## 3 Discussion  
### The Slow-Growing Ps Exhibits Enhanced Resistance to N Deficiency  
Root morphological traits are strongly affected by the soil N availability [23; 25]. Some studies have found that high N supply promotes root development and results in a greater root biomass [26]. However, conflicting results have also been reported, with the nitrogen supply having little or no effect on root growth [27]. In addition, the N supply leads to different effects on root growth; N deficiency induced longer roots, greater surface area and greater biomass in the slow-growing Ps (Table 1). Consistent with root growth, low N supply had only minor effects on the photosynthetic pigments (_A_, _gs_, _E_). Metabolites of C generated from photosynthesis are major building blocks for growth and contribute to differences in acclimation to N limitation in poplars. In Ps, under a limiting N supply, photosynthate allocation was prioritized to root growth. Indeed, in Ps, root sucrose concentrations were higher with low N supply than with medium or high N supply. Low N supply might have caused greater export of photosynthate from source to sink tissues in Ps. Consistent with this hypothesis, Ps also showed a lower foliar sucrose content with low N supply. A higher total C concentration induced SPS and HxK activity in the leaves in Ps with abundant N supply, which indicated that C metabolism is stimulated by a sufficient N supply (Figure 3D). Indeed, it has also been shown in other plants that an increase in protein biosynthesis results in a greater demand for C skeletons [28; 29]. However, the activities of HxK, CWI, and VI in the Ps roots were induced with low N supply, suggesting accelerated glycogen metabolism and respiration in the Ps roots in response to N deficiency. The sucrose in the roots is transported from the source tissue, hydrolys and phosphorylated to glucose-6-phosphate, which is the key substrate for glycolysis and respiration [30]. These results were consistent with the greater root development seen in Ps with low N supply because more energy was required to support root growth. Fractionation of nitrogen isotopes between a plant and its environment occurs during uptake and assimilation of inorganic nitrogen [20]. \(\delta^{15}\)N at natural abundance levels acts as a tracer, and significant discrimination is positive in most biological systems. For instance, _Populus \(\times\) euramericana_ and _Populus \(\times\) cathayana_ display increased \(\delta^{15}\)N under drought condition [31]. In other plants, \(\delta^{15}\)N is also increased by an N deficiency [32; 33]. Low N supply leads to higher \(\delta^{15}\)N and decreases the total N concentration, suggesting that \(\delta^{15}\)N is enriched in N-deficient poplars. Under N deficiency, plants retard their N uptake and assimilation processes and are forced to utilize \({}^{15}\)N to meet N demands; thus, \(\delta^{15}\)N is enriched in the tissues [34; 35]. The higher \(\delta^{15}\)N under low N supply compared with those under medium and high N supply is probably associated with slowing of the N metabolism, leading to less depletion of \(\delta^{15}\)N under low N availability (Figure 6B,D). These results were consistent with the decreased net NH\({}_{4}\)+ and NO\({}_{3}\)- flux, the total N concentration and the NR activity. However, low N supply induced the activities of enzymes involved in NH\({}_{4}\)+ assimilation (i.e., GS, GOGAT, and GDH) in Ps leaves. The slow-growing Ps is widespread on the Loess plateau. The soil in this area is alkaline and low in N. NH\({}_{4}\)+ can stimulate root development and Ps may have evolved a mechanism to utilize limiting NH\({}_{4}\)+ even in a nutrient deficient soil [23; 36]. These results are consistent with a flourishing root system in Ps in the presence of low N.  
The transcriptional regulation of key genes involved in C/N metabolism is crucial for herbaceous plants in response to a complex and volatile N supply [37; 38].