Elevated carbon dioxide concentrations increase the risk of Cd exposure in rice

Since the Industrial Revolution, crops have been exposed to various changes in the environment, including elevated atmospheric carbon dioxide (CO₂) concentration and cadmium (Cd) pollution in soil. However, information about how combined changes affect crop is limited. Here, we have investigated the changes of japonica and indica rice subspecies seedlings under elevated CO₂ level (1200 ppm) and Cd exposure (5 μM Cd) conditions compared with ambient CO₂ level (400 ppm) and without Cd exposure in CO₂ growth chambers with hydroponic experiment. The results showed that elevated CO₂ levels significantly promoted seedling growth and rescued the growth inhibition under Cd stress. However, the elevated CO₂ levels led to a significant increase in the shoot Cd accumulation of the two rice subspecies. Especially, the increase of shoot Cd accumulation in indica rice was more than 50% compared with control. Further investigation revealed that the decreases in the photosynthetic pigments and photosynthetic rates caused by Cd were attenuated by the elevated CO₂ levels. In addition, elevated CO₂ levels increased the non-enzymatic antioxidants and significantly enhanced the ascorbate peroxidase (APX) and glutathione reductase (GR) activities, alleviating the lipid peroxidation and reactive oxygen species (ROS) accumulation induced by Cd. Overall, the research revealed how rice responded to the elevated CO₂ levels and Cd exposure, which can help modify agricultural practices to ensure food security and food safety in a future high-CO₂ world.