Introduction

Chromium (Cr) is one of the most toxic heavy metals found in the Earth’s crust. Although it is used in a variety of applications, such as electroplating, steel production, and leather tanning, it has a harmful effect on the environment (Hayat et al., 2012; Altundogan, 2005); therefore, increasingly severe environmental contamination has become a growing concern in the past several years (Zayed and Terry 2003). Previously, Cr6+ has been shown to be toxic to plants (Shanker et al., 2005), aquatic animals (Velma et al., 2009), and microorganisms (Cheung and Gu, 2007). Moreover, a Cr6+ salt was reported to have higher mobility than Cr3+ salt and was therefore considered to be associated with decreased plant growth and changes in plant morphology (Hayat et al., 2012). Although Cr6+ toxicity to plants is considered a major environmental problem (Sundaramoorthy et al., 2010), the mechanism underlying its action is not fully understood.

It has been shown that Cr6+ exerts a wide range of toxic effects on the growth and metabolism of plants (Shanker et al., 2004). A previous study revealed that the accumulation of Cr6+ appeared as electron-dense granules in the roots of some plants (Shanker et al., 2004); however, there is a paucity of reliable information on Cr6+ deposits in Cr stress-induced chloroplasts. Studies assessing the effects of Cr6+ on chloroplast organization, which is important to understanding the physiological alterations of photosynthesis in damaged plants, are scarce. Meanwhile, Cr6+ stress leads to physiological disorders, with plants showing abnormal stomatal conductance, reduced intercellular space, and reduced growth and yield (Azmat and Khanum, 2005). A previous work revealed that germination of Triticum aestivum L. seeds was reduced under 100 mg L−1 of Cr6+ (Dey et al., 2009). In addition, Liu et al. (2008) found that Cr6+ exposure decreased the uptake of the micronutrients manganese, iron, copper, and zinc (Mn, Fe, Cu, and Zn, respectively) in Amaranthus viridis L.; however, results of these studies on the effects of Cr6+ on photosynthesis status are rare. Furthermore, reactions with aldehydic products of lipid peroxidation were defined as evidence of oxidative stress (Sandalio et al., 2001). The induction of reactive oxygen species (ROS) scavenging enzymes constitute an important protective mechanism, which can be used as a biomarker that reflects the affinity between antioxidative capacity and Cr6+ tolerance (Zhang et al., 2010). Among those, superoxide dismutase ((SOD) EC 1.15.1.1) is the first line of defense against oxidative stress. Catalase ((CAT) EC 1.11.1.6) and guaiacol peroxidase ((POD) EC 1.11.1.7) and consists of mainly eliminating hydrogen peroxide (H2O2) systems in plants. It has been documented that POD and CAT are increased in the Cr-sensitive mung bean after treatment with different Cr6+ concentrations (Rout et al., 1997); meanwhile, they decreased in Cr-tolerant cultivars (Samantary, 2002). The one which has a higher affinity for eliminating of H2O2 is the focus of current research against heavy metal stress, with less information available in cereal crops, such as rice (Qiao et al., 2013).

This study investigated the responses of Oryza sativa L. to exogenous Cr6+ by measuring (1) changes in poor plant growth; (2) changes in photosynthetic pigment, Cr6+ deposits, and chloroplast ultrastructure; (3) changes in photosynthesis and the contents of soluble protein; (4) changes in malondialdehyde (MDA), superoxide (O2̄), and H2O2 content; and (5) changes in SOD, CAT, and POD activity.

Materials and methods

Plant materials and growth conditions

A hydroponic experiment was performed in the greenhouse of the Xianlin campus, Nanjing Normal University, Nanjing, China. Seeds from rice cultivar LYPJ obtained from the Institute of Food Crops, Jiangsu Academy of Agricultural Sciences, were sterilized with 3.0 % H2O2 for 20 min, rinsed with distilled water three times, and transferred to an incubator with a 12-h light/12-h dark photoperiod. Uniformly germinated seeds were selected and transferred to plastic pots containing Kimura B nutrient solution. Nutrient solutions prepared with deionized-nanopure water contained 48.2 mg L−1 (NH4)2SO4, 24.8 mg L−1 KH2PO4, 18.5 mg L−1 KNO3, 15.9 mg L−1 K2SO4, 59.9 mg L−1 Ca(NO3)2, 134.8 mg L−1 MgSO4·7H2O, 7.45 mg L−1 ethylenediaminetetraacetic acid (EDTA)-Na2, 20 mg L−1 Na2SiO3, 2.86 mg L−1 H3BO3, 0.08 mg L−1 CuSO4·5H2O, 0.22 mg L−1 ZnSO4·7H2O, 1.81 mg L−1 MnCl2·4H2O, and 0.09 mg L−1 (NH4)6Mo7O24·4H2O. After 7.0 days of growth, the plants were treated with different concentrations of Cr6+ (0, 25, 50, 100, and 200 μM). The Cr6+ levels assessed in the present study were based on preliminary findings, where Cr6+ concentrations >200 μM were lethal to plants. The plastic pots were then placed in a growth chamber under the conditions mentioned above for 7.0 days. All solutions were refreshed every 2.0 days, and experiments were performed in triplicate.

Plant growth

Growth was measured in terms of height, root length, fresh weight, and dry weight after 7.0 days of treatment. Dry weight was obtained after drying at 80 °C until a constant weight was recorded.

Pigment content determination

The contents of chlorophyll (Chl) a, Chl b, and carotenoids as well as the Chl a/Chl b percentage were determined according to a previous study (Lichtenthaler, 1987). Chl and carotenoid (Car) were extracted using ice-cold 80 % (v/v) acetone. The extract was centrifuged at 3000×g (SIGMA 3K30, Goettingen, Germany) for 5.0 min, and absorbance of the supernatant was measured at 470, 645, and 663 nm with a UV-754 spectrophotometer (Shanghai Institute of Plant Physiology, China). Pigment contents were calculated in microgram per gram of fresh weight.

Cr6+ deposits and chloroplast ultrastructural observation

The leaves of the control and chromium-treated plants were cut into small pieces (approximately 0.1 × 0.5 cm) and initially fixed for 2.0 h at 4.0 °C in 2.5 % (v/v) glutaraldehyde in a 0.1-M phosphate buffer solution (PBS, pH 7.3), followed by post-fixation with 5.0 % (w/v) aqueous osmium tetroxide for 2.0 h. The samples were rapidly dehydrated using an ascending ethanol gradient and finally embedded in Epon 812 resin. Thin sections were obtained using an LKB-V ultramicrotome (LKB Ultrascan XL, Bromma, Sweden) with a diamond knife (LKB Products, Vienna, Austria) and stained in 2.0 % (w/v) uranyl acetate and lead citrate before examination using a Hitachi H-7650 transmission electron microscope (TEM).

Chloroplast isolation

Chloroplasts were isolated according to a method described by Ketcham et al. (1984), with minor modifications. Plant material (5.0 g) was minced and placed in a precooled mortar containing the extraction medium (50 mM Tris–HCl, pH 7.6, 5.0 mM MgCl2, 10 mM NaCl, 0.4 M sucrose, and 0.1 % bovine serum albumin (BSA)). The homogenate was centrifuged at 1000×g for 2.0 min and the supernatant was recentrifuged at 2000×g for 2.0 min. After the supernatant was removed, the pellet was supplemented with the extraction medium and the tube slightly rotated on ice blocks to obtain a uniform suspension of chloroplasts, which was then kept in the dark on ice for use in subsequent procedures.

Photophosphorylation level, adenosine triphosphate content, and calcium-ATPase and magnesium-ATPase activities

The photophosphorylation level was measured using a luminescence meter (FG-300, Shanghai Institute of Plant Physiology, Shanghai, China) as previously proposed (Ketcham et al., 1984). Approximately 0.1 mL chloroplast suspension was added to 0.9 mL reaction buffer with 10 mM K3Fe(CN)6, 0.2 M tricine (pH 8.0), 20 mM MgCl2, 20 mM Na2HPO4, and 20 mM adenosine diphosphate (ADP) and illuminated (50 μmol m−2 s−1) for 1.0 min; 0.5 mL 20 % trichloroacetic acid (TCA) was then added to stop the reaction. The resulting solution was centrifuged at 1000×g for 5.0 min, and 0.1 mL supernatant was mixed with 9.9 mL 0.02 M Tris–HCl (pH 7.5). Approximately 0.2 mL of the latter sample and 0.8 mL luciferase were mixed to determine the level of photophosphorylation.

The adenosine triphosphate (ATP) content was measured using the bioluminescence method described by Zhu et al. (2001). Calcium (Ca2+)-ATPase and magnesium (Mg2+)-ATPase activities were measured as described by Vallejos et al. (1983). To assess Mg2+-ATPase activity, a 0.1-mL chloroplast suspension was added to activate the solution with 0.25 M (pH 8.0) Tris–HCl, 0.5 M NaCl, 0.05 M MgCl2, 0.05 M dithiothreitol, and 0.05 M phenazine methosulfate, and the mixture was activated by illuminating (50 μmol m−2s−1) it for 6.0 min at 25 °C. Approximately 0.5 mL activated solution was mixed with 50 mM Tris–HCl, 2.0 mM MgCl2, and 10 μmol ATP at pH 8.0 and 36 °C for 10 min; the reaction was stopped with 20 % TCA and centrifuged. The supernatants were used for isoelectric point determination. The Ca2+-ATPase activity of soluble chloroplast coupling factor was determined by essentially using the same method, except that MgC12 was replaced with CaCl2.

Chlorophyll a fluorescence parameters

Chl a fluorescence was determined in vivo using a Handy-PEA instrument (Handy PEA, Hansatech, UK) as previously described (Perreault et al., 2011). The experiment was conducted at 10:00 a.m., with the leaves allowed to adapt to the dark conditions for at least 20 min at room temperature before measurements were taken, which allowed all the reaction centers to open. The leaves were then exposed to red light of 650 nm through LED at an excitation irradiance of 3000 μmol m−2s−1 for 800 ms. The following parameters were calculated: maximum quantum yield of photosystem II (PSII) (Fv/Fm), absorption flux of photons per cross section (ABS/CSM), phenomenological fluxes for trapping per cross section (TR0/CSM = φP0 [ABS/CSM]), potential electron transport per cross section (ET0/CSM = φE0 [ABS/CSM]), and dissipation per excited leaf cross section (DI0/CSM = [ABS/CSM]−[TR0/CSM]). The measurements were replicated 15 times.

Protein extraction and sodium dodecyl sulfate–polyacrylamide gel electrophoresis analysis

Proteins were extracted on ice with a mortar and pestle in cool extracting medium (50 mM Tris–HCl, pH 7.6, 5.0 mM MgCl2, 10 mM NaCl, 0.4 M sucrose, and 0.1 % BSA) and mixed with an equal volume of sodium dodecyl sulfate (SDS)-loading buffer, consisting of 10 % glycerol (w/v), 5.0 % mercaptoethanol (v/v), 2.3 % SDS, 6.25 mM Tris–HCl (pH 6.8), and 0.01 % (w/v) bromophenol blue (René et al., 2011). The samples were then boiled for 5.0 min, and equal amounts of protein were loaded onto 12 % polyacrylamide gel. The gel was run at 120 V, and protein bands were detected using Coomassie Brilliant Blue R-250 staining.

Determination of malondialdehyde, O2 −, and H2O2 contents

MDA contents were estimated as described by Wu et al.(2003). Leaves were homogenized in 10 % (w/v) TCA solution and centrifuged at 5000×g for 10 min. The resulting supernatant was mixed with the same volume of 0.5 % (w/v) thiobarbituric acid solution in 20 % (w/v) TCA. The mixture was then heated at 100 °C for 30 min and quickly cooled on ice. The absorbance was measured at 532 and 600 nm at 25 °C, and the concentration of MDA was expressed as micromole per gram of fresh weight (μmol g−1 FW).

O2̄ generation rates were measured by monitoring nitrite formation from hydroxylamine according to Wang and Luo (1990). Plant material of 0.2 g weight was homogenized in 3.0 mL of 65 mM K-phosphate buffer (pH 7.8) and then centrifuged at 5000×g for 10 min at 4 °C. The supernatants (0.75 mL) were mixed with 0.675 mL of the above phosphate buffer and 0.075 mL of 10 mM hydroxylamine chlorhydrate. After incubated at 25 °C for 20 min, 0.5 mL mixture was added with 0.5 mL of 17 mM sulfanilamide and 0.5 mL of 7.0 mM α-naphthylamine and then the mixture was incubated at 25 °C for 20 min again before adding the same volume of ether. The mixture was centrifuged at 1500×g for 5 min. Absorbance was measured at 530 nm with the aqueous phase, and O2̄ formation rates were calculated from the standard curve of NaNO2. The results were expressed as micromole of nitrite formation per gram of fresh weight (μmol nitrite g−1 FW).

H2O2 contents were measured according to Shen et al. (2015) by monitoring titanium sulfate. Samples (0.5 g) were homogenized with phosphate buffer (50 mM, pH 6.8) and centrifuged. The supernatants were then mixed with 0.1 % (w/v) titanium chloride in 20 % (v/v) H2SO4. The absorbance was measured at 410 nm, and H2O2 amounts were derived from the H2O2 standard curve. The results were expressed as micromole per gram of fresh weight (μmol g−1 FW).

Activities of antioxidant enzymes

For the extraction of antioxidant enzymes, plant material (0.5 g) was homogenized on ice with a precooled mortar and pestle in 0.05 M PBS. The homogenate was centrifuged at 10,000×g for 20 min at 4.0 °C, and the supernatant was used to measure the activities of SOD, CAT, and POD (Fu et al., 2014). Enzyme activity assays were performed in 3.0 mL reaction volumes at 25 °C.

SOD activity was measured according to Stewert and Bewley (1980). A 3.0-mL reaction mixture was set up with 50 mM phosphate buffer (pH 7.8), 13 mM methionine, 75 mM nitro blue tetrazolium (NBT), 0.1 mM EDTA, 50 mM Na2CO3, and 0.1 mL enzyme extract. The reaction was started by adding 2.0 μmol riboflavin and allowed to proceed for 15 min, illuminated with two 20-W fluorescent tubes. The absorbance of the mixture was determined at 560 nm. One unit of activity was defined as the amount of enzyme required to inhibit 50 % of the initial reduction of NBT under light (unit g−1 FW).

CAT activity was measured at 240 nm by recording the decomposition of H2O2 according to Fu et al. (2014). A 3.0-mL reaction mixture was set up with 50 mM phosphate buffer (pH 7.0), 15 mM H2O2, and 0.1 mL enzyme extract. One unit of CAT activity was defined as the decomposition of 1.0 μmol H2O2 per minute per milligram of fresh weight (μmol H2O2 min−1 mg−1 FW) at 25 °C.

POD activity was assayed according to the guaiacol method (Maehly, 1955). A 3.0-mL reaction mixture was set up with 16 mM guaiacol, 0.15 M phosphate buffer (pH 6.1), 2.0 mM H2O2, and 0.1 mL enzyme extract. Enzyme activity was calculated by the increment in absorbance at 470 nm per minute per milligram of fresh weight (ΔOD470nm min−1 mg−1 FW) at 25 °C.

Statistical analysis

Values are the mean ± standard deviation from at least three individual experiments. The coefficients of correlation (r value) were calculated by using Microsoft Excel 2003. Data were assessed by one-way analysis of variance using SPSS 17.0 (SPSS Inc., Chicago, IL, USA).

Results

Cr6+ treatment of rice seedlings results in poor plant growth

Figure 1 and Table 1 show single plant fresh weight, dry weight, plant height, and root length of rice seedlings treated with increasing concentrations of Cr6+. After treating with 200 μM Cr6+, the single plant fresh and dry weights were decreased by 54.8 and 47.4 %, respectively; meanwhile, plant height and root length were decreased by 27.5 and 41.0 %, respectively.

Fig. 1
figure 1

Effects of Cr6+ stress on plant growth in rice seedling leaves

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Table 1 Effects of Cr6+ on growth characteristics in rice seedling leaves
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Photosynthetic pigment, Cr6+ deposits and chloroplast development is defective in Cr6+-treated rice seedlings

Decreased Chl a (p < 0.05), Chl b (p < 0.05), and Car (p < 0.05) contents were observed with increasing Cr6+ levels in the nutrition medium (Table 2). In Table 2, Chl a, Chl b, Car contents, and Chl a/Chl b were decreased to 48.42, 63.58, 44.60, and 22.7 %, respectively, after treatment with increasing concentrations of Cr6+ (maximum Cr6+ concentration was 200 μM).

Table 2 Effects of Cr6+ on photosynthetic pigments in rice seedling leaves
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Transmission electron micrographs of chloroplasts under normal conditions and those of treated plants are shown in Fig. 2. There was no Cr6+ deposit in the chloroplasts of the control (Fig. 2a); however, Cr6+ deposits were found and gradually increased in plants treated with 25, 50, 100, and 200 μM Cr6+ (Figs. 2b–e).

Fig. 2
figure 2

Cr6+ deposits changes in the ultrastructure of chloroplasts in leaves in control and Cr6+ treatment groups. a, b, c, d, e 0, 25, 50, 100, 200 μM Cr6+ concentration, respectively. CW cell wall, G Grana thylakoids, P plastoglobule, SL stroma lamellae

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Under normal conditions, chloroplasts are lens-shaped and intact, with an orderly arrangement of grana and stroma thylakoids. They display well-developed membrane systems consisting of grana connected by stroma lamellae (Fig. 2a). After treatment with 50 μM Cr6+, the chloroplasts swelled to an abnormal shape. When treated with 100 μmol Cr6+, their inner structure was disorganized; the chloroplast matrix zone expanded and the lamellae structure disintegrated (Fig. 2c). Higher Cr6+ concentrations increased this damage, with chloroplasts swelling even more and appearing to be globular (Fig. 2e). Finally, the inner structure of the chloroplasts was considerably disrupted.

Photophosphorylation level, ATP content, and Ca2+-ATPase and Mg2+-ATPase activities in rice seedlings are affected by Cr6+ treatment

As shown in Fig. 3, the photophosphorylation level, ATP content, and Ca2+-ATPase and Mg2+-ATPase activities overtly decreased (r p  = −0.983, p < 0.01; r ATP  = −0.9478, p < 0.01; r Ca 2+ = −0.967, p < 0.01; r Mg 2+ = −0.993, p < 0.01). After treatment with 200 μmol Cr6+, these parameters markedly decreased, respectively, by 69.84, 68.03, 51.37, and 39.92 % compared with those of the control.

Fig. 3
figure 3

Effects of Cr6+ on photophosphorylation level (a), ATP content (b), Ca2+-ATPase activity (c), and Mg2+-ATPase activity (d) in rice seedling leaves. Data are mean±SD (n = 3). Different small letters indicate significant differences p < 0.05, by using the t test

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Photochemical efficiency of PSII in rice seedlings decreases after Cr6+ treatment

To further elucidate the disruption of PSII in rice seedlings exposed to Cr6+ toxicity, PSII efficiency in primary photochemistry was assessed using Chl a fluorescence. As shown in Table 3, Fv/Fm decreased progressively at all Cr6+ concentrations (r = −0.994, p < 0.01), with a maximum loss of 45.99 % from treatment with 200 μM Cr6+; these data indicate the severe destruction of PSII reaction centers. In addition, treatment with 200 μM Cr6+ significantly reduced ABS/CSM values after 7.0 days (r = −0.983, p < 0.01), suggesting that high Cr6+ concentrations result in damaged antenna pigments in rice seedlings. Indeed, DI0/CSM was 16.39 % higher than that of the control after treatment with 200 μM Cr6+, indicating that the energy was dissipated as heat, and light utilization changed significantly compared with that of the control plants. In addition, TR0/CSM and ET0/CSM decreased by 19.32 and 38.96 %, respectively, under 200 μmol Cr6+.

Table 3 Chlorophyll fluorescence parameters in the leaves of rice seedlings exposed to different Cr6+ concentrations
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Photosynthesis-related proteins are affected by Cr6+ treatment

The analysis of total protein extracts from the leaves of rice seedlings by SDS-PAGE showed that in Cr-treated (7.0 days) leaves, exogenous Cr6+ induced a massive decrease in protein levels (Fig. 4), and protein content decreased with increasing Cr6+ concentrations. Specifically, the amounts of four polypeptides with apparent molecular weights of 43, 38, 36, and 33 kDa starkly diminished in response to Cr6+ treatment.

Fig. 4
figure 4

Decreased protein amounts after Cr6+ treatment of leaves in rice seedlings

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MDA, O2 −, and H2O2 contents in rice seedlings increased by Cr6+ treatment

The lipid peroxidation status in plants was evaluated by the MDA content. Compared with the control, treatment with Cr6+ led to a rapid increase in MDA levels in the leaves of rice seedlings (r = −0.989, p < 0.01) (Fig. 5a), peaking at 53.9 % after treatment with 200 μM Cr6+. A similar trend was observed with O2̄· (r = −0.8564, p < 0.01) (Fig. 4b), with a maximum increase of 25.2 %. H2O2 content (Fig. 4c) reached a peak value at 25 μmol Cr6+, which increased by 263 % over that of the control plants; however, it then declined.

Fig. 5
figure 5

Effects of Cr6+ on MDA content (a), generation rates of O2 − (b), and H2O2 (c) in rice seedling leaves. Data are mean±SD (n = 3). Different small letters indicate significant differences p < 0.05, by using the t test

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Antioxidant enzyme activities in rice seedlings increased by Cr6+ treatment

Various Cr6+ stress caused pronounced effects on the activities of antioxidant enzymes in rice seedlings (Fig. 6). SOD activity was overtly changed after exposure to different Cr6+ concentrations; specifically, at 100 μM Cr6+, a significant decrease in SOD activity was observed (Fig. 6a). CAT activity was significantly increased. There was a peak value at 100 μM Cr6+, but it was decreased with other concentrations. POD activity was increased at all concentrations except 100 μM Cr6+ (Fig. 6c).

Fig. 6
figure 6

Effects of Cr6+ on the antioxidant enzymes SOD (a), CAT (b), and POD (c) in rice seedling leaves. Data are mean±SD (n = 3). Different small letters indicate significant differences p < 0.05, by using the t test

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Discussion

Cr6+ is one of the most toxic heavy metals in the environment. Several studies have assessed the chemistry of Cr6+ in soil and its uptake by plants and have concluded that it is harmful to plant growth (Arun et al., 2005). Indeed, previous studies have shown that Cr6+ toxicity in plants affects growth and development, including the germination process and growth of roots, stems, and leaves, which results in decreased total dry matter production and yield (Choudhury and Panda 2005); therefore, we evaluated Cr-treated rice seedlings for photosynthesis and ultrastructural alterations to determine the mechanism by which Cr6+ affects crops.

Ultrastructure of chloroplasts are damaged under Cr6+ stress

Our study showed that Cr6+ treatment distinctly inhibited plant growth, Chl contents, photosynthesis parameters, and chloroplast development in rice seedlings (Figs. 1 and 2, Tables 1, 2, and 3). Photosynthetic pigment content is considered a sensitive parameter under metal stress conditions and is used as a potential biomarker for heavy metal stress (Qiao et al., 2013). As shown in Table 2, Chl a, Chl b, and Car contents in the leaves of rice seedlings decreased as Cr6+ increased from 25 to 200 μM; similar results were obtained in Lolium perenne L. (Vernay et al., 2007). The decreased Chl content induced by Cr6+ might be a result of an inhibition of the enzymes involved in Chl biosynthesis (Singh et al., 2006; Sivaci et al., 2008). Moreover, Cr6+ stress induced the apparent accumulation of Cr6+ and significantly damaged chloroplast ultrastructure (Figs. 2b–e). Here, observations of ultrastructures showed that Cr6+ toxicity damages the ultrastructure of chloroplasts and thylakoids, as demonstrated by swollen chloroplasts, which were far from the cell wall, with 200 μM Cr6+ showing maximum severity (Fig. 2e). Photosynthesis was inevitably blocked when the integrity of the chloroplast ultrastructure in cells was destroyed under environmental stress. These changes were similar to the disorganization of the thylakoid membranes observed in Brassica napus L. (Gill et al., 2014).

These findings demonstrated a relationship between the structure and function of rice seedlings and that the influence of Cr6+ on chloroplasts was important for understanding physiological alterations. Changes in chloroplast ultrastructure were another important reason for the decrease in Chl content. Both the loss in pigment and damage to the chloroplasts ultimately cause the disturbances in photosynthetic capacity (Qiao et al., 2013).

Photosynthesis is inhibited under Cr6+ stress

Photophosphorylation activity is the capacity of chloroplasts to produce ATP under light conditions, using light energy to transform ADP into ATP (Zhang et al., 2007), a reaction catalyzed by ATP-synthase in chloroplast thylakoid membranes. The altered photophosphorylation activity suggests that Cr6+ treatment of rice seedlings decreases the capacity of chloroplasts to produce ATP and transform light energy into chemical energy (Fig. 3a). This was confirmed by the variation tendency of Ca2+-ATPase and Mg2+-ATPase activities (Figs. 3c, d). The decreased photosynthesis performance of Cr6+-stressed seedlings might be interpreted as a decreased utilization of light energy by the treated plants toward sustaining photochemical reactions, which denotes a decrease in light-use efficiency (Wang et al., 2014). To clarify the alterations of PSII in rice seedlings treated with Cr6+, PSII efficiency in primary photochemistry was assessed using Chl a fluorescence. The Fv/Fm, representing the maximum quantum yield of PSII, shows a decreasing trend with increasing concentrations of Cr6+ (Table 3). The decreased Fv/Fm might be a result of photo-inhibition or other injury to PSII components in rice seedlings under Cr6+ stress (Roháček, 2002; Rodriguez et al., 2012). A similar response was observed in ABS/CSM, TR0/CSM, and ET0/CSM (Table 3). DI0/CSM markedly increased, suggesting that the energy was dissipated as heat, and the utilization of light strikingly changed compared with those of the control seedlings. These findings corroborate a previous report by Liu et al. (2008).

Abiotic stressors, such as heavy metals, can affect plants by altering soluble protein contents (Yang et al., 2011). In the present investigation, protein contents decreased dramatically in the leaves of rice seedlings treated with ≥25 μM Cr6+. The effect depends on the quantities as well as the types of heavy metals applied and the plants species. SDS-PAGE also revealed that Cr6+ treatment decreases the amount and intensity of proteins (Fig. 6). The decreased protein contents in the test plants that received higher concentrations of Cr6+ are most likely the result of the toxic effects of ROS (Verma and Dubey, 2003) or to the increased protease activity activated under metal stress (Xu et al., 2010). In addition, the four polypeptides with apparent molecular weights of 43, 38, 36, and 33 kDa are protein subunits of the PSII reaction center. The decrease in these four protein subunits demonstrates that the PSII reaction center is most likely damaged under Cr6+ stress. Ultimately, photosynthesis is inhibited.

Antioxidant enzyme systems of rice seedlings lose their intrinsic balance under Cr6+ stress

It is known that heavy metals cause oxidative damage resulting from the production of ROS (Malecka et al., 2001). MDA is a decomposition product of polyunsaturated fatty acids in biomembranes. It has been used as a biomarker for lipid peroxidation resulting from oxidative stress (Mittler, 2002). As shown above, when rice seedlings are subjected to elevated Cr6+ concentrations, MDA levels prominently increase (Fig. 5a). The increased MDA content suggests that Cr6+ deminifies the cell membrane and induce oxidative stress, which is shown to be related to induced production of O2̄·and H2O2 (Wang et al., 2008). O2̄· and H2O2 contents increase gradually with Cr6+ concentration (Fig. 5b, c), in agreement with previous findings (Zhang et al., 2010), which suggested that Cr6+ induced MDA production in wheat seedlings; therefore, membrane lipid peroxidation should be considered a component of Cr6+ toxicity. This effect was also evidenced by the decrease in photosynthetic pigments (Table 2) and proteins (Fig. 4). Increased MDA, O2̄, and H2O2 contents result in increased oxidative and lipid peroxidation, damage the structure and function of the membranes and disturb metabolic processes, and finally inhibit the growth and physiological processes in rice seedlings. The activities of antioxidant enzymes caused by heavy metals are considered to be important defense systems of plants against oxidative stress (Zhang et al., 2010). The protective mechanisms adapted by plants to scavenge free radicals and peroxides consist of several antioxidant enzymes (POD, CAT, and SOD) (Allen, 1995). Antioxidant enzymes are important components in preventing oxidative stress, and the activity of one or more of these enzymes generally increases in plants when exposed to toxic metals (Singh et al., 2006). SOD converts superoxide anion into another reactive intermediate (H2O2). In addition, H2O2 is removed by CAT in the peroxisomes or by membrane-bound POD (Foyer et al., 1997). The increase in SOD activities detected in the leaves of rice seedlings following Cr6+ treatment could be considered to be circumstantial evidence of enhanced production of free radicals. The results herein indicate that the tolerance capacity of plants to toxic metals depends on the equilibrium between the production of ROS and the quenching activity of the antioxidants (Xu et al., 2010). In addition, the activities of SOD decrease at high Cr6+ concentrations. It is evident from our study that the antioxidant enzyme systems of rice seedlings lose their intrinsic balance and are unable to effectively resist the generation of excess ROS. Moreover, POD is considered as having a higher capacity for H2O2 than CAT. In our results, the POD activity increases, except under 50 μM Cr treatment (Fig. 5c), while CAT activity is activated only at 50 μM Cr6+ to compensate for POD insufficiency (Fig. 5b), suggesting that CAT, not POD, might play a main role during this period, which is consistent with the previous viewpoint (Zhang et al., 2009) These relationships appear to also be connected with gene expression. Further experiments will be carried out to test this possibility.