Photosynthetic Characteristics and Antioxidative Metabolism of Flag Leaves in Responses to Nitrogen

A two-factorial experiment was conducted with two wheat cultivars, SN1391 (large spike and large grain) and GC8901 (multiple spike and medium grain), and two nitrogen (N) application rates (12 and 24 g N m-2), to investigate the responses of photosynthetic characteristics and antioxidative metabolism to nitrogen rates in flag leaves of field-grown wheat during grain filling. The results showed that the content of N and chlorophyll (Chl) in wheat flag leaves decreased after anthesis and the net photosynthetic rate (Pn), effective quantum yield of PS II (ΦPS II), efficiency of excitation capture by open PS II reaction centers (Fv′/Fm′), and photochemical quenching coefficient (qp) began to decrease at 14 days after anthesis. However, the maximal efficiency of PS II photochemistry (Fv/Fm) decreased slightly until the late period of senescence and the nonphotochemical quenching coefficient (NPQ) increased during flag leaves’ senescence. As a result, a conflict came into being between absorption and utilization to light energy in flag leaves during senescence, which might accelerate the senescence of flag leaves. Compared with GC8901, the lower plant population of SN1391 during grain filling was helpful to maintain the higher content of photosynthetic pigment, activity of PS II, and Pn in flag leaves during senescence. The delayed decrease in antioxidative enzyme activity and the lower degree of membrane lipid peroxidation in the senescing leaves of SN1391 were beneficial to protect the photosynthetic apparatus, which lead to the prolonged duration of CO2 assimilation. With the increase of N application, the Chl content of SN1391 flag leaves and the efficiency of excitation captured by open PS II centers increased. At the same time, the thermal dissipation in SN1391 flag leaves at high N (HN) treatment decreased and ΦPS II improved greatly, which were favorable to the increase of Pn. The SOD, POD, CAT and APX activities in the flag leaves of SN1391 increased markedly at