Responses of wheat and rice grain mineral quality to elevated carbon dioxide and canopy warming

The impact of climate change on grain quality is a critical issue for human health. However, how grain chemical element composition responds to climate change stresses has been poorly addressed. An open-air field experiment was conducted to investigate the impacts of factorial combinations of [CO2] (elevated [CO2] (500 ppm) vs ambient [CO2] (410 ppm)) and canopy air warming ( + 2 degrees C) on grain chemical element concentrations for two consecutive years in a rice-wheat rotation system. Overall, elevated [CO2] decreased the concentrations of most chemical elements by 4.2-20.1% in grain, but remarkably increased grain Mn, Mo, Cr, Ni, Cd and Pb concentrations by 9.2-72.9% for both crops. In contrast, warming consistently increased chemical element concentrations by 4.0-81.6% for both crops. Under concurrent elevated [CO2] and warming, macro-elements concentrations were unaffected throughout two consecutive years. Clearly, the positive effect of canopy warming moderately mitigates the negative impact of elevated [CO2] on grain chemical elements. However, greater increases in Mn, Mo, Cr, Ni, Cd and Pb concentrations were observed in concurrent elevated [CO2] and warming treatment compared to elevated [CO2] or warming alone. This indicates that elevated [CO2] and canopy warming act synergistically on heavy metal concentrations, which aggravates the threat of heavy metal toxicity in grain. Therefore, improving grain quality should be endeavored to address the adverse effect from simultaneously elevated [CO2] and warming.