Study on the water level ecological amplitude of the wetland plant Triarrhena lularioriparia

Abstract Water level plays an important role in the growth and development of wetland plants. To study the ecological responses of the growth and physiology of Triarrhena lularioriparia under different water levels, seedlings were used as the study materials and in 13 water level gradient treatment groups, −67, −54, −42, −34, −29, −16, −3.5, 10, 20, 30, 40, 50, 60 cm. The results showed that (1) the plant height, leaf number and root activity of T. lularioriparia first increased and then decreased with the increment of the water level gradient, and the highest values appeared at a water level of approximately 0 cm; (2) the concentrations of chlorophyll a (Chl a), chlorophyll b (Chl b), chlorophyll (a + b) (Chl a + b) and biomass first increased and then decreased with the increment of the water level gradient. The malondialdehyde (MDA) and antioxidant enzyme concentrations of T. lularioriparia first increased and then decreased, and both reached their maximums at a water level of approximately 20 cm. The soluble protein decreased and then increased with the change in the water level gradient and reached its lowest value at 31 cm; soluble protein increased slightly at 30–40 cm but showed a stable trend after the waterlogging depth exceeded 40 cm. (3) The water level ecological amplitude of T. lularioriparia was [−38.62, 41.07 cm], and the optimum water level ecological amplitude was [−11.04, 20.15 cm]. This study demonstrates that T. lularioriparia could adapt to drought and flooding stress to a certain extent, but it could be stressed if the water level exceeded the suitable range.


Introduction
Water level is a basic environmental controlling factor in lake wetland ecosystems.Water level changes play a significant role in the development of vegetation and ecosystems (Coops et al. 2003).The response of wetland plants to changes in water level is an important area in wetland ecology.Water depth and soil water content play important roles in the growth of plant communities and the temporal framework that affects their distribution in water bodies along with temporospatial changes.Changes in water level gradients fundamentally determine the structure and characteristics of wetlands, such as the plant population structure (Laine et al. 2007), species diversity (Riis et al. 2002) and community succession.Different water level gradients also affect soil moisture and air content, as well as biological, physical and chemical processes in the growth environment, thus affecting spatial distribution.Water level gradients cause changes in the soil environment and therefore have an effect on the spatial distribution and ecological characteristics of vegetation (Kellogg et al. 2003;Xi et al. 2016).
Triarrhena lularioriparia (T.lularioriparia) is a perennial herb of Poaceae and adapts to warm, humid and sunny environments (Dang et al. 2012) and usually grows in wetlands and hilly slope areas with an elevation of 12.0 to 17.0 m (Li et al. 2006;Hu et al. 2010).T. lularioriparia is one of the dominant species in Poyang Lake, China (Ge et al. 2011), and is interlaced with reeds and distributed in a small area with incomplete continuity (Liu et al. 2013).Previous studies have shown that the survival rate of T. lularioriparia decreased after more than 120 days of flooding stress, and it completely died after 180 days of flooding enviroments (Li et al. 2018).Studies on the relationship between T. lularioriparia and hydrology are mainly related to the effects of water level conditions on its organic carbon components and distribution (Xiong et al. 2002), the species richness in its seed bank under flooding conditions and so on (Chen et al. 2020).However, under the influence of environmental pressures, the water level of Poyang Lake has fluctuated frequently in recent years, and the overall water level has appeared to decline.The distribution and succession of vegetation in Poyang Lake Beach are affected by water level changes (Tan et al. 2016).As one of the dominant communities in Poyang Lake, the responses of vegetation to the environmental changes have crucial influences on the biodiversity and ecosystem stability in Poyang Lake.
At present, there are a few studies of the wetland plants ecological amplitude, and previous researches have focused on the water ecological amplitude and water level ecological amplitude (Cao et al. 2015;Zhen, 2018).In order to reveal the hydrological ecological thresholds of morphological and physiology indicators of T. lularioriparia, and provide scientific basis for field planting of it, This study investigates the ecological amplitude and the ecological characteristics of the wetland plant T. lularioriparia from the perspective of water level.And it is helpful to understand the appropriate water level conditions and reveal the response mechanisms of T. lularioriparia.

Test materials and design
The experiment was performed in the plant sunlight cultivation room in the Key Laboratory of Poyang Lake Wetland and Watershed Research, Ministry of Education.Rhizomes of the T. lularioriparia to be tasted were collected from the typical distribution area of T. lularioriparia in Poyang Lake Nanji Wetland National Nature Reserve (28.9 N, 116.32 E) in April 2017.Rhizomes of uniform size were selected for precultivation before the experiment.The soil of the culture media used for preculture was taken from Nanji Wetland and was configured into sandy loam with medium fertility and a pH of 5.4 with soil in the laboratory.The collected T. lularioriparia rhizomes were cultivated in an experimental basin (35 cm Â 26 cm Â 13 cm) to keep the soil moisture content at approximately 45%, and the cultivation lasted for approximately 10 days.In May, T. lularioriparia seedings with average plant height of 54.7 cm were selected and transplanted to 42 experimental pots (35 cm Â 26 cm Â 13 cm) with 10 plants per basin.
The 42 pots of precultured seedlings of T. lularioriparia were randomly placed into 13 treatment groups along the following gradients: À67, À54, À42, À34, À29, À16, À3.5, 10, 20, 30, 40, 50, and 60 cm (the above measured values were all measured from the soil surface).When the water level is higher than the soil surface, it is usually referred to as the water level, i.e. a positive value; however, when it is located below the soil surface, it is usually called the groundwater depth.These two parameters were fused.If the result is positive, it indicates that the water level is above the soil surface (Figure 1), while negative numbers indicate that the water level is below the soil surface.The treatment groups with a positive water level were placed in the test basin in a plastic vat with a height of 70 cm, an upper diameter of 57 cm and a lower diameter of 45 cm, and three replication groups were established for each treatment group.The test was conducted for 30 days from May 2 to June 1, 2017.At the beginning of the test, the water level of each treatment group was observed every day, and water was replenished to the water levels set by the test.The soil moisture content was measured by a HH2 soil Moisture Meter, and the soil moisture content lost to evaporation was replenished according to the measured soil volume moisture content.Through the end of the experiment, the morphological and physiological indexes of each treatment group were obtained.

Index determination
Ten plants with a good growth condition were randomly selected to measure the height of straightened leaves as the growth index by using a ruler.All leaves that were not yellowed were counted in the number of leaves.The physiological indexes were all determined for successfully growing T. lularioriparia plants, and the 3rd through 5th leaves from the top to the bottom were collected.A SPAD-502 chlorophyll meter was used to measure the relative chlorophyll concentrations of T. lularioriparia.An ethanol acetone mixture was applied to extracting chlorophyll from the ground leaves that had been directly soaked and ground by mixing with acetone and anhydrous ethanol in equal amounts to obtain the extracted solution.The concentrations of chlorophyll a, chlorophyll b, total chlorophyll and carotenoids were obtained by the improved Arnon method (Wang and Huang 2015).The MDA concentrations were determined by the thiobarbituric acid method.The activity of superoxide dismutase (SOD) was determined by using the nitrogen blue tetrazole method.The peroxidase (POD) activity was measured by the guaiacol method.The potassium permanganate titration method was used to measure catalase (CAT) activity.The soluble protein content was determined by the Coomassie blue G-250 staining method.Root activity was measured by the TTC method (Zhang, 2004).

Data processing
Based on the growth and physiological characteristic of T. lularioriparia measured in the experiment, a Gaussian model is used to obtain the relationship between species and environmental factors.The Gaussian model is: where y is an index that represents biological or ecological characteristics of the plant species; A þ y 0 represents the maximum value of the corresponding index; X c indicates the optimum value of the plant species for a certain environmental factor; W represents the degree of resistance of the species and can also be described as the physical amplitude of the species.Generally speaking, the ecologically suitable range is [x c -2w, x c þ2w] and the optimum ecological range is [x c -w, x c þw].
OriginPro 9 was used to perform GaussAmp fitting on 12 indexes of the T. lularioriparia seedlings, including plant height, number of leaves, chlorophyll a, chlorophyll b, total chlorophyll, Soil and Plant Analyzer Development (SPAD), aboveground biomass, SOD, POD, CAT, MDA, soluble protein and root activity.GaussAmp equation has been used to calculate the water level ecological amplitude based on each index.

Ecological response of the morphological characteristics of T. lularioriparia seedlings to different water levels
Water level is a limiting factor for the growth of T. lularioriparia.The plant height and leaf number of T. lularioriparia first increased and then decreased with increasing water level, showing a single peak in growth.Plant height reached a maximum of 88.7 cm at a water level of À3.06 cm, while the average leaf number reached a maximum of 8 at a water level of 3.69 cm (Figure 2).

Ecological response of photosynthetic pigments and SPAD in seedlings of T. lularioriparia to different water levels
The Chl a, Chl b, Chl (a þ b), and SPAD increased then decreased with an increase of water level.Chl a reached its maximum value of 11.15 mg/g À1 at a water level of 16.52 cm, Chl b reached its maximum value of 4.17 mg/g À1 at a water level of 18.21 cm, and Chl (a þ b) reached its maximum value of 15.32 mg/g À1 at a water level of 16.97 cm.The chlorophyll relative value content SPAD of T. lularioriparia reached its maximum value of 33.8 mg/g À1 at a water level of À0.92 cm (Figure 3).
3.3.Ecological response of the antioxidative enzyme system in T. lularioriparia seedlings under different water level gradients The antioxidative enzyme system (SOD, POD and CAT) of T. lularioriparia showed a significant normal distribution with the increment of the water level gradient, showing a single peak type change.The maximum value of SOD was 241.11UÁg À1 at a water level of À29.76 cm; the maximum value of POD was 1,217.34U/(gÁmin) À1 at a water level of 22.88 cm; the maximum value of CAT was 61.64 U/(gÁmin) À1 at a water level of 24.84 cm (Figure 4).

Ecological response of malondialdehyde (MDA) and the aboveground biomass of T. lularioriparia seedlings to different water levels
The MDA concentration and aboveground biomass of T. lularioriparia increased and then decreased with the increment of the water level gradient, showing a significant normal distribution and an unimodal change.the MDA concentration was at a maximum (104.57nmol/g À1 ) at a water level of 27.99 cm; the aboveground biomass was at a maximum (47.87/gÁplantÀ1 ) at a water level of 1.56 cm (Figure 5).

Ecological response of soluble protein and root activity of T. lularioriparia seedlings to different water level gradients
The soluble protein decreased at the beginning stage and then increased with the increment of the water level gradient, while the root activates increased then decreased with an increase of water level.The soluble protein reached a minimum value of 1.22 mg/g À1 at a water level of 31.57cm, and the root activates reached the maximum value of 1.22 mg(gh) À1 at a water level of 31.57cm (Figure 6).

Water level ecological amplitude for seedling growth of T. lularioriparia
According to the changes of the ecological characteristics of T. lularioriparia seedlings with different water levels, Gaussian equation fitting was performed for the 13 abovementioned indexes across the water level gradient (Table 1).For each index, the lowest interval takes the maximum value of each index measurement value and the highest interval takes the minimum value of each index measurement value to determine the water level ecological amplitude for the growth of the seedlings of T. lularioriparia.Thus, the optimal water level ecological amplitude of T. lularioriparia was within the range of À11.04 to 20.15 cm, and the limit of the water level ecological amplitude of T. lularioriparia was [-38.62 cm, 41.07 cm] (Table 2).

Response of the morphological indexes of T. lularioriparia to the water level gradient
The Gaussian fit of plant height, leaf number and root activity was good, and all of their values increased first and then decreased with the increase of water gradient (from drought to flooding), and reached the maximum at about 0 cm water level, indicating that the soil moisture environment near saturation is the most suitable for the above four indexes.
Leaf number affects the total amount of fixed solar energy of plants and thus affects plant growth and development.Under certain stress conditions, plant height grows slowly  and leaf shedding increases to reduce energy loss.When stress is further enhanced, plant growth may stop growing or even die directly (Wang 2014).In this experiment, plant height and leaf number increased first and then decreased with the increase of water level, which showed the adaptation to drought and flooding stress.
Root system is the link between individual plant and soil, and it is an important organ for plants to absorb water and nutrients, and its activity affects the growth of plants.root activity was the index with the narrowest ecological amplitude among the 13 indicators measured in this experiment, and was most affected by water level, which reduced the hydrological ecological threshold of T. lularioriparia to a large extent (Table 2).
The highest values of plant height, leaf number and root activity all appeared at a water level of approximately 0 cm, which proves that the soil moisture environment near saturation is the most suitable for the growth of T. lularioriparia.This study found that with the increment of the water level gradient, the morphological indexes of T. lularioriparia all showed an increasing and then a decreasing trend, which proved that T. lularioriparia could resist the stress environment by adjusting its own structure and reducing the size of individual plant and the number of leaves under drought and flooding stress, which is consistent with the research results for Carex cinereus (Yang et al. 2015).

Response of the physiology indexes of T. lularioriparia to the water level gradient
The concentrations of photosynthetic pigments will affect the photosynthetic efficiency and the normal growth and development of plants (Sun et al. 2018).In this study, the value of Chl a, Chl b, Chl (a þ b) and SPAD all increased at the beginning stage and then decreased with the increment of the water level gradient.An extreme water level gradient environment will inhibit the synthesis of photosynthetic pigments (Liu et al. 2013), which may be due to the reduction of the amount of light energy capture to resist the extreme environment and to ensure that the photosynthetic apparatus is not damaged by photooxidation (Guo et al. 2016).With the increment of the water level gradient, the biomass of T. lularioriparia also first showed an increasing trend and then a decreasing trend, which proved that an extreme stress environment strongly inhibits the photosynthetic pigments, thus preventing the accumulation of biomass, as reported by Hu Jia (Hu 2018).
The water level and ecological threshold are relatively consistent when the MDA and antioxidant enzyme concentrations of T. lularioriparia reached their maximum values.With the increment of the water level gradient, the content showed an increasing and then a decreasing trend, and both reached their maximums at a water level of approximately 20 cm.This finding indicated that under flooding stress, the antioxidant enzyme system and malondialdehyde content in T. lularioriparia increase to help it resist stress.Soluble protein decreased in the beginning stage and then increased with the increasing in the water level and reached the lowest value at 31 cm, which proved that the waterlogging stress environment inhibits the synthesis of soluble protein; soluble protein also increased slightly at 30-40 cm, suggesting that T. lularioriparia has a corresponding adaptation to the waterlogging environment at this stage, consistent with previous studies (Li et al. 2013;Ji et al. 2019).However, soluble protein showed a stable trend after the waterlogging depth exceeded 40 cm, and the extreme waterlogging environment still had very serious inhibitory effects on the growth and development of T. lularioriparia, consistent with the researches of Zhang and Li (Zhang et al. 2012;Li et al. 2018) .

Water level ecological amplitude of T. lularioriparia
The water level ecological amplitude of T. lularioriparia is [-38.62 cm, 41.07 cm], and the optimum water level ecological amplitude is [À11.04,20.15 cm].The SOD, CAT and root activity of T. lularioriparia are more sensitive to changes of the water level gradient, and root activity is the most sensitive to these changes (Zhang et al. 2018).T. lularioriparia is not a flood-tolerant or drought-tolerant plant, but it can adapt to certain drought and flooding environments and has a weak adaptability to extreme environments.When the water level exceeds 41.07 cm or is less than À38.62 cm, the survival of T. lularioriparia will be greatly affected, which is consistent with the observed effect of flooding on T. lularioriparia seedlings in this study, with the optimum soil moisture content of T. lularioriparia of 45%, and with the growth of T. lularioriparia in the field.
Plants have a self-regulating mechanism in the face of adversity, but when external stress is too strong, plants will stop growing or even die.Generally speaking, the wider the ecological range, the greater the ability of plants to cope with stress conditions.In this paper, 13 indicators were used to study the ecological amplitude of T. lularioriparia and explore its hydrological ecological threshold.increasing the test indexes and experimental gradients could explore the hydrological ecological amplitude and ecological threshold of T. lularioriparia more accurately.

Conclusion
In summary, the water level ecological amplitude of T. lularioriparia is [À38.62,41.07 cm], and the optimum water level ecological amplitude is [À11.04,20.15 cm], which shows that T. lularioriparia has some tolerance to drought and flooding environments and explains why T. lularioriparia can still survive under water level fluctuations.This research results regarding the ecological response and adaptation mechanism of T. lularioriparia under different environments provide reference for wetland conservation and restoration in Poyang Lake.In future researches, multiple environmental factors should be studied, variable gradients should be increased, and the investigation and study of the actual situation in the field should be strengthened to further combine indoor tests with field tests.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Funding
This research was supported by the Natural Scientific Foundation of China (42261010, 42061021).

Notes on contributors
Yun Cao received his PhD from Nanjing Normal University in 2007.His research interests are in plant ecology and ecological restoration in wetlands.He is currently working on a research project about the ecological amplitude of beach-typical vegetation in Poyang Lake under a water level gradient.
Na Guan is a graduate student at Jiangxi Normal University.Her research interest is in submerged vegetation restoration.
Xiaojia Tang is a graduate student at Jiangxi Normal University.Her research interest is in submerged vegetation restoration.
Minli Xu is a undergraduate student at Jiangxi Normal University.Her research interest is in submerged vegetation restoration.

Figure 1 .
Figure 1.Schematic diagram of testing setup design.

Figure 2 .
Figure 2. Heights and leaves number of Triarrhena lularioriparia under different water levels.

Figure 3 .
Figure 3. Photosynthetic pigments (Chl a þ b, Chl a and Chl b) and Soil and Plant Analyzer Development (SPAD) contents of Triarrhena lularioriparia leaves under different water levels.

Figure 6 .
Figure 6.Soluble protein contents and root activity of Triarrhena lularioriparia under different water levels.

Table 1 .
Gauss regression equations for morphological and physiological indices of Triarrhena lularioriparia.