Cadmium: Oxidative Stress and Photosynthesis Efficiency

Published: 2021-08-13 18:15:07
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Abstract
Cadmium (Cd) contamination is a worldwide threat for plant growth and healthy of human beings. Selenium (Se) is beneficial for plant growth under various stressful conditions such as heavy metal contamination. In this study, we investigated the mitigating effects of exogenous Se on Cd induced damage in tall fescue. Cd (30 mg/L, as CdSO4·8/3 H2O) and Se (0.1 mg/L, as Na2SeO3) treatments were employed individually or in combination using hydroponics system. The results showed that Cd stressed plants displayed obvious toxicity symptoms. Cd stress significantly increased malondialdehyde (MDA) content and electrolyte leakage (EL), and remarkably decreased chlorophyll content, antioxidant enzymes activity, photosynthetic efficiency and expression of relative genes. However, exogenous Se improved the overall physiological and photosynthetic behaviors of tall fescue under Cd stress. Se significantly increased chlorophyll content, enhanced CAT and SOD activities, but decreased the levels of MDA, EL and the uptake or accumulation of Cd in tall fescue under Cd stress. In addition, Se regulated gene transcription and improved photosynthetic efficiency under Cd treatment. Therefore, Se was effective in mitigating the deleterious effects of Cd in tall fescue, and the Se mediated up-regulation of antioxidant system and photosynthesis activities might help tall fescue to improve the Cd resistance.
Keywords: Tall fescue; Cadmium; Selenium; Oxidative stress; Photosynthesis activity; Gene expression
Introducation
Cadmium (Cd) is one of the most toxic heavy metal elements. It has been reported that the soil Cd concentration has remarkably increased during the past decades (Chen et al., 2013b), and the area of Cd contaminate arable land has increased to approximately 20 million hm2 (Liu et al., 2015a). Cd in soil is readily absorbed and accumulated by plants. Excess Cd usually inhibits plant growth and interferes with cellular functions through disrupting their metabolism (Yourtchi and Bayat, 2013). Cd can also result in leaf chlorosis, decline in photosynthesis rate, reduced water and nutrient uptake and plant death (Yourtchi and Bayat, 2013). Moreover, Cd leads to many other adverse effects in plants, including oxidative stress, DNA damage, protein dysfunction and the decrease in the efficiency of electron transport chain (Liu et al., 2013). In the agricultural practice, some chemical reagents can be applied to suppress Cd uptake and alleviates the Cd phytotoxic effect, which has been considered to be an economical and effective way for safe crop production (Sun et al., 2016).
Selenium (Se) is a trace element which is essential for both human beings and animals (White and Brown, 2010). At the same time, with its antioxidative property, Se is beneficial to plant growth and development at low concentration (Djanaguiraman et al., 2005, Filek et al., 2008, Nawaz et al., 2015). It has been proved that exogenous Se plays a positive effect on various plant physiological processes (Ismael et al., 2018), therefore enhances plant biomass under either stressed or non-stressed environments (Chu et al., 2010; Malik et al., 2011). Se supplementation increased plant growth, chlorophyll content, photosynthetic performance, and nitrogen content in tobacco (Liu et al. 2015b). Se application also improved the reproductive capacity of Brassica rapa by increasing seed production and seed viability (Lyons et al., 2009). Because of these merits, Se was often used as an exogenous protectant against diverse abiotic stresses in plant (Hartikainen et al., 2000; Feng et al., 2013). Numerous studies had indicated that Se application enhanced plant resistance against environmental stresses, such as salinity (Jiang et al., 2017), drought (Nawaz et al., 2014; 2015), high temperature (Djanaguiraman et al., 2010), ultraviolet-B (Yao et al., 2013) and water deficit (Andrade et al., 2018). Recently, increasing attention has been paid to the potential utility of Se in the alleviation of heavy metal toxicity especially Cd toxicity.
Previous investigations have demonstrated that Se could promote plant growth, decrease Cd level and enhance photosynthesis performance in plants exposed to Cd stress. For instance, it was found that Se supplementation could remarkably restrain the accumulation of Cd in lettuce (Lactuca sativa) (He et al., 2004), reduce Cd content in pepper (Capsicum annuum L.) fruits (Mozafariyan et al. 2014) and rice (Oryza sativa) shoots (Wan et al., 2016), decrease the Cd concentration in maize plant, and alleviates the Cd-induced growth damage (Sun et al., 2013). Under Cd stress, Se application significantly increased the fresh weight of roots in cucumber (Cucumis sativus L.) (Hawrylak-Nowak et al., 2014), and the inhibition of plant height, root length and biomass by Cd could be mitigated after treated with 3µM selenite (Sun et al. 2016). Se is important in the control of the Cd toxicity in seaweed (Gracilaria dura) (Kumar et al., 2012). 50 ?M Se pretreatment improved Cd tolerance in rapeseed seedlings by increasing the antioxidant defense and methylglyoxal detoxification system (Hasanuzzaman et al. 2012). In addition, low concentration of Se could also protect membrane lipids and recover the envelope membrane structure of rape (Brassica napus L.) under Cd stress treatment (Filek et al. 2010). However, the mechanism of Se in protecting plants subjected to Cd stress has not been completely illuminated. Previous reports suggested that the mitigation of Cd toxicity by Se might be associated with the reduced Cd uptake, decreased accumulation of ROS and a better nutrient balance in plant (Zhu et al., 2009; Feng et al., 2013). By enhancing enzymatic (Saidi et al., 2014; Filek et al., 2008) and non-enzymatic (Schiavon et al., 2013) antioxidants, Se application could alleviate Cd induced ROS stress. Furthermore, Se could induce the production of phytochelatins, which hinder Cd translocation from the roots to shoots (Hawrylak-Nowak et al., 2014). At the same time, under Cd stress, Se was involved in the regulation of the recovery of photosynthetic system, as well as the reconstruction of cell membranes and chloroplasts (Feng et al., 2013).
Tall fescue (Festuca arundinacea Schreb) is an important cool season grass species that is widely utilized in temperate regions, either for turf or for forage use. As a perennial grass, tall fescue possesses fast reproduction and outstanding resistance to various abiotic stresses. In addition, it was observed that tall fescue tolerated and enriched certain heavy metals, such as Cd, Pb and so on (Begonia et al., 2005; Xu and Wang, 2013). Recently, the perfect Cd tolerance of tall fescue was reported (Xu and Wang, 2014), suggesting its great prospects in future phytoremediation. However, so far there is no investigation on the effects of exogenous Se on the responses of tall fescue plants under Cd stress. Therefore, the aim of this study was to evaluate the effects of exogenous Se on various physiological processes and related gene expressions in tall fescue under Cd-stress. Cd and Se accumulation, chlorophyll and MDA content, electrolyte leakage, antioxidant enzyme activities, chlorophyll a fluorescence were measured. To investigate gene expression, genes encoding antioxidant enzymes and related genes were analyzed. The result will be beneficial for understanding the mechanisms of Se-induced resistance against Cd stress in tall fescue and other grass species.
Materials and methods
Plant material, Cd and Se treatments
The seeds of tall fescue (commercial cultivar “Houndog 5”) were sowed in plastic pots (7.5?cm in diameter and 8.5?cm depth) containing pearl stone and vermiculite (1:1 v:v). After germination, the seedlings were maintained in a greenhouse with 14 h photoperiod (300 ?mol photons m?2 s?1) and 24/22 °C (day/night) temperature for 40 days. The seedlings were watered daily to field capacity level, and fertilized with a 1/2 strength Hoagland solution once a week. For accommodation, the seedlings were then transplanted into conical flasks filled with 1/2 Hoagland solution, and grown for a week. On the 7th day of accommodation, , Cd (as CdSO4·8/3 H2O) and Se (as Na2SeO3) were added to the solutions to form 4 treatment regimes: (1) control, 1/2 Hoagland solution (CK); (2) Se, CK+0.1 mg/L Se (3) Cd, CK +30 mg/L Cd; and (4) Cd+Se, CK+30 mg/L Cd +0.1 mg/L Se. Each treatment was performed with three replicates, and the solution was renewed every other day. After 7 days of treatment, plant samples were collected to determine Cd and Se concentrations, enzyme activities, and gene expression analysis.
Determination of Cd and Se concentration
After 7 days of treatment, the plant leaves and roots were sampled separately. To remove the possible ion contamination from the root surface, roots were soaked with 20 mM Na2-EDTA solution for 20 min and washed with deionized water. All samples were then dried at 70 °C to the constant weight, and subsequently milled and digested with 2 N HCl. The concentrations of Cd and Se were then measured using an inductively coupled plasma mass spectrometry (ICP-MS, 7700ce, Agilent Technologies, SantaClara, CA, USA) according to the method of Wan et al. (2016).
Measurement of Chlorophyll content
Chlorophyll content was determined following the method of Hiscox and Israeltem (1979). About 0.1 g of fresh leaves were cut into small pieces and transferred into a 15 ml tube containing 10 ml dimethylsulfoxide (DMSO), and the tubes were then kept in the dark for 48 h. Accordingly, the absorbance of the chlorophyll extract was measured at 663 and 645 nm using a spectrophotometer (UV-2600, UNICO Instruments Co., Ltd., Shanghai, China). The chlorophyll content (Chl a, Chl b and Chl total) was calculated according to the formula of Hiscox and Israeltem (1979).
Electrolyte leakage (EL) measurement
The EL level was measured using the method described by Hu et al (2018). In brief, about 0.15 g fresh leaves were cut into small pieces and transferred into a 50 mL centrifuge tube containing 25 mL deionized water. The initial conductivity (Ci) was measured using a conductance meter (JENCO-3173, Jenco Instruments, Inc., San Diego, CA, United States) after shaken the tubes for 24 h at room temperature. Subsequently, the tubes were autoclaved for 15 min at 121°C to release all the electrolytes, and the maximum conductivity (Cmax) was measured after the tubes cooled down to room temperature. The following formula was used to calculate the relative EL.
Relative EL% = Ci/Cmax*100%.
Determination of enzyme activities, soluble protein and MDA content
Crude enzyme extract was prepared by homogenizing 0.3 g fresh leaves with 4 ml cooled potassium phosphate buffer (0.15 M, pH 7.0) using a pre-chilled mortar and pestle. The homogenate was then transferred into a 10 mL tube, and centrifuged at 12,000 rpm for 20 min at 4 °C. The supernatant was used for determination of soluble protein and malondialdehyde (MDA) content, and superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) activity.
Soluble protein content was measured following the method of Bradford (1976), and the MDA content was determined according the method of previous report (Fan et al., 2015b). The activities of SOD and POD were measured according to the method described by Fan et al. (2015b), and CAT activity was determined by the method of Chance and Maehly (1955).
Chlorophyll a (Chla) fluorescence transient analysis
Chl a fluorescence transient was examined using a pulse-amplitude modulation fluorometer (PAM 2500, Heinz Walz GmbH) with high time resolution (10 µs). Each treatment was measured at least five times using different leaves. After 30 min of leaves dark accommodation, measurement was performed under a saturating light intensity of 2000 µmol photons m-2 s-1. The chl a fluorescence emission triggered by the strong light pulses was measured and digitized between 10 µs and 300 ms according to Korres et al. (2003). Subsequently, the OJIP transient analysis was performed using the JIP-test following the method of Chen et al. (2013a).
The JIP-Test
According to Strasser et al. (2004) the JIP-test is a multi-parametric analysis of the OJIP transient. Chlorophyll fluorescence kinetics curve comprises the changing process from initial fluorescence intensity (F0) to maximal fluorescence intensity (FP). When illuminated by high intensity actinic light, the dark-accommodated plants and other photosynthetic organisms will display a rise of OJIP curves. A typical JIP-test usually contains 4 phrases: O-J (0.05–5 ms), J-P (5–50 ms) and I-P (50–1000 ms). Chlorophyll fluorescence kinetics curve reflects the intensity of stress effects on photosynthesis function. In this study, chlorophyll a fluorescence transient analysis was performed using the JIP-test according to the method described by Chen et al. (2013a). The fluorescence intensities at 50 µs, 2 ms (J-step) and 30 ms (I-step) were designated as FO, FJ, FI, respectively, and the maximal fluorescence (P-step) was denoted as FP. Selected JIP-test parameters were calculated using the method described by Ni et al. (2012).
Real-time quantitative PCR (Q-PCR) analysis
Total RNA was isolated from leave samples using Trizol reagent (Invitrogen,USA). About 2 µg of total RNA was used for the first strand cDNA synthesis using cDNA synthesis kit (Fermentas, Canada). Gene-specific primers for Q-PCR were listed in Table S1. Q-PCR procedure was performed according to our previous method (Li et al., 2017).
Statistical analysis
At least three biological replicates were used in all the experiments. Results were expressed as mean ± standard deviation. Statistical analysis was carried out by one-way ANOVA with the statistical package for social science (SPSS ver.11.5), Origin 7.5 and Excel 2003 for Windows. The statistical effects of treatments were evaluated by Duncan’s multiple range tests at a significance level of P ? 0.05.
Results
Effect of exogenous Se on Cd accumulation in tall fescue
Under Cd stress, tall fescue plants showed obvious toxicity symptoms. Cd treated plants showed yellow or withered leaves, decrease in plant height and root length as compared to the control (CK). However, exogenous Se obviously mitigated Cd induced toxic effects (see Fig.S1). The Cd accumulation was under the detection limit in both CK and Se treatment, either in shoots or in roots. Under Cd treatment, shoots and roots showed high level of Cd content (0.375 and 4.041 mg·g-1), respectively. However, exogenous Se significantly decreased the Cd content by 30.7 % and 28.5 % both in shoots and roots, respectively, when compared to Cd treatment alone (Table. 1). In addition, our results showed that Cd supplementation significantly decreased the shoot Se level to under the detection limit, but increased remarkably the accumulation of Se in the root of tall fescue (Table. 1).
Effect of exogenous Se on MDA content and EL
As shown in Fig.1, Cd stress significantly increased both MDA and relative EL in tall fescue leaves. Under Cd treatment, MDA content and EL value were 166 % and 63.2 % higher than that of the CK, respectively. However, Se supplementation significantly reduced the MDA content by 52 % and relative EL value by 28.6 % in tall fescue leaves, when compared to Cd treatment alone. At the same time, exogenous Se had no affect on both MDA content and relative EL value under CK.
Chlorophyll content and soluble protein
In regarding to chlorophyll content, there was no obvious change between plants treated with Se alone and the control. Cd stress significantly decreased Chl a, Chl b and Chl total content by 22.6 %, 26.7 % and 22.9 %, respectively, when compared with the CK. However, the Cd induced decline in chlorophyll content was remarkably recovered by exogenous application of Se. Under Cd stress, Se treatment restored Chl b content to the control level, and surprisingly increased Chl a and Chl total content higher than that of CK (Fig. 2A).
In addition, Cd stress significantly decreased soluble protein content as compared to the CK (p ? 0.05) (Fig. 2B). While the application of Se promoted the accumulation of soluble protein in tall fescue under Cd stress. The soluble protein content in the plants under Se + Cd treatment was significantly higher than that of Cd stress alone (p ? 0.05), and reached to the control level. However, there was no significant difference between Se and CK treatment groups.
Effect of exogenous Se on antioxidant enzyme activities
The antioxidant enzyme activities in tall fescue were varied in different treatment regimes (Fig. 3). Cd stress significantly decreased the SOD activity by 10.1%, but, Se supplementation greatly enhanced the SOD activity by 29.6% when compared to the CK. Under Cd stress, Se supplemented plants showed significantly higher SOD activity than that of CK,, and was comparable to only Se treated plants.
Similar to SOD, CAT activity was sharply increased by approximately 2 folds with Se application alone (p ? 0.05), but significantly reduced by Cd treatment alone when compared with the CK. Furthermore, as compared to the Cd stress, exogenous Se improved the CAT activity by 40 %, which was almost equal to CK. Unlike SOD or CAT, the POD activity was significantly increased with individual Se supplementation and Cd stress by 46.2 % and 136.6 %, respectively, when compared to the CK. However, under Cd stress, Se decreased the activity of POD by 15% in tall fescue leaves, but, much higher than CK and Se treatment groups.
Effect of Se on the expression of photosynthetic and antioxidant enzyme related genes
To study the possible effect of Se on gene expression involved in photosynthetic system and antioxidant enzymes in tall fescue under Cd stress, three genes related to the photosynthetic system and five genes related to antioxidant enzymes were analyzed by Q-PCR.
Cd or Se alone had no effect on the expression of psbA in tall fescue when compared to the CK. However, under Cd stress, Se significantly enhanced the gene transcription level of psbA as compared to the CK (Fig. 4A). Individually, both Cd and exogenous Se significantly down-regulated the expression of psbB in tall fescue leaves, but, Cd treated plants supplemented with Se showed higher psbB transcription level when compared to the CK. In addition, under Cd stress, exogenous Se significantly enhanced the abundance of psbB when compared to the Cd stress alone. Similar to psbB, the expression of psbC was significantly inhibited by both Cd and Se treatment individually, and the reduction of transcription level was more sharply in Cd treated plants. Just like psbB, psbC was also significantly up-regulated by Se application in Cd treated plants when compared to Cd stress alone, but its transcription level was lower than that of CK and Se treatment groups.
The expression of ChlCu/ZnSOD was significantly up-regulated in Se supplemented tall fescue plants, but sharply inhibited in Cd treated plants as compared to the CK (Fig. 4B). Se supplementation enhanced the expression of ChlCu/ZnSOD in tall fescue plants exposured to Cd stress, but, its transcription level was much lower than that of the CK. Similarly, the abundance of CytCu/ZnSOD was significantly decreased by Cd treatment, but the transcription level increased almost to CK level with the addition of Se under Cd stress. However, Se alone had no affect on the expression of this gene (Fig. 4B).
GPX and pAPX shared a very similar expression profile. Both genes were significantly down-regulated by Cd treatment as compared to the CK (Fig. 4C). However, Se application enhanced their expression levels to a certain extent in plants under Cd stress when compared to Cd stress alone. Meanwhile, there was no difference in the expression levels of the two genes between Se and CK regimes. Unlike the above genes, Cd stress significantly increased the transcription level of GR. By contrast, application of Se alone or Se combined with Cd did not apparently affect its expression (Fig. 4C).
Effect of Se on OJIP transient curves
Se suplmentation alone had no effect on the OJIP curve of tall fescue, but Cd stress alone obviously decreased the OJIP curve as compared to the CK, which was partially ameliorated by Se supplementation (Fig. 5).
Furthermore, the JIP-test was performed to analyze the parameters of OJIP transient curves, and thereby to quantify the photosynthesis in the leaves of tall fescue. The basic parameters including F0, FK, FJ, FI, FP and M0 were extracted and listed in Table 2. Cd stress significantly enhanced the F0, Fk, and M0 with or without Se supplementation. Meanwhile, Se application alone increased the Fk and M0, but reduced the F0. There were no obvious differences in the FJ and FI between Cd and CK regimes. However, Se application apparently increased the value of FJ and FI, especially in Cd stressed tall fescue plants,when compared to CK and Cd alone regimes. Unlike those above parameters, the value of Fp was much lower in Cd treatment regime, but Se supplementation restored Fp to the normal level.
Cd stress alone significantly increased the value of specific energy fluxes such as ABS/RC and TP0/RC, but remarkably reduced with exogenous Se supplementation.
However, under both CK and Se application alone, there were no obvious differences in ABS/RC and TP0/RC. On the other hand, ET0/RC and RE0/R were both significantly reduced by Cd and Se application, either alone or in combination when compared to the CK.
Parameters related to quantum yield and efficiencies contained ?Ro, ?Eo, ?Po (namely FV/FM), and ?RC. Cd stress alone significantly decreased the values of ?Po. However, there was no obvious difference in ?Po between Se alone and CK . Meanwhile, ?RC showed similar trends with ?Po. Compared to the CK, ?Eo value was dramatically declined in all the other three treatments. Similarly, both Cd and Se treatment alone decreased the value of ?Ro. However, no distinct difference was observed in this parameter between CK and Cd+Se regime.
Both PItotal and PIABS are important performance indices that demonstrate the overall PSII activity. In this study, both parameters were decreased in Se and Cd regimes when compared to the CK. However, the values were slightly higher in Se suplmented regime when compared with the Cd stress alone.
Discussion
In the present study, Se supplementation significantly decreased the Cd uptake in both roots and shoots of tall fescue, while improved its growth. For the first time, this paper provides information about the beneficial role of exogenous Se in reducing Cd uptake or translocation, as well as in enhancement of photosynthesis performance and antioxidative protection against Cd toxicity in tall fescue plants.
Se increased the chlorophyll content in tall fescue exposed to Cd
In our study, Cd stress significantly reduced the chlorophyll content of tall fescue. Similarly, the reduction of chlorophyll concentrations was reported in sunflower (Helianthus annuus) and pepper (Mozafariyan et al., 2014; Saidi et al., 2014) under Cd stress. This decrease was considered to be associated with lipid damage in the chloroplast membranes and/or the inhibition of chlorophyll biosynthesis (Feng et al., 2013; Hawrylak-Nowak et al., 2014).
In this study, Se application enhanced the chlorophyll content of tall fescue leaves under Cd treatment. Under Cd stress, similar results were reported in barley (Hordeum vulgare L.), spinach (Spinacia oleraceae L.), broccoli (Brassica oleracea), cucumber, seaweeds, and tomato (Solanum lycopersicum) with Se suplmentation (Pedrero et al., 2008; Hawrylak-Nowak 2009; Akbulut and Cakir, 2010; Kumar et al., 2012; Safaryazdi et al., 2012; Mozafariyan et al., 2014). Moreover, pre-soaking of seeds with Se could alleviate Cd-induced pigment loss in sunflower plants exposed to Cd (Saidi et al. 2014).
The oxidative stress and antioxidants of tall fescue exposed to Se and Cd
MDA is the oxidized product of membrane lipids, and its content is generally considered as an indicator of lipid peroxidation and oxidative stress level. It has been reported that exogenous Se reduced lipid peroxidation at low doses (Djanaguiraman et al., 2005). In this study, the MDA content was significantly increased with Cd treatment in tall fescue leaves. However, under Cd treatment, application of Se significantly decreased the MDA content (Filek et al., 2008). Similar to our result, many studies had indicated that optimal exogenous Se could decline the MDA concentration in diverse plant species under various stresses (Pukacka et al., 2011; Seppänen et al., 2003; Iqbal et al., 2015). The decrease in lipid peroxidation by Se might be attributed to its positive effects on plant’s antioxidant capacity (Feng et al., 2013). In addition to the induction of lipid peroxidation, membrane dysfunctioning was also generally observed in plants exposed to stress (Sofo et al. 2004). In this study, Cd treatment increased the EL of tall fescue, and the result was in accordance with those results in mustard (Brassica juncea) (Asgher et al. 2014), rapeseed (Hasanuzzaman et al. 2012), Cassia italica (Hashem et al. 2016) and tomato (Alyemeni et al., 2018). However, just like MDA, the EL level was also reduced by Se application in tall fescue exposed to Cd stress. Similar result was reported in tomato (Alyemeni et al., 2018). The result indicated that Se supplementation was beneficial for the maintenance of structural and functional integrity of membranes, and consequently prevents electrolyte leakage in plants. Furthermore, Filek et al. (2008) suggested that the greater membrane stability in Se-supplemented plants might be associated with the increased production of unsaturated fatty acid.
In response to oxidative stress, plants have developed enzymatic antioxidant system, which regulates oxidative stress through the catalytic reduction of ROS by antioxidant enzymes such as SOD, CAT, POD, etc (Feng et al., 2013). In the present study, Cd treatment significantly reduced the activities of both SOD and CAT of tall fescue leaves. However, Se supplementation increased the activities of both enzymes under non-stressed and Cd stressed conditions. Similar changes of CAT activity were reported in rapeseed seedlings when treated with Cd alone or Cd combined with Se (Hasanuzzaman et al., 2012). In addition, the activity reduction of these antioxidant enzymes was reported in Brassica chinensis under Cd treatment, but, the reduction was not restored by Se supplementation (Yu et al., 2018). Unlike SOD and CAT, the activity of POD was enhanced by Cd and Se treatment, either individually or in combination when compared to the CK.
The expression of antioxidant enzymes was up-regulated under Cd treatment in B. juncea (Asgher et al., 2014), wheat (Triticu. aestivum) (Khan et al. 2015), sunflower (Abd-Allah et al. 2015), tomato (Alyemeni et al., 2018) and ryegrass (Lolium perenne) (Hartikainen et al., 2000). Furthermore, Se increased the activity of antioxidant enzyme including SOD and CAT in tomato when exposed to Cd stress (Alyemeni et al., 2018), which was in accordance with our result. In B. napus, exogenous Se improved Cd tolerance through increasing both the activity of antioxidant enzymes and the content of non-enzymatic antioxidant components (Filek et al. 2008). These investigations indicated that Cd may have complex effects on antioxidant enzymes depending on the difference in Cd concentration, the plant species and the management methods. At the same time, optimal dosage of Se was beneficial to enhance the antioxidant capacity, and therefore alleviates the Cd induced oxidative stress in tall fescue as well as many other plants.
The effect of Se on the photosynthesis activity of tall fescue under Cd stress
Photosynthesis is a sensitive process in tall fescue, and has been inhibited by diverse abiotic stresses including Cd treatment (Huang et al., 2017; 2018; Zhang et al., 2017; Hu et al., 2018). PSII is an important membrane structure in photosynthesis system, and is vulnerable to stress condition (Chen et al., 2014; Huang et al., 2017). Here, the effects of Cd and Se on tall fescue photosystem were investigated using Chl a fluorescence transient.
The photosynthesis activities of tall fescue subjected to different treatments were evaluated by fluorescence transient curves. As shown in Table 2 and Fig. 5, there was subtle difference between the Cd treatment and CK. However, Cd stress influenced the PSII function through altering the F0, Fk, Fp and M0. For example, Fo was significantly enhanced by Cd stress, which was in agreement with previous studies (Huang et al., 2017; 2018; Zhuo et al., 2017). The possible reason might be the dissociation of light-harvesting complex II from PSII complex and accumulation of inactive RCs of PSII (Yamane et al., 1997), or increased back electron transfer from QB to QA (Kou?il et al., 2004). By contrast, FM was much lower in Cd treated tall fescue. Similarly, Huang et al (2018) reported the decrease of FM induced by chromium (Cr) treatment in tall fescue. However, exogenous Se recovered the inhibition or enhancement of Cd induced changes in these parameters, which suggested that exogenous Se plays a positive role in protecting tall fescue PSII from the destruction by Cd stress. Previous studies also reported that exogenous regulators like nitric oxide and Spd alleviated the damage of PS II in tall fescue under various abiotic stresses (Huang et al., 2018; Zhang et al., 2017).
In addition, we analyzed Chl a fluorescence transient data using JIP-test (Yusuf et al., 2010; Chen et al., 2014). The parameters associated with quantum yields and efficiencies were evaluated including maximum quantum yield for primary photochemistry (?Po, also known as FV/FM), quantum yield of the electron transport flux from QA to QB (?Eo), quantum yield for reduction of end electron acceptor at the PSI acceptor side (?Ro) and the probability that a PSII Chl molecule functions as RC (?RC). It was observed that Cd treatment alone significantly declined the values of these parameters, and therefore inhibited the efficiency of electron transportation as well as the Chl molecule functions of PSII. This result was in agreement with that of Huang et al (2017), who reported that different concentrations of Cd treatment decreased the FV/FM, ?Eo and ?RC in tall fescue. However, in Cd treated tall fescue plants, the decline of ?Ro, FV/FM and ?RC was restored to the control level with Se supplementation. Recently, Alyemeni et al. (2018) reported that Se could enhance the value of FV/FM in tomato under Cd stress, which was in consistent with our result.
It was reported that decrease in PSII photochemical efficiency might be partially attributed to the destruction of antennae pigments (Calatayud and Barreno, 2004). In this study, specific energy fluxes were analyzed to examine the functional properties of PSII. The result indicated that Cd stress significantly increased the ABS/RC and TP0/RC, but Se supplementation, recovered the two parameters almost to the normal level. By contrast, Cd stress decreased the ET0/RC and RE0/RC, and was not resumed with Se supplementation. These changes were in accordance with that of Chl content in our study (Fig. 2A).
Both PIABS and PItotal are performance indxices describing the overall behaviors of photosynthetic activities, which were usually used for assessing the photochemical activities of stressed plants (Fan et al., 2015a). In the present study, both Cd and Se treatment
significantly decreased the PITotal and PIABS values. Our results were in consistent with previous studies on plants under heavy metal stresses (Mathur et al., 2016; Huang et al., 2018). This decrease might be due to the ROS burst induced by Cd. However, the values of PIABS and PItotal were slightly higher in Cd + Se regime when compared with the Cd stress alone. This result suggests that exogenous Se may have a positive effect.

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