Annals of Botany 100: 1507–1515, 2007
doi:10.1093/aob/mcm234, available online at www.aob.oxfordjournals.org
Contrasting Physiological Responses of Six Eucalyptus
Species to Water Deficit
A ND R E W M E R C HA N T 1, * , A N D R E W CA L L I S T E R 2 , S T E FAN AR ND T 2 ,
MI CH AEL TAU SZ 2 and MA RK AD AMS 1
1
School of Biological, Earth and Environmental Sciences, University of New South Wales, Sydney,
NSW 2052, Australia and 2School of Forest and Ecosystem Science, The University of Melbourne,
Water St, Creswick, Victoria, 3363, Australia
Received: 12 June 2007 Returned for revision: 4 July 2007 Accepted: 6 August 2007 Published electronically: 27 September 2007
† Background and Aims The genus Eucalyptus occupies a broad ecological range, forming the dominant canopy in
many Australian ecosystems. Many Eucalyptus species are renowned for tolerance to aridity, yet inter-specific variation in physiological traits, particularly water relations parameters, contributing to this tolerance is weakly characterized only in a limited taxonomic range. The study tests the hypothesis that differences in the distribution of
Eucalyptus species is related to cellular water relations.
† Methods Six eucalypt species originating from (1) contrasting environments for aridity and (2) diverse taxonomic
groups were grown in pots and subjected to the effects of water deficit over a 10-week period. Water potential, relative water content and osmotic parameters were analysed by using pressure– volume curves and related to gas
exchange, photosynthesis and biomass.
† Key Results The six eucalypt species differed in response to water deficit. Most significantly, species from high
rainfall environments (E. obliqua, E. rubida) and the phreatophyte (E. camaldulensis) had lower osmotic potential
under water deficit via accumulation of cellular osmotica (osmotic adjustment). In contrast, species from low rainfall
environments (E. cladocalyx, E. polyanthemos and E. tricarpa) had lower osmotic potential through a combination
of both constitutive solutes and osmotic adjustment, combined with reductions in leaf water content.
† Conclusions It is demonstrated that osmotic adjustment is a common response to water deficit in six eucalypt
species. In addition, significant inter-specific variation in osmotic potential correlates with species distribution in
environments where water is scarce. This provides a physiological explanation for aridity tolerance and emphasizes
the need to identify osmolytes that accumulate under stress in the genus Eucalyptus.
Key words: Eucalyptus, water potential, osmotic adjustment, water deficit.
IN TROD UCT IO N
Most woody plant genera show considerable inter-specific
variation in leaf osmotic parameters (Roberts and Knoerr,
1977; Bahari et al., 1985; Abrams, 1988, 1990; Parker
and Pallardy, 1988; Fan et al., 1994; Myers et al., 1997)
that reflects, at least partly, their distribution in relation to
climatic and edaphic conditions. The genus Eucalyptus
forms the dominant canopy in many forest and woodland
ecosystems across the Australian continent. Over 900
species of eucalypt are formally recognized (Boland
et al., 2006) and together occupy a broad range of environmental conditions. Whilst sections of the genus are
renowned for their tolerance to arid conditions and capacity
to cope with extremely low water potentials, others are confined to moist areas. However, inter-specific differences in
the physiological and/or chemical traits that may confer
such tolerance have been inadequately characterized.
For eucalypts, changes in osmotic potential in response
to water deficits have been assessed using the standard technique of pressure – volume ( pV) analysis (Schulte and
Hinckley, 1985). There is considerable inter-specific variation within the genus in the capacity to regulate osmotic
potential (Myers and Neales, 1986), and this extends to
* For correspondence. E-mail [email protected]
variation among provenances (Tuomela, 1997; Li, 1998)
and even between clones (Pita and Pardos, 2001).
Variation in the capacity to modify osmotic potential
among eucalypt species has been proposed to explain differing salt (Grieve and Shannon, 1999) and drought
(Li, 1998) tolerances, leading to suggestions that osmotic
potential may be used as a selection criterion for high performing individuals (van der Moezel et al., 1991; Lemcoff
et al., 1994). The use of osmotic potential as a measure of
acclimation to water deficit and its application to the wider
genus have been hindered by changes in measurement techniques (Callister et al., 2006; Lenz et al., 2006) and the
small number of species investigated.
Despite these limitations, reductions in osmotic potential
of between 0.2 and 0.8 MPa appear common to all eucalypt
species (Bachelard, 1986; Myers and Neales, 1986;
Lemcoff et al., 1994; Myers et al., 1997; Prior and
Eamus, 1999; White et al., 2000). The magnitude of this
adjustment is generally greater in higher plants (Taiz
and Zeiger, 1998). Water potentials below 23.5 MPa are
common for many Eucalyptus species of low rainfall
environments (Bell and Williams, 1997), and reductions
in osmotic potential are likely to act in combination with
morphological adaptations enabling eucalypts to withstand
low external water potential. Traits such as the regulation
of cell-wall elasticity (measured using pV analysis as the
# The Author 2007. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved.
For Permissions, please email: [email protected]
1508
Merchant et al. — Eucalyptus Responses to Water Deficit
maximum change in bulk elastic modulus) also assist some
eucalypts to maintain tissue hydration (Stoneman et al.,
1994; Prior and Eamus, 1999; Pita and Pardos, 2001) and
are likely to vary widely between species due to differences
in leaf structures.
Changes in osmotic potential have been quantified on
a diversity of eucalypt species; however, individual
studies have focused on species of similar edaphic conditions, e.g. E. nitens and E. globulus (White et al.,
1996), E. behriana and E. polyanthemos (Myers and
Neales, 1986), E. microcarpa (Clayton-Greene, 1983;
Myers and Neales, 1986), E. melliodora (Clayton-Greene,
1983), and E. camaldulensis, E. grandis, E. viminalis and
E. tereticornis (Lemcoff et al., 1994). Selection has also
focused on species that are often closely related. The magnitude of differences in osmotic potential between species
is not sufficient to allow conclusions as to contrasting
growth strategies under stress conditions in the broader
genus. Only recently (e.g. White et al., 2000) have comparative studies begun on eucalypt species from a wide
range of environmental conditions, as well of different
taxonomic groups. White et al. (2000) identified contrasting
responses to seasonal drought stress for four, distantly
related field-grown eucalypts: E. camaldulensis and
E. saligna avoided water deficit by increased rooting
depth whilst E. leucoxylon and E. platypus had inherently
low osmotic potentials, and also significantly adjusted
maximum bulk elastic modulus.
Eucalypt diversity is largely attributable to substantial
outbreeding (Boland et al., 2006) and, coupled with the
strong influence of edaphic and climatic conditions on
species distributions (e.g. Adams, 1996), results in a strong
association between species of Eucalyptus and the environment. In light of observations by White et al. (2000), this
suggests considerable variation in physiology that are inadequately characterized.
The present study investigates the hypothesis that eucalypts originating from contrasting environments for water
availability possess distinct physiological responses to
water deficit. To test this hypothesis, species from environments differing in water availability, and species from
contrasting taxonomic groupings were chosen (e.g.
Merchant et al., 2006). Pressure– volume analysis was
used to determine leaf osmotic parameters along with
water potential and tissue hydration, and related them to
leaf gas exchange, photosynthesis and biomass allocation.
MATE RIA L AN D M ET HO DS
Plant selection
Eucalypt species were selected for study (1) as representative of major subgeneric classifications within the genus
Eucalyptus and (2) to encompass species from contrasting
environments, particularly regarding water availability
(Table 1).
Seeds were sown in early August, 2003. The trees were
raised in an open-ended greenhouse in Creswick,
Victoria, Australia (37.43 8S, 143.89 8E) for 5 months
before commencement of treatments. Irradiance inside
the greenhouse was 85– 90 % of ambient (in the range
400– 700 nm), and temperatures were a maximum of 2 8C
higher than outside. Trees were transplanted into a mix of
50 : 50 peat/coarse river sand (v/v) in 9-L pots. Macro- and
micro-nutrients were supplied as 5 g of slow-release fertilizer
‘Nutricotew’ per litre of potting mix. Plants were watered
daily to soil capacity before starting treatments.
Experimental design
Twelve uniform plants from each species were selected
and assigned to either of two treatments, ‘well watered’
(WW, six plants) and ‘water deficit’ (WD, six plants).
WW plants were watered to field capacity daily, at dusk.
WD plants received 20% of the average water used by WW
plants of that species. Water usage was determined gravimetrically by the difference in weight of the WW plant
( plus soil and pot) following watering (once through-flow
had ceased) and its weight before watering the following
day. Pots were fully randomized and rotated twice weekly
to minimize differences in light. An additional six plants
of each species were harvested at the beginning of the
experiment for biomass measurements (see below).
Plant height, diameter and biomass measurements
Measurements of plant height, diameter and leaf area
were made after 10 weeks of treatment. Height was
measured as the distance between the stem base and the
shoot tip. Stem diameter was measured (to 0.1 mm) 2 cm
above the soil surface with electronic callipers.
Approximately 10 % of the leaves (by weight) was randomly selected from each plant and their area measured
using a Li-Cor 3050A/4 model leaf area meter (Li-Cor,
USA). Dry biomass of leaves, roots and stems (including
branches and twigs, and the 10 % sub-sample of leaves)
was determined after drying at 75 8C for 24 h. Specific
leaf area (SLA) was calculated from area and mass. Total
leaf area was calculated using total foliar biomass. At the
beginning of the experiment, six additional plants from
each species were harvested and the total biomass (dry
weight) determined. Biomass gain during the treatment
period is expressed as relative growth increment (RGI,
Table 2), calculated as the average biomass gain during
the treatment period divided by the average initial plant
biomass.
Leaf water relations
Leaf water relations were determined using the youngest
fully expanded leaves on a terminal branchlet. Both before
and after the water deficit, pre-dawn shoot water potential
(cpdwn) of each tree was measured using a Scholander
pressure bomb (PMS Corvallis, OR, USA). Leaf relative
water contents (RWCs) were determined on three leaves
from each plant: these were removed, sliced into 5-mm sections and weighed (fresh weight), then placed in deionized
water at 4 8C for 12 h to allow full hydration. Leaf samples
were then lightly blotted dry with tissue paper and
re-weighed (saturated weight), before drying at 36 8C for
Merchant et al. — Eucalyptus Responses to Water Deficit
1509
TA B L E 1. Characteristics of eucalypt species studied
Species
Subgenus
Section
Section climatic
distribution
Environmental
classification
Annual
rainfall (mm)
Growth habit
Species edaphic
conditions
Species of cool,
moist climates.
Annual rainfall
100–1500
distributed
throughout the
year so there is
no marked dry
season
No species of the
group extends
into the tropics or
the arid zone
Mesic
Tall to very tall
tree, 45–90 m
600– 2400
. . . on a wide range
of soils with best
development on
good quality loams
Mesic
550– 1400
Occurs on a wide
range of soil types
Exsertaria
Occupy a wide
variety of habitats
Phreatophytic
Varies from
small . . . to
taller and
straighter (to
35 m)*
Medium-sized to
tall tree (to
45 m)
150– 1100
Adnataria
Species of the
group are widely
distributed over
mainland
Australia. There
are none in the
higher rainfall
areas
Xeric
Small to
medium-sized
tree,* 15– 25 m
450– 970
Xeric
Small to
medium-sized
woodland tree
10–25 m
500– 800*
Xeric
A small to large
tree,* 8 –35 m
380– 650
along inland rivers
or dry watercourses
and floodplains . . .
preferring deep,
moist subsoils with
clay content*
in drier open forests
. . . up to 650 m in
foothills mostly on
shallow soils of
sedimentary origin
. . . on hillsides,
gullies or open flats
(with deep loamier
soils)*
. . . in drier, open
forests on welldrained skeletal soils
(often gravely with
quartz on low ridges
or adjacent flat
country)*
Soils are mainly
skeletal or
frequently rather
shallow or deep
sands or ironstone
gravels
E. obliqua L’Hér
Eucalyptus
Eucalyptus
E. rubida Deane
and Maiden
Symphyomyrtus
Maidenaria
E. camaldulensis
Dehnh
Symphyomyrtus
E. polyanthemos
Schauer
Symphyomyrtus
E. tricarpa L.A.S
Johnson
Symphyomyrtus
E. cladocalyx
F. Muell
Symphyomyrtus
Sejunctae
Single species
taxa. Natural
distribution on
sandy soils of
Kangaroo Island
and Flinders
Ranges*
Species from different taxonomic groups were chosen and from a range of habitats (particularly water availability), and of different growth habit.
References taken from Boland et al. (2006) and *Costermans (1992).
48 h and re-weighing (dry weight). RWC was calculated
from:
RWC ¼ ðFresh weight Dry weightÞ=ðSaturated weight
Dry weightÞ 100:
Leaf osmotic potential at full turgor ( pft), leaf osmotic
potential at turgor loss point ( ptlp) and leaf relative water
content at turgor loss point (RWCtlp) were determined
using pV analysis (Tyree and Hammel, 1972; Tyree and
Richter, 1982; Turner, 1988). Sections of branch were cut
under water (whilst keeping leaves dry) and placed in the
dark for 3 h at room temperature to re-saturate. Samples
were considered to have re-hydrated if their water potentials
were between 20.1 and 20.3 MPa. Maximum bulk elastic
modulus (1) was calculated according to Turner (1988) and
White et al. (2000) as the slope of the relationship between
pressure potential and RWC in the positive turgor range
(Turner, 1988). Apoplastic water content was estimated
by extrapolation of the linear phase of the pV curve (that
which is below the turgor loss point) to the x-axis. A pV
curve for three WW and three WD trees were determined
both before (WW0, WD0) and after 10 weeks after
(WW10, WD10) water deficit.
Measurements of Amax and gs
Light-saturated photosynthesis (Amax) and stomatal conductance at light saturation (gs) were measured with a Li-Cor 6400
on two consecutive clear days between 1100 and 1430 h in
Mean and s.e. are shown together with the significance of the difference between treatments from Tukey’s HSD test (*P ¼ 0.05– 0.01,**P ¼ 0.01–0.001,***P , 0.001). All trees at 0 weeks and
well-watered trees at 10 weeks were watered daily to field capacity of the potting material. Plants exposed to water deficit received 20% of the average water volume used by well-watered plants of that
species.
Merchant et al. Eucalyptus Responses to Water Deficit.
34.82 + 0.63***
45.16 + 1.42***
69.73 + 2.94***
37.98 + 0.52***
31.21 + 0.89***
41.91 + 1.03***
85.12 + 3.15
96.97 + 2.70
141.93 + 4.01
75.03 + 1.09
74.58 + 1.72
72.51 + 1.83
0.3 + 0.03**
0.32 + 0.01***
0.21 + 0.03***
0.24 + 0.02***
0.41 + 0.04***
0.35 + 0.02***
0.92 + 0.18*
0.78 + 0.02**
1.00 + 0.10**
0.75 + 0.04***
0.89 + 0.05**
0.80 + 0.04**
E. obliqua
E. rubida
E. camaldulensis
E. cladocalyx
E. polyanthemos
E. tricarpa
1.32 + 0.08
0.98 + 0.05
1.60 + 0.16
1.14 + 0.06
1.21 + 0.09
1.18 + 0.11
10.65 + 1.05
14.12 + 0.99
18.43 + 0.60
11.17 + 0.92
15.83 + 1.30
13.67 + 0.78
7.95 + 0.64*
10.10 + 0.18**
13.98 + 0.94**
8.25 + 0.75**
11.87 + 0.40**
10.13 + 0.53**
0.57 + 0.05
0.75 + 0.05
0.70 + 0.07
0.55 + 0.04
0.99 + 0.12
0.79 + 0.05
40.30 + 2.09
65.14 + 0.86
38.51 + 2.04
42.47 + 1.60
37.52 + 1.48
30.54 + 1.97
31.54 + 1.17**
54.17 + 1.42***
36.40 + 2.63
41.15 + 1.65
30.36 + 0.63**
30.12 + 0.48
WD
WW
WD
WD
WW
WD
WW
WD
WW
Species
Height (m)
Diameter (mm)
Total leaf area (m2)
WW
SLA (g21 m22)
RGI (g21 g21)
Merchant et al. — Eucalyptus Responses to Water Deficit
TA B L E 2. Average height, diameter, leaf area, specific leaf area (SLA) and relative growth rate (RGI) of six Eucalyptus species after 10 weeks of well watered (WW)
and water deficit (WD) treatments (n ¼ 6)
1510
the 10th week of the experiment. Two measurements were
made on first fully expanded (FFE) leaves on each plant.
Irradiance was set at 1800 mmol m22 s21, which was
roughly ambient around midday. Block temperature was set
at 22 8C, resulting in leaf temperatures between 20 and
22 8C, and air flow rate through the chamber was
300 mL min21. Because the differences in transpiration
between WW and WD plants would have caused large differences in the water vapour pressure deficit in the chamber, a
Li-Cor dew-point generator was used to control vapour
pressure deficit of the incoming air in the range 1–1.4 kPa.
Readings were taken once steady state was achieved, usually
less than 6 min.
Statistical analysis
Effects of treatments were assessed by analysis of variance using SPSS analytical software (Version 13 for
Windows, Chicago, IL, USA). Homogeneity of variances
was tested using Levene’s test with the null hypothesis
that all variances are equal across groups. Multiple comparisons between species were made using Tukey’s honestly significant difference test ( pooled variance) with the
initial assumption that all mean values are equal (null
hypothesis). Comparisons between WD and WW variants
within species were made with Fisher’s least significant
difference (LSD) post-hoc test.
R E S U LT S
Plant growth
Plants used for the experiment were uniform: initial biomass
for each species (+s.e.) was: E. obliqua 18.97 + 1.85 g,
E. rubida 11.50 + 1.60 g, E. camaldulensis 11.45 + 1.28 g,
E. polyanthemos 17.09 + 0.97 g, E. tricarpa 18.93 +
1.72 g, E. cladocalyx 16.10 + 1.02 g. All plants grew
during the treatment period (Table 2). However, when subjected to water deficit, relative growth rate over the 10-week
treatment period was reduced by approx. 50% for each
species (Table 2). The distribution of total mass among
plant organs (leaf, stem and roots) did not differ between
treatments: around 40% was in the roots, 40% in the stems
and branches, and 20% in the leaves for all species.
After 10 weeks, plant height, diameter and leaf area were
significantly smaller in WD compared with WW treatments
for all species (Table 2). Leaf-area-to-weight ratios (SLA)
were significantly less in WD than in WW treatments for
E. obliqua, E. rubida and E. polyanthemos but not for the
other species (Table 2).
Photosynthesis and stomatal conductance
All plants reduced photosynthesis and stomatal conductance during the treatment period (Fig. 1). At the
end of the experiment, Amax for control plants was
between 15 mmol CO2 m22 s21 (E. camaldulensis) and
30 mmol CO2 m22 s21 (E. polyanthemos) whilst in WDtreated plants, Amax was between 5 mmol CO2 m22 s21
(E.
polyanthemos)
and
10 mmol
CO2 m22 s21
Merchant et al. — Eucalyptus Responses to Water Deficit
1511
F I G . 1. Light-saturated photosynthesis rate (Amax) and light-saturated stomatal conductance (gs,max) of six Eucalyptus species. Means and s.e. of n ¼ 6
trees for each data point.
(E. tricarpa). The gs value ranged between 450 and
300 mmol H2O m22 s21 (E. obliqua and E. camaldulensis,
respectively) in the control treatment whilst for WDtreated plants it varied between 150 mmol H2O m2 s21
(E. tricarpa) and 50 mmol H2O m2 s21 (E. cladocalyx).
Leaf water relations
cpdwn and RWC were significantly reduced in all species
in the WD compared with the WW treatment (Fig. 2).
Neither cpdwn nor RWC differed significantly between
species at the beginning of the experiment. Analysis of variance for osmotic potential at full turgor ( pft), osmotic
potential at turgor loss point ( ptlp) and maximum bulk
elastic modulus (1) showed a significant effect of species
and treatment (P , 0.05) with no significant interaction.
RWCtlp differed among species but not with treatment
and showed no interaction.
After 10 weeks, withdrawal of water (WD treatments)
significantly reduced pft and ptlp in all species (Fig. 3).
Differences in pft and ptlp between treatments were of a
similar magnitude for each species. Correspondingly,
there were no differences in pft and ptlp between WW treatments at the start of the experiment (WW0) or after 10
weeks (WW10). RWCtlp and 1 did not vary significantly
with treatment for any species.
Calculated apoplastic water content varied considerably
from near zero in E. camaldulensis (control) to 18 % in
E. obliqua (control) (data not shown). Apoplastic water
content did not show consistent patterns with treatment or
species.
Post-hoc testing for differences among the species delineated two distinct groups in both pft and ptlp (Fig. 3).
E. cladocalyx, E. polyanthemos and E. tricarpa had consistently lower osmotic potentials than E. obliqua, E. rubida and
E. camaldulensis. This was not observed for RWCtlp and 1.
RWCtlp was remarkably consistent among species
(Fig. 3). That of E. obliqua was greater and that of
E. rubida, this difference being smaller than the narrow
range shown by other species. However, although 1
varied widely among species, post-hoc analysis separated
only E. polyanthemos as having significantly reduced 1
(and thereby greater tissue elasticity) and E. obliqua as
having significantly greater 1 (and smaller tissue elasticity)
compared with the other species. Both E. obliqua and
E. camaldulensis displayed low maximum tissue elasticity
prior to treatment; however, maximum elasticity increased
in these species over the treatment period. E. rubida,
E. polyanthemos and E. tricarpa all showed increasing
but highly variable maximum elasticity (reduced 1) in
response to water deficits.
DISCUSSION
During this experiment, the osmotic potential at both full
turgor and the point of turgor loss of all six Eucalyptus
species decreased significantly in response to water
deficit. This agrees with previous investigations demonstrating that decreased (more negative) osmotic potential is a
common response within the genus Eucalyptus, and is considered to help ameliorate the effects of water deficit
(Clayton-Greene, 1983; Lemcoff et al., 1994, 2002;
Stoneman et al., 1994; White et al., 1996, 2000; Tuomela,
1997; Li, 1998; Guarnaschelli et al., 2003).
A second pattern in the present data suggests that eucalypt species from environments where water is relatively
1512
Merchant et al. — Eucalyptus Responses to Water Deficit
F I G . 2. Pre-dawn water potential (MPa) and relative water content (%) of six Eucalyptus species for well-watered treatment (WW0) and water-deficit
treatment (WD0) before commencement of treatments, (18 December, 2003) and well-watered (WW10) and water-deficit plants (WD10) at week 10
(post-water-deficit treatment, 2 March, 2004) (n ¼ 6). Mean and s.e. are shown together with the significance of the difference between treatments
and sampling dates obtained from Tukey’s HSD test (*P ¼ 0.05– 0.01, **P ¼ 0.01– 0.001, ***P , 0.001). All trees at 0 weeks and well-watered
trees at 10 weeks were watered daily to field capacity of the potting material. Plants exposed to water deficit received 20% of the average water
volume used by well-watered plants of that species.
scarce (E. cladocalyx, E. polyanthemos and E. tricarpa)
generally exhibit more negative osmotic potentials (both
pft and ptlp) than species from environments where water
is readily available (E. obliqua, E. rubida and
E. camaldulensis). Interestingly, the magnitude of this
difference between species groups was relatively constant,
between 0.2 and 0.5 MPa. Previous studies investigating
diverse eucalypt species show a similar pattern of osmotic
potential (Clayton-Greene, 1983; Myers and Neales,
1986; Lemcoff et al., 1994; Tuomela, 1997; White et al.,
2000; Pita and Pardos, 2001). This supports the hypothesis
that eucalypts from contrasting environments differ in
osmotic regulation. Species from environments where
water is scarce maintain significant concentrations of
osmotica under all conditions (i.e. constitutive osmotica).
In the present experiment, osmotic traits showed a clearer
response to water deficit than other commonly accepted
measures of response. For example, in agreement with
Ngugi et al. (2003) but in contrast to Osorio et al. (1998),
the current data revealed no change in the distribution of
mass as a result of treatment. Similarly, in contrast to
Ngugi et al. (2004), Amax and gs showed no consistent
trend between eucalypt species. While Amax and gs were
substantially reduced in all species, the continued growth
throughout the experiment suggests that regulation of
water content via osmotic adjustment plays a significant
role in physiological homeostasis.
In Eucalyptus, the identity of solutes contributing
to changes in osmotic potential is not established.
Compatible solutes, sensu Paul and Cockburn (1989), are
commonly associated with osmotic adjustment in plants,
and include proline (Storey and Wyn Jones, 1977; Hare
and Cress, 1997), glycine betaine (Storey and Wyn Jones,
1977; Sakamoto and Murata, 2002) and cyclitols (Popp
et al., 1997). Large concentrations of one or more cyclitols
accumulate in many eucalypts in response to salinity (e.g.
Adams et al., 2005; Merchant and Adams, 2005).
Concentrations of quercitol in leaves of a considerable
number of Eucalyptus spp. (Adams et al., 2005; Merchant
et al., 2006) showed similar taxonomic distribution to that
identified here and would readily account for the additional
0.2 – 0.5 MPa of osmotic potential occurring in the species
adapted to low rainfall.
Despite some deserved notoriety for inconsistency (see
Wardlaw, 2005) the pV method used here to assess 1 gave
results similar to those of Stoneman et al. (1994), Prior
and Eamus (1999) and Pita and Pardos (2001) for eucalypts.
In the present study, there was no significant difference in 1
between the WW and WD plants or between species, partly
because of large variances. A trend of greater maximum
elasticity (reduced 1) over the treatment period can be
observed in the data for E. obliqua and E. camaldulensis,
in agreement with its role in turgor maintenance for
E. platypus (White et al., 2000) and E. globulus (White
et al., 1996), suggesting that elasticity is a key mechanism
of turgor maintenance. However, the present data also
suggest that this trend is related to tissue age, as similar
reductions in maximum elasticity are also observed in
Merchant et al. — Eucalyptus Responses to Water Deficit
1513
F I G . 3. Osmotic potential at full turgor ( pft, MPa), osmotic potential at turgor loss point (ptlp, MPa), relative water content at turgor loss point
(RWCtlp, %) and mean maximum bulk elastic modulus (1) for six Eucalyptus species subjected to well-watered treatment (WW0) before treatment,
(18 December, 2003) and well-watered (WW10) and water deficit plants (WD10) at week 10 (post water deficit treatment, 2 March, 2004) (n ¼ 6).
Mean and standard error are shown together with the significance of the difference between treatments and sampling dates obtained from Tukey’s
HSD test (**P ¼ 0.01–0.001, ***P , 0.001). Well-watered trees at 0 weeks and well-watered trees at 10 weeks were watered daily to field capacity.
Plants exposed to water deficit received 20% of the average water volume used by well-watered plants of that species. Species post-hoc groupings
represent differences between species for all treatments at all times using Fisher’s least squared difference post-hoc test.
WW treatments. Increases in 1 (thus reductions in elasticity) are frequently related to tissue age; however, in contrast to the present results, tissues are commonly thought to
become less elastic with age. Prior and Eamus (1999)
demonstrated a strong age effect on 1 in E. tetradonta
and it was thus surprising that the relatively young leaf
tissues of E. obliqua and E. camaldulensis were highly
inelastic prior to treatment. Equally, some eucalypts show
no adjustment of 1 (e.g. E. marginata; Stoneman et al.,
1994) in response to water deficit. Overall, the varying
results suggest that changes in 1 work in concert
with changes in p to maintain leaf turgor, depending on
tissue age.
None of the species studied showed improved ability to
maintain turgor as water supply declined. The lack of a discernible pattern in RWCtlp in response to treatment suggests
that improved tolerance to lower external water potentials
(assuming little or no physiological function below the
turgor loss point) is largely dependent on the ability to
maintain leaf RWC above approx. 80– 90 %. Figure 2
shows that after 10 weeks, RWCs were below those of
RWCtlp (Fig. 3). Growth under these conditions may
well be dependent upon the temporary release of stress provided by watering ‘events’. The hypothesized mechanism is
thus that eucalypts differ in their ability to tolerate water
deficits that arise between waterings by the maintenance
1514
Merchant et al. — Eucalyptus Responses to Water Deficit
of cellular function through decreased osmotic potential.
Such a mechanism would rely on the accumulation of
solutes that are compatible with cellular function and
adjustments in 1.
Periods of water deficit pose additional hazards for the
maintenance of plant growth. As primary capture of
photon energy is relatively insensitive to stress, plants
under adverse conditions are frequently exposed to light
intensities in excess of those that can be used for carbon
assimilation (Hare et al., 1998). This excess energy can
lead to the generation of damaging reactive oxygen
species. Previous studies among a range of plant genera
have indicated that approximately 20 – 30 % of absorbed
light is dissipated by photosynthesis and that this proportion
is reduced during water deficit (Flexas and Medrano,
2002). Accumulation of solutes that may help to disperse
light energy is thus a benefit (in addition to osmotic adjustment) that may accrue in some species (Wyn Jones et al.,
1977).
Adjustment of SLA during water deficit has been
detected among eucalypts (Li et al., 2000; Pita and
Pardos, 2001; Ngugi et al., 2004) albeit for a limited
range of species. The current study of a broader range of
species indicates reductions in SLA particularly among
species with high SLA at the beginning of the experiment.
The most significant reductions in SLA were found in
species from areas with high rainfall (E. obliqua and
E. rubida). This pattern was also detected by Ngugi et al.
(2003). In that study the reduction in dry matter accumulation by E. cloeziana was roughly twice that recorded for
E. argophloia. These results suggest that reduction in
SLA is a common mechanism of adjustment to water
deficit by trees from high rainfall environments, and
raises interesting questions regarding the capacity of eucalypts to adjust morphologically and physiologically to
environmental conditions. The evidence presented here,
and that of Ngugi et al. (2003), suggests that species from
high rainfall environments possess greater plasticity of
SLA as a mechanism to enhance growth during productive
conditions.
On a whole-plant basis, the concurrent reduction in stem
growth and leaf area of all species supports the hypothesis
that eucalypts adjust transpiration capacity to suit water
availability. Breakdown and redistribution of cellular constituents to regions of new growth could explain the more
negative osmotic potential in leaves exposed to water
deficit. Overall, the concurrent decreases in osmotic potential and reductions in total leaf area suggest clear coordination of physiological and structural adaptations.
In conclusion, Eucalyptus species regulate osmotic
potential probably through changes in the concentration
of osmotica. Changes in other physiological parameters
(1, RWCtlp) play a smaller role in the maintenance of cell
turgor under periods of water deficit. Osmotic potential is
considered the clearest measure of physiological differences
among Eucalyptus spp. from contrasting water environments. We have confirmed, for the species studied, the
hypothesis that eucalypt species from contrasting environments differ in osmotic characteristics enabling maintenance
of growth under water deficit.
ACK N OW L E D G E M E N T S
We thank the Landcare Nursery in Creswick for providing
space for the experiment, Dr Charles Warren for assistance
with the watering regime and, together with Najib Ahmady
and Douglas Rowell, for assistance with physiological
measurements. This work was supported by funding from
the School of Forest Ecosystem Science (SFES). SFES is
supported by the Victorian Government’s Department
of Sustainability and Environment and The University of
Melbourne. A.M. gratefully acknowledges the support of
a School of Forest and Ecosystem Science scholarship.
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