Higher Atmospheric CO₂ Levels Favor C₃ Plants Over C₄ Plants in Utilizing Ammonium as a Nitrogen Source

Photosynthesis of wheat and maize declined when grown with NH₄ ⁺ as a nitrogen (N) source at ambient CO₂ concentration compared to those grown with a mixture of NO₃ ^- and NH₄ ⁺, or NO₃ ^- as the sole N source. Interestingly, these N nutritional physiological responses changed when the atmospheric CO₂ concentration increases. We studied the photosynthetic responses of wheat and maize growing with various N forms at three levels of growth CO₂ levels. Hydroponic experiments were carried out using a C₃ plant (wheat, Triticum aestivum L. cv. Chuanmai 58) and a C₄ plant (maize, Zea mays L. cv. Zhongdan 808) given three types of N nutrition: sole NO₃ ^- (NN), sole NH₄ ⁺ (AN) and a mixture of both NO₃ ^- and NH₄ ⁺ (Mix-N). The test plants were grown using custom-built chambers where a continuous and desired atmospheric CO₂ (C ) concentration could be maintained: 280 μmol mol^-1 (representing the pre-Industrial Revolution CO₂ concentration of the 18th century), 400 μmol mol^-1 (present level) and 550 μmol mol^-1 (representing the anticipated futuristic concentration in 2050). Under AN, the decrease in net photosynthetic rate (P ) was attributed to a reduction in the maximum RuBP-regeneration rate, which then caused reductions in the maximum Rubisco-carboxylation rates for both species. Decreases in electron transport rate, reduction of electron flux to the photosynthetic carbon [Je(PCR)] and electron flux for photorespiratory carbon oxidation [Je(PCO)] were also observed under AN for both species. However, the intercellular (C ) and chloroplast (C ) CO₂ concentration increased with increasing atmospheric CO₂ in C₃ wheat but not in C₄ maize, leading to a higher Je(PCR)/ Je(PCO) ratio. Interestingly, the reduction of P under AN was relieved in wheat through higher CO₂ levels, but that was not the case in maize. In conclusion, elevating atmospheric CO₂ concentration increased C and C in wheat, but not in maize, with enhanced electron fluxes towards photosynthesis, rather than photorespiration, thereby relieving the inhibition of photosynthesis under AN. Our results contributed to a better understanding of NH₄ ⁺ involvement in N nutrition of crops growing under different levels of CO₂.