Zinc-lysine Supplementation Mitigates Oxidative Stress in Rapeseed (Brassica napus L.) by Preventing Phytotoxicity of Chromium, When Irrigated with Tannery Wastewater

Abstract

Contamination of soil and water with metals and metalloids is one of the most serious problems worldwide due to a lack of a healthy diet and food scarcity. Moreover, the cultivation of oilseed crops such as rapeseed (Brassica napus L.) with tannery wastewater could contain a large amount of toxic heavy metals [e.g., chromium (Cr)], which ultimately reduce its yield and directly influence oilseed quality. To overcome Cr toxicity in B. napus, a pot experiment was conducted to enhance plant growth and biomass by using newly introduced role of micronutrient-amino chelates [Zinc-lysine (Zn-lys)], which was irrigated with different levels [0% (control), 33%, 66%, and 100%] of tannery wastewater. According to the results of present findings, very high content of Cr in the wastewater directly affected plant growth and composition as well as gas exchange parameters, while boosting up the production of reactive oxygen species (ROS) and induced oxidative damage in the roots and leaves of B. napus. However, activities of antioxidants initially increased (33% of wastewater), but further addition of tannery wastewater in the soil caused a decrease in antioxidant enzymes, which also manifested by Zn content, while the conscious addition of wastewater significantly increased Cr content in the roots and shoots of B. napus. To reduce Cr toxicity in B. napus plants, exogenous supplementation of Zn-lys (10 mg/L) plays an effective role in increasing morpho-physiological attributes of B. napus and also reduces the oxidative stress in the roots and leaves of the oilseed crop (B. napus). Enhancement in different growth attributes was directly linked with increased in antioxidative enzymes while decreased uptake and accumulation of Cr content in B. napus when cultivated in wastewater with the application of Zn-lys. Zn-lys, therefore, plays a protective role in reducing the Cr toxicity of B. napus through an increase in plant growth and lowering of Cr uptake in various plant organs. However, further studies at field levels are required to explore the mechanisms of Zn-lys mediated reduction of Cr and possibly other heavy metal toxicity in plants.

Keywords: antioxidant enzymes; growth; heavy metals; micronutrient-amino chelates; oil seed crop.

Figure 1

The effect of different levels of tannery wastewater on total chlorophyll contents (A), carotenoid contents (B), transpiration rate (C), stomatal conductance (D), net photosynthesis (E) and water use efficiency (F) under the same concentration of Zinc-lysine application in B. napus. Values are demonstrated as means of three replicates along with the standard deviation (SD; n = 3). One-way ANOVA was performed, and the mean differences were tested by HSD (p < 0.05). Different lowercase letters on the error bars indicate a significant difference between the treatments. Different treatments used in the figures are as follows: Ck (without irrigation with wastewater + 0 mg/L Zn-lysine), T1 (without irrigation with wastewater + 10 mg/L Zn-lysine), T2 (33% irrigation with wastewater + 0 mg/L Zn-lysine), T3 (33% irrigation with wastewater + 10 mg/L Zn-lysine), T4 (66% irrigation with wastewater + 0 mg/L Zn-lysine), T5 (66% irrigation with wastewater + 10 mg/L Zn-lysine), T6 (100% irrigation with wastewater + 0 mg/L Zn-lysine), and T7 (100% irrigation with wastewater + 10 mg/L Zn-lysine).

Figure 2

The effect of different levels of tannery wastewater on MDA contents (A) H₂O₂ contents (B) and EL leakage (C) in the roots and leaves of B. napus under the same concentrations of Zinc-lysine application. Values are demonstrated as means of the three replicates along with the standard deviation (SD; n = 3). One-way ANOVA was performed, and mean differences were tested by HSD (p < 0.05). Different lowercase letters on the error bars indicate a significant difference between the treatments. Different treatments used in the figures are as follows: Ck (without irrigation with wastewater + 0 mg/L Zn-lysine), T1 (without irrigation with wastewater + 10 mg/L Zn-lysine), T2 (33% irrigation with wastewater + 0 mg/L Zn-lysine), T3 (33% irrigation with wastewater + 10 mg/L Zn-lysine), T4 (66% irrigation with wastewater + 0 mg/L Zn-lysine), T5 (66% irrigation with wastewater + 10 mg/L Zn-lysine), T6 (100% irrigation with wastewater + 0 mg/L Zn-lysine), and T7 (100% irrigation with wastewater + 10 mg/L Zn-lysine).

Figure 3

The effect of different levels of tannery wastewater on the activities of SOD (A), POD (B), CAT (C) and APX (D) in the roots and leaves under the same concentrations of Zinc-lysine application in B. napus. Values are demonstrated as the means of three replicates along with the standard deviation (SD; n = 3). One-way ANOVA was performed, and the mean differences were tested by HSD (p < 0.05). Different lowercase letters on the error bars indicate a significant difference between the treatments. Different treatments used in the figures are as follow: Ck (without irrigation with wastewater + 0 mg/L Zn-lysine), T1 (without irrigation with wastewater + 10 mg/L Zn-lysine), T2 (33% irrigation with wastewater + 0 mg/L Zn-lysine), T3 (33% irrigation with wastewater + 10 mg/L Zn-lysine), T4 (66% irrigation with wastewater + 0 mg/L Zn-lysine), T5 (66% irrigation with wastewater + 10 mg/L Zn-lysine), T6 (100% irrigation with wastewater + 0 mg/L Zn-lysine) and T7 (100% irrigation with wastewater + 10 mg/L Zn-lysine).

Figure 4

The effect of different levels of tannery wastewater on the uptake of Cr (A) and Zn (B) content in the roots and leaves under the same concentrations of Zinc-lysine application in B. napus. Values are demonstrated as the means of three replicates along with standard deviation (SD; n = 3). One-way ANOVA was performed, and the mean differences were tested by HSD (p < 0.05). Different lowercase letters on the error bars indicate a significant difference between the treatments. Different treatments used in the figures are as follow: Ck (without irrigation with wastewater + 0 mg/L Zn-lysine), T1 (without irrigation with wastewater + 10 mg/L Zn-lysine), T2 (33% irrigation with wastewater + 0 mg/L Zn-lysine), T3 (33% irrigation with wastewater + 10 mg/L Zn-lysine), T4 (66% irrigation with wastewater + 0 mg/L Zn-lysine), T5 (66% irrigation with wastewater + 10 mg/L Zn-lysine), T6 (100% irrigation with wastewater + 0 mg/L Zn-lysine) and T7 (100% irrigation with wastewater + 10 mg/L Zn-lysine).

Figure 5

Correlation between different attributes studied in this study. H2O2-R (H₂O₂ initiation in roots), CR-R (Cr content in roots), EL-R (electrolyte leakage in roots), MDA-R (MDA content in roots), CR-S (Cr content in shoots), EL-L (electrolyte leakage in leaves), MDA-L (MDA content in leaves), H2O2-L (H₂O₂ initiation in leaves), Zn-R (zinc content in roots), NP (net photosynthesis), SOD-L (SOD activity in leaves), WUE (water use efficiency), TR (transpiration rate), SC (stomatal conductance), Zn-L (zinc content in leaves), SOD-R (SOD activity in roots), PH (plant height), POD-R (POD activity in roots), CAT-L (CAT activity in leaves), CAT-R (CAT activity in roots), RL (root length), Carot (carotenoid content), APX-R (APX activity in roots), POD-L (POD activity in leaves), TC (total chlorophyll content), APX-L (APX activity in leaves), LA (leaf area), LFW (leaves fresh weight), LDW (leaves dry weight), NOL (number of leaves), RFW (root fresh weight), RDW (root dry weight).

Figure 6

Heatmap histogram correlation between different parameters studied in this experiment. (Ck) Cr 0%, Zn-lys 0 mg L⁻¹ (T1) Cr 0%, Zn-lys 10 mg L⁻¹ (T2) Cr 33%, Zn-lys 0 mg L⁻¹ (T3) Cr 33%, Zn-lys 10 mg L⁻¹ (T4) Cr 66%, Zn-lys 0 mg L⁻¹ (T5) Cr 66%, Zn-lys 10 mg L⁻¹ (T6) Cr 100%, Zn-lys 0 mg L⁻¹ (T7) Cr 100%, Zn-lys 10 mg L⁻¹. The abbreviations are as follow: CR-R (Cr content in roots), MDA-R (MDA content in roots), PH (plant height), SOD-R (SOD activity in roots), TC (total chlorophyll content), and Zn-R (zinc content in roots).

Figure 7

Score (a) and loading plots (b) of principal component analysis (PCA) on different studied attributes of B. napus grown in tannery wastewater soil. Score plot represents separation of treatments (1) Cr 0%, Zn-lys 0 mg L⁻¹ (2) Cr 0%, Zn-lys 10 mg L⁻¹ (3) Cr 33%, Zn-lys 0 mg L⁻¹ (4) Cr 33%, Zn-lys 10 mg L⁻¹ (5) Cr 66%, Zn-lys 0 mg L⁻¹ (6) Cr 66%, Zn-lys 10 mg L⁻¹ (7) Cr 100%, Zn-lys 0 mg L⁻¹ (8) Cr 100%, Zn-lys 10 mg L⁻¹. The abbreviations are as follows: Zn-R (zinc content in roots), TC (total chlorophyll content), PH (plant height), SOD-R (SOD activity in roots), CR-R (Cr content in roots), and MDA-R (MDA content in roots).