The differential tolerance of C3 and C4 cereals to aluminum toxicity is faded under future CO2 climate

Industrial activities have led to a gradual and global increase in soil aluminum (Al) and atmospheric CO2 concentrations. Al bioavailability strongly depends on the soil pH, which in turn is affected by atmospheric CO2 levels. In spite of the concurrent impact which Al and elevated CO2 (eCO2) could have on plants, their interaction and how it might affect the growth of economically important crop species has not been investigated. Here, we have investigated the combined impact of soil Al and eCO2 exposure on key C3 (wheat, oat) and C4 (maize, sorghum) crops, at the physiological and biochemical level. Compared to C3 plants, C4 plants accumulated less Al by stimulating soil Al retention through exudation of root organic acids. Consequently, Al-exposed C4 plants maintained photosynthetic performance and anti-oxidative capacity. Exposure to eCO2 reduced the stress responses of C3 and C4 crops to Al exposure. Elevated CO2 decreased Al accumulation and oxidative damage in all cereals, and ameliorated C3 plant growth. This was reflected on the biochemical level, where eCO2 inhibited ROS production and restored RuBisCo activity in C3 crops only. Overall, our data suggest that, compared to C3 crops, C4 cereals are more tolerant to soil Al exposure under current ambient CO2 (aCO2) levels whereas future eCO2 levels might stimulate Al tolerance in C3 crops.