Heavy metal tolerance in Atlantic Forest species: Antioxidants, phytochelatins, or root exudates?

Environmental contamination by heavy metals (HM) threatens plant physiology; however, some species tolerate it via strategies like root exudation of organic acids (ROAs), phytochelatin (PC) synthesis, and antioxidants (ascorbic acid - AA, glutathione - GSH). We hypothesized that plant life-form and growth-rate may influence HM-tolerance. We experimentally studied five Atlantic Forest (a hotspot for biodiversity conservation) species: fast-growth pioneer tree (Schinus terebinthifolia), slow-growth non-pioneer tree (Cariniana legalis), fast-growth herbaceous (Seemannia sylvatica), fast-growth liana (Passiflora edulis), and slow-growth epiphyte (Aechmea fasciata). Plants were hydroponically exposed to Cu, Ni, and Zn (CuZnNi) for 45 days, and their physiological responses (gas exchange, chlorophyll fluorescence, pigments, biomass) and HM-tolerance strategies (PCs, ROAs, AA, GSH) were measured. Distinct HM-tolerance levels and mechanisms were found. The pioneer tree and epiphyte were the most tolerant species, showing mild physiological disturbances. The pioneer tree combined avoidance (phytic acid exudation), immobilization (leaf PC3), and mitigation (AA) mechanisms. The epiphyte relied on avoidance (diverse ROAs) and immobilization (leaf PC2). The non-pioneer tree was moderately tolerant, using immobilization (leaf PC6) and mitigation (AA) strategies, but still exhibited significant physiological imbalances. Despite their respective defense mechanisms, the herbaceous (avoidance: oxalic acid; immobilization: leaf PC4) and liana (mitigation: GSH) were highly sensitive, showing severe toxicity. Our results suggest that plant life-form influences heavy-metal accumulation and tolerance, whereas growth rate was not a reliable predictor. Broader investigations, including additional species, metals, and environmental conditions, are needed to determine whether these patterns reflect life-form effects or taxon-specific responses.