Journal of
Plant Ecology
Volume 6, Number 5,
Pages 418–427
october 2013
doi:10.1093/jpe/rtt010

available online at
www.jpe.oxfordjournals.org
Grazing alters warming effects on
leaf photosynthesis and respiration
in Gentiana straminea, an alpine
forb species
Haihua Shen1,2,*, Shiping Wang3 and Yanhong Tang1
1
Center for Environmental Biology and Ecosystem Studies, National Institute for Environmental Studies, Onogawa 16-2,
Tsukuba, Ibaraki 305-8506, Japan
2
State Key Laboratory of Vegetation and Environmental Change, Institute of Botany, Chinese Academy of Sciences, Beijing
100093, China
3
Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100085, China
*Correspondence address: State Key Laboratory of Vegetation and Environmental Change, Institute of Botany,
Chinese Academy of Sciences, Beijing 100093, China. Tel: +86-10-62836896; E-mail: [email protected]
Abstract
Aims
Vast grasslands on the Tibetan Plateau are almost all under livestock
grazing. It is unclear, however, what is the role that the grazing will
play in carbon cycle of the grassland under future climate warming. We found in our previous study that experimental warming can
shift the optimum temperature of saturated photosynthetic rate into
higher temperature in alpine plants. In this study, we proposed and
tested the hypothesis that livestock grazing would alter the warming
effect on photosynthetic and respiration through changing physical
environments of grassland plants.
Methods
Experimental warming was carried by using an infrared heating system to increase the air temperature by 1.2 and 1.7°C during the
day and night, respectively. The warming and ambient temperature
treatments were crossed over to the two grazing treatments, grazing
and un-grazed treatments, respectively. To assess the effects of grazing and warming, we examined photosynthesis, dark respiration,
maximum rates of the photosynthetic electron transport (Jmax), RuBP
carboxylation (Vcmax) and temperature sensitivity of respiration Q10
in Gentiana straminea, an alpine species widely distributed on the
Tibetan grassland. Leaf morphological and chemical properties
were also examined to understand the physiological responses.
Important findings
1) Light-saturated photosynthetic rate (Amax) of G. straminea showed
similar temperature optimum at around 16°C in plants from all
experimental conditions. Experimental warming increased Amax at
all measuring temperatures from 10 to 25°C, but the positive effect
of the warming occurred only in plants grown under the un-grazed
conditions. Under the same measuring temperature, Amax was
significantly higher in plants from the grazed than the un-grazed
condition.
2) There was significant crossing effect of warming and grazing on the temperature sensitivity (Q10) of leaf dark respiration.
Under the un-grazed condition, plants from the warming treatment
showed lower respiration rate but similar Q10 in comparison with
plants from the ambient temperature treatment. However, under
the grazed condition Q10 was significantly lower in plants from the
warming than the ambient treatment.
3) The results indicate that livestock grazing can alter the warming effects on leaf photosynthesis and temperature sensitivity of
leaf dark respiration through changing physical environment of the
grassland plants. The study suggests for the first time that grazing
effects should be taken into account in predicting global warming
effects on photosynthesis and respiration of plants in those grasslands with livestock grazing.
Keywords: alpine plant, grazing, photosynthesis, respiration,
temperature acclimation
Received: 24 June 2012 Revised: 21 January 2013 Accepted: 26
January 2013
© The Author 2013. Published by Oxford University Press on behalf of the Institute of Botany, Chinese Academy of Sciences and the Botanical Society of China.
All rights reserved. For permissions, please email: [email protected]
Shen et al. | Grazing alters warming effects on understory forb419
Introduction
Plant photosynthesis and respiration are the dominant biological processes regulating biosphere–atmosphere carbon
exchange (Ryan 1991; Valentini et al. 2000; Silim et al. 2010).
Increasing temperatures as a result of global warming can
have profound impacts on these two processes, which in turn
affect CO2 concentrations in the atmosphere and hence global
warming (Dewar et al. 1999; Gunderson et al. 2000; Atkin
and Tjoelker 2003). Understanding the two processes in relation to temperature is needed in order to predict the carbon
budget of ecosystems in response to future global warming.
Plant photosynthesis and respiration rates are strongly
temperature dependent. As instantaneous response, the
photosynthetic rate generally rises with temperature
increases up to an optimum temperature and then decreases
with further increases in temperature. As acclimation, this
relationship between instantaneous photosynthetic rate and
measuring temperature varies with growth temperature of
plants; for example, the thermal optimum of photosynthesis is typically adjusted to approximate growth temperature
(e.g. Pearcy 1977; Berry and Björkman 1980; Hikosaka et al.
2006). Similarly, as instantaneous response, the respiration
rate (R) increases exponentially with temperature, and the
relationship also varies with growth temperature; R usually
decreases with the increase of growth temperature under
the same measuring temperature (Atkin and Tjoelker 2003;
Atkin et al. 2005; Tjoelker et al. 2009). Since the magnitude
of plant responses to temperature differs between photosynthesis and respiration, the carbon balance and plant
growth can change under warming conditions (Morison and
Morecroft 2006; Bruhn et al. 2007; Way and Sage 2008; Shen
et al. 2009). Consequently, the temperature dependencies of
these two processes become a primary determinant of how
plants will respond to global warming (Silim et al. 2010).
The temperature dependence of photosynthesis and respiration also varies with growth environments other than temperature, such as radiation and water availability (Tranquillini
et al. 1986; Robakowski et al. 2003; Bauerle et al. 2007; Alonso
et al. 2009; Yamori et al. 2010). For example, growth light
can affect temperature dependency of photosynthetic rate
because light regime changes nitrogen allocation between
photosynthetic components (Yamori et al. 2010). In grasslands, the growth environments including radiation regimes
can be often modified by livestock grazing (Jones 2000; Shen
et al. 2009; Jeddi and Chaieb 2010); for example, reduced vegetation coverage by grazing increased irradiance for some forb
plants within grasslands (Dahlgren and Driscoll 1994; Wan
et al. 2002; Klein et al. 2005). It is, therefore, reasonable to
expect that temperature response and/or acclimation of photosynthesis and respiration can differ under grazing compared
with un-grazed condition. However, to our knowledge, little
information is available for addressing how livestock grazing affects temperature response and acclimation of the two
physiological processes in alpine grasslands, though many
observations have been reported focusing on temperature
response and acclimation of photosynthesis and respiration
under various other environments (e.g. Larigauderie and
Körner 1995; Atkin et al. 2000; Atkin et al. 2006; Shen et al.
2009).
Tibetan Plateau, extending over 2.5 million km2, holds the
largest alpine grassland in the world. The grassland here, like
other Eurasian grasslands, is almost all under livestock grazing
(Cao et al. 2004). Because of the vast area and high sensitivity
of the alpine grassland in response to climate change, clarifying the effect of livestock grazing on plant carbon budget will
be needed for further assessment on the uncertainty of terrestrial carbon cycle under global warming conditions. We have
previously examined the warming effect on photosynthesis
and respiration of Gentiana straminea, a typical understory forb
in most meadows on the Tibetan Plateau, through an open
top chamber experiment and found that the experimental
warming shifted the optimum temperature of photosynthesis (Shen et al. 2009). In the current study, we hypothesized
that livestock grazing may change temperature response and/
or acclimation of photosynthesis and respiration of grassland
plants through changing physical environments. We tested
the hypothesis by examining the combined effects of experimental warming and grazing on photosynthesis and respiration of G. straminea. We also examined leaf morphology and
biochemical components to explain the effects of experimental treatments on the two physiological responses.
MATERIALS AND METHODS
Study site and plant materials
The field site was an alpine meadow (37°37’N, 101°12’E,
3250 m elevation) located at the northeastern edge of Tibetan
Plateau in a large valley surrounded by the Qilian Mountains.
The mean annual temperature for the 25 years from 1976 to
2001 was −1.7°C, and mean annual precipitation was 560 mm,
over 80% of which falls during the summer monsoon season
(Wang et al. 1999). The soil of the study site is classified as
a Mat Cry-gelic Cambisol (Chinese Soil Taxonomy Research
Group 1995).
The controlled warming–grazing experiment by infrared
heating system (FATE) was started from May 2006. The set
point of canopy temperature difference between heated
and corresponding reference plots was 1.2°C during the
day and 1.7°C at night (Luo et al. 2010). To investigate the
independent and/or combined effect of warming and grazing,
a two-factorial design (warming and grazing) was used, with
four replicates for each of four treatments: ambient with
un-grazed (A/U), ambient with grazing (A/G), warming
with un-grazed (W/U) and warming with grazing (W/G). In
total, 16 plots with 3-m diameter were used in a complete
randomized block distribution in the field. Livestock grazing
regimes were achieved by fencing one adult Tibetan sheep
in the plot for ~2 h in 15 August 2006, and by fencing two
adult Tibetan sheep for ~1 h in every month from July to
420
Journal of Plant Ecology
September since 2007. More details about the experimental
design are described by Luo et al. (2010).
Gentiana straminea is one of the most common forb species
of meadows of the Tibetan Plateau and the Himalayan region.
This species is a non-palatable plant that the livestock—sheep,
yak and horses—tend to avoid. The linear-shaped leaves of
this species grow diagonally from the soil surface to the top
of the canopy and are 20–30 cm long when mature, which
cannot reach the top of canopy in un-grazed treatment (Shen
et al. 2009).
Assessment of the micro-environment
To examine the effect of warming and grazing on micro-environments, in each treatment regime, we installed and fixed
one quantum sensor (GaAsP Photodiode G1118; Hamamatsu
City, Japan) on a horizontal plane at the average height of
the G. straminea leaves. All quantum sensors were calibrated
in sunlight and in artificial shade against a standard quantum
sensor (Li-Cor Model 190S; Li-Cor, Lincoln, NE) before the
test measurements (Shen et al. 2009). We recorded photosynthetic photon flux density (PPFD) and leaf temperature at
1-min intervals from 3 August to 8 August 2009 using a datalogger (Thermic 2300A; EtoDenki Ltd., Tokyo, Japan). We
measured leaf temperature by attaching two copper-constantan thermocouples firmly on the adaxial side of the leaf with
ventilated bandage that is highly permeable to CO2 and H2O.
Leaf morphology and biochemistry
Leaf morphology and biochemical components provide basic
information for explaining any change in photosynthesis
and respiration (Björkman 1981; Boardman 1997). Many
leaf traits are often important indicators for assessing grazing
effect (e.g. Diaz et al. 2001; Diaz et al. 2007). In July 2008, to
examine the change in leaf size under different treatments
without destruction of leaves, we first scanned 165 leaves
from plants growing outside the experimental site to a computer and estimated the individual leaf size (i.e. the area of
an individual leaf) by using ImageJ software (ImageJ 1.42q,
National Institutes of Health, Washington, DC). The length
and width of these sample leaves were measured with a digital caliper. From the measurements, we obtained the following statistical model:
Leaf size = 0.69 × leaf length × leaf width
(R
2
= 0.97; P < 0.001, n = 165)
To obtain individual leaf size and leaf mass per unit area
(LMA, gm−2) controlling for changes due to leaf age, we sampled 12 individual uppermost fully expanded leaves for each
of the four treatment regimes. These leaves are randomly
determined from different ramets in different plots of each
treatment. We then estimated the leaf size from the above
regression model and measured the dry weight (oven dried
at 80°C for 24 h) for each individual leaves. LMA was then
obtained for each leaf.
To measure the content of chlorophyll a and b and carotenoid concentrations, the leaves were cut into small pieces after
we had taken the photosynthetic measurements and were
then placed in separate bottles containing 10 ml of ethanol,
acetone and water (4.5:4.5:1 [v/v/v]). The resulting solution
was stabilized in tightly closed bottles that were kept in a refrigerator (about 5°C) in the dark for nearly 3 weeks. Absorbance
of the chlorophyll and carotenoid solution was analyzed at
wavelengths of 663, 645 and 440 nm by using a UV-Vis spectrophotometer (UV-1601; Shimadzu, Tokyo, Japan). The chlorophyll a and b and carotenoid concentrations were calculated
according to methods provided by Zhu (1990).
Photosynthetic response to PPFD, temperature
and intercellular CO2 concentration
Leaf gas exchange was measured on fully expanded intact
leaves in the field with an LI-6400 portable photosynthesis
measurement system (Li-Cor, Inc.) in July 2008. Four plants
were randomly decided in each treatment regime for measurements under PPFD conditions of 0, 25, 50, 100, 250, 500,
1000 and 1500 µmol m−2 s−1, each combined with four temperatures (10, 15, 20, and 25°C) in the sample chamber. During
these measurements, we maintained the CO2 concentration
at 370 µmol mol−1. The relative humidity was controlled to
between 60 and 70% by using a water-bubbling system or
desiccant to adjust the inflowing air (Shen et al. 2009). We
fitted the maximum photosynthetic rate (Amax) to the measured temperature by two-dimensional polynomial curves and
estimated the optimum temperature of Amax and relevant Amax
from these two-dimensional polynomial functions.
To obtain the maximum rates of photosynthetic electron
transport (Jmax) and RuBP carboxylation (Vcmax), following
the method by Yamori et al. (2005), we measured the CO2
dependence of photosynthesis under CO2 concentrations of
0, 50, 100, 150, 370 and 1000 µL L−1 in the ambient air (Ca).
Light intensity was set to 1000 µmol m−2 s−1 at three temperatures (15, 20 and 25°C). At each CO2 concentration, net
photosynthetic rate (A) and intercellular CO2 concentration
were calculated.
We calculated Jmax and Vcmax from the following equations
(Farquhar et al. 1980):


O 
Vc max = (IS )×Γ * +K c 1 + 


K o 

J max =
( A1000 + Rd )×(4C i + 8Γ *)
Ci − Γ *
where, A1000 is the net photosynthetic rate at the highest Ca
of 1000 µL L−1 CO2. IS is the initial slope of the A versus Ci
curve obtained with the data measured at Ca of 0, 50, 100 and
150 µL L−1. The Other details are referred elsewhere (Yamori
et al. 2005; Shen et al. 2009).
Shen et al. | Grazing alters warming effects on understory forb421
Response of respiration rate to temperature
The short-term temperature sensitivity of the respiration rate
is described by Q10:


10


 R (T −T0 )

Q10 =  T 
,
 RT 
0
where, RT0 and RT are values of leaf dark respiration (R) measured on a reference leaf temperature (T, T0), respectively, and
a second leaf T, where R is induced by rapid warming.
Data analysis
We used analysis of variance (ANOVA) of repeated-measures
and least significant different (LSD) multiple comparison
to identify treatment effect in environmental parameters,
leaf morphology, biochemical and physiological parameters
between plants from different treatments using SPSS Version
11.5 (SPSS Inc., Chicago, IL, USA). Because the photosynthetic rate within grazed and un-grazed conditions varied,
we compared the relative responses by converting the results
into the percentage of change between warming and ambient
regimes—for example, percent of warming-induced change in
Amax (the relative warming effect on Amax, RWEAmax = (Amax.w−
Amax.a)/Amax.a) —and then compared these percentages to
assess the effect of grazing on the changes with the photosynthetic rate in response to temperature. P-value above 0.05
was judged to be statistically significant in this study.
RESULTS
Growth irradiance and leaf temperature
PPFD measured on a horizontal plane at the average height
of the G. straminea leaves was significantly higher under the
grazed condition than in the un-grazed condition (Fig. 1a),
which was about eight times higher on a clear day and
about three times higher on a cloudy day (data not shown).
Approximately 90% of the PPFD data from the un-grazed
condition were <250 µmol m−2 s−1, and no readings above
750 µmol m−2 s−1 were obtained (Fig. 1b).
Leaf temperatures measured by either daily mean (24-h
mean), or daytime mean (from 8:00AM to 20:00PM, local
time), or nighttime mean were all significantly higher under
the warming treatments than in the ambient temperature treatments within grazed and/or un-grazed treatments
(Fig. 1c). However, the difference of the daytime and nighttime mean leaf temperatures between the two temperature
treatments was significantly smaller under the grazed conditions than the un-grazed conditions (Fig. 1d).
Leaf morphology and biochemistry
The experimental grazing significantly reduced individual leaf
size and total leaf area per ramet (Tables 1 and 2). However,
the grazing effect on leaf size and total leaf area did not depend
on the temperature treatment (Table 2).
The experimental warming significantly decreased the
total chlorophyll content (Chl a+b), the ratio of chlorophyll a
and b (Chl a/b) and carotenoid content, while grazing tended
to slightly increase all the above biochemical components
(Tables 1 and 2). However, the interaction effect of warming
and grazing was not statistically significant (Table 2).
Response of light-saturated photosynthesis and
respiration
Plants from all the four experimental treatments showed
an increase in light-saturated photosynthesis (Amax) with
an increase in measuring temperature from 10 to 15°C and
then a decrease in Amax under higher measuring temperatures (Fig. 2a). This decrease in Amax with higher temperatures was more notable in plants from the grazed conditions
(Fig. 2a).
Amax at the optimum temperature was significantly higher
in plants from the grazed than from the un-grazed conditions.
In ambient temperature treatments, for example, the Amax
was about 22% higher in plants from the grazed than the ungrazed treatment (Fig. 2a; Tables 1 and 2).
The relative warming effect (RWEAmax) on Amax was significantly higher in the un-grazed conditions than in the grazed
conditions (Fig. 2b). In the un-grazed conditions, Amax was
consistently about 9% higher in plants from the warming
plots than in those from the ambient temperature plots when
measuring temperatures were 10, 15 and 20°C (Fig. 2b).
However, under the grazed conditions, Amax was almost
similar between plants from the two different temperature
treatments.
The dark respiration rates (R) increased exponentially
with increasing temperature in all treatments (Fig. 3). In
plants from the un-grazed conditions, R tended to be about
0.15 µmol m−2 s−1 constantly smaller at all measuring temperatures in plants from the warming than the ambient
temperature treatment, which resulted in similar Q10 of R
for leaves from the two treatments in the un-grazed conditions. However, in plants from the grazed conditions,
the Q10 was significantly higher in plants in ambient temperature plots than under the experimental warming plots
(Tables 1 and 2).
Temperature response of RuBP regeneration and
RuBP carboxylation
To understand the underlying biochemical mechanisms
involved in the response of photosynthesis to temperature,
we compared the maximum rate of electron transport (Jmax)
and the maximum rate of carboxylation (Vcmax) between
the four experimental treatments (Fig. 4). Jmax, which was
derived from CO2 response curves at different measuring
temperatures, was consistently higher at all the measurement
temperatures under the ambient than the warming regimes
in un-grazed treatment, but this difference was not observed
plants from grazed treatment (Figs 4a and b). The maximum
rate of carboxylation (Vcmax) showed higher values in plants
422
Journal of Plant Ecology
Figure 1: physical environmental variation under different experimental conditions. (a) Daily total photosynthetic photon flux density (PPFD)
under grazed and un-grazed conditions. (b) Frequency distribution of PPFD under grazed and un-grazed conditions. (c): Leaf temperatures
measured on two leaves of Gentiana straminea for each of four experimental treatments. (d) Increases in leaf temperature under un-grazed
(W/U–A/U) and grazed (W/G–A/G) conditions. Different lower-case letters (a, b and c) indicate statistically significant difference at P < 0.05
from least significant difference multiple comparison.
from the ambient compared with warming treatment under
both grazing conditions. The difference was statistically
significant under measurement temperatures of 15 and 20°C
in plants from the un-grazed treatment but those of 20–25°C
from the grazed treatment (Figs 4c and d).
Discussion
Photosynthetic response and acclimation to
temperature changes
The instantaneous response of Amax to measuring temperature
in G. straminea followed a similar pattern with our previous
report: Amax increased with the increase of temperature below
around 16°C and then decreased with temperature increase
(Shen et al. 2009). The optimum temperature of Amax is very
close to the average daily temperature of the alpine meadow
observed during the growing season by Kato et al. (2004).
However, the experimental warming effects on photosynthesis, that is, the acclimation of photosynthesis to growth
temperature, differed with the observation in our previous
study (Shen et al. 2009). The optimum temperature of Amax
was similar in plants grown under the two temperature
conditions in this study but shifted into higher temperature
in plants grown from the high temperature in the previous observation. This discrepancy may be due to the small
temperature difference in the current study. Temperature
elevation was greater in the OTC experiment than in the
current warming experiment. In general, an increase of 1ºC
in growth temperature can lead to a shift of 0.10–0.59ºC in
the optimum temperature in different plants (see review
by Hikosaka et al. 2006). This means that a shift of 1°C in
the thermal optimal of Amax needs a change in the temperature environment of 5–10°C. In our previous study, OTCs
increased the mean daytime leaf temperature by 3.3°C,
whereas the FATE system used in the current study only
increased the mean daytime leaf temperature by about 1.6°C
(Fig. 1).
Many other studies suggest that there is a high variability
in the degree of photosynthetic acclimation to experimental
warming among species, with some species exhibiting full
acclimation and others incapable of even partial acclimation
(e.g. Pearcy 1977; Berry and Björkman 1980; Larigauderie
and Körner 1995; Xiong et al. 2000; Hikosaka et al. 2006;
Dillaway and Kruger 2010). Our observation from this and
the previous studies suggests that within the same species,
the temperature optimum of Amax varies with the changed
magnitudes of growth temperature. A significant change
of the optimum may require a constant change around
3°C in this alpine species, G. straminea. The lower thermal
acclimation is consistent with some alpine plants and
Shen et al. | Grazing alters warming effects on understory forb423
Table 1: the leaf morphology, chlorophyll content, photosynthetic and respiration parameters of Gentiana straminea growing under
different environmental conditions
A/U
W/U
A/G
W/G
Leaf morphology
Individual leaf size (cm2)
14.2 ± 0.7b
2
13.7 ± 1.1b
b
12.9 ± 1.1ab
ab
10.9 ± 0.5a
61.3 ± 4.9
a
55.0 ± 5.9
53.6 ± 3.7a
83.3 ± 3.2a
Total leaf area per ramet (cm )
69.5 ± 3.6
LMA (g/m2)
76.6 ± 2.9ab
77.4 ± 3.1ab
71.3 ± 5.1b
1.40 ± 0.10ab
1.19 ± 0.05a
1.52 ± 0.14b
1.26 ± 0.08ab
a
b
3.67 ± 0.06ab
b
Leaf biochemistry
Chlorophyll (a+b) content (mg/FWg)
b
The ratio of chlorophyll a and b
3.71 ± 0.02
3.55 ± 0.05
b
Carotenoid content (mg/FWg)
3.74 ± 0.04
a
0.34 ± 0.02
0.28 ± 0.01
0.37 ± 0.03
0.31 ± 0.01ab
15.8 ± 0.2a
15.9 ± 0.2a
16.1 ± 0.2a
16.1 ± 0.3a
b
Leaf physiology
Optimal temperature for Amax (°C)
−2 −1
15.1 ± 0.6
16.8 ± 1.0
16.9 ± 0.5b
11.4 ± 0.7a
14.3 ± 0.4ab
16.1 ± 0.8bc
17.0 ± 1.1c
1.76 ± 0.07a
1.91 ± 0.07a
2.41 ± 0.10b
1.98 ± 0.10a
Amax at the optimal temperature (µmol m s )
13.6 ± 0.2
Amax/R at the optimal temperature
Q10 of dark respiration
a
ab
LMA = Leaf mass per unit area; A/U = ambient temperature with un-grazed regime; W/U = warming treatment with un-grazed regime;
A/G = ambient temperature with grazed regime, W/G = warming treatment with grazed regime.
Data represent means ± standard error.
Different lower case letters (a, b and c) indicate statistically significant differences between experimental treatments (Least significant difference
multiple comparison, P < 0.05).
Table 2: two-way analysis of variance (ANOVA) for estimated effects of warming and grazing on the morphological, biochemical and
physiological parameters of Gentiana straminea
F and (P) value of two-way ANOVA
Warming
Grazing
Warming × Grazing
Leaf morphology
Individual leaf size (cm2)
2.34 (ns)
2
5.92 (0.016)
0.79 (ns)
Total leaf area per ramet (cm )
1.10 (ns)
5.74 (0.023)
0.54 (ns)
Leaf mass per unit area (LMA, g/m2)
3.16 (ns)
0.01 (ns)
2.40 (ns)
Chlorophyll (a+b) content (mg/FWg)
5.88 (0.032)
1.00 (ns)
0.06 (ns)
The ratio of chlorophyll a and b
6.74 (0.023)
2.90 (ns)
1.06 (ns)
Carotenoid content (mg/FWg)
12.16 (0.005)
3.25 (ns)
0.03 (ns)
Leaf biochemistry
Leaf physiology
Optimal temperature for Amax (°C)
0.55 (ns)
Amax at the optimal temperature (µmol m−2 s−1)
0.93 (ns)
12.06 (0.005)
Amax/R at the optimal temperature
3.74 (0.077)
16.34 (0.002)
Q10 of dark respiration
2.55 (ns)
17.02 (0.001)
cold-insensitive species. Alpine plants have lower thermal
acclimation compared with lowland plants (Atkin et al. 2006),
and cold-acclimated crop species showed higher temperature
homeostasis of photosynthesis than cold-sensitive species
(Campbell et al. 2007; Yamori et al. 2009). Our study suggests
that modelers may be able to predict temperature optimum
of photosynthesis of alpine plants on the plateau without
considering the effects of growth temperature if the average
temperature changes within 1–2°C.
0.00 (ns)
0.15 (ns)
0.59 (ns)
0.55 (ns)
10.87 (0.006)
Livestock grazing effects on the photosynthesis
and its temperature response
Our result indicates however that grazing significantly
affected Amax at the optimal temperature and tended to reduce
the relative effect of warming on the Amax (Tables 1, 2). Since
G. straminea is a non-palatable forb and we measured fullsized healthy leaves, the differences in the photosynthetic
capacity in plants between the grazed and un-grazed plots are
not likely to be the direct effect of grazing. There are several
424
Journal of Plant Ecology
indirect environmental factors that might have affected Amax.
The most decisive effect of grazing was perhaps the change in
growth irradiance. Leaves of G. straminea in the grazed plot
received about three to eight times the PPFD received in the
un-grazed plot (Figs 1a, b), which can also lead to a high photosynthetic rate, and a high Vcmax, but not necessarily Jmax. This
is also supported by the results that plants from the grazed
treatment tended to have ‘sun leaves’ with smaller leaf size
and higher chlorophyll content compared with leaves from
the un-grazed condition (Tables 1 and 2). Nitrogen availability
could be another factor because sheep excrement and urine
may increase soil N in the grazed plots. The increase of soil
N can affect the photosynthetic performance, which may be
responsible for the higher chlorophyll content in the grazed
plots in the study.
Livestock grazing alter the temperature response
of respiration
Figure 2: (a) Light-saturated photosynthetic rate (Amax) as a function
of measuring temperature in Gentiana straminea having grown under
ambient temperature with un-grazed treatment (A/U,○); under
warming with un-grazed treatments (W/U, ● ); under ambient temperature with grazed treatment (A/G, △); and under warming with
grazed treatment (W/G, ▲). Values represent mean ± standard error
of four samples. The curves were fitted by two-dimensional polynomial function between Amax and measuring temperature. (b) Relative
warming effect of Amax (RWEAmax) in plants with un-grazed (● ) grazed
(▲) treatments. RWEAmax is the ratio of (Amax.w−Amax.a) /Amax.a, where
Amax.w is the averaged Amax of 4 plants in the warming treatment and
Amax.a is the averaged Amax of 4 plants in the ambient temperature
treatment. The least significant difference comparison indicated a significant difference between the grazed and the un-grazed treatment
(P < 0.05).
Grazing altered temperature acclimation of dark respiration
in leaves of G. straminea (Fig. 3; Table 1). Atkin and
Tjoekler (2003) tried to distinguish two types of respiratory
acclimation to temperature, with type I acclimation changing
predominately in Q10 values, but nothing under low test
temperatures and type II acclimation having respiration rate
changes under all test temperatures (Atkin and Tjoelker
2003; Shen et al. 2009). In the current study, the temperature
acclimation of respiration in the un-grazed regime appeared
to be type II; that is, R was lower under warming conditions
than under ambient conditions at all test temperatures
though there was a large difference between individual
plants, whereas Q10 was not significantly different between
the ambient and warming treatments (Fig. 3a; Table 1).
However, in grazed plots, Q10 was significantly lower under
warming conditions than under ambient conditions, and
thus temperature acclimation appears to have been type
I. Although the mechanisms for the two types have not yet
been clarified, a number of studies have shown that Q10 values
Figure 3: dark respiration rate (R) as a function of measurement temperature in Gentiana straminea under warming or ambient conditions with
un-grazed treatments (a) and warming or ambient conditions with grazed treatments (b). Mean ± standard error for four leaves of different
ramets. Abbreviations and symbols are the same as in Fig. 2a.
Shen et al. | Grazing alters warming effects on understory forb425
Figure 4: effect of temperature on the maximum rate of electron transport (Jmax, a, b) and the maximum rate of carboxylation (Vcmax, c, d)
from un-grazed regimes (a, c) and grazed regimes (b, d). Data represent means ± standard error, n = 4. Asterisks indicate statistically significant
differences of contrasting regimes for each measured temperature (Least significant difference comparison, *P < 0.05).
are highest at high concentrations of respiratory substrates
(e.g. Covey-Crump et al. 2002), whereas a similar response was
observed in mitochondria isolated from soybean cotyledons
(Atkin et al. 2002). The high Q10 values found in the grazed
plots in our study can be explained by the fact that leaves
receiving high levels of light have increased concentrations of
mitochondrial protein per unit leaf area and mass (Noguchi
et al. 2005), as well as increased substrate availability (AzcónBieto and Osmond 1983), and/or increased ATP turnover
(Lambers 1985; Noguchi et al. 1996). However, some other
studies have also shown that the Q10 values of leaf respiration
are not dependent on growth irradiance and concurrent rates
of photosynthesis (Bolstad et al. 1999; Hartley et al. 2006;
Zaragoza-Castells et al. 2007).
Conclusion and implication of the study
In conclusion, this study indicates that livestock grazing can
alter the effect of temperature elevation on leaf morphology
and physiology of understory plants through changing light
and other physical environments of these plants, implying the
important consequences of change in plant community structure under climate change.
Despite of extensive livestock grazing all over the Eurasian
grasslands, we have little information to assess the effects of
livestock grazing on the response of plant physiology to global
warming in these grasslands. Our results first demonstrated
the importance of livestock grazing on photosynthesis and
respiration. The study suggests that we need to take the
effects of livestock grazing into account in future assessments
of the impacts of climate change on carbon cycle in grassland
ecosystems. Further studies are needed to examine how
different grazing intensities affect temperature response and
acclimation of grassland species.
Funding
National Natural Science Foundation of China (31000177)
and Sumitomo Foundation of Japan, a strategic JapaneseChinese Cooperative Program on ‘Climate Change’: Integrated
assessment and prediction of carbon dynamics in relation to
climate changes in grasslands on the Qinghai-Tibetan and
Mongolian Plateaus, and the National Basic Research Program
(2010CB833502).
Acknowledgements
We are grateful to Izumi Washitani and Nishihiro Jun for providing us
with LI-6400 in the field measurement. We also thank Dr Song Gu,
Dr Shengbo Shi, Dr Yingnian Li and Dr Xinquan Zhao for their valuable comments and assistance in this study.
426
References
Alonso A, Pérez P, Martínez-Carrasco R (2009) Growth in elevated
CO2 enhances temperature response of photosynthesis in wheat.
Physiol Plant 135:109–20.
Atkin OK, Bruhn D, Hurry VM, et al. (2005) The hot and the cold:
unraveling the variable response of plant respiration to temperature. Funct Plant Biol 32:87−105.
Atkin OK, Holly C, Ball MC (2000) Acclimation of snow gum
(Eucalyptus pauciflora) leaf respiration to seasonal and diurnal variations in temperature: the importance of changes in the capacity and temperature sensitivity of respiration. Plant Cell Environ
23:15−26.
Atkin OK, Scheurwater I, Pons TL (2006) High thermal acclimation
potential of both photosynthesis and respiration in two lowland
Plantago species in contrast to an alpine congeneric. Glob Change
Biol 12:500−15.
Atkin OK, Tjoelker MG (2003) Thermal acclimation and the dynamic
response of plant respiration to temperature. Trends Plant Sci
8:343–51.
Atkin OK, Zhang Q, Wiskich JT (2002) Effect of temperature on rates
of alternative and cytochrome pathway respiration and their relationship with the redox poise of the quinone pool. Plant Physiol
128:212–22.
Azcón-Bieto J, Osmond CB (1983) Relationship between photosynthesis and respiration: the effect of carbohydrate status on the rate
of CO2 production by respiration in darkened and illuminated
wheat leaves. Plant Physiol 71:574–81.
Bauerle WL, Bowden JD, Wang GG (2007) The influence of temperature on within-canopy acclimation and variation in leaf
photosynthesis: spatial acclimation to microclimate gradients
among climatically divergent Acer rubrum L. genotypes. J Exp Bot
58:3285–98.
Berry JA, Björkman O (1980) Photosynthetic response and adaptation to temperature in higher plants. Annu Rev Plant Physiol
31:491−543.
Björkman O (1981) Response to different quantum flux densities. In
Lange OL, Nobel PS, Osmond CB, and Ziegler H (eds) Physiological
Plant Ecology I. Encyclopedia of Plant Physiology. Vol. 12A. Berlin:
Springer, 57–107.
Boardman NK (1997) Comparative photosynthesis of sun and shade
plants. Ann Rev Plant Physiol 28:355−77.
Bolstad PV, Mitchell KA, Vose JM (1999) Foliar temperature-respiration response functions for broad-leaved tree species in the southern Appalachians. Tree Physiol 19:871–8.
Bruhn D, Egerton JJC, Loveys BR, et al. (2007) Evergreen leaf respiration acclimates to long-term nocturnal warming under field conditions. Glob Change Biol 13:1216−23.
Campbell C, Atkinson L, Zaragoza-Castells J, et al. (2007) Acclimation
of photosynthesis and respiration is asynchronous in response to
changes in temperature regardless of plant functional group. New
Phytol 176:375–89.
Cao GM, Tang YH, Mo WH, et al. (2004) Grazing intensity alters soil
respiration in an alpine meadow on the Tibetan Plateau. Soil Biol
Biochem 36:237−43.
Chinese Soil Taxonomy Research Group (1995) Chinese Soil Taxonomy.
Beijing: China Agricultural Science and Technology Press, 58–147.
Journal of Plant Ecology
Covey-Crump EM, Attwood RG, Atkin OK (2002) Regulation of root
respiration in two species of Plantago that differ in relative growth
rate: the effect of short- and long- term changes in temperature.
Plant Cell Environ 25:1501−13.
Dahlgren RA, Driscoll CT (1994) The effects of whole-tree clear-cutting on soil processes at the Hubbard Brook Experimental Forest,
New Hampshire, USA. Plant Soil 158:239−62.
Dewar RC, Medlyn BE, McMurtrie RE (1999) Acclimation of the
respiration photosynthesis ratio to temperature: insights from a
model. Glob Change Biol 5:615−22.
Diaz S, Lavorel S, McIntyre SUE, et al. (2007) Plant trait responses to
grazing? a global synthesis. Glob Change Biol 13:313−41.
Diaz S, Noy-Meir I, Cabido M (2001) Can grazing response of herbaceous plants be predicted from simple vegetative traits? J Appl Ecol
38:497−508.
Dillaway DN, Kruger EL. (2010) Thermal acclimation of photosynthesis: a comparison of boreal and temperate tree species along a
latitudinal transect. Plant Cell Environ 33:888–99.
Farquhar GD, von Caemmerer S, Berry JA (1980) A biochemical
model of photosynthetic CO2 assimilation in leaves of C3 species.
Planta 149:78–90.
Gunderson CA, Norby RJ, Wullschleger SD (2000) Acclimation of
photosynthesis and respiration to simulated climatic warming in
northern and southern populations of Acer saccharum: laboratory
and field evidence. Tree Physiol 20:87–96.
Hartley IP, Armstrong AF, Murthy R, et al. (2006) The dependence of respiration on photosynthetic substrate supply and temperature: integrating
leaf, soil and ecosystem measurements. Glob Change Biol 12:1954−68.
Hikosaka K, Ishikawa K, Borjigidai A, et al. (2006) Temperature acclimation of photosynthesis: mechanisms involved in the changes
in temperature dependence of photosynthetic rate. J Exp Bot
57:291–302.
Jeddi K, Chaieb M (2010) Changes in soil properties and vegetation
following livestock grazing exclusion in degraded arid environments of South Tunisia. Flora 205:184−9.
Jones A (2000) Effects of catter grazing on North American arid ecosystems: a quantitative review. West N Am Nat 60:155−64.
Kato T, Tang Y, Gu S, et al. (2004) Seasonal patterns of gross primary
production and ecosystem respiration in an alpine meadow ecosystem on the Qinghai-Tibetan Plateau. J Geophys Res 109:D12109
doi:12110.11029/12003JD003951.
Klein J, Harte J, Zhao XQ (2005) Dynamic and complex microclimate
response to warming and grazing manipulations. Glob Change Biol
11:1440−51.
Lambers H (1985) Respiration in intact plants and tissues: its regulation and dependence on environmental factors, metabolism and
invaded organisms. In Douce R, Day DA (eds) Higher Plant Cell
Respiration. Berlin: Springer, 418−73.
Larigauderie A, Körner CH (1995) Acclimation of leaf dark respiration to temperature in alpine and lowland plant species. Ann Bot
76: 245−52.
Luo C, Xu G, Chao Z, et al. (2010) Effect of warming and grazing on
litter mass loss and temperature sensitivity of litter and dung mass
loss on the Tibetan Plateau. Glob Change Biol 16:1606−17.
Morison JIL, Morecroft MD (2006) Plant Growth and Climate
Change: Significance of Temperature in Plant Life. Boston: Blackwell
Publishing.
Shen et al. | Grazing alters warming effects on understory forb427
Noguchi K, Sonoike K, Terashima I (1996) Acclimation of respiratory properties of leaves of Spinacia oleracea L., a sun species, and
of Alocasia macrorrhiza (L.) G. Don., a shade species, to changes in
growth irradiance. Plant Cell Physiol 37:377−84.
Wan S, Luo Y, Wallace LL (2002) Changes in microclimate induced
by experimental warming and clipping in tall grass prairie. Glob
Change Biol 8:754−68.
Noguchi K, Taylor NL, Millar AH, et al. (2005) Response of mitochondria to light intensity in the leaves of sun and shade species. Plant
Cell Environ 28:760−71.
Wang D, Sun R, Wang Z, et al. (1999) Effects of temperature and photoperiod on thermogenesis in plateau pikas (Ochotona curzoniae)
and root voles (Microtus oeconomus). J Comp Physiol B, Biochem Syst
Environ Physiol 169:77–83.
Pearcy RW (1977) Acclimation of photosynthetic and respiratory carbon dioxide exchange to growth temperature in Atriplex lentiformis
(Torr) Wats. Plant Physiol 59:795−9.
Way DA, Sage RF (2008) Thermal acclimation of photosynthesis
in black spruce [Picea mariana (Mill.) B.S.P.]. Plant Cell Environ
31:1250–62.
Robakowski P, Montpied P, Dreyer E (2003) Plasticity of morphological and physiological traits in response to different levels of irradiance in seedlings of silver fir (Abies alba Mill). Trees 17:431−41.
Xiong FS, Mueller EC, Day TA (2000) Photosynthetic and respiratory
acclimation and growth response of Antarctic vascular plants to
contrasting temperature regimes. Am J Bot 87:700–10.
Ryan MG (1991) Effects of climate change on plant respiration. Ecol
Appl 1:157−67.
Yamori W, Evans JR, Von Caemmerer S (2010) Effects of growth and
measurement light intensities on temperature dependence of CO2
assimilation rate in tobacco leaves. Plant Cell Environ 33:332–43.
Shen HH, Klein J, Zhao XQ, et al. (2009) Leaf photosynthesis and
simulated carbon budget of Gentiana straminea from a decade-long
warming experiment. J Plant Ecol 2:207−16.
Silim SN, Ryan N, Kubien DS. (2010) Temperature responses of photosynthesis and respiration in Populus balsamifera L.: acclimation
versus adaptation. Photosyn Res 104:19–30.
Yamori W, Noguchi K, Hikosaka K, et al. (2009) Cold-tolerant crop
species have greater temperature homeostasis of leaf respiration
and photosynthesis than cold-sensitive species. Plant Cell Physiol
50:203–15.
Tjoelker MG, Oleksyn J, Lorenc-Plucinska G, et al. (2009) Acclimation
of respiratory temperature responses in northern and southern
populations of Pinus banksiana. New Phytol 181:218–29.
Yamori W, Noguchi K, Terashima I (2005) Temperature acclimation of
photosynthesis in spinach leaves: analyses of photosynthetic components and temperature dependencies of photosynthetic partial
reactions. Plant Cell Environ 28:536−47.
Tranquillini W, Havranek WM, Ecker P (1986) Effects of atmospheric humidity and acclimation temperature on the temperature
response of photosynthesis in young Larix decidua Mill. Tree Physiol
1:37–45.
Zaragoza-Castells J, Sánchez-Gómez D, Valladares F, et al. (2007) Does
growth irradiance affect temperature dependence and thermal
acclimation of leaf respiration? Insights from a Mediterranean tree
with long-lived leaves. Plant Cell Environ 30:820–33.
Valentini R, Matteucci G, Dolman AJ, et al. (2000) Respiration as the
main determinant of carbon balance in European forests. Nature
404:861–5.
Zhu GL (1990) Separation and determination of photosynthetic pigments. In Zhu GL (ed) The Plant Physiological Experiments. Beijing:
Beijing University Press, 51−4.