Higher Atmospheric CO(2) Levels Favor C(3) Plants Over C(4) Plants in Utilizing Ammonium as a Nitrogen Source

Photosynthesis of wheat and maize declined when grown with NH(4)(+) as a nitrogen (N) source at ambient CO(2) concentration compared to those grown with a mixture of NO(3)(–) and NH(4)(+), or NO(3)(–) as the sole N source. Interestingly, these N nutritional physiological responses changed when the atmospheric CO(2) concentration increases. We studied the photosynthetic responses of wheat and maize growing with various N forms at three levels of growth CO(2) levels. Hydroponic experiments were carried out using a C(3) plant (wheat, Triticum aestivum L. cv. Chuanmai 58) and a C(4) plant (maize, Zea mays L. cv. Zhongdan 808) given three types of N nutrition: sole NO(3)(–) (NN), sole NH(4)(+) (AN) and a mixture of both NO(3)(–) and NH(4)(+) (Mix-N). The test plants were grown using custom-built chambers where a continuous and desired atmospheric CO(2) (C(a)) concentration could be maintained: 280 μmol mol(–1) (representing the pre-Industrial Revolution CO(2) 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(n)) 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(i)) and chloroplast (C(c)) CO(2) concentration increased with increasing atmospheric CO(2) in C(3) wheat but not in C(4) maize, leading to a higher Je(PCR)/ Je(PCO) ratio. Interestingly, the reduction of P(n) under AN was relieved in wheat through higher CO(2) levels, but that was not the case in maize. In conclusion, elevating atmospheric CO(2) concentration increased C(i) and C(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 NH(4)(+) involvement in N nutrition of crops growing under different levels of CO(2).