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Persian or English walnut (Juglans regia L.), the walnut species cultivated for nut production, is one of the oldest food sources known and is grown worldwide in temperate areas. France is the 7th leading producer as of 2016 with 39 kt. Deciphering walnut genetic diversity and structure is important for efficient management and use of genetic resources. In this work, 253 worldwide accessions from the INRA walnut germplasm collection, containing English walnut and several related species, were genotyped using 13 SSR (Single Sequence Repeat) markers selected from the literature to assess diversity and structure. Genetic diversity parameters showed a deficiency of heterozygotes and, for several SSRs, allele-specificities among the accessions tested. Principal Coordinate Analysis (PCoA) showed the 253 accessions clustered in largely in agreement with the existing botanical classification of the genus. Among the 217 J. regia accessions, two main clusters, accessions from Eastern Europe and Asia, and accessions from Western Europe and America, were identified using STRUCTURE software. This was confirmed by Principal Coordinate Analysis and supported by Neighbor-Joining tree construction using DARwin software. Moreover, a substructure was found within the two clusters, mainly according to geographical origin. A core collection containing 50 accessions was selected using the maximum length sub-tree method and prior knowledge about their phenotype. The present study constitutes a preliminary population genetics overview of INRA walnut genetic resources collection using SSR markers. The resulting estimations of genetic diversity and structure are useful for germplasm management and for future walnut breeding programs.

Background Walnut ( Juglans regia L.) breeding programs aim to develop new genotypes that exhibit superior agronomic traits, including high yield, improved nut quality, and favorable phenological traits. One of the primary methods used in these programs is hybridization, which involves controlled crosses between selected parent varieties. In reciprocal cross, understanding the genetic contributions of both maternal and paternal parents is crucial, as these contributions significantly influence the phenotypic traits of the resulting progeny. This knowledge allows breeders to predict and select genotypes that best meet the desired breeding objectives, ultimately enhancing agricultural productivity and sustainability. Results This study analyzed the pomological traits of F1 plants derived from four different hybrid combinations: Pedro × Maraş 18, Pedro × Sütyemez 1, Maraş 18 × Pedro, and Sütyemez 1 × Pedro. The assessment focused on key nut traits, including nut length, nut diameter, nut weight, kernel weight, and kernel percentage. Statistical analyses revealed significant variations in these traits among the hybrid combinations, with these differences determined at the p  < 0.05 significance level. Kernel weight exhibited the highest coefficient of variation (CV = 33.63%), indicating substantial variability in this trait among the hybrids. Nut diameter had the lowest variability (CV = 12.82%), suggesting greater consistency across the hybrid combinations. Other traits, such as nut weight, nut length, and kernel percentage, showed intermediate levels of variability, with CVs of 27.33%, 13.45%, and 18.59%, respectively. The study found that maternal parents played a more partially dominant role in determining nut traits in most hybrid combinations. However, when Sütyemez 1 and Maraş 18 were used as maternal parents, their influence on the inheritance of some nut traits was relatively greater than the other parent variety (Pedro). Conclusion The findings emphasize the crucial role of parent selection in walnut hybridization breeding programs, with parental effects being relatively prominent in influencing pomological traits, underscoring the need for careful selection of maternal parents to achieve the desired outcomes. Among the varieties studied, Pedro, Maraş 18, and Sütyemez 1 were identified as promising parent genotypes for improving key nut traits. The variability observed in traits such as kernel and nut weights suggests potential for further selection and genetic improvement. This variation highlights the genetic diversity present in the studied hybrids, which can be effectively utilized in breeding efforts. These results not only contribute to the improvement of walnut varieties but also have broader implications for global walnut production, providing valuable guidance for breeding programs aiming to improve nut quality in walnut hybridization programs.

• Not all roots born as first-order branches are the same and this has important consequences for overall function. We hypothesized that, compared with fibrous roots, pioneer roots are built to live longer at the expense of absorptive capacity. • We tested this hypothesis by investigating pioneer and fibrous roots in their first 14 d of life in the arbuscular mycorrhizal tree species: Acer negundo, Acer saccharum, Juglans nigra, Liriodendron tulipifera and Populus tremuloides. Root observations were made with root-access boxes that allowed roots to be sampled at known ages in field-grown trees. • Compared to fibrous roots, pioneer roots had larger diameter, lower specific root length, greater average length and a lack of mycorrhizal or nonmycorrhizal fungal colonization. Pioneer roots < 14 d old had more layers of hypodermis with a lower percentage of putative passage cells and more protoxylem groups than similar age fibrous roots. • Our results suggest that pioneer roots are constructed for defense against biotic and abiotic challenges, exploration of soil distal to the stem, high fibrous root branching and secondary development with high axial hydraulic conductivity at the expense of mycorrhizal colonization and high absorptive capacity for water and nutrients.

Background Soil drought stress is a limiting factor of productivity in walnut ( Juglans regia L). Ferredoxin (Fd) level decreases under adverse environmental stress. Functional replacement of decreased Fd by Fld (Flavodoxin) had been shown to have protective effect under abiotic stress condition. This study aimed to evaluate four transgenic lines (L3, L4, L13 and L17) along with non-transgenic line under three osmotic stresses levels (0, 10 and 12% PEG). Methods and results This experiment carried out based on a completely randomized design with four replications. To confirm that the Fld gene is successfully integrated into the walnut genome, PCR and dot blot analysis were carried out. The transgenic lines of walnut expressing Fld displayed increased tolerance to osmotic stress at 10 and 12% PEG condition. Lines expressing Fld exhibited increasing tolerance to drought stress and maintained health of plants under osmotic conditions. Results of real time PCR showed that expression level of Fld gene in L4 was higher than the others. Among transgenic lines, L4 was more tolerant than other lines under osmotic stress. Conclusions These findings indicate that expression of Fld gene can increase tolerance to osmotic stress in Persian walnut and is useful tool for walnut production in arid and semi-arid regions.

Walnut shell suture strength directly impacts the ability to maintain shell integrity during harvest and processing, susceptibility to insect damage and other contamination, and the proportion of kernel halves recovered during cracking. Suture strength is therefore an important breeding objective. Here, two methods of phenotyping this trait were investigated: 1) traditional, qualitative and rather subjective scoring on an interval scale by human observers, and; 2) quantitative and continuous measurements captured by a texturometer. The aim of this work was to increase the accuracy of suture strength phenotyping and to then apply two mapping approaches, quantitative trait loci (QTL) mapping and genome wide association (GWAS) models, in order to dissect the genetic basis of the walnut suture trait. Using data collected on trees within the UC Davis Walnut Improvement Program (n = 464), the genetic correlation between the texturometer method and qualitatively scored method was high (0.826). Narrow sense heritability calculated using quantitative measurements was 0.82. A major QTL for suture strength was detected on LG05, explaining 34% of the phenotypic variation; additionally, two minor QTLs were identified on LG01 and LG11. All three QTLs were confirmed with GWAS on corresponding chromosomes. The findings reported in this study are relevant for application towards a molecular breeding program in walnut.

Leaf gas exchange and stem xylem hydraulic and mechanical properties were studied for unburned adults and resprouting burned Juglans californica (southern California black walnut) trees 1 year after a fire to explore possible trade-offs between mechanical and hydraulic properties of plants. The CO₂ uptake rates and stomatal conductance were 2-3 times greater for resprouting trees than for unburned adults. Both predawn and midday water potentials were more negative for unburned adult trees, indicating that the stems were experiencing greater water stress than the stems of resprouting trees. In addition, the xylem specific conductivity was similar in the two growth forms, even though the stems of resprouting trees were less vulnerable to water-stress-induced embolism than similar diameter, but older, stems of adult trees. The reduced vulnerability may have been due to less cavitation fatigue in stems of resprouts. The modulus of elasticity, modulus of rupture and xylem density were all greater for resprouts, indicating that resprouts have greater mechanical strength than do adult trees. The data suggest that there is no trade-off between stem mechanical strength and shoot hydraulic and photosynthetic efficiency in resprouts, which may have implications for the success of this species in the fire-prone plant communities of southern California.

BACKGROUND: and Aims Heterodichogamy differs from normal dichogamy, in that it involves two mating types (protogyny and protandry) that occur at a 1 : 1 ratio in a population. Flowering phases of the two mating types are synchronized and reciprocal, which was considered to ensure between-type outcrossing. This study aims to quantify the flowering pattern and pollination efficacy in Juglans mandshurica, a wind-pollinated heterodichogamous tree. METHODS: The pattern of flowering phenology was monitored within individual trees and pollen traps were used to measure air-borne pollen loads during the spring in 2003 and 2004. Pollen longevity was determined by staining technique. Also a pollen supplementation experiment was performed in 2004 to assess pollen limitation of fruit production. Key Results There was no overlap between sexual functions within individual trees. Flowering periods of the two mating types were reciprocal and synchronous in both 2003 and 2004. Air-borne pollen loads were large, and protogynous and protandrous individuals each produced a high pollination peak, consistent with the two blooming periods. Maximum pollen longevity was about 4 h for protandrous individuals, and 3 h for protogynous individuals. Pollen supplementation did not increase fruit production in either protogynous or protandrous individuals. CONCLUSIONS: Heterodichogamous flowering in Juglans mandshurica effectively avoids selfing, promotes between-type outcrossing, and leads to efficient pollination in a natural population.

Architectural analysis of 840 Slovenian walnut (Juglans regia L.) genotypes was performed to determine the most typical and frequent morphological types and to evaluate their vegetative and generative potential. Four branching and fruiting patterns (I–IV) were detected. A 3‐year‐old fruiting branch, consisting of a 3‐year‐old shoot plus corresponding 2‐year‐old and 1‐year‐old shoots, was used as a structural unit for quantitative analysis. In the intermediate fruit‐bearing types with mesotonic and acrotonic branching pattern (types II and III), the total lengths of 3‐, 2‐ and 1‐year‐old shoots were 385 and 380 cm, respectively, compared with 275 and 253 cm in the terminal and lateral‐fruiting types (types I and IV). In type I, 1‐year‐old shoots had significantly fewer nodes than in other types. In addition, they had a thinner basal diameter than types III and IV, and their angles were the most erect (39°). Only 0·4 out of 3·6 1‐year‐old shoots were flowering with one mixed bud with 1·9 female flowers. In type IV, 2‐year‐old shoots had significantly more nodes and a larger basal diameter than other types. One‐year‐old shoots in type IV are thicker than those in other types. Ratios between the number of flowering and the total number of 1‐year‐old shoots were 0·7 in type IV, 0·6 in type III, 0·5 in type II and 0·1 in type I. On 1‐year‐old shoots in type IV, 1·7 mixed buds with a mean of three female inflorescences per bud were counted. Consequently, the generative potential is highest in type IV and lowest in type I. In types II and III, growth and the ability to bear fruits are more balanced.

The organogenetic cycle of shoots on main branches of 4‐year‐old Juglans regia trees was studied. Mono‐ and bicyclic floriferous and vegetative annual shoots were analysed. Five parent annual shoot types were sampled between October 1992 and August 1993. Organogenesis of summer growth units was monitored between 16 Jun. and 3 Aug. 1993. Variations over time in the number of nodes, cataphylls and embryonic green leaves of terminal buds were studied. The number of nodes of parent shoot buds was compared with the number of nodes of shoots derived from parent shoot buds. The spring growth units of mono‐ and bicyclic shoots consist exclusively of preformed leaves which were differentiated, respectively, during the spring flush of growth (mid‐April until mid‐May) or the summer flush of growth (mid‐June until early August) in the previous growing season. Thus, winter buds may consist of flower and leaf primordia differentiated in two different periods during annual shoot extension. The summer growth units of bicyclic shoots consist of preformed leaves that were differentiated in spring buds during the spring flush of growth in the current growing season. Bud morphology is compared between spring and summer shoots.

The current study investigated the impact of cold stress on the morphological, physiological, and phytochemical properties of Juglans regia L. (J. regia) using in vitro microclone cultures. The study revealed significant stress-induced changes in the production of secondary antioxidant metabolites. According to gas chromatography–mass spectrometry (GC–MS) analyses, the stress conditions profoundly altered the metabolism of J. regia microclones. Although the overall spectrum of metabolites was reduced, the production of key secondary antioxidant metabolites significantly increased. Notably, there was a sevenfold (7×) increase in juglone concentration. These findings are crucial for advancing walnut metabolomics and enhancing our understanding of plant responses to abiotic stress factors. Additionally, study results aid in identifying the role of individual metabolites in these processes, which is essential for developing strategies to improve plant resilience and tolerance to adverse conditions.