Excess Zinc Supply Reduces Cadmium Uptake and Mitigates Cadmium Toxicity Effects on Chloroplast Structure, Oxidative Stress, and Photosystem II Photochemical Efficiency in Salvia sclarea Plants

Salvia sclarea L. is a Cd(2+) tolerant medicinal herb with antifungal and antimicrobial properties cultivated for its pharmacological properties. However, accumulation of high Cd(2+) content in its tissues increases the adverse health effects of Cd(2+) in humans. Therefore, there is a serious demand to lower human Cd(2+) intake. The purpose of our study was to evaluate the mitigative role of excess Zn(2+) supply to Cd(2+) uptake/translocation and toxicity in clary sage. Salvia plants were treated with excess Cd(2+) (100 μM CdSO(4)) alone, and in combination with Zn(2+) (900 μM ZnSO(4)), in modified Hoagland nutrient solution. The results demonstrate that S. sclarea plants exposed to Cd(2+) toxicity accumulated a significant amount of Cd(2+) in their tissues, with higher concentrations in roots than in leaves. Cadmium exposure enhanced total Zn(2+) uptake but also decreased its translocation to leaves. The accumulated Cd(2+) led to a substantial decrease in photosystem II (PSII) photochemistry and disrupted the chloroplast ultrastructure, which coincided with an increased lipid peroxidation. Zinc application decreased Cd(2+) uptake and translocation to leaves, while it mitigated oxidative stress, restoring chloroplast ultrastructure. Excess Zn(2+) ameliorated the adverse effects of Cd(2+) on PSII photochemistry, increasing the fraction of energy used for photochemistry (Φ(PSII)) and restoring PSII redox state and maximum PSII efficiency (Fv/Fm), while decreasing excess excitation energy at PSII (EXC). We conclude that excess Zn(2+) application eliminated the adverse effects of Cd(2+) toxicity, reducing Cd(2+) uptake and translocation and restoring chloroplast ultrastructure and PSII photochemical efficiency. Thus, excess Zn(2+) application can be used as an important method for low Cd(2+)-accumulating crops, limiting Cd(2+) entry into the food chain.