Effect of Cadmium and Copper Exposure on Growth, Physio-Chemicals and Medicinal Properties of Cajanus cajan L. (Pigeon Pea)

Abstract

Soil contamination with heavy metals is an emerging concern in the modern era, affecting all forms of life. Pigeon pea is a multi-use shrub with medicinal and nutritional values. On the basis of a randomized complete design, we investigated in the current project the combined cadmium (Cd) and copper (Cu) effect on plant growth and physio-chemical/medicinal properties of pigeon pea. Three-week-old seedlings were grown in combined Cd and Cu amended soil with increasing metal concentrations (control, 20 + 30 mg/kg, 40 + 60 mg/kg, and 60 + 90 mg/kg) for three months. At high-dose metal cumulative stress (60 + 90 mg/kg), plant shoot and root growth in terms of plant height as well as fresh and dry weight were significantly inhibited in association with decreased photosynthetic attributes (chlorophyll a and b contents, net photosynthesis, transpiration rate, stomatal conductance, intercellular CO₂ concentrations) and diminished nutrient contents. Cd and Cu at high amounts inflicted oxidative stresses as assessed in elevated lipid peroxidation (MDA), hydrogen peroxide (H₂O₂), and electrolyte leakage contents. Antioxidant enzyme activities, namely, those of superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), and glutathione peroxidase (GPX), were enhanced, along with proline content with increasing metal quantity. Phenolics and flavonoids exhibited a diverse response regarding metal concentration, and their biosynthesis was significantly suppressed at high Cd and Cu cumulative stress. The reduction in secondary metabolites may account for declined medicinal properties of pigeon pea as appraised in reduced antibacterial, 2, 2-diphenyl-1-picrylhydrazyl (DPPH), and ferric-reducing antioxidant potential (FRAP) activities. Our results clearly demonstrate that the exposure of pigeon pea to Cd- and Cu-contaminated soil might affect consumers due to the presence of metals and the negligible efficacy of the herbal products.

Keywords: antioxidant enzymes; medicinal properties; metals cumulative stress; oxidative damage; pigeon pea.

Figure 1

The effect of different Cd and Cu concentrations (0, Cd 20 + Cu 30 mg/kg, Cd 40 + Cu 60 mg/kg, and Cd 60 + Cu 90 mg/kg) on the chlorophyll a (A), chlorophyll b (B), net-photosynthesis (C), transpiration rate (D), intercellular CO₂ (E), and stomatal conductance (F) on the leaves of the pigeon pea. Different letters represent significant differences between the treatments at p ≤ 0.05.

Figure 2

The impact of different Cd and Cu stress concentrations (0, Cd 20 + Cu 30 mg/kg, Cd 40 + Cu 60 mg/kg, and Cd 60 + Cu 90 mg/kg) on the shoot’s FRAP (A) and DPPH (B) contents of pigeon pea plants. Bars represent means from the three independent replicates (±, n = 3); different letters indicate significant differences between the treatments at p < 0.05.

Figure 3

The effect of different Cd and Cu treatments (0, Cd 20 + Cu 30 mg/kg, Cd 40 + Cu 60 mg/kg, and Cd 60 + Cu 90 mg/kg) on the shoot (A,C,E) and root (B,D,F) H₂O₂, EL, and MDA contents of pigeon pea plants. Bars represent means from the three independent replicates (±, n = 3); different letters indicate significant differences between the treatments at p < 0.05.

Figure 4

The effect of different cadmium and copper concentrations (0, Cd 20 + Cu 30 mg/kg, Cd 40 + Cu 60 mg/kg, and Cd 60 + Cu 90 mg/kg) on shoots and roots’ antioxidant enzymes, namely, SOD (A), POD (B), CAT (C), and GPX (D) contents in pigeon pea plant. Bars represent means from the three independent replicates (±, n = 3); different letters indicate significant differences between the treatments at p < 0.05.

Figure 5

The effect of different cadmium and copper concentrations (0, Cd 20 + Cu 30 mg/kg, Cd 40 + Cu 60 mg/kg, and Cd 60 + Cu 90 mg/kg) on shoot (A) and root (B) metal accumulation of cadmium and copper in pigeon pea plant. Bars represent means from the three independent replicates (±, n = 3); different letters indicate significant differences between the treatments at p < 0.05.

Figure 6

Effect of different cadmium and copper concentrations (0, Cd 20 + Cu 30 mg/kg, Cd 40 + Cu 60 mg/kg, and Cd 60 + Cu 90 mg/kg) on shoot macro-nutrient K⁺, Ca⁺, and Mg²⁺ (A–C) and macro-nutrient Fe⁺, Zn, and Mn (D–F) contents in pigeon pea plant. Bars represent means from the three independent replicates; different letters indicate significant differences between the treatments at p < 0.05.