Heavy Metal Toxicity in Medicinal Solanaceae: A Critical Comparative Appraisal of Accumulation, Tolerance and Consequences for Herbal Quality

Abstract

The Solanaceae supply several of the most heavily traded medicinal plants, among them Withania somnifera, Solanum nigrum, Datura stramonium, Hyoscyamus niger and Capsicum annuum, yet the same family contains taxa that accumulate cadmium and other potentially toxic elements to concentrations deliberately exploited in soil remediation. That dual identity generates a problem which has attracted surprisingly little integrated attention: the physiological traits making a species attractive for phytoextraction are largely the traits that compromise the safety of material harvested for medicinal use. This review critically appraises evidence on metal uptake, partitioning, tolerance and phytochemical consequences across medicinal Solanaceae, and evaluates how far current findings support inferences about the safety and consistency of herbal products. Literature was identified through structured searching of openly accessible scholarly indexes and metadata registries, supplemented by backward and forward citation tracking and by institutional sources, with a final search date of 10 June 2026. The retrieved evidence is dominated by short-term cadmium experiments on S. nigrum and Nicotiana tabacum conducted in hydroponic or pot systems, while the taxa most heavily traded as herbal raw material remain comparatively neglected. Mechanistic work converges on a limited set of processes, including non-selective root transport, xylem loading, cell-wall binding, vacuolar sequestration, glandular trichome secretion and thiol-dependent detoxification, but the quantitative contribution of each process is poorly resolved and has rarely been compared across species under common conditions. A recurring and under-acknowledged observation is that sub-lethal metal exposure alters concentrations of pharmacologically active constituents, including tropane alkaloids, in directions that are not reliably predictable from dose alone. Contamination surveys of finished products confirm exposure, yet most report total element concentrations without speciation, extraction behaviour or bioaccessibility, leaving the step from plant burden to human dose weakly supported. Research priorities include multi-species field comparisons under shared protocols, routine reporting of metal transfer into the extract phase, and explicit separation of accumulator germplasm from medicinal germplasm.