Abstract: Nitrogen nutrient influences crop growth and grain yield through its direct effects on chlorophyll, rubisco, and photosynthetic structure. Many researches have focused on photosynthetic rate and dark reaction in winter wheat (Triticum aestivum L.) and explained in details the photosynthetic physiology of the flag leaf and canopy in late growing stages. In the present field experiment, a large-spike wheat cultivar, Lankaoaizao 8, was used for photoreaction study, which was seldom mentioned in former reports. Four nitrogen application rates with 0 (N1), 90 (N2), 180 (N3), and 360 kg N ha^-1 (N4) were respectively arranged in a randomized block design with three replicates. In each plot (3 m´5 m), half of the nitrogen fertilizer was applied before sowing, and the other half was topdressed at jointing stage. Additional P₂O₅ (150 kg ha^-1) and K₂O (150 kg ha^-1) were supplied as basal fertilizer. Since the 5th day after anthesis, flag leaves on ten stems from each plot were sampled every five days to determinate contents both of Chl and Car. Six fluorescence dynamic parameters of the flag leaf, fluorescence maximum (F_m), fluorescence origin (F_o), PSⅡ potential activity (F_v/F_o), the maximal quantum efficiency of PSⅡ photochemistry (F_v/F_m), photochemical quenching coefficient (qp), and non-photochemical quenching coefficient (qN), were measured at booting, flowering, early filling, and harvest stages, respectively. Content of Chl (a+b) decreased gradually with the process of grain filling in N1 treatment (control), but increased (P<0.01) at 5–15 days after anthesis (DAA), then decreased (P<0.01) in all three nitrogen treatments, and reached the peak in N3 treatment. Content of Car in the control decreased sharply at 20–30 DAA, and obtained the maximum value in N3 treatment. These results indicated that nitrogen application could avail photosynthetic pigments and delay leaf senescence. F_v/F_o, F_v/F_m, and qP in both control and nitrogen treatments reached their peaks at flowering stage, but most of them in nitrogen treatments were significantly (P<0.05) higher than those in control at the same growing stage. The qN showed a ascending trend from booting to harvest stages with its peak appearing at harvest stage in all the treatments. Compared with the control (N1), the N3 and N4 treatments showed the biggest effects on F_v/F_o and F_v/F_m, qP, and qN, respectively. It is suggested that proper nitrogen application rate (180 kg ha^-1) may promote grain yield by enhancing photosynthetic pigment contents, PSⅡ potential activity, and the maximal quantum efficiency of PSⅡ photochemistry, as well as reducing non-photochemical quenching coefficient.

Key words: Winter wheat, Photosynthetic characteristics, Nitrogen effect, Yield