Ecotypic variation and inducible accumulation responses of Leersia hexandra Swartz under Cu(II) and Ni(II) co-contamination.

Heavy metal co-contamination is a global challenge for phytoremediation, particularly in regions impacted by mining and electroplating industries, where the selection of plant materials with stable and resilient accumulation traits remains a key bottleneck. This study investigated the differential responses of two ecotypes of L. hexandra, a historically contaminated-site ecotype and an uncontaminated cropland-site ecotype, to combined Cu(II) and Ni(II) stress in a hydroponic system. The contaminated-site ecotype exhibited significantly higher metal accumulation capacity than the cropland-site ecotype (p < 0.05). Its aboveground tissues accumulated up to 17.6-fold more Cu(II) in stems and 23.5-fold more Cu(II) in leaves, with Ni(II) showing a similar trend. Physiologically, the contaminated-site ecotype showed an organ-specific antioxidant response, characterized by relatively strong root enzymatic defense involving POD (5.6-fold higher) and CAT (14.0-fold higher), whereas leaf antioxidant enzyme activities remained comparatively low. These patterns suggest that the contaminated-site ecotype retained stronger apparent metal accumulation and organ-level allocation capacity, together with a more pronounced root-level antioxidant response, after historical habitat remediation. Most notably, although the accumulation capacity of the contaminated-site ecotype appeared lower than the historical maximum reported for the same habitat, it still showed stronger Cu(II)/Ni(II) accumulation and organ-specific antioxidant responses than the cropland-site ecotype under renewed exposure. These results suggest that historically selected metal-adaptive traits in L. hexandra may persist to some extent after environmental pressure declines and may be reflected in stronger short-term responses upon renewed metal exposure. This study provides an ecotype-oriented basis for selecting locally adapted L. hexandra germplasm for phytoremediation of Cu(II)/Ni(II)-co-contaminated environments.