Comparison of the Photosynthetic Capacity of Phragmites australis in Five Habitats in Saline‒Alkaline Wetlands

Water shortages have an important impact on the photosynthetic capacity of Phragmites australis. However, this impact has not been adequately studied from the perspective of photosynthesis. An in-depth study of the photosynthetic process can help in better understanding the impact of water shortages on the photosynthetic capacity of P. australis, especially on the microscale. The aim of this study is to explore the photosynthetic adaptation strategies to environmental changes in saline‒alkaline wetlands. The light response curves and CO2 response curves of P. australis in five habitats (hygrophilous, xerophytic, psammophytic, abandoned farmland, paddy field drainage) in saline‒alkaline wetlands were measured at different stages of their life history, and we used a nonrectangular hyperbolic model to fit the data. It was concluded that P. australis utilized coping strategies that differed between the growing and breeding seasons. P. australis in abandoned farmland during the growing season had the highest apparent quantum efficiency (AQE) and photosynthetic utilization efficiency for weak light because of the dark environment. The dark respiration rate of P. australis in the drainage area of paddy fields was the lowest, and it had the highest values for photorespiration rate, maximum photosynthetic rate (Pmax), photosynthetic capacity (Pa), biomass, maximum carboxylation rate (Vcmax), and maximum electron transfer rate (Jmax). The light insensitivity of P. australis increased with the transition from growing to breeding season, and the dark respiration rate also showed a downward trend. Moreover, Vcmax and Jmax would decline when Pmax and Pa showed a declining trend, and vice versa. In other words, Vcmax and Jmax could explain changes in the photosynthetic capacity to some extent. These findings contribute to providing insights that Vcmax and Jmax can directly reflect the variation in photosynthetic capacity of P. australis under water shortages in saline‒alkaline wetlands and in other parts of world where there are problems with similarly harmful environmental conditions.


Introduction
Vegetation is a fundamental part of wetlands, so it is important to study the photosynthetic response mechanisms of vegetation for wetland protection [1,2]. Phragmites australis is one of the typical wetland plants and, therefore, studying the photosynthetic process of P. australis in differing environments is helpful in further understanding the response strategies of plants to different environmental conditions [3,4].

Characteristics of Light Response Curve in the Growing Season
During the growing season, the saturated light intensity (Im) of HP (hygrophilous type of P. australis) was the lowest, and the Im of PP (P. australis in drainage area of paddy field) was the highest. The apparent quantum efficiency (AQE) of FP (P. australis in abandoned farmland) had the highest value, indicating that P. australis in abandoned farmland had the highest photosynthetic efficiency under low light conditions. The value for maximum net photosynthetic rate (Pmax) of PP was the highest, and its biomass (Bm) per unit area was also higher than that of other habitats (Table 1 and Figure 1). The rate of dark respiration (Rd) of PP was also the lowest among the five habitats. Thus, higher Pmax and lower Rd appear to be beneficial for the accumulation of biomass in the growth stage. Table 1. Photosynthetic physiological characteristics of P. australis in the growing season (Im: saturated light intensity; AQE: apparent quantum efficiency; Rd: rate of dark respiration; Pmax: maximum net photosynthetic rate; Bm: biomass; HP: hygrophilous type of P. australis; XP: xerophytic type of P. australis; SP: psammophytic type of P. australis; FP: P. australis in abandoned farmland; PP: P. australis in drainage area of paddy field). Different letters after the values indicate statistically significant differences between five habitats in the same row. LSD: use "least significant difference" as a method when test the differences between variables. Note: Means (n = 10) followed by different letters are significantly different by LSD (p < 0.05).

Characteristics of Light Response Curve in the Growing Season
During the growing season, the saturated light intensity (Im) of HP (hygrophilous type of P. australis) was the lowest, and the Im of PP (P. australis in drainage area of paddy field) was the highest. The apparent quantum efficiency (AQE) of FP (P. australis in abandoned farmland) had the highest value, indicating that P. australis in abandoned farmland had the highest photosynthetic efficiency under low light conditions. The value for maximum net photosynthetic rate (Pmax) of PP was the highest, and its biomass (Bm) per unit area was also higher than that of other habitats (Table 1 and Figure 1). The rate of dark respiration (Rd) of PP was also the lowest among the five habitats. Thus, higher Pmax and lower Rd appear to be beneficial for the accumulation of biomass in the growth stage. Table 1. Photosynthetic physiological characteristics of P. australis in the growing season (Im: saturated light intensity; AQE: apparent quantum efficiency; Rd: rate of dark respiration; Pmax: maximum net photosynthetic rate; Bm: biomass; HP: hygrophilous type of P. australis; XP: xerophytic type of P. australis; SP: psammophytic type of P. australis; FP: P. australis in abandoned farmland; PP: P. australis in drainage area of paddy field). Different letters after the values indicate statistically significant differences between five habitats in the same row. LSD: use "least significant difference" as a method when test the differences between variables.  Figure 1. Fitting results of light response curves of (a) hygrophilous type of P. australis, (b) xerophytic type of P. australis, (c) psammophytic type of P. australis, (d) P. australis in abandoned farmland, and (e) P. australis in paddy field drainage during the growing season. PAR: photosynthetically active radiation.

Characteristics of Light Response Curve in the Breeding Season
During the breeding season, the saturated light intensity (Im) of PP was the lowest, and the Im of FP was the highest. There were no significant differences in the rate of dark respiration (Rd) between HP (hygrophilous type of P. australis) and XP (xerophytic type of P. australis). SP (psammophytic type of P. australis) had the lowest biomass. The order of AQE was FP < HP < XP < PP < SP, and the order of Pmax was FP < PP < HP < XP < SP. SP had the highest values for AQE and Pmax. Furthermore, SP also had the lowest Rd. However, PP still had the largest biomass (Table 2 and Figure 2). Therefore, there may be other factors that determined the accumulation of biomass in the stage of breeding. At the same time, after entering the breeding season, the change in Pmax varied depending on the habitat. The Pmax of SP and PP changed greatly. The Pmax of SP increased by around 50% while the Pmax of PP decreased by 50.5%. However, the Pmax of the others did not change significantly; the Pmax of HP and FP decreased while that of XP increased. During the breeding season, the saturated light intensity (Im) of PP was the lowest, and the Im of FP was the highest. There were no significant differences in the rate of dark respiration (Rd) between HP (hygrophilous type of P. australis) and XP (xerophytic type of P. australis). SP (psammophytic type of P. australis) had the lowest biomass. The order of AQE was FP < HP < XP < PP < SP, and the order of Pmax was FP < PP < HP < XP < SP. SP had the highest values for AQE and Pmax. Furthermore, SP also had the lowest Rd. However, PP still had the largest biomass (Table 2 and Figure 2). Therefore, there may be other factors that determined the accumulation of biomass in the stage of breeding. At the same time, after entering the breeding season, the change in Pmax varied depending on the habitat. The Pmax of SP and PP changed greatly. The Pmax of SP increased by around 50% while the Pmax of PP decreased by 50.5%. However, the Pmax of the others did not change significantly; the Pmax of HP and FP decreased while that of XP increased. Note: Means (n = 10) followed by different letters are significantly different by LSD (p < 0.05).  During the growing season, the order of CO 2 saturation point (Cm) was PP < SP < XP < HP < FP; the order of CO 2 compensation point (Cc) was SP < PP < XP < HP < FP (Table 3 and Figure 3). FP had the highest Cm and Cc, indicating that FP could make use of a wide range of concentrations of CO 2 . However, the quantum efficiency of CO 2 (ϕCO 2 ) of FP was the lowest, indicating that the utilization efficiency of low-concentration CO 2 for FP was lower than compared to other conditions; this may be why FP needed to make use of a wide range of concentrations of CO 2 . PP had the highest value of ϕCO 2 , and its rate of respiration (Rl) and photosynthetic capacity (Pa) were also the largest among the five habitats. However, as mentioned before, PP still had the largest biomass; a higher respiratory rate would not be enough to affect the biomass accumulation. Table 3. Characteristics of CO 2 response curve of P. australis in the growing season (Cm: CO 2 saturation point; Cc: CO 2 compensation point; ϕCO 2 : the highest quantum efficiency of CO 2 ; Rl: rate of respiration; Pa: photosynthetic capacity; HP: hygrophilous type of P. australis; XP: xerophytic type of P. australis; SP: psammophytic type of P. australis; FP: P. australis in abandoned farmland; PP: P. australis in drainage area of paddy field). Different letters after the values indicate statistically significant differences between five habitats in the same row. LSD: use "least significant difference" as a method when test the differences between variables.  During the growing season, the order of CO2 saturation point (Cm) was PP < SP < XP < HP < FP; the order of CO2 compensation point (Cc) was SP < PP < XP < HP < FP (Table 3 and Figure 3). FP had the highest Cm and Cc, indicating that FP could make use of a wide range of concentrations of CO2. However, the quantum efficiency of CO2 (φCO2) of FP was the lowest, indicating that the utilization efficiency of low-concentration CO2 for FP was lower than compared to other conditions; this may be why FP needed to make use of a wide range of concentrations of CO2. PP had the highest value of φCO2, and its rate of respiration (Rl) and photosynthetic capacity (Pa) were also the largest among the five habitats. However, as mentioned before, PP still had the largest biomass; a higher respiratory rate would not be enough to affect the biomass accumulation. Table 3. Characteristics of CO2 response curve of P. australis in the growing season (Cm: CO2 saturation point; Cc: CO2 compensation point; φCO2: the highest quantum efficiency of CO2; Rl: rate of respiration; Pa: photosynthetic capacity; HP: hygrophilous type of P. australis; XP: xerophytic type of P. australis; SP: psammophytic type of P. australis; FP: P. australis in abandoned farmland; PP: P. australis in drainage area of paddy field). Different letters after the values indicate statistically significant differences between five habitats in the same row. LSD: use "least significant difference" as a method when test the differences between variables. Note: Means (n = 10) followed by different letters are significantly different by LSD (p < 0.05).

Characteristics of CO 2 Response Curve in the Breeding Season
During the breeding season, the order of CO 2 saturation point (Cm) was SP < PP < XP < HP < FP, and the order of CO 2 compensation point (Cc) was SP < HP < XP < PP < FP (Table 4 and Figure 4); hence, the values of Cm and Cc were still the highest for FP. Moreover, the quantum efficiency of CO 2 (ϕCO 2 ) of FP was also the lowest, indicating that FP still needed to make use of a wide range of CO 2 concentrations. The photosynthetic capacity (Pa) was similar to that of the growing season, i.e., under conditions of sufficient light and CO 2 , P. australis showed strong photosynthetic capacity in five habitats, especially in the drainage area of paddy field (PP). However, compared with the growth period, the photosynthetic capacity (Pa) of PP, HP, and FP decreased, while that of XP and SP increased. Table 4. Characteristics of CO 2 response curve of P. australis in the breeding season (Cm: CO 2 saturation point; Cc: CO 2 compensation point; ϕCO 2 : the highest quantum efficiency of CO 2 ; Rl: rate of respiration; Pa: photosynthetic capacity; HP: hygrophilous type of P. australis; XP: xerophytic type of P. australis; SP: psammophytic type of P. australis; FP: P. australis in abandoned farmland; PP: P. australis in drainage area of paddy field). Different letters after the values indicate statistically significant differences between five habitats in the same row. LSD: use "least significant difference" as a method when test the differences between variables.

Photosynthetic
Note: Means (n = 10) followed by different letters are significantly different by LSD (p < 0.05).
Plants 2020, 9, x FOR PEER REVIEW 6 of 17 under conditions of sufficient light and CO2, P. australis showed strong photosynthetic capacity in five habitats, especially in the drainage area of paddy field (PP). However, compared with the growth period, the photosynthetic capacity (Pa) of PP, HP, and FP decreased, while that of XP and SP increased. 0.021 ± 0.003a 0.039 ± 0.002b 0.017 ± 0.003c 0.011 ± 0.002d 0.049 ± 0.003e Note: Means (n = 10) followed by different letters are significantly different by LSD (p < 0.05).

Photosynthetic Characteristics of P. australis in the Growing Season
The fitting results for the light response curve demonstrated that apparent quantum efficiency (AQE) was one of the most important indicators for characterizing the ability of plants to assimilate CO2 under low light conditions. The slope of the light response curve at the weak light stage (i.e., the smaller PAR value interval) was calculated as the apparent quantum efficiency, which means the

Photosynthetic Characteristics of P. australis in the Growing Season
The fitting results for the light response curve demonstrated that apparent quantum efficiency (AQE) was one of the most important indicators for characterizing the ability of plants to assimilate CO 2 under low light conditions. The slope of the light response curve at the weak light stage (i.e., the smaller PAR value interval) was calculated as the apparent quantum efficiency, which means the average amount of CO 2 assimilated by one photon [17,18]. During the growing season, the AQE of FP was the highest (0.047 ± 0.004) in five habitats, indicating that FP had a strong ability to utilize weak light. Field investigations also revealed that in abandoned farmland, P. australis mainly grew in a shady environment (shading by trees is one of the reasons leading to the abandonment of farmland). Furthermore, the high AQE also indicated that P. australis adapted to long-term shading, indicating that it had higher photosynthetic efficiency in weak light. The rate of respiration represents the rate at which plants consume organic matter. It is generally believed that a higher respiration rate is not conducive to the accumulation of organic matter. Maximum photosynthetic rate (Pmax) represents the ability to assimilate CO 2 under sufficient light; the higher the value of Pmax, the higher the rate of carbon sequestration, and the more favorable it is for the accumulation of organic matter [19,20]. PP had the highest Pmax and the lowest respiration rate. Therefore, the general rule of biomass accumulation of P. australis in five habitats was as follows: the higher the value of Pmax, the larger the biomass. Studies have found that rich soil nutrient content is conducive to the accumulation of photosynthetic carbon sequestration of plants [21,22]. The reason for PP's greater Pmax is presumed to be related to the use of fertilizers in paddy fields, which indirectly results in higher N, P, and other nutrient elements in soil than in other habitats, which is more conducive to the fixation of CO 2 and accumulation of organic matter [23]. However, PP had the lowest Pmax. Some studies have shown that P. australis can regulate its genes, to some extent, to adapt to high-salinity environments. These genetic regulations include higher relative expression levels of genes associated with photosynthesis and lignan biosynthesis, indicative of a greater ability to maintain growth under saline conditions [24]. At the same time, the distribution of photosynthetically fixed C in roots and soils also changes, for example, with lower contents of photosynthetically fixed C in roots and higher contents in soil [25].
Photosynthetic capacity (Pa) is one of the most important indicators for analyzing the characteristics of the CO 2 response curve. It is used to characterize the maximum potential of fixing CO 2 under conditions of sufficient light and CO 2 . Photorespiration refers to the consumption of superfluous substances by respiration when high amounts of [H] and ATP accumulate in the photoreaction but the photosynthetic dark reaction is inhibited so as to prevent their accumulation, affecting plant metabolism [26][27][28][29]. Therefore, the rate of photorespiration (Rl) in plants can reflect their photoreaction rate to a certain extent, and this then affects the final net photosynthetic rate. During the growing season, the general rule for the photosynthetic capacity of P. australis in the five studied habitats was that the higher the Rl value, the higher the Pa value. The Rl of PP was the highest, and the Pa of PP was also the highest among the five habitats. Moreover, the CO 2 quantum efficiency (ϕCO 2 ) of PP was also the highest, indicating that it had the highest photosynthetic efficiency for low concentrations of CO 2 .
By further fitting the CO 2 response curve, the limits for the photosynthetic rate in the dark reaction process were obtained for different intercellular CO 2 concentrations (Ci). Vc represents the limitation of Rubisco carboxylase and J represents the limitation of RuBP (ribulose bisphosphate) regeneration. Therefore, the intersection point (Ci_transition) of the Vc-limit curve (blue) and J-limit curve (red) was the demarcation between the limitation of Rubisco carboxylase and the limitation of RuBP regeneration. When C < Ci_transition, the photosynthetic rate is mainly limited by Vc, and when C > Ci_transition, the photosynthetic rate is mainly limited by J [30,31]. According to the fitting results, during the growing season, the Vcmax and Jmax of PP were the highest among the five habitats, and the photosynthetic capacity (Pa) of PP was also the highest (Table 5 and Figure 5). Moreover, the value of Ci_transition of PP was 308 ppm, which was lower than the general environmental CO 2 concentration (around 400 ppm). Therefore, the photosynthetic rate of PP was mainly determined by Vc and J, while the photosynthetic rates of others were mainly determined by Vc. Table 5. Characteristics of photosynthetic dark reaction of P. australis in the growing season (Vcmax: maximum carboxylation rate; Jmax: maximum electron transfer rate; Ci_transition: intersection point of the Vc-limit curve (blue) and J-limit curve (red); HP: hygrophilous type of P. australis; XP: xerophytic type of P. australis; SP: psammophytic type of P. australis; FP: P. australis in abandoned farmland; PP: P. australis in drainage area of paddy field). Different letters after the values indicate statistically significant differences between five habitats in the same row. LSD: use "least significant difference" as a method when test the differences between variables.  Table 5. Characteristics of photosynthetic dark reaction of P. australis in the growing season (Vcmax: maximum carboxylation rate; Jmax: maximum electron transfer rate; Ci_transition: intersection point of the Vc-limit curve (blue) and J-limit curve (red); HP: hygrophilous type of P. australis; XP: xerophytic type of P. australis; SP: psammophytic type of P. australis; FP: P. australis in abandoned farmland; PP: P. australis in drainage area of paddy field). Different letters after the values indicate statistically significant differences between five habitats in the same row. LSD: use "least significant difference" as a method when test the differences between variables.

Photosynthetic Characteristics of P. australis in the Breeding Season
By comparing the characteristics of light response curves, reeds in all habitats showed an increase in the value of the light saturation point (Im) and a decrease in the value of AQE after entering the breeding season, which meant a gradual adaptation to and utilization of the high-light environment. It was believed that the general downward trend observed for the value of AQE during the process of plant growth may be related to the increase in average solar radiation intensity [32]. Meanwhile, the rates of dark respiration of P. australis in all habitats were lower than those of the growing season, and the biomass showed accumulation with a decrease in the rates of dark respiration. Moreover, the rate of biomass accumulation of FP was the highest among the five habitats (386.7%). However, the Pmax of HP, FP, and PP showed a downward trend. The decrease in photosynthetic rate was related to the decrease in stomatal conductance, and the decrease in stomatal conductance was related to the increase in salinity [33]. Some studies have shown that reeds could adapt to a saline and alkaline environment by rapid ecological evolution and phenotypic differentiation. At the same time, reeds could also adapt to a harsh environment by reducing the photosynthetic rate or chlorophyll concentration and increasing the K + concentration in leaves [34][35][36].
By comparing the characteristics of CO2 response curves, reeds in all habitats showed a decrease in the value of CO2 quantum efficiency (φCO2) after entering the breeding season, which represents an adaptation to high concentrations of CO2. Except for HP, reeds in all habitats showed an increase in the value of CO2 compensation points (Cc), which meant decreased photosynthetic sensitivity to low concentrations of CO2. In addition, PP had the highest φCO2, Rl, and Pa in both the growing and breeding seasons. Moreover, Vcmax and Jmax as well as Pmax and Pn of XP and SP showed an upward trend while showing a downward trend for HP, FP, and PP (Table 6 and Figure 6). The results showed that the fitting results of the light response curves and the CO2 response curves were consistent. It was also found that XP and SP entered the withering season later than HP, FP, and PP during the field investigation, which may be related to the later decline in ability to undergo dark reaction (Vcmax, Jmax) of XP and SP. Therefore, Vcmax and Jmax, as important indicators reflecting the characteristics of photosynthetic dark reaction, could explain the changes in photosynthetic rate to some extent [37]. However, Vcmax and Jmax represent only the dark reaction part of photosynthesis, and if combined with the chlorophyll fluorescence parameters, i.e., the characteristics

Photosynthetic Characteristics of P. australis in the Breeding Season
By comparing the characteristics of light response curves, reeds in all habitats showed an increase in the value of the light saturation point (Im) and a decrease in the value of AQE after entering the breeding season, which meant a gradual adaptation to and utilization of the high-light environment. It was believed that the general downward trend observed for the value of AQE during the process of plant growth may be related to the increase in average solar radiation intensity [32]. Meanwhile, the rates of dark respiration of P. australis in all habitats were lower than those of the growing season, and the biomass showed accumulation with a decrease in the rates of dark respiration. Moreover, the rate of biomass accumulation of FP was the highest among the five habitats (386.7%). However, the Pmax of HP, FP, and PP showed a downward trend. The decrease in photosynthetic rate was related to the decrease in stomatal conductance, and the decrease in stomatal conductance was related to the increase in salinity [33]. Some studies have shown that reeds could adapt to a saline and alkaline environment by rapid ecological evolution and phenotypic differentiation. At the same time, reeds could also adapt to a harsh environment by reducing the photosynthetic rate or chlorophyll concentration and increasing the K + concentration in leaves [34][35][36].
By comparing the characteristics of CO 2 response curves, reeds in all habitats showed a decrease in the value of CO 2 quantum efficiency (ϕCO 2 ) after entering the breeding season, which represents an adaptation to high concentrations of CO 2 . Except for HP, reeds in all habitats showed an increase in the value of CO 2 compensation points (Cc), which meant decreased photosynthetic sensitivity to low concentrations of CO 2 . In addition, PP had the highest ϕCO 2 , Rl, and Pa in both the growing and breeding seasons. Moreover, Vcmax and Jmax as well as Pmax and Pn of XP and SP showed an upward trend while showing a downward trend for HP, FP, and PP (Table 6 and Figure 6). The results showed that the fitting results of the light response curves and the CO 2 response curves were consistent. It was also found that XP and SP entered the withering season later than HP, FP, and PP during the field investigation, which may be related to the later decline in ability to undergo dark reaction (Vcmax, Jmax) of XP and SP. Therefore, Vcmax and Jmax, as important indicators reflecting the characteristics of photosynthetic dark reaction, could explain the changes in photosynthetic rate to some extent [37]. However, Vcmax and Jmax represent only the dark reaction part of photosynthesis, and if combined with the chlorophyll fluorescence parameters, i.e., the characteristics of the light reaction part of photosynthesis, the variations in the photosynthetic rate will be explained more comprehensively. Hence, it is necessary to conduct further studies on the specific photosynthetic process of P. australis. Table 6. Characteristics of photosynthetic dark reaction of P. australis in the breeding season (Vcmax: maximum carboxylation rate; Jmax: maximum electron transfer rate; Ci_transition: intersection point of the Vc-limit curve (blue) and J-limit curve (red); HP: hygrophilous type of P. australis; XP: xerophytic type of P. australis; SP: psammophytic type of P. australis; FP: P. australis in abandoned farmland; PP: P. australis in drainage area of paddy field). Different letters after the values indicate statistically significant differences between five habitats in the same row. LSD: use "least significant difference" as a method when test the differences between variables. Note: Means (n = 10) followed by different letters are significantly different by LSD (p < 0.05).

Photosynthetic
Plants 2020, 9, x FOR PEER REVIEW 10 of 17 of the light reaction part of photosynthesis, the variations in the photosynthetic rate will be explained more comprehensively. Hence, it is necessary to conduct further studies on the specific photosynthetic process of P. australis. Table 6. Characteristics of photosynthetic dark reaction of P. australis in the breeding season (Vcmax: maximum carboxylation rate; Jmax: maximum electron transfer rate; Ci_transition: intersection point of the Vc-limit curve (blue) and J-limit curve (red); HP: hygrophilous type of P. australis; XP: xerophytic type of P. australis; SP: psammophytic type of P. australis; FP: P. australis in abandoned farmland; PP: P. australis in drainage area of paddy field). Different letters after the values indicate statistically significant differences between five habitats in the same row. LSD: use "least significant difference" as a method when test the differences between variables. Note: Means (n = 10) followed by different letters are significantly different by LSD (p < 0.05).

Study Area
Niuxintaobao Wetland (45°13′-45°16′ N, 123°13′-123°21′ E) is located in the west of Songnen Plain in Northeastern China (Figure 7). Administratively, it is within the provinces of Jilin and Heilongjiang of China. It is formed by water accumulation in the interfluvial lowlands caused by the hydraulic movement of Huolin and Taoer Rivers. It is moderately saline-alkaline, with an area of around 33 km 2 . The main source of water supply is Taoer River [38]. P. australis saline-alkaline marshes are distributed in the study region, and it is characterized by a typical semiarid and moderate monsoon climate with distinctive seasons; the total annual sunlight is 5259 MJ/m 2 , the frostfree period is 137 d of the year [39], and it is one of the typical distribution areas of reeds in inland China.
A field survey was carried out during May (growing season) and August (breeding season). Reed habitats were classified according to the measured soil moisture as follows: hygrophilous (HP), xerophytic (XP), psammophytic (SP), abandoned farmland (FP), or paddy field drainage (PP) [40]. Ten stands (5 m × 5 m) in each habitat were selected and used as replicates for all habitats (Table 7 and Figure 7).  Figure 6. Modeling results of photosynthetic rate limitations of (a) hygrophilous type of P. australis, (b) xerophytic type of P. australis, (c) psammophytic type of P. australis, (d) P. australis in abandoned farmland, and (e) P. australis in paddy field drainage during the breeding season. A c is the gross photosynthetic rate when Rubisco activity is limiting; A j is the gross photosynthetic rate when RuBP regeneration is limiting (RuBP: ribulose bisphosphate; Ci: intercellular CO 2 concentration).

Study Area
Niuxintaobao Wetland (45 • 13 -45 • 16 N, 123 • 13 -123 • 21 E) is located in the west of Songnen Plain in Northeastern China (Figure 7). Administratively, it is within the provinces of Jilin and Heilongjiang of China. It is formed by water accumulation in the interfluvial lowlands caused by the hydraulic movement of Huolin and Taoer Rivers. It is moderately saline-alkaline, with an area of around 33 km 2 . The main source of water supply is Taoer River [38]. P. australis saline-alkaline marshes are distributed in the study region, and it is characterized by a typical semiarid and moderate monsoon climate with distinctive seasons; the total annual sunlight is 5259 MJ/m 2 , the frost-free period is 137 d of the year [39], and it is one of the typical distribution areas of reeds in inland China.
A field survey was carried out during May (growing season) and August (breeding season). Reed habitats were classified according to the measured soil moisture as follows: hygrophilous (HP), xerophytic (XP), psammophytic (SP), abandoned farmland (FP), or paddy field drainage (PP) [40]. Ten stands (5 m × 5 m) in each habitat were selected and used as replicates for all habitats (Table 7 and Figure 7).

Biomass Collection
There are non-destructive sampling methods using remote sensing spectroscopy for measuring plant biomass, and these methods are mainly used in the macro or large-scale research [41][42][43][44][45][46]. In order to directly reflect the characteristics of biomass, combined with the sampling methods commonly used by previous researchers [47][48][49][50][51][52], we chose the harvesting method to measure the biomass. That is, the aboveground parts of P. australis in five habitats were mowed in a 0.5 m × 0.5 m square and then dried in an oven at 75 °C for 48 h. The final weight was recorded when the weight showed no further reductions.

Measurement of Light Response Curve
The third top leaves of 10 shoots from each stand were used as replicates. The relative humidity was 45-50% and the temperature was around 25 °C. The light response curve was measured by LI-6400XT (LICOR, Lincoln, NE, USA) at 9:00-11:00 on a bright, clear day in May and August. Full light induction was carried out after installing the red and blue light source leaf chamber (6400-02B). After successful induction, the stable photo values under 15 light intensity (PAR, μmol/m 2 /s) gradients (2000, 1800, 1600, 1400, 1200, 1000, 800, 600, 400, 200, 150, 100, 50, 25, and 0) were selected and recorded in the file. Photo values were recorded in order of light intensity, from high to low. The standard of photo value recording is that the intake concentration of the instrument is stable without leakage; the stomatal conductance (Cond), intercellular CO2 concentration (Ci), and transpiration rate (Tr) of line C are all positive, the value of Cond is between 0 and 1; and the change rate of photo value (△P) is less than 2%.

Biomass Collection
There are non-destructive sampling methods using remote sensing spectroscopy for measuring plant biomass, and these methods are mainly used in the macro or large-scale research [41][42][43][44][45][46]. In order to directly reflect the characteristics of biomass, combined with the sampling methods commonly used by previous researchers [47][48][49][50][51][52], we chose the harvesting method to measure the biomass. That is, the aboveground parts of P. australis in five habitats were mowed in a 0.5 m × 0.5 m square and then dried in an oven at 75 • C for 48 h. The final weight was recorded when the weight showed no further reductions.

Measurement of Light Response Curve
The third top leaves of 10 shoots from each stand were used as replicates. The relative humidity was 45-50% and the temperature was around 25 • C. The light response curve was measured by LI-6400XT (LICOR, Lincoln, NE, USA) at 9:00-11:00 on a bright, clear day in May and August. Full light induction was carried out after installing the red and blue light source leaf chamber (6400-02B). After successful induction, the stable photo values under 15 light intensity (PAR, µmol/m 2 /s) gradients (2000, 1800, 1600, 1400, 1200, 1000, 800, 600, 400, 200, 150, 100, 50, 25, and 0) were selected and recorded in the file. Photo values were recorded in order of light intensity, from high to low. The standard of photo value recording is that the intake concentration of the instrument is stable without leakage; the stomatal conductance (Cond), intercellular CO 2 concentration (Ci), and transpiration rate (Tr) of line C are all positive, the value of Cond is between 0 and 1; and the change rate of photo value ( P) is less than 2%.

Measurement of CO 2 Response Curve
The leaves were the same as those used in measuring the light response curve. After full light induction, the CO 2 mixer was used to control the CO 2 concentration gradient of 2000, 1800, 1600, 1400, 1200, 1000, 800, 600, 400, 200, 150, 100, 50, 25, and 0. The saturated light intensity (2000 µmol/m 2 /s) was chosen as the light intensity. However, the photo values were recorded in the following order: 400, 200, 150, 100, 50, 25, 0, 400, 600, 800, 100, 1200, 1400, 1600, 1800, and 2000. The standard of recording the photo value is the same as that of the measurement of the light response curve.

Fitting Light Response Curve
The fitting of light response curve is based on the nonrectangular hyperbolic model (Equation (1)) [53,54]: where Pn is the photosynthetic rate, I is the light intensity, a is the apparent quantum efficiency (AQE), Pmax is the maximum photosynthetic rate, Rd is the respiratory rate, and θ is the correction coefficient. According to the formula, Ic is set as the light compensation point, i.e., the value of I when Pn(I) = 0, Im is the light saturation point, i.e., the value of I when Pn'(I) = 0, and Pn'(I) is the first derivative of the function Pn(I).

Fitting the CO 2 Response Curve
A nonrectangular hyperbolic model was also used to fit the CO 2 response curve (Equation (2)) [55], but there are corresponding deformations when calculating Vcmax (represented by Ac in Equation (3)) and Jmax (represented by Aj in Equation (3)) [56]: where Pn is the photosynthetic rate, C is the CO 2 concentration, a is the CO 2 quantum efficiency (ϕCO 2 ), Pa is the photosynthetic capacity, Rl is the respiratory rate, and θ is the correction coefficient. According to the formula, Cc is set as the CO 2 compensation point, i.e., the value of C when Pn(C) = 0, Cm is the CO 2 saturation point, i.e., the value of C when Pn'(C) = 0, and Pn'(C) is the first derivative of the function Pn(C).
where Am is the hyperbolic minimum of Ac and Aj, Ac is the gross photosynthetic rate when Rubisco activity is limiting, Aj is the gross photosynthetic rate when RuBP regeneration is limiting, Rl is the respiratory rate, and θ is the correction coefficient.

Statistical Analysis
The least squares method was used to estimate the fit of the experimental data. The test of fitting results could be divided into a goodness of fit test and a significance test for the regression equation. The decision coefficient R 2 was used to verify the goodness of fit, and the F test was used to verify the significance of the regression equation. One-way ANOVA was used to test the differences in photosynthetic characteristics of P. australis in different habitats. The confidence intervals of all the analyses were 95%. Statistical software SPSS22.0 for Windows (IBM Corp., Armonk, NY, USA) was used for the above statistical analyses, and the experimental data and regression model were also plotted and analyzed by R language software package "plantecophys", written by Remko Duursma [57] (v.3.4.2; R Foundation for Statistical Computing, Vienna, Australia).

Conclusions
This study was the first attempt to compare the response of P. australis to environmental changes from the perspective of the photosynthetic process. The findings indicate that with the transition from the growing season to the breeding season, P. australis showed decreased photosynthetic sensitivity, the rate of dark respiration also showed a downward trend, and plants were more conducive to the accumulation of biomass. P. australis in the drainage area of a paddy field benefited from abundant nutrition; its biomass and photosynthetic capacity were the highest. Moreover, the maximum photosynthetic rate and photosynthetic capacity of P. australis in all five habitats had the same trend of variation, and the trend was consistent with that of Vcmax and Jmax. Overall, our results suggest that study of Vcmax and Jmax is beneficial for exploring the photosynthetic adaptation strategies to harsh environmental changes, such as water shortages in saline-alkaline wetlands, and in other areas facing the same problems in the world. However, if combined with the chlorophyll fluorescence parameters, i.e., the characteristics of the light reaction part of photosynthesis, the variation in photosynthetic capacity can be explained more comprehensively. Hence, the specific photosynthetic process of P. australis deserves further research.