Oxidative Biomarkers and Antioxidant Systems in Heavy Metal Phytoremediation: A Critical Appraisal of Diagnostic Value and Causal Contribution

Abstract

Measurements of oxidative damage and antioxidant activity have become near-obligatory components of experimental phytoremediation research. Malondialdehyde concentration, hydrogen peroxide accumulation, electrolyte leakage and the activities of superoxide dismutase, catalase and ascorbate peroxidase are reported in a large majority of studies describing metal uptake by candidate remediation species. The interpretive weight placed on these measurements has grown faster than the evidence justifying it. This review examines two related propositions that underpin much of the field: that oxidative biomarkers indicate a physiological cost of metal accumulation in a way that predicts remediation performance, and that antioxidant capacity is a determinant, rather than a consequence, of metal tolerance and accumulation. Literature was identified through searches of biomedical, multidisciplinary and open-access scholarly indexes, supplemented by targeted retrieval of foundational methodological and conceptual works, and appraised for design adequacy, exposure realism, endpoint selection and consistency with independent evidence. The available evidence supports a diagnostic role for oxidative biomarkers only under narrowly specified conditions, and the thiobarbituric acid method that dominates the literature carries interference and specificity problems that are seldom addressed in phytoremediation reports. Comparisons between hyperaccumulating and non-accumulating taxa give inconsistent support for superior enzymatic antioxidant defence, whereas thiol metabolism, and glutathione in particular, shows a more reproducible association with tolerance together with a defensible causal basis derived from genetic manipulation. Interventions that raise antioxidant activity frequently improve biomass, and gains in metal removal are usually attributable to that biomass response rather than to redox status as such. Field studies rarely measure oxidative biomarkers, and where plant condition has been assessed at field scale, growth-based measures have discriminated soil treatments more effectively than physiological indices. Redox measurements are best understood as descriptors of plant condition rather than as predictors of extraction efficiency. Research priorities include compartment-resolved and genotype-anchored measurement, reporting of cumulative metal removal alongside redox endpoints, and deliberate testing of redox manipulations under aged field contamination.