 and
S2 treatments were designed to reach soil water potentials of
–0.5 and –1.0 MPa, respectively. Young rooted cuttings may
Growth and morphological traits
Five plants per clone and treatment were harvested on the day
that treatments were imposed (Day 0) and also on Days 60 and
95. Plant height, dry weight of leaves, roots and stems and leaf
area were determined. Specific leaf area (SLA) was calculated
as the ratio of leaf area to leaf dry weight. Mean leaf size (LM)
was calculated as the ratio of total leaf area to the number of
leaves per plant. Apices were excluded from the LM calculations.
Transpiration
Transpiration was estimated gravimetrically from the weights
of each plant and its container weight, measured on alternate
days. Data were pooled separately for two periods: Days
25–27 and Days 62–89.
Tissue water relations
Pressure–volume curves were developed by the free-transpiration method (Pallardy et al. 1991). In total, 180 curves were
obtained on Days 49–59 and 84–94 (3–6 curves per clone, set
and treatment, more than three curves in 90% of the cases). A
single fully expanded leaf from the fourth whorl was excised
under deionized water and rehydrated for 1–3 h in the dark.
Data from preliminary experiments indicated that longer periods of rehydration occasionally resulted in infiltration. Leaves
were covered with Parafilm for the first measurements (until a
water potential of approximately –1.4 MPa was reached). Afterward, the Parafilm was removed and leaves were allowed to
dry in air. Because juvenile E. globulus leaves are sessile, the
leaf lamina was trimmed and the main vein was used as a petiole. Leaf water potential (Ψl) was measured with a pressure
chamber (PMS Instrument Co., Corvallis, OR). Leaf fresh
weight (WF) was recorded before each water potential determination. Fourteen pairs of measurements (Ψl, WF) were made
for each pressure–volume curve.
After measurements, leaves were oven-dried at 60 °C to
constant weight (Wd). Turgid weight (WT) was estimated from
the linear relationship between the first 6 ± 2 pairs of Ψl and
WF values. Tissue relative water content (RWC) was calculated as: RWC = (WF – Wd)/(WT – Wd). Osmotic potential at
full turgor (Π100), osmotic potential at the turgor loss point
(Π0) and relative water content at the turgor loss point (RWC 0)
were deduced from each pressure–volume curve (Pallardy et
al. 1991). Osmotic potential (Π) was estimated from the linear
relationship between the last 6 ± 1.7 pairs of Ψl and WF values.
Turgor pressure (P) was calculated from P = Ψl – Π. Bulk
elastic modulus (ε) was calculated after Koide et al. (1989) as:
ε = (∆P/∆RWC)(RWCm – RWC a), where RWC m is mean relative water content over a calculation interval, ∆P/∆RWC is the
slope of the turgor pressure versus RWC relationship over the
same interval, and RWC a is relative apoplastic water content
derived from pressure–volume curves. Bulk elastic modulus
was calculated over successive intervals, and maximum val-
TREE PHYSIOLOGY VOLUME 21, 2001
PHYSIOLOGICAL RESPONSES OF EUCALYPTUS GLOBULUS TO DROUGHT
ues (ε max) determined. Osmotic adjustment was calculated as
the difference in mean Π100 between water-stressed plants and
controls.
Statistical analysis
Two-way and one-way analyses of variance (ANOVA) were
performed to determine effects of water availability and genotype on growth and tissue water relations. All statistical comparisons were considered significantly different at P < 0.05
and means were compared by the Bonferroni test. Relationships between variables were analyzed by simple linear regression.
Results
Growth
The drought treatments resulted in a significant decrease in
plant leaf area from Day 60 onward (Figure 1), but drought-induced decreases in dry weight were statistically significant
only at the end of the experiment (Day 95). In response to
drought, total plant leaf area and total plant dry weight on Day
95 were reduced by 56 and 36%, respectively (Figure 1).
On Day 0, there were no significant differences in plant biomass between clones. However, there were highly significant
(P < 0.001) differences between clones, between harvests and
between treatments for total dry weight, leaf area and plant
height on Days 60 and 95 (Table 1). There were also significant clone × treatment interactions for dry weight, leaf area
and plant height, but not for clone × harvest. At the end of the
experiment, mean height, leaf area and dry weight of Clones
115.3, 131.2 and 413.7 were higher than the corresponding
clonal means in the control treatment. Growth of Clones 131.2
and 115.3 was reduced markedly in response to the drought
treatments (Table 1). Consistently high values for height, leaf
area and dry weight were measured in Clone 161.3 in the
drought treatments, whereas consistently low values were
measured in Clone 334.1 in all of the treatments (Table 1).
Leaf expansion and specific leaf area.
Differences in mean leaf size (LM) between treatments were
highly significant (P < 0.005) from the first harvest (Day 60)
onward. Differences between clones were only significant
(P < 0.001) at the second harvest (Day 95). The course of leaf
601
expansion differed between clones and treatments (Figure 2).
On Day 95, mean leaf size of control plants was 1.6 times that
of water-stressed plants, whereas mean leaf size of S2 plants
was slightly lower than that of S1 plants. Under well-watered
conditions, leaf expansion was greater in Clones 413.7,
115.3 and 131.2 than in the other clones. Drought-induced reductions in leaf expansion were greatest in these same clones
and least in Clone 161.3 (Figure 2).
Mean leaf size increment between harvests was 405 and
265 mm 2 for plants in treatments S1 and S2, respectively. The
increment for Clones 161.3 and 115.3 was higher than the
clonal mean for both drought treatments, but for Clones
131.3 and 413.7 the increment was higher for only one drought
treatment. Mean leaf size increment of Clones 115.2,
131.2 and 334.1 was lower than the mean for both drought
treatments (Figure 2).
Specific leaf area of control plants was significantly higher
than that of water-stressed plants, but differences between
drought treatments were not statistically significant. Clonal
differences in SLA were not significant at any harvest, and the
clone × treatment interaction effect was only significant (P <
0.05) on Day 60. The highest proportional decrease in SLA between control and water-stressed plants was in Clone
161.3 (30%) and the lowest proportional decrease in SLA was
in Clones 334.1 and 131.2 (Table 2).
Transpiration
Between Days 25 and 57, transpiration rates of control plants
were 1.6 and 2.2 times those of S1 and S2 plants, respectively.
Between Days 62 and 89, transpiration rates of control plants
were 3.3 and 3.5 times those of S1 and S2 plants, respectively
(Table 3). Transpiration rates of control plants increased from
26.7 to 46.1 g plant –1 day –1 between Days 25–57 and Days
62–89. In contrast, transpiration rates in water-stressed plants
decreased in some clones between Days 25–57 and Days
62–89, despite the slightly greater size of plants harvested on
Day 95 (Table 3).
Different clones showed different patterns of transpiration
during the study (Table 3). Clone 413.7 was the only clone in
which transpiration rates measured between Days 62 and 89
were higher than those measured between Days 25 and 57 in
both drought treatments. In contrast, transpiration rates of S1
and S2 plants of Clone 131.2 decreased between Days 25–57
Figure 1. Effects of three watering regimes (C = well-watered, S1 and S2 =
water deficit) on dry matter (left) and
leaf area (right) production during the
experiment. Each value is the mean for
35 plants ± standard errors.
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602
PITA AND PARDOS
Table 1. Clonal ranking in height (H, mm), total dry weight (DW, g) and leaf area (LA, cm 2 ), 60 and 95 days after the beginning of treatments. Abbreviations: C = well watered; S1 = Stress 1; S2 = Stress 2; and *** = significant differences between clones (P < 0.001). Symbols refer to clones
as follows: ◆ = 115.2; ■ = 115.3; ▲ = 131.2; ¨ = 131.3; ● = 161.3; ¡ = 334.1; and ∆ = 413.7.
H (mm)(***)
Day 60
C
S1
Day 95
S2
C
S1
S2
Day 60
Day 95
C
C
S1
■
▲
∆
◆
S1 S2
+40%
■
+30%
+20%
+10%
+5%
■
∆
●
Mean
404
–5%
▲
◆
¨
¡
–10%
●
¨
◆
∆
●
∆
374 365
¡
◆
■
■
▲
▲
–20%
–30%
LA (cm2)(***)
DW (g)(***)
∆
■
∆
●
▲
∆
¨
◆
●
538 427 415
●
¨
¡
◆
¨
■
▲
■
¡
¡
▲
●
∆
●
¨
●
4.1
3.8 3.7
¨
∆
◆
▲
◆
■
¡
¡
4.4
◆
∆
▲
■
¡
▲
S2
∆
●
4.2
▲
¨
¡
◆
¨
■
▲
¡
Tissue water relations.
Osmotic potential at full turgor (Π100), osmotic potential at the
turgor loss point (Π0) and relative water content at the turgor
loss point (RWC0) as well as the maximum bulk modulus of
elasticity (ε max) were significantly different for plants in the
control and water stress treatments. Control plants reached
lower RWC0 and ε max and higher Π0 and Π100 than water-stressed plants. Absolute values of all of these parameters
were higher on Days 84–94 than on Days 49–59, for all the
treatments (Figure 4).
Significant differences between clones in Π100, Π0 and ε max
S2
●
●
●
¨
¨
◆
438.6 293.7 288.0
◆
◆
▲
¡
S2
¨
◆
●
∆
●
780.6 348.4 342.7
▲
∆
¡
■
¡
S1
∆
∆
■
■
▲
∆
¡
C
■
▲
∆
■
◆
●
S1
Day 95
■
–40%
and Days 62–89. Behavior of Clone 161.3 was similar to that
of Clone 413.7, and the other clones showed an intermediate
response.
Mean transpiration was positively correlated with mean leaf
area (Figure 3), and the relationship was highly significant
(P < 0.005) for plants in every treatment on Day 95, but only
significant (P = 0.04) for control plants on Day 60. The slope
of the regression for S2 plants was 44 and 35% that of control
and S1 plants, respectively (Figure 3).
C
¨
∆
●
6.7
Day 60
■
▲
●
¨
▲
◆
◆
¡
¨
¡
¡
were found only for the S1 treatment on Days 49–59 (P <
0.005). Differences between clones in RWC 0 were not significant for any treatment or measurement day. On Days 49–59,
the lowest Π0 and Π100 values were reached in Clones 413.7
and 161.3 and the highest in Clones 334.1 and 131.2 (Table 4).
Maximum values of ε max were measured in Clones 413.7 and
115.3.
The maximum degree of osmotic adjustment was 0.41 MPa.
The highest values of osmotic adjustment (OA) and bulk elastic modulus (ε max) were measured in clones that exhibited the
highest leaf expansion between harvests. Conversely, the lowest ε max and OA values were obtained in the clones with the
lowest leaf expansion between Days 60 and 95 (Figure 5). The
highest OA and ε max values were measured in clones that
showed moderate to good growth under field conditions,
whereas the lowest values were measured in clones that
showed poor growth under field conditions (data not shown).
Discussion
Drought significantly decreased plant biomass, plant leaf area,
TREE PHYSIOLOGY VOLUME 21, 2001
PHYSIOLOGICAL RESPONSES OF EUCALYPTUS GLOBULUS TO DROUGHT
Figure 2. Mean leaf size (LM) measured on Day 60 (open) and mean
LM increment from Day 60 to Day 95 (solid fill = increments higher
than clonal mean, shaded = increments lower than clonal mean for
each treatment). Abbreviations: C = control; S1and S2 = water stress.
Vertical bars are standard errors of LM values measured on Days 60
and 95.
specific leaf area (SLA) and leaf expansion in all E. globulus
clones tested. Significant treatment differences in whole-plant
dry mass accumulation were found between clones on Day 60,
the fastest growing clones being the most affected by drought.
Although differences in SLA among clones were not significant, a significant clone × treatment interaction was found on
Day 60. Among clones, the smallest proportional drought-induced decreases in SLA were measured in Clones 131.2 and
334.1. The highest SLA values were recorded in the same
clones. Xeric genotypes generally produce leaves with lower
SLA than mesic genotypes (Abrams 1994). Gibson et al.
(1991) found that a semi-arid provenance of E. camaldulensis
Dehnh. responded to water deficit mainly with morphological
changes (decreases in SLA). Similarly, Lauteri et al. (1997)
reported lower SLA in drought-adapted Castanea sativa Mill.
provenances than in drought-sensitive provenances. Thus, the
responses of Clones 131.2 and 334.1 suggest that they are less
able to withstand drought than the other clones. Consistent
with this suggestion, the lowest leaf expansion between Days
603
60 and 95 was measured in Clones 131.2 and 334.1 in the
drought treatments.
Significant differences between clones in mean leaf size
were found on Day 95. Among clones tested, Clones 413.7,
115.3 and 131.2 had the largest leaves and exhibited the greatest biomass growth under well-watered conditions and the
highest proportional decrease in mean leaf size, leaf area and
plant biomass under drought conditions. Several studies have
shown that genotypes from xeric areas generally have smaller
leaves than genotypes from more mesic habitats (Abrams
1994, Teklehaimanot 1998). A small leaf size is considered an
adaptive response to high temperatures (Zeiger 1993, Potts
and Jordan 1994). However, Gibson et al. (1991) found that
E. camaldulensis seedlings from a semi-arid provenance produced fewer but larger leaves than seedlings from more mesic
provenances. Hybrid E. camaldulensis × E. globulus rooted
cuttings produced larger leaves than E. globulus clones and
shed more leaves under severe drought in a greenhouse experiment (Pita 1998). We conclude that clones with inherently
large leaves (Clones 413.7, 115.3 and 131.2) must be classified as high-risk clones for planting in drought-prone areas.
Differences among clones in transpiration were related to
differences in leaf area under optimum conditions. Because
yield is correlated with transpiration (Calder et al. 1992,
Eldridge et al. 1993), the relationship between transpiration
and leaf area confirms the importance of early leaf development for maximizing productivity (Mebrahtu and Hanover
1991, Rhodenbaugh and Pallardy 1993, Osorio et al. 1998).
Under drought conditions, the relationship between leaf area
and transpiration was only statistically significant on Day 95.
We note that the slope of the relationship between leaf area
and transpiration was similar for control and S1 plants on Day
95, but less than half this value for S2 plants. Thus, rooted cuttings of E. globulus show a double response to chronic water
deficits: a reduction in leaf expansion and a decrease in transpiration rate.
The drought treatments had significant effects on all of the
tissue water relations measured. Maximum bulk modulus of
elasticity (ε max) varied between 3.16 and 7.80 MPa in water-stressed plants. These values are within the range reported
by White et al. (1996) and Correia et al. (1989) for recently expanded leaves of E. globulus growing in the field. Water stress
increased ε in contrast to findings by White et al. (1996) and
Correia et al. (1989) for E. globulus seedlings growing in the
field. Generally, drought reduces cell wall elasticity (Turner
and Jones 1980); however, water deficits have been reported
to increase elasticity in some tree species (Blake et al. 1991).
Table 2. Mean specific leaf area (m2 kg –1) and standard errors for each clone and treatment measured on Day 60.
Treatment
Control
Stress 1
Stress 2
Clone
115.2
115.3
131.2
131.3
161.3
334.1
413.7
19.61(0.54)
16.34(0.70)
16.13(0.55)
20.88(0.46)
16.31(0.52)
14.47(1.5)
18.87(1.20)
18.02(0.88)
16.16(0.59)
20.79(0.35)
15.60(0.90)
16.58(0.23)
21.05(0.80)
14.88(0.75)
4.73(0.31)
17.42(1.1)
15.85(0.55)
17.39(1.1)
17.89(1.0)
14.88(0.80)
15.27(0.46)
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604
PITA AND PARDOS
Table 3. Mean transpiration rates (g H2O plant –1 day –1) between Days 25 and 57 (November 16–December 18), and between Days 62 and 89 (December 23–January 19). An asterisk denotes statistically significant differences between clones based on one-way ANOVA (P < 0.05).
Time interval
Days 25–57
Days 62–89
Treatment ANOVA
C
S1
S2
C
S1
S2
*
NS
NS
*
*
NS
Clone
Clonal mean
115.2
115.3
131.2
131.3
161.3
334.1
413.7
24.61
17.14
12.56
33.01
18.56
12.02
33.74
15.89
10.66
57.21
14.37
12.98
26.73
15.86
13.2
57.39
9.52
11.7
23.58
15.23
11.83
37.18
9.18
16.44
27.76
16.95
11.97
43.7
16.94
13.6
21.41
16.46
11.58
37.38
7.88
11.58
29.17
17.67
11.72
57.13
21.23
14.1
A decrease in leaf tissue elasticity may be an important adaptive response to maintain water uptake when soil water is depleted, because the change in water potential for a given
change in relative water content is greater for plants with a
high ε than for plants with a low ε. In agreement with this, the
lowest ε max values were measured in those clones that exhibited the smallest increases in transpiration and leaf size in the
drought treatments.
Both Π100 and Π0 were reduced by drought (cf. White et al.
1996). Values of Π100 varied between –0.84 and –1.21 MPa in
water-stressed plants. These values are slightly higher than
those found by White et al. (1996) for E. globulus in the field.
Osmotic potential at the turgor loss point (Π0) varied between
–1.14 and –1.56 MPa, and the maximum degree of osmotic adjustment was 0.41 MPa. Osmotic adjustment in pot-grown
seedlings of several Eucalyptus species ranged from 0.24 MPa
(E. camaldulensis) to 0.50 MPa (E. tereticornis Sm.) (Lemcoff et al. 1994). A similar degree of osmotic adjustment
26.71
16.46
11.93
46.14
13.95
13.20
(0.4 MPa) was found in pot-grown seedlings of E. marginata
J. Donn ex Sm. (Stoneman et al. 1994) and E. microtheca F.J.
Muell. (Tuomela 1997). The degree of osmotic adjustment
that we observed was higher than that found in E. globulus
trees growing at low rainfall sites in Tasmania (–0.12 MPa,
White et al. 1996) and in Portugal’s Atlantic region
(–0.29 MPa, Correia et al. 1989) and close to the maximum
values measured in any species of this genus. The observed
decreases in osmotic potential and the reduction in tissue elasticity (increase in ε max) between Days 49–59 and 84–94 may
reflect tissue maturation (Salleo and Lo Gullo 1990, Anderson
and Helms 1994). Osmotic potential may also change during
leaf development in the absence of drought (Parker et al. 1982,
Abrams 1988).
Despite the small difference between drought treatments in
our study, differences in tissue water relations between clones
were detected only in the less severe (S1) drought treatment.
Minimum Π0 values were measured in those clones that exhib-
Figure 3. Relationship between mean
transpiration and mean leaf area of
seven E. globulus clones. Symbols refer to clones as follows: ◆ = 115.2; ■
= 115.3; ▲ = 131.2; ¨ = 131.3; ● =
161.3; ¡ = 334.1; and ∆ = 413.7.
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PHYSIOLOGICAL RESPONSES OF EUCALYPTUS GLOBULUS TO DROUGHT
605
Figure 4. Leaf water relations of control
(open), S1 (shaded) and S2 plants (solid
fill) measured just before plants were
harvested on Days 60 and 95.
ited the highest leaf expansion in the same treatment (Clones
161.3 and 413.7). An earlier and greater reduction in osmotic
potential in response to drought was found by Abrams and
Knapp (1986) for Quercus macrocarpa Michx. compared
with the less drought-resistant Celtis occidentalis L. growing
in the same field. Abrams (1988) found significantly lower
values of osmotic potential for the relatively xeric provenances of Cercis canadensis L. compared with a relatively
mesic forest provenance during both early and later stages of
drought. For three populations of Eucalyptus viminalis Labill.,
Ladiges (1975) found that the more drought-resistant populations tended to reach low water potentials more rapidly than
the less-resistant population. Moreover, seedlings of the
drought-resistant population maintained higher transpiration
rates for longer periods during drought than the other populations. Our results support these findings.
Tissue water relations of Clones 131.2 and 334.1 were unaffected by 49 days of drought treatment, and these clones
showed poor growth under drought. The response of Clone
161.3 differed slightly from the rest of the clones in that it
reached the highest degree of osmotic adjustment without
marked changes in leaf elasticity. Osmotic adjustment may be
achieved by an increase in the osmotically active solutes per
cell (“true” osmotic adjustment, Turner and Jones 1980) or
through a passive concentration of solutes linked to changes in
cell size (Correia et al. 1989). Similarly, increases in the bulk
elastic modulus are related to increases in cell wall thickness
or reductions in cell size, or both. Production of small cells
may be related to a decrease in leaf expansion and a reduced final leaf size (Hinckley et al. 1989, Metcalfe et al. 1990). The
smallest drought-induced reduction in mean leaf size was
measured in Clone 161.3. Thus, it appears that osmotic adjustment in Clone 161.3 occurred mainly by active solute concentration rather than through changes in leaf size.
In summary, intraspecific differences in tissue water relations were found in E. globulus clones subjected to water
stress. Clonal ranking according to tissue water relations was
consistent with clonal differences in leaf expansion and partially in agreement with the observed behavior of the clones
under field conditions. However, to establish the relevance of
these results to E. globulus breeding programs, it will be necessary to conduct additional studies with an increased number
Table 4. Osmotic potential at full turgor (Π100, MPa), osmotic potential at the turgor loss point (Π0, MPa) and maximum bulk modulus of elasticity
(εmax, MPa) for E. globulus clones exposed for 49 days to well-watered (C) and water-stressed (S1) conditions. Values are means (± standard errors) for n = 3–5 observations. Means followed by different letters within columns are significantly different according to the Bonferroni test.
Clone
115.2
115.3
131.2
131.3
161.3
334.1
413.7
C
S1
Π100
Π0
ε max
Π100
Π0
ε max
–0.82a ± 0.06
–0.84a ± 0.02
–0.80a ± 0.04
–0.74a ± 0.05
–0.80a ± 0.06
–0.82a ± 0.05
–0.82a ± 0.06
–1.17a ± 0.03
–1.09a ± 0.03
–1.14a ± 0.03
–1.09a ± 0.11
–1.22a ± 0.08
–1.20a ± 0.03
–1.06a ± 0.05
3.02a ± 0.63
2.78a ± 0.18
3.15a ± 0.39
2.39a ± 0.44
4.26a ± 0.78
3.82a ± 0.69
4.30a ± 0.67
–0.95ab ± 0.09
–1.14b ± 0.03
–0.84a ± 0.03
–1.04ab ± 0.05
–1.21b ± 0.04
–0.87a ± 0.06
–1.17b ± 0.08
–1.24ab ± 0.11
–1.43bc ± 0.02
–1.14a ± 0.04
–1.36abc ± 0.06
–1.61c ± 0.08
–1.21ab ± 0.09
–1.56c ± 0.04
4.96abc ± 0.17
6.95c ± 1.12
3.16a ± 0.26
5.08abc ± 0.51
5.33abc ± 0.64
3.36ab ± 0.63
5.93bc ± 0.79
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606
PITA AND PARDOS
Figure 5. Clonal classification according to maximum bulk modulus
of elasticity (ε max, MPa) and the osmotic adjustment (OA, MPa) measured on Days 49–59 in S1 plants. Increases in mean leaf size (∆LM)
between Days 60 and 95 in S1 plants are also shown.
of clones and to relate the results to quantitative data from
field trials.
Acknowledgments
Financial support for this project was provided by the Ministerio de
Industria y Energia of Spain and CYCIT-CDTI. The authors thank
Professor I. Trnkova Farrell for checking the English version of the
manuscript, and I. Aranda, M. Fernández, A. Royo, L. Serrano,
F. Montes and F. Masedo for their generous technical assistance.
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