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.

Ilektra Sperdouli,Anelia Dobrikova,Michael Moustakas,Ioannis-Dimosthenis Adamakis,Emilia Apostolova,Anetta Hanć

doi:10.3390/toxics10010036

Abstract

Salvia sclarea L. is a Cd2+ tolerant medicinal herb with antifungal and antimicrobial properties cultivated for its pharmacological properties. However, accumulation of high Cd2+ content in its tissues increases the adverse health effects of Cd2+ in humans. Therefore, there is a serious demand to lower human Cd2+ intake. The purpose of our study was to evaluate the mitigative role of excess Zn2+ supply to Cd2+ uptake/translocation and toxicity in clary sage. Salvia plants were treated with excess Cd2+ (100 μM CdSO4) alone, and in combination with Zn2+ (900 μM ZnSO4), in modified Hoagland nutrient solution. The results demonstrate that S. sclarea plants exposed to Cd2+ toxicity accumulated a significant amount of Cd2+ in their tissues, with higher concentrations in roots than in leaves. Cadmium exposure enhanced total Zn2+ uptake but also decreased its translocation to leaves. The accumulated Cd2+ 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 Cd2+ uptake and translocation to leaves, while it mitigated oxidative stress, restoring chloroplast ultrastructure. Excess Zn2+ ameliorated the adverse effects of Cd2+ on PSII photochemistry, increasing the fraction of energy used for photochemistry (ΦPSII) and restoring PSII redox state and maximum PSII efficiency (Fv/Fm), while decreasing excess excitation energy at PSII (EXC). We conclude that excess Zn2+ application eliminated the adverse effects of Cd2+ toxicity, reducing Cd2+ uptake and translocation and restoring chloroplast ultrastructure and PSII photochemical efficiency. Thus, excess Zn2+ application can be used as an important method for low Cd2+-accumulating crops, limiting Cd2+ entry into the food chain.

Highlights

Increased industrial and agricultural human activities, such as mining and smelting, electroplating, wastewater irrigation, and chemical fertilizers, have resulted in high environmental content of cadmium (Cd2+) [1,2,3]
We evaluated the light energy distribution pattern in photosystem II (PSII) for photochemistry (ΦPSII) for regulated non-photochemical energy loss—that is, for photoprotective heat dissipation (ΦNPQ)—and for non-regulated energy loss in PSII (ΦNO)
The increased H2O2 generation in Cd2+ + Zn2+-treated plants (Figure 5b) seems to have triggered a defense response that lowered Cd2+ toxicity effects on PSII photochemistry by increasing the fraction of energy used for photochemistry (ΦPSII) (Figure 2), restoring the PSII redox state (Figure 3b) and the maximum PSII efficiency (Fv/Fm) (Figure 3a), suggesting beneficial effects of reactive oxygen species (ROS) production [59]

Summary

Introduction

Increased industrial and agricultural human activities, such as mining and smelting, electroplating, wastewater irrigation, and chemical fertilizers, have resulted in high environmental content of cadmium (Cd2+) [1,2,3]. Zinc is involved in various metabolic processes, playing catalytic, regulatory, and structural roles with several crucial functions in the cell [9,10,11,12,13,14,15,16]. It performs a fundamental role in anti-oxidative defense and retains the membranous structure of various cell organelles [14,17]. The zinc homeostasis mechanism is not global within plants since most plants contain between 30 and 100 μg Zn2+ g−1 dry weight (DW), but some other species are accumulating more than 10,000 μg Zn2+ g−1 DW without showing symptoms of toxicity [18], despite the fact that concentrations above 300 μg Zn2+ g−1 DW are considered toxic to plants [1,9]

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