## 3 Discussion
### The Fast Growth of Pe Relies on Sufficient N Supply  
In our study, the root length and surface area were higher in Pe than in Ps with sufficient N supply, but were similar between the two poplars with low N supply (Table 1). In addition, Pe had greater biomass than Ps with a sufficient N supply. These results indicate that the fast-growing Pe was more sensitive to the N supply than the slow-growing Ps and that Pe had a greater capacity for nitrogen acquisition and assimilation. This characteristic of the fast-growing Pe may be critical for supporting its rapid metabolism and growth [43]. With rapid metabolism and growth, Pe may develop a flourishing root system that results in greater nutrient absorption to satisfy metabolic consumption. Photosynthesis is the primary limiting factor of plant productivity. N is a constituent of the photosynthetic machinery, and N-containing compounds play an essential role in CO2 fixation [44]. Plant productivity depends on the plant and the metabolic expenditure [45]. The chlorophyll content and the net photosynthetic rate in Ps were higher than in Pe at low N, suggesting that the fast-growing Pe has low resistance to N deficiency. However, the total carbohydrates and the net photosynthetic rate were higher in Pe than in Ps, indicating that Pe has a greater carbon assimilation capacity than does Ps (Figure S1). Actually, most fructose and glucose content measurements in the roots and leaves in Pe were similar to, or higher than, those in Ps at all three N levels. However, with low N supply, the root sucrose content was lower in Pe than in Ps. Sucrose is the form in which carbohydrates are transported from the source to the sink tissues and reflects the C export status [10]. Compared with the fast-growing Pe, Ps may have evolved an adaptive C transport strategy in the presence of low N. These results indicate that the fast-growing Pe was superior to the slow-growing Ps in utilizing N to stimulate growth and produce biomass. Additionally, with a sufficient N supply, the activities of enzyme involved in C metabolism (i.e., SPS, SUS, HxK, CWI, and VI activities) were also higher in Pe than in Ps, indicating acceleration of the physiological processes of C metabolism in Pe compared with Ps (Figure 3). N physiological process is critical for plant growth and may differ between species. Indeed, the NH4+, NO3-, and total N concentrations were generally higher in the roots and leaves of Pe than in Ps. Consistent with these results, the net flux of NH4+ and NO3- in the Pe roots was also higher than in the Ps roots, reflecting the ability of the fast-growing Pe to acquire NH4+ and NO3- more rapidly. However, \(\delta^{15}\)N in the Pe roots was lower than in Ps. N assimilation does not favour \({}^{15}\)N enrichment and results in the depletion of \({}^{15}\)N in the plant dry matter compared with the heavier N isotope in the soil [35; 46]. Previous studies showed that slow-growing poplars accumulate N via a decrease in the nitrogen-use efficiency, resulting in greater N storage [5; 47]. These results were consistent with lower N uptake (i.e., the net flux and concentration of NH4+ and NO3-), and lower total N concentration and assimilation (i.e., the NR, GS, GDH activity) in Ps compared with Pe. Associated with the different morphological and physiological responses to the N supply, different transcriptional regulation of the genes and post-transcriptional/translational mechanisms involved in C/N metabolism are also expected in the two poplar species [48]. In our study, the regulation of most key genes was more responsive to N addition in Pe than in Ps. Many studies have also reported differences among poplar species in gene expression responses to high temperature, drought stress and nutrient availability [3; 23; 49]. For example, N fertilization leads to the acceleration of N physiological processes via _AMTs_ and _NRTs_ in _Populus popularis_, but not in _Populus alba \(\times\) Populus glandulosa_[8].