Abstract: Abstract: 【Objective】Under field conditions, this paper studied the photosynthetic characteristics and antioxidative metabolism during flag leaf senescence and nitrogen effects of winter wheat. 【Method】Two winter wheat cultivars, SN1391 (a large-spike cultivar) and GC8901 (a multiple-spike cultivar), were studied under two nitrogenous fertilizer rate conditions, which were 12 gN•m-2 (LN) and 24 gN•m-2 (HN). 【Result】The results showed that the nitrogen (N) content, chlorophyll content, photosynthetic rate and activities of PSⅡ in flag leaves decreased significantly with senescence processing, but the maximal efficiency of PSⅡ photochemistry (Fv/Fm) decreased slightly at the final stage of senescence. So there was a conflict between absorption and utilization of light energy in senescent flag leaves, which might induce the senescence of wheat flag leaves. In senescent wheat leaves, there were several ways to dissipate excess excitation energy and clean active oxygen, such as increasing thermal dissipation, maintaining a reasonable carotinoid content and working in phase in the system of scavenging active oxygen. With the increase of N application, the chlorophyll content of SN1391 flag leaves and the efficiency of excitation captured by open PSII centers increased. At the same time, the thermal dissipation in SN1391 flag leaves of high N (HN) treatment decreased, but the effective quantum yield of PSⅡ (ΦPSⅡ) improved greatly, which made for the increase of photosynthetic rate (Pn). The SOD, POD, CAT and APX activities in the flag leaves of SN1391 increased markedly in HN treatment, so that it could clean more active oxygen and decreased the degree of membrane lipid peroxidation. In this way, the ability of SN1391 to protect photosynthetic apparatus improved in HN treatment. In the HN treatment, the activities of PSⅡ in flag leaves of GC8901 increased greatly and the thermal dissipation increased, which induced Pn. At the same time, antioxidative enzymes activities decreased and the degree of membrane lipid peroxidation increased, which influenced CO2 assimilation indirectly. Compared with GC8901, the percentages of fertile tillers in SN1391 was lower, so that its plant population was smaller. In senescent leaves of SN1391, the content of photosynthetic pigment was higher, activity of PSⅡ decreased slowly and Pn was higher than those in GC8901. The antioxidative enzymes activities decreased slightly and the degree of membrane lipid peroxidation was low in the senescent flag leaf of SN1391, which protected the photosynthetic apparatus effectively and prolonged the time of CO2 assimilation. 【Conclution】The conflict between absorption and utilization of light energy in senescent flag leaves might induce the senescence of wheat flag leaves. Effects of nitrogen rate on the photosynthetic and senescent characteristics of flag leaves varied with wheat type.

Key words: Winter wheat（Triticum aestivum L.）, Senescence, Photosynthetic characteristics, Antioxidative metabolism, Nitrogen