Cadmium toxicity reduction in rice (Oryza sativa L.) through iron addition during primary reaction of photosynthesis

Cadmium (Cd) pollution is a worldwide concern due to its biotoxicity. Because Cd and Fe are closely associated during plant photosynthesis, this study aims at investigating the mechanism governing Cd toxicity during photosynthetic primary reaction in rice by adjusting Fe concentration. The results show that moderate Fe concentration (1.0 g kg⁻¹) added to soil can increase the stomatal conductance (Gs) and SPAD value by stimulating the stomatal opening and chlorophyll synthesis. Moderate Fe concentration can also improve the maximum fluorescence (Fm) and the maximal photochemical efficiency (Fv/Fm) to keep the high reaction center activity and electronic transfer efficiency in photosystems I and II. Thus, moderate Fe can eliminate Cd-induced decrease in Gs, intercellular CO₂ concentration (Ci) and net photosynthetic rate (Pn) as well as the disorder of antioxidative system under Cd concentration of 2.0 mg kg⁻¹ in the soil. When its application is increased to 2.0 g kg⁻¹, Fe can notably decrease Pn, and result in remarkable decrease in the biomass of shoots and grains. Decrease in Pn can be mainly attributed to high Fe concentration which can greatly destroy chloroplast structure and, meanwhile, inhibit the electron transfer between acceptor and donator in photosynthetic chain especially from quinone A (Q_A) to quinone B (Q_B). Unlike the situation under moderate Fe concentration, the high Fe application cannot mitigate the Cd-induced decrease in photosynthetic index. Our results indicate that the moderate Fe application is necessary to promote rice performance and production and, in the meantime, to inhibit Cd toxicity in the extensively polluted soils.