Shau-Lian Wong · Chung-Wei Chen · Meng-Yuan Huang · Jen-Hsien Weng
Relationship between photosynthetic CO
2uptake rate and electron transport rate in two C
4perrenial grasses under different nitrogen fertilization, light and temperature conditions
(pink text: revised) S.-L. Wong · C.-W. Chen
Division of Botany, Endemic Species Research Institute, Nantou, Taiwan
M.-Y. Huang
Biodiversity Research Center, Academia Sinica, Taipei, Taiwan
J.-H. Weng ( )
Graduate Institute of Ecology and Evolutionary Biology, China Medical University, Taichung, Taiwan. e-mail: [email protected]
Tel.: Tel: +886-4-22053366 #8102; Fax: +886-4- 2071507 1
2 3 4 5 6 7 8 9 10 11 12 13 14 15
Abstract We examined the feasibility of using chlorophyll fluorescence to estimate
CO2 exchange (A) of C4 perennial grasses under different environmental as well as physiologic conditions by using Pennisetum purpureum and Miscanthus floridulus, capable of year-round growth, to determine the association of electron transport rate (ETR) and A. The grasses were fertilized with 3 levels of nitrogen, and measurement involved the top 2 fully expanded leaves, with chlorophyll content 0.18–0.55 g m-2. Chlorophyll fluorescence, CO2 and H2O exchange were measured simultaneously at 4 seasonal temperatures (30–15 °C, Sept.–Jan.), 6 levels of photosynthetic photon flux density (PPFD) (0–2,000 mol m–2 s–1) and 2 levels of relative humidity [60% (15–30
°C) and 40% (30 °C alone)]. Variables were recorded when A was stable. Most leaves with high chlorophyll content showed high A at the same PPFD and seasonal temperature. Despite a broad range of A obtained because of both stomatal and non- stomatal factors, ETR was still highly correlated linearly with net photosynthetic rates, when combining data for the same species for analysis. Thus, ETR could be used to assess the dynamic A of C4 perennial species through different seasons, even under varied light intensity, seasonal temperature, humidity, nitrogen fertilization and phenological stage.
Keywords: C4 · electron transport rate · light intensity · nitrogen fertilization · photosynthetic rate · seasonal temperature.
Abbreviations A CO2 fixation α Leaf absorption
Ca Ambient CO2 concentration 16
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Ci Intercellular CO2 concentration ETR Electron transport rate
f The fraction of absorbed quanta used by PSII.
ФCO2 Photosynthetic rate per absorbed quantum ФPSII Photosystem II efficiency
gs Stomatal conductance N Nitrogen
PN Net photosynthetic rate
PPFD Photosynthetic photon flux density PSII Photosystem II
RH Relative humidity.
Introduction
Photosynthesis, one of the major determinants of biomass production and terrestrial carbon budgets, is influenced by many environmental and physiological factors (Berry and Downton 1982). Understanding the biomass production of crops and vegetation under different conditions requires monitoring spatial and temporal variations in photosynthetic capacity. However, the traditional measurement of photosynthetic capacity, especially in the field, is limited by cumbersome techniques used to measure leaf gas exchange.
Recently, chlorophyll fluorescence quenching analysis has been found to be a fast, simple, non-invasive, and reliable method to assess changes in photosystem II (PSII) function under different environmental and physiological conditions (Roháček and Barták 1999; Maxwell and Johnson 2000). Among chlorophyll fluorescence variables, electron transport rate (ETR), calculated from the product of PSII efficiency and absorbed light, expresses the relative rate of electron transport through PSII (Krall and Edwards 1992).
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Electron flow through PSII is consumed mainly by carbon assimilation (Krall and Edwards 1992; Oberhuber and Edwards 1993). When CO2 fixation (A) is inhibited by certain environmental and physiological stresses, leaves may downregulate their PSII efficiency (ФPSII), mainly through xanthophyll-dependent non-photochemical quenching to avoid damage caused by excess absorbed energy (Demmig-Adams et al.
1996; Kato et al. 2003; Adams et al. 2004). Thus, a linear correlation between ETR and A can be obtained for many C3 and C4 plants. However, in C3
plants, both CO2 fixation and photorespiration are major sinks for electrons from PSII.
Therefore, with increased photorespiration, the ratio of ETR to A [or ФPSII to photosynthetic rate per absorbed quantum (ФCO2)] greatly increases with decreasing CO2 partial pressure (Krall and Edwards 1990, Cornic and Briantais 1991) or increasing temperature (Oberhuber and Edwards 1993; D'Ambrosio et al. 2006; Wong et al. 2012) as well as O2 partial pressure (D'Ambrosio et al. 2006; Ripley et al. 2007).
In C4 plants, photorespiration is restricted,
4
44444444444444444444444444444444444444444444444444444444444444444444
4444444444444444444444444444444444444444444444444444444444. ,,absorbed photons may be mostly used to drive the CO2 fixation (Krall and Edwards 1992).
Numerous studies have concluded a strong linear relationship between ETR and A.
This relationship appears to be stable under many conditions (Edwards and Baker 1993), even when merging data from different genotypes (Earl and Tollenaar 1998) or with differences in both CO2 partial pressure and temperature (Kakani et al. 2008) across a broad range of light intensity. However, the ETR–A relationships in C4
species may be complicated because of uncertain ETR based on fluorescence measurements: chlorophyll fluorescence may occur from both the mesophyll and 69
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bundle sheath cells (Romanowskaa et al. 2006). Moreover, in addition to A and photorespiration, electrons from PSII have several energy sinks, such as the water–
water cycle (Asada 1999) and the cyclic electron flow within PSII (Miyake and Okamura 2003) as well as nitrogen (N) assimilation (Robinson 1990). Furthermore, CO2 leakage from bundle sheath cells is another possibility for electron usage in C4 plants and is influenced by many conditions (Kubien et al. 2003; Naidu and Long 2004; Yin et al. 2011b). Thus, the ETR–A (or
ФPSII–ФCO2) relationship of C4 plants may vary by genotype (D'Ambrosio et al. 2003), leaf N (Khamis et al. 1990), growth season or growth conditions (Earl and Tollenaar 1998; Fryer et al. 1998; Baerlocher et al. 2004; Farage et al. 2006) or even conditions during measurement (Earl and Tollenaar 1998).
Leaf N is an important factor affecting photosynthesis. Many C4 and C3 species show a strong positive correlation between A and leaf N content (Weng and Chen 1987; Makino and Osmond 1991; Weng and Hsu 2001; Lawlor 2002). Most leaf N is allocated to chloroplasts (Makino and Osmond 1991; Lawlor 2002), and lower A under limited N is often attributed to reduced chlorophyll content and enzyme activities (Verhoeven et al. 1997; Lawlor 2002; Cheng 2003; Huang et al. 2004;
Netto et al. 2005; Zhao et al. 2005), which leads to decreased light absorption and light utilization by leaves (Verhoeven et al. 1997; Cheng et al. 2001). N deficiency also results in significant decreases in ФPSII (Cheng et al.
2001) and increases the electron flux to alternative sinks (Kumagai et al. 2010). However, for C4 plants, the effect of leaf N on the ETR–A (or ФPSII–ФCO2) relationship has not been studied in detail. Khamis et al. (1990) noted a small decrease in ФCO2 at any given ФPSII in N-limited maize leaves. However, Cheng et al. (2001) noted that when leaves of apple (a C3 plant) were measured under non- 94
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photorespiratory conditions and quantum yield was expressed on an absorbed PPFD basis, a close ФPSII–ФCO2 relationship could be found for leaves with a wide range of N content.
Eco-physiological studies require knowledge of photosynthetic characteristics of plants at different environmental and physiological conditions. Under well-water conditions, light intensity, temperature and leaf N status are important factors in photosynthesis. Chlorophyll fluorescence measurement is a fast and simple method for estimating photosynthesis. However, for C4 plants, the combined effects of leaf N, light intensity and seasonal temperature on the ETR–A relationship is not clear, especially for subtropical perennial C4 grasses. In addition, stomatal conductance (gs) is another important limiting factor in photosynthesis. Stomata prevent water loss and facilitate CO2 diffusion to mesophyll cells. To optimize water-use efficiency, gs and A may be closely related under many conditions, including changing light intensity (D’Ambrosio et al. 2003; Huxman and Monson 2003), temperature (Pittermann and Sage 2000, 2001) and leaf N status (Weng and Hsu 2001; Mohotti and Lawlor 2002).
In this study, we used two perennial C4 grasses to elucidate the ETR–A relationship in varied seasons under different seasonal temperature, photosynthetic photon flux density (PPFD), and relative humidity. We also discuss the effects of stomatal and non-stomatal limitations.
Materials and methods Plant materials
Two perennial C4 plants, Pennisetum purpureum (cv. A146), a cultivated herbage, and Miscanthus floridulus, a wild grass, were used as materials. The former was
propagated from cuttings, and the latter was collected from the lowlands of central Taiwan. Plants were potted (38 cm-diameter) in a mixture of soil : vermiculite : sand 119
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(1:1:1), and placed outdoors to receive regular water and full sunlight in the nursery of the Endemic Species Research Institute, Chichi Township, Nantou County, Taiwan (23°49′N, 120°48′E, 250 m a.s.l.). To obtain leaf samples with different photosynthetic capacity, 0, 1 and 2 g N fertilizer was applied to pots at 4 and 2 weeks before measurement. During the growth period (May 2010−Jan. 2011), the mean monthly temperature was 26.6−28.9°C (May−Aug.) and 28.5−14.8°C (Sept.−Jan.) (data from the Endemic Species Research Institute).
Measurements
Measurements were carried out from September 2010 to January 2011. Pot-grown plants were exposed to outdoor sunlight before measurement. We measured the top 2 fully expanded leaves with different chlorophyll content [0.18–0.55 g m–2, determined by use of a chlorophyll meter (SPAD 502, Minolta Camera Co., Osaka, Japan)]. From 9:30 to 15:00 h, photosynthesis, stomatal conductance and chlorophyll fluorescence were measured by use of a portable, open-flow gas exchange system (LI-6400, LI- COR, Lincoln, NE, USA) with an integrated fluorescence chamber head (LI-6400-40)
stepwise from high to low levels (2,000; 1,200; 800, 400, 200 and 0 μmol m–2 s–1) of PPFD. The values of A, gs, ratio of intercellular to atmospheric CO2 concentration (Ci/Ca) and chlorophyll fluorescence variables were recorded when A was stable. Leaf temperature was maintained at 15, 20, 25 and 30 °C.
Measurements were taken in the season with the climate temperature close to the leaf temperature. The 30°C measurement was in September to October and the 25 °C, 20 °C and 15 °C measurements were in November, December and January, respectively. Throughout the measurements, CO2 concentration in the chamber was kept at 350–400 μmol mol–1 (no control). Relative humidity (RH) of air entering the 145
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chamber was 60% (for all 4 temperatures) and 40% (for 30 °C only), which was controlled by passing temperature-controlled water.
The actual PSII efficiency (∆F/Fm’) was calculated as (Fm’–F)/Fm’. F and Fm’are the actual and maximal levels of fluorescence during illumination, respectively; the former was determined under each PPFD level, and the latter was determined by applying a 0.8-s pulse of saturating flashes of approximately 6,000 μmol quanta m–2 s–
1. ETR was calculated as ΦPSII×PPFD×α×f (Maxwell and Johnson 2000). The variables α and farethe fractions of incident PPFD absorbed by the leaf and absorbed PPFD used by PSII, respectively. We used the mean leaf absorption of 0.84 for green leaves (Björkman and Demmig 1987; see Discussion section), and 0.4 for f of C4
plants (Yin and Struik 2012).
Statistical analysis
We measured 5 to 10 (usually 6–7, Fig. 3) leaves from 4 pots for each species. Data were analyzed by linear regression by use of Sigma Plot v10.0.
Results
We used P. purpureum measured at 30 °C and 15 °C and 60% RH as an example for illustrating the light-response curves of PN, gs, ETR and Ci/Ca (Figs. 1 and 2).
Typically, PN, gs and ETR showed a hyperbolic increase with increasing PPFD, and leaves with higher chlorophyll content always had higher PN, gs and ETR, the closest relation being between ETR and PN. PN, gs, and ETR values were lower at low than high seasonal temperature in all leaves. The Ci/Ca for both species decreased with increasing PPFD and stabilized somewhat at about 1,200 μmol m–2 s–1 PPFD with most treatments (Fig. 3). However, as compared with a given level of PPFD, low temperature and high RH produced high Ci/Ca for both species.
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Yet, under 30 °C and 60% RH, even leaves with high chlorophyll content had higher gs but still showed lower Ci/Ca than leaves with low chlorophyll content.
Although PN was influenced by many conditions, including stomatal and non- stomatal factors, ETR was still highly correlated with PN in both species when merging data from leaves with different chlorophyll content and measured under different PPFD, temperature and RH (Fig. 4a, c). However, both the slope and determination coefficient (r2) for the ETR–PN correlation were greater for P.
purpureum than M. floridulus. In contrast, the gs–PN correlation was not as close as ETR–PN correlation for both species. At low temperature, many PN values were low in the gs–PN regression lines (Fig. 4b, d).
Discussion
We examined the association of ETR and A for 2 C4 perennial grasses by merging data from varied seasons and different N fertilization, light intensity, seasonal temperature and RH. The top two fully expanded leaves with high chlorophyll content showed high PN at the same PPFD and seasonal temperature (Figs. 1 and 2, and data not shown). This result might be due to both leaf chlorophyll content and photosynthetic characteristics being influenced by leaf N (Verhoeven et al. 1997;
Lawlor 2002; Cheng 2003; Huang et al. 2004; Ding et al. 2005; Netto et al.
2005; Zhao et al. 2005).
ETR is calculated as the product of PSII efficiency and absorbed light. α may vary by leaf chlorophyll content. Previously, we found that the correlation between α and SPAD value (Minolta SPAD-502 chlorophyll meter reading) could be fitted with the following equation (Jiang 2007):
α = 0.0034•SPAD + 0.7266 (1)
We used the data shown in Fig. 1 to compare the association of PN and ETR 196
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calculated with the mean of α (0.84) (Björkman and Demmig 1987) or with the result of equation 1. Even in leaves with a broad range of chlorophyll content (0.18–0.55 g m–2, SPAD = 22.4–49.5 and α = 0.80–0.89), we found a similar association of PN and ETR regardless of use of α = 0.84 or 0.80–0.89 for calculating ETR (Fig. 5). Thus, we used α = 0.84 in the present study.
Many reports have indicated that chlorophyll content always increases linearly with increasing leaf N (e.g., Cheng 2003; Netto et al. 2005; Zhao et al. 2005). Chlorophyll content can be detected easily and quickly by use of a chlorophyll meter (e.g., Netto et al. 2005). Thus, we used chlorophyll content measured by a chlorophyll meter as an indicator ofleaf N.
Photosynthesis is limited by both stomatal and non-stomatal factors. The former is associated with decreased leaf Ci caused by stomata closure, and the latter is associated with decrease in photochemical efficiency and CO2 fixation (Berry and Downton 1982; Brodribb 1996). Both A and gs decrease under N deficiency (Huang et al. 2004; Ding et al. 2005; Zhao et al. 2005). However, the response of Ci accompanying the decreases in A and gs are not uniform: such leaves may show higher Ci (Huang et al. 2004; Ding et al. 2005) or lower Ci (Zhao et al. 2005) than leaves with non-deficient N supply. We found similar results. Under all conditions, most leaves with high chlorophyll content had high PN
and gs (Figs. 1, 2 and data not shown). However, at 30 °C and 60% RH, Ci/Ca was greater in leaves with low than high chlorophyll content (Fig. 1d). This finding might due to reduced carboxylation efficiency (Huang et al. 2004; Ding et al. 2005). In contrast, at 15 °C, all tested leaves showed high Ci/Ca, and the difference in Ci/Ca was less than at 30 °C. This finding might be due to key C4 enzymes, such as the cold 222
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liability of pyruvate Pi dikinase is sensitive to low temperature (Pittermann and Sage 2000; Kubien et al. 2003, Kakani et al. 2008) and leads to the carboxylation efficiency of all leaves being more limited by low temperature than gs (Pittermann and Sage 2000, 2001; Kubien et al. 2003; Kościelniak and Biesaga-Kościelniak 2006). As well, many PN values, obtained at low temperature, were low in the gs–PN regression lines (Fig. 4b, d). In addition, gs may be reduced in response to increased vapor pressure difference between leaf and air (Morison and Gifford 1983; Yong et al. 1997;
Maherali et al. 2003). We showed that low RH could decrease Ci/Ca (Fig. 3).
For many C4 plants, including Miscanthus, A increases up to a Ci of about 200–
250 μmol mol–1 (Ziska and Bunce 1997; Huxman and Monson 2003; Naidu and Long 2004). We measured photosynthesis under 350–400 μmol mol–1 CO2 and found that Ci/Ca could decrease to < 0.5 when measured at 30 °C, especially under 40% RH.
Under these conditions, Ci was lower than the saturated portion of the A–Ci curve for many C4 plants. Thus, A might be limited by low Ci. In C4 or C3 plants under non- photorespiration (e.g., low O2 and saturated CO2), because A is a major sink for electrons from PSII, the allocation electron flow between A and other alternative sinks in C4 plants remains unchanged under many conditions. This situation explains the close association of ETR and A (or ФPSII and ФCO2) in many cases, including under changed Ci value (Krall and Edwards 1992; Oberhuber and Edwards 1993; Naidu and Long 2004; Farage et al. 2006; Kakani et al. 2008).
In terms of the effect of N on the ETR–A (or ФPSII–ФCO2) association in C4 plants, maize leaves under low N showed a small decrease in ФCO2 at any given ФPSII (Khamis et al. 1990). In addition, the partitioning of electron flow between CO2 assimilation and photorespiration was not affected by leaf N content in leaves of apple (C3 fruit tree) (Cheng et al. 2001). However, probably because of PSII heterogeneity and 247
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changes in electron flow to sinks other than CO2 assimilation, the ФPSII–ФCO2
relationship under non-photorespiratory conditions was curvilinear when merging data from apple leaves with a wide range of leaf N content and measured at 87–1,990 μmol m–2 s–1 PPFD, low 2% O2 and saturated CO2 (1,300 μmol mol–1) (Cheng et al. 2001). In the present study, we derived a broad range of leaf-scale photosynthetic rate from variation in N fertilizer and seasonal temperature, as well as PPFD and RH during measurement. Even when merging all data for the same species, PN still showed a close linear correlation with ETR (Fig. 4a, c). This result indicated no change in proportion of electron flow to CO2 assimilation and alternative sinks.
The allocation of electron flow between A and other alternative sinks may be altered in low-temperature-grown C4 plants (Fryer et al. 1998; Farage et al. 2006). However, the threshold temperature for change in allocation electron flow varies by species. For example, PSII electron transport relative to A increased for 17°C-grown Cyperus longus but for 10°C-grown Miscanthus × giganteus (Farage et al. 2006). In the
present study, the mean monthly temperature of the coldest month (January) remained near 15 °C. Probably, this temperature was not low enough to change the electron flow of the tested species. In addition, the ETR/A (or ФPSII/ФCO2) ratio may increase with advancing growth for annual C4 crops (Earl and Tollenaar 1998) and winter- dormant perennial C4 grasses grown at high altitude (45°50‘N) (Baerlocher et al.
2004). However, in the lowlands of subtropical Taiwan, perennial C4 grasses can grow year-round. We measured the top 2 fully expanded leaves. Probably for these reasons, the ETR–A relation of the 2 tested species measured in different seasons was not influenced by phenological stages. Also, we found no change in ETR/A ratio with temperature for either species (Fig. 4 a, c), which suggests no marked effects on CO2
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leakage and dark respiration under light (Naidu and Long 2004).
However, the slope for the ETR–PN correlation was higher for P. purpureum than M. floridulus (Fig. 4 a, c). C4 maize cultivars have shown differences in the ФPSII–ФCO2
relationship, due to differences in allocation of electron flow between A and other alternative sinks (D'Ambrosio et al. 2003). As well, PN is determined by subtracting dark respiration from gross photosynthesis; thus, in addition to the effects of alternative sinks, the ETR–PN relation may be affected by dark respiration. According to Yin et al. (2011a), dark respiration under light is closely related to the intercept value at the PN-axis of the ETR–PN linear relationship. The absolute value of this intercept was higher for P. purpureum than M. floridulus (Fig. 4 a, c), so P.
purpureum had higher dark respiration under light than M. floridulus. Thus, the higher
slopes for the ETR–A correlation in P. purpureum than M. floridulus was not due to low dark respiration. The ETR–PN relation was not influenced markedly by both gs
and carboxylation efficiency within a species (Fig. 4 a, c), and more electrons were not diverted to non-photochmical quencing in M. floridulus than P. purpureum (data not shown). The correlation coefficient between ETR and PN greater for P.
purpureum than M. floridulus may be due to the lower proportion of electron flow to
alternative sinks (Fryer et al. 1998; D'Ambrosio et al. 2003; Farage et al. 2006). From Fig. 4 a, c and Yin et al. (2011a), the ratio of ETR to gross photosynthetic rate for P.
purpureum and M. floridulus was about 3.8 and 4.3, respectively. The latter value was
similar to that for both maize, an annual C4 crop (Earl and Tollenaar 1998) and Spartina alterniflora, a high-altitude–grown perennial C4 grass, in the early part of the growth season (Baerlocher et al. 2004), but lower than that for maize at the late grain- filling stage (Earl and Tollenaar 1998) and growth under controlled environmental conditions in a climate chamber (D'Ambrosio et al. 2003). However, the ETR/A value 297
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for M. floridulus was only 3.8, smaller than the theoretical minimum value (4, Edwards and Baker 1993). Because we used a lower flash strength (6000 μmol m–2 s–
1) than saturating (Loriaux et al. 2013) to assess Fm', ФPSII might have been under- measured for both tested species.
In conclusion, we tested 2 perennial C4 grasses that can sustain year-round growth in subtropical Taiwan to determine the association of ETR and A. Despite a broad range of A obtained because of both stomatal and non-stomatal factors caused by different light, humidity, seasonal temperature, phenological stage and N fertilization, ETR was still highly correlated linearly with net photosynthetic rates, when data from the same species were combined for analysis. Thus, ETR could be used to assess the dynamic A for some perennial C4 species through different seasons.
However, the ETR–A correlation may vary by many conditions and may not apply to all C4 plants under all conditions.
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Fig. 1 Light-response curves for Pennisetum purpureum measured in September to
October at 30 °C and 60% relative humidity. PN represents net photosynthetic rate;
ETR represents electron transport rate; gs represents stomatal conductance; Ci
represents intercellular CO2 concentration; Ca represents atmospheric CO2
concentration; closed and open symbols represent high (0.39–0.50 g m–2) and low (0.18–0.33 g m–2) chlorophyll content, respectively. Each point represents the value for one leaf. Numeric values in panel A are chlorophyll content for each leaf.
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Fig. 2 Light-response curves for Pennisetum purpureum measured in January at 15°C and 60% relative humidity. PN represents net photosynthetic rate; ETR represents electron transport rate; gs represents stomatal conductance; Ci represents intercellular CO2 concentration; Ca represents atmospheric CO2 concentration; closed and open symbols represent high (0.39–0.55 g m–2) and low (0. 0.25–0.33 g m–2) chlorophyll content, respectively. Each point represents the value for one leaf. Numeric values in panel A are chlorophyll content for each leaf.
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B Pennisetum purpureum
PPFD (mol m-2 s-1)
0 500 1000 1500 2000
Ci/Ca
A Miscanthus floridulus
0 500 1000 1500 2000
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
(6) (6)
(6) (6)
(7) (10)(6)
(7)(6)
(5)
Fig. 3 Effect of photosynthetic photon flux density (PPFD) on ratio of intercellular to
atmospheric CO2 concentration (Ci/Ca) at different seasonal temperatures. ●, ▲, ○ and △ represent at 60% relative humidity and 15 °C (in January), 20 °C (in December),25 °C (in November) and 30 °C (in September–October), respectively; ▽ represents at 40% relative humidity and 30 °C (in September–October). Data are mean SD; numeric value within the parentheses are sample size of each measurement.
520
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ETRSLC (mol e- m-2 s-1)
0 20 40 60 80 100 120 140 160 PN (mol CO2 m-2 s-1 )
0 10 20 30 40
gs (mol H2O m-2 s-1)
ETR (mol e- m-2 s-1)
0 20 40 60 80 100 120 140 160 0
10 20 30 40
gs (mol H2O m-2 s-1)
0.0 0.1 0.2 0.3 0.4
Miscanthus floridulusPennisetum purpureum
y = 0.235 x - 0.633 r2 = 0.952***
y = 88.664 x + 1.660 r2 = 0.698***
y = 0.265 x -1.189
r2 = 0.985*** y = 106.751 x + 0.230 r2 = 0.742***
A
D B
C
Fig. 4 The correlation of electron transport rate (ETR) and stomatal conductance (gs) with net photosynthetic rate (PN) for Miscanthus floridulus and Pennisetum purpureum. ●, ▲, ○ and △ represent at 60% relative humidity and 15 °C (in
January), 20 °C (in December), 25 °C (in November) and 30 °C (in September–
October), respectively; ▽represents at 40% relative humidity and 30 °C (in September–October). Each point represents the value for each leaf measured under each level (0–2,000 μmol m–2 s–1) of photosynthetic photon flux density. ***
represents P < 0.001.
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ETR (mol e- m-2 s-1)
40 60 80 100 120 140 160 180
PN (mol CO 2 m-2 s-1 ) 10 20 30 40
y = 0.220x - 1.471 r2 = 0.990***
y = 0.209 x - 1.075 r2 = 0.992***
Fig. 5 The association of electron transport rate (ETR) and net photosynthetic rate
(PN) for Pennisetum purpureum at 60% relative humidity and 30 °C. Data from Fig. 1 [SPAD value (Minolta SPAD-502 chlorophyll meter reading) = 22.4–49.5]. Closed symbol and solid line vs. open symbol and dotted line – α = 0.84 and α = 0.0034•SPAD + 0.7266 (α = 0.80–0.89), respectively, used for calculating ETR. Each point represents the value for each leaf measured under each level (0–2,000 μmol m–2 s–1) of photosynthetic photon flux density. *** represents P < 0.001.
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