Differential Effects of Lead and Cadmium Contamination on the Antioxidant Systems of Key Plant Species in South Caucasus Vegetation Communities

Abstract

Heavy metal contamination is an increasing threat to plant communities worldwide, yet most studies examine antioxidant stress responses in single target species. Here we report the effects of lead and cadmium contamination on the antioxidant system of ten plant species representing key elements of local vegetation in the South Caucasus (Georgia, the Caucasus), using sites with contrasting contamination levels near Tbilisi and the Bolnisi copper mining area respectively. Eleven antioxidant compounds and three associated enzyme activities were measured in fresh leaves, and data were standardised using the log response ratio (lnRR) to allow comparisons across species. Correlation of lnRR values between leaf heavy metal content and antioxidant variables was used to distinguish stress-specific (cross-species) from species-specific (idiosyncratic) responses. Under lead stress, 8 of 15 antioxidant variables showed cross-species correlations: most were negative, consistent with suppression under severe contamination, while ascorbic acid and nitrate reductase activity were positively correlated, suggesting active upregulation. Under cadmium stress, 4 of 13 variables correlated across species; phenolics and nitrate reductase activity were negatively correlated, while proline — a known osmoprotectant — showed a consistent positive response. The remaining variables responded idiosyncratically under both metals, reflecting substantial inter-species variation. Malva was the most sensitive species to lead and Rumex to cadmium, while Xanthium showed the least differential cadmium uptake between contamination levels. These findings highlight the coexistence of conserved and species-specific antioxidant strategies in plant communities exposed to heavy metals, and point to ascorbic acid, proline, and nitrate reductase activity as promising candidates for community-level biomonitoring.