Botanical Studies (2008) 49: 225-233.
*
Corresponding author: E-mail: sbzhang@mail.kib.ac.cn;
Tel: 86-0871-5223002; Fax: 86-0871-5223005.
INTRODUCTION
The majority of the 49 species in the genus Meconopsis
grow at high elevations (2,135-5,795 m) in the Himalayas
and other mountains in western China. Only M. cambrica
can be found in Europe (Chuang, 1981). As famous
horticultural plants bearing large and beautiful flowers,
Meconopsis have attracted the attention of botanists. Some
Meconopsis species can be used as traditional herbal
medicine, for they possess anti-inflammatory and analgesic
activities (Samant et al., 2005). Several members of the
genus have been cultivated over 100 years, but cultivating
Meconopsis is not an easy task because of the poor
performance at lower altitude, especially in summer (Ren,
1993; Still et al., 2003). In addition, habitat destruction
has increasingly threatened these valuable gene pools,
which are now limited to a narrow range of distribution
(Sulaiman and Babu, 1996).
Empirical observations suggest that high temperature
during the growing season is an important determinant
limiting the growth and development of Meconopsis
(Norton and Qu, 1987; Ren, 1993). However, the
adaptation of Meconopsis to temperature is significantly
different across species (Ren, 1993). Both M. punicea
and M. betonicifolia grown in colder temperatures have
a larger dry weight and flower size than those grown
in warmer conditions (Still et al., 2003). Meconopsis
integrifolia can flower in its native habitat even in the
snow. This remarkable tolerance for low temperatures
would lead to poor adaptation in warm temperatures. The
growth and survival of plants can be determined by the
Photosynthetic adaptation of Meconopsis integrifolia
Franch. and M. horridula var. racemosa Prain
Shi-Bao ZHANG* and Hong HU
Kunming Institute of Botany, Chinese Academy of Sciences, Kunming 650204, P. R. China
(Received May 29, 2007; Accepted March 5, 2008)
ABSTRACT.
Both Meconopsis integrifolia Franch. and M. horridula var. racemosa Maxim. are native
to the Himalayas and prized as ornamentals and medicinal plants. Cultivating Meconopsis is difficult at
lower altitudes owing to its intolerance to hot summers. To develop a cultivation strategy and predict plant
performance for introduction, we compared the photosynthetic capacity of M. integrifolia and M. horridula
as well as their photosynthetic responses to light and temperature in the nursery at an altitude of 3,260 m.
Meconopsis integrifolia was more sensitive to high temperature than M. horridula while M. horridula reached
a peak photosynthetic rate at a higher light level than M. integrifolia. Compared with M. integrifolia, M.
horridula showed a higher light saturated photosynthetic rate, maximum RuBP saturated rate of carboxylation,
light saturated rate of electron transport, stomatal conductance, leaf dry mass, and N content per unit area. The
mesophyll conductance and leaf N content per unit mass of the two species were not significantly different.
The differences in photosynthetic capacity between two Meconopsis species were correlated with their
biochemical efficiency and leaf thickness, but not chlorophyll content or mesophyll conductance. The results
suggest that, at lower altitudes, introducting and cultivating M. horridula could be easier owing to its wider
physiological adaptation.
Keywords: Chlorophyll fluorescence; Leaf traits; Meconopsis; Photosynthesis; Physiological adaptation.
Abbreviations: AQE, apparent CO
2
quantum efficiency (mol CO
2
mol photons
-1
); Chl, chlorophyll content
per unit area (mg dm
-2
); F
v
/F
m
, potential quantum yield of PSII; LMA, leaf dry mass per unit area (g m
-2
); N
a
,
nitrogen content per unit area (g m
-2
); N
m
, nitrogen content per unit mass (mg g
-1
); T
opt
, optimal temperature
for photosynthesis; PPFD, photosynthetic photon flux density (£gmol m
-2
s
-1
); £pPSII, effective quantum yield
of PSII; F
v
¡¦/F
m
¡¦, efficiency of excitation energy capture by open reaction centre; ETR, apparent rate of elec-
tron transport of PSII (£gmol m
-2
s
-1
); qP, photochemical quenching; NPQ, non-photochemical quenching; P
N
,
photosynthetic rate (£gmol m
-2
s
-1
); P
Nmax
, light-saturated P
N
(£gmol m
-2
s
-1
); g
m
, mesophyll conductance (mol
m
-2
s
-1
); g
s
, stomatal conductance (mol m
-2
s
-1
); J
max
, light saturated rate of electron transport (£gmol m
-2
s
-1
);
V
cmax
, maximum RuBP saturated rate of carboxylation (£gmol m
-2
s
-1
).
PHYSIOLOGY
pg_0002
226
Botanical Studies, Vol. 49, 2008
thermo-tolerance capabilities of photosynthesis because
photosynthetic traits govern carbon acquisition (Sharkey,
2000; Kuo-Huang et al., 2007). High temperature has
been shown to alter thylakoid membrane structure,
decrease Rubisco activity and RuBP regeneration capacity,
perturb photosynthetic electron transport, increase dark
respiration and photorespiration, and sequentially affect
carbon assimilation (Sharkey, 2000; Wise et al., 2004).
Photosynthetic apparatuses can be protected against heat
damage by dissipating excessive light energy (Hamerlynck
and Knapp, 1996). However, the protective mechanisms
are not uniform among plant species, and this affects their
acclimation to a new environment (Streb et al., 1998).
Because of their dependency on the availability of solar
radiation, plants often show various adaptive strategies
to maximize photosynthetic efficiency, depending on
their light environments (Givnish, 1988). Insufficient
light may reduce carbon gain and growth of plant by
limiting photosynthesis. Conversely, high light levels may
damage photosynthetic apparatuses (Hjelm and Ogren,
2004). From our field observation, M. integrifolia under
full sunlight grows poorly when compared to nearby M.
horridula. Intensive light appears to accelerate leaf aging
in M. integrifolia. Obviously, these two Meconopsis
species have divergent adaptation to light, but the nature
of this adaptation has not been studied.
After transplanting from natural habitat to nursery,
plants may be exposed to uncomfortable environments
(Zhang et al., 2005). The growth of plant in the altered
environment depends on the physiological tolerance
and genetic differentiation since almost all dry weight
accumulation is the result of photosynthetic carbon
fixation (Wu and Campbell, 2006). Photosynthetic
responses to light and temperature have been used to
predict plant performance and physiological tolerances
to environmental conditions and select growth conditions
suitable for different species. Chlorophyll fluorescence
measurement is used to assess heat-induced alteration
of thylakoid membranes and thermal damage to PSII
(Hamerlynck and Knapp, 1996). To date, however, only
limited studies on the cultivation and photosynthesis of
Meconopsis have appeared (Ren, 1993; Still et al., 2003).
Our experiment was designed to compare the
photosynthetic capacity and related leaf traits of
M. integrifolia and M. horridula, as well as their
photosynthetic responses to light, temperature, and CO
2
concentration in the nursery at an altitude of 3,260 m.
Meconopsis integrifolia (Maxim.) Fr. is a perennial
herb found in rocky or grassy places in sparse woods at
altitudes of 3,300-5,100 m in western China and Burma.
The leaves and stems are covered with dense hairs. Most
plants reach about 50 cm in height. Typically, the basal
rosette-leaves may grow to 15-32 cm long and up to 5 cm
wide while the upper ones are 5-11 cm long and 0.5-1.0
cm wide. This species usually bears 4-5 yellow flowers
from May to June, and the fruits mature from June to
August. Meconopsis horridula var. racemosa (Maxim.)
Prain is also a perennial herb and has sharp spines on the
leaves and stems. The stem is branched with many blue
or purple flowers. This species occurs on rocky slopes at
altitudes of 3,000-4,900 m in southwestern China. The
flowering period is June to July, and fruit-setting period
July to September. The main goals of this study were to
determine: (1) how photosynthetic light and temperature
responses differ between the two species, and (2) whether
Meconopsis species differed in their biochemical capacity
for photosynthesis.
MATERIALS AND METHODS
The study was conducted in Zhongdian Experimental
Station of Alpine Flower at an altitude of 3,260 m
(atmospheric pressure = 69 kPa) in southwest China. For
May to September (growing period), mean air temperature
and precipitation were 12.3 ¡Ó 2.1¢XC (mean ¡Ó SD) and 430
¡Ó 55 mm in 2005, respectively. Relative air humidity from
May to September averaged 78.2% during the 1958-2000
period (data obtained from Meteorological Station of
Zhongdian County).
The seeds of M. horridula var. racemosa were collected
from Hong Mountain in southwestern China at an
altitude of 3,900 m in August 2003 while the seeds of M.
integrifolia came from Big Snow Mountain at an altitude
of 4,200 m in September 2003. The seeds of two species
were sown in the nursery in March 2004. The seedlings
of both species were grown under full sunlight, fertilized
monthly with a liquid nutrient solution during the summer,
and watered every 2-3 days as needed. The plants typically
took 2 years to reach the flowering stage.
Gas exchange and chlorophyll fluorescence in response
to different environmental conditions were measured on
fully expanded leaves using a portable photosynthesis
system (LI-6400, LI-COR, Lincoln, NE, USA) equipped
with a fluorescence chamber (LI-6400-40) in June 3-16
(flowering period), 2005. Before measurement, the leaf
was adapted to the dark for more than 10 h. After the
minimum fluorescence (F
o
) was determined by a weak
modulated light, a 0.8 s saturating light was used on the
dark-adapted leaf to determine the maximum fluorescence
(F
m
). The leaf was then illuminated by an actinic light of
1200 £gmol m
-2
s
-1
(10% blue, 90% red) for 15 min. Then
the maximum chlorophyll fluorescence in the light (F
m
¡¦)
was determined by applying an 0.8 s saturating light pulse.
After the actinic light had been switched off, a far-red light
was exerted to determine the minimal level of fluorescence
(F
o
¡¦). The photosynthetic light response curves (P
N
-PPFD)
were made using an automated protocol built into LI-6400.
The program was configured to advance to the next step
if the sum of three coefficients of variation (CO
2
, water
vapor, and flow rate) was less than 0.3%, with a minimum
waiting time of 3 min. Each leaf was equilibrated to initial
conditions by waiting at least 15 min before executing
the automated protocol. P
N
-PPFD curves of three leaves
were measured at the light intensities of 2000, 1600, 1200,
pg_0003
ZHANG and HU ¡X Photosynthesis of
Meconopsis
227
1000, 800, 600, 400, 300, 200, 100, 50 and 0 £gmol m
-2
s
-1
under controlled levels of CO
2
(350 £gPa Pa
-1
), flow rate
(500 £gmol s
-1
), leaf temperature (20¢XC), and vapor pressure
deficit (1.0-1.5 kPa).
Photosynthetic CO
2
response curves (P
N
-C
i
) and P
N
-
PPFD curves were determined with the same leaves. After
completion of P
N
-PPFD curve measurement, the leaf was
induced at 1200 £gmol m
-2
s
-1
light intensity and 350 £gPa
Pa
-1
CO
2
concentration for 15 min. Photosynthetic rates
versus CO
2
response curves together with chlorophyll
fluorescence were measured at a leaf temperature of 20¢XC,
PPFD of 1200 £gmol m
-2
s
-1
, and vapor pressure deficit of
1.0-1.5 kPa. P
N
-C
i
response measurements were started
at ambient CO
2
concentration, decreased gradually to
0 £gPa Pa
-1
, returned to ambient CO
2
concentration, and
then increased to a higher concentration to ensure that
the stomata stayed open throughout the measurement.
The photosynthetic rate and chlorophyll fluorescence
were measured at different CO
2
concentrations using the
automated protocol built into LI-6400.
The temperature responses of photosynthesis and
chlorophyll fluorescence were measured between 10 and
35¢XC at PPFD of 1200 £gmol m
-2
s
-1
, CO
2
concentration of
350 £gPa Pa
-1
and vapor pressure deficit of 1.0-1.5 kPa.
Leaf temperature was adjusted to the desired level using
the internal heating/cooling system of the analyzer. Each
sample was first dark-adapted for 10 h. Thereafter, the
weak measuring light beam was switched on to determine
the minimum fluorescence yield F
o
. Subsequently, the
maximum fluorescence yield F
m
was determined by
applying a 0.8s saturating light pulse. The light source
of gas analyzer was then switched on and the sample
was illuminated at PPFD of 1200 £gmol photons m
-2
s
-1
.
Leaf temperature was adjusted to 10
o
C until steady-
state photosynthesis was achieved. The values of gas
exchange and chlorophyll fluorescence were first recorded
immediately. In the experiment with gradually increased
temperature, leaf temperature was first elevated from 10¢XC
to the next higher selected temperature at a 5¢XC increment
and maintained constant for 10 min. The same procedure
was repeated for each of six tested leaf temperatures in
the ranges from 10 to 35¢XC, with the same leaf being
used throughout the entire measurement. The optimal
temperature for photosynthesis (T
opt
) was calculated from
the polynomial curve fitted to the temperature response
data.
For the recovery experiment, the sample leaf was
adapted in the dark at 20¢XC for 30 min before F
o
and
F
m
were measured. After the leaf was illuminated at
2000 £gmol m
-2
s
-1
and 20¢XC for 30 min, the light source was
switched off, and the values of F
o
and F
m
were measured
immediately. Then the leaf was recovered in the dark at 20
¢XC, and the values of F
o
and F
m
were recorded after 10, 15,
20, 25, 30, 35, 40 and 60 min of recovery, respectively.
P
N
-PPFD curves were fitted by a non-rectangular
hyperbola. Light saturated photosynthetic rate (P
Nmax
), dark
respiration (R
d
) and apparent CO
2
quantum yield (AQE)
were determined for each leaf using Photosyn Assistant
v.1.1 (Dundee Scientific, Scotland, UK), which follows the
method of Prioul and Chartier (1977).
The mesophyll conductance from the sub-stomatal
cavity to chloroplasts (g
m
) was estimated according to the
method of Harley et al. (1992) as,

;
=

d
N
d
N
i
N
m
R
P
4
ETR
R
P
8
ETR
*
A
C
P
g

(1)
Where the rate of respiration (R
d
) was calculated from P
N
-
PPFD curve in the same leaf. .* was the hypothetical CO
2
compensation point in the absence of R
d
. The values of
.* at 20¢XC was derived from the value at 25¢XC according
to Bernacchi et al. (2001). g
m
was calculated from
photosynthetic rate at C
i
100-350 £gPa Pa
-1
, and the average
value of g
m
was determined for each leaf. Over this C
i
range, the values of g
m
are stable (Niinemets et al., 2005).
The chloroplast CO
2
concentration (C
c
) was calculated
from Equation 2 (Bernacchi et al., 2002).
m
N
i
c
/g
P
C
C

(2)
Then using chloroplast CO
2
concentration (C
c
) instead
of intercellular CO
2
concentration (C
i
), the maximum car-
boxylation rate by Rubisco (V
cmax
) and light-saturated elec-
tron transport (J
max
) were calculated from P
N
-C
c
response
curves using by Photosyn Assistant software based on the
photosynthetic model of von Caemmerer and Farquhar
(1981). The values of C
c
were adjusted according to the
atmospheric pressure at study site when the photosynthetic
parameters were calculated. As the software allows us
to enter the values of the Michaelis-Menten constant for
RuBP carboxylation (K
c
), RuBP carboxylation (K
o
), and
Rubisco specificity factor (£n), the values of K
c
, K
o
and £n
derived from the literature of Bernacchi et al. (2001) were
used to calculate V
cmax
and J
max
.
The chlorophyll fluorescence parameters were calcu-
lated as: (1) potential quantum yield of PSII: F
v
/F
m
= (F
m
-
F
o
)/ F
m
; (2) effective quantum yield of PSII: £p PSII = (F
m
¡¦
- F
s
)/ F
m
¡¦; (3) efficiency of excitation energy capture by
open reaction centre: F
v
¡¦/F
m
¡¦ = (F
m
¡¦ - F
o
¡¦)/ F
m
¡¦; (4) appar-
ent rate of electron transport of PSII: ETR = 0.5£pPSIIQ
abs
,
where 0.5 was a factor assuming an equal distribution of
absorbed photons between PSI and PSII for C
3
plants.
Q
abs
was the absorbed light energy that was calculated as
PPFD * leaf absorbance, and leaf absorbance was taken as
0.85; (5) photochemical quenching: qP = (F
m
¡¦ - F
s
) / (F
m
¡¦ -
F
o
¡¦); (6) non-photochemical quenching: NPQ = (F
m
- F
m
¡¦)
/ F
m
¡¦.
The leaves from the sampled plants previously used in
photosynthetic measurements were harvested. Leaf areas
were measured using a leaf area meter (LI-3000A, LI-
COR, Lincoln, NE, USA). Dry mass was determined after
drying for 48 h at 70¢XC. Then, leaf N content was analyzed
using a LecoFP-428 N analyzer (Leco Corporation, MI,
USA). LMA was calculated as leaf dry mass per unit area.
pg_0004
228
Botanical Studies, Vol. 49, 2008
Chlorophyll was extracted using the technique of Moran
and Porath (1980). Chlorophyll content was analyzed
with a spectrophotometer (UV-2550, Shimadzu, Japan)
and calculated using equations developed by Inskeep and
Bloom (1985).
Statistical analysis was performed using SPSS
12.0 (SPSS Inc., Chicago, USA). The differences in
photosynthetic parameter and leaf trait among species
were tested using an independent t-test. The relationships
between photosynthetic parameters and leaf traits were
assessed using linear regression analysis.
RESULTS
Photosynthetic rate increased with increasing PPFD
(Figure 1). The maximum photosynthetic rate was obtained
at PPFD of 1040 £gmol m
-2
s
-1
for M. integrifolia and 1350
£gmol m
-2
s
-1
for M. horridula. The photosynthetic light
saturation level of M. integrifolia was lower than that of
M. horridula (t=5.063, p=0.007). This could be confirmed
by chlorophyll fluorescence. For both species, the values
of F
v
¡¦/F
m
¡¦, £pPSII and qP decreased with increasing PPFD.
At low irradiance, the values of qP, £pPSII and ETR
differed little, but above 400 £gmol m
-2
s
-1
the values for
M. horridula were higher than those of M. integrifolia.
The electron transport rate of M. integrifolia reached the
maximal at 800 £gmol m
-2
s
-1
PPFD, with M. horridula at
1600 £gmol m
-2
s
-1
PPFD. From 0 to 2000 £gmol m
-2
s
-1
, the
NPQ of M. horridula increased at nearly all times while
the NPQ of M. integrifolia did not increase above the
PPFD of 1200 £gmol m
-2
s
-1
.
Dark acclimated value of F
v
/F
m
reflects the potential
quantum efficiency of PSII and can be used as a sensitive
indicator of photosynthetic performance. At ambient
condition, the F
v
/F
m
values of both species were not
significantly different (Figure 2). Howeve, after exposure
to 2000 £gmol m
-2
s
-1
PPFD for 30 min, the decrease in
F
v
/F
m
of M. integrifolia was more pronounced than in
M. horridula. The F
v
/F
m
of M. horridula recovered from
photo-inhibition more rapidly than in M. integrifolia in the
dark.
Both M. integrifolia and M. horridula showed an initial
increase in P
N
as leaf temperature climbed above 10¢XC
(Figure 3). The optimal temperature for photosynthesis
(T
opt
) was 23.8¢XC in M. horridula, and 21.7¢XC in M .
integrifolia. Meconopsis horridula had higher T
opt
relative
Figu re 1. Photosynthetic and chlorophyll fluorescence light
response curves for Meconopsis integrifolia (.) and M.
horridula (¡´) at 360 £gPa Pa
-1
CO
2
concentration and 20¢XC leaf
tem perature. Each point represents the mean ¡Ó 1S E of three
measurements.
Figure 2. T h e m a x i m um q u a n t u m yi e l d o f p r i m a r y
photochemistry of PSII in Meconopsis horridula and M.
integrifolia monitored during the recovery period following a
30-min light stress treatment at 2000 £gmol m
-2
s
-1
PPFD. Each
point represents the mean ¡Ó 1SE of three measurements.
Figure 3. E ffec ts of te mp era ture on pho tos ynt hes is a nd
fluores cenc e param eters of M econops is hor ri dul a and M.
integrifolia. The measurements were conducted at a 1200 £gmol
m
-2
s
-1
light intensity and a 350 £gPa Pa
-1
CO
2
concentration. Each
point represents mean ¡Ó 1SE of three measurements.
pg_0005
ZHANG and HU ¡X Photosynthesis of
Meconopsis
229
t o M. integrifolia (t =3.263, p=0.0282). The P
N
at 10¢XC
for M. integrifolia was 75.2% of the maximal value while
for M. hurridula the P
N
at 10¢XC was 62.4% of maximum.
At 35¢XC, the P
N
of M. integrifolia decreased to 63.4% of
maximal value, and M. horridula to 70.8%. The maximal
value of stomatal conductance (g
s
) was found at 20¢XC in M.
horridula and at 15¢XC in M. integrifolia. For both species,
the g
s
did not change significantly below 25¢XC, but it
decreased above 25¢XC. The g
s
at 35¢XC for M. integrifolia
was 54.3% of the maximal value while the g
s
at 35¢XC was
61.1% of the maximum in M. horridula. Although the
two species showed the maximal values of F
v
¡¦/F
m
¡¦, £pPSII,
qP and ETR about 20¢XC, the photochemical efficiency of
M. horridula was higher than that of M. integrifolia. The
lowest NPQ was found at 20¢XC for M. integrifolia, but at
25¢XC for M. horridula.
The photosynthetic CO
2
response curves indicated that
M. horridula had a higher biochemical capacity (including
V
cmax
and J
max
) for photosynthesis than M. integrifolia
(Figure 4). At a C
i
concentration of 520 £gPa Pa
-1
, t he
values of F
v
¡¦/F
m
¡¦, £pPSII, and ETR in M. integrifolia
peaked and remained stable above this concentration while
the values of F
v
¡¦/F
m
¡¦, £pPSII, and ETR in M. horridula
peaked at a C
i
concentration of 400 £gPa Pa
-1
and decreased
slightly above this C
i
concentration. The qP value of M.
horridula peaked at a lower C
i
concentration than M.
integrifolia. At a high C
i
concentration, the NPQ values
of the two species were similar, but that of M. integrifolia
decreased from low C
i
to high C
i
concentration, the NPQ
of M. horridula decreased to the lowest from 0 to 400 £gPa
Pa
-1
and then increased to 1600 £gPa Pa
-1
.
At saturating light, the P
N
of M. horridula was
higher than that of M. integrifolia. The higher P
Nmax
i n M. horridula was supported by greater V
cmax
and
J
max
. Meconopsis horridula had higher g
s
value than
M. integrifolia, but the g
m
of the two species was not
significantly different. N content per leaf mass of two
species revealed no significant differences while N
content per leaf area in M. horridula exceeded that in M.
integrifolia owing to the higher dry mass per unit area.
The chlorophyll content per unit area in M. horridula was
lower relative to that in M. integrifolia (Table 1).
DISCUSSION
Differences in photosynthetic performance and
adaptation are both genetically determined and
environmentally induced (Hamerlynck and Knapp, 1996).
Meconopsis horridula showed higher light requirements
and could recover from high-light inhibition more rapidly
than M. integrifolia. The higher qP of M. horridula under
high PPFD conditions indicated that its photo-protective
Figure 4. Photosynthesis and fluores cence parameters of M.
horridula and M. integrifolia in response to intercellular CO
2
concentration at 1200 £gmol m
-2
s
-1
light intensity and 20¢XC leaf
temperature. Each data point represents mean ¡Ó SE of three
measurements.
Table 1. Comparison of photosynthetic parameters and related leaf traits of Meconospsis integrifolia and M. horridula. Different
letters indicate mean statistically different values p<0.05 as determined by t-test between species.
Parameters
integrifolia
horridula
t
p
P
Nmax
(£gmol m
-2
s
-1
)
15.07¡Ó1.73
a
23.40¡Ó1.42
b
3.721
0.020
AQE (mol CO
2
mol photons
-1
)
0.051¡Ó0.002
a
0.065¡Ó0.010
b
3.066
0.037
V
cmax
(£gmol m
-2
s
-1
)
51.67¡Ó2.33
a
113.33¡Ó15.32
b
3.979
0.016
J
max
(£gmol m
-2
s
-1
)
128.00¡Ó10.41
a
228.67¡Ó5.24
b
8.639
0.001
g
s
(mol m
-2
s
-1
)
0.154¡Ó0.004
a
0.196¡Ó0.007
b
3.979
0.016
g
m
(mol m
-2
s
-1
)
0.229¡Ó0.052
a
0.209¡Ó0.057
a
0.268
0.802
N
m
(mg g
-1
)
23.73¡Ó0.87
a
26.18¡Ó1.01
a
1.839
0.140
N
a
(g m
-2
)
1.98¡Ó0.09
a
4.62¡Ó0.23
b
10.927
0.000
Chl (mg dm
-2
)
3.951¡Ó0.118
a
2.647¡Ó0.069
b
9.539
0.001
LMA (g m
-2
)
83.25¡Ó2.67
a
176.49¡Ó5.13
b
16.126
0.000
pg_0006
230
Botanical Studies, Vol. 49, 2008
strategy seemed more efficient. A major contributor to
the protection of the photosynthetic apparatus in plants
growing in excess excitation energy is an increased ability
to dissipate this energy via non-photochemical quenching
such as the xanthophylls cycle, which is one of the
mechanisms protecting chloroplasts from excess PPFD
(Hjelm and Ogren, 2004). The NPQ of M. integrifolia
increased rapidly below 1000 £gmol m
-2
s
-1
and remained
relatively constant, indicating that the lower NPQ would
limit the photosynthetic performance under high light
condition. This difference could be explained by the sharp
spines on the stem and leaves of M. horridula, since
their presence would reduce the absorbed light, thereby
increasing the adaptation to high light. In addition, the
lower chlorophyll content in M. horridula would reduce
photon capture and avoid photo-damage by excess energy
under high PPFD conditions.
Plants from habitats that differ in temperature regimes
had different temperature response characteristics; such
that the photosynthetic rate at temperatures close to their
normal growth temperatures may be maximized (Cabrera
et al., 1998). In the present study, M. horridula h ad a
higher T
opt
than M. integrifolia. The P
N
of the former at
high temperature was significantly higher than that of the
latter. The difference in T
opt
can reflect the influence of
more hairs on the stems and leaves (Tsukaya and Tsuge,
2001). Across temperatures, stomatal conductance and
photosynthetic rate in M. horridula (r
2
=0.228, p=0.045)
and M. integrifolia (r
2
=0.236, p=0.041) enjoyed a close
relationship. In particular, the reduced g
s
above 25
o
C
would exert an important effect on the inhibition of
photosynthesis. NPQ is thought to be a good indicator of
the concentration of dissipating complexes and the ability
of plants to dissipate light energy in excess of that required
for CO
2
assimilation (D¡¦Ambrosio et al., 2006). Stress
conditions such as high light or unfavorable temperatures
markedly promote non-photochemical quenching. At
moderate light intensity, non-photochemical quenching
is dependent on the temperature (Bilger and Bjorkman,
1991). The variation in NPQ of M. integrifolia and M.
horridula confirmed previous findings. The values of NPQ
in the two Meconopsis species were increased to avoid
photo-damage when exposed to unfavorable temperatures.
Acclimating to different temperatures affected the growth
of Meconopsis plants after growing at the lower altitude.
Although the growth of M. integrifolia and M. horridula
is limited by high summer temperatures (Ren, 1993), M.
horridula had higher photosynthesis at high temperatures
and might be better at tolerating them.
The electron transport rate (ETR) of the two
Meconopsis species increased with increasing C
i
, bu t
the ETR of M. horridula above saturated C
i
decreased
slightly, while M. integrifolia remained constant. This
indicated a triose phosphate utilization (TPU) limitation in
M. horridula, but not in M. integrifolia. Triose phosphate
utilization by end-product (sucrose, starch) synthesis may
exert short-term feedback control of photosynthesis in the
field at the extreme of source/sink imbalance before long-
term adaptive mechanisms re-establish greater equilibrium.
Hence the plants grown with CO
2
enrichment tend towards
phosphate (P
i
) limitation (Paul and Foyer, 2001). Low
P
i
reduces the capacity of the photosynthetic carbon
reduction cycle to use nicotinamide adenine dinucleotide
phosphate-oxidase (NADPH). Obviously, the electron
transport rate is regulated by feedback (Sharkey, 1990), as
excess electron transport capacity increases vulnerability
to damage due to the formation of active oxygen species,
particularly when exposed to environmental stress.
Anything that restricts triose phosphate utilization can
limit photosynthesis (Paul and Foyer, 2001). Short-term,
low-temperature stress results in an inhibition of sucrose
biosynthesis, which leads to a restriction in phosphate
recycling and photophosphorylation. This is because
the maximum capacity for triose phosphate utilization
is temperature dependent, and chloroplast phosphate
requirements increase at low temperature (Sage and
Sharkey, 1987). Restriction in TPU would trigger feedback
mechanisms that reduce the rate of photosynthetic electron
transport, limit ATP supply, and thus reduce the rate of
photosynthetic carbon assimilation. During the growing
season at high elevation, the plants of Meconopsis
frequently experience low temperatures (often below 10
o
C)
at night, followed by high light and optimal temperatures
during the day. As M. horridula had a weaker tolerance to
low temperature than M. integrifolia, the triose phosphate
utilization and photosynthetic electron transport in M.
horridula would be suppressed by low temperature under
high C
i
conditions.
The variation in NPQ of M. horridula below 800 £gPa
Pa
-1
was larger than that of M. integrifolia, indicating its
greater sensitivity to CO
2
. Golding and Johnson (2003)
suggested that the electron transport is downregulated to
match the reduced requirement for electrons when the CO
2
fixation is inhibited under low C
i
conditions. Induction
of NPQ at low C
i
would quench the excess energy to
minimize reactive oxygen production and alleviate the
excitation pressure on PSII, as the pH gradient is needed
to support the increase in NPQ. In the short-term, the
restriction of P
i
serves to increase the transthylakoid £GpH
gradient, preventing over-reduction of PSI and increasing
energy dissipation (Paul and Foyer, 2001). As stomatal
movement regulated by CO
2
concentration affects the
entry of CO
2
into leaf and consumption of photosynthetic
electron transport, the difference in the response of
chlorophyll fluorescence to CO
2
is linked to stomatal
conductance (Lawson et al., 2002).
Meconopsis horridula exhibited higher photosynthetic
capacity and photochemical efficiency than M. integrifolia
across all CO
2
, light, and temperature levels examined.
The cause of difference in photosynthesis among species
is partly biochemical (Zhang et al., 2006). Leaf N content
and photosynthetic capacity are strongly related because of
the large proportion of leaf N present in the photosynthetic
apparatus (Evans, 1989). Both N content per leaf area
pg_0007
ZHANG and HU ¡X Photosynthesis of
Meconopsis
231
(N
a
) and N content per unit mass (N
m
) were significantly
correlated to light saturated P
N
of two Meconopsis spe-
cies (r
2
=0.860, p=0.008; r
2
=0.878, p=0.006, respectively).
There was a positive correlation between N
a
and V
cmax
(r
2
=0.897, p=0.004) or J
max
(r
2
=0.942, p=0.001). V
cmax
is
related to the content and activity of Rubisco while J
max
is
related to its regeneration. The higher leaf N content in M.
horridula resulted in the higher Rubisco content and P
Nmax
.
The low CO
2
partial pressure at high altitude
would have a negative influence on photosynthetic
CO
2
assimilation because CO
2
is the substrate for the
carboxylation reaction of Rubisco (Korner, 1999). Some
previous works on alpine plants showed that the negative
impact of low CO
2
partial pressure on photosynthesis can
be partly compensated for by the increase in diffusion of
all gas-phase molecules at lower atmospheric pressure, and
thus by the increased diffusion rates of CO
2
into the leaf
(Smith and Donahue, 1991; Terashima et al., 1995). The
photosynthetic capacities of alpine plants are not inferior
to those of their lowland relatives (Korner, 1999; Zhang et
al., 2007). Compared with the results of Shi et al. (2006),
the values of P
Nmax
, V
cmax
and J
max
in M. horridula were
close to those of Buddleja davidii at a similar altitude.
Shi et al. (2006) suggested that photosynthetic capacity
is correlated with CO
2
diffusional conductance, and the
decreasing temperature with increasing altitude affects
CO
2
diffusion and the ratio of C
c
to C
a
.
Most of previous studies have shown that
photosynthetic capacity is directly correlated with stomatal
conductance and mesophyll conductance (Niinemets et al.,
2005; Warren, 2006). Meconopsis horridula had higher
g
s
than M. integrifolia, but the g
m
of the two species was
not significantly different. The difference in P
Nmax
was
positively related to the stomatal conductance (r
2
=0.758,
p =0.024), but not mesophyll conductance (r
2
=0.107,
p =0.528). Thickness of leaf pubescence would increase
leaf boundary layer resistance to CO
2
movement (Meinzer
et al., 1985). Although the longer diffusive path found
in thicker leaves increases resistance and thus reduces
CO
2
concentrations at the sites of carboxylation (Kao and
Chang, 2001), high LMA is often associated with higher
amounts of mesophyll cell, N content, Rubisco content,
and thereby photosynthetic rate (Friend and Woodward,
1990). The LMA of the two Meconopsis species was sig-
nificantly related to P
Nmax
(r
2
=0.770, p=0.002).
In conclusion, M. integrifolia appeared to have a lower
adaptive capacity to grow at lower altitudes because of
fewer effective protective mechanisms against photo-in-
hibition. Meconopsis horridula was proven more resistant
to photo-inhibition as it had more photosynthesis at higher
temperatures and so might be relatively better at tolerating
them. The capability of M. horridula to preserve the
functional potential of the photosynthetic apparatus under
heat stress conditions was probably an important factor in
its capacity to grow and survive warmer habitats.
Acknowledgements. This project is supported by the
National Natural Science Foundation of China (30770226),
the Natural Science Foundation of Yunnan (2006C0043Q),
and the West Light Foundation of Chinese Academy of
Sciences. The authors thank Prof. Chong-Shih Tang at the
University of Hawaii for improving the English.
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