## 2 Materials and Methods  
### Plant Materials  
Our study utilized the mapping population data from _Populus euphratica_ that have been previously published [34]. The population is a full-sib F1 family, resulting from the artificial crossbreeding of two dioecious tree specimens. The maternal parent _Populus euphratica_, Pe-1, and the paternal parent, 0046, originated from the banks of the Tarim River in Korla, Xinjiang, 31 km from each other. In 2014, the male flower branches of paternal parent 0046 were hydroponically cultivated in an artificial climate chamber at Beijing Forestry University (BFU). When the pollen matured, it was collected in EP tubes and refrigerated at \(-20\)\({}^{\circ}\)C. The maternal parent Pe-1, _Populus euphratica_, was transplanted to the greenhouse of BFU. The pollen of paternal parent 0046 was hand-pollinated with the pollen of female flowers at the time of maturity, and mature seeds were successfully harvested in mid-June. Collected seeds were cultured under aseptic conditions in glass tubes, which measured 40 mm in diameter and 400 mm in height. The tubes were filled with a 1/2 Murashige and Skoog medium supplemented with 0.4 mg/L of IBA, 25 g/L of sucrose, and 8 g/L of agar, adjusted to pH 6.0. After being cultivated in tubes for 4 months, 408 seedlings were successfully obtained and transferred to a greenhouse seedbed at BFU for further growth. Following a series of experiments involving histoculture asexualization, expansion, and preservation, a final count of 156 clonal lines demonstrated the plants' ability to effectively root in the growth medium and mature into fully developed specimens. To identify the optimal concentration of salt stress, a preliminary experiment was conducted by randomly selecting 20 clonal lines from a pool of 156 _Populus euphratica_ clones. The experiment established four gradients of NaCl concentrations: 0.1%, 0.3%, 0.5%, and 0.7%, corresponding to 17.1116 mM, 51.3347 mM, 85.5578 mM, and 119.781 mM, respectively. These concentrations were tested on the 20 randomly selected clonal lines (with 5 replicates each) through a tube germination culture, aiming to determine the suitable concentration for the trials by observing the growth and development phenotypes of both the aerial parts and the root system. Apical buds, approximately 10 mm in length and containing 4-6 leaves, were inoculated into tubes for cultivation. After 45 days of culture, root formation and survival rates were assessed. The root formation rates under the four NaCl concentration gradients--0.1%, 0.3%, 0.5%, and 0.7%--were found to be 100%, 72%, 5%, and 2%, respectively. Comparing these rates with those of 20 clones under control conditions revealed that a 0.1% NaCl concentration exerted stress on the root growth of _Populus euphratica_, shortening the taproot length significantly. Consequently, 0.1% NaCl (17.1116 mM) was determined as the necessary concentration for inducing salt stress, ensuring that _Populus euphratica_ saplings could root and grow under such conditions. Moreover, it was observed that most of the tissue-cultured apical buds began to root by the 13 th day post-inoculation, which also established the starting point for collecting root trait data. In vitro inoculation experiments were conducted on 156 clonal lines. Uniformly grown single plants from each clonal line were selected, and apical buds showing consistent growth (approximately 10 mm in length, four to six leaflets) were inoculated into cylindrical flat-bottomed glass tubes measuring 300 mm in length and 45 mm in diameter. Each tube was loaded with 260 milliliters of a growth substrate, and salt-treated tubes contained an additional 0.1% NaCl. Tubes were grouped in sets of 24, and the racks were designed in a 3 by 8 grid pattern. All tube-grown saplings were cultivated in the tissue culture room under uniform conditions with a temperature of 26 \({}^{\circ}\)C, light intensity of 1500 lx, and a photoperiod of 16 h of light followed by 8 h of darkness. Data collection commenced on the 13 th day post-inoculation.