American Journal of Plant Sciences, 2011, 2, 716-725
doi:10.4236/ajps.2011.25086 Published Online November 2011 (http://www.SciRP.org/journal/ajps)
Temperature Dependency of Photosynthesis of
Sphagnum spp. Distributed in the
Warm-Temperate and the Cool-Temperate Mires
of Japan
Akira Haraguchi, Nanae Yamada
Faculty of Environmental Engineering, The University of Kitakyushu, Kitakyushu, Japan.
Received August 22nd 2011; revised September 25th, 2011; accepted October 25th, 2011.
ABSTRACT
We investigated the temperature dependency of photosynthetic rates for five Sphagnum species: Sphagnum palustre, S.
fimbriatum in the Tadewara mire (south-western Japan in a warm-temperate zone) and S. papillosum, S. fuscum, S.
fallax in the East Ochiishi mire (north-eastern Japan in a cool-temperate zone) measuring photosynthetic light response
within a temperature range between 5 and 40˚C. The maximum photosynthetic rate was obtained at T = 35˚C for S.
palustre, S. fuscum and S. papillosum, and at T = 30˚C for S. fimbriatum and S. fallax. Photosynthetic rates of all these
species showed a maximum at 300 - 500 μmol·m–2·s–1 of PPFD and it decreased at higher PPFD (>500 μmol·m–2·s–1)
under low temperature (5˚C - 10˚C). These results imply that Sphagnum species are not fully physiologically adapted to
low temperature environments, although Sphagnum species distribute mostly in the circumpolar region.
Keywords: Carbon Dioxide Exchange, Photosynthesis, Sphagnum, Temperature Dependence
1. Introduction
Carbon accumulation by peat-forming plants is a great
concern for the global carbon cycle [1,2]. Sphagnum
plants are major components of vegetation in boreal mires
and form thick peat layers containing huge amounts of
organic carbon. Decomposition of peat causes the emis-
sion of carbon into the atmosphere, whereas deposition
of peat promotes accumulation of carbon in the soil. In
addition, the balance of peat formation and decomposi-
tion controls net carbon flow between the rhizosphere
and the atmosphere.
The growth and primary productivity of Sphagnum
spp. have been investigated und er different conditions of
water deficiency, nutrition, acidity, temperature, UV ra-
diation and atmospheric pollution from pollutants such as
ozone or bisulphate. Prior eco-physiological studies of
Sphagnum plants clarified that most of the Sphagnum
spp. grow well under acidic and poor nutritional condi-
tions [3,4]. Sphagnum spp. are acidophilic species and
the plants change the habitat acidity by exchanging solu-
ble cations with protons at the cell walls of the plants.
Only some few species such as S. squarrosum and S. fim-
briatum can colonize under calcareous conditions [5].
Colonization of these pioneer Sphagnum spp. acidifies
the habitat and then other sp ecies are able to colonize the
acidic habitat. The growth of Sphagnum declined in
growth media with high nutrient concentrations, espe-
cially those containing high levels of inorganic phospho-
rus [4].
Although th e growth responses of Sphagnum plants to
various environmental parameters have already been
investigated, physiological responses of Sphagnum spp.
such as photosynthetic activity h ave not been fully inves-
tigated. The water availability of habitats or the water
contents of plants are among the limiting factors of the
photosynthetic activity o f Sphagnum [6-8 ]. Many Sphag-
num spp. have optimal water contents for photosynthesis
at 600% - 800% of dry weight and the photosynthetic
rate rapidly decreases when the water content becomes
lower than the optimal. Desiccation tolerance of Sphag-
num plants can be evaluated by the minimum water con-
tent in plants that maintain photosynthetic activity and
the duration of dehydration from which the plants can
recover physiological activity after rehydration [9,10].

Temperature Dependency of Photosynthesis of Sphagnum spp. Distributed in the Warm-Temperate and the 717
Cool-Temperate Mires of Japan
Desiccation tolerance depends on the species and it de-
termines the distribution of Sphagnum species along the
gradients of microtopography within a mire community
[3]. Hummock-forming species have a high tolerance for
desiccation. Sphagnum spp. in section Acutifolia have
high desiccation tolerance because the green cells face
the adaxial surface and in addition the green cells are
covered with hyaline cells which face the abaxial surface.
Species growing on hollow or submerged species are
poorly adapted to dry conditions, although the species can
survive under dry conditions for a short period. Sphag-
num spp. in section Cuspidata have low desiccation to-
lerance because of their leaf anatomical structure is the
opposite of that found in species in section Acutifolia.
Recent studies on the relationship between Sphagnum
growth and nutrition have been focused on atmospheric
nitrogen deposition [11-16] and many studies showed
that enhanced nitrogen deposition accelerates the photo-
synthetic rate of Sphagnum plants, whereas growth of
plants was limited by the deposition of atmospheric ni-
trogen.
Effects of environmental pollutants on the photosyn-
thesis of Sphagnum plants have also been of great con-
cern. Bisulphite (3) in the atmospheric deposition
has an extremely inhibiting effect on Sphagnum photo-
synthesis [17-19], and 5.0 mM of 3 has a comple-
tely fatal effect on Sphagnum. Iron deposition accompa-
nying bisulphite deposition from the atmosphere de-
creases the effect of bisulphite on Sphagnum, and thus
plants growing in polluted areas have higher tolerance
capability for atmospheric pollution [20].
HSO
HSO
Ozone also has an inhibiting effect on the photosyn-
thesis of Sphagnum plants. Sphagnum elongation and
photosynthetic rate declined after 6 - 9 weeks of O3 ex-
posure at 70 - 80 ppb [21]. Respiration rate of Sphagnum
plants increased after exposure to O3, although the pho-
tosynthetic rate was not affected by O3 exposure [21,22].
Increased UV-B radiation inhibited elongation and
dark respiration, but the effect of UV-B radiation in-
crease on net photosynthetic rate per unit dry weight of
plants was not significant [23]. Reduction of solar UV-B
increased height growth but decreased volumetric den-
sity in S. magellanicum; furthermore, UV-B reduction
had little effect on biomass production [24,25].
In addition to the effects of environmental pollution
and UV radiation, global environmental change’s influ-
ence on the photosynthetic activity of Sphagnum plants
is also of great concern because the growth and decom-
position of senescent plant materials determine the ac-
cumulation rate of organic carbon in peat soil and the
consequent global carbon balance. As for the photosyn-
thetic capacity of individual Sphagnum plants, the re-
sponse of growth and photosynth etic rate to the tempera-
ture regime has been investigated by various researchers.
Investigation on the growth of S. fuscum, S. balticum, S.
cuspidatum and S. magellanicum under different tempe-
rature conditions rang ing from T = 11.2˚C to 21.4˚C, and
all of these Sphagnum plants showed maximum growth
at T = 21.4˚C [26]. Temperature dependence of Sphag-
num photosynthetic rates by using experimental mea-
surement data as well as a theoretical model showed that
maximum net photosynthetic rate along a particular tem-
perature regime differed according to light intensity [27].
The optimal temperature in low light inten sity conditions
(PPFD = 20 mol·m–2·s–1) was T = 0˚C to 5˚C, whereas
in high light intensity conditions (PPFD = 500 mol·m–2·s–1)
the optimal temperature was between 20˚C and 25˚C.
These studies show that the photosynthetic capacity of
Sphagnum plants has a rather high optimal temperature
range at 20˚C to 25˚C, even though Sphagnum plants
successfully distribute in the circum po lar region.
In this paper we investigated the temperature depen-
dence of the carbon assimilation rate of five Sphagnum
species growing in warm-temperate and cool-temperate
zones in Japan. We measured the response of the CO2
exchange rate of the Sphagnum species along a tempera-
ture range of 5˚C - 40˚C, a much wider range compared
to those previously reported. Furthermore, we tried to
test the hypothesis that Sphagnum species can tolerate
higher temperature conditions than those in their native
distribution areas. We also tried to clarify the differences
in temperature dependence for the photosynthetic rates
found among various species and climatic zones. Mea-
suring optimum temperatures for photosynthetic rates
and making interspecific comparisons provided informa-
tion on the adaptation of Sphagnum plants to low tem-
perature environments. Additionally, we discussed the
adaptive traits of Sphagnum species and the responses of
community structure to global climatic change.
2. Materials and Methods
2.1. Plant Materials
Sphagnum samples were collected from the Tadewara
mire in south-western Japan (33.07˚N, 131.14˚E, 1000 m
a.s.l.) in June 2007, and the East Ochiishi mire in north-
eastern Japan (43.16˚N, 145.50˚E, 50 m a.s.l.) in Sep-
tember 2007.
The Tadewara mire (measuring 1.0 km east to west,
and 3.0 km north to south) is located within the Kujyu
volcanic mountain area at 1000 m a.s.l. The mire deve-
loped on the gentle slope at the end an alluvial fan. An-
nual mean temperature in the mire was 9.1˚C, average
daily maximum temperature was 25.6˚C in August, av-

Temperature Dependency of Photosynthesis of Sphagnum spp. Distributed in the Warm-Temperate and the 718 Cool-Temperate Mires of Japan
erage daily minimum temperature was –6.2˚C in January,
annual precipitation was 2720 mm, and maximum snow
accumulation was 12 cm in January (data from the Japan
Meteorological Agency between 1971 and 2000). Annual
precipitation in the Kujyu mountain area is reported to be
up to 3500 mm [28]. The upper part (southern area) of
the Tadewara mire is covered with a grassland domi-
nated by Miscanthus sinensis Anderss. and the lower part
(northern area) of the mire is covered by mesotrophic to
ombrotrophic mire vegetation dominated by Phragmites
australis (Cav.) Trin. ex Steud., Moliniopsis japonica
(Hack.) Hayata, and Hydrangea paniculata Sieb. et Zucc.
Two Sphagnum species, Sphagnum palustre L. (section
Palustria or Sphagnum) and Sphagnum fimbriatum Wils.
ex Wils. & Hookf. (section Acutifolia) distribute in the
central area of the mire community. The depth of the
peat layer at the center of the Sphagnum dominated mire
was 4.2 m and the layer lower than 163 cm contains
large amounts of volcanic mineral deposition (C conten ts
< 5% w/w; Haraguchi et al. , un p ubl ished data ).
Another sampling site, the East Ochiishi mire was in
the Ochiishi district, Nemuro, in north-eastern Japan.
Some ombrogenous mires were established on the coa-
stal terrace at 40 - 50 m a.s.l, having been formed by the
Nemuro Group in the Upper Cretaceous. The annual
mean temperature in the East Ochiishi mire was 5.7˚C,
average daily maximum temperature was 20.5˚C in Au-
gust, average daily minimum temperature was –9.3˚C in
February, annual precipitation was 1031 mm, and maxi-
mum snow accumulation was 23 cm in February (data
from the Japan Meteorological Agency between 1971
and 2000). A Picea glehnii (Fr. Schm.) Masters. forest
was estab- lished on the mire in conjunction with the
Sphagnum- dominated mire [29]. The site description
and description of the vegetation structure of the mires in
the Ochiishi district appear in [29]. Sphagnum plants
were collected from the East Ochiishi mire, which has a
peat accumula- tion of 1.3 - 1.5 m. Several tephra layers
are included in the peat layer, and the latest layer of the
Holocene tephra originating from Tarumae Volcano
(Ta-a; 1739 AD) ap- pear s at a depth of 10 - 15 cm. The
dominant Sphagnum species in the mire are Sphagnum
fuscum (Schimp.) Klinggr. (section Acutifolia) and S.
papillosu m Lindb. (section Palustria). Sphagnum fallax
Klinggr. (section Cuspidata), S. girgensohnii Russ. (sec-
tion Acutifolia) and S. squarrosum Crome (section
Squarrosa) were abundant on the P. glehnii forest floor.
We collected S. papillosum, S. fuscum and S. fallax from
the East Ochiishi mire.
Sphagnum samples of 10 cm leng th with capitula were
excised in the field and stored in air-tight plastic bags
immediately after sample collection. Samples were trans-
ferred to the laboratory at room temperature and samples
were cultivated at T = 25˚C under 12L12D illuminated
with PPFD is 40 mol·m–2·s–1 until photosynthesis mea-
surements were made. Plants were submerged in pure
water except for the capitula and water was supplied at 1 -
2 day intervals, thus preventing plants from entering the
atmosphere. Photosynthesis measurements were made
within 10 days fro m the beginn ing of cultivatio n and the
plant samples were kept in good physiological condition
for the measurement.
2.2. Measurements of Photosynthesis and Dark
Respiration Rates
Capitula segments of 1 cm from the apex were excised
from the shoots of Sphagnum plants and regularly ar-
ranged on Petri dishes. Twelve capitula were used for
measurements for S. palustre, S. fimbriatum, S. papillo-
sum and S. fallax, whereas 15 capitula were used for S.
fuscum. Plant materials were submerged in water and
preliminary illumination was provided in the growth
chamber at T = 20˚C for more than 24 hours at PPFD =
90 mol·m–2·s–1. Water in the Petri dish was removed
just before the photosynthesis measurements.
Photosynthetic rates and dark respiration rates for the
Sphagnum spp. were measured by CO2 exchange rate in
a chamber with a constant flow of air. Ambient air (CO2
concentration 420 - 470 ppm) was pooled in a gas bag
(3.5 m3) and supplied the air at 1.0 L·min–1 to the cham-
ber after saturation of humidity by flushing through wa-
ter. Concentration of CO2 before and after the chamber
was measured by infrared CO2 analyzer (LI-COR, LI-
840). The chamber (13 cm × 10 cm × 4.5 cm) was made
of polypropylene and the upper surface of the chamber
was made of acryl transparency plate (10.2 cm × 7 cm).
The chamber was placed in a 100 L water bath and water
temperature was controlled by a temperature control sy-
stem (TAITEC, CL-150R). Irradiation was provided by
four 500 W halogen lamps and the photon flux density of
photosynthetically active radiation (PPFD) was measured
by a PPFD sensor in water (LI-COR, LI- 250A).
Petri dishes with Sphagnum capitula were placed in
the plastic chamber. The chamber was submerged in wa-
ter and water temperature was controlled by circulating
water. Measurement of CO2 concentration started 30 min
after setting up the chamber. Readings of the CO2 con-
centration became stable within 30 min. Concentrations
of CO2 were recorded at 1 sec intervals. CO2 concentra-
tions flowing both in and out of the chamber were alter-
nately measured at 10 sec intervals. PPFD at the surface
of plant samples and water temperature just at the surface
of the chamber were recorded for each measurement.
PPFD changed from ca. 800 mol· m–2·s–1 to 0

Temperature Dependency of Photosynthesis of Sphagnum spp. Distributed in the Warm-Temperate and the 719
Cool-Temperate Mires of Japan
mol·m–2·s–1 and CO2 concentrations were measured at
15 different PPFD levels. Dark respiration was measured
by covering the chamber with a dark net. Temperature
was ch anged from 5 ˚C to 40˚C at 5˚C intervals. A cham-
ber without plant samples was used as a reference and
the CO2 concentration of the reference chamber was used
as a blank.
After measurement of the CO2 exchange rate, plant
samples were dried at T = 70˚C for 24 hours and dry
weights of the plants were measured. Photosynthetic and
respiration rates were calculated on a dry-weight basis.
3. Results
Absolute photosynthetic rates of Sphagnum spp. showed
high variance between individual plants, and we pre-
sented PPFD-photosynthetic rate response curves by the
relative rate (%) per maximum rate within the measure-
ments using the same plant samples (Figure 1). Absolute
values for photosynthetic rate appear in Figure 2. PPFD-
gross photosynthetic rate response curves showed signi-
ficant temperature dependence for every species. S. fim-
briatum from the Tadewara mire showed the maximum
photosynthetic rate at T = 25˚C, and the rate maintained
higher values with in the temperature range of T = 15˚C -
35˚C (Figure 1(a)). Within the temperature range of
15˚C to 35˚C, the maximum photosynthetic rate was ob-
tained at PPFD = 320 - 450 mo l ·m–2·s–1 and the rate
decreased at PPFD > 450 mol·m–2·s–1. The gross pho-
tosynthetic rate of S. fimbriatum showed lower values at
T = 5˚C, 10˚C and 40˚C, and the maximum photosynthetic
rates were obtained at PPFD = 200 - 250mol ·m–2·s–1 at T
= 5˚C and 10˚C.
S. palustre from the Tadewara mire showed maximum
photosynthetic rates at T = 25˚C - 30˚C and the rates re-
mained almost the same within the temperature range
(Figure 1(b)). At T = 35˚C, photosynthetic rates for S.
palustre significantly decreased at PPFD > 490 mol·m–2·s–1,
whereas the decrease in photosynthetic rates at PPFD >
660 mo l ·m–2·s–1 was not significant at T = 25˚C and
30˚C. Photosynthetic rates for S. palustre at T = 5˚C
were the lowest, and the rate decreased at PPFD > 120
mol·m–2·s–1. The gross photosynthetic rate decreased at
PPFD > 80 mol·m–2·s–1 at T = 40˚C.
S. fuscum from the East Ochiishi mire showed maxi-
mum gross photosynthetic rate at T = 35˚C and the higher
rate was maintained within the temperature range of T =
30˚C - 40˚C (Figure 1(c)). At T = 30˚C, the gross photo-
synthetic rate of S. fuscum slightly decreased at PPFD >
550 mo l·m–2·s–1, whereas no decrease in photosynthetic
rate at high light intensity was observ ed at T = 35˚C and
40˚C. The photosynthetic rate for S. fuscum at T = 5˚C
was the lowest.
S. papillosum from the East Ochiishi mire showed a
maximum photosynthetic rate at T = 35˚C, and the rate
maintained higher values within the temperature range of
T = 30˚C - 35˚C (Figure 1(d)). Even at T = 40˚C, the
photosynthetic rate of S. papillosum remained almost the
same as that measured at T = 25˚C. The photosynthetic
rate of S. papillosum showed its lowest value at T = 5˚C,
and the rate decreased at PPFD > 530 mol·m–2·s–1.
S. fallax from the East Ochiishi mire showed a maxi-
mum photosynthetic rate at T = 30˚C, and the rate main-
tained higher values within a temperature range of T =
25˚C - 40˚C (Figure 1(e)). The photosynthetic rate of S.
fallax showed its lowest value at T = 5˚C, and the rate
decreased at PPFD > 270 mol·m–2·s–1.
Light intensity at th e saturated photosynthetic rate was
200 - 300 mol·m–2·s–1 (Figure 1) and the value was
quite similar to the reported values [30-32]. Many of the
respons e cur ves show ed a de cre ase in pho tosyn the tic ra te
when PPFD exceeded 300 mol·m–2·s–1. Thus, neither
the hyperbolic response curve nor the non-rectangular
hyperbola model [33] was applicable for the regression
of the PPFD-photosynthesis response curve obtained
here. At first we applied the non-rectangular hyperbola
model excluding the experimental data for higher PPFDs
than that at which the maximum photosynthetic rate was
obtained. The non-rectangular hyperbola model is re-
presented by the formula:


 
2
smaxds
maxdd s
PP IRP
IP1RRP0




 
where Ps is the net CO2 assimilation rate (mol· C O 2·m–2·s–1),
I is the PPFD (mol-photon·m–2·s–1), Rd is the rate of
dark respiration expressed as CO2 production (mol-
CO2·m–2·s–1), Pmax is the light-saturated rate of gross CO2
assimilation (mol - C O 2·m–2·s–1), and
 and  are con-
stants determining curvature at the shoulder and apparent
quantum efficiency (initial slope at zero irradiance of the
light-photosynthetic rate relation ship curve), respectively.
Rd was obtained from the experimental data, and the
light response curve of the gross pho tosynthetic rate was
obtained by substitu ting Rd = 0 in the equation.
Maximum photosynthetic rates (Pmax) estimated by the
non-rectangular hyperbola model (excluding data for
higher PPFDs if the PPFD-photosynthesis curves showed
a decreasing tendency at higher PPFDs) were obtained at
T = 35˚C for S. palustre, S. fuscum and S. papillosum,
and at T = 30˚C for S. fimbriatum and S. fallax (Figure
2(a)). Although temperature dependence for Pmax was
significant (p < 0.05) only for S. palustre by the Kruskal-
Wallis test, Pmax at T = 40˚C was 70% - 85 % of Pmax at T
= 35˚C for northern species (S. fuscum, S. fallax and S.

Temperature Dependency of Photosynthesis of Sphagnum spp. Distributed in the Warm-Temperate and the
Cool-Temperate Mires of Japan
720
Figure 1. Relative gross photosynthetic rate (% per maximum) and PPFD relationship curve of (a) Sphagnum fimbriatum; (b)
S. palustre (from the Tadewara mire in northern Kyushu, Japan); (c) S. fuscum; (d) S. papillosum; (e) S. fallax (from the East
Ochiishi mire in north-eastern Hokkaido, Japan) at temperatures of 5˚C, 10˚C, 15˚C, 20˚C, 25˚C, 30˚C, 35˚C, and 40˚C.

Temperature Dependency of Photosynthesis of Sphagnum spp. Distributed in the Warm-Temperate and the 721
Cool-Temperate Mires of Japan
Figure 2. (a) Temperature dependence of gross photosynthetic rate (Pmax), (b) dark respiration (Rd) and (c) apparent quan-
tum efficiency (slope of PPFD-photosynthesis response curve at 0 irradiation; (
 ) of Sphagnum fimbriatum (●), S. palustre
(○), S. fuscum (■), S. papillosum (□), and S. fallax (▲). Pmax and  were estimated using a non-rectangular hyperbola
model excluding data for higher irradiation if PPFD-photosynthetic rate curves showed decreasing tendencies at higher irra-
diation. Rd was measured direc tly.
papillosu m), whereas the ratio was 30% - 40% for
southern species (S. fimbriatum and S. palustre). Relative
gross Pmax (% per maximum) determined by using the
data for relative photosynthetic rate within the same
plant samples presented in Figure 1 showed significant
temperature dependence for every species: S. fimbriatum
(p < 0.01), S. palustre (p < 0.01), S. fuscum (p < 0.01), S.
papillosu m (p < 0.01) and S. fallax (p < 0.05) by the
Kruskal-Wallis test.
Dark respiration (Rd) for S. fimbriatum increased over
T = 30˚C and showed a maximum at T = 40˚C (Figure
2(b)). S. fallax showed a tendency similar to that of S.
fimbriatum and Rd increased over T = 30˚C and showed
a maximum at T = 40˚C. Rd for S. fuscum and S. papil-
losum showed a maximum around T = 15˚C - 20˚C, al-
though the temperature dependence was not significant.
S. palustre showed a maximum Rd at T = 30˚C, but the
extremely high Rd value for S. palustre could be due to
some error in measurement. Temperature dependence for
Rd was significant (p < 0.05) only for S. palustre.
Apparent quantum efficiency (  ) did not show signi-
ficant temperature dependence (p > 0.05) for any of the
species (Figure 2(c)). However, S. palustre and S. fus-
cum showed extremely high values at T = 40˚C. Slope of

Temperature Dependency of Photosynthesis of Sphagnum spp. Distributed in the Warm-Temperate and the 722 Cool-Temperate Mires of Japan
the PPFD-photosynthetic rate response curve at zero ir-
radiance determined by using the relative photosynthetic
rate (% per maximum) that corresponds to  also showed
no significant temperature dependence (p > 0.05) for any
of the species.
After the estimation of photosynthetic parameters us-
ing the non-rectangular hyperbola model, a fourth func-
tion equation was applied for the experimental data, in-
cluding all the data at higher PPFDs. The equation pro-
vided the best fit for most of the PPFD-photosynthesis
response curves which showed a decline of photosyn-
thetic rate at higher PPFDs. Then we used the fourth
function equation to obtain an estimated value. However,
this equation does not have any physiological meaning
for describing the regression of photosynthetic rates in
relation to PPFDs.
Photosynthetic rates for the five Sphagnum species
distributed in the Tadewara mire and the East Ochiishi
mire showed obvious temperature dependence (Figure
3). Fits for the photosynth esis response cu rves in relation
to PPFD were found using the fourth function equation
and were used to estimate the rates at low light intensity
(PPFD = 150 mol· m–2·s–1) and high light intensity
(PPFD = 500 mol· m–2·s–1). S. fimbriatum, S. palustre
and S. fuscum showed significant differences in photo-
synthetic rates among various temperature conditions by
the Kruskal-Wallis test, both at the low and high light
intensities (p < 0.05 for S. palustre at PPFD = 150
mol·m–2·s–1, p < 0.001 for S. fuscum at PPFD = 500
mol·m–2·s–1, p < 0.01 for others). S. fallax showed signi-
ficant differences in photosynthetic rates among various
temperature conditions at PPFD = 500 mol·m–2·s–1 (p <
0.05; Figure 4). S. papillosum did not show significant
dependence between its photosynthetic rate and the vary-
ing temperature conditions (p > 0.05).
Although temperature dependence at optimal PPFDs
that showed maximum rates for gross photosynthesis at
each temperature was significant only for S. palustre (p <
0.05), PPFD at Pmax tended to increase with increasing
temperature. S. palustre showed the lowest value at T =
40˚C.
4. Discussion
The temperature dependence of photosynthetic rate was
observed for every species of Sphagnum studied here.
Apparent quantum efficiency (
 ) did not show signifi-
cant temperature dependence (Figure 2(c)), and hence
biochemical reaction is the rate-determining reaction de-
pending on temperature. Temperature dependence levels
in relation to the photosynthetic rate were quite similar
for all the species at a temperature of T < 25˚C, and the
Q10 value from T = 10˚C to 20˚C was 1.70 - 2.47 (Figure
2(a)).
Temperature dependence of the photosynthetic rates at
high temperature (T > 25˚C) showed differences among
the species. S. fimbriatum and S. palustre from the Ta-
dewara mire showed significant decreases of photosyn-
thetic rate at T = 40˚C compared to the results for T =
35˚C and the rates of S. fimbriatum and S. palustre at T =
40˚C were 36% and 43% of those at T = 35˚C, respec-
tively, whereas the rates at T = 40˚C for S. fuscum, S.
papillosum and S. fallax from the East Ochiishi mire
were slightly lower than the rates at T = 35˚C. The rates
for S. fuscum, S. papillosum, and S. fallax at T = 40˚C
Figure 3. Temperature dependence of gross photosynthetic rate (Pmax) of Sphagnum fimbriatum (●), S. palustre (○), S. fus-
cum (■), S. papillosum (□), and S. fallax (▲) at (a) PPFD = 150 mol·m–2·s–1 and (b) PPFD = 500 mol·m–2·s–1 estimated
based on regression using a fourth function e quation.

Temperature Dependency of Photosynthesis of Sphagnum spp. Distributed in the Warm-Temperate and the 723
Cool-Temperate Mires of Japan
Figure 4. Optimal PPFDs showing maximum rates of pho-
tosynthesis in PPFD-gross photosynthesis response curves
measured at each temperature. Species are Sphagnum fim-
briatum (●), S. palustre (○), S. fuscum (■), S. papillosum
(□), and S. fallax (▲).
were 69%, 68% and 84% of those obtained at T = 35˚C,
respectively.
Thus we found that the ph otosynthetic rates of the two
species from the Tadewara mire decreased when tem-
perature exceeded 35˚C, whereas the rates of the three
species from the East Ochiishi mire did not decrease
when the temperature was 40˚C.
Especially under low temperature conditions, the pho-
tosynthetic rates for Sphagnum spp. under conditions of
high PPFD were lower than rates obtained under low
PPFD conditions. This shows that photosynthesis was
inhibited by high PPFD in low temperature environments.
Inhibition of photosynthesis by high PPFD levels has
been reported for some species, e.g., tomato, Nicotiana
tabacum, and Camellia sinensis at T = 0˚C - 15˚C [34-37].
Most of these reports are for vascular plants sensitive to
low temperature stress. Our observations showed that
inhibition of photosynthesis by high PPFDs under low
temperature conditions was evident for Sphagnum spp.
whose distribution was mostly in the circumpolar region.
Apparent quantum efficiencies (
 ) were not signifi-
cantly different among the various temperature condi-
tions created for each of the species. This implies that
photoreaction in Sphagnum plants is not significantly
affected by temperature.
Dark respiration (Rd) commonly shows a tendency to
increase with increasing temperature, and the net photo-
synthetic rate and the consequent net primary production
showed maximum levels at temperatures lower than the
optimum temperature for gross photosynthetic rate.
However, Rd in our measurements of Sphagnum spp. did
not significantly increase up to T = 40˚C. Respiration in
Sphagnum plants showed an adaptive trend as photosyn-
thetic capacity at higher temperatures.
Although Sphagnum species dominate in mires in low
temperature regions and these species are the successful
plants in low temperature environments, the physiologi-
cal properties of Sphagnum may not have fully adapted
to low temperature conditions. Considering the fact that
Sphagnum spp. distribute also in tropical lowland areas
with high temperatures [38], Sphagnum species may
have potential for distribution in tropical regio ns.
5. Acknowledgements
Authors thank to the member of the University of Kita-
kyushu for their assistance in field study. This study was
partly funded by the Seven-Eleven Midorinokikin Fund,
JSPS Grant-in-Aid for Scientific Research 18208019 &
20248033, and the River Fund in Charge of the Founda-
tion of River and Watershed Environment Management,
Japan.
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