Ameliorative Effects of Exogenous Proline on Photosynthetic Attributes, Nutrients Uptake, and Oxidative Stresses under Cadmium in Pigeon Pea (Cajanus cajan L.)

Abstract

Proline plays a significant role in the plant response to stress conditions. However, its role in alleviating metal-induced stresses remains elusive. We conducted an experiment to evaluate the ameliorative role of exogenous proline on cadmium-induced inhibitory effects in pigeon pea subjected to different Cd treatments (4 and 8 mg/mL). Cadmium treatments reduced photosynthetic attributes, decreased chlorophyll contents, disturbed nutrient uptake, and affected growth traits. The elevated activity of antioxidant enzymes (superoxide dismutase, catalase, and glutathione peroxidase), in association with relatively high contents of hydrogen peroxide, thiobarbituric acid reactive substances, electrolyte leakage, and endogenous proline, was measured. Exogenous proline application (3 and 6 mM) alleviated cadmium-induced oxidative damage. Exogenous proline increased antioxidant enzyme activities and improved photosynthetic attributes, nutrient uptake (Mg²⁺, Ca²⁺, K⁺), and growth parameters in cadmium-stressed pigeon pea plants. Our results reveal that proline supplementation can comprehensively alleviate the harmful effects of cadmium on pigeon pea plants.

Keywords: Cajanus cajan; antioxidant enzymes; exogenous proline; growth traits; metal stress.

Figure 1

Hydrogen peroxide (H₂O₂, mmol g⁻¹ FW) and lipid peroxidation (TBRS) contents (mmol g⁻¹ FW), and electrolyte leakage (EL%) in the leaves (A,C,E) and the roots (B,D,F) of pigeon pea exposed to different Cd and exogenous proline treatments. Bars represent the mean of three replicates ± SD. Cd 1 (4 mg), Cd 2 (8 mg), Cd 1 + Pro 1 (4 mg Cd + 3 mM proline), Cd 1 + Pro 2 (4 mg Cd + 6 mM proline), Cd 2 + Pro 1 (8 mg Cd + 3 mM proline), Cd 2 + Pro 2 (8 mg Cd + 6 mM proline). Different small letters show significant differences, and the same small letters show non-significant differences according to Duncan’s multiple range test (p ≤ 0.05).

Figure 2

Proline contents (mmol g⁻¹ fresh weight (FW)) (A) in the leaves and (B) in the roots of pigeon pea plants, under different cadmium and exogenous proline treatments. Bars represent the mean of three replicates ± SD. Cd 1 (4 mg), Cd 2 (8 mg), Cd 1 + Pro 1 (4 mg Cd + 3 mM proline), Cd 1 + Pro 2 (4 mg Cd + 6 mM proline), Cd 2 + Pro 1 (8 mg Cd + 3 mM proline), Cd 2 + Pro 2 (8 mg Cd + 6 mM proline). Different small letters show significant differences (p ≤ 0.05) according to Duncan’s multiple range test.

Figure 3

Superoxide dismutase (SOD) activity (U mg⁻¹ protein), peroxide reductase catalase (CAT) activity (U mg⁻¹ protein), glutathione peroxidase (GPX) activity (U mg⁻¹ protein), and ascorbate peroxidase (APX) activity (U mg⁻¹ protein) in leaves (A,C,E,G) and roots (B,D,F,H) of pigeon pea plants subjected to various cadmium and exogenous proline treatments. Bars represent the mean of three replicates ± SD. Cd 1 (4 mg), Cd 2 (8 mg), Cd 1 + Pro 1 (4 mg Cd + 3 mM proline), Cd 1 + Pro2 (4 mg Cd + 6 mM proline), Cd 2 + Pro1 (8 mg Cd + 3 mM proline), Cd 2 + Pro 2 (8 mg Cd + 6 mM proline). Different small letters show significant differences (p ≤ 0.05) according to Duncan’s multiple range test.