Tree
Physiology
6,221-221
0 1990 HCIWI Puhli.shir,~-Vic,ro/.iu,
Cutmdu
Leaf growth of Eucalyptus globulus seedlingsunder water deficit
J. C. METCALFE,‘,?
W. J. DAVIES’ and J. S. PEREIRA*
Received
May
12, 1989
Summary
Eucalyptus glohulus
Labill. seedlings grown under field conditions
in Portugal were watered either daily
(control)
or every 6 days (drought-treated).
Relative
to those of control plants, rates of leaf production
and leaf biomass accumulation
were reduced by almost half in drought-treated
plants. However,
whereas
expansion
of new leaves on control plants slowed toward the end of the 30.day experiment,
expansion
of leaves of the same age on drought-treated
plants accelerated
as a change in weather conditions
resulted
in midday plant water potentials
above -3.0 MPa. In plants that were left unwatered
until they wilted and
were then watered daily, expansion
of the fifth leaf pair from the apex was slower than that of the same
pair of leaves of plants watered daily throughout;
but it continued
for about twice as long and resulted in
the same final leaf area. Drought
treatment
also caused a substantial
reduction
in the rate of leaf
production,
which,
in part, accounted
for the effect of drought
on leaf biomass
production.
In a
greenhouse
study, witholding
water for 15 days had only a slight effect on the length or width of adaxial
epidermal
cells, and the effect was quickly
reversed on rewatering.
Introduction
There is a strong linear relationship between intercepted radiation and total aboveground biomass in Eucalyptus glohulus Labill. (Linder 1985), suggesting that productivity is closely related to total leaf area (cf. Pereira et al. 1989). Physiological
components known to determine leaf area include the rate and duration of individual-leaf growth and the rate of leaf production. These factors must in turn depend on
rates of cell division and expansion. In most cases, however, the relative importance
of these components is not well understood.
Increase in leaf area at both the single plant and the stand level is closely related
to water availability (Grier and Running 1977, Pereira et al. 1989). Hsiao and
Acevedo (1974) showed that reduced growth in maize leaves caused by water deficit
may be reversed if the deficit is neither too severe nor too prolonged. In other species,
however, water deficit has a negative effect on both leaf expansion rate and final leaf
size (Rawson et al. 1980, Takami et al. 198 1, Mazzoleni and Dickmann 1988).
Eucalypt species are planted as a commercial crop in many areas where there is a
summer drought. This study therefore investigated the way in which drought afffects
leaf area accretion in seedlings of Eucalyptus globulus.
3 Author- to M,hom cowespondewe should he addressed
Downloaded from http://treephys.oxfordjournals.org/ at Pennsylvania State University on April 21, 2014
’ Institute of Envir-onmental and Biological Sciences, Unir~ersity of Lancaster-, Lancaster
LA1 4YQ, UK
’ Department of Forestry, Institute Superior de Agronomiu, P-1399 Lishou Codex, Portugal
222
METCALFE,
DAVIES
AND
PEREIRA
Methods
Field study
Greenhouse study
Seedlings were grown outdoors in 15-cm pots containing John Innes II compost.
When the plants were 3 months old and approximately 35 cm tall, they were
transferred to a greenhouse with a minimum night temperature of 10 “C and a
maximum day temperature of 28 “C. Natural lighting was supplemented by fluorescent tubes (approximately 200 pmol mm2 SK’) to provide a photoperiod of 14 hours.
During the first 7 days in the greenhouse the plants were watered daily. Thereafter
they received either no water until the fifteenth day when they were watered
(drought-treated), or were watered to field capacity daily (control). Approximately
every fifth day, leaves were sampled, sectioned transversely, stained with calcafluor
(CAS No. 4404-43-7) and examined under a fluorescence microscope. At least 30
cells were measured with a graticule on several sections from each leaf. Values are
means of five replications per treatment. Measurements were made on very young
expanding leaves (third pair from the terminal bud), in the most rapid phase of
growth, and on leaves that were slightly older (8th leaf from the terminal bud).
Results
The effects of irrigation regime on height growth, leaf number and leaf biomass
production of outdoor-grown Eucalyptus glohulus seedlings are shown in Table 1.
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Eucalyptus glohulus seedlings were planted in 5-liter pots of fertile soil and grown
outdoors in Lisbon, Portugal. The pots were covered with plastic film to prevent loss
of soil water by evaporation, or rewetting of the soil by rain or dew. The seedlings,
which were approximately 50 cm tall, were watered either daily (control), or every
6 days (drought-treated).
The lengths and widths of leaves not yet fully expanded were measured with
calipers every 2 days, and leaf areas were calculated by means of regression
equations constructed from linear leaf dimensions and leaf areas measured with a
Li-Cor leaf area meter. Leaves too small and delicate to be measured with calipers
(< 1 cm long) were classed as new leaves, and were counted in pairs up to the
terminal bud. Production of new leaves was followed on each plant until rewatering
of the drought-treated plants. On each rewatering day, leaf area measurements were
begun on leaves that had reached a sufficient size for measurement with calipers.
A separate set of plants was left unwatered until they wilted and then watered daily
as were the contol plants. In these plants, growth of the fifth leaf pair from the apex
was followed until expansion was complete.
Midday xylem water potentials of young leaves were measured with a pressure
chamber before plants were rewatered. At the end of the experimental period, all
plants were harvested and measurements made of leaf number, stem length and the
dry weight of leaves produced.
WATER
DEFICIT
AND
Table I. Increments
treated E. ~lohulus
5).
LEAF
GROWTH
in leaf number,
seedlings grown
Treatment
OF EUCALYPTUS
leaf dry weight and total stem length of well-watered
and droughtoutdoors
for 30 days. Values are means of five replicates
k SE (n =
No. of new leaves
Control
Drought-treated
223
Leaf DW
95k5
60+4
(8)
increment
Total
(cm)
stem length
410+22
236 k 17
830 * 10
648 2~ 32
L5
Ll
L3
ws
0
15
30
Q
15
30
u
15
30
0
15
30
Days
Figure 1. Time course of expansion
of individual
leaves of well-watered
(WW) and drought-treated
E. glohulus
seedlings.
L6 denotes the youngest
leaf pair at the start of the experiment,
Ll
youngest
leaf pair at the end of the experimental
period.
(WS)
denotes
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Compared with control plants watered every day, drought-treated plants, which were
watered every 6 days, produced little more than half the number and dry mass of new
leaves. Height growth was also restricted by drought treatment, but not as severely
as leaf growth.
The time course of expansion of individual leaves is shown in Figure 1. The data
presented are for successive leaf pairs on five separate plants. Leaves of the well-watered plants all showed the same pattern of rapid early expansion followed by a
declining rate of expansion after about 25 days. In contrast, expansion of leaves of
drought-treated plants was very slow for the first 16 days and then considerably
faster, although on average their rate of expansion was at all times less than that of
leaves of well-watered plants.
The pattern of expansion of leaves of the drought-treated plants appears to have
been related to weather conditions. For the first 16 days of the experiment the soil in
pots containing the drought-treated plants dried severely in the 6-day intervals
between waterings. Before plants were rewatered, the leaves wilted and midday leaf
224
METCALFE,
DAVIES
AND
PEREIRA
Table 2. Midday
water potentials
(MPa) of young leaves (Leaf
~luhulus
seedlings.
Drought-treated
seedlings
were watered
made just before watering.
Values are means of five replicates
Day
6
12
17
24
Midday
water
potential
3) of well-watered
every sixth day
f SE.
and drought-treated
and measurements
(MPa)
Well-watered
Drought-treated
-1.24 f 0.09
-1.32f0.04
-1.09 * 0.03
-1.17f0.07
-3.94 * 0.12
-3.83+0.19
-3.48kO.17
-2.33 k 0.02
E.
were
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water potentials fell to approximately -4.0 MPa (Table 2). Rewatering during this
period did not result in a detectable acceleration of leaf expansion (Figure l),
although it allowed the leaves to regain turgor. Toward the end of the experiment,
however, a change in the weather resulted in less rapid transpiration and higher
minimum leaf water potentials, and this was reflected in an increased rate of leaf
expansion in the droughted plants, although the rate remained much lower than that
of the leaves of well-watered plants (Figure 1).
Judging by the minimum leaf water potentials recorded in Table 2, it is unlikely
that leaf growth of droughted plants occurred during the daytime in the period
immediately before rewatering. However, under well-watered conditions, the expansion of E. globulus leaves was similar in both the light and the dark (data not shown).
Expansion of leaves of droughted plants subject to severe midday water deficit may
thus have occurred at night following partial recovery from daytime water deficit.
Expansion of the fifth leaf pair from the apex in well-watered control plants and
in plants first left unwatered until they wilted and then watered daily is shown in
Figure 2. Although expansion of leaves of the drought-treated plants was slower than
that of leaves of well-watered plants they eventually attained the same final area.
Production of new leaves (Figure 3) was extremely sensitive to soil water availability. Well-watered plants produced new leaves at a steady rate, one new leaf pair
emerging approximately every 8 days. In comparison, the rate of leaf production by
drought-treated plants was substantially reduced. The increase in rate of new leaf
production evident in both treatments toward the end of the experiment reflects the
change in weather that reduced transpiration rates and increased midday minimum
leaf water potentials (Table 2).
Dimensions of adaxial epidermal cells of leaves of greenhouse-grown E. glohulus
that were either watered daily, or kept unwatered for 15 days, are shown in Table 3.
Epidermal cells were larger in the older leaves (L8) than in young leaves (L3).
However, Table 3 indicates that leaves of similar age are very similar in epidermal
cell dimensions, despite different treatments. Slight variations in cell size in well-watered plants were due to small differences in the size of the leaves sampled. Only
small differences were detected in epidermal cell sizes of the youngest leaves taken
from control or drought-treated plants. After 10 days without water there was
evidence of a restriction in cell length in older leaves of unwatered plants, though
WATER
AND
LEAF
nsd
GROWTH
nsd
l
10
20
22s
OF EUCALYPTUS
30
40
50
Days
Figure 2. Leaf area of the 5th leaf of E. glohul~~~
unwatered
until they wilted and then watered daily
of r-tests are shown for leaf areas at the beginning
watered plants (Day 2 I) and maximum
leaf size of
IYYULLS Day 42 for drought-treated
plants; * = P <
‘i
0
4
8
12
16
seedlings.
Plants were watered
daily (o), or kept
(M). Values are means of five replicates
f SE. Results
of the experiment
(Day l), maximum
leaf size of welldrought-treated
plants (Day 28 for well-watered
plants
0.05; nsd = P > 0.05).
20
24
28
Days
Figure 3. Production
of new leaves by well-watered
(0) and drought-treated
(m) E. ~lohul~s
Arrows
indicate days on which drought-treated
plants were watered and when “new”
leaves
I cm in length were no longer included
in the count of new leaves.
seedlings.
more than
well-watered and rewatered plants showed comparable cell sizes soon after rewatering. There were significant effects of imposed water deficit on cell length, width and
area during the experiment.
Discussion
Water deficit reduced leaf biomass of Eucalyptus ~lobulus and this was a function of
reduced rates of both leaf production and leaf expansion. Similar results have been
reported by Borralho et al. (1989) for E. glohulus clones growing in the field in
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lo-
DEFICIT
226
METCALFE,
DAVIES
AND
Table 3. Dimensions
(length and width, k k SE) of adaxial epidermal
cells measured
on the
8th (L8) leaves of juvenile
E. glohulus
seedlings,
either watered, unwatered
or unwatered
then rewatered.
The analysis
of variance
indicated
that effects of treatment
on length and
significant
(P= O.Ol), and effects of measurement
day on length and width were significant
Day
Well-watered
L3
3rd (L3) and
for 15 days
width were
(P = 0.001).
Rewatered
Unwatered
L8
PEREIRA
L8
L3
L8
17.6 k 0.3
14.4 * 0.3
32.7k
1
27.8 kO.8
-
-
12.1 kO.2
-
28.0 k 3
-
12.8 f 2
14.7 + 2
32.8 2~ 1
33.4 * 1
17.8 2 0.2
22.7k0.4
-
-
17.5 * 0.3
16.2 k 0.2
-
18.6kO.2
19.0 * 0.3
-
18.8 + 0.2
18.6 + 0.2)
20.3 IL 0.3
19.9 2 0.3
Lewk W
5
10
15
16
17.0
14.8
14.9
14.9
+
f
k
i
0.4
0.3
0.2
0.2
33.1
33.1
31.7
34.2
f
rt
f
f
1
1
0.8
1
5
10
15
16
Width
18.1
18.2
18.6
18.7
(p)
f 0.3
zk 2
f 0.2
rt 0.2
22.8
22.8
19.1
19.4
f
f
+
*
0.3
0.5
0.3
0.4
Portugal. They found a threefold increase in standing biomass under irrigation
compared to rainfed controls, the increase in biomass being linearly related to leaf
area index. In several annual species, Boyer (1968, 1970) found that as leaf water
potential and turgor fell, leaf enlargement was inhibited before photosynthesis and
respiration. Our results confirm the sensitivity of leaf area growth to water deficit
and show that in E. ~1ohulu.s this results from effects of water deficit on the rates of
both leaf production and leaf enlargement (cf. Metcalfe et al. 1990).
Ridge et al. (1986) found that, in poplar hybrids, both cell size and cell number
were genetically determined traits. Our observations suggest that the same is true of
juvenile E. glohulus leaves. Not only was final leaf area similar in well-watered and
drought-treated plants, but adaxial epidermal cells of leaves of comparable age were
similar in length and width in both well-watered and drought-treated plants (Table 3).
The slight reduction in the linear dimensions of epidermal cells of older leaves
caused by prolonged water deficit was completely reversed once the water deficit
was terminated.
Leaves of Eucalyptus globulus differ from those of certain Populus clones in
possessing the ability to resume expansion after a period of growth-inhibiting
water
deficit and to attain the same size as leaves on plants that have been continuously
well watered. Likewise, Bachelard (1986) found that, in a range of Eucalypt species,
there was no change in specific leaf area with decreasing soil water potential. The
reduction caused by water deficit in the final size of poplar leaves (Van Volkenburgh
1987, Mazzoleni and Dickmann 1988) appears to be a consequence of reduced cell
division not size, as Van Volkenburgh (1987) found that final epidermal cell size was
unaffected by water deficit.
The capacity of E. globulus leaves to resume expansion following a growth-inhib-
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L3
WATER
DEFICIT
AND
LEAF
GROWTH
OF EUCALYPTUS
227
iting water deficit, may enhance the interception of radiation and thus fixation of
carbon in a drought-prone habitat.
Acknowledgments
We thank SandraBarreto
and Luisa
from the EEC is acknowledged.
Antunes
for help with collection
and analysis
of data. Project
support
References
Downloaded from http://treephys.oxfordjournals.org/ at Pennsylvania State University on April 21, 2014
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