Key Factors Controlling Cadmium and Lead Contents in Rice Grains of Plants Grown in Soil with Different Cadmium Levels from an Area with Typical Karst Geology

Cadmium (Cd) is a naturally occurring element often associated with lead (Pb) in the Earth’s crust, particularly in karst regions, posing significant safety hazards for locally grown rice. Identifying the key factors controlling Cd and Pb content in local rice is essential under the natural soil condition, as this will provide a crucial theoretical foundation for implementing security intervention measures within the local rice-growing industry. This study collected three types of paddy field soils with varying Cd concentrations from karst areas for pot experiments. The rice varieties tested included a low-Cd-accumulating variety, a high-Cd-accumulating variety, and a locally cultivated variety. Soil physicochemical properties and plant physiological indices were monitored throughout the rice growth stages. These data were used to construct a segmented regression model of Cd and Pb levels in rice grains based on the plant’s metabolic pathways and the structure of polynomial regression equations. Stepwise regression identified the key factors controlling Cd and Pb accumulation in rice grains. In conclusion, the key factors controlling Cd and Pb levels in rice grains should be classified into two categories: (i) factors influencing accumulation in roots and (ii) factors regulating transport from roots to grains. The aboveground translocation abilities for Cd, Pb, zinc (Zn), iron (Fe), manganese (Mn), calcium (Ca), and magnesium (Mg) in soil among the three rice varieties showed no significant interspecific differences under identical soil conditions. Soil Mg uptake by rice roots may represent a key mechanism for inhibiting soil Cd uptake by rice roots. In karst areas with high background soil Cd, increased soil organic matter (SOM) levels enhance Pb bioavailability. Additionally, the rice YXY may possess a potential for low Cd accumulation.