Aims Heavy metal toxicity in crops is a global concern that impairs plant growth and reduces yield. Melatonin (MT), a novel plant growth regulator, plays a key role in plant defense responses. This study aimed to evaluate the effects of different concentrations of MT (0, 50, 100, and 200 μM) on the physiological and biochemical mechanisms underlying okra’s cadmium (Cd) stress tolerance. Methods Pot experiments were conducted to assess the effects of MT on okra under cadmium chloride (CdCl2) stress. Four MT concentrations (0, 50, 100, and 200 μM) were used, and additional treatments with 100 μM MT, 5 mM diphenyleneiodonium (DPI, an NADPH oxidase inhibitor suppressing H2O2 production), or their combination were set to confirm the optimal MT concentration. Physiological and biochemical indicators related to okra growth, osmotic adjustment, photosynthesis, reactive oxygen species (ROS) accumulation, and antioxidant systems were determined, and hierarchical cluster and correlation analyses were performed. Results Under CdCl2 stress, no MT application led to okra plant death, while 100 μM MT significantly promoted okra growth. Treatments with 100 μM MT, 5 mM DPI, or their combination all enhanced Cd tolerance, confirming 100 μM as the optimal MT concentration. MT increased osmotic adjustment substances (proline, soluble protein, soluble sugar) and photosynthetic pigments (chlorophyll a/b, total chlorophyll, carotenoids) in Cd-exposed okra. CdCl2 exposure induced ROS accumulation, causing lipid peroxidation, membrane integrity loss, and cell death; however, MT strongly inhibited ROS accumulation, reducing oxidative stress indicators (H2O2, O2-, malondialdehyde [MDA], electrolyte leakage). Additionally, MT activated enzymatic (superoxide dismutase [SOD], glutathione peroxidase [GPX], catalase [CAT], glutathione reductase [GR], ascorbate peroxidase [APX], peroxidase [POD]) and non-enzymatic (ascorbic acid [AsA], glutathione [GSH]) antioxidant systems, and elevated AsA/DHA and GSH/GSSG ratios, maintaining redox homeostasis. Hierarchical cluster and correlation analyses revealed that exogenous MT induces Cd tolerance through osmotic adjustment, photosynthesis protection, and ROS scavenging. Conclusions Exogenous MT plays an integral role in regulating okra’s Cd stress resistance, with 100 μM being the optimal concentration. Its protective effects are achieved by enhancing osmotic adjustment capacity, protecting photosynthetic systems, inhibiting ROS accumulation, activating antioxidant systems, and maintaining redox homeostasis. This study expands the biological function of MT as an innovative biostimulant, providing a theoretical basis for improving crop productivity in heavy metal-polluted agricultural environments.

The role of melatonin in enhancing cadmium stress tolerance in okra: Physiological and biochemical insights

期刊论文
发表日期：

2026年3月2日

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Feibing Wang,
Zhengrong Wei,
Suhang Yao,
Siqi Xie,
Lin Wang,
Jie Ji

摘要

Aims

Heavy metal toxicity in crops is a global concern that impairs plant growth and reduces yield. Melatonin (MT), a novel plant growth regulator, plays a key role in plant defense responses. This study aimed to evaluate the effects of different concentrations of MT (0, 50, 100, and 200 μM) on the physiological and biochemical mechanisms underlying okra’s cadmium (Cd) stress tolerance.

Methods

Pot experiments were conducted to assess the effects of MT on okra under cadmium chloride (CdCl2) stress. Four MT concentrations (0, 50, 100, and 200 μM) were used, and additional treatments with 100 μM MT, 5 mM diphenyleneiodonium (DPI, an NADPH oxidase inhibitor suppressing H₂O₂ production), or their combination were set to confirm the optimal MT concentration. Physiological and biochemical indicators related to okra growth, osmotic adjustment, photosynthesis, reactive oxygen species (ROS) accumulation, and antioxidant systems were determined, and hierarchical cluster and correlation analyses were performed.

Results

Under CdCl2 stress, no MT application led to okra plant death, while 100 μM MT significantly promoted okra growth. Treatments with 100 μM MT, 5 mM DPI, or their combination all enhanced Cd tolerance, confirming 100 μM as the optimal MT concentration. MT increased osmotic adjustment substances (proline, soluble protein, soluble sugar) and photosynthetic pigments (chlorophyll a/b, total chlorophyll, carotenoids) in Cd-exposed okra. CdCl2 exposure induced ROS accumulation, causing lipid peroxidation, membrane integrity loss, and cell death; however, MT strongly inhibited ROS accumulation, reducing oxidative stress indicators (H₂O₂, O₂-, malondialdehyde [MDA], electrolyte leakage). Additionally, MT activated enzymatic (superoxide dismutase [SOD], glutathione peroxidase [GPX], catalase [CAT], glutathione reductase [GR], ascorbate peroxidase [APX], peroxidase [POD]) and non-enzymatic (ascorbic acid [AsA], glutathione [GSH]) antioxidant systems, and elevated AsA/DHA and GSH/GSSG ratios, maintaining redox homeostasis. Hierarchical cluster and correlation analyses revealed that exogenous MT induces Cd tolerance through osmotic adjustment, photosynthesis protection, and ROS scavenging.

Conclusions

Exogenous MT plays an integral role in regulating okra’s Cd stress resistance, with 100 μM being the optimal concentration. Its protective effects are achieved by enhancing osmotic adjustment capacity, protecting photosynthetic systems, inhibiting ROS accumulation, activating antioxidant systems, and maintaining redox homeostasis. This study expands the biological function of MT as an innovative biostimulant, providing a theoretical basis for improving crop productivity in heavy metal-polluted agricultural environments.