Alcea rosea L. responses to Cd and Pb stress: phenotypic, physiological, subcellular, chemical speciation, and ultrastructural analyses

Cadmium (Cd) and lead (Pb) are highly toxic heavy metals that contaminate soils and aquatic environments, thereby inhibiting plant growth. Moreover, the phenomenon of combined pollution by Cd and Pb frequently occurs in natural environments. Hollyhock ( Alcea rosea ), a biennial herbaceous ornamental plant with a prolonged flowering period and abundant flowering, exhibits heightened tolerance to Cd and Pb. Although A. rosea is neither a hyperaccumulator nor a typical accumulator of Cd and Pb, its large biomass, strong adaptability, and high ornamental value make it a promising candidate for phytoremediation of soils with low to moderate contamination, achieving simultaneous pollution mitigation and landscape beautification. This study examined the phenotype, physiological responses, subcellular distribution, metal ion distribution and speciation, and root ultrastructure of A. rosea following treatment with Cd (30 μmol/L), Pb (200 μmol/L), and combined stress (Cd + Pb). Cd, Pb, and their combination inhibited the growth of A. rosea . The plants enhanced their tolerance against metals by increasing antioxidant enzyme activities, osmoregulatory compounds, and cellular chelators. The roots accumulated significantly more Cd and Pb than the shoots, and combined stress further raised Cd and Pb levels. Cd was primarily accumulated in the root cell walls and soluble fractions, while Pb was localized to the cell walls. The key chemical forms of Cd were FNaCl and FHAc, while FHCl and FHAc dominated for Pb. Combined stress markedly increased the contents of Cd in FE, FNaCl, FHAc, and FHCl, and Pb in FW, FHAc, and FHCl, which synergistically enhanced their toxicity and mobility (FE:80% ethanol extract; FW: double-distilled aqueous extract; FNaC1: l mol/L NaC1 extract; FHAc: 2% glacial acetic acid extract; FHCI:0.6 mol/L hydrochloric acid extract; FR: residual fraction). The ultrastructural damage included a reduction in heterochromatin, damage to the membranes, and detachment of the cell walls. This study demonstrates the potential of A. rosea to remediate soils co-contaminated with Cd and Pb, thereby contributing to ecological restoration efforts.