Polyamine Metabolism, Photorespiration, and Excitation Energy Allocation in Photosystem II Are Potentially Regulatory Hubs in Poplar Adaptation to Soil Nitrogen Availability

Nitrogen fertilization is common for poplar trees to improve growth and productivity. The utilization of N by poplar largely depends on fertilizer application patterns; however, the underlying regulatory hubs are not fully understood. In this study, N utilization and potentially physiological regulations of two poplar clones (XQH and BC5) were assessed through two related experiments (i: five levels of N supply and ii: conventional and exponential N additions). Poplar growth (leaf area) and N utilization significantly increased under fertilized compared to unfertilized conditions, whereas photosynthetic N utilization efficiency significantly decreased under low N supplies. Growth characteristics were better in the XQH than in the BC5 clone under the same N supplies, indicating higher N utilization efficiency. Leaf absorbed light energy, and thermal dissipation fraction was significantly different for XQH clone between conventional and exponential N additions. Leaf concentrations of putrescine (Put) and acetylated Put were significantly higher in exponential than in conventional N addition. Photorespiration significantly increased in leaves of XQH clone under exponential compared to conventional N addition. Our results indicate that an interaction of the clone and N supply pattern significantly occurs in poplar growth; leaf expansion and the storage N allocations are the central hubs in the regulation of poplar N utilization.