Reference-quality genomes of both sexes are essential for studying sex determination and sex-chromosome evolution, as their gene contents and expression profiles differ. Here, we present independent chromosome-level genome assemblies for the female (XX) and male (XY) genomes of desert poplar (Populus euphratica), resolving a 22.7-Mb X and 24.8-Mb Y chromosome. We also identified a relatively complete 761-kb sex-linked region (SLR) in the peritelomeric region on chromosome 14 (Y). Within the SLR, recombination around the partial repeats for the feminizing factor ARR17 (ARABIDOPSIS RESPONSE REGULATOR 17) was potentially suppressed by flanking palindromic arms and the dense accumulation of retrotransposons. The inverted small segments S1 and S2 of ARR17 exhibited relaxed selective pressure and triggered sex determination by generating 24-nt small interfering RNAs that induce male-specific hyper-methylation at the promoter of the autosomal targeted ARR17. We also detected two male-specific fusion genes encoding proteins with NB-ARC domains at the breakpoint region of an inversion in the SLR that may be responsible for the observed sexual dimorphism in immune responses. Our results show that the SLR appears to follow proposed evolutionary dynamics for sex chromosomes and advance our understanding of sex determination and the evolution of sex chromosomes in Populus.

Euphrates poplar (Populus euphratica Oliv.) has heteromorphic leaves including strip, lanceolate, ovate, and broad‐ovate leaves from base to top in the mature canopy. To clarify how diameter at breast height (DBH) and tree height affect the functional characteristics of all kinds of heteromorphic leaves, we measured the morphological anatomical structure and physiological indices of five crown heteromorphic leaves of P. euphratica at 2, 4, 6, 8, 10, and 12 m from the same site. We also analysed the relationships between morphological structures and physiological characteristics of heteromorphic leaves and DBH and the height of heteromorphic leaves. The results showed that the number of abnormalities regarding blade width, leaf area, leaf thickness, leaf mass per area, cuticle layer thickness, palisade tissue thickness, and palisade tissue/sponge tissue ratio increased with size order and sampling height gradient. Net photosynthetic rate, transpiration rate, stomatal conductance, instantaneous water use efficiency, stable delta carbon isotope ratio, proline and malondialdehyde (MDA) content increased with DBH and sampling height. By contrast, blade length, leaf shape index, and intercellular CO2 concentration decreased with the increase in path order and sampling height gradient. Although MDA content and leaf sponge thickness were not correlated with DBH or sampling height, other morphological structure and physiological parameters were significantly correlated with these variables. In addition, correlations were found among leaf morphology, anatomical structure, and physiological index parameters indicating that they changed with path order and tree height gradient. The differences in the morphology, anatomic structure and physiological characteristics of the heteromorphic leaves ofP. euphratica are related to ontogenesis stage and coronal position.

Populus euphratica Oliv., a pioneer species of desert riparian forest, is characterized heterophylly. To understand the adaptation strategies of the heteromorphic leaves of P. euphratica to soil drought, we assessed the structural and functional characteristics of the heteromorphic leaves at different heights in suitable soil moisture conditions (groundwater depth 1.5 m) and drought conditions (groundwater depth 5 m), which include morphology, anatomical structure, photosynthetic capacity, water use efficiency, osmotic adjustment capacity, and endogenous hormones. These results indicate that leaf area, leaf thickness, fence tissue, palisade-to-sea ratio, main vein xylem area, vessel area, net photosynthetic rate, transpiration rate, and proline, MDA, IAA, GA₃, and ZR contents showed a positive correlation with the tree height under the two soil moisture conditions, but leaf shape index, leaf water potential (LWP), and ABA content showed a decreasing trend. In addition, the main vein vascular bundle area, main vein xylem area, and contents of malondialdehyde, ABA, GA₃, and IAA were significantly greater under soil drought conditions than normal soil water content. Under soil drought stress, the heteromorphic leaves of P. euphratica showed more investment in anatomical structure and greater water use efficiency, proline, and hormone contents, and synergistic changes to maintain high photosynthetic efficiency. This is an adaptation strategy to water stress caused by soil drought and tree height changes.

The ability of plants to alter specific combinations of leaf traits during development and in response to abiotic stress is crucial for their success and survival. While there are numerous studies on the variation of leaf traits within the canopies of Populus species, the application of network analysis to understand the variation and combinations of these traits across different growth stages is rare. The leaves of Populus euphratica, a dominant species in arid regions, exhibit notable morphological variations at different developmental stages and canopy heights in response to water scarcity and climate change. In this study, 34 leaf traits (morphological, chemical, photosynthetic, and hydraulic) and their roles in drought adaptation were investigated in 60 Populus euphratica plants at five developmental stages and five canopy heights using leaf trait network (LTN) analysis. The aim was to analyze adaptive strategies to arid environments at different developmental stages and canopy heights through the interdependence of leaf traits. The results showed that the internal coordination capacity of leaf trait networks decreased and then increased with each developmental stage, while the functional modules of leaf trait networks were loosely connected and aggregated with the increase in tree diameter at breast height. With increasing canopy height, the coordination linkage’s ability between leaf traits showed an increasing then decreasing trend, and the traits of the leaves in the canopy at 6 m were more closely connected, less modular, and simpler in topology compared with those in the other layers. Leaves form functional modules by coordinating specific traits that promote growth and resist drought. Leaf photosynthesis, water transport, and nutrient traits were central to different developmental stages, whereas leaf morphology, nutrient metabolism, and drought-resistance-related traits were central to the canopy height. Leaf morphology and osmoregulatory traits play key roles in leaf trait network regulation, including leaf length and width, leaf shape index, soluble sugars, and soluble proteins, which are important “intermediary traits” in the Populus euphratica leaf network. Further analysis revealed that structural traits were important at different developmental stages and canopy heights. When resources are limited, the leaf preferentially maintains a stable connection between structural traits to enhance photosynthesis, and these traits and their combinations might confer drought resistance. During the rapid development stage, the connection between chemical traits becomes important, and the leaf grows by rapidly accumulating nutrients. In summary, this study provides new perspectives and insights into the drought adaptation strategies of P. euphratica at different developmental stages and canopy heights by analyzing leaf trait networks.

Populus pruinosa Schrenk has the biological characteristics of heteromorphic leaves and is a pioneer species for wind prevention and sand fixation. The functions of heteromorphic leaves at different developmental stages and canopy heights of P. pruinosa are unclear. To clarify how developmental stages and canopy height affect the functional characteristics of leaves, this study evaluated the morphological anatomical structures and the physiological indicators of leaves at 2, 4, 6, 8, 10, and 12 m. The relationships of functional traits to the developmental stages and canopy heights of leaves were also analyzed. The results showed that blade length (BL), blade width (BW), leaf area (LA), leaf dry weight (LDW), leaf thickness (LT), palisade tissue thickness (PT), net photosynthetic rate (Pn), stomatal conductance (Gs), proline (Pro), and malondialdehyde (MDA) content increased with progressing developmental stages. BL, BW, LA, leaf dry weight, LT, PT, Pn, Gs, Pro, and the contents of MDA, indoleacetic acid, and zeatin riboside had significant positive correlations with canopy heights of leaves and their developmental stages. The morphological structures and physiological characteristics of P. pruinosa leaves showed more evident xeric structural characteristics and higher photosynthetic capacity with increasing canopy height and progressive developmental stages. Resource utilization efficiency and the defense ability against environmental stresses were improved through mutual regulation of each functional trait.

Populus euphratica is a key community-building species in the desert riparian forests of Northwest China, exhibiting exceptional resistance to stress and playing a vital role in soil and water conservation as well as maintaining ecological balance in arid regions. To investigate the ecological processes underlying the composition of P. euphratica communities and to identify their community construction mechanisms, this study analyses the species diversity and phylogenetic diversity of 58 P. euphratica communities, exploring their assembly processes and key influencing factors. This research aims to elucidate the relationship between community structure from the perspective of species evolution and analyse the construction mechanisms of P. euphratica communities across different clusters in arid environments. The results show that the species diversity of P. euphratica clusters in Northwest China is relatively low, and a significant correlation is noted with phylogenetic diversity (PD). The Shannon–Wiener and Margalef indices exhibit similar trends, whereas Simpson’s index show the opposite trends. Pielou’s index range from 0.7 to 0.85. Notably, the PD and species diversity of the P. euphratica–Haloxylon ammodendron association group (Group 4) is significantly higher (p < 0.05) compared to that of the other groups. Additionally, net relatedness index (NRI) and nearest taxon index (NTI) peaked in the P. euphratica–H. ammodendron association group (Group 4) and the Populus pruinosa–Tamarix ramosissima–Phragmites australis association group (Group 1) (p < 0.05). A Pearson correlation analysis indicated that PD was significantly positively correlated with Margalef’s index, Shannon–Wiener’s index, and Pielou’s index, but was significantly negatively correlated with Simpson’s index, while also being associated with environmental factors. Key factors influencing the diversity of P. euphratica communities in Northwest China include total phosphorus, pH, soil moisture content, total potassium, the mean temperature of the coldest quarter, precipitation of the wettest month, and precipitation seasonality. Soil factors primarily affected the Pielou and Simpson indices of species diversity, whereas climatic factors mainly influenced the Margalef and Shannon–Wiener indices. PD and structure were mainly influenced by climatic factors. The combined effects of soil and climatic factors play a crucial role in sustaining the diversity and ecological adaptation of these plant communities. In summary, P. euphratica communities may exhibit a significant ecological niche conservation in response to environmental changes, and competitive exclusion might be the primary process shaping community structure. Climatic factors were shown to be important regulators of community diversity and phylogenetic structure.