Cadmium (Cd), a toxic heavy metal, has recently gained more attention due to various human activities, such as mining, smelting, sewage sludge, chemical fertilizers, and pesticides (Bin et al. 2013, Römkens et al. 2009). Cd is present in many foods, as it can be absorbed by grain crops, accumulate in the edible parts, and then enter the food chain (Liu et al. 2015). Rice is a staple food in China, especially in the south, and it is the primary exposure source of Cd in humans due to the ingestion of contaminated rice grown in high Cd soils (Feng et al. 2010). Long-term consumption of Cd-polluted rice may cause serious health problems, such as anemia, cancer, heart attack, proteinuria, lung disorder, emphysema, and osteoporosis (Mei et al. 2017). In recent years, the occurrence of notorious itai-itai disease in Japan was connected to the intake of Cd-contaminated rice (Järup &Åkesson 2009).
Given its Karst topography, the Guizhou province has a high Cd content in the soil related to the high local geochemical background (Chen et al. 2015). According to China National Environmental Monitoring Centre (CNEMC 1990), the background content of Cd in Guizhou is 1.244 mg/kg, the highest concentration in China. Cd is mainly sourced from the weathering and pedogenesis of carbonate rocks with high Cd and Calcium (Ca) content (Li et al. 2020). Pu et al. (2015) and Fang et al. (2007) found that most soil samples in Guizhou Province were rich in Ca2+, Mg2+, CO32−/HCO3−, and soil solutions contained an abundance of Ca2+ Mg2+ ions.
Given that calcium oxide (CaO) mainly controls soil pH, researchers found that the Ca2+ ions in soil were responsible for its increased pH (Hamid et al. 2020). Some studies have suggested that calcium ion channels may allow Cd to enter into plant cells (Liu et al. 2020, Perfus-Barbeoch et al. 2002). Ca plays an essential role in plant growth as a significant nutrient element. Meanwhile, studies have shown considerable competition between Ca and Cd at the transport sites of plant cell membranes (Qin et al. 2020). It is believed that Ca alleviates the toxicity of Cd in plants through three processes: First, Ca alleviates the accumulation of Cd in plants by regulating plant growth; Secondly, Ca increases the activity of antioxidant enzymes in plants to relieve oxidative stress caused by Cd; Thirdly, Ca enhances photosynthetic, physiological, and metabolic activities in plants to regulate the signaling pathways that depend on calcium transmission (Guo et al. 2018, Huang et al. 2017, Khan et al. 2020, Rahman et al. 2016). However, some studies have shown that calcium can also increase the absorption of Cd by Sesbania sesban and Brassica juncea (Franziska &Hans 2015, Suzuki 2005). Therefore, whether Ca2+ ions decrease Cd accumulation in rice plants requires further study.
Magnesium (Mg), a component of the chlorophyll molecule, significantly influences the synthesis of nucleic acids and proteins for photosynthetic energy (Chen et al. 2017b, Ismail et al. 2008). Mg is the fourth essential nutrient element for plant growth and coexists with Ca, potassium (K), sodium (Na), and other elements in the soil. All of these elements inhibit the uptake of Cd in plants. Mg competes with heavy metal ions during uptake by the roots and affects the accumulation and transport of Cd in the plant. Applying Mg fertilizer to Cd-polluted land inhibits the availability of Cd in the soil. Studies found that Mg-deficiency increased the absorption and accumulation of Cd in rice seedlings while adding Mg2+ ions alleviated the Cd stress of rice seedlings. Mg deficiency can influence the physiological metabolism of rice, leading to the increased expression of Cd transporter OsiRT1, Oszip1, and Oszip3 in the plant, inducing increased Cd uptake and translocation by the roots (Chou et al. 2011).
External environmental factors (such as soil pH, Eh, and organic matter) can also impact the uptake of Cd in rice and its translocation processes (Chen et al. 2017a, Honma et al. 2016, Liu et al. 2013). As a result, soil remediations often focus on reducing the availability of heavy metals in the soil by increasing its pH. Calcium magnesium phosphate can promote the development of iron plaque on the root surface, thereby decreasing the Cd concentration and increasing the free amino acid concentration in grains, thus significantly limiting the accumulation of Cd in rice root protoplasts (Cai et al. 2021, Zhao et al. 2020). However, the potential influence of Ca2+ and Mg2+ ions under the same concentration conditions, which have a more significant blocking effect on cadmium, remains unclear. We conducted a hydroponic experiment to investigate: (i) the uptake, translocation, and accumulation of Cd in rice plants; (ii) the effect of Ca2+ and Mg2+ ions on the absorption, distribution, and translocation of Cd in rice; and (iii) the effect difference between Ca and Mg on Cd in rice.