Background: Nitrogen (N) is one of the main factors limiting the wood yield in poplar cultivation. Understanding the molecular mechanism of N utilization could play a guiding role in improving the nitrogen use efficiency (NUE) of poplar. Results: In this study, three N-efficient genotypes (A1-A3) and three N-inefficient genotypes (C1-C3) of Populus deltoides were cultured under low N stress (5 μM NH₄NO₃) and normal N supply (750 μM NH₄NO₃). The dry matter mass, leaf morphology, and chlorophyll content of both genotypes decreased under N starvation. The low nitrogen adaptation coefficients of the leaves and stems biomass of group A were significantly higher than those of group C (p < 0.05). Interestingly, N starvation induced fine root growth in group A, but not in group C. Next, a detailed time-course analysis of enzyme activities and gene expression in leaves identified 2062 specifically differentially expressed genes (DEGs) in group A and 1118 in group C. Moreover, the sensitivity to N starvation of group A was weak, and DEGs related to hormone signal transduction and stimulus response played an important role in the low N response this group. Weighted gene co-expression network analysis identified genes related to membranes, catalytic activity, enzymatic activity, and response to stresses that might be critical for poplar’s adaption to N starvation and these genes participated in the negative regulation of various biological processes. Finally, ten influential hub genes and twelve transcription factors were identified in the response to N starvation. Among them, four hub genes were related to programmed cell death and the defense response, and PodelWRKY18, with high connectivity, was involved in plant signal transduction. The expression of hub genes increased gradually with the extension of low N stress time, and the expression changes in group A were more obvious than those in group C. Conclusions: Under N starvation, group A showed stronger adaptability and better NUE than group C in terms of morphology and physiology. The discovery of hub genes and transcription factors might provide new information for the analysis of the molecular mechanism of NUE and its improvement in poplar.

In the present study, using the BLUP-GGE approach, southern-type (ST) Populus deltoides genotypes were analyzed and evaluated, and variations in growth traits, seedling height (H), and ground diameter (GD) were studied in various climatic regions, which could facilitate the increase of the breeding range of ST. The test materials were 119 one-year-old ST genotypes, and the test sites were Ningyang (NY) and Hainan (HN). A linear mixed-effects model was constructed, and the BLUP values of H and GD were obtained using the linear unbiased prediction (BLUP) method. GGE-Biplots were generated. The H variation was greater than the GD variation. The effects of environment, block, and genotype–environment interaction (G×E) were highly significant. The goodness of fit of the GGE-Biplots obtained by extracting the BLUP values was >95%. According to the GGE-biplot results, the performance of each genotype varied considerably. The genotype No. 13 had the highest average GD and the highest average H. In NY, the genotypes No. 93 and 115 had the highest H and GD, and in HN, the genotype No. 9 had the highest H and GD. ST had a better second-year survival rate in NY than in HN. The hybridization of tropical Populus deltoides can be performed using the No. 13 and 117 genotypes, which grow rapidly and have high yields.