Botanical Studies (2008) 49: 127-137.
*
Corresponding author: E-mail: ybgao@nankai.edu.cn; Tel:
022-23508249.
PHYSIOLOGY
Water relations, hydraulic conductance, and vessel
features of three Caragana species of the Inner Mongolia
Plateau of China
Jing LI, Yu-Bao GAO*, Zhi-Rong ZHENG, and Zeng-Lu GAO
College of Life Science, Nankai University, Tianjin 300071, P. R. China
(Received June 20, 2007; Accepted November 21, 2007)
ABSTRACT.
From early May to late September 2005, diurnal and seasonal changes in water relations, hy-
draulic conductance, vessel features, photosynthesis (DCA), transpiration (DCE), and water use efficiency
(WUE) were studied in the field for three Caragana speciesXC. microphylla Lam, C. davazamcii Sancz and
C. korshinskii KomXgrowing in different habitats in the Inner Mongolia Plateau of China. The three species
were generally exposed to severe environmental drought during most of the growing period. Among them, C.
korshinskii had the largest vessels and specific hydraulic conductivity (K
s
) in one-year-old twigs while the low-
est were recorded in C. microphylla. All three species had the highest K
s
in the summer and the lowest in the
spring. For the three species, the best leaf water status occurred in the autumn indicated by the largest diurnal
mean leaf water potential (Z
L
) and the lowest leaf relative water deficit (RWD) while the severest leaf water
stresses occurred in the spring. Caragana microphylla had larger Z
L
than the other two species, and the lowest
Z
L
as well as the largest RWD occurred in C. korshinskii. The lowest RWD were found in C. davazamcii. The
three species had the largest DCA and DCE in the autumn while the lowest values occurred in the spring for C.
microphylla and C. davazamcii. Caragana korshinskii had the lowest DCA and DCE in the summer, which re-
sulted in a decrease in soil water loss. Leaf stomatal conductance (g
s
) and transpiration rate (E) were found to
be correlated to hydraulic conductance in soil-leaf continuum (G
t
) more closely than K
s
in one-year-old twigs.
Generally, the seasonal changes in G
t
were in accordance with those of DCA and DCE, with the exception of
C. davazamcii, which had the largest G
t
in the spring. Among the three species, C. davazamcii had the high-
est values of seasonal mean DCE and G
t
, and the lowest drought resistance during the growing period, which
was in line the higher soil water content in its habitat. In the spring, despite the high soil water content, C.
microphylla had the greatest resistance to the leaf water deficit raised by the low G
t
. In the autumn, the great-
est drought resistance occurred in C. korshinskii, which was exposed to the severest water stress due to low
soil water content. By analyses of seasonal changes of K
s
and G
t
as well as their relationships with leaf gas
exchange, G
t
was approved to be more important than specific subportions (one-year-old twigs in this study)
in terms of leaf water supply.
Keywords: Caragana davazamcii; Caragana korshinskii; Caragan microphylla; Diurnal and seasonal change;
Growth rhythm; Hydraulic conductance; Leaf gas exchange; Vessel size.
INTRODUCTION
Numerous studies have been done on plant water
relations over a long time period (Kramer, 1983;
Zimmermann, 1983; Holbrook and Putz, 1996; Donovan
et al., 2001), and it is widely recognized that plants
can regulate their transpiration by decreasing stomata
conductance in response to water deficit (Sperry, 2000).
In addition to the traditional theory that the signals of
chemicals like abscissic acid (ABA) (Ackerson and Radin,
1983; Zhang and Davies, 1987, 1990; Tardieu and Davies,
1992) can drive stomatal regulation, hydraulic signals
are being recognized as another mechanism (Fuchs and
Livingston, 1996; Meinzer et al., 1995, 1997; Borghetti et
al., 1998; Tausend et al., 2000).
There is substantial evidence that stomatal behavior
is positively correlated with hydraulic conductance of
the soil-leaf continuum (G
t
) in diverse plant species
and growth forms (Meinzer et al., 1990; Sperry and
Pockman, 1993; Irvine et al., 1998; Bond and Kavanagh,
1999; Sohan et al., 1999; Comstock, 2000; Sperry, 2000;
Hubbard et al., 2001). Such a close relationship between
vapor and liquid phase conductance results from an active
response of stomata to G
t
. When G
t
was experimentally
increased by partial defoliation or shading, stomatal
conductance (g
s
) and transpiration of the untreated foliage
pg_0002
128
Botanical Studies, Vol. 49, 2008
increased (Tschaplinski and Blake, 1989; Ovaska et al.,
1992; Pataki et al., 1998). When G
t
was decreased by root
pruning, stem notching, or freeze thawing, g
s
decreased
(Hammel, 1967; Meinzer and Grantz, 1990; Sperry et al.,
1993). Such close coordination between leaf gas exchange
and liquid phase conductance can dampen variation of
daily leaf water potential under a wide range of conditions
(Mernzer et al., 1992; Sperry et al., 1993; Sperry, 2000).
For trees and shrubs, most of the hydraulic pathway is
composed of xylem, which contains vessels or tracheids.
The properties of vessels or tracheids strongly influence
water transport efficiency. Numerous studies have
demonstrated the great reduction in hydraulic conductance
raised by xylem embolism. At the same time, a decrease in
leaf gas exchange was observed in many species (Sperry
et al., 1998; Nardini and Salleo, 2000; Pockman and
Sperry, 2000; Sperry, 2000; Jordi et al., 2002). Since the
transpiration stream also flows in extra-xylem pathways
in the root and leaf, where embolism does not occur, the
changes in root and leaf tissues during water stress can
have important consequences for a plants G
t
(Sperry,
2000). Telling which subportions are primarily responsible
for the hydraulic resistance is difficult. Generally, stomatal
behavior has been found to be more correlated with
total resistance than with the resistance of any specific
subportion of the pathway (Comstock, 2000).
In this study, the leaf gas exchange, leaf water status,
and hydraulic conductance of three Caragana species
were investigated during different periods of a growing
season. The three species, Caragana microphylla Lam,
C. davazamcii Sancz, and C. korshinskii Kom, grow in
different habitats of Chinas Inner Mongolia Plateau.
Their close interspecific relationships have been proved by
morphology, physiology, and molecular biology, and some
taxonomists even regarded them as one species (Wang et
al., 1994; Wei et al., 1999; Ma et al., 2003a, 2003b; Zhao,
2005). All three Caragana species have been shown to
adapt well to drought conditions (Li and Zhang, 1996;
Xiao and Zhou, 2001; Zhou et al., 2001).
The aims of this study were: (1) to learn the relationship
between leaf gas exchange and hydraulic conductance
from two levels, i.e. G
t
and the hydraulic conductivity of a
specific subportion of the pathway (K
s
); (2) to investigate
the anatomical fundamentals of K
s
adopted in the present
study; (3) to understand the growth strategies by which the
three Caragana species survive unfavorable environments
in terms of their seasonal variations in photosynthesis,
transpiration, and hydraulic conductance.
MATERIALS AND METHODS
Study site and plant materials
The three study sites were located in the Inner Mon-
golia Plateau of China, which has a temperate continental
climate, with distinct dry (October to April) and rather wet
seasons (July to September). The growing period for most
plants is from late April to early September.
The shrubs of C. microphylla were investigated at the
Inner Mongolia Grassland Ecosystem Research Station
(IMGERS) (43.95XN, 116.07XE, 1,100 m above sea level)
in the central part of a typical steppe in Xilingol Plateau.
The average annual precipitation is 350 mm, and the aver-
age annual temperature is 0.20XC (from 1961 to 2000).
Studies on C. davazamcii were carried out at the Huangfu-
chuan Station of the Inner Mongolia Hydrology Research
Institute (IMHS) in Zhungeer Banner (39.45XN, 111.07
XE, 1,130 m above sea level) in the eastern part of a warm-
temperate steppe in Ordos Plateau. The average annual
precipitation is 369 mm. The average annual temperature
is 6.20XC (from 1953 to 1990), and the soil is of a chest-
nut type. Studies on C. korshinskii were conducted at the
Ordos Sandland Ecological Station (OSES) in Yijinhuole
Banner (39.21XN, 109.49XE, 1,300 m above sea level)
in the central part of a warm-temperate steppe in Ordos
Plateau. The annual precipitation is 360 mm. The average
annual temperature is 6.30XC (from 1960 to 2000), and
the maximum temperature in summer can be as high as 40
XC. The soil is of an aeolian sand type. As shown in Table
1, the climatic factors of the three sites varied to a large
extent during the growing period, and the precipitation of
IMGERS was significantly lower than that of the other two
sites (Table 1).
Table 1. Environmental factors recorded in the three study sites in the growing period of 2005.
Environmental factor
Study site
April May June July August September October
Total precipitation in a month
(mm)
IMGERS
0.6 15.6 25.7 37.3 11.2
18.0
2.7
IMHS
16.4 61.4 48.3 82.6 53.0
43.1
2.6
OSES
4.7 76.3 27.3 73.9 105.3
17.8
8.2
Monthly mean air temperature
(
J
)
IMGERS
5.9 12.4 19.6 22.0 21.1
14.5
6.2
IMHS
11.4 17.4 23.4 24.7 21.7
17.1
8.8
OSES
10.5 16.2 22.6 23.5 20.2
15.7
7.3
Monthly mean atmospheric
relative humidity (%)
IMGERS
30.0 38.0 46.0 61.0 53.0
46.0
42.0
IMHS
30.0 40.0 43.0 53.0 66.0
63.0
51.0
OSES
24.0 38.0 34.0 50.0 64.0
59.0
50.0
pg_0003
LI et al. X Water relations, hydraulic conductance, and vessel features of three
Caragana
species
129
Since most of the root systems were located at a soil
depth of 10-100 cm in the soil (Niu et al., 2003), we col-
lected the soil samples from depths of 30 cm, 60 cm and
100 cm and measured the water contents separately. The
mean values were taken as the final soil water content.
Generally, the study sites at IMGERS and IMHS had
same seasonal tendency, i.e. spring>summer>autumn (Fig-
ure 1). However, the greatest soil water content of OSES
occurred in the summer (4.50%), and the smallest values
were observed in the autumn (1.40%). Of the three sites,
IMHS had the highest soil water content, except in the
summer. The study site at OSES had the highest soil water
content in the summer, but the lowest values in the spring
and autumn.
From each study site, samples were taken of ten healthy
adult shrubs for the investigation of plant water relations.
Four of them were labeled for the examination of hydrau-
lic architecture and xylem anatomy. Some quantitative
characteristics of Caragana plants are shown in Table 2.
Caragana korshinskii shrub covered the smallest area and
had the greatest height among the three species. The shrub
area of C. microphylla was significantly larger than that
of the other two species, and its height was a little greater
than that of C. davazamcii. The highest shrub density was
found in C. davazamcii, and the lowest in C. korshinskii.
All the Caragana shrubs are more than ten years old.
The studies were carried out in May (early spring), July
(mid-summer), and September (early autumn) of 2005,
which covered most of the growing period of the three
Caragana species. In each season, diurnal changes in
transpiration, leaf water potential, leaf water deficit, and
hydraulic architecture in each species were followed for
three or more sunny days. Measurements were made at in-
tervals of 2 h, from 6:00 to 20:00 during those days.
Photosynthetic rate, water relations and soil
water content
Leaf photosynthetic rate (A), leaf transpiration rate (E)
and stomata conductance (g
s
) were recorded with a por-
table photosynthesis system (LI-6400, LI-COR, Lincoln,
Nebraska, USA) operating in an open flow mode. The di-
urnal cumulative values of net photosynthesis (DCA) and
transpiration (DCE) (from 6:00 to 18:00) were calculated
following the two formulae (Ma et al., 2004b): (a) DCA =
U net photosynthetic rate 7200; (b) DCE = U transpira-
tion rate 7200. The water use efficiency (WUE) was ob-
tained following the formula: WUE = DCA / DCE.
Leaf relative water deficit (RWD) was measured fol-
lowing Stockers method (Liu, 1983). Leaf samples (4-6
g) were taken, and their fresh weight (W
f
) was determined.
After soaking them in water for 24 h, the saturated weight
of leaf was measured (W
sat
). Leaf dry weight (W
d
) was de-
termined by oven-drying the sample (60XC for 48 h). The
leaf water deficit was calculated from the formula: RWD =
(W
sat
-W
f
) / (W
sat
-W
d
) 100%.
Leaf water potential, Z
L
, was measured with a water
potential system (Psypro, Wescor, Amer). Predawn Z
L
(6:00) was used to estimate soil water potential (Z
soil
) (Tar-
dieu and Simonneau, 1998; Tausend et al., 2000). The sea-
sonal variations of soil water content were recorded. 30-40
soil samples (30-50 g each) were collected, and the fresh
weight of soil samples was measured. After being dried in
an oven (105XC for 24 h), the dry weight of the soil was
determined, and the soil water content was calculated.
Hydraulic conductance
Four one-year-old twigs were sampled from the
labeled shrubs every 2 h, and were taken to the laboratory
immediately. Segments of 5 cm in length and 3 mm in
diameter were obtained by re-cutting the twigs under
water. Meanwhile the segment length (L) was measured
accurately, and the leaves from the twigs were collected
to measure leaf dry weight. The segments were placed in
a conductivity apparatus (Sperry and Tyree, 1988), which
permitted the measurement of the flow rate of solution
Table 2. Quantitative characteristics of three Caragana shrubs in different habitats.
Species
Density (plant/100 m
2
) Shrub height (cm) Shrub area (m
2
) Age of plant (years)
C. microphylla
19
90.79
b
2.025
a
10-15
C. davazamcii
34
79.70
b
1.193
b
15
C. korshinskii
13
168.6
a
0.967
b
10
Same letter denotes non-significant difference while different letter denotes a significant difference (\=0.05).
Figure 1. Soil water content of the three study sites during dif-
ferent seasons. Means are given SD (n=27).
pg_0004
130
Botanical Studies, Vol. 49, 2008
(w, g min
-1
) in response to the pressure difference (GP,
MPa). In this study, specific hydraulic conductivity, K
s
,
was adopted to describe the hydraulic architecture of one-
year-old twigs: K
s
= w L / (G P A
w
), where A
w
is a cross-
sectional area of wood (Zotz et al., 1997a, 1997b).
Leaf area-specific total hydraulic conductance of the
soil/leaf pathway (G
t
) was determined as:
G
t
= E / GZ, where GZ is the difference between soil
water potential (Z
soil
) and leaf water potential (Z
L
) at a
given time (Borghetti et al., 1998; Sperry, 2000; Tausend
et al., 2000).
Anatomical analysis
The segments (about 3 mm in diameter) of one-year-
old twigs used in the experiments on hydraulic architec-
ture were collected and stored with FAA solution (formol-
acetic-alcohol fixative) until sectioning. Sections 10 gm
thick were obtained with a rotary microtome, and stained
with safranin and fast green FCF. Quantitative data were
analyzed with Microscopic Image Analysis.
Because a small number of large conduits contributes
much more to conductivity than do many small ones, we
employed the statistic D
95
according to the method intro-
duced by Tyree et al. (1994). D
100
is the mean diameter of
all vessels, and D
95
is the mean diameter of vessels respon-
sible for about 95% of the total stem conductance. All ves-
sels in one sample were categorized into small, medium
and large size classes according to the method of Gorsuch
et al. (2001). Vessels in the small size class contributed
<1% to total flow. Medium-sized vessels contributed be-
tween 1 and 2% to total flow while large vessels contrib-
uted >2% to total flow.
Statistical analysis
The comparisons of parameters among species or
seasons were made using a one-way ANOVA followed
by a significant difference test (at P< 0.05). Relationships
between G
t
and leaf gas exchange of three Caragana
species were determined with SPSS software 11.0.
RESULTS
Vessel features and hydraulic architectures of
one-year-old twigs
In general, the D
100
and D
95
o f C. microphylla were
significantly lower than those of the other two species, and
the largest D
100
and D
95
were found in C. korshinskii (Table
3). Though the greatest contribution to water transport
was from the large size vessels of all three species, the
contribution from large vessels of C. microphylla was
much higher than that of the other two species. Vessel
density differed significantly among species (Table 3). The
species with the narrowest vessels, C. microphylla, had
the highest vessel density. C. korshinskii had a little higher
vessel density than C. davazamcii.
As shown in Table 4, all the three species had
the greatest K
s
in the summer, and the lowest values
occurred in the spring. In the summer, C. korshinskii had
significantly greater K
s
than the other two species. In the
spring and autumn, C. davazamcii and C. korshinskii
had similar K
s
values, and these exceeded that of C.
microphylla.
Table 3. Vessel size and density in one-year-old twigs of three Caragana species. Data are means SD. Vessel density of each
species was determined on four segments, and vessel width was determined on 50 vessels per segment.
Species
Vessel element size
Vessel density
(no./mm
2
)
Diameter range
(
g
m)
D
100
(
g
m)
D
95
(
g
m)
Water transport contribution of
three size classes (%)
Large Medium Small
C. microphylla
4.93~49.28 13.50
7.73
b
23.84
7.56
bc
69.05 15.24 15.71
1109.52
C. davazamcii
5.64~60.63 19.09
8.93
ab
26.77
7.33
b
46.97 24.71 28.32
836.17
C. korshinskii
5.66~60.67 21.01
12.08
a
35.66
9.23
a
49.56 19.84 31.60
951.27
Same letter denotes non-significant difference while different letter denotes a significant difference among the three species (\ =
0.05).
Table 4. Diurnal mean values of K
s
of one-year-old twigs for the three Caragana species during the growing period.
C. microphylla
C. davazamcii
C. korshinskii
Spring
0.409
b
1.209
a
1.267
a
Summer
1.168
c
2.078
b
4.129
a
Autumn
0.708
b
1.562
a
1.501
a
Same letter denotes non-significant difference while different letter denotes a significant difference among the three species (\ =
0.05).
pg_0005
LI et al. X Water relations, hydraulic conductance, and vessel features of three
Caragana
species
131
Leaf water status and gas exchange
As shown in Figure 2, there was no difference in
diurnal or seasonal patterns of Z
L
for the three spe-
cies. The highest diurnal Z
L
occurred in the morning
and evening, and the lowest values occurred at midday.
The mean Z
L
of the three species varied with seasons:
spring<summer<autumn. Though the lowest diurnal mean
Z
L
occurred in the spring, the lowest values of Z
L min
(diur-
nal minimum of Z
L
) were always observed in the summer.
The diurnal patterns of RWD were the inverse of those of
Z
L
. All three species had the greatest RWD in the spring,
and the lowest in the autumn. Among the three species, C.
korshinskii had the lowest Z
L
and highest RWD; the high-
est Z
L
and lowest RWD occurred in C. davazamcii.
In the spring and summer, all three species had the
greatest E in the morning (between 8 a.m. and 10 a.m.).
In the autumn, the diurnal greatest E occurred at midday.
Meanwhile, an abnormal high was observed in C. korshin-
skii in the early morning, when the photon flux density
was relatively low. With the exception of C. microphylla,
which had the greatest E in the summer, both C. davazam-
cii and C. korshinskii had the greatest E in the autumn. The
lowest E occurred in the spring for C. micriphylla and C.
davazamcii but in the summer for C. korshinskii. Among
the three species, the sequence of E in the spring was C.
davazamcii > C. korshinskii > C. microphylla. In the sum-
mer and autumn, the sequence was C. microphylla > C.
davazamcii > C. korshinskii (Figure 3). As for g
s
, the diur-
nal and seasonal patterns generally were the same as those
of E among the three species. In the spring, C. davazamcii
had higher g
s
than the other two species, and the lowest
values occurred in C. microphylla. Caragana korshinskii
had the lowest g
s
in the summer and the highest g
s
in the
autumn. The values of C. microphylla were higher than
those of C. davazamcii in the summer, but in the autumn,
the two species had a similar g
s
.
A significant positive dependence of g
s
and E on G
t
was
found in all three species throughout the growing period.
Figure 4A shows the exponential correlations between g
s
and G
t
(r
2
> 0.45, P < 0.005). The optimal models of E on
G
t
were power functions as shown in Figure 4B (r
2
> 0.50,
P < 0.005). The dependence of g
s
and E on G
t
was differ-
ent among the three species. Under the same G
t
, C. micro-
phylla had higher g
s
and E than the other two species. In
contrast with the dependence of leaf gas exchange on G
t
,
little or no dependence of g
s
or E on K
s
of one-year-old
twigs was obtained for each species (r
2
< 0.20, P >0.05).
As shown in Table 5, each Caragana species showed
rather different DCA, DCE, WUE, GZ and G
t
during the
whole growing period. Interspecific differences were also
recorded during the same season.
In C. microphylla, the lowest values of DCA and DCE
occurred in the spring, together with the lowest G
t
. How-
ever, the species had the highest WUE in the spring. In
the summer, though the GZ decreased a little, the G
t
was
much higher than in the spring, and the DCA had roughly
doubled. The DCE had increased even more, directly re-
sulting in a significant decrease in WUE. In the autumn,
C. microphylla had the highest G
t
and lowest GZ during
the growing period, which resulted overall in a decrease in
DCE. Since the DCA was unchanged from the summer, the
WUE increased a little in the autumn. The seasonal varia-
tion of DCE in C. davazamcii was the lowest among the
three species. Though the species had the largest G
t
in the
spring, the lowest GZ resulted in the smallest DCE during
the growing period. With the increase in GZ in the sum-
mer, the DCE was higher than in the spring. However, the
low DCA resulted in decreased WUE in the summer. The
greatest photosynthesis and transpiration occurred in the
autumn, when the species had the greatest WUE. Among
the three species, C. korshinskii had the most significant
seasonal variations in photosynthesis and transpiration:
its lowest DCA and DCE occurred in the summer, and its
highest in the autumn. This was similar to the seasonal pat-
terns of C. davazamcii. The highest WUE of C. korshinskii
occurred in the autumn, when the species had the largest
G
t
and the lowest GZ during the whole growing period.
Comparing all three species, C. microphylla had the
highest DCA and DCE during most of the growing period;
the lowest values were found in C. korshinskii. The GZ of
C. davazamcii was the lowest, but the G
t
was higher than
Figure 2. Diurnal and seasonal changes of leaf water potential
(Z
L
) and relative water deficit (RWD) of three Caragana species.
Figure 3. Diurnal patterns of transpiration rate (E) and stomatal
conductance (g
s
) in different seas ons for the three Caragana
speices. Symbols are as in Figure 2.
pg_0006
132
Botanical Studies, Vol. 49, 2008
in the other two species. The GZ and G
t
of C. korshinskii
were generally lower than those of C. microphylla, but the
G
t
was higher in the spring. As for WUE, the lowest values
of the whole growing period occurred in C. davazamcii. In
the spring, C. microphylla had the highest WUE while C.
korshinskii had it in the autumn.
DISCUSSION
Effect of vessel features on hydraulic
conductance
According to the Hagen-Poiseuille Law, maximum
hydraulic conductivity of xylem (K
h
) is linearly
proportional to the mean hydraulic diameter of the
conduits raised to the fourth power: K
h
.
d
h
4
.
n
c
, where
n
c
is the number of conduits (Tyree et al., 1994; Jordi et
al., 2002). If the xylem area is proportional to d
2
and n
c
,
maximum specific hydraulic conductivity (K
s
) would scale
with the square of mean conduit diameter. In this study, C.
korshinskii had the greatest D
100
and D
95
while the lowest
D
100
and D
95
were obtained in C. microphylla. Thus, the
expected sequence of maximum K
s
of one-year-old twigs
was C. korshinskii > C. davazamcii > C. microphylla,
which was in accordance with the actual observations of
K
s
.
Furthermore, the analysis of three size classes helps us
understand the seasonal variations in K
s
of one-year-old
twigs of three Caragana species. As shown in Figure 5A,
the quantitative proportions of large vessels was no less
than 7.50% for all three species, and most of the vessels
(>85%) were categorized as small. With regards to water
transport contribution, large vessels were responsible for
the majority of water transport Figure 5B. Though large
vessels are highly efficient for water transport, they are
fragile and vulnerable to the embolism caused by water
deficit or low temperatures (Tyree et al., 1994). In contrast,
small vessels can successfully withstand large amounts of
negative tension in the xylem though they transport water
less efficiently. The coexistence of large vessels in small
quantities and small vessels in large quantities embodies a
strong adaptation to drought in three Caragana species.
Relationships of leaf gas exchange and
hydraulic conductance
Numerous studies have demonstrated a positive
correlation of E and g
s
with G
t
(Meinzer and Grantz, 1990;
Meinzer et al., 1995; Saliendra et al., 1995; Sperry, 2000).
When G
t
is experimentally increased by partial defoliation
Figure 5. Three vessel size classes (small, medium and large) in
xylem. (A) Ratio of vessel number of each size class to total ves-
sel number. (B) Contribution of three vessel size classes to total
conductivity in each Caragana species.
Figure 4. Relationship between g
s
and E with G
t
of three Caragana species during the whole growing period. Solid lines are fitted
nonlinear regressions: (A) C. microphylla y=0.0657e
0.1895x
, r
2
=0.62, P=0.000, C. davazamcii y=0.0644e
0.1156x
, r
2
=0.452, P=0.004 and
C. korshinskii y=0.026e
0.2681x
, r
2
=0.650, P=0.000 (B) C. microphylla y=1.305x
0.8364
, r
2
=0.80, P=0.000, C. davazamcii y=1.681 x
0.5253
,
r
2
=0.56, P=0.000, and C. korshinskii y=1.3924x
0.5809
, r
2
=0.72, P=0.000. Symbols are as in Figure 2.
pg_0007
LI et al. X Water relations, hydraulic conductance, and vessel features of three
Caragana
species
133
or shading, g
s
and E of the untreated foliage increase;
when G
t
is decreased by stem notching or root pruning, g
s
decreases accordingly (Meinzer and Grantz, 1990; Pataki
et al., 1998). The results obtained from our experiments
on the three Caragana species during the entire growing
period also demonstrated that both E and g
s
were closely
related to G
t
(Figure 5). Similar positive relationships
between E and g
s
and G
t
in the three species suggest that
leaf stomatal behavior and transpiration were limited by
G
t
over the entire range observed. The regression curves
in this study were non-linear, which is similar to what has
been observed in some other studies (Sperry and Pockman,
1993; Meinzer et al., 1995). The G
t
values of all three
Caragana species varied greatly with seasons and species.
The highest values exceeded 20 mmol H
2
O m
-2
s
-1
MPa
-1
,
greater than those reported in other studies (Meinzer et al.,
1995; Tausend et al., 2000).
The greatest G
t
generally occurred in the early autumn,
when the leaf gas exchange and photosynthesis rate
reached the highest level of the whole growing period,
with the exception of C. davazamcii, which had the
greatest G
t
in the spring. Meanwhile, the three Caragana
species had the best leaf water status in the early autumn,
indicated by relatively high leaf water potential and the
lowest water deficit. Caragana microphylla had the lowest
diurnal mean value of G
t
in the spring, which was in line
with its lowest g
s
, E and A. Caragana korshinskii had
the lowest G
t
in the summer, when the lowest leaf gas
exchange and A occurred. Among the three species, C.
davazamcii had a greater G
t
than the other two species, but
the greatest E occurred in C. microphylla due to its high
GZ and G
t
. Caragana korshinskii had the least G
t
, which
was in accordance with its lower g
s
and E during most of
the growing season.
As for the hydraulic conductance of a specific subpor-
tion of the pathway, its influence on leaf gas exchange may
be different from G
t
. Generally, there were no statistically
remarkable correlations in this study between K
s
in one-
year-old twigs and g
s
or E (r
2
< 0.20, P > 0.05). As for the
seasonal changes of K
s
, apparent variations were observed
for the three Caragana species, which can be interpreted
as the result of embolism and refilling in the vessels (Tyree
and Sperry, 1988). By comparison, C. korshinskii had
greater K
s
than the other two Caragana species, especially
in the summer. In the autumn, because of the cumulating
embolism in the xylem, the decrease in K
s
occurred for all
three Caragana species. In contrast, the highest G
t
usually
occurred in the autumn, when the E and g
s
were the great-
est too.
The difference in the response of leaf gas exchange to
G
t
and K
s
of one-year-old twigs can be interpreted from
the hydraulic resistance located in the different parts of
Table 5. Seas onal changes of photos ynthesis, trans piration and WUE together with GZ and G
t
of Caragana microphylla, C.
davazamcii and C. korshinskii plants.
Parameter
Species
Spring Summer Autumn
Diurnal cumulative value of net Photosynthesis (DCA)
(mmol CO
2
m
-2
)
C. microphylla 446.32
b
888.21
a
881.43
a
C. davazamcii 583.36
b
454.10
b
803.33
a
C. korshinskii 483.53
b
234.71
b
963.54
a
Diurnal cumulative value of transpiration (DCE) (mol H
2
O
m
-2
)
C. microphylla 113.36
b
362.82
a
318.72
a
C. davazamcii 235.15
a
285.43
a
293.98
a
C. korshinskii 190.40
ab
95.53
b
233.92
a
Water use efficiency (WUE) (mmol CO
2
mol
-1
H
2
O)
C. microphylla
4.01
a
2.45
b
2.77
b
C. davazamcii
2.48
a
1.59
b
2.73
a
C. korshinskii
2.53
b
2.46
b
4.12
a
Diurnal mean water potential difference between soil to leaf
(GZ ) (MPa)
C. microphylla
0.97
a
0.92
ab
0.83
b
C. davazamcii
0.66
b
0.80
a
0.75
a
C. korshinskii
0.81
b
1.09
a
0.65
c
Diurnal mean hydraulic conductance of soil-leaf continuum
(G
t
) (mmol H
2
O m
-2
s
-1
MPa
-1
)
C. microphylla
2.73
b
7.95
a
8.11
a
C. davazamcii
12.05
a
9.56
b
9.90
b
C. korshinskii
5.09
b
2.61
c
7.12
a
Different letters denote a significant difference following Duncans multiple test (a=0.05).
pg_0008
134
Botanical Studies, Vol. 49, 2008
the sap flow pathway. Xylem composes most of the wa-
ter transport pathway. Meanwhile, since the transpiration
stream also flows in extra-xylary pathways in root and
leaf, where cavitation does not occur, changes of hydraulic
resistance in root and leaf tissues during water stress can
have important consequences for plants (Sperry, 2000). It
is impossible to tell here whether this hydraulic resistance
was primarily in root, stem, leaf veins, or symplastic por-
tions to the pathway associated with movement from veins
to evaporative sites, which have been identified as sites of
unusually high hydraulic resistance in past studies (Zim-
mermann, 1983; Tyree et al., 1993; Yang and Tyree, 1993).
Overall, stomatal behavior was generally much better
correlated with total resistance than was any specific sub-
portion of the pathway (Comstock and Ehleringer, 1988;
Comstock, 2000).
Specific growth rhythm in relation to hydraulic
conductance in plant
For most plants growing in arid and semi-arid areas,
water and heat are the most important environmental
factors. The former determines their distribution and
survival, and the latter determines the length of the
growing period. Though C. microphylla, C. davazamcii,
and C. korshinskii share a close interspecific relationship,
it is hard to find two Caragana species in one habitat due
to their geographical replacement in the Inner Mongolia
Plateau of China. During its long-term adaptation to
the specific water and heat conditions of this habitat, a
Caragana species would have had to develop a particular
growth rhythm.
Growing mainly in the east of the Inner Mongolia
Plateau, C. microphyllas growing period was about a
month shorter than that of the other two species found
in the west. Its leaf elongation begins in late May, two
weeks later than the other two species, and apparent
defoliation occurs usually in mid September. Its shorter
growing period required C. microphylla to develop a rapid
growth strategy, which was demonstrated by the great
photosynthesis and transpiration rate observed in the field.
To meet the water demands of leaf growth, highly efficient
water transport was also necessary. Except for the lower
values at the beginning of its growing period due to the
severe embolism in its xylem, its G
t
value was high at all
times. Though the soil water content was much higher in
the spring, the transpiration rate in leaves was still lower
because of the higher hydraulic resistance from soil to
leaves. However, DCA and WUE also demonstrated its
high adaptability to leaf desiccation, which originated
from low hydraulic efficiency in the spring. In the summer
and autumn, C. microphylla had a higher DCA and DCE,
accompanied by its higher G
t
.
Among the three species, C. davazamcii lived in the
most favorable environmental conditions, with a good
amount of rainfall and relatively suitable air temperatures
and humidity. The growing period started in early May
and finished by the end of September. Though its DCA and
DCE were not as high as those in C. microphylla in the
summer and autumn, its seasonal mean values during the
growing period were greater. On account of its relatively
steady environmental conditions, the seasonal variations
in DCE and DCA were not as dramatic as in the other
two species. Reflecting its need to supply a large quantity
of water to leaves in the spring during active growth, C.
davazamcii had the greatest G
t
to complement the lower
differences in water potential from the soil to the leaves.
The highest DCE and DCA occurred in mid September,
when the leaves started to defoliate. G
t
also increased
accordingly to meet the increased water demand from the
leaves. Generally, C. davazamcii had the smallest WUE,
and perhaps the least drought resistance among the three
species, which is in line with the better water conditions in
its habitat.
The most particular species is C. korshinskii, which
grew in the habitat with soil drought and atmospheric
drought imposed upon it asynchronously. Since its study
site was only about 120 kilometers away from that of C.
davazamcii, there was not much difference in rainfall.
However, the air temperature of the OSES was higher than
that in IMHS, which could have been the main cause of
low DCA and DCE of C. korshinskii during most of the
growing season. Consistent with its lower metabolic rate
in the leaves, C. korshinskii had the least efficient water
transport system, i.e. lower G
t
than the other two species.
The species entered the growing period in early May,
which was about two weeks earlier than C. microphylla in
IMGERS. By late May, the species already had higher G
t
due to the refilling of embolized vessels, but the low soil
water content limited its rapid growth. In the summer, G
t
decreased greatly, which might have acted as a hydraulic
signal and resulted in the dramatic decreases in g
s
and
E. According to observations by Wang et al. (1996),
the photosynthesis of C. korshinskii is very sensitive to
high air temperatures. So in the summer, to avoid the
dysfunction of photosynthetic organ and unnecessary
water loss, it closes most of its stomata. The decreased
E in leaves also saved some soil water under shrubs.
When the air temperatures went lower in late summer, C.
korshinskii recovered its rapid growth, which resulted in
continuous water loss till September, when the soil water
content was as low as 1.40%. Due to having the highest
G
t
in the autumn, the species still had the greatest E and
A. Furthermore, the calculated WUE was as great as
4.12 mmol CO
2
.
mol
-1
.
H
2
O, which demonstrated the high
resistance of C. korshinskii to soil drought in the autumn.
Conclusions
The three Caragana species growing in arid or semi-
arid habitats displayed different hydraulic conductance,
vessel features, leaf water status, gas exchange, and
growth rhythm during an entire growing season in 2005.
Overall, C. korshinskii had the largest vessels and K
s
in
one-year-old twigs, and C. microphylla had the lowest
values, but there were no significant relationships between
pg_0009
LI et al. X Water relations, hydraulic conductance, and vessel features of three
Caragana
species
135
leaf gas exchange and K
s
. Additionally, the seasonal
changes of photosynthesis and transpiration were not in
line with K
s
. In contrast, the leaf gas exchange (E and g
s
)
was positively correlated to total hydraulic conductance
of the soil/leaf pathway (G
t
) for the three Caragana
species, and the seasonal patterns of G
t
were generally in
accordance with those of DCA and DCE. That is, the leaf
stomatal behavior of one-year-old twigs was governed by
G
t
rather than K
s
. Among the three species, C. davazamcii
had the best leaf water status and highest G
t
, and the least
drought resistance during the growing period. Caragana
microphylla was exposed to the severest leaf water deficit
in the spring, which resulted from its lowest G
t
. Though
the DCA and DCE were lower than those of the other two
species, the WUE was the highest. Caragana korshinskii
had the lowest DCA, DCE and G
t
among the three species.
In the autumn, C. korshinskii had the greatest drought
resistance, indicted by a WU E higher than the other two
species.
Acknowledgements. This work was financially sup-
ported by National Basic Research Program of China
(2007CB106802).
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pg_0012