Ecological and photosynthetic characteristics of Carex schmidtii with respect to hydrological fluctuations

Abstract Hydrological fluctuations are key abiotic stresses that influence plant growth and photosynthetic processes of wetland plants. However, the response mechanism of the plant characteristics and photosynthesis of Carex schmidtii, with respect to hydrological fluctuations, are still unclear. Comparative studies of plant size parameters, biomass parameters, and photosynthesis parameters of C. schmidtii under the interactive influence of initial water depth (WD), water-level amplitude (WA), and duration time, were performed. The coupling relationship between any two factors of the plant characteristics of C. schmidtii was also examined. Generally, the results showed that the WD and WA treatments, and the duration time, significantly affected plant size, biomass, and photosynthesis of C. schmidtii. The exception was the effect by the WA on the biomass and the NPQ. Furthermore, the biomass and photosynthesis of C. schmidtii were significantly affected by the interactive effects by WD and WA. The plant size, biomass, and photosynthesis parameters of C. schmidtii first increased and then decreased over time. Larger values of plant size, biomass, and photosynthesis parameters (except for qL and NPQ) were found for the treatment with an initial water depth of 0 cm. As was the situation with WD, the largest values of plant height, chlorophyll content, Fv/Fm, and ΔFv/Fm’ were obtained for a water-level amplitude of 0 cm. Furthermore, the chlorophyll content was positively related to the plant size parameters, Fv/Fm, ΔFv/Fm’, and qL. Except for qL, ΔFv/Fm’ had a significant relationship with all plant factors. Lower water levels and smaller water level changes are more conducive to the growth and photosynthese of C. schmidtii. Plant size, biomass accumulation and photosynthesis of C. schmidtii collectively coped with hydrological fluctuations. The findings have improved understanding of the response of C. schmidtii to hydrological fluctuations. They have thereby provided invaluable information for the restoration, management and conservation of tussock wetlands.

hydrological fluctuations are key abiotic stresses that influence plant growth and photosynthetic processes of wetland plants.however, the response mechanism of the plant characteristics and photosynthesis of Carex schmidtii, with respect to hydrological fluctuations, are still unclear.comparative studies of plant size parameters, biomass parameters, and photosynthesis parameters of C. schmidtii under the interactive influence of initial water depth (WD), water-level amplitude (Wa), and duration time, were performed.the coupling relationship between any two factors of the plant characteristics of C. schmidtii was also examined.Generally, the results showed that the WD and Wa treatments, and the duration time, significantly affected plant size, biomass, and photosynthesis of C. schmidtii.the exception was the effect by the Wa on the biomass and the NPQ.Furthermore, the biomass and photosynthesis of C. schmidtii were significantly affected by the interactive effects by WD and Wa. the plant size, biomass, and photosynthesis parameters of C. schmidtii first increased and then decreased over time.larger values of plant size, biomass, and photosynthesis parameters (except for ql and NPQ) were found for the treatment with an initial water depth of 0 cm.as was the situation with WD, the largest values of plant height, chlorophyll content, Fv/ Fm, and ΔFv/Fm' were obtained for a water-level amplitude of 0 cm.Furthermore, the chlorophyll content was positively related to the plant size parameters, Fv/Fm, ΔFv/Fm' , and ql.except for ql, ΔFv/ Fm' had a significant relationship with all plant factors.lower water levels and smaller water level changes are more conducive to the growth and photosynthese of C. schmidtii.Plant size, biomass accumulation and photosynthesis of C. schmidtii collectively coped with hydrological fluctuations.the findings have improved understanding of the response of C. schmidtii to hydrological fluctuations.they have thereby provided invaluable information for the restoration, management and conservation of tussock wetlands.

Introduction
Hydrological fluctuations with changes in water depth, inundation frequency, and inundation duration are controlling factors for the wetland ecosystem (Casanova and Brock 2000;Mitsch and Gosselink 2007;Lou et al. 2015).They thereby affect the process of seed germination, seedling planting, plant growth, and reproduction of wetland plants.In addition, they change the community distribution pattern and biodiversity and regulate the wetland structure and function (Yan et al. 2015;Zhang et al. 2020a;Qi et al. 2021;Jing et al. 2023).Moreover, plants are crucial for the maintenance of structure and function of the wetland ecosystems (Bai et al. 2017;Lou et al. 2018).The plant characteristics, in addition to photosynthesis, are key indicators that characterize the plant ecological and physiological processes and their corresponding functions.For instance, wetland plants change their size (i.e.height, stem diameter and leaf size), nutrient stoichiometry and photosynthesis to adapt to hydrological fluctuations (Zhang et al. 2019a(Zhang et al. , 2019b(Zhang et al. , 2021a;;Yao et al. 2021).Plant morphology plasticity, biomass allocation, and photosynthetic adaptation are common growth strategies for wetland plants in response to hydrological fluctuations (Bizet et al. 2015;Li et al. 2021;Song et al. 2021).Thus, the coupling relationship between the plant performance and hydrological fluctuations is an important factor when studying the response and adaptation of plants to hydrological fluctuations.
Plant characteristics and photosynthesis positively or negatively respond to hydrological fluctuation in wetlands (Zhang et al. 2019b;Bai et al. 2021;Zhang et al. 2023).In fact, the coupling involving the relationship between plant growth and hydrological fluctuations, as well as the coupling between different plant characteristics, have become hotspots for the wetland research (Holmquist et al. 2021;Zhang et al. 2021a).Previous studies have shown that there is a hydrological threshold for plants, and the value of this threshold varies depending on the type of plant species, growth stage, and other environmental factors (Jing et al. 2017;Zhang et al. 2020b;Ma et al. 2022).Hydrological fluctuations around the hydrological threshold exert stress on plant growth and, especially, on plant characteristics and photosynthesis (Li et al. 2022).In response to hydrological fluctuations, wetland plants change their characteristics and photosynthesis within a certain limit to maintain their growth, reproduction, and development (Yan et al. 2015;Bai et al. 2021;Yao et al. 2021).It has been found that the hydrological fluctuations change the range of light, temperature, gas exchange, and nutrient resources, which are recognized as important raw materials and conditions for the photosynthetic process of wetland plants (Colmer et al. 2013;Li et al. 2017).In addition, the hydrological fluctuations decrease the availability of raw materials, severely inhibit the photosynthesis of wetland plants, and result in a decrease in energy sources for the support of life (Luo et al. 2011;Zhang et al. 2019b).Moreover, hydrological fluctuations change the redox conditions of the soil by excessive consumption of soil oxygen.They also strongly affect the process of nutrient absorption and accumulation of potential toxic compounds in the soil, which influence plant growth and photosynthesis (Baastrup-Spohr et al. 2016;Zhang et al. 2019b).It is, therefore, of utmost importance to conduct in-depth research about the ecological and photosynthetic responses of the wetland plants to hydrological fluctuations.
Carex schmidtii is a native tussock-forming species in the riparian wetlands and mountainous wetlands in the northeast of China (Yan et al. 2015;Zhang et al. 2021a).It has an abundant root system and forms a microtopography that supports a strong accumulation of biological elements and a rich diversity of species (Zhang et al. 2022;Li et al. 2023;Wang et al. 2023).C. schmidtii, with a hydrological fluctuation tolerance, can survive along a fresh-water flooding gradient in the frequently flooded zone (Zhang et al. 2019a(Zhang et al. , 2020a)).
It is, therefore, recognized as a desirable restoration species.Previous studies have reported how leaf morphological and structural characteristics, nutrient trade-off, biotic stresses, and photosynthesis of C. schmidtii for varied hydrological conditions including water-level fluctuations, alternating flooding-drought conditions, hydrological fluctuation, hydrological and microtopographic effects, consecutive drought and re-flooding, flooding depth (Yan et al. 2015;Zhang et al. 2019aZhang et al. , 2019bZhang et al. , 2020aZhang et al. , 2020bZhang et al. , 2021aZhang et al. , 2022;;Qi et al. 2021).In addition, previous studies also have reported how plant diversity, community structure, community pattern and their landscape in response to changes in hydrological conditions (Wang et al. 2019;Zhang et al. 2021b;Qi et al. 2022;Wang et al. 2023).In relation to the hydrological conditions, the degradation and restoration of the tussock wetlands (with an emphasize on the latter) have also received attention from both government and scientists (Zhang et al. 2019a(Zhang et al. , 2019b;;2021c;Bohnen et al. 2022).However, this is not sufficient for the determination of the response mechanism of Carex tussocks to hydrological changes.
Ecological and photosynthetic characteristics in response to hydrological fluctuations are, instead, important for the determination of the hydrological response mechanism and restoration of C. schmidtii (Zhang et al. 2019b;Yao et al. 2021;Huang et al. 2022).Therefore, in the present study, an indoor simulation experiment has been performed, focusing on the plant characteristics and photosynthesis under specific hydrological conditions.The main objectives were: (1) To examine the effects of initial water depth (WD), water-level amplitude (WA), duration and their interactions on plant size parameters, biomass parameters, and photosynthesis of C. schmidtii; (2) To determine the cultivation time for the growth maximum of C. schmidtii under different hydrological conditions; and (3) To examine the relationships between any two plant parameters of C. schmidtii.The findings helped to understand the response and restoration mechanism of C. schmidtii with respect to hydrological fluctuations, and provided supporting information for the management and restoration of tussock wetlands.

Plant materials
Carex schmidtii tussocks and soil were collected from the riparian wetland along the Nenjiang River in the Momoge Wetland Nature Reserve (45°50′-46°18′ N, 123°55′-124°4′ E) in northeastern China.The hummocks were cut into uniform pieces of 20 cm in height and 15 cm in diameter.Each of them was then planted in plastic containers (50 cm × 50 cm × 50 cm) together with 30 kg of a mixed soil.Half of the hummocks were planted within the soil and the other half were planted on top of the soil.All these plants were, thereafter, cultivated in a greenhouse in the Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences (Figure 1; Zhang et al. 2020a).
At the beginning of the experiments, the growing plants had an average height of 25.2 cm and an average biomass of 0.028 g.The average value of the soil pH was 7.34.In addition, the average total amount of organic carbon, nitrogen, phosphorus, and carbon 13 (δ 13 C) in the soil were 33.63%, 4.27 g .kg −1 , 0.52 g .kg −1 and −27.39 ± 0.04‰, respectively.Furthermore, the tap water that was used in the experiment had a pH value of 7.34, an electrical conductivity of 6.10 S .m −1 , a total amount of dissolved solids of 0.47 g .L −1 , a salinity of 0.35‰, a total amount of nitrogen of 0.46 mg .-1,and a total amount of phosphorus of 0.02 ± 0.00 mg .L −1 (Zhang et al. 2019a).The air temperature changed between 25 and 38 °C, and the relative humidity ranged from 36% to 58% in the greenhouse (Zhang et al. 2020a).

Experimental design
The initial water depth (WD) was set to 0 cm (WD0) above the soil surface at the start of the experiments, i.e. at day 0 (Figure 1).Also, the soil water content in WD0 ranged from 37.11% to 41.62%.Water-level amplitude (WA) treatments were implemented in these experiments on day 10, with a hydrological fluctuation period of 40 days.Furthermore, water was added to these soil samples day 0, day 50, and day 90 to increase the water level by 0, 5, and 10 cm, respectively.Furthermore, water was drained from the plastic containers day 30, day 70, and day 110, to decrease the water level by 0, 5, and 10 cm, respectively.Thus, a larger water depth (WD + WA) was set for these soil samples from day 10 to day 30, from day 50 to day 70, and from day 90 to day 110.In addition, the soil samples recovered from the WD treatments from day 30 to day 50 and from day 70 to day 90.A total of nine experimental processes with six duplicates were thereby conducted (Figure 1).The whole experiment lasted for 110 days, covering the key growth stages of C. schmidtii and ensuring complete hydrological cycles.

Determination of plant size parameters and biomass parameters
The plant height and stem diameter of C. schmidtii were determined by using a flexible rule and vernier caliper, respectively, every 20 day.In addition, samples from the plants were collected every 20 day.As the second leaf on the plants was determined to have the best growth and full extension, it was collected from the plants.These leaf samples and remaining plant samples were individually placed in envelopes for marking, and dried at 65 °C until a constant weight was achieved, and then plant dry weight and leaf dry weight were obtained by weighing.

Determination of photosynthesis parameters
The chlorophyll content was determined by using an ultraviolet spectrophotometer.0.2 g of fresh leaves from C. schmidtii were soaked in 10 ml of 96% ethanol for two days in the dark, and then centrifuged at 2000 rpm for 15 min.Following Arnon (1949), the absorbance of the supernatant was, thereafter, measured at 663, 645, and 470 nm.
The photosynthesis parameters, including the maximum quantum yield of photosystem II (Fv/Fm), the effective quantum yield (ΔFv/Fm'), the photochemical fluorescence quenching (qL), and the non-photochemical energy quenching (NPQ), were determined by using chlorophyll fluorescence measurements (Zhang et al. 2019b;Helm et al. 2020).The measurements were performed on fully expanded leaves, between 09:00 am and 11:30 am, by using a portable fluorometer (PAM-2500, Walz, Effeltrich, Germany).Prior to these measurements, the C. schmidtii leaves were adapted to darkness for 15 min using leaf clips.Based on the light response curve, the minimal fluorescence (Fo) was measured by using a weak red-measuring beam, and the maximum fluorescence (Fm) was measured by using a saturated light pulse.Furthermore, Fv/Fm and ΔFv/Fm' were calculated by using Equations ( 1) and ( 2), respectively: where the maximum Fm′ and fluorescence F′ were measured for light-adapted leaves.Additionally, qL and NPQ were calculated by using Equations ( 3) and ( 4), respectively:

Statistical analyses
IBM SPSS Statistics 26 software, Origin 9.3 software and R 4.3.1 were used to analysis and plot.Prior to further analyses, the plant size parameter (i.e.plant height and stem diameter), biomass parameters (i.e.plant dry weight and leaf dry weight), and photosynthesis parameters (i.e.chlorophyll content, Fv/Fm, ΔFv/Fm, qL, and NPQ) of C. schmidtii were checked for normality and homogeneity.Some of these parameters were then log-transformed or square root-transformed to meet the assumptions of homoscedasticity.However, the permutational analysis of variance (ANOVA) had to be implemented in the R software (within the lmPerm package) if the assumptions of homoscedasticity were not met al.so, the multifactorial ANOVA was used to evaluate the effects of WD, WA, duration time, and their interactions on the plant size, biomass, and photosynthesis parameters of C. schmidtii.Furthermore, a polynomial fitting was used to describe the relationship between the plant parameters and the duration time.In addition, the time (TP) to obtain the maximum values of the plant parameters was calculated by using the formula of the symmetry axes of the binary Linear equation.Significantly positive, of negative, relationships between any two plant parameters of C. schmidtii were examined by using the Origin 9.3 software.

Plant height and stem diameter of Carex schmidtii
The WD, WA, and duration time were found to have significant effects on the plant height and stem diameter of C. schmidtii (Figure 2 and Table 1).Based on all data that were collected from all growth stages, the plant height rapidly increased from 25.2 cm to 62.5 cm in the early growth stage between day 0 and day 20.It reached the highest value of 69.7 cm on day 40, and decreased to 53.2 cm at the end of the experiment (Figure 2).As the WD value increased, the plant height of C. schmidtii corresponded to the following the order: WD5 < WD10 < WD0.As was the situation with the WD treatments, the WA5and WA10-induced plant heights were significantly lower than the WA0-induced one.Moreover, the stem diameter of C. schmidtii did first increase to 0.20 cm from day 0 and day 40, and then decreased to 0.15 cm on the 100th day.Also, the stem diameter decreased with an increasing WD value.In addition, the WA0-and WA5-induced stem diameters were 0.18 cm and 0.19 cm, respectively, which was significantly larger than the WA10-induced one (0.16 cm).

Plant dry weight and leaf dry weight of Carex schmidtii
The plant dry weight and leaf dry weight of C. schmidtii were found to be significantly affected by WD, duration time, and the interactive effects of WD and WA (Figure 3).The plant dry weight did first increase and then decrease with an increase in time.The largest value of 0.33 g was measured on the 80th day, which was 2.2 times larger than the lowest value of 0.15 g that was measured on the 20th day.Also, the plant dry weight did gradually decrease with an increasing WD value.The plant dry weight after the WD10 + WA10 and WD10 + WA0 treatments was significantly lower than for the other seven treatments, and the largest value of the plant dry weight was obtained after the WD0 + WA10 treatment.Moreover, the leaf dry weight increased from 0.014 g (day 0) to 0.091 g (day 80).Also, it showed a similar variation for an increased WD value as was the situation with the plant height.The leaf dry weight after the WD0 + WA10 (0.2121 g) and WD0 + WA0 (0.2182 g) treatments were significantly larger than the leaf dry weight after the WD10 + WA10 (0.3000 g) and WD10 + WA0 (0.3249 g) treatments (Figure 3).

The TP for Carex schmidtii
The TPs ranged from 49.1 days to 63.4 days for the plant height, from 41.5 days to 63.4 days for the stem diameter, from 49.8 days to 100 days for the plant dry weight, and from 66 days to 100 days for the leaf dry weight (Table 2).

Photosynthesis parameters of Carex schmidtii
The WD and WA treatments and the duration time, in addition to the interactive WD and WA treatment, was found to have significant effects on the chlorophyll content of C. schmidtii (Figure 4 and Table 1).The chlorophyll content of C. schmidtii decreased from 2.09 mg g −1 to 0.65 mg g −1 with an increase in time.The chlorophyll content on the 80th day was 1.55 mg  g −1 , which was significantly lower than the values obtained from day 0 to day 60.However, it was higher than the value obtained on the 100th day.The chlorophyll content of C. schmidtii after the WD0 treatment was 2.06 mg g-1, which was 1.26 times larger than the content obtained after the WD5 treatment, and 1.27 times larger than the content obtained after the WD10 treatment.Additionally, the chlorophyll content decreased from 1.90 mg g −1 to 1.67 mg g −1 for increasing WA treatments.As a result of the interactive WD and WA treatments, the chlorophyll content after WD0 + WA0 and WD0 + WA5 treatments were 2.36 mg g −1 and 1.97 mg g −1 , respectively.These values were significantly larger than the values obtained after WD10 + WA10 (1.49 mg g −1 ) and WD5 + WA5 (1.51 mg g −1 ) treatments.Moreover, the Fv/Fm and ΔFv/Fm' parameters of C. schmidtii were significantly affected by the WD and WA treatments and the duration time, in addition to their interactive combinations (Figure 4 and Table 1).The Fv/Fm parameter decreased with an increasing WD treatment, and the highest value of Fv/Fm was 0.67.This value was 89% larger than the smallest value obtained after WD10 treatment.The Fv/Fm value after the WA0  treatment had the highest value of 0.67, which was 1.20 times larger than the smallest value obtained after the WA5 treatment.Additionally, the Fv/Fm value decreased with an increase in time.For the interactive effects of WD and WA treatments, the largest value of Fv/Fm (0.76) was obtained after the WD0 + WA0 treatment, and the smallest value (0.49) was obtained after the WD10 + WA10 treatment.Furthermore, the Fv/Fm value after the WD0 + WA0 treatment was basically unchanged from day 0 to day 70, and it thereafter decreased.The variation of ΔFv/Fm' for hydrological fluctuations was similar to the corresponding variation of Fv/Fm.The ΔFv/Fm' values ranged from 0.39 (WD10) to 0.52 (WD0) after WD treatments, and from 0.40 (WA5) to 0.51 (WA0) after WA treatment.For the interactive effect of WD and WA treatments, the ΔFv/Fm' values ranged from 0.35 (WD10 + WA10) to 0.58 (WD0 + WA0).
With one exception, the qL and NPQ parameters were significantly affected by WD, WA, and duration time, and for the interactive effects of WD and WA treatments (Figure 4 and Table 1).As the exception, the NPQ parameter was not affected by the WA treatment.The qL value after WD5 treatment was significantly larger than after WD0 and WD5 treatments.The qL values ranged from 0.65 (WA10) to 0.68 (WA5) under WA treatments, t.The largest qL value obtained after all interactive WD + WA treatments was obtained for the WD5 + WA0 treatment.Furthermore, the NPQ values ranged from 0.37 (WD5) to 0.47 (WD10).

Relationships between any two plant parameters of Carex schmidtii
The plant height showed a significantly positive relationship with the stem diameter, chlorophyll content, ΔFv/Fm' , and NPQ (Figure 5).The chlorophyll content and ΔFv/Fm' increased with an increase in the stem diameter.Also, the plant dry weight was positively related to the leaf dry weight and NPQ, but negatively related to ΔFv/Fm' .Furthermore, the ΔFv/Fm' and qL values increased with an increase in the leaf dry weight, but the NPQ value decreased.Also, the chlorophyll content increased with an increase in the Fv/ Fm, ΔFv/Fm' , and qL values.The Fv/Fm value increased with an increase in the ΔFv/Fm' value and a decrease in the qL value.In addition, the ΔFv/Fm' and qL values decreased with an increase in the NPQ value.

Discussion
Hydrology fluctuation is a controlling factor that affects the plant growth, community assembly, and community pattern in the wetland system (Baastrup-Spohr et al. 2016;Lou et al. 2015Lou et al. , 2018)).In general, the hydrological fluctuation is a common press that limits the plant growth, reproduction, and development.It thereby influences the structure and function of the wetlands (Yao et al. 2021;Li et al. 2022;Zhang et al. 2023).However, wetland plants with a strong hydrological fluctuation tolerance can survive from hydrological events (Bai et al. 2021;Huang et al. 2022).In the present study, all treatments have resulted in a living C. schmidtii.However, high WD and WA treatments resulted in a low plant performance.An appropriate hydrological threshold can improve the plant performance (Lou et al. 2018;Zhang et al. 2020b;Ma et al. 2022).Once this threshold is exceeded, the plants will be subjected to hydrological stress and their performance will be damaged.Hydrological fluctuations are the most common form of environmental stress in natural wetlands, and hydrological conditions that exceed the optimal growth threshold of C. schmidtii occur now and then (Zhang et al. 2019b;Jing et al. 2023).Therefore, C. schmidtii has formed a series of systematic and effective survival strategies to survive from sudden hydrological disaster events (Zhang et al. 2020a).According to the findings in the present study, it can be confirmed that the response of C. schmidtii to hydrological fluctuations gradually weakens with the increase in time.As being consistent with results in previous studies, C. schmidtii has a stronger adaptability to hydrological fluctuations in the early growth stage of the plant (Zhang et al. 2019a(Zhang et al. , 2020a)).Based on its overall performance, a comprehensive evaluation of plant adaptability is an important goal of the research in this field.More specifically, the plant characteristics and photosynthesis are essential and important components for a comprehensive evaluation of the plant adaptability.
Morphological plasticity is a common strategy for the plants to adapt to environment fluctuations (Bizet et al. 2015;Yuan et al. 2017).In the present study, plant height and stem diameter did significantly change under the effect of the WD and WA treatments and the duration time, in addition to their interactive effects.Previous studies have found that the plant height increases with an increasing water depth, and the extended plant stems and leaves above the water surface will then obtain sufficient photosynthetic materials and oxygen for their growth (Xie et al. 2008;Khanday et al. 2017).It was in the present study found that C. schmidtii had a peak value of plant height and stem diameter from day 0 to day 20.This rapid growth ensured that the plants had a stronger possibility to survive from hydrological fluctuations.This result is consistent with the leaf responses that was reported by Zhang et al. (2019b).They observed a rapid growth of leaves in the early growth stage of C. schmidtii.Additionally, it was also found in the present study that the growth rate of the plant height and stem diameter varied for the differently applied hydrological conditions.Although the adaptability of plant height and stem diameter to hydrological changes became reduced in the later growth stage, the changes of these plant parameters for various hydrological conditions were still significant, especially at the end of the experiment.Thus, it could be concluded that hydrological fluctuations will change the plant height and stem diameter by regulating the plant growth rate.
The plant biomass and leaf biomass are key factors for material conservation, demonstrating another indication of plant morphology plasticity (Li et al. 2021;Pan et al. 2023).In the present study, both the plant dry weight and leaf dry weight were found to decrease with an increasing WD value.This observation was consistent with previous findings that showed that the biomass of Carex brevicuspis decreased with an increasing water depth (Li et al. 2018).The plants changed their biomass allocation to maintain the relative size of the developing canopy (Hayes et al. 2017;Pan et al. 2023).The photosynthesis and respiration were thereby ensured.However, the large water depth inhibited the plant growth and biomass accumulation.The resulting plant dry weight and leaf dry weight after WD10 + WA10 and WD10 + WA0 treatments were significantly lower than for the other treatments.The plants invested too much energy to withstand the hydrological fluctuations, and reduced the accumulated dry material in their bodies (Voesenek et al. 2016;Sasidharan et al. 2018;Zhang et al. 2020a).Thus, the change in biomass allocation is another indicator of plant morphology plasticity for plants to adapt to environmental hydrological fluctuations.
In the present study, the TPs have been used to determine the time when C. schmidtii reached its growth maximum for the different treatments (Zhang et al. 2020b).The TPs for the plant height, stem diameter, and plant dry weight were of largest importance in the evaluation of the plant peaks.However, the correlations amongst the different TPs showed that the hydrological fluctuations resulted in a dispersion of the TPs, thereby weakening their correlations.However, the TPs for the plant parameters (i.e.plant height and stem diameter) demonstrated a significant correlation for most of the hydrological treatments.Thus, the development of this secondary indicator was found to be beneficial for further analyses of the effect of environmental stress on the plant growth.The TPs could effectively describe the plant growth maxima.
Furthermore, the plant size variations were in the present study found to be external indicators of wetland plants in response to hydrological fluctuations, while the photosynthesis capacity was the internal adaptation mechanism of the wetland plants (Luo et al. 2011;Zhang et al. 2019b).Photosynthetic pigments and parameters are important for the photosynthesis capacity, effectively reflecting the physiological process and material accumulation in the wetland plants (Benavente-Valdés et al. 2016;Zhang et al. 2020a).It has in the present study been found that the WD and WA treatments, and the duration time, have significant effects on the chlorophyll content of C. schmidtii.Previous studies have shown that a long-term drought will restrict the synthesis of chlorophyll and reduce the chlorophyll content in the plants (Yuan et al. 2017;Zhang et al. 2019b).However, it has here been found that higher WD and WA values will also reduce the chlorophyll content in C. schmidtii.Thus, a small water depth were more conducive to increase the chlorophyll content.In general, the chlorophyll content was closely related to the photosynthesis parameters.The Fv/Fm and ΔFv/Fm' parameters of C. schmidtii displayed a response pattern that was similar to the chlorophyll content.The largest Fv/Fm and ΔFv/Fm' values were obtained after the interactive WD0 + WA0 treatment, while the smallest values were obtained after the interactive WD10 + WA10 treatment.This could be explained by the following: 1) Large water depths limited the growth of the plant above the ground (especially the leaves), which resulted in a damage of the photosynthetic sites; 2) Flooding limited the plant respiration, causing a lack of energy for plant life activities (especially photosynthesis) (Voesenek et al. 2016;Sasidharan et al. 2018).Overall, a wet environment was more conducive to the synthesis of photosynthetic pigments and the photosynthesis process of C. schmidtii.
The coupling relationships between any two plant parameters have been overlooked in previous studies, being equally important as the response mechanism of individual plant parameters to hydrological fluctuations (Holmquist et al. 2021).In the present study, the chlorophyll content and ΔFv/Fm' of C. schmidtii have been found to experience significant correlations with other plant parameters, and can be used as an important indicator for plant growth.Individual variations of parameters reflect the flexibility of plant response, while correlations between parameters show common adaptations.All these parameter modifications ensure that the plants can survive hydrological events.In addition, the relationship between plant parameters can directly display the linkage effect under environmental stress.This linkage effect is also a key component of the plant response mechanism.The complexity of this mechanism makes it necessary to conduct further in-depth research studies in this field in the future.
In field, C. schmidtii expericed hydrological fluctuations in freshwater marshes (Zhang et al. 2019a).Water-level fluctuation is considered as master factor affecting management and restoration of C. schmidtii tussocks (Zhang et al. 2020a;Qi et al. 2021).Previous studies has reported the effects of single flooding or drought conditions, as well as alternating drought-flooding conditions, on the growth, physiological traits, and nutrient trade-off of C. schmidtii, providing valuable information for the restoration and conservation of C. schmidtii tussocks, eapecially water-saving irrigation measures for semi-arid region (Yan et al. 2015;Zhang et al. 2019aZhang et al. , 2019bZhang et al. , 2020bZhang et al. , 2021a)).However, all of these overlooked periodic hydrological fluctuations during the growth period of C. schmidtii, which limits the development of restoration technology based on hydrological fluctuation for C. schmidtii wetlands.Our previous research reported growth and physiological responses of C. schmidtii to water-level fluctuation in view of leaf traits (Zhang et al. 2020a).Considering the plant size, biomass and photosynthetic characteristics of C. schmidtii with respect to hydrological fluctuations, we found lower water levels and smaller water level changes are more conducive to promoting the ecological and physiological resopnses of C. schmidtii to hydrological fluctuations.

Conclusion
With just a few exceptions, it has in the present study been shown that the initial water depth (WD), water-level amplitude (WA), duration time, and their interactions have significant effects on the plant characteristics and photosynthesis of Carex schmidtii.The exceptions are the WA effects on the plant dry eright, leaf dry eright, and NPQ.The plant height, biomass parameters, and photosynthesis parameters did first increase, and then decrease, for an increasing time under different hydrological fluctuation conditions.Also, the WD0 treatment contributed to an improved increase of the plant size, biomass, and photosynthesis (Fv/Fm and ΔFv/Fm') parameters.Additionally, the plants obtained the largest plant height, chlorophyll content, Fv/Fm value, and ΔFv/Fm' value as a result of the WA0 treatment.With an exception of the NPQ parameter, the chlorophyll content became positively related to the plant size and photosynthesis parameters.Furthermore, the ΔFv/Fm' parameter showed a significant relationship with all plant parameters, except qL.Moreover, the WD and WA treatment, and their interactions, did significantly affect the plant dry weight and leaf dry weight, except for the AW-induced effect on the plant dry weight.The plant dry weight did first increase, and then decrease, with an increasing time.A similar pattern was obtained for the leaf dry weight.The highest values of the plant dry weight and leaf dry weight were observed on the 80th day, which indicated that C. schmidtii adopted a material conservation strategy in the late growth stage.This result was consistent with our previous findings that were based on a compromise between the specific leaf area and leaf dry content.Lower water levels and smaller water level changes are more conducive to the growth of C. schmidtii and can promote the capacity of plant size, biomass accumulation and photosynthesis of C. schmidtii coping with hydrological fluctuations together.Thus, lower water levels and smaller water level changes is preferred for wetland restoration.

Figure 5 .
Figure 5. significant relationships between any two plant traits of C. schmidtii.fv/fm: the maximum quantum yield of photosystem II; Δfv/fm': the effective quantum yield; ql: the photochemical fluorescence quenching; nPQ: the non-photochemical energy quenching.

Table 1 .
results (f values) of three-way anoVas about the effects of water depth (Dw), change in amplitude of water level (aw), duration time (t) and their interactions on the ecological and photosynthetic traits of Carex schmidtii.

Table 2 .
changes in plant height, stem diameter, plant dry weight and leaf dry weight of Carex schmidtii over time.
** p < 0.01.* wD: initial water depth, wa: water-level amplitude.tPx: the time point to obtain the maximum value of plant traits.α represents that the max time threshold that tPx can retrieve.