Ecol Res (2009) 24: 645–653
DOI 10.1007/s11284-008-0535-8
O R I GI N A L A R T IC L E
Xiaoyong Cui Æ Song Gu Æ Jing Wu Æ Yanhong Tang
Photosynthetic response to dynamic changes of light and air humidity
in two moss species from the Tibetan Plateau
Received: 29 December 2007 / Accepted: 1 July 2008 / Published online: 26 August 2008
The Ecological Society of Japan 2008
Abstract Bryophytes are widely distributed in alpine
meadow and wetlands on the Tibetan Plateau, where
light intensity and air moisture are highly variable in
time. To address how bryophytes respond to their light
and moisture environments, we examined dynamic
photosynthesis in two moss species, Distichium inclinatum common in meadow, and Encalypta alpine frequently found in wetland. Photosynthetic induction
response was faster in the two moss species than in
most vascular species. In both species the 90% induction time after a sudden light increase from 50 to
600 lmol m2 s1 was within 3 min. The induction was
faster in mosses experiencing a period of weak light
than in those in total darkness. E. alpina, the wetland
species, showed more rapid induction response and
shorter post-illumination CO2 fixation to sunflecks than
D. inclinatum, the meadow species. Photosynthetic rate
(Amax) under saturated light in the two species increased linearly with increasing air relative humidity
(RH). The meadow species D. inclinatum showed higher
Amax under low RHs, but exhibited lower Amax under
high RHs in comparison with the wetland moss E.
alpina. Moreover, the quantum efficiency increased
linearly with increasing RH, indicating that air
humidity plays a critical role in photochemical activities
in the alpine mosses. The study suggests that there are
X. Cui Æ S. Gu
Northwest Institute for Plateau Biology,
Chinese Academy of Sciences, 810001 Xining,
People’s Republic of China
J. Wu Æ X. Cui (&)
Graduate University of Chinese Academy of Sciences,
A19 Yuquanlu, 100049 Beijing, People’s Republic of China
E-mail: [email protected]
Tel.: +86-10-88256497
Fax: +86-10-88256153
Y. Tang
National Institute for Environmental Studies,
Onogawa 16-2, Tsukuba, Ibaraki 305-8506, Japan
acclimations in dynamic photosynthesis in response to
light and humidity, and the acclimations would benefit
a high leaf carbon gain in the two alpine moss species
in their common habitats.
Keywords Alpine meadow Æ Carbon gain Æ Moss Æ
Photosynthetic induction Æ Relative humidity
Introduction
Bryophytes can be found in various terrestrial ecosystems from polar to tropical regions. Studies have focused on the ecological function of bryophytes in
different ecosystems, including desert, bog, tundra, and
forests (Kellner 2001; Nakatsubo 2002; Malmer et al.
2003; Sedia and Ehrenfeld 2003; Heijmans et al. 2004;
Cornelissen et al. 2007). In arid and semiarid deserts,
many bryophytes species are major components of biotic
crusts, which are the pioneers in succession of desert
communities and thus play an important role in sand
dune fixation (Barker et al. 2005; Xu et al. 2005). As
dominant species, the bryophytes play a critical role in
the development and maintenance of bogs, alpine tundra, and Antarctic ecosystems (Harley et al. 1989;
Uchida et al. 2002; Pannewitz et al. 2003; Bragazza et al.
2005; Dunn and Robinson 2006; Toet et al. 2006;
Gornall et al. 2007). Even in forest ecosystems where
bryophytes tend to be neglected, the group of species are
found to contribute largely to species diversity and
carbon fixation (DeLucia et al. 2003). There are also
reports on diverse aspects of bryophytes in grasslands
(Rincon and Grime 1989; Ingerpuu et al. 1998; Csintalan et al. 1997, 2000; Virtanen et al. 2000; Bates et al.
2005; Ingerpuu et al. 2005; Peintinger and Bergamini
2006).
In grasslands on the Tibetan Plateau, bryophytes
distribute with high coverage and have been reported to
contribute more than one third of annual vegetation
productivity (Li and Qi 1993). In most habitats, bryophytes grow under grass and forest canopies or even
646
grow beneath the litter and snow during the winter
season. In these regimes, light intensity is often highly
variable (e.g. Cui et al. 2006). Many studies have suggested that it is important for leaves of vascular plants
growing under low light regimes to use variable photon
density for leaf carbon gain (e.g. Pearcy et al. 1996;
Tang et al. 2003; Tausz et al. 2005). Among these
studies, it is evident that low stomatal conductance is a
major limitation of dynamic photosynthesis responding
to changes in photon density (e.g. Knapp and Smith
1989; Pearcy and Seemann 1990; Naumberg and Ellsworth 2000). Unlike vascular plants, bryophytes are
characterized by dominance of the gametophytic stage,
and gametophyte shoots do not have stomata. We thus
hypothesized that mosses should respond more quickly
to variable light, and therefore the light-use efficiency
should be higher under variable light than for vascular
plants.
On the Tibetan Plateau, in addition to the variable
light, the moisture regime for moss species can be also
highly variable in time. Mosses, however, are poikilohydrous plants that are not able to maintain water.
Their photosynthetic activity is sensitive to, and highly
dependent on, environmental water availability (Davey
1997; Csintalan et al. 1999; Marschall and Proctor 1999,
2004; Rice and Schneider 2004; Hájek and Beckett
2008). Some studies have tried to address how rapidly
bryophytes respond to change of soil or atmospheric
moisture (Proctor and Smirnoff 2000; Proctor 2004).
However, it is not clear whether moss species growing
under different water regimes exhibit different characteristics in dynamic photosynthesis in response to
change of air humidity.
In this pilot study, we investigated photosynthesis
response to dynamic light and air humidity in two moss
species from an alpine meadow and an alpine wetland.
We tried to evaluate photosynthetic benefit for leaf
carbon gain in bryophytes common in two contrasting
water environments on the Tibetan Plateau.
Materials and methods
Plant materials
Two moss species, Distichium inclinatum and Encalypta
alpina were taken, respectively, from a Kobresia humilis
meadow and a wetland at Haibei Alpine Meadow Ecosystem Research Station of the Chinese Academy of
Sciences (lat. 3729¢N, long. 10112¢E) in the northeast
part of the Qinghai-Tibetan Plateau. The altitude is
about 3,250 m. Annual mean air temperature is 2C
and annual precipitation is 500 mm. The details for the
alpine meadow and wetland can be found elsewhere
(Klein et al. 2001; Hirota et al. 2004). Mosses were
cultured in a growth chamber at 20C/5C (day/night)
with 14-h-day length with PPFD 620 lmol m2 s1 for
about five months before we conducted the following
measurements.
Experimental procedure
An LI-6400 IRGA analyzer (LI-COR, Lincoln, NE,
USA) was used for photosynthesis measurements in this
study. The leaf-temperature sensor was removed and
moss temperature was calculated by the energy balance
method (Ehleringer 1989; Rochette et al. 1990; LI-COR
2005). A cluster of moss in its natural density was gently
fastened together and inserted in the center of the
chamber, where the leaf temperature sensor was originally mounted. The plants were then illuminated by
LEDs about 1.5 cm above the plants in a status similar
to that under natural conditions. The hole was connected to a short tube, which was immersed in tap water
to avoid air leaking from the leaf chamber. A slice of
paper towel was placed in the tube with one end
touching moss rhizoids and the other end in water to
maintain high water content in mosses. In a preliminary
experiment in winter we took intact moss turfs into the
laboratory at the field station. After one night in darkness at a temperature of around 0C, the plants were
exposed to different temperatures or different moisture
levels within a small chamber in darkness for around
2 h. Chlorophyll a fluorescence parameter Fv/Fm was
measured in darkness at the end of the treatments using
a PAM-2000 (Heinz Walz, Germany). In the measurements at different temperatures, air humidity was not
modified and plants were fully watered. In the measurements with different water contents, samples were
maintained at about 14C. We measured 4–8 replicates
(different moss turfs) in each treatment. Moss samples
were oven-dried and water content was determined after
Fv/Fm measurement. The results showed that Fv/Fm
(maximum photochemical efficiency) of these mosses
was stable at air temperatures between 0 and 30C
(P = 0.29, F test with df1 = 4, df2 = 18) and for water
content above 60% (Fig. 1). Therefore, the water content of mosses was maintained at about 200%, and air
temperature was around 22C, during the measurements
to avoid negative effects of water or temperature stress
on photosynthesis. High water content of moss samples
was achieved by fully watering rhizoids and the lower
parts of stems with paper towel soaked in water. The
CO2 concentration was constantly 400 lmol mol1
during photosynthesis measurements.
In the photosynthetic induction measurement, moss
was kept in darkness or under low light of 50 lmol
m2 s1 for more than 30 min before measurement.
Photosynthetic rate was continuously recorded during
change of PPFD from 0 to 600 or 50 to 600 lmol m2
s1 to obtain the light induction response curve. Postillumination CO2 fixation was determined by tracing
photosynthetic rate to stabilization after changing PPFD
from 600 to 0 lmol m2 s1. The RH of air entering the
leaf chamber was controlled to be 68.4 ± 0.5% during
the measurement. Moss temperature was 25.1 ± 1.1C.
There were five replicates in each treatment.
To characterize response of mosses to air humidity,
photosynthetic light response curve (A/I curve) was
647
simulated sunflecks, t is the length of time from the
beginning of the simulated sunfleck, and t63% is a
characteristic time constant (time for 63% change of gas
exchange rate). A60 (photosynthesis rate 60 s after the
simulated sunfleck) and t90% (t till 90% of Amax is
reached) were calculated according to Eq. (1). IS60 s
(induction state 60 s after the simulated sunfleck) is
evaluated from (A60 Ashade)/(Amax Ashade).
Induction loss and post-illumination gain of photosynthesis was calculated on the basis of Eqs. (2) and (3):
Amax ðT 2 T 1Þ ZT 2
AðtÞdt
ð2Þ
T1
ZT 4
AðtÞdt Ashade ðT 4 T 3Þ
ð3Þ
T3
where T1 is the start of illumination and T2 is the time
when maximum photosynthesis is reached. T3 is the end
of illumination and T4 is the time when minimum
photosynthesis is reached in darkness. A(t) is the time
course of the photosynthetic rate during the experiment.
A/I curves under different air RH were fitted by a
non-rectangular hyperbolic model (Dewar et al. 1998).
As a value of zero for the convexity term fitted the data
best, the model turned into:
Fig. 1 Response of photochemical efficiency of photosynthetic
photosystem II (PSII) to air temperature (a) and thalliform water
content (b) in mosses on the Tibetan Plateau. There were 4–8
replicates for each treatment. Error bars indicate standard
deviation (STDEV in SPSS) in individual treatment. The curve
fitted in b was y ¼ 17:13x=ðx þ 0:73Þ 16:37(R = 0.94). Moss
samples were taken from the field and measured within 2 days in
the laboratory in winter. Intact moss turfs from both alpine
meadow and wetland, some of which contained co-existing moss
species, were used and species were not identified
determined at different air relative humidity with the
following light intensity sequence: 600, 400, 200, 100, 50,
25, and 0 lmol m2 s1. Photosynthetic rate was recorded after it reached its ‘‘steady state’’ for each PPFD.
For moss response to humidity fluctuations, light
intensity was maintained at 600 lmol m2 s1 while
incoming air RH varied from about 46% to below 10%
and then to near 64% (as shown in Fig. 4). Moss temperature was 26.3 ± 0.9C. Three to five replicates were
conducted for each species in these two experiments.
Data analysis
Induction parameters were calculated according to Allen
and Pearcy (2000a). The induction curve (as shown in
Fig. 2) was modeled by the following equation:
A ¼ Ashade þ ðAmax Ashade Þ ð1 expðt=t63% ÞÞ
ð1Þ
where Ashade is the initial photosynthetic rate in the dark
or in shade (0 and 50 lmol m2 s1), Amax is the maximum photosynthetic rate achieved in response to the
A¼
Amax aI
Amax þ aI
ð4Þ
where A is the net photosynthetic rate, and Amax is the
maximum rate of net photosynthesis at each RH. a is the
quantum efficiency of photosynthesis. I is light intensity.
All statistical tests were performed with SPSS. Moss
species and low light regime were regarded as the two
factors in air humidity response. Each factor had two
levels. Main and interaction effects of the two factors
were included in univariate analysis of the general linear
model. Duncan’s post hoc test was used to compare
different treatments.
Results
Light induction of photosynthesis
Mosses showed a clear induction response to a sudden
increase of photon flux density (Fig. 2). In the first
minute after the light increase, both species attained
more than 50% of the full induction. The increase of
CO2 uptake was more rapid in plants with low-light
induction under 50 lmol m2 s1 than in those without
induction in the dark (Fig. 3). To achieve 90% of the full
induction, it took less than 3 min for mosses with lowlight induction, but it needed about 5 min for plants
without induction (Fig. 3).
Distichium inclinatum from meadow demonstrated a
slightly lower induction rate than Encalypta alpine, the
648
induction rate was higher and the carbon loss was lower
for plants with low-light induction than for those without induction (Figs. 3, 4). The average time length for
90% induction and induction carbon loss under light
change from 0 to 600 lmol m2 s1 were approximately
twice those under light change from 50 to 600 lmol
m2 s1.
Response to air humidity
Fig. 2 Response of CO2 fixation rate (thin solid line) to change of
light intensity (broken line) in a cluster of D. inclinatum. A similar
pattern was obtained for E. alpine but is not shown in the figure.
Modeled photosynthetic rate during light induction is also shown
(thick solid line) to demonstrate calculation of light induction
parameters
The preliminary experiment showed that PSII photochemical efficiency of mosses from Tibetan Plateau
ecosystems decreased during desiccation (Fig. 1b).
Photosynthetic rate of the mosses also responded
quickly to variation of air humidity (Fig. 5). Decrease of
RH led to a drastic reduction of CO2 fixation rate, which
recovered after an increase of RH. Decrease of photosynthetic rate on reduction of RH was much slower than
its recovery on increasing RH (Fig. 5). Both moss species showed similar linear relationships between RH and
photosynthetic rate (Fig. 6a). Photosynthetic quantum
effficiency was linearly correlated with RH (Fig. 6b).
The quantum efficiency was lower in D. inclinatum than
in E. alpina, but the species difference diminished with
increasing RH.
Discussion
Photosynthetic light induction
Fig. 3 Comparison of light induction responses in the two moss
species D. inclinatum and E. alpine from alpine meadow and alpine
wetland, respectively. 0 0–600, 50 50–600 lmol m2 s1. IS(60 s)
induction degree after 60 s illumination, t(63%) and t(90%) time
for 63 and 90% induction. There were five replicates in each
treatment. Results from Duncan’s post hoc test are shown and
different letters in the four treatments for a parameter indicate
statistically significant differences at the level P = 0.05
species from wetland (Fig. 3). The difference was statistically significant for t63% and t90% (P < 0.10),
whereas it was not significant for IS60 s (Table 1).
In comparison with the accumulated carbon gain,
assuming an immediately increase of CO2 uptake after
light increase, D. inclinatum lost about two times more
leaf carbon gain during the induction than E. alpina, the
species from wetland (P < 0.10, Table 1). Post-illumination carbon gain was also some two times higher in D.
inclinatum than in E. alpine (P < 0.05, Table 1).
Light intensity experienced before a sudden increase
of light to 600 lmol m2 s1 had a statistically significant effect on the photosynthetic induction rate and thus
also on the carbon loss (P < 0.05, Table 1). The
The 90% induction time in the two moss species was
faster than for most understory vascular plants, which
were between 3 and 37 min for 90% induction (Horton
and Neufeld 1998; Naumberg and Ellsworth 2000;
Tausz et al. 2005). In vascular plants, photosynthetic
induction response includes both biochemical induction
and stomatal induction. The biochemical reaction in
response to PPFD variation is very fast (Roháček and
Barták 1999). Stomatal conductance increases more
slowly than biochemical induction (Whitehead and
Teskey 1995). The low stomatal induction often limits
photosynthetic rate under sunflecks in many cases,
especially when stomatal conductance is below a critical
value (Allen and Pearcy 2000a, b). Generally, stomatal
induction was quicker in species living in an environment with a higher temporal heterogeneity of light
(Knapp and Smith 1990). Mosses in this study respond
rapidly to dynamic light, mainly because thalli do not
have cuticle or stomata. Photosynthetic induction response would become fast if the induction depended
mainly on the removal of photochemical limitations
(Tang and Liang 2000).
It is difficult to separate biochemical induction from
stomatal induction in vascular higher plants (Allen and
Pearcy 2000a, b). In case of small or no stomatal limitation, biochemical induction velocity was similar in
649
Table 1 Comparison of photosynthetic induction rate (IS60s, t63%, t90%), induction carbon loss, and post-illumination and carbon gain
between moss species from alpine wetland and meadow
Parameters
Source
Sum of squares
Degrees of freedom
Mean square
F value
Significant level
IS60
Species
Lighta
Species
Error
Total
Species
Light
Species
Error
Total
Species
Light
Species
Error
Total
Species
Light
Species
Error
Total
Species
Error
Total
0.0546
0.1176
0.0642
0.3327
7.2821
0.4421
0.8966
0.0049
2.2624
17.0009
3.3426
5.9626
0.0092
12.9720
98.4812
6.0756
9.4536
0.5232
23.1805
90.5454
0.2695
0.2215
3.2130
1
1
1
16
20
1
1
1
16
20
1
1
1
16
20
1
1
1
16
20
1
6
8
0.0546
0.1176
0.0642
0.0208
2.625
5.654
3.086
0.125
0.030
0.098
0.4421
0.8966
0.0049
0.1414
3.126
6.341
0.035
0.096
0.023
0.855
3.3426
5.9626
0.0092
0.8107
4.123
7.354
0.011
0.059
0.015
0.917
6.0756
9.4536
0.5232
1.4488
4.194
6.525
0.361
0.057
0.021
0.556
0.2695
0.0369
7.300
0.035
s
t63%
t90%
Induction carbon loss
Post-illumination carbon gain
· light
· light
· light
· light
ANOVA was conducted by the method of univariate analysis in general linear model in SPSS
Low light regime, i.e. 0 or 50 lmol m2 s1, before photosynthetic induction
a
Fig. 4 Comparison of carbon loss during light induction (Induction
loss) and carbon gain by post-illumination CO2 fixation (Post gain)
in the two moss species D. inclinatum and E. alpine from alpine
meadow and alpine wetland, respectively. 0 0–600, 50 50–
600 lmol m2 s1. Post gain for 50–600 lmol m2 s1 was not
determined. There were five replicates in each treatment, except
three replicates for post gain in We-0. Error bars indicate STDEV
for individual treatments. Results from Duncan’s post hoc test are
also shown and different letters in the four treatments for a
parameter indicate statistically significant differences at the level
P = 0.05
both sun and shade leaves of rainforest species Nothofagus cunninghamii (Tang and Liang 2000; Tausz et al.
2005). The biochemical induction time was similar in the
mosses in this study. High stomatal conductance and
high photosynthetic capacity explain the fast response of
N. cunninghamii to sunflecks (Tausz et al. 2005). With
little stomatal limitation, photosynthetic induction gives
Fig. 5 Response of CO2 fixation rate (A) to air humidity in E.
alpina. The pattern was similar for D. inclinatum and is not shown
in the figure
the potential speed of photochemical induction. Moss
provides an ideal model to study photochemical induction response to sudden light increase without interference from stomatal limitation. The consistent results
from N. cunninghamii and alpine mosses in this study
perhaps further suggest that the potential photochemical
induction time should be less than 3 min after induction
in low-light. Moss thalli are often covered with a thin
layer of water, which limits CO2 diffusion into thalli.
Variation of water layer thickness affects the d13C value
of moss thalli (Proctor et al. 1992; Rice 2000). Water
layer thickness should be stable in this study, because of
the steady environmental conditions in the leaf chamber
650
efficiency in the two moss species (Fig. 4). The meadow
moss, D. inclinatum, loses twice as much carbon during
light induction owing to slower photosynthetic induction than E. alpina, the species from alpine wetland
(Fig. 4). Although the carbon loss due to the induction
can be partly compensated by post-illumination carbon
gain, the net carbon fixation amount is less in D. inclinatum than in E. alpina.
Response to air humidity
Fig. 6 Relationship between air humidity and photosynthetic rate
(a) and quantum efficiency (b) in D. inclinatum and E. alpine from
alpine meadow and alpine wetland on the Tibetan Plateau. The
fitted lines were D. inclinatum: y = 0.0329x 0.8108,
R2 = 0.8164, E. alpina: y = 0.05x 0.9816, R2 = 0.9246 in a
and D. inclinatum: y = 0.0012x 0.0275, R2 = 0.7298, E. alpina:
y = 0.008x 0.004, R2 = 0.7444 in b
and plentiful water supply to the mosses; this is confirmed by stability of measured transpiration rates (data
not shown).
Photochemical induction state seems high in mosses
even after long-period dark acclimation (Fig. 2).
Numerous studies have shown that physiological activities in many mosses are able to recover quickly after
relief from stresses (Carballeira et al. 1998; Meyer and
Santarius 1998; Deltoro et al. 1999; Proctor and Tuba
2002). Maintenance of a high level of photosynthetic
enzyme activity in stressful conditions may play an
essential role in carbon gain.
Photochemical induction can be species-dependant
(Allen and Pearcy 2000b; Naumberg and Ellsworth
2000; Tausz et al. 2005). Activation of photosynthetic
enzymes, mainly ribulose 1,5-bisphosphate carboxylase/
oxygenase (Rubisco), and buildup of Calvin cycle
intermediates during sunfleck may be species and environment-dependent (Lei and Lechowicz 1997; Naumberg and Ellsworth 2000; Yin and Johnson 2000). It is
interesting that slower induction gain is accompanied by
slower induction loss (Figs. 3, 4). The deactivation rate
of Rubisco accounts for the divergent dynamic light
utilization capacity between two vascular species
(Ernstsen et al. 1997).
The difference in photosynthetic induction can
directly affect CO2 fixation and then sunfleck utilization
Physiological activities of moss are sensitive to thallus
dehydration (Oechel and Lawrence 1985; Schipperges
and Rydin 1998; Wasley et al. 2001; Hamerlynck et al.
2002; Proctor and Tuba 2002; Benkô et al. 2002;
McNeil and Waddington 2003). However, little is
known about photosynthetic response to air humidity
in moss. In vascular plants, photosynthetic capacity is
generally reduced by either soil water stress or low air
humidity. The limitation of photosynthetic reduction
under low water availability varies with species, growing season, and duration of water stress (Singh and
Sasahara 1981; Woledge et al. 1989; Tardieu and
Simonneau 1998; Franks and Farquhar 1999; Oren
et al. 1999). Reduction of stomatal conductance is the
major, or even sole, factor that explains the photosynthetic depression induced by low water regimes
(Banon et al. 2003; Ennahli and Earl 2005; Flexas et al.
2006). Decrease of photochemical activity is observed
in some cases by means of gas-exchange measurement
(Dai et al. 1992). However, this may be an artifact
resulting from patchiness in stomatal opening, while
photochemical electron transport does not change (Dai
et al. 1992; Shirke and Pathre 2004; Ennahli and Earl
2005). Photosynthetic photosystem II electron-transport processes can tolerate acute water deficiency
(Germ et al. 2005).
In this study, air humidity dramatically affected CO2
fixation in the mosses (Fig. 5). Moss thallus lacks stomata and is often covered by a thin water film. Low
water availability can reduce the thickness of the water
film, which thus facilitates CO2 diffusion into chloroplasts (Rice 2000). Air humidity also affects the thickness of the water film on plant leaves, with a thinner
water film at lower air RH (van Hove and Adema 1996).
Therefore, CO2 fixation rate is expected to respond inversely to RH variation when water supply is sufficient.
The current study, however, shows that CO2 fixation
rate increases almost linearly with increasing air
humidity (Fig. 6a). This implies that low air humidity
limited photosynthetic capacity of the two moss species,
which may be because of a reduction of photochemical
activity as a consequence of the low water availability. It
is unfortunate that we were unable to measure photochemical activity in this experiment. In vascular plants,
photochemical activity is not affected by low air
humidity because of feedforward decline of stomatal
conductance before change of leaf water content (Shirke
651
and Pathre 2004). In an intertidal alga that lacks stomata, lower humidity causes inhibition of photosynthesis because a small amount of water is removed and
because of loss of recovery after re-exposure to seawater
(Kawamitsu et al. 2000). O2 evolution from sunflowers
with leaf discs stripped away from the lower epidermis is
remarkably depressed at lower water potential, indicating inhibition of photochemical activity because of
metabolic limitation (Tang et al. 2002). Loss of the
activities of membrane-associated reactions is the main
cause of metabolic inhibition under water stress (Schwab
et al. 1989; Ortiz-Lopez et al. 1991; Gunasekera and
Berkowitz 1993; Tezara et al. 1999; Flexas and Medrano
2002; Lawlor and Cornic 2002; Parry et al. 2002). This
study and other studies all seem to suggest that the response of photosynthesis to cell water status in mosses is
substantially the same as in vascular plants (Dilks and
Proctor 1979; Tuba et al. 1996; Proctor 2000).
Photosynthetic response to changes of light intensity
and air humidity is quite fast in the two moss species
(Figs. 2, 4). Air humidity tends to vary quickly with
changes of solar radiation, temperature, rainfall, wind,
and other environmental factors. The fast response of
photosynthesis to fluctuation of light and humidity may
provide some insights into our understanding of the
ecophysiology of moss in harsh alpine environments.
Acknowledgments The study was partly supported by the Natural
Science Foundation of China (30670318). It was also part of a joint
research project between the National Institute for Environment
Studies, Japan, and the Northwest Plateau Institute of Biology,
China, supported by the projects ‘‘Integrated Study for Terrestrial
Carbon Management of Asia in the 21st Century Based on Scientific Advancements’’ and ‘‘Early detection and prediction of climate warming based on the long-term monitoring of alpine
ecosystems on the Tibetan Plateau’’. We declare that the experiments in this paper comply with the current laws of the People‘s
Republic of China in which the experiments were performed.
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