Silicon (Si) foliar treatment modulates Capsicum annuum L. (green chilli) growth and stress responses under cadmium and lead stress

Abstract

Silicon (Si) plays a crucial role in improving plant resilience against abiotic stresses including heavy metals (HMs). However, little is known about its role in chilli plants during HM stresses like cadmium (Cd) and lead (Pb). The present study aimed to evaluate the role of Si in chilli plants grown under different concentrations of Cd and Pb respectively. Based on our findings, the increased levels of Cd and Pb adversely affected the physiological and biochemical traits in chilly plants. For instance, at 100 mg kg^-1, Cd and Pb significantly reduced the seed germination (62.5% and 50%), vigor indices (67.2% and 56.8%), root biomass (88% and 66%), chlorophyll a (75.5% and 55.5%) and carotenoids (56.4% and 48.7%) in chilly plants. However, supplementation of Si in chilli plants aids them in recovering from Cd and Pb side effects by improving their physiological and biochemical traits. At 25 mg kg^-1 soil Cd and Pb, Si significantly (p ≤ 0.05) improved the root length (17% and 24%), root biomass (23% and 27%), and carotenoids (16% and 19%) of chilli plants compared to control plants. Moreover, Si application significantly (p ≤ 0.05) reduced the oxidative stress markers; malondialdehyde (MDA), electrolyte leakage (EL), hydrogen peroxide (H₂O₂), and superoxide radical (O₂-) in Cd and Pb stressed chilli plants as compared to non-Si treated plants. Interestingly, Si foliar application in Cd and Pb-treated chilli plants upregulates the transcript levels of POD, SOD, CAT, and GPX genes. Additionally, Si reduces the HM-induced phytotoxicity by decreasing Cd and Pb uptake in roots and shoots of chilli plants, as well as metal translocation (TF) and bioconcentration (BCF) factors. In summary, these results highlight the protective role of Si in chilli plants by mitigating the side effects of Cd and Pb stress. Hence, Si fertilizers can be used in sustainable agriculture to mitigate HM toxicity and improve crop productivity.

Keywords: heavy metals; metal uptake; resilience mechanism; silicon (Si); stress responsive gene.

Figure 1

Effect of exogenously applied silicon (Si) on germination efficiency (A), and seedling vigour index (B) in chilli plants treated with Cd and Pb. Here, C, control; Si, silicon; Cd, cadmium; Pb, lead. The bar diagrams represent the mean values of three replicates (n = 3). Corresponding error bars represents standard deviation (S.D.). Letters a, b, c, d etc. in figure depict that mean values are significantly different from each other according to Duncan’s multiple range test (DMRT).

Figure 2

Effect of silicon (Si) root length (A), shoot length (B), root fresh weight (C), shoot fresh weight (D), root dry weight (E) and shoot dry weight (F) of chilli plants treated with different Cd and Pb concentrations. Here, C, control; Si, silicon; RL, root length; SL, shoot length; RFW, root fresh weight; SFW, shoot fresh weight; RDW, root dry weight; SDW, shoot dry weight; Cd, cadmium; Pb, lead. Each experiment was independently repeated thrice. The bar and scatter diagrams represent the mean values of three replicates (n = 3), with sample size (n =5). Corresponding error bars represents standard deviation (S.D.). Letters a, b, c, d etc. in figure depict that mean values are significantly different from each other according to DMRT test.

Figure 3

Impact of Si on leaf pigments; chlorophyll a (A), chlorophyll b (B) total chlorophyll (C) and carotenoid content (D) of chilli plants exposed to varying Cd and Pb concentrations. Each experiment was independently repeated thrice. The bar and scatter diagrams represent the mean values of three replicates (n = 3), with sample size (n =5). Corresponding error bars represents standard deviation (S.D.). Letters a, b, c, d etc. in figure depict that mean values are significantly different from each other according to DMRT test.

Figure 4

Antioxidant enzymes; catalase (A), guaiacol peroxidase (B), peroxidase (C), and superoxide dismutase (D) and defense responsive gene expressions i.e. CAT (E), GPX (F), POD (G) and SOD (H) of chilli plants treated with Si and increasing Cd and Pb concentrations. Each experiment was independently repeated thrice. The bar and scatter diagrams represent the mean values of three replicates (n = 3), with sample size (n =5). Corresponding error bars represents standard deviation (S.D.). Letters a, b, c, d etc. in figure depict that mean values are significantly different from each other according to DMRT test.

Figure 5

Effect of Si application on proline (A), MDA (B), electrolyte leakage (C) and ROS generation; hydrogen peroxide (D) and superoxide radical (E) content of CD and Pb treated chilli plants. Each experiment was independently repeated thrice. The bar and scatter diagrams represent the mean values of three replicates (n = 3), with sample size (n =5). Corresponding error bars represents standard deviation (S.D.). Letters a, b, c, d etc. in figure depict that mean values are significantly different from each other according to DMRT test.

Figure 6

Uptake of Cd (A), Pb (B) in roots and shoots tissues, HM tolerance index (C) and translocation factor (D) of metal-treated and Si applied plants. Each experiment was independently repeated thrice. The bar and scatter diagrams represent the mean values of three replicates (n = 3), with sample size (n =5). Corresponding error bars represents standard deviation (S.D.). Letters a, b, c, d etc. in figure depict that mean values are significantly different from each other according to DMRT test.

Figure 7

Heat map matrix representing the association between the analysed parameters of chilli plants treated with Cd, Pb and silicon (Si). RL, root length; SL, shoot length; RFW, root fresh weight; SWF, shoot fresh weight; RDW, root dry weight; SDW, shoot dry weight; chl, chlorophyll; EL, electrolyte leakage; H₂O₂, hydrogen peroxide; MDA, malondialdehyde; POD, peroxidase; SOD, superoxide dismutase; CAT, catalase; GPX, guaiacol peroxidase; Cd, cadmium; Pb, lead; Si, silicon; TF, translocation factor; TI, tolerance index. Scale bar represents normalized values, uses min-max scaling with negative values indicating lower-than-average and positive values showing higher-than-average. I, represents the main cluster grouping, IA and IB, represents a sub-cluster within cluster I.

Figure 8

Pearson correlation matrixfor the analysed parameters of chilli plants after Cd and Pb stress and Si treatment. RL, root length; SL, shoot length; RFW, root fresh weight; SWF, shoot fresh weight; RDW, root dry weight; SDW, shoot dry weight; chl, chlorophyll; EL, electrolyte leakage; H₂O₂, hydrogen peroxide; MDA, malondialdehyde; POD, peroxidase; SOD, superoxide dismutase; CAT, catalase; GPX, guaiacol peroxidase; Cd, cadmium; Pb, lead; Si, silicon; TF, translocation factor; TI, tolerance index.

Figure 9

Principal component analysis (PCA) for the analysed parameters of chilli plants treated with Cd and Pb and exogenously applied Si. RL, root length; SL, shoot length; RFW, root fresh weight; SWF, shoot fresh weight; RDW, root dry weight; SDW, shoot dry weight; chl, chlorophyll; EL, electrolyte leakage; H₂O₂, hydrogen peroxide; MDA, malondialdehyde; POD, peroxidase; SOD, superoxide dismutase; CAT, catalase; GPX, guaiacol peroxidase; Cd, cadmium; Pb, lead; Si, silicon; TF, translocation factor; TI, tolerance index.