1 Introduction

Toxic heavy metals have become a focal point in food safety as their presence in the food chain threatens human health, resulting in illnesses, such as kidney failure and cancer (Sytar et al. 2013; Saifullah et al. 2014; Edelstein and Ben-Hur 2018). Cd is ranked as the most dangerous among eight inorganic pollutants, with a standard rate of 7%, making it the most severe soil contaminant in China (Wang et al. 2015). Compared with other heavy metals, Cd is more mobile and active in soil (Liu et al. 2019). Once discharged into the environment, Cd is absorbed by plant roots and transported to other parts of the plant, thereby adversely affecting the growth and development of several plant organs, including reduction in shoot and root biomass, quality, yield, water use efficiency, and mineral uptake (Agamin and Mohamed 2013; Rizwan et al. 2018; Zhou et al. 2021).

At the cellular level, Cd toxicity results in oxidative damage to DNA and organelle ultrastructure, protein denaturation, and cell death (Riaz et al. 2021). Cd-induced oxidative stress is a major cause of cellular toxicity, as it directly increases the production of hydrogen peroxide (H2O2), hydroxyl (OH−), and superoxide (O2−) radicals (Rahman et al. 2016). Lin et al. (2012) and Hasanuzzaman et al. (2017a) used histochemical staining to demonstrate Cd-induced oxidative damage in tissues. Plants have developed an enzymatic antioxidant system, including superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), ascorbate–glutathione (AsA–GSH) cycle enzyme ascorbate peroxidase (APX), and glutathione peroxidase (GPX), to detoxify reactive oxygen species (ROS) induced by biotic and abiotic stresses. Huang et al. (2020) reported that Cd stress significantly inhibited the activity of antioxidant enzymes (CAT and SOD) in the leaves and roots of rice plants, which can be attributed to Cd binding to the carbonyl and sulfhydryl groups of proteins to replace specific cofactors, causing enzyme inactivation (Mostofa et al. 2015; Rizwan et al. 2017).

Several strategies are employed to reduce Cd accumulation and toxicity in plants, including exogenous application of plant growth regulators, microbes, grafting, and mineral-based fertilizers, as well as selecting for low Cd-accumulating cultivars and use of organic amendments. Se application is one of the most efficient and cost-effective methods as it alleviates Cd toxicity in several plant species by reducing ROS accumulation and cell membrane damage through antioxidant defense system; decreasing Cd bioavailability; maintaining chloroplast ultrastructure; improving plant growth parameters and photosynthetic traits; and playing a key role in the regulation of Cd transporters (Rizwan et al. 2017; Riaz et al. 2021). In addition, Zhang et al. (2020) verified that Se application improves the yield of strawberry plants under Cd stress. Besides Cd stress, exogenous Se also can improve millet biomass under salt stress which exhibited strong accommodation of Se on plants growth under abiotic stress (Rasool et al. 2020; Shah et al. 2020).

Tomato (Solanum lycopersicum L.), which belongs to the family Solanaceae, originates from western South America and is the most economically important fruit worldwide (Zhu et al. 2018; Londoñno-Giraldo et al. 2021). Tomato varieties can be classified into two types: common (S. lycopersicum ‘commune’) and cherry (S. lycopersicum ‘cerasiforme’). Cherry tomatoes are more popular because of their small size, delicate taste, and variety of colors, which make them attractive for fresh-produce consumption and salad-making (Figàs et al. 2015; Liu et al. 2018). Furthermore, cherry tomatoes are rich in vitamin C, lycopene, β-carotene, and lutein, which contribute to their extensive marketing and consumer consumption (Klee and Giovannoni 2011). Therefore, reducing Cd content in cherry tomato plants is necessary to improve food safety.

Previous studies used the tomato cultivar ‘Micro-Tom’ to determine the mechanisms underlying Cd-stress alleviation using Se (Alves et al. 2020). However, the effects of Se on photosynthesis in Cd-stressed cherry tomatoes still remain unexplored. Therefore, the aim of this study was to investigate the responses of two Cd-accumulating cherry tomato cultivars, with different Cd-accumulation rates, to Cd toxicity and the effects of Se on Cd toxicity.

2 Materials and Methods

2.1 Materials

The experiment was carried out from March to June 2019 at the Chengdu campus of Sichuan Agricultural University (30°71′ N, 103°87′ E). Two cherry tomato cultivars, Hanluzhe (HLZ) and Lvfeicui (LFC), which were purchased from Shouguang Cricket Agricultural Technology Extension Service Center, Shandong province, China, were screened from previous study (Su et al. 2021). Cherry tomato seeds were surface sterilized in 2% hydrogen peroxide (H2O2) for 30 min and then rinsed with deionized water (Huang et al. 2020). After sprouting upon germination at 25 °C in an incubator, the seeds were germinated in a plug tray in a greenhouse. The growing conditions were as follows: 12 h daylight; 300 μmol·(m2 s)−1 light intensity; 26 ± 3 °C and 18 ± 3 °C daytime and nighttime temperatures, respectively; and 65–85% relative humidity. After plants had four true leaves, seedlings with uniform growth were transferred to plastic containers containing 5 L of nutrient solution, with six seedlings per plastic container. The nutrient solution was prepared according to the procedure reported by Lin et al. (2012); the composition of the nutrient solution was NH4NO3 (114.3 mg L−1), NaH2PO4·2H2O (50.4 mg L−1), K2SO4 (89.3 mg L−1), CaCl2 (110.8 mg L−1), MgSO4·7H2O (405 mg L−1), MnCl2·4H2O (1.88 mg L−1), (NH4)6Mo7O24·4H2O (0.09 mg L−1), H3BO3 (1.17 mg L−1), ZnSO4·7H2O (0.05 mg L−1), CuSO4·5H2O (0.04 mg L−1), and the pH was adjusted to 6.5 ± 0.1.

2.2 Experimental Design

We employed four treatments: (1) control (CK): basal nutrient solution (BNS); (2) Se treatment: 2.5 μmol L−1 Na2SeO3 + BNS; (3) Cd treatment: 50 μmol L−1 CdCl2 + BNS; (4) Cd + Se treatment: 50 μmol L−1 CdCl2 + 2.5 μmol L−1 Na2SeO3 + BNS. The concentrations of Cd were chosen according to previous studies (Lin et al. 2012; Cai et al. 2011). The experiment was conducted in a completely randomized design with three replicates (each replicate contained six seedlings in one plastic container), and the nutrient solution was replenished every 3 days. Other environmental conditions were similar to those described above. After treatment for 15 days, some of the cherry tomato seedlings were washed thoroughly with deionized water; their roots, stems, and leaves separated; and then stored at – 80 °C for further analyses.

2.3 Detection of Cd Content

The method applied followed that of Lin et al. (2012). To eliminate iron on the root surface, 20 mM Na2-EDTA and deionized water were used to wash the roots thrice. The root stems and leaf samples were oven-dried at 75 °C for 2 days. Dried samples (0.5 g) were powdered and digested in an HNO3/HClO4 mixture (4:1, v/v). Cd concentrations were determined using an iCAP 6300 ICP spectrometer (Thermo Scientific, Waltham, MA, USA) with a detection limit of ≤ 0.006 μg mL−1. The Cd concentrations in the leaves, stems, and roots were calculated as the weight/number of moles of Cd per dry weight (DW) of the plant body.

2.4 Histochemical Detection and Determination of H2O2 and O2 −

The 1 mg/mL 3,3-diaminobenzidine (DAB) dye was used to detect H2O2 accumulation in leaves, and 0.5 mg mL−1 nitroblue tetrazolium (NBT) dye was used to detect O2− accumulation in leaves (Ahammed et al. 2013). H2O2 content in leaves was detected according to the process reported by Alexieva et al. (2001), and the O2− production rate in leaves was determined according to the procedure reported by Wu et al. (2010).

2.5 Assay for Chlorophyll Content

The content of chlorophyll a, chlorophyll b, and carotenoids were determined following the acetone-ethanol mixture immersion method followed by Harmut (1987). Fresh leaf tissue (0.1 g) was soaked in 80% acetone-ethanol mixture and then treated in darkness for 24 h. After leaves completely became colorless, the solution was measured at 663, 645, and 470 nm in spectrophotometer.

2.6 Measurements of Chlorophyll Fluorescence and Photosynthetic Parameter

The photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), and intercellular CO2 concentration (Ci) were measured using a LI-6400 portable photosynthesis system (LI-COR 6400, Lincoln, NE, USA).

The chlorophyll Fluorescence parameters (maximum efficiency of photosystem II photochemistry (Fv/Fm), the relative PSII electron transport rate (ETR), coefficients of photochemical quenching (qP), and non-photochemical quenching values (NPQ)) were measured with a portable fluorescence meter (Walz, PAM-2500, Effeltrich, Germany).

2.7 Subcellular Localization

The differential centrifugation technique method was applied followed by Dai et al. (2020). Frozen samples (0.2 g) of tomato tissues were homogenized in 20 mL cold (4 ℃) extraction buffer. The extraction buffer contained 50 mmol L−1 Tris–HCl (pH 7.5), 250 mmol L−1 sucrose, 1.0 mmol L−1 dithiothreitol (C4H10O2S2), 5.0 mmol L−1 ascorbic acid, 1.0% (w/v) polyvinylpyrrolidone (PVP). Briefly, the homogenate was centrifuged at 3000 r min−1 for 10 min at 4 ℃, and the pellet was designated the cell-wall fraction. The supernatant was further centrifuged at 12,000 r min−1 for 25 min at 4 ℃, and the resultant pellet and supernatant were designated as the organelle-containing fraction and soluble fraction, respectively. The different fractions were oven-dried at 105 ℃ for 24 h then wet-digested in HNO3/HClO4 mixture (4:1, v/v). Cd content of different fraction was detected.

2.8 Extraction of Cd in Different Chemical Forms

The method applied followed that of Zhao et al. (2015). To determine which chemical forms of Cd were present in cherry tomatoes, six chemical forms of Cd were extracted step by step using a sequence of designated extractants in the following order: (1) 80% ethanol (FE), extracting inorganic Cd and aminophenol cadmium; (2) deionized water (FW), extracting water-soluble Cd of organic acid complexes and Cd(H2PO4)2; (3) 1 mol L−1 NaCl (FNaCI), extracting Cd integrated with pectate and protein; (4) 2% acetic acid (FHAc), extracting insoluble CdHPO4, Cd3(PO4)2 and other Cd-phosphate complexes; (5) 0.6 mol L−1 HCl (FHCI), extracting oxalate acid-bound Cd; (6) any Cd remaining was considered residual Cd (Fr). Frozen tissues (0.2 g) were homogenized in extraction solution with a mortar and a pestle, and then shaken for 22 h at 25 ℃ in 50 mL centrifugal tube. The homogenate was then centrifuged at 3500 r min−1 for 15 min. The sedimentation was re-suspended in extraction solvent, centrifuged at 3500 r min−1 for 15 min. The supernatant from each of the three repetitions was then pooled for each of the five extraction solutions. The concentration of each Cd chemical form in each fraction was determined.

2.9 Determination of Malondialdehyde and Electrolyte Leakage Rate

The malondialdehyde (MDA) content was detected using the thiobarbituric acid method reported by Ahmad et al. (2018). Fresh leaves were macerated in 0.1% trichloroacetic acid (TCA), and the homogenate was centrifuged at 10,000 r min−1 for 5 min. We reacted 1 mL of supernatant with 4 mL thiobarbituric acid (5% TBA prepared in 20% TCA) at 100 °C for 30 min. Thereafter, samples were cooled in an ice bath and were again centrifuged for 10 min at 10,000 r min−1. Optical density was read at 532 and 600 nm (Ahmad et al. 2018). The electrolyte leakage rate in the leaves of two cultivars was measured using a conductivity meter via the method described by Ahmad et al. (2018).

2.10 Assays for Non-Enzymatic Antioxidants

Fresh leaves (0.5 g) were ground into a homogenate with precooled 5% metaphosphoric acid (including 1 mM EDTA), and the homogenate was centrifuged at 4 °C and 12,000 r min−1 for 20 min. The supernatant was used to determine antioxidant content. Glutathione disulfide (GSSG) and glutathione (GSH) contents were measured according to the method reported by Griffith. (1980). Ascorbate (AsA) and dehydroascorbate (DHA) contents were measured as described by Gossett et al. (1994).

2.11 Assays of Antioxidant Enzyme Activities

Frozen leaves (0.5 g) were ground into a homogenate with 5 mL 50 mM phosphoric acid buffer containing 0.2 mM EDTA and 2% polyvinylpyrrolidone (PVP), and the homogenate was centrifuged at 4 °C and 12,000 r min−1 for 20 min. The supernatant was used to determine the antioxidant enzyme activities. Superoxide dismutase (SOD, EC 1.15.1.1), peroxidase (POD, EC 1.11.1.7), catalase (CAT, EC 1.11.1.6), and ascorbate peroxidase (APX, EC 1.11.1.11) activity were determined via the methods reported by Colak et al. (2019). Dehydroascorbate reductase (DHAR, EC 2.5.1.18) and monodehydroascorbic acid reductase (MDHAR, EC 1.6.5.4) activity was determined via the procedure described by Hasanuzzaman et al. (2012). Glutathione peroxidase (GPX, EC 1.11.1.9) and glutathione reductase (GR, EC 1.6.4.2) activity was measured via the method described by Khanna et al. (2019).

2.12 Assays of Phytochelatins and Glutathione Synthesis Enzyme Activities

Phytochelatins (PCs), phytochelatin synthesis enzymes (PCSase), γ-glutamylcysteine synthetase (γ-ECS, EC 6.3.2.2), and glutathione synthetase (GSS, EC 6.3.2.3) activity in the leaves of two cultivars were measured using an enzyme immunoassay ELISA kit (http://www.mmbio.cn/, China).

2.13 Statistical Analysis

The data from all determinations were subjected to analysis of variance (ANOVA) to identify the response of the two cherry tomato cultivars to Cd and Se (Huang et al. 2021). Lowercase letters in figures indicate significant differences between treatments in the same species at the 0.05 significance level.

3 Results

3.1 Se Altered Cd Content in the Two Cherry Tomato Cultivars

Cd was not detected in the control and Se-treated groups of both cultivars (Fig. 1). Under Cd stress, leaf Cd concentrations of HLZ and LFC were 87.9 mg kg−1 DW and 148.4 mg kg−1 DW, respectively. Moreover, Se significantly (P < 0.05) decreased Cd concentrations in Cd-stressed HLZ and LFC leaves by 18.3% and 13.4%, respectively. The root Cd concentrations of Cd-stressed HLZ and LFC plants were 462.7 mg kg−1 DW and 461.1 mg kg−1 DW, respectively. However, under Se + Cd treatment, the root Cd concentrations of HLZ and LFC plants were 13.9% and 7.7% (P < 0.05) lower than those of plants exposed to Cd treatment, respectively. These results suggest that although Se significantly decreased the Cd content in the leaves and roots of the two cultivars, it exhibited no effect on stem Cd concentrations.

Fig. 1
figure 1

Cadmium content of Hanluzhe (HLZ) and Lvfeicui (LFC). Cd content in leaf (A), stem (B), and root (C) of HLZ and LFC, where blue bars represent HLZ and yellow bars represent LFC. Different lowercase letters indicate significant differences (P < 0.05) in the same species. Not detected (Nd)

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To determine the mechanism underlying the mitigation of Cd accumulation by Se, we determined the subcellular distribution (Table 1) and chemical forms of Cd (Table 2) in the leaves and roots of both cultivars. We observed Cd primarily in the cell wall and soluble fractions, with only traces in the organelles. Under Cd treatment, exogenous Se significantly (P < 0.05) reduced Cd in the cell wall fraction in the HLZ leaves by 21.74% compared to Cd treatment, and 31.37% in the roots; meanwhile, the organelles fraction had 28.22% and 27.05% (P < 0.05) less Cd in the HLZ leaves and roots compared to Cd treatment, respectively. Se application also decreased the organelles Cd fraction by 29.78% in the LFC leaves and decreased Cd content in both cell-wall (28.67%) and organelles Cd fractions (19.26%) in the LFC roots.

Table 1 Subcellular distribution of Cd (mg kg−1 FW) in two cherry tomatoes
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Table 2 Chemical forms of Cd (mg kg−1 FW) in two cherry tomatoes
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The highest proportion of Cd was extracted from the leaves and roots of both cultivars using NaCl. However, exogenous Se significantly decreased Cd proportions that were extracted using NaCl in HLZ leaves and LFC roots. Furthermore, Cd proportions extracted using 80% ethanol and deionized water were significantly decreased upon Se treatment in both cultivars. Thus, Se altered the proportion of different forms of Cd extracted from the two cultivars.

3.2 Se alleviates Cd-Induced Oxidative Damage

Oxidative damage was the major Cd-induced injury in the two cultivars. Thus, histochemical detection and determination of H2O2 and O2− radicals were used to determine Cd-induced and Se-alleviated oxidative damage in the leaves of the two cultivars (Fig. 2A–D). Under Cd stress, the O2− production rates of HLZ and LFC increased by 34.8% and 84.4% (P < 0.05), respectively, compared with the control group. In contrast, under Cd + Se treatment, the O2− production rates of HLZ and LFC were 8.2% and 10.3% (P < 0.05) lower than those exposed to Cd stress, respectively (Fig. 2F). A similar trend was observed for H2O2. Under Cd stress, H2O2 content increased by 67.9% and 153.6% (P < 0.05) in HLZ and LFC plants, respectively, compared with the control group, whereas exogenous Se significantly (P < 0.05) decreased H2O2 content by 10.3% and 19.1% in Cd-stressed HLZ and LFC plants, respectively (Fig. 2E).

Fig. 2
figure 2

Oxidative damage in leaves of two cultivars under different treatment. Histochemical detection O2− in A Hanluzhe (HLZ) and B Lvfeicui (LFC). Histochemical detection of H2O2 in C HLZ and D LFC. H2O2 content (E), generation rate of O2− (F), MDA content (G), and electrolyte leakage rate (H) in HLZ and LFC leaves, where blue bars represent HLZ leaves and yellow bars represent LFC leaves. Different lowercase letters indicate significant differences (P < 0.05) in the same species

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MDA content indicates the degree of membrane lipid peroxidation. We observed that Cd significantly increased MDA content in the two cultivars compared with the control group (Fig. 2G). Additionally, Se + Cd treatment decreased MDA content by 4.0% and 7.4% (P < 0.05) in HLZ and LFC, respectively. Hence, Cd induced more severe oxidative damage in the high Cd-accumulating cultivar LFC than in the low Cd-accumulating cultivar HLZ. Moreover, the alleviation of Cd-induced oxidative damage by Se was more efficient in the high Cd-accumulating cultivar LFC than in the low Cd-accumulating cultivar HLZ.

3.3 Chlorophyll Content, Fluorescence, and Photosynthetic Parameters of the Two Cultivars

Chlorophylls participate in energy production and transformation through photosynthesis and thus aid in plant growth. Under Cd stress, the content of chlorophyll a, b, total chlorophyll, and carotenoid markedly decreased in both cultivars (Fig. 3). However, exogenous Se significantly (P < 0.05) increased chlorophyll a, total chlorophyll, and carotenoid content by 14.5%, 15.3%, and 15.6% in HLZ and 14.5%, 14.8%, and 9.7% in LFC compared to Cd treatment, respectively. However, Se did not significantly affect the chlorophyll b content in both cultivars exposed to Cd stress.

Fig. 3
figure 3

Effect of application of Se on chlorophyll contents of A Hanluzhe (HLZ) and B Lvfeicui (LFC). Different lowercase letters indicate significant differences (P < 0.05)

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Photosynthesis is one of the primary metabolic plant processes that influences crop production. Cd stress significantly decreased Pn, Gs, and Tr parameters and increased Ci parameters in the two cultivars to varying degrees (Table 3). Although exogenous Se did not affect photosynthetic parameters in LFC, it increased Pn by 19.1% and decreased Ci by 4.1% in the Cd-stressed HLZ plants.

Table 3 Effect of application of Se on photosynthetic and fluorescence parameters of two cultivars
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Furthermore, Cd stress significantly decreased Fv/Fm, ETR, and qP, and enhanced NPQ parameters in the two cultivars to varying degrees (Table 3). In contrast, exogenous Se enhanced Fv/Fm and ETR, and decreased NPQ parameters in both cultivars exposed to Cd stress. Thus, exogenous Se improved PSII electron transport rate during the light reaction to mitigate Cd-induced photosynthetic inhibition.

3.4 Antioxidant Content and Antioxidant Enzyme Activity in the Two Cultivars

Modulation of antioxidant content is a pivotal mechanism to balance osmotic stress caused by abiotic stressors. In this study, although Cd significantly increased the content of GSSG, GSH, and DHA, it decreased AsA content in the two cultivars, compared with the control (Table 4). Se + Cd treatment decreased the DHA content by 31.2% and 11.6% (P < 0.05), and increased the content of GSSG by 10.8% and 22.0% and GSH by 32.1% and 22.6% in the HLZ and LFC plants, respectively, compared with Cd-stressed plants. Nonetheless, Se + Cd treatment did not affect AsA content compared with Cd stress, suggesting that Se regulated Cd-induced osmotic stress.

Table 4 Antioxidant content of two cherry tomatoes under different treatment
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We further determined the activities of several antioxidant enzymes to investigate the mechanism underpinning the mitigation of oxidative damage by Se. Cd decreased the activities of SOD, CAT, POD, MDHAR, GPX, and APX in the two cultivars (Fig. 4). However, compared with Cd-stressed plants, Se + Cd treatment significantly (P < 0.05) increased SOD, POD, CAT, GPX, and APX activity in HLZ plants by 37.0%, 50.6%, 18.5%, 28.3%, and 26.6%, respectively. Similarly, Se + Cd treatment significantly (P < 0.05) increased SOD, POD, CAT, MDHAR, GPX, and APX activity in LFC plants by 48.9%, 30.4%, 28.6%, 21.1%, 30.4%, and 47.4%, respectively, compared with Cd-stressed plants. Moreover, Cd accelerated the activities of GR and DHAR in the two cultivars. Comparably, Se + Cd treatment significantly increased GR activity by 31.2% and 16.3% in HLZ and LFC, and decreased DHAR activity by 8.5% and 9.6% in HLZ and LFC, respectively, compared with Cd-stressed plants.

Fig. 4
figure 4

Antioxidant enzyme activity of Hanluzhe (HLZ) and Lvfeicui (LFC), where blue bars represent HLZ leaves and yellow bars represent LFC leaves. SOD enzyme activity (A), POD enzyme activity (B), GR enzyme activity (C), GPX enzyme activity (D), CAT enzyme activity (E), MDHAR enzyme activity (F), DHAR enzyme activity (G), APX enzyme activity (H) of two cultivars. Different lowercase letters indicate significant differences (P < 0.05) in the same species. SOD superoxide dismutase, POD peroxidase, GR glutathione reductase, GPX glutathione peroxidase, CAT catalase, MDHAR monodehydroascorbate reductase, DHAR dehydroascorbate reductase, APX ascorbate peroxidase

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3.5 Biosynthesis of GSH and PCs

Two crucial GSH biosynthesis enzymes are γ-ECS and GSS. Compared with the control group, Cd increased γ-ECS and GSS activities in the two cultivars (Fig. 5). Furthermore, Se + Cd treatment significantly increased γ-ECS and GSS activities in HLZ and LFC plants compared with plants exposed to Cd treatment.

Fig. 5
figure 5

Activity of γ-glutamylcysteine synthetase (A) and glutathione synthetase (B), phytochelatins (C) content, and phytochelatin synthase enzyme activity (D) in Hanluzhe (HLZ) and Lvfeicui (LFC), where blue bars represent HLZ leaves and yellow bars represent LFC leaves. Different lowercase letters indicate significant differences (P < 0.05) in the same species

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PCs are important compounds that chelate Cd in the vacuole. Under Cd stress, PCs content increased in the two cultivars compared with that in the control group. Se + Cd treatment increased PCs content in HLZ and LFC plants by 15.1% and 42.4% (P < 0.05), respectively, compared with the Cd-stressed plants. However, Se exhibited different effects on PCSase activity in the two cultivars upon exposure to Cd stress; Se significantly increased PCSase activity in Cd-stressed LFC plants by 16.2% (P < 0.05); however, no significant effect was observed in Cd-stressed HLZ plants. Thus, the increase in PCs content to chelate Cd upon exogenous Se application was higher in the high Cd-accumulating cultivar LFC than in the low Cd-accumulating cultivar HLZ.

3.6 Se Improved Nutrient Content in the Two Cultivars Exposed to Cd Stress

Compared to the control, Cd stress significantly decreased the content of Ca, Fe, Zn, and Cu in the shoot and roots of both cultivars to varying degrees (Table 5). Additionally, Mg concentration varied among the two cultivars upon Cd stress; although Cd increased Mg content in HLZ plants, it inhibited Mg accumulation in LFC plants compared to the control. Exogenous Se significantly improved shoot and root Mg, Fe, and Cu content of HLZ plants; shoot Ca, Mg, Fe, and Cu content, and root Ca and Fe content of LFC plants. Overall, Se promoted both root absorption and improved root to shoot transportation of nutrients under Cd stress (Fig. 6).

Table 5 Effect of Cd and Se on macro- and micro-element concentrations of two cherry tomatoes
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Fig. 6
figure 6

Schematic diagram of Se mitigation on photosynthesis and oxidative stress under Cd stress. H2O2 hydrogen peroxide, O2− superoxide radicals, GSSG glutathione disulfide, GSH glutathione, AsA ascorbate, DHA dehydroascorbate, SOD superoxide dismutase, POD peroxidase, CAT catalase, GR glutathione reductase, MDHAR monodehydroascorbate reductase, APX ascorbate peroxidase, GPX glutathione peroxidase, PCs phytochelatins, γ-ECS γ-glutamylcysteine synthetase, GSS glutathione synthetase, PCSase phytochelatin synthase enzyme, MDHA monodehydroascorbate, DHAR dehydroascorbate reductase, ROS reactive oxygen species, e− electrons, PC plastocyanin

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4 Discussion

Cd pollution has become a growing concern in recent years. Generally, low Cd content implies low toxicity and more optimized growth conditions. Therefore, minimization of Cd content in crop plants is essential to prevent Cd damage and improve food safety (Zhou et al. 2021). Riaz et al. (2021) reported that the Se-mediated reduction in Cd transportation can be ascribed to the establishment of Cd–thiol complexes, which result in the disruption of upward movement of Cd in the shoot. In this study, exogenous Se decreased Cd content in the leaves and roots of the two cherry tomato cultivars treated with Cd, and the reduction rate in the low Cd-accumulating cultivar HLZ was higher than that in the high Cd-accumulating cultivar LFC.

Photosynthesis is an important plant metabolic process that provides energy for plants (Zou et al. 2020). Photosynthesis requires light and light is captured by light-harvesting chlorophyll a/b-binding (Lhc) proteins (Zou et al. 2020). High chlorophyll levels often result in high crop yields (Alam et al. 2019). In this study, Cd significantly decreased the chlorophyll content of the two cherry tomato cultivars, which were cultivated with rice and strawberry plants (Wu et al. 2021; Huang et al. 2021). However, exogenous Se increased the total chlorophyll content in the two cherry tomato cultivars by increasing chlorophyll a content under Cd stress, a phenomenon also observed in rice plants (Huang et al. 2021). Thus, the increase in chlorophyll content upon Se application improved tomato yield under Cd stress (Xie et al. 2021).

Photosynthesis has two steps: light reaction and CO2 assimilation. In the light reaction, electrons (e−) are extracted from H2O and delivered to NADP+, which is coupled with the translocation of protons (H+) from the stroma to the lumen to generate a proton motive force that is used to drive ATP synthesis (Kramer et al. 2004). In this study, Cd significantly reduced ETR, Fv/Fm, qP, and increased NPQ parameters in the two cherry tomato cultivars, which were cultivated with wheat and maize (Ozfidan-Konakci et al. 2018; Zhao et al. 2018). These results revealed that Cd altered the e− transfer efficiency of the light reaction center.

In the photosynthetic chain, the photosynthetic cytochrome b6f complex, an Fe-containing protein that consists of eight subunits and seven prosthetic groups, catalyzes H+-coupled e− transfer across the thylakoid membrane (Bhaduri et al. 2019). Plastocyanin (PC) is a copper-containing protein that transfers e− from cytochrome b6f complex to photosystem I (Viola et al. 2021). In this study, Cd significantly decreased Fe and Cu content in the two cherry tomato cultivars, suggesting that Cd altered e− transfer by decreasing the abundance of cytochrome b6f complex and PC. Nonetheless, Se application significantly improved Fe and Cu content in the shoots of the two cherry tomato cultivars. Moreover, exogenous Se increased Fv/Fm, ETR, and decreased NPQ parameters of the two cherry tomato cultivars exposed to Cd stress. Thus, exogenous Se enhanced e− transfer efficiency by improving the content of macro- and micro-elements in the two cultivars under Cd stress.

During CO2 assimilation, plant cells use NADPH and ATP as substrates to convert CO2 into sugar (Hu et al. 2021). Increased Ci and decreased Gs and Pn parameters indicated that Cd altered both the light reaction and CO2 assimilation in the two cherry tomato cultivars. Furthermore, Mg is required for the synthesis of Rubisco, which indirectly affects CO2 fixation by regulating Rubisco abundance (Tränkner et al. 2018). Thus, Mg deficiency can limit CO2 assimilation (Jamali Jaghdani et al. 2021). In this study, Mg content varied in the two Cd-stressed cherry tomato cultivars; Cd decreased and increased Mg content in the high Cd-accumulating cultivar LFC and the low Cd-accumulating cultivar HLZ, respectively. This suggests that Cd-tolerant cultivars exhibit a compensatory mechanism to resist Cd toxicity. Nevertheless, Se application significantly improved Mg content in the shoots of the two cherry tomato cultivars. Therefore, combined with the reduced Ci parameters, we hypothesized that exogenous Se enhanced CO2 assimilation by improving Mg accumulation, which might have improved Pn in HLZ.

Cd stress decreased the efficiency of photosynthetic light conversion, resulting in high levels of ROS that caused oxidative damage to nucleic acids, lipids, proteins, and cofactors (Singh et al. 2019). MDA content reflects the degree of membrane lipid peroxidation, which indirectly validates the degree of cell damage caused by free radicals (Mohamed et al. 2012). In this study, Cd increased the generation rate of O2−, H2O2, and MDA content in the two cherry tomato cultivars. A similar phenomenon was observed in mung bean (Hossain et al. 2010) and cucumber (Hawrylak-Nowak et al. 2014). However, the response to Cd stress varied in the two cherry tomato cultivars; the low Cd-accumulating cultivar HLZ was more tolerant to Cd-induced oxidative damage compared with the high Cd-accumulating cultivar LFC, which is consistent with the findings of Hassan and Mansoor (2014). Previous studies have also demonstrated that Se alleviated Cd-induced oxidative stress, as indicated by reduced O2−, H2O2, and MDA in rice (Lin et al. 2012) and mustard plants (Ahmad et al. 2016). Similar Se alleviation was observed in the cherry tomato cultivars; however, the mitigation efficiency of Se in LFC exceeded that in HLZ. These results suggest that the mitigation of Cd-induced oxidative damage by Se varied among cultivars of the same species.

To cope with the adverse effects of Cd stress, plants have evolved a well-developed antioxidant defense system that contains non-enzymatic and enzymatic antioxidants. The non-enzymatic antioxidant defense system primarily comprises GSH, AsA, carotenoids, and tocopherols (Gill and Tuteja 2010), whereas the enzymatic antioxidant defense system primarily comprises SOD, CAT, APX, and POD (Noctor et al. 2020). In plants, SOD converts O2− to H2O2, which provides frontline protection against ROS. In addition, POD and CAT scavenge H2O2 and convert it into H2O (Hasanuzzaman et al. 2011). Several studies have reported that Cd stress inhibits the activity of antioxidant enzymes (Hasanuzzaman et al. 2017a; Huang et al. 2020; Li et al. 2020). In this study, the activities of SOD, POD, and CAT were inhibited to varying degrees in both Cd-stressed cherry tomato cultivars. This Cd-induced inhibition may attribute to the replacement of Fe by Cd from the active center of antioxidant enzymes, which rendered them inactive (Karam et al. 2017). However, exogenous Se significantly increased the activities of SOD, POD, and CAT in the two cherry tomato cultivars, which is consistent with the results of previous studies on rice, Chinese cabbage, and wheat plants (Huang et al. 2020; Wu et al. 2017; Zhou et al. 2021).

The AsA–GSH pathway is another major antioxidant defense pathway that plays a role in scavenging H2O2 to mitigate oxidative damage, in addition to CAT and POD (Foyer and Noctor 2005, 2011). In this pathway, GSH, GSSG, AsA, and DHA are the primary antioxidant components (Hasanuzzaman et al. 2011; Smirnoff and Wheeler 2000). Similar to pak choi, Cd decreased AsA content but increased DHA content (Lou et al. 2017) in the cherry tomato cultivars. Nonetheless, exogenous Se alleviated alterations in antioxidant content in the two cherry tomato cultivars, which is consistent with the study on rapeseed by Hasanuzzaman et al. (2012). GSH, which synthesized from cysteine by γ-ECS and GSS, is an essential metabolite with several functions in plants (Fässler et al. 2011). In this study, both Cd and Se increased the activities of γ-ECS and GSS in the two cherry tomato cultivars to varying degrees, resulting in the increase in GSH and GSSG content upon Cd and Se treatment.

GPX, GR, DHAR, MDHAR, and APX are antioxidant enzymes that play important roles in the AsA–GSH pathway (Hasanuzzaman et al. 2011; Smirnoff and Wheeler 2000). GPX and APX are essential enzymes that detoxify H2O2. APX efficiently scavenges ROS owing to its high binding affinity for H2O2, even at low concentrations. APX uses AsA as an e− donor to convert H2O2 into H2O while AsA is transformed into MDHA (Szarka et al. 2012), whereas GPX utilizes GSH as a substrate to transform H2O2 into H2O while GSH is converted to GSSG (Zsigmond et al. 2011). Cd stress significantly inhibits the activity of APX in strawberry leaves after fruit maturation (Zhang et al. 2020). In this study, Cd-induced inhibition of APX and GPX was similar to that reported in the previous study. Additionally, the application of exogenous Se protected plants from Cd-induced oxidative stress by increasing APX and GPX activity, which is consistent with previous studies on rice, rapeseed, and radish (Amirabad et al. 2020; Chao et al. 2010; Hasanuzzaman et al. 2012). Furthermore, the increase in APX and GPX activity was higher in LFC than in HLZ.

In the AsA–GSH pathway, GR, DHAR, and MDHAR aid MDHA and GSSG to regenerate AsA and GSH and maintain the antioxidant defense system, which confers long-term stress tolerance (Gao and Zhang 2008; Huang et al. 2005). Several studies have reported that although Cd improves the activity of the AsA–GSH pathway enzyme GR, it inhibits the activities of MDHAR and DHAR compared with the control groups (Hasanuzzaman et al. 2012, 2017a, 2017b; Zhang et al. 2020). The effect of Cd on the activity of GR and MDHAR in the two cherry tomato cultivars was consistent with these findings (Hasanuzzaman et al. 2012; 2017b). However, the impact of Cd on the activity of DHAR in the two cultivars did not agree with the finding of a previous study (Hasanuzzaman et al. 2012), which may be attributed to species variations. The activity of DHAR, MDHAR, and GR, with increased levels of AsA and GSH, was induced upon exogenous Se under Cd stress, which alleviated oxidative stress (Hasanuzzaman et al. 2012, 2017b; Saidi et al. 2014). In this study, the effects of Se on the two cherry tomato cultivars were complex. Se significantly increased GR activity in both cultivars. However, the activity of DHAR in the two cultivars was inconsistent with the findings of a previous study (Hasanuzzaman et al. 2012), which was lower in plants exposed to Se + Cd treatment compared with those exposed to Cd treatment. Nonetheless, the effects of Se on the activity of MDHAR varied between the two cultivars. Overall, Se mitigated Cd-induced oxidative damage by accelerating the AsA–GSH pathway in the two cherry tomato cultivars. However, Se accelerated AsA–GSH pathway more efficiently in the high Cd-accumulating cultivar LFC than in low Cd-accumulating cultivar HLZ.

Besides antioxidant defense, PCs aid plants in the management of stress induced by toxic elements (Va´zquez et al. 2009). A number of metal ions are involved in the activation of PCSase in plants; however, the strongest activator is Cd (Akhter et al. 2012). PCs are synthesized from GSH by PCSase (Xiao et al. 2020), and chelate and sequester Cd ions in vacuoles (Jia et al. 2011). Zhang et al. (2019) reported that Cd stress increased PCs content in the roots and shoots of Zea mays. In this study, the exogenous application of Se elevated PCs content, and LFC exhibited a higher PCs content than HLZ, which suggests that exogenous Se efficiently increased Cd immobilization in the high Cd-accumulating cultivar LFC.

5 Conclusions

In this study, we determined the effects of Se on Cd content, photosynthesis, antioxidant enzyme activity, and nutrient content in two cherry tomato cultivars with different Cd-accumulation rates (HLZ and LFC) under Cd stress. The alleviation of Cd-induced oxidative stress by Se was more efficient in the high Cd-accumulating cultivar LFC than in the low Cd-accumulating cultivar HLZ. Furthermore, Se-mediated alteration in Cd content and improvement in Pn was higher in the low Cd-accumulating cultivar HLZ than in the high Cd-accumulating cultivar LFC. Thus, screening low Cd-accumulating cultivars with exogenous Se is a promising method to reduce Cd accumulation in cherry tomatoes.