Effects of nitrogen application methods on photosynthesis and senescence characteristics of flag leaves in wheat under limited irrigation

Under limited irrigation, the effects of different nitrogen application methods on photosynthetic parameters and senescent characteristics of flag leaves of two winter wheat cultivars (SX733 with low drought tolerance and SM 12 with high drought tolerance ) was studied. Compared to single nitrogen application at the early stage of jointing (single fertilization, SF), split nitrogen application at the early stage of jointing and flag leaf expansion (3/4 amount at the early stage of jointing and 1/4 amount at flag leaf expansion, DF) increased the chlorophyll contents (Chl), soluble protein contents (SP), RuBPCase activities, stomatal conductance (Gs), and photosynthetic rate (Pn), prolonged the relative steady phase of Chl (RSP), photosynthetic active duration (PAD) and enhanced leaf source capacity (LSC) of flag leaves in both cultivars. The activities of superoxide dismutase (SOD) and peroxidase (POD) were higher, at mid- and late-growth stage of flag leaves in DF than in SF, while the active oxygen amount (O.2 -) and malondialdehyde (MDA) contents were lower in DF. The regulation effects of DF on photosynthetic parameters and high-value duration of photosynthetic functions of flag leaf in Shixin733, were more pronounced than in Shimai12. The 1000-grain weights, yields and water use efficiencies in DF treatment in both tested cultivars were all higher than in SF, and the yield and water use efficiency in DF was significantly higher than in SF in Shimai12. It is suggested that suitable amount of nitrogen application at flag leaf expansion could increase the activities of the cellular protection enzymes, such as superoxide dimutase and peroxidase, and sustain a higher ROS scavenging capacities, consequently improve the photosynthetic assimilation capability of flag leaves, delay the flag leaf senescence, and increase the kernel dry matter accumulation and yield under limited irrigation.