To investigate the salt tolerance of Populus talassica × Populus euphratica, morphological and physiological parameters were measured on the second day after the 15th, 30th and 45th days of NaCl treatment, revealing significant effects of NaCl on growth. To further elucidate the mechanisms underlying salt tolerance, transcriptomic and metabolomic analysis were conducted under different NaCl treatments. The results of morphological and physiological indexes showed that under low salt treatment, P. talassica × P. euphratica was able to coordinate the growth of aboveground and belowground parts. Under high salt concentration, the growth and water balance of P. talassica × P. euphratica were markedly inhibited. The most significant differences between treatments were observed on the second day after the 45th day of NaCl treatment. Transcriptomic analysis showed that the pathways of gene enrichment in the roots and stems of P. talassica × P. euphratica were different in the salt resistance response. And it involves several core pathways such as plant hormone signal transduction, phenylpropanoid biosynthesis, MAPK signaling pathway-plant, plant- pathogen interaction, carbon metabolism, biosynthesis of amino acids, and several key Transcription factors (TFs) such as AP2/ERF, NAC, WRKY and bZIP. Metabolomic analysis revealed that KEGG pathway enrichment analysis showed unique metabolic pathways were enriched in P. talassica × P. euphratica under both 200 mM and 400 mM NaCl treatments. Additionally, while there were some differences in the metabolic pathways enriched in the roots and stems, both tissues commonly enriched pathways related to the biosynthesis of secondary metabolites, biosynthesis of cofactors, biosynthesis of amino acids, flavonoid biosynthesis, and ABC transporters. Association analysis further indicated that biosynthesis of amino acids and plant hormone signal transduction pathway play key roles in the response of P. talassica × P. euphratica to salt stress. The interactions between the differentially expressed genes (DEGs) and several differentially accumulated metabolites (DAMs), especially the strong association between LOC105124002 and Jasmonoyl-L-Isoleucine (pme2074), were again revealed by the interactions analysis. In this study, we resolved the changes of metabolic pathways in roots and stems of P. talassica × P. euphratica under different NaCl treatments and explored the associations between characteristic DEGs and DAMs, which provided insights into the mechanisms of P. talassica × P. euphratica in response to salt stress.

In order to elucidate the salt tolerance mechanism of Populus talassica × Populus euphratica, the growth, physiology and anatomical characteristics of P. talassica × P. euphratica were studied under different concentrations of NaCl-stress treatments. In this study, the annual seedlings of Populus talassica × Populus euphratica were used as the test material in a field potted control experiment. The basic salt content of the culture soil was the control (CK), and two NaCl treatments of 200 mmol/L and 400 mmol/L were established. The pot experiment showed that: (1) Compared with CK, the 200 mmol/L NaCl-stress treatment significantly increased the growth parameters of P. talassica × P. euphratica, such as leaf area, plant height, ground diameter, biomass, root length, root surface area, root fork number and root-shoot ratio. However, compared with CK, the 400 mmol/L NaCl-stress treatment significantly reduced most growth parameters. (2) The 200 and 400 mmol/L NaCl-stress treatments significantly decreased various physiological parameters such as relative water content (RWC), chlorophyll content, water potential, stomatal opening and photosynthetic parameters and increased the accumulation of MDA and Pro compared with CK. The 200 mmol/L NaCl-stress treatment significantly increased the activity of antioxidant enzymes, and the 400 mmol/L NaCl-stress treatment significantly decreased the activity of antioxidant enzymes. (3) Compared with CK, 200 and 400 mmol/L NaCl-stress treatments significantly improved the leaf palisade tissue thickness and palisade-to-sea ratio, as well as the stem xylem and stem phloem thickness and pith diameter, and significantly increased the root xylem thickness, root phloem thickness, and root cross-cutting diameter of P. talassica × P. euphratica. The growth, physiological characteristics and anatomical characteristics of P. talassica × P. euphratica under NaCl-stress treatments showed that it had good salt tolerance and adaptability, and the 200 mmol/L NaCl-stress treatment promoted the growth of P. talassica × P. euphratica to a certain extent. This study provided a theoretical basis for the study of the salt-tolerant mechanism of P. talassica × P. euphratica.

The exogenous plant growth regulators (PGRs) represent a useful strategy for reducing the adverse effects of salt stress in plants. In order to investigate the regulatory effect of exogenous PGRs on the salt tolerance of Populus talassica × Populus euphratica seedlings, in this study, the effects of different foliar spray concentrations of ABA (5 mg·L⁻¹, 15 mg·L⁻¹, 25 mg·L⁻¹), PP₃₃₃ (300 mg·L⁻¹, 900 mg·L⁻¹, 1500 mg·L⁻¹), and SA (40 mg·L⁻¹, 120 mg·L⁻¹, 200 mg·L⁻¹) on P. talassica × P. euphratica seedlings under salt stress (soil salt concentration of 2%) were determined. The results showed that the dry weight, total root length, root surface area, root volume, total Chl content, and photosynthetic parameters of P. talassica × P. euphratica seedlings significantly decreased under salt stress and increased their contents of malondialdehyde (MDA), hydrogen peroxide (H₂O₂), superoxide anion (O₂⁻), free proline (Pro), superoxide dismutase (SOD), and peroxidase (POD). However, ABA, PP₃₃₃, and SA can mitigate the adverse effects of salt stress on these indicators. Especially, the 15 mg·L⁻¹ ABA, 900 mg·L⁻¹ PP₃₃₃, and 120 mg·L⁻¹ SA treatments had the best effect on alleviating salt stress, with significant increases in dry weight, root parameters, total Chl content, and the photosynthetic parameters of the P. talassica × P. euphratica seedlings, improving their photosynthetic characteristics; meanwhile, increased Pro content and enzyme activity and decreased MDA, H₂O_2, and O₂⁻ content protected the integrity of membrane system and enhanced the salt tolerance of the seedlings. SA resulted in a better ameliorative effect on salt stress compared to ABA and PP₃₃₃.

A new Populus variety with a strong salt tolerance was obtained from cross breeding P. talassica as the female parent and P. euphratica as the male parent. In order to elucidate the molecular mechanism and find out the major differentially expressed genes of salt tolerance of P. talassica × P. euphratica, after being subjected to salt stress, at 0, 200, and 400 mmol/L NaCl, the root, stem, and leaf transcriptomes (denoted as R0, S0, and L0; R200, S200, and L200; and R400, S400, and L400, respectively) of P. talassica × P. euphratica were sequenced. In total, 41,617 differentially expressed genes (DEGs) were identified in all the comparison groups with 21,603 differentially upregulated genes and 20,014 differentially downregulated genes. Gene Ontology analysis showed that DEGs were significantly enriched in biological processes that may be involved in salt stress, such as ‘cell communication’, ‘ion transport’, ‘signaling’, and signal ‘transmission’. Kyoto Encyclopedia of Genes and Genomes analysis showed that DEGs were mainly enriched in pathways of ‘plant–pathogen interaction’, ‘carbon metabolism’, and ‘plant hormone signal transmission’. The pathways and related gene information formed a basis for future research on the mechanisms of salt stress, the development of molecular markers, and the cloning of key genes in P. talassica × P. euphratica.

The compensatory growth and defensive capabilities of woody plants after damage are crucial to their large-scale promotion and economic value. Here, Populus talassica × Populus euphratica were subjected to artificial defoliation treatments that simulated leaf damage [25% (D₂₅), 50% (D₅₀), and 75% (D₇₅) leaf removal] to study the growth, anatomical, and physiological characteristics. The results showed that D₂₅ and D₅₀ treatments significantly increased the growth parameters, such as leaf length, leaf area, and specific leaf area, but did not affect the distributions of root and stem biomasses compared with the CK. However, the D₇₅ treatment significantly decreased most growth parameters. The time required for the chlorophyll content to recover increased along with the damage intensity as follows: D₂₅, high-flat-high; D₅₀, low-high-flat; and D₇₅, low-flat-high. Furthermore, leaf damage significantly reduced stomatal density, whereas the stomatal width, area, opening, and Pn significantly increased by 8.59%, 8.40%, 23.27%, and 31.22%, respectively, under the D₅₀ treatment, generating a photosynthetic compensation response. The leaf anatomical parameters increased along with damage intensity, except spongy tissue thickness, which decreased, while the stem anatomical parameters showed trends of first increasing and then decreasing, reaching maxima under the D₅₀ treatment. The enzymes showed an increasing and then decreasing trend as the damage time increased. After 1 d of treatment, CAT, POD, and PAL activities peak at D₇₅, in contrast to a peak of SOD activity at D_50. Overall, these findings indicate that it is advisable to keep the amount of leaf damage within 50%. The leaf damage can have an impact on the growth of P. talassica × P. euphratica. They adjusted their resource allocation strategy and physiological defense capacity by increasing the chlorophyll content, improving photosynthetic capacity, changing stem and leaf anatomy, and increasing defense enzyme activity levels, thereby improving their damage tolerance and adaptability.