Decoding Phytotoxicity: The Predictive Power of Total Soil Copper Content in Long-Term Pepper Growth in Copper-Polluted Soils

Abstract: The predictive accuracy of total metal content in contaminated soils for determining phytotoxicity has long been debated. Attempts to determine the “phytoavailable” metal fraction have yielded inconsistent results. Further complications arise in contaminated soils containing multiple metals, making interpretation of results difficult. Therefore, our study focused on an agricultural field in El Melón, Valparaíso region, central Chile, which is predominantly contaminated with copper due to the destruction of a tailings dam by the 1965 earthquake. Our primary objective was to determine which soil copper pool, either soluble or total, controls copper phytotoxicity at this unique site. Total copper ranged from 76 to 1672 mg/kg, while soluble copper (extracted by 0.1 M KNO₃) ranged from 0.11 to 0.34 mg/kg. Using a prolonged 128-day ecotoxicity assessment with pepper (Capsicum annuum L.), our results indicate that total soil copper content emerges as a robust predictor of various plant responses. Regressions showed significant relationships for shoot copper content (R² = 0.77, P < 0.001), shoot dry weight (R² = 0.56, P = 0.02), xylem thickness (R² = 0.33, P = 0.08), and leaf thickness (R² = 0.29, P = 0.10). Conversely, the influence of soluble copper concentration on pepper responses and shoot copper content was not statistically significant (P > 0.1). Our discussion underscores that plant element uptake depends not only on the concentrations in the soil solution (intensity), but also on the total element content in the soil (quantity) and its supply kinetics (capacity). Therefore, the total metal content of the soil was found to be a more reliable predictor of plant responses than the soluble copper fraction in the soil. The anatomical changes observed in this study represent, to the best of our knowledge, the first report of metal-induced stress in Capsicum annuum. From this novel perspective, the results of our study are significant, especially for plant water relations, given their dependence on xylem and leaf thickness.