Abstract: Wheat (Triticum aestivum) plants were pot-cultured in open top chambers at the nitrogen application rate of 0 and 200 mg·kg^-1 soil and the atmospheric CO₂ concentration of 400 and 760 μmol·mol^-1. Through the determination of flag leaf nitrogen and chlorophyll contents, photosynthetic rate (P_n)-intercellar CO₂ concentration (C_i) response curve, and chlorophyll fluorescence parameters at heading stage, the photosynthetic electron transport rate and others were calculated, aimed to investigate the effects of nitrogen application and elevated atmospheric CO₂ concentration on the photosynthetic energy partitioning in wheat flag leaves. Elevated atmospheric CO₂ concentration decreased the leaf nitrogen and chlorophyll contents, compared with the ambient one, and the chlorophyll a/b ratio increased at the nitrogen application rate of 200 mg·kg^-1. With the application of nitrogen, no evident variations were observed in the maximal photochemical efficiency (F_v/F_m), maximal quantum yield under irradiance (F_v′/F_m′) of PSⅡ reaction center, photochemical fluorescence quenching coefficient (q_p), and actual PSⅡ efficiency under irradiance (Φ_PSⅡ) at elevated atmospheric CO₂ concentration, and the total photosynthetic electron transport rate (J_F) of PSⅡ reaction center had no evident increase, though the non-photochemical fluorescence quenching coefficient (NPQ) decreased significantly. With no nitrogen application, the F_v′/F_m′, Φ_PSⅡ, and NPQ at elevated atmospheric CO₂ concentration decreased significantly, and the J_F had a significant decrease though the F_v/F_m and q_p did not vary remarkably. Nitrogen application increased the J_F and photochemical electron transport rate (J_C); while elevated atmospheric CO₂ concentration decreased the photorespiration electron transport rate (J₀), Rubisco oxidation rate (V₀), ratio of photorespiration to photochemical electron transport rate (J₀/J_C), and Rubisco oxidation/carboxylation rate (V₀/V_C), but increased the photochemical electron transport rate (J_C) and Rubisco carboxylation rate (V_C). It was concluded that elevated atmospheric CO₂ concentration decreased the leaf nitrogen and chlorophyll contents, while nitrogen application increased the photosynthetic electron transport rate of PSⅡ reaction center significantly, and promoted the photosynthetic electron flow towards photochemistry, making more photosynthetic electron take part in Rubisco carboxylation and leading to the significant increase of P_n.

Key words: atmospheric CO₂ concentration, nitrogen, photosynthetic electron transport rate, energy partitioning, wheat