Redox homeostasis and trace metal regulation drive allantoin-induced tolerance against combined microplastic and cadmium stress in pea (Pisum sativum L.).

BACKGROUND

The combined toxicity of polyvinyl chloride microplastics (MP) and cadmium (Cd) is widespread in the environment, but their effects on plants are poorly understood. Allantoin (ALA) is a growth regulator that acts as a nitrogen reservoir, osmolyte, and radical scavenger under stress. This study aimed to investigate the role of allantoin in mitigating the phytotoxic effects of MP and Cd in pea plants.

METHODS

Foliar application of ALA (300 mg L1) was given to plants challenged with 100 mg kg1 Cd and 200 mg kg1 MP, as single or combined treatments. Growth, chlorophyll concentration, photosynthesis efficiency, leaf relative water content, nutrient acquisition, radical detoxification, oxidative stress markers, antioxidant enzyme activities, and anatomical features were measured. Histochemical staining was used to detect reactive oxygen species (ROS) and membrane damage, while concentrations of hydrogen sulfide and nitric oxide were assessed.

RESULTS

Exposure to Cd and MP, as single or combined treatments, severely impaired plant performance by diminishing growth, photosynthetic efficiency, and nutrient acquisition. Parllel to this, MP and Cd induced severe oxidative stress, marked by elevated ROS levels (O2•, H2O2, and •OH) and lipid peroxidation, which caused significant membrane damage as confirmed by histochemical analyses. Exogenous ALA countered these impairments by upregulating hydrogen sulfide and nitric oxide production and enhancing antioxidant enzyme activities. These ALA-mediated shifts restored nutrient acquisition, enhanced cellular protection, and mitigated the vascular and structural damage induced by MP and Cd toxicity.

CONCLUSION

Allantoin effectively mitigates the synergistic phytotoxicity of MP and Cd in pea plants by enhancing antioxidant defense, maintaining ion homeostasis, protecting microstructure, and facilitating nutrient uptake.