Effects of Domestic Sewage on the Photosynthesis and Chromium Migration of Coix lacryma-jobi L. in Chromium-Contaminated Constructed Wetlands

Abstract

To investigate the effects of domestic sewage on the photosynthesis and chromium migration of plants in chromium-contaminated constructed wetlands, small vertical flow constructed wetlands of Coix lacryma-jobi L. were set up. These wetlands were used to treat wastewater containing 0, 20, and 40 mg/L of hexavalent chromium (Cr (VI)), prepared with domestic sewage (DS), 1/2 Hoagland nutrient solution (NS), and 1/2 Hoagland nutrient solution prepared with domestic wastewater (DN), respectively. The aim was to investigate the effects of domestic sewage on indicators, such as plant growth and chromium accumulation. The results were as follows: (1) Plant heights were significantly inhibited under 20 mg/L and 40 mg/L Cr (VI) treatments, and stem diameters were not significantly affected. The use of domestic sewage in treatment alleviated the inhibition of Cr (VI) on the growth of Coix lacryma-jobi L. (2) Indicators such as root activity, photosynthetic gas exchange, and chlorophyll fluorescence properties significantly decreased with the increase in Cr (VI) concentration. The values of these photosynthetic gas exchange parameters under the DN treatment were the greatest, followed by NS and DS. On the 70th day of Cr (VI) treatment, the net photosynthetic rate (Pn) under the DN treatment was significantly higher than that under NS and DS treatments. (3) Glutathione (GSH) content in roots, stems, and leaves of Coix lacryma-jobi L. significantly increased with the increase in Cr concentration, and it increased more significantly under the DN and DS treatments than under the NS treatment. (4) With the same Cr treatment, the Cr content in roots, stems, and leaves of Coix lacryma-jobi L. under the NS treatment was the highest, followed by DS and DN. The total Cr content in the substrate under the DN treatment was the highest, followed by DS and NS. (5) The addition of domestic sewage reduced the Cr (VI) content in the water sample and increased the organic matter content. The Cr (VI) content in the water sample under the NS treatment was the highest, followed by DS and DN. The addition of domestic sewage increased the accumulation of chromium in the substrate, decreased the absorption of chromium by plants, increased GSH content in roots, stems, and leaves, alleviated the damage of Cr (VI) to plants, and thus benefited the growth of Coix lacryma-jobi L. in the constructed wetlands and ensured the sustainable and stable operation of the wetlands.

1. Introduction

Chromium (Cr) exists mainly in the forms of Cr (III) and Cr (VI) in nature. Cr (III) is relatively stable in soil. A small amount of it in the human body can promote carbohydrate metabolism, but an excessively high level can be toxic to the human body [1]. Cr (VI) is highly mobile in soil and easily enriched in crops and may ultimately be absorbed by the human body. Cr (VI) is highly toxic to the human body, with strong mutagenic, carcinogenic, and teratogenic effects [2,3].

Cr is widely used in industries, such as electroplating, printing and dyeing, papermaking, and leathermaking. Both urban and rural industrialization accelerate amid social development, and improper discharge of Cr-containing wastewater can still be seen in China due to supervision, technology, and other reasons. For example, in January 2019, several business owners in Henan Province were reported by Dahebao to have discharged wastewater with an excessive level of Cr during sheepskin dyeing, causing serious environmental pollution [4]. In March 2022, the Wanxiu Court of Wuzhou City reported an incident of illegal discharge of pollutants containing elements, such as nickel and Cr from an electroplating workshop in Wuzhou City [5]. Improper discharge of industrial Cr not only threatens the safety of crop production, but also poses a risk to human health.

Taking technical measures to reduce the Cr content in wastewater is an important way to prevent and control Cr pollution. Constructed wetlands, with the advantages of low construction and operation costs, easy maintenance and management, and high ecological benefits [6], have been widely used in the purification of industrial wastewater and domestic sewage. However, in underdeveloped regions or countries, such as India, Brazil, and South Africa, it is common to find mixed discharge of industrial wastewater and domestic sewage [7,8,9]. Therefore, some researchers have noticed the influence of domestic sewage on the role of constructed wetlands in removing heavy metals from wastewater. For example, a study by Lou suggests that mixing domestic sewage can improve the ability of constructed wetlands to remove Fe, Zn, Cu, Cd, and Ni from acid mine wastewater [10]. A study by Chen et al. shows that adding domestic sewage or straw decomposition products can improve the ability of constructed wetlands to remove heavy metals from wastewater [11]. Wang et al. found that the coexistence of domestic sewage and acid mine wastewater can improve the ability of constructed wetlands to remove heavy metals from wastewater [12]. Constructed wetlands play a good role in purifying Cr in wastewater [13]. Li Kai found that a higher content of domestic sewage in mixed polluted wastewater can better benefit the purification of Cr (VI) and Ni (II) by Leersia hexandra Swartz constructed wetlands [14]. A study by Li Shuai shows that the use of domestic sewage to prepare 1/2 Hoagland nutrient solution can significantly improve the ability of Coix lacryma-jobi L. constructed wetlands to remove Cr from wastewater, which has a better treatment effect and results in better plant growth than irrigation with only domestic sewage or 1/2 Hoagland nutrient solution [15]. Organic matter and microorganisms in domestic sewage may facilitate the transformation of Cr (VI) to Cr (III) in the substrate, increase the adsorption of Cr by the substrate, and reduce the absorption of Cr by plants [16]. Therefore, it is believed that adding domestic sewage may reduce the absorption of Cr by plants, promote the accumulation of Cr in the substrate, and thus alleviate the harm of Cr on plants and benefit the sustainable and efficient operation of constructed wetlands. However, not many studies have been conducted in this field.

Constructed wetlands mainly rely on the absorption of plants and substrates and the effects of microorganisms to remove pollutants from wastewater. How the plants grow is crucial for the continuous and efficient treatment of wastewater by constructed wetlands [17]. Photosynthetic capacity directly reflects the growth of plants, and photosynthetic parameters are extremely sensitive to adversity and stress. Sun et al. [18] found that Cr (VI) stress significantly reduced stomatal conductance (Gs) and the activity of photosystem II (PSII) in wheat leaves, leading to a significant decrease in photosynthetic rate (Pn). Under Cr (VI) stress, plants produce excessive reactive oxygen species (ROS), which can cause serious damage to the photosynthetic system of plants. As a result, the photosynthetic gas exchange and chlorophyll fluorescence properties of plants are inhibited, and the degree of inhibition can reflect the degree of Cr (VI) stress on plants [19]. Glutathione (GSH) generated in plants is a low-molecular-weight antioxidant that can enhance important functions, such as reducing intracellular peroxides, scavenging free radicals, and chelating heavy metals. Fatma et al. [20] found that the toxicity of Cr to the photosynthesis of wheat can be mitigated by the exogenous addition of S and NO, essentially by increasing the level of GSH, and thus mitigating the toxic effects of Cr on photosynthesis and the activity of Calvin cycle enzymes in leaves. Whether the effect of Cr (VI) on the photosynthesis of Coix lacryma-jobi L. is mitigated by regulating the GSH content in leaves under the condition of domestic sewage added needs to be studied in depth.

In this study, miniature Coix lacryma-jobi L. vertical flow constructed wetlands were set up, and wastewater containing different concentrations of Cr (VI) was prepared by adding domestic sewage, 1/2 Hoagland nutrient solution, and 1/2 Hoagland nutrient solution prepared with domestic wastewater. The effects of adding domestic sewage and nutrients on plant growth, photosynthetic system, and GSH content of the plants in chromium-contaminated constructed wetlands were studied to reveal the physiological mechanism of domestic wastewater mitigating the effect of Cr (VI) on plants. The results can provide a theoretical basis and new ideas for the efficient treatment of Cr-polluted wastewater by constructed wetlands.

2. Material and Methods

2.1. Material

Coix lacryma-jobi L., a wild wetland plant in Guangxi, is provided by the Crops Research Institute of Guangxi Academy of Agricultural Sciences.

2.2. Experiment Design

This study was conducted from March to September 2022 at the teaching and research base of the College of Agriculture, Guangxi University. Miniature vertical flow constructed wetlands were constructed with a study by Li et al. as a reference [21]. Large plastic barrels with a top diameter of 71 cm, a bottom diameter of 45 cm, and a height of 61 cm were used. The barrels were filled with 10 cm of large cobblestones (diameters ranging from 3 to 5 cm) and 40 cm of river sand (particle size ranging from 0.25 to 0.5 mm) from the bottom up, and a tap was installed 10 cm above the bottom for drainage. Twelve uniformly grown Coix lacryma-jobi L. seedlings were planted in each barrel. Within 1 month after planting, the seedlings were irrigated with 1/2 Hoagland nutrient solution. Cr (VI)-containing wastewater treatment was performed after the seedlings grew to 20 cm. Domestic wastewater (DS, main indicators, COD: 200~205 mg/L, TP: 0.78~1.18 mg/L, TN: 8.16~12.33 mg/L, NH₃-N: 15.98~22.56 mg/L. Wastewater was collected from sewage pipes on each water inflow day, with its main indicators measured. The wastewater was not pretreated). Thereafter, 1/2 Hoagland nutrient solution (NS) and 1/2 Hoagland nutrient solution prepared with domestic wastewater (DN) were used as mother liquor, respectively. To ensure the stability of Cr (VI) in the incoming water, K₂Cr₂O₇ solution was added to the mother liquor 2 h before each inflow to simulate wastewater containing 0, 20, and 40 mg/L of Cr (VI) (no significant change in Cr (VI) content was detected within 2 h). After entering the constructed wetlands, the wastewater containing Cr (VI) remained for 3 days after the water inflow, followed by a 4-day drying cycle, making a 7-day intermittent cycle. Each treatment was repeated three times, on five wetland pools per repetition. Each wetland tank was filled with 30 L of water by each water inflow.

2.3. Sample Collection

Coix lacryma-jobi L. plant samples and water samples were collected 10, 40, and 70 days after Cr (VI) treatment, and substrate samples were collected 48 days after the treatment. The roots, stems, and leaves of the samples were separated, rinsed with deionized water, and filled into sample bags after the surface was dried. They were kept fresh at 4 °C and brought to the laboratory. The samples were cut into small pieces and stored in sealed bags in a freezer with an ultra-low temperature of −80 °C for further measurement of the total chromium content in various organs of Coix lacryma-jobi L. The root tip was used for root activity measurement. The water samples were collected at the outlet and stored in PE plastic bottles with a capacity of 100 mL, and the bottles were stored in an insulation box with ice packs and sent to the laboratory. After being filtered through a 0.45 μm water filter membrane, the water samples were placed in centrifuge tubes with a capacity of 50 mL at 4 °C for the determination of total Cr content in the water discharged. The surface substrate was randomly sampled to obtain 200 g of fresh sample, which was placed in sealed bags and air-dried in a drying room. The substrate samples were used for the measurement of total Cr and total organic carbon content in the substrate.

Root, stem, and leaf samples were collected 20 and 50 days after the Cr (VI) treatment for GSH content measurement in different organs.

2.4. Experimental Methods

2.4.1. Measurement of Stem Diameters and Plant Heights

Six uniformly grown plants were selected from each repetition 10, 40, and 70 days after Cr (VI) treatment, and plant heights and stem diameters were measured. The plant height is the above-ground length, and the stem diameter is the diameter of the second above-ground node. According to the method developed by Long et al. [22], the inhibition rate of Cr (VI) treatment on plant height and stem diameter was calculated as follows:

$$\mathrm{Inhibition} \mathrm{rate} \mathrm{of} \mathrm{plant}(\%)=\frac{\mathrm{Plant} \mathrm{height} \mathrm{of} \mathrm{CK} -\mathrm{Height} \mathrm{of} \mathrm{plants} \mathrm{treated} \mathrm{with} \mathrm{Cr} \left(\mathrm{VI}\right)}{ \mathrm{Plant} \mathrm{height} \mathrm{of} \mathrm{CK}} \times 100\%$$

(1)

$$\mathrm{Inhibition} \mathrm{rate} \mathrm{of} \mathrm{stem} \mathrm{diameter}(\%)=\frac{ \mathrm{Stem} \mathrm{diameter} \mathrm{of} \mathrm{CK}-\mathrm{Stem} \mathrm{diameter} \mathrm{treated} \mathrm{with} \mathrm{Cr} \left(\mathrm{VI}\right)}{ \mathrm{Stem} \mathrm{diameter} \mathrm{of} \mathrm{CK}} \times 100$$

(2)

2.4.2. Measurement of Root Activity

The TTC (triphenyl tetrazolium chloride) method was used to measure root activity [23]. Briefly, 0.5 g of Coix lacryma-jobi L. root tip sample was cut into small pieces and added to 5 mL of 0.4% TTC and 5 mL of phosphate buffer (pH = 7.0). The sample was incubated at 37 °C for 3 h and extracted in ethyl acetate for 15 min. The characteristic absorption peak at 458 nm was measured using a spectrophotometer.

2.4.3. Measurement of Photosynthetic Parameters and Chlorophyll Fluorescence Parameters

Photosynthetic parameters were measured using an LI-6400XT portable photosynthesis meter (LI-COR, Lincoln, NE, USA) from 9 to 11 a.m. 10, 40, and 70 days after Cr (VI) treatment, respectively [24]. The chlorophyll fluorescence parameters were measured using an AMP-2100 portable chlorophyll fluorometer (Walz, Effeltrieh, Germany), and the initial fluorescence (Fo), maximum fluorescence (Fm), and maximum photochemical quantum yield of PS II (Fv/Fm) were measured. The same parts of nine effective leaves were selected for each treatment, and one leaf was recorded three times to obtain the mean value for analysis.

2.4.4. Measurement of GSH Content in Different Parts of Coix lacryma-jobi L.

GSH was measured using the colorimetric method developed by Xie et al. [25]. Briefly, 1 g of fresh leaf sample was placed in a mortar, and 3 mL of pre-cooled 5% TCA solution and a small amount of quartz sand were added. The sample was thoroughly ground in an ice bath and then centrifuged at a low temperature (4 °C) for 15 min. Then, 0.25 mL of the supernatant liquid was obtained and mixed with 0.5 mL of Tris-HCl buffer solution (0.25 mol·L⁻¹, pH = 8). Thereafter, 0.25 mL of 3% formaldehyde was added, and the solution was shaken well and stood at room temperature for 20 min. Moreover, 3 mL of the DTNB solution pre-warmed in a constant temperature water bath at 25 °C was added and mixed well. After 5 min of standing, the measurement was obtained at 412 nm using a spectrophotometer, and GSH content was calculated based on its standard curve.

2.4.5. Measurement of Cr Content in Different Parts of Coix lacryma-jobi L., Substrate, and the Water Discharged

Based on the method developed by Wang [26], 0.3 g of the sample was placed into a boiling tube and soaked with 6 mL of HNO₃ and 1.5 mL of HClO₄ overnight. Thereafter, the sample was boiled and neutralized in a graphite furnace. After cooling, the solution was diluted to 50 mL with 0.2% HNO₃ and filtered. The chromium content was measured using an inductively coupled plasma emission spectrometer (model: ICP-5000, manufacturer: Focused Photonics Inc., origin: Beijing, China).

Based on the method developed by Liu [27], the total Cr content in substrate was measured using an inductively coupled plasma emission spectrometer after the samples were boiled and filtered.

Based on the method developed by Ribas [28], the Cr (VI) content in water was measured using diphenylcarbazide spectrophotometry. The colorimetric tubes used were soaked in 10% dilute nitric acid for over 16 h to prevent Cr (VI) adsorption on the inner walls of the tubes. Based on the method developed by Yue [29], the spectrophotometric method was used to measure the total Cr content in the water discharged.

2.4.6. Measurement of Organic Matter Content in Substrate

Based on the method developed by Wang [30], the potassium dichromate oxidation-external heating method was used to measure the total organic carbon content in the soil samples.

2.5. Statistical Analysis

Origin 2021 was used for plotting, Excel 2019 was used to collate data, SPSS Statistics 25 was used for calculation and statistical analysis, and Duncan’s test was used for multiple comparisons of significant differences (p < 0.05).

3. Results and Analysis

3.1. Plant Height and Stem Diameter

As shown in

Table 1. Plant heights and stem diameters under different treatments.

Treatment Time (d)	Irrigation Moisture	Cr (VI) Concentration (mg/L)	Plant Height (cm)	Plant Height Inhibition Rate (%)	Stem Diameter (mm)	Stem Diameter Inhibition Rate (%)
10	DN	CK	22.5 ± 0.72 a		8.77 ± 0.21 a
		20	21.3 ± 0.72 a	5.33	8.7 ± 0.1 abc	0.76
		40	18.73 ± 0.57 b	16.74	8.47 ± 0.12 cd	3.42
	NS	CK	22.17 ± 0.49 a	-	8.73 ± 0.15 ab
		20	18.67 ± 0.68 b	15.79	8.5 ± 0.1 bcd	2.67
		40	16.23 ± 0.7 c	26.77	8.4 ± 0.1 d	3.82
	DS	CK	22.33 ± 1.07 a	-	8.67 ± 0.06 abc
		20	19.23 ± 0.45 b	13.88	8.47 ± 0.15 cd	2.31
		40	17.33 ± 0.55 c	22.39	8.4 ± 0.1 d	3.08
40	DN	CK	92.3 ± 1.74 a		11.43 ± 0.25 a
		20	83.23 ± 1.48 b	9.82	11.17 ± 0.06 abc	2.33
		40	69.73 ± 1.04 d	24.45	11.03 ± 0.15 bcd	3.5
	NS	CK	91.6 ± 1.73 a	-	11.33 ± 0.25 ab
		20	76.3 ± 0.8 c	16.70	10.93 ± 0.15 cd	3.53
		40	65.47 ± 1.4 e	28.53	10.83 ± 0.15 cde	4.41
	DS	CK	83.37 ± 0.9 b	-	10.87 ± 0.25 cde
		20	70.6 ± 1.47 d	15.31	10.53 ± 0.21 de	3.07
		40	64.53 ± 0.87 e	22.59	10.7 ± 0.2 e	3.68
70	DN	CK	140.03 ± 3.01 a		12.77 ± 0.06 a
		20	129.4 ± 1.05 b	7.59	12.3 ± 0.2 b	3.66
		40	109.23 ± 1.29 e	21.99	12.03 ± 0.06 bcd	5.74
	NS	CK	139.7 ± 1.41 a	-	12.73 ± 0.15 a
		20	118.43 ± 0.83 d	15.64	12.17 ± 0.31 bc	4.45
		40	102.4 ± 0.7 f	22.81	11.93 ± 0.06 cde	6.28
	DS	CK	126.57 ± 2.15 c	-	12.27 ± 0.25 b
		20	106.77 ± 1.19 e	15.22	11.73 ± 0.15 de	4.35
		40	98.2 ± 1.47 g	22.41	11.67 ± 0.06 e	4.89

Note: Inhibition rate (%) = $\frac{\mathrm{Plant} \mathrm{height} \mathrm{of} \mathrm{CK} -\mathrm{Height} \mathrm{of} \mathrm{plants} \mathrm{treated} \mathrm{with} \mathrm{Cr} \left(\mathrm{VI}\right)}{ \mathrm{Plant} \mathrm{height} \mathrm{of} \mathrm{CK}}$ × 100%. Different lowercase letters indicate significant differences between treatments at the same time (p < 0.05).

, the plant heights of CK Coix lacryma-jobi L. ranged from 22.16 to 140.03 cm, and the plant heights under 20 mg/L and 40 mg/L Cr (VI) treatments ranged from 18.67 to 129.4 cm and 16.23 to 109.23 cm, respectively, with all being significantly shorter than CK. The degree of inhibition was greater with a higher Cr (VI) concentration (p < 0.05). The differences between treatments with different Cr (VI) concentrations were significant. With the same Cr concentration, the plant heights of Coix lacryma-jobi L. were greatest under 1/2 Hoagland nutrient solution prepared with domestic wastewater (DN), followed by 1/2 Hoagland nutrient solution (NS) and domestic sewage (DS). The plants under the DN treatment were significantly taller than those under NS and DS treatments (p < 0.05). With 20 mg/L of Cr (VI), the inhibition rates of Coix lacryma-jobi L. plant heights under DN, NS, and DS were 5.33%~75.9%, 15.64%~16.7%, and 13.88%~15.31%, respectively. With 40 mg/L of Cr (VI), the inhibition rates under DN, NS, and, DS were 16.74%~24.45%, 22.81%~28.52%, and 22.39%~26.7%, respectively. The higher the Cr concentration, the more significantly the plant growth is inhibited. The plant heights of Coix lacryma-jobi L. irrigated with domestic sewage were less inhibited than those under NS.

The stem diameters of Coix lacryma-jobi L. ranged from 8.4 to 12.73 mm, and the diameters under DN were the largest, followed by NS and DS, with insignificant differences.

3.2. Root Activity

The root activity of Coix lacryma-jobi L. ranged from 0.475 to 2.418 mg/gFW-h (Figure 1) and decreased with the increase in Cr (VI) concentration. The root activity of the plant showed a trend of increasing and then decreasing as the time of Cr (VI) treatment extended. In each period, the root activity under DN was the highest, followed by NS and DS, and the 40th day of treatment saw the highest root activity. On the 10th day of Cr treatment and with 20 and 40 mg/L of Cr (VI), the differences in root activity under different inflow conditions were not significant. On the 70th day, the root activity under DN was significantly greater than that under NS and DS (p < 0.05). Compared with CK, the root activity of Coix lacryma-jobi L. under DN, NS, and DS with 20 mg/L of Cr (VI) decreased by 18.53%~42.67%, 23.74%~53.62%, and 18.61%~41.08%, respectively, and that with 40 mg/L of Cr (VI) decreased by 39.07%~61.09%, 42.62%~72.27%, and 36.85%~65.76%, respectively. With the same Cr treatment, the root activity under NS dropped to a greater degree than under DN and DS.

3.3. Photosynthetic Gas Exchange Parameters of Coix lacryma-jobi L.

As shown in

Table 2. Photosynthetic gas exchange parameters under different treatments.

Photosynthetic Gas Exchange Parameters	Treatment Time (d)	Water Management	CK	Cr 20 mg/L	Cr 40 mg/L
Net Photosynthetic Rate (μmol/m2·s)	10	DN	19.2 ± 0.96 a	14.73 ± 0.51 b	11.56 ± 0.48 c
		NS	19 ± 0.98 a	12.12 ± 1.05 c	9.23 ± 0.73 d
		DS	13.96 ± 1.54 b	9.56 ± 1.52 d	6.79 ± 1.09 e
	40	DN	24.32 ± 1.12 a	19.52 ± 0.46 bc	16.13 ± 0.35 ef
		NS	24.22 ± 1.1 a	18.14 ± 1.25 cd	14.34 ± 1.23 fg
		DS	21.29 ± 1.31 b	17.53 ± 0.93 de	14.06 ± 1.43 g
	70	DN	18.6 ± 0.54 a	14.17 ± 0.9 b	11.77 ± 0.75 c
		NS	18.16 ± 1 a	10.17 ± 1.02 d	9.38 ± 0.92 d
		DS	13.94 ± 1.12 b	8.71 ± 1.07 d	6.9 ± 0.66 e
Stomatal conductivity (mmol·m−2·s−1)	10	DN	0.177 ± 0.005 a	0.126 ± 0.011 b	0.116 ± 0.005 bc
		NS	0.169 ± 0.013 a	0.118 ± 0.014 bc	0.098 ± 0.016 cd
		DS	0.131 ± 0.017 b	0.109 ± 0.015 bc	0.085 ± 0.008 d
	40	DN	0.183 ± 0.016 a	0.143 ± 0.013 bc	0.124 ± 0.008 bcd
		NS	0.182 ± 0.016 a	0.138 ± 0.009 bc	0.119 ± 0.019 cd
		DS	0.15 ± 0.013 b	0.124 ± 0.012 bcd	0.105 ± 0.014 d
	70	DN	0.101 ± 0.007 a	0.073 ± 0.004 bc	0.064 ± 0.007 c
		NS	0.097 ± 0.007 a	0.064 ± 0.003 c	0.049 ± 0.004 d
		DS	0.081 ± 0.011 b	0.053 ± 0.004 d	0.043 ± 0.004 d
Intercellular CO2 Concentration (μmol/mol)	10	DN	228.7 ± 15.97 a	200.16 ± 17.52 ab	163.49 ± 19.5 cde
		NS	225.49 ± 17.25 a	191.74 ± 16.27 bc	155.87 ± 17.39 de
		DS	182.08 ± 16.38 bcd	167.51 ± 22.77 bcde	147.29 ± 14.94 e
	40	DN	273.84 ± 24.18 a	238.79 ± 16.91 ab	213.15 ± 26.66 bc
		NS	272.93 ± 22.5 a	231.44 ± 18.14 bc	207.63 ± 24.05 bc
		DS	244 ± 22.26 ab	217.99 ± 17.75 bc	192.75 ± 20.76 c
	70	DN	176.43 ± 15.26 a	150.15 ± 19.67 abc	124.49 ± 12.42 cd
		NS	176.4 ± 16.15 a	147.33 ± 19.8 abc	120.74 ± 11.32 cd
		DS	163.66 ± 16.05 ab	136.98 ± 16.92 bcd	115.19 ± 14.79 d
Transpiration Rate (μmol/m2·s)	10	DN	2.64 ± 0.22 a	2.1 ± 0.15 b	1.8 ± 0.24 bcd
		NS	2.61 ± 0.21 a	1.97 ± 0.18 bc	1.62 ± 0.24 cd
		DS	2.13 ± 0.21 b	1.69 ± 0.23 cd	1.46 ± 0.19 d
	40	DN	3.69 ± 0.17 a	3.36 ± 0.16 ab	2.99 ± 0.19 bcd
		NS	3.66 ± 0.19 a	3.23 ± 0.19 b	2.82 ± 0.25 cd
		DS	3.13 ± 0.19 bc	2.98 ± 0.28 bcd	2.66 ± 0.23 d
	70	DN	2.42 ± 0.29 a	1.99 ± 0.25 b	1.66 ± 0.04 bcd
		NS	2.41 ± 0.3 a	1.83 ± 0.19 bc	1.52 ± 0.17 cd
		DS	1.96 ± 0.17 b	1.57 ± 0.21 cd	1.34 ± 0.11 d

Note: Different lowercase letters indicate significant differences between treatments at the same time (p < 0.05).

, the net photosynthetic rate (Pn) of Coix lacryma-jobi L. leaves ranged from 6.787 to 24.32 μmolCO₂/m²·s, stomatal conductance (Gs) ranged from 0.043 to 0.183 mmol/m⁻²·s⁻¹, intercellular CO₂ concentration (Ci) ranged from 115.188 to 273.836 μmol/mol, and transpiration rate (Tr) ranged from 1.32 to 3.69 μmol/m²·s. All these parameters significantly dropped as the Cr (VI) concentration rose, with similar changing trends. The various photosynthetic parameters increased and then decreased as the Cr (VI) treatment time extended, reaching the highest value on the 40th day of Cr (VI) treatment. With the same Cr concentration, the values of these photosynthetic gas exchange parameters under the DN treatment were the greatest, followed by NS and DS. On the 70th day of Cr (VI) treatment, the net photosynthetic rate (Pn) under DN was significantly greater than that under NS and DS. The gas exchange level under the condition of adding domestic sewage increased more significantly than that under NS from 10 to 40 d of Cr (VI) treatment. From 40 to 70 d, the gas exchange level under DN and DS decreased more significantly than that under NS, indicating that the gas exchange of Coix lacryma-jobi L. leaves under the condition of adding domestic sewage maintained a relatively high increase when the plants grew fast, and the rate of gas exchange slightly decreased when the plants grew relatively slow. As the concentration of Cr (VI) increased, the degree to which gas exchange of Coix lacryma-jobi L. leaves irrigated by domestic sewage or the mixture containing domestic sewage was inhibited was smaller than that of leaves irrigated by the pure nutrient solution.

3.4. Chlorophyll Fluorescence Properties of Coix lacryma-jobi L.

Initial Fluorescence (Fo) and Maximum Fluorescence (Fm)

The initial fluorescence (Fo) of leaves reflects the fluorescence yield when the PSII reaction center is fully open, and the Fo of Coix lacryma-jobi L. leaves increased significantly (p < 0.05) with the increase in Cr (VI) concentration and decreased. Then, it increased as the time of Cr (VI) treatment extended (

Table 3. Chlorophyll fluorescence parameters under different treatments.

Chlorophyll Fluorescence Parameters	Treatment Time (d)	Water Management	CK	Cr 20 mg/L	Cr 40 mg/L
Fo	10	DN	0.225 ± 0.016 d	0.249 ± 0.006 cd	0.276 ± 0.02 bc
		NS	0.226 ± 0.014 d	0.264 ± 0.011 bc	0.292 ± 0.019 ab
		DS	0.264 ± 0.013 bc	0.278 ± 0.017 bc	0.314 ± 0.021 a
	40	DN	0.189 ± 0.012 e	0.229 ± 0.01 d	0.258 ± 0.012 abc
		NS	0.198 ± 0.017 e	0.246 ± 0.013 bcd	0.271 ± 0.019 ab
		DS	0.241 ± 0.011 cd	0.25 ± 0.015 bcd	0.282 ± 0.012 a
	70	DN	0.25 ± 0.014 d	0.318 ± 0.01 c	0.333 ± 0.009 bc
		NS	0.251 ± 0.011 d	0.331 ± 0.029 bc	0.358 ± 0.012 ab
		DS	0.31 ± 0.016 c	0.328 ± 0.025 bc	0.37 ± 0.027 a
Fm	10	DN	1.444 ± 0.062 a	1.291 ± 0.061 b	1.19 ± 0.029 bc
		NS	1.431 ± 0.065 a	1.245 ± 0.074 bc	1.121 ± 0.075 cd
		DS	1.178 ± 0.078 bc	1.126 ± 0.068 cd	1.014 ± 0.074 d
	40	DN	1.779 ± 0.182 a	1.601 ± 0.042 b	1.511 ± 0.017 bc
		NS	1.773 ± 0.163 a	1.485 ± 0.059 bc	1.41 ± 0.053 c
		DS	1.476 ± 0.064 bc	1.395 ± 0.051 c	1.343 ± 0.057 c
	70	DN	1.362 ± 0.067 a	1.216 ± 0.055 bc	1.078 ± 0.056 cd
		NS	1.357 ± 0.072 a	1.16 ± 0.098 bcd	1.071 ± 0.095 d
		DS	1.245 ± 0.089 ab	1.148 ± 0.093 bcd	1.06 ± 0.04 d
Fv/Fm	10	DN	0.844 ± 0.017 a	0.807 ± 0.006 b	0.768 ± 0.018 cd
		NS	0.841 ± 0.016 a	0.788 ± 0.011 bc	0.738 ± 0.029 d
		DS	0.775 ± 0.026 bc	0.752 ± 0.03 cd	0.69 ± 0.003 e
	40	DN	0.894 ± 0.006 a	0.857 ± 0.007 b	0.829 ± 0.01 cd
		NS	0.888 ± 0.013 a	0.834 ± 0.015 bc	0.807 ± 0.021 de
		DS	0.837 ± 0.014 bc	0.821 ± 0.017 cd	0.79 ± 0.013 e
	70	DN	0.816 ± 0.018 a	0.738 ± 0.012 bc	0.69 ± 0.012 cde
		NS	0.815 ± 0.017 a	0.712 ± 0.043 bcd	0.664 ± 0.039 de
		DS	0.751 ± 0.017 b	0.712 ± 0.043 bcd	0.65 ± 0.039 e

Note: Different lowercase letters indicate significant differences between treatments at the same time (p < 0.05).

). The maximum fluorescence (Fm) can show the electron transfer of PSII, and the trend of Fm is opposite to that of Fo. Fv/Fm is the light energy conversion efficiency in the PSII reaction center, which is related to the degree of photoinhibition of Coix lacryma-jobi L. The Fv/Fm of Coix lacryma-jobi L. leaves ranged from 0.65 to 0.888, which decreased significantly (p < 0.05) with the increasing Cr (VI) concentration, indicating a decrease in photosynthetic capacity. Compared with the control, the maximum photochemical quantum yields under DN, NS, and DS under the 20 mg/L Cr (VI) treatment dropped by 4.14%~9.56%, 6.04%~12.6%, and 1.9%~5.2%, respectively, and by 7.21%~15.38%, 9.07%~18.53%, and 5.57%~13.44%, respectively under the 40 mg/L Cr (VI) treatment. With the same Cr concentration, the decrease under NS was the most significant, followed by DN and DS.

3.5. Glutathione (GSH) Content in Different Organs of Coix lacryma-jobi L.

According to

Table 4. The GSH content (μg/gFW) in different parts of Coix lacryma-jobi L. under different treatments.

Organs	Treatment Time (d)	Water Management	CK	Cr 20 mg/L	Cr 40 mg/L
Root	20	DN	36.915 ± 1.954 e	42.296 ± 2.197 de	55.099 ± 4.183 c
		NS	36.563 ± 2.489 e	74.323 ± 5.431 b	90.784 ± 5.431 a
		DS	40.959 ± 1.847 de	47.022 ± 2.409 d	61.037 ± 4.262 c
	50	DN	40.024 ± 6.117 f	57.49 ± 4.041 e	69.312 ± 2.781 cd
		NS	41 ± 4.818 f	86.561 ± 6.624 b	103.137 ± 7.757 a
		DS	41.532 ± 4.356 f	61.265 ± 6.836 de	78.218 ± 5.273 bc
Stem	20	DN	36.915 ± 1.954 e	42.296 ± 2.197 cd	55.099 ± 4.183 bc
		NS	36.563 ± 2.489 de	74.323 ± 5.431 ab	90.784 ± 5.431 a
		DS	40.959 ± 1.847 de	47.022 ± 2.409 bc	61.037 ± 4.262 a
	50	DN	40.024 ± 6.117 e	57.49 ± 4.041 d	69.312 ± 2.781 b
		NS	41 ± 4.818 e	86.561 ± 6.624 bc	103.137 ± 7.757 a
		DS	41.532 ± 4.356 e	61.265 ± 6.836 c	78.218 ± 5.273 a
Leaf	20	DN	36.915 ± 1.954 f	42.296 ± 2.197 e	55.099 ± 4.183 bc
		NS	36.563 ± 2.489 f	74.323 ± 5.431 c	90.784 ± 5.431 a
		DS	40.959 ± 1.847 f	47.022 ± 2.409 d	61.037 ± 4.262 b
	50	DN	40.024 ± 6.117 e	57.49 ± 4.041 d	69.312 ± 2.781 bc
		NS	41 ± 4.818 e	86.561 ± 6.624 b	103.137 ± 7.757 a
		DS	41.532 ± 4.356 e	61.265 ± 6.836 cd	78.218 ± 5.273 b

Note: Different lowercase letters indicate significant differences between treatments at the same time (p < 0.05).

, GSH content in the roots, stems, and leaves of Coix lacryma-jobi L. ranged from 36.56 to 103.14 μg/gFW, 18.64 to 53.13 μg/gFW, and 200.47 to 434.79 μg/gFW, respectively. With the same Cr concentration, the values of GSH content in roots, stems, and leaves of the plant were the highest under NS, followed by DS and DN (except for CK). Specifically, the GSH content in roots and leaves under NS was significantly higher than that under DN and DS (p < 0.05). GSH content in various parts of the plant increased to certain degrees as the time of treatment extended. With the same Cr concentration, the GSH content under NS increased from 5.57% to 18% over time, while that under DS and DN increased from 8.28% to 35.92% (except for CK). With domestic sewage added, the GSH content increased more significantly over time than under NS.

Compared with the control, values of GSH content in roots, stems, and leaves significantly increased after treatment with both 20 mg/L and 40 mg/L of Cr (VI). On the 20th and 50th days of Cr treatment with 40 mg/L of Cr (VI), the values of GSH content in roots, stems, and leaves under DN were 30.27% and 20.56%, 13.96% and 13.58%, and 30.28% and 13.08% higher than those under the treatment with 20 mg/L of Cr (VI); 9.98% and 7.13%, 11.46% and 11.14%, and 5.88% and 6.33% higher than the latter under NS; 29.8% and 27.67%, 19.06% and 15.26%, and 24.63% and 14.29% higher than the latter under DS. With domestic sewage added, the GSH content increased more significantly than under NS.

3.6. Cr Content in Roots, Stems, and Leaves of Coix lacryma-jobi L.

The total Cr content in roots, stems, and leaves of Coix lacryma-jobi L. ranged from 9.16 to 236.16, 1.94 to 80.14, and 3.22 to 91.67 mg/kg, respectively, with the highest Cr content in roots, followed by that in leaves and stems. The Cr content in roots, stems, and leaves increased as the Cr (VI) concentration increased and the treatment time extended. In addition, there were significant differences between treatments with different Cr (VI) concentrations (p < 0.05) (Figure 2).

Under DN, the Cr content in roots, stems, and leaves of Coix lacryma-jobi L. ranged from 9.16 to 144.7, 2.09 to 57.99, and 3.39 to 58.66 mg/kg, respectively. The figures under NS and DS were 9.48~236.16, 1.94~80.14, and 3.22~91.67 mg/kg, and 9.16~166.16, 2.21~63.45, and 3.34~64.7 mg/kg. With the same Cr (VI) concentration, the values of Cr content in roots, stems, and leaves of Coix lacryma-jobi L. under NS were higher than those under DN and DS.

3.7. Organic Matter Content in Substrate

As shown in

Table 5. Organic matter content (mg/kg) under different treatments.

Treatment Time (d)	Water Management	CK	Cr 20 mg/L	Cr 40 mg/L
10	DN	26.32 ± 2.13 a	21.77 ± 2.37 bc	18.17 ± 1.87 cde
	NS	15.17 ± 1.23 ef	12.31 ± 2.76 f	11.27 ± 1.58 f
	DS	24.95 ± 2.49 ab	19.91 ± 2.58 cd	16.94 ± 2.23 de
40	DN	32.69 ± 1.74 a	25.98 ± 2.81 bc	23.24 ± 2.01 cd
	NS	20.15 ± 2.9 de	16.16 ± 2.57 ef	14.23 ± 3.32 f
	DS	30.33 ± 1.77 ab	24.06 ± 2.6 cd	21.66 ± 3.64 cd
70	DN	34.12 ± 3.62 a	27.31 ± 3.29 c	24.94 ± 3.58 bcd
	NS	27.11 ± 4.11 bc	21.07 ± 3.65 cd	18.78 ± 2.82 d
	DS	31.03 ± 3.14 ab	25.68 ± 4.09 bc	22.38 ± 3.62 cd

Note: Different lowercase letters indicate significant differences between treatments at the same time (p < 0.05).

, the organic matter content under the three treatments ranged from 18.17 to 34.12 mg/kg (DN), 11.27 to 27.11 mg/kg (NS), and 16.94 to 31.03 mg/kg (DS). Organic matter content decreased as the Cr (VI) concentration increased and increased as the treatment time extended. The organic matter content under both DN and DS was greater than that under NS with the same treatment time and Cr (VI) concentration. The organic matter content was significantly greater (p < 0.05) under DN and DS than under NS on the 10th and 40th days of the Cr (VI) treatment, but on the 70th day, the differences in organic matter content between different conditions were not significant.

3.8. Total Cr Content in Substrate and the Water Discharged

The total Cr content in substrate (

Table 6. Total Cr content in substrate and total Cr content and Cr (VI) content in the water discharged under different treatments.

Treatment Time (d)	Water Management	Cr Content in the SUBSTRATE (mg/kg)			Cr content in the Water Discharged (mg/L)			Cr (VI) Content in the Water Discharged (mg/L)
		CK	Cr 20 mg/L	Cr 40 mg/L	CK	Cr 20 mg/L	Cr 40 mg/L	CK	Cr 20 mg/L	Cr 40 mg/L
10	DN	6.74 ± 1.57 d	42.82 ± 3.16 bc	66.55 ± 5.23 a	0.06 ± 0.02 d	0.24 ± 0.03 c	0.43 ± 0.05 b	0.03 ± 0.01 d	0.08 ± 0.01 c	0.15 ± 0.02 b
	NS	3.8 ± 0.59 d	39.79 ± 5.46 c	62.78 ± 6.16 a	0.03 ± 0.01 d	0.3 ± 0.04 c	0.56 ± 0.03 a	0.03 ± 0.01 d	0.16 ± 0.01 b	0.19 ± 0.01 a
	DS	7.21 ± 1.4 d	47.89 ± 3.8 b	69.23 ± 5.86 a	0.07 ± 0.03 d	0.27 ± 0.06 c	0.5 ± 0.07 ab	0.03 ± 0.01 d	0.15 ± 0.02 b	0.15 ± 0.02 b
40	DN	8.69 ± 1.48 c	76 ± 4.37 b	141.87 ± 6.59 a	0.08 ± 0.03 e	0.36 ± 0.04 d	0.65 ± 0.05 b	0.04 ± 0.01 d	0.14 ± 0.02 c	0.19 ± 0.02 b
	NS	4.79 ± 0.59 c	72.88 ± 9.11 b	136.16 ± 7.28 a	0.04 ± 0.02 e	0.45 ± 0.04 c	0.77 ± 0.05 a	0.04 ± 0.02 d	0.22 ± 0.02 b	0.31 ± 0.02 a
	DS	9.95 ± 0.96 c	82.27 ± 6.09 b	147.45 ± 10.94 a	0.1 ± 0.03 e	0.41 ± 0.09 cd	0.69 ± 0.05 ab	0.05 ± 0.01 d	0.2 ± 0.02 b	0.2 ± 0.02 b
70	DN	9.7 ± 1.17 d	135.27 ± 5.44 c	183.44 ± 11.62 ab	0.11 ± 0.05 e	0.44 ± 0.04 d	0.72 ± 0.06 b	0.05 ± 0.02 e	0.19 ± 0.02 d	0.24 ± 0.02 c
	NS	4.95 ± 0.62 d	129.67 ± 8.4 c	174.67 ± 7.81 b	0.04 ± 0.01 e	0.57 ± 0.02 c	0.86 ± 0.06 a	0.04 ± 0.01 e	0.29 ± 0.02 b	0.38 ± 0.02 a
	DS	10.62 ± 0.83 d	140.47 ± 8.47 c	189.78 ± 10.48 a	0.12 ± 0.04 e	0.53 ± 0.06 c	0.79 ± 0.08 ab	0.05 ± 0.01 e	0.24 ± 0.02 c	0.25 ± 0.02 c

Note: Different lowercase letters indicate significant differences between treatments at the same time (p < 0.05).

) ranged from 6.74 to 183.44 mg/kg under DN, 3.8 to 174.67 mg/kg under NS, and 7.21 to 189.78 mg/kg under DS, and the total Cr content in substrate increased significantly (p < 0.05) as the concentration of Cr (VI) treatment increased and the treatment time extended. The total Cr content in the discharged water ranged from 0.06 to 0.72 mg/L under DN, 0.03~0.86 mg/L under NS, and 0.07~0.79 mg/L under DS, and the Cr (VI) content in the discharged water under the three treatments was 0.03~0.24 (DN), 0.03~0.38 (NS), and 0.03~0.25 mg/L (DS), respectively (Table 6). NS saw the highest total Cr content in substrate and Cr and Cr (VI) content in the discharged water, followed by DS and DN. Therefore, the DN treatment had the best effect in treating chromium-containing sewage.

4. Discussion

Cr is not an essential element for plants, and excessive accumulation can inhibit plant growth. Cr (VI) can greatly inhibit plant growth due to its strong oxidative properties [31]. A study by Adhikari et al. showed that Cr (VI) stress increases reactive oxygen species (ROS) in maize seedlings and significantly decreases biomass in sand culture [32]. Liu et al. found that the growth of Coix lacryma-jobi L. is significantly inhibited when the Cr (VI) content reaches 20 mg/L [27]. However, studies have also shown that adding domestic sewage to prepare 1/2 Hoagland nutrient solution to constructed wetlands can alleviate the inhibitory effect of Cr (VI) on plant growth [11]. The present study showed that the growth of Coix lacryma-jobi L. was significantly inhibited by treatment with both 20 mg/L and 40 mg/L of Cr (VI) and the degree of inhibition increased significantly with the increase in Cr (VI) concentration. Hussain et al. also found that the growth of rice was severely inhibited under Cr (VI) stress [33]. However, the degree of inhibition of the growth of Coix lacryma-jobi L. was lower under DN and DS conditions than under the NS conditions, indicating that the use of domestic sewage to prepare 1/2 Hoagland nutrient solution can effectively alleviate the inhibitory effect of Cr (VI) on the growth of the plant. Similarly, a study by Saleem et al. found that the use of press mud as an organic amendment can effectively reduce the impact of Cr (VI) on the growth of Helianthus annuus L. [34]. Photosynthesis is an important factor determining how plants grow, and the growth of Coix lacryma-jobi L. is inhibited under Cr stress, which is closely related to the inhibition of photosynthesis [35].

Photosynthesis is highly sensitive to heavy metal stress, and many studies have used changes in the parameters of photosynthetic gas exchange as important indicators of plant response to heavy metal stress [36,37]. When plants are subjected to heavy metal stress, the efficiency of photosynthetic gas exchange is significantly inhibited, leading to a decrease in their photosynthetic capacity [38]. A study by Vernay et al. showed that Cr (VI) stress leads to a significant decrease in photosynthetic gas exchange parameters in Datura innoxia [39]. The present study found that Pn, Gs, Ci, and Tr of Coix lacryma-jobi L. leaves were significantly inhibited under Cr (VI) treatment, which is consistent with the results of previous studies. Chlorophyll fluorescence parameters are important indicators for assessing the effects of adversity and stress on the photosynthetic system of plants. Elevated Fo means reversible inactivation or irreversible damage of PS II reaction centers [40]. Fv/Fm indicates the maximum photochemical efficiency of PS II when all reaction centers are open. When plants are subjected to heavy metal stress, Fv/Fm significantly decreases, and photoinhibition of PS II increases [41]. In this study, Fo of Coix lacryma-jobi L. increased under Cr (VI) stress, indicating that the PSII reaction center was damaged to some extent. Fv/Fm decreased with the increasing intensity of Cr (VI) stress, indicating that the plants were subjected to photoinhibition. The parameters of light energy conversion efficiency of Coix lacryma-jobi L. leaves decreased significantly as the Cr (VI) increased, indicating that Cr stress causes photoinhibition of plants, which is consistent with previous findings. This may be an important reason for the inhibition of photosynthesis in this study. The DN and DS treatments with domestic sewage can reduce the extent to which photosynthetic gas exchange and chlorophyll fluorescence parameters are inhibited under Cr (VI) stress, indicating that domestic sewage may maintain high photosynthetic capacity by alleviating the damage of Cr (VI) to the photosystem, which may be an important reason for normal plant growth. Domestic sewage contains a large amount of organic matter and microorganisms. Previous studies have shown that organic matter and microorganisms can promote the conversion of Cr (VI) to Cr (III). Cr (III) easily binds with anions or organic matter in substrate to generate an insoluble form, which reduces the absorption of Cr by plants [42]. This may be an important reason why plant growth is less inhibited with domestic sewage added.

The root system of plants is an important part that directly contacts and accumulates Cr, which directly affects the growth of the aboveground parts. Root activity reflects the ability of plants to absorb water and nutrients, and is usually inhibited under heavy metal stress [43]. Guo et al. found that under Cd and Pb stress, the root activity of Matricaria chamomilla L. significantly decreased and was positively correlated with the degree of stress [44]. The extent to which the growth of Coix lacryma-jobi L. is inhibited by Cr (VI) can be obtained by studying the effect of Cr (VI) on root activity [45]. In this study, the root activity of Coix lacryma-jobi L. decreased with the increase in Cr (VI) concentration. Compared with CK, the degree of root activity inhibition under DS and DN was lower than that under NS, indicating that the root activity of Coix lacryma-jobi L. was less inhibited with domestic sewage added. The root activity under DN was the highest, and that under NS was higher than under DS. Root activity is closely related to the amount of heavy metals absorbed by plants. A study by Liu et al. suggested that with the increase in Cd accumulation in rice roots, root activity significantly decreased [46]. This study found a similar trend. Root activity under DN and NS was greater than that under DS, indicating that good nutritional conditions facilitate root growth. Under the condition with domestic sewage added, the root activity of Coix lacryma-jobi L. was less inhibited, which may be due to the fact that the plant absorbed less Cr.

In this study, Cr content in different parts of Coix lacryma-jobi L. under DS and DN was lower than that under NS, while Cr content in substrate under DS and DN was higher than that under NS, indicating that adding domestic sewage to chromium-containing wastewater may promote the conversion of Cr into forms in substrate that are difficult for plants to use. Meanwhile, the absorption of Cr by the root system and transfer of Cr from roots to aboveground parts both decrease, which may be an important reason why the growth, photosynthesis, and root activity of Coix lacryma-jobi L. were less inhibited under the condition with domestic sewage added. A study by Yu showed that an increase in organic matter content could promote the chemical form transformation of cadmium and reduce the toxicity of the element in soil [47]. Cr (VI) content in constructed wetlands may be closely related to organic matter content. Xiao found that soluble organic matter is an excellent reducer of Cr (VI), which can promote the conversion of Cr (VI) to Cr (III) [48]. A study by Choppala et al. showed that cattle manure and S can promote the reduction in Cr (VI) and its transformation into forms that are difficult for plants to absorb [49]. Microorganisms play an important role in the chemical form transformation of Cr, which can promote the transformation of Cr (VI) into Cr (III) and reduce its mobility [50]. Domestic sewage contains abundant organic matter and microorganisms, which facilitate the chemical form transformation of Cr. In this study, the organic matter content in the substrate under the treatment with domestic sewage added was significantly higher than when the plants were irrigated with nutrient solution only, which may promote the conversion of Cr (VI) and reduce Cr absorption by plants in the constructed wetlands. Finally, the decrease in the amount of Cr absorbed by Coix lacryma-jobi L. may be an important reason why the plant was less inhibited under the condition with domestic sewage added.

Cr (VI) can produce reactive oxygen species in plants due to its strong oxidative properties, leading to peroxidation of cell membrane lipids and causing chain reactions that lead to cell damage [51]. GSH generated in plants can act as an antioxidant and complex with heavy metals, which is very pivotal for plants to alleviate toxicity [52]. Jan et al. found that Cr (VI) treatment could maintain a high level of GSH content, and GSH content in plants indicates the degree of Cr (VI) stress on plants [53]. The results of this study showed that GSH content in roots, stems, and leaves increased under Cr treatment. The GSH content in various organs of Coix lacryma-jobi L. under DN and DS was lower than that under NS, and the DN treatment saw the lowest values, indicating that plants under NS were subjected to the highest degree of Cr (VI) stress, which is similar to the previous findings. When plants are under adversity or stress, they adapt to or mitigate the damage caused by stress by regulating the generation of GSH [54]. A study by Qiu showed that exogenous GSH can effectively alleviate Cr toxicity in rice [55]. Jan’s study showed that exogenous 24-Epibrassinolide can improve the antioxidative ability and increase the GSH content of plants under Cr stress [56]. The results of this study showed that domestic sewage improved the capability of Coix lacryma-jobi L. leaves to generate GSH, significantly increased GSH content, and improved the antioxidative ability of the plant, which is consistent with previous findings. Additionally, photosynthetic gas exchange parameters and chlorophyll fluorescence parameters of leaves under DN were higher than those under NS, indicating that domestic sewage may mitigate the effect of Cr (VI) on photosynthesis by increasing GSH content. This may be related to the complex composition of domestic sewage. However, further research is needed to investigate how domestic sewage increases GSH content.

5. Conclusions

Adding domestic sewage to chromium-containing wastewater increased the organic matter content in substrate, promoted Cr accumulation in substrate, reduced Cr uptake by plants, increased GSH content in roots, stems, and leaves of Coix lacryma-jobi L., improved indicators, such as photosynthetic gas exchange parameters of leaves, chlorophyll fluorescence parameters, and root activity, and alleviated the inhibition of plant growth by Cr (VI) stress. Under the condition with 1/2 Hoagland nutrient solution, indicators, such as plant growth, photosynthetic parameters, and fluorescence parameters of leaves were better in the constructed wetlands with domestic sewage added than under treatment with the nutrient solution or domestic sewage alone. The capability to remove Cr from wastewater under DN was better than that under NS and DS, indicating that under good nutritional conditions, adding domestic wastewater from external sources can improve the capacity of constructed wetlands to remove Cr (VI) from wastewater and contribute to the sustainable and efficient operation of the wetlands. However, the mechanism of promoting the chemical form transformation of Cr to mitigate the damage of Cr (VI) to plants and improve the effect of Cr (VI) treatment under a condition with domestic wastewater added needs further research.