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

Feng Wang; Jingwen Gao; Jean W H Yong; Qiang Wang; Junwei Ma; Xinhua He

doi:10.3389/fpls.2020.537443

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

Front Plant Sci. 2020 Dec 2:11:537443. doi: 10.3389/fpls.2020.537443. eCollection 2020.

Authors

Feng Wang^{1

2

3}, Jingwen Gao^{1

3}, Jean W H Yong^{3

4}, Qiang Wang¹, Junwei Ma¹, Xinhua He^{2

3}

Affiliations

¹ Institute of Environmental Resources, Soil and Fertilizer, Zhejiang Academy of Agricultural Sciences, Hangzhou, China.
² Centre of Excellence for Soil Biology, College of Resources and Environment, Southwest University, Chongqing, China.
³ School of Biological Sciences, The University of Western Australia, Perth, WA, Australia.
⁴ Department of Biosystems and Technology, Swedish University of Agricultural Sciences, Alnarp, Sweden.

Abstract

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 _a ) 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 _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 ) 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 _n 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 _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 NH₄ ⁺ involvement in N nutrition of crops growing under different levels of CO₂.

Keywords: NH4+ stress; Triticum aestivum; Zea mays; atmospheric CO2; ecophysiology; electron transport; photosynthesis.