Persian or common walnut (Juglans regia L.) is a fruit tree of significant agricultural importance and is considered highly drought-resistant. However, the effects of different irrigation treatments and tree height on the physiology, growth and fruit quality of the walnut tree remain largely elusive. In the presently study, we selected ‘Wen 185’, one of the main walnut cultivars, as the target plant species. We established three irrigation treatments (deficit (DI), conventional (CI) and excess irrigation (EI)) from April to September of 2020 and measured leaf hydraulic traits, photosynthetic characteristics, soluble sugar (SS) content, leaf area, branch growth, fruit morphology and the no and deflated kernel (NDK) rate of walnut trees in each treatment. Our results showed that: (1) midday leaf water potential (Ψ_md) decreased significantly under the DI treatment and declined significantly with increasing tree height; upper canopy Ψ_md in the DI group decreased by 18.40% compared to the lower canopy; (2) the light compensation point, light saturation point, maximum net photosynthetic rate, maximum photochemical efficiency and chlorophyll SPAD values of trees in the DI group decreased slightly but did not differ significantly from the CI and EI treatments; (3) reduced irrigation did not significantly affect the soluble sugar content of leaves (LSs) and fine roots (RSs), but the soluble sugar content of walnut kernels (FSs) was significantly higher in the DI treatment than under the CI and EI treatments and also increased with tree height; the average soluble sugar content across heights was 6.61% in the EI group, 7.19% in the CI group and 9.52% in the DI group; (4) branch terminal leaf area (LA) was significantly reduced at the end of new branches, and Huber values (HV) were significantly higher under the DI treatment; compared to the EI group, LA was reduced by 52.30% in the DI group and 32.50% in the CI group; HV increased by 79.00% in the DI group and 15.70% in the CI group; (5) reduced irrigation did not significantly affect fruit morphology but did increase the NDK rate of walnuts, which also increased with tree height; the average NDK rate across all heights was 4.63% in the EI group, 5.04% in the CI group, and 8.70% in the DI group; the NDK rate was 41.75% higher in the upper part of the canopy compared to the lower part in the DI group. Our results indicate that walnut trees suffer greater water stress in the upper canopy than in the middle and low parts of the canopy. By increasing HV, walnut trees maintained relatively stable photosynthetic capacity under drought. However, water deficit had a significant effect on NDK rates, particularly at greater tree heights.

As global climate change escalates, horticultural crops, especially walnuts, face increased vulnerability to frost damage. Cold hardiness—a crucial trait for survival—is influenced by complex physiological and biochemical mechanisms. This study assessed the cold hardiness of five walnut cultivars—‘Xinxin 2’, ‘Wen 81’, ‘Wen 185’, ‘Zha 343’, and ‘Xinzaofeng’—under simulated low-temperature stress, focusing on differences in freezing tolerance. One-year branches were gradually cooled to temperatures as low as −30 °C. Key physiological metrics, including electrolyte leakage (EL) and regrowth (RG) potential, along with biochemical metrics like antioxidant enzyme activities and osmoregulatory compounds, were used to evaluate cold hardiness. A comprehensive cold resistance indicator, derived using the subordination function method, highlighted cultivar resilience. Results showed significant variation in cold tolerance, with ‘Wen 185’ and ‘Wen 81’ exhibiting superior resilience, while ‘Xinxin 2’ was the most susceptible. Logistic regression analysis of relative electrolyte conductivity (REC) data estimated the semi-lethal temperature (LT50), identifying ‘Wen 81’ as the most cold-tolerant cultivar (LT50 = −21.73 °C). Antioxidant enzymes and osmoregulatory compounds were crucial for maintaining cellular stability and recovery after freezing. These findings offer practical insights for breeding cold-resistant cultivars and strategies to mitigate frost damage.

This study focused on the “Xinlu” walnut and explored the molecular regulatory mechanism of lignin synthesis in the endocarp, aiming to explain the formation of bared-nut walnuts through morphological, metabolomic, and transcriptomic techniques. It was found that the synthesis of lignin, cellulose, p-coumaryl alcohol, and sinapyl alcohol was severely inhibited in the pulpy-hue (PUH) of the endocarp. We obtained 14 modules (gene sets) significantly correlated with the lignification factor (LIG) and 1548 hub genes. Additionally, we identified a MEplum3 module involved in endocarp lignin synthesis, primarily participating in phenylalanine biosynthesis and the lignin biosynthetic process. Meanwhile, we constructed a gene co-expression network for the MEplum3 module and identified a key hub gene for lignin synthesis—JrCAD10. Among the different tissues of “Xinlu”, the expression level of JrCAD10 in the scleritic-hue (SCH) was significantly higher than in other tissues, with a relative copy number (RCN) of 3.2. However, JrCAD10 expression was severely suppressed in the PUH. The suppression of JrCAD10 expression led to the inhibition of lignin monomer synthesis, which further resulted in inhibited lignin synthesis, thus forming the bared-nut walnut. Our findings provide new insights into understanding the regulation of lignin synthesis and offer a possible explanation for the formation of bared-nut walnuts.

Background: The phylogenetic placement and chloroplast-inferred maternal relationships of commercial pluot cultivars remain unclear, largely because plastome-level evidence is limited for assessing their affinities with Prunus salicina and Prunus ussuriensis. Although chloroplast genome structure has been well characterized in angiosperms and in several Prunus species, complete plastome resources and comparative genomic evidence for commercial pluot cultivars remain scarce. Methods: Here, we assembled the complete chloroplast genomes of six commercial pluot cultivars and performed comparative genomic, phylogenomic, and divergence time analyses using representative Prunus species. Results: All genomes exhibited the typical circular quadripartite structure and ranged from 157,865 to 158,138 bp in length. Genome organization, GC content, and gene content were highly conserved, whereas the IR regions showed an elevated GC content of approximately 42.6%, owing to rRNA gene enrichment. IR boundary comparison revealed contraction at the IRb/SSC boundary in P. ussuriensis, while pluot cultivars were structurally more similar to P. salicina. In total, 370 SSR loci and four hypervariable regions, namely rpoB–trnC-GCA, petN–psbM, trnV-UAC–trnM-CAU, and trnP-UGG–psaJ, were identified as candidate molecular markers for Prunus germplasm identification and genetic analysis. Phylogenomic analysis resolved four major clades within Prunus and showed that ‘Flavor King’, ‘Flavor Supreme’, and ‘Flavor Queen’ grouped with P. ussuriensis, whereas ‘Flavorosa’, ‘Dinosaur Egg’, and ‘Flavorich’ grouped with P. salicina. Conclusion: Overall, this study provides the first comparative plastome analysis of six commercial pluot cultivars and offers chloroplast-level evidence for their maternal affinities within Prunus, together with useful marker resources for cultivar identification and germplasm evaluation.