Abstract: Grain yield is mainly contributed by photosynthate post anthesis in wheat (Triticum aestivum L.), but assimilate accumulation before flowering also plays an important role, especially under stress conditions. The effects of nitrogen fertilizer on assimilation and partitioning of carbohydrate at maturity have been studied in different experiments with different conclusions. To evaluate the dynamic changes of assimilation and carbohydrate partitioning in wheat organs under different nitrogen application rates, a poly-spike cultivar Yumai 49-198 and a large-spike cultivar Lankao Aizao 8 were used in a pot experiment in 2005–2006 growth season. The nitrogen fertilizer was applied (half basic application and half topdressing at jointing) at two levels of 18 g m^-2 and 36 g m^-2 respectively. Flag leaves were labeled by ¹⁴CO₂ at pre-anthesis stage. The results indicated that the ¹⁴C assimilates photosynthesized at pre-anthesis mainly distributed in stem and sheath, next in grain at maturity. The proportion of ¹⁴C partitioned to stem and sheath, grain was 44.31–60.96% and 31.81–40.67% respectively. In comparison, the proportion of ¹⁴C partitioned to grain was increased, while to stems and sheath was reduced by increased N application. The percentage of assimilated ¹⁴C in stems and sheaths of Lankao Aizao 8 was greater than that of Yumai 49-198. There was 34.94% of the assimilates in grains de-rived from the reallocation of assimilate stored at pre-anthesis and 65.06% from photosynthate post anthesis. Moreover, transloca-tion amount, translocation proportion and contribution of assimilates before flowering to grains decreased with higher nitrogen application rate, while the contribution of assimilates post anthesis and grain weight per spike increased. The results also showed that the percentage of post anthesis assimilation to grains was 77.84% and 56.29% for Lankao Aizao 8 and Yumai 49-198, respec-tively, indicating that assimilates photosynthesized post anthesis in Lankao Aizao 8 was greater than that in Yumai 49-198. Finally, grain yield in 36 g m^-2 N treatment was higher than that in 18 g m^-2 N treatment. Meanwhile, increase range in grain yield of Lankao Aizao 8 both in treatments of 36 g m^-2 and 18 g m^-2 was greater than that of Yumai 49-198, indicating that the effect of nitrogen fertilizer on Lankao Aizao 8 was significantly greater than that of Yumai 49-198.

Key words: Winter wheat, Nitrogen application rate, Assimilation, Accumulation, Translocation, Distribution