Effect of Nitrogen Rates on Wheat Photosynthesis, Anatomical Parameters and Photoassimilate Partitioning in North China Plain

Evaluating the effects of nitrogen (N) on photosynthesis characteristics and photoassimilate partitioning via vascular bundles (VB) under water-saving irrigation is crucial to maximum grain yield of environment-friendly wheat production in the North China Plain (NCP). Field experiments were conducted with four N application rates 0 (N0), 180 (N180), 240 (N240), and 300 (N300) kg N ha−1 under 70% and 65% relative soil water content (RSWC) at jointing and anthesis, respectively. Results showed that, compared with N0, N180 significantly improved the photosynthetic parameters and chlorophyll fluorescence of flag leaf after anthesis. N application improved the anatomical parameters both in the flag leaf and the stem. Total transverse area of VB in the flag leaf and in the stem internode below the ear under N180 was 17.95–23.70% and 37.91–43.90% larger than those of N0, respectively, due to the increased numbers and transverse area of VB. Furthermore, N180 had higher dry matter assimilation after anthesis (DMA) and its contribution to the grain. N180 increased grain yield by 14.23–23.24% compared with N0, and N supply exceeding 180 kg N ha−1 did not further increase yield. Moreover positively correlations were showed among photosynthesis characteristics after anthesis, total transverse area of VB in the flag leaf and in the stem internode below the ear, DMA and grain yield. In summary, the recommended N application rate was 180 kg N ha−1 under RSWC-based supplemental irrigation that can produce grain yield over 7500 kg ha−1 for at least 2 years in the NCP.