Increasing CO₂ differentially affects essential and non-essential amino acid concentration of rice grains grown in cadmium-contaminated soils

Environmental pollution by both ambient CO₂ and heavy metals has been steadily increasing, but we do not know how fluctuating CO₂ concentrations influence plant nutrients under high Cd pollution, especially in crops. Here, we studied the effects of elevated CO₂ and Cd accumulation on proteins and amino acids in rice under Cd stress. In this pot experiment, we analyzed the amino-acid profile of 20 rice cultivars that accumulate Cd differently; the plants were grown in Cd-containing soils under ambient conditions and elevated CO₂ levels. We found that although Cd concentrations appeared to be higher in most cultivars under elevated CO₂ than under ambient CO₂, the effect was significant only in seven cultivars. Combined exposure to Cd and elevated CO₂ strongly decreased rice protein and amino acid profiles, including essential and non-essential amino acids. Under elevated CO₂, the ratios of specific amino acids were either higher or lower than the optimal ratios provided by FAO/WHO, suggesting that CO₂ may flatten the overall amino-acid profile, leading to an excess in some amino acids and deficiencies in others when the rice is consumed. Thus, Cd-tainted rice limits the concentration of essential amino acids in rice-based diets, and the combination with elevated CO₂ further exacerbates the problem. - Highlights: • Elevated CO₂ increased Cd (cadmium) accumulation in 20 rice cultivars. • Cd decreased total amino acid and protein content in most cultivars. • Elevated CO₂ exacerbated the decrease, particularly among essential amino acids. • Elevated CO₂ flattens the amino acid profile-decreasing some and increasing others. - Combined exposure to Cd and elevated CO₂ strongly decreased both essential and non-essential amino acids in 20 rice cultivars.