Walnut Breeding Research Articles

Identification of a major QTL region and candidate genes responsible for harvest date combining QTL mapping and QTL-seq approaches in walnut (Juglans regia L.)

Walnut (Juglans regia L.) takes place within the Juglandaceae family and is one of the most popular nut species. In walnut breeding, several limiting factors need to be revealed regarding the genetic mechanism underlying important agronomical traits. In the current report, the monitoring of the ‘Chandler x Kaplan-86’ F1 walnut population was carried out four successive years (2015, 2016, 2017, 2018) and the distribution of the plants for harvest date from four years was ranged from August 27th to October 22nd. Then, we dissected the genomic regions associated with harvest date using classical Quantitative Trait Loci (QTLs) mapping and a rapid and cost-effective QTL-seq approach in ‘Chandler x Kaplan-86’ F1 walnut population for the first time. In classical QTL mapping, a major QTL was recognized in LG4 corresponding to Chr1 in Chandler female map with 27.6 (%) phenotyping variance. In QTL-seq, we bulked the DNAs of the genotypes with extreme harvest date separately, the whole genome resequencing was performed for a parent Chandler and both bulks (early and late) to calculate Delta SNP indexes for each Single Nucleotide Polymorphisms (SNPs) and Insertion/Deletion (InDels). Approximately 48.04 million paired reads for Chandler, 149.72, and 150.14 million paired reads were obtained from early and late harvest bulks, respectively. A total of 229,307 SNPs and 45,223 InDels were mined from the merged sequences data covered whole chromosomes. The responsible genomic fragments were identified in Chr2 and Chr11 in QTL-seq. The highest linked InDel locus detected in QTL-seq in Chr11 around 17 million base pairs distinguished the plants according to harvest date. InDel Sequence Characterized Amplified Region (SCAR) marker is a candidate for marker-assisted selection for future walnut breeding studies.

The RNA-seq and BSA-seq analysis reveals genes associated to the control of early fruiting in walnut (Juglans regia L.)

Walnut is an important nut worldwide. The duration of the juvenile phase has a considerable impact on walnut yield. Floral transition is critical for walnut flowering and fruit setting. To improve walnut breeding, it is critical to study the mechanisms of early blooming and fruiting in early-fruiting (EF) walnuts and shorten their juvenile stages. This work used EF and later-fruiting (LF) walnuts to screen for genes associated with walnut early fruiting using RNA sequencing (RNA-seq) and bulk segregant analysis sequencing (BSA-seq). Thirty-seven key differential genes were analyzed using weighted gene co-expression network association (WGCNA) in RNA-seq, and FT, CAL, FUL, and APRR9 genes were found to be able to shorten the juvenile phase of plants by affecting the floral transition process and influencing plant fruiting time. BSA-seq was utilized to identify 3305656 SNPs and 596640 InDels in EF walnut and 925347 SNPs and 514751 InDels in LF walnut. A combined analysis of RNA-seq and BSA-seq revealed 49 structurally varied DEGs in both kinds, with 47 genes in the NC_049914.1 region, one gene in NC_049901.1, one gene locus in NC_049908.1, and eight structurally variant DEGs found solely in early fruiting walnuts. The analysis of candidate genes and verification of RT-qPCR expression revealed that JrLOC109015871 (JrPBL12), JrLOC108983551 (WRI1-like), JrLOC109016788 (WRKY12), and AP2-like genes are likely to be related to early fruiting of walnut, with WRKY12 being expressed at the highest level during the flowering peak of early fruiting walnut. It is likely that the WRKY12 gene regulates the flowering transition of walnut through the ageing pathway, allowing walnut to complete the transition from nutrient growth to reproductive growth more quickly and realize early fruiting to increase walnut yield, laying the groundwork for the study of the molecular mechanism of walnut's early fruiting trait.

Utilizing SSR markers to examine the population structure and molecular genetic diversity of walnut (Juglans regia L.) genotypes in the Northwestern Himalayan region of Jammu and Kashmir

By using 16 simple sequence repeat (SSR) markers, the genetic relatedness of 21 exceptional walnut genotypes was assessed. A significant degree of genetic diversity was observed within a given population, as indicated by the number of alleles per locus ranging from 2 to 4. WGA-1, WGA-4, and WGA-79 contained the greatest number of alleles (4), followed by WGA-118, WGA-202, and WGA-42. Conversely, WGA-27, WGA-69, and WGA-32 contained the fewest alleles. The range of the PIC value was 0.11 to 0.38. Using model-based cluster analysis, all genotypes were categorized into two primary clusters according to the UPGMA dendrogram, with varying degrees of sub-clustering. All the genotypes were categorized into six genetically distant subpopulations. The genotypes were genetically distinct but had variable degrees of admixture. The anticipated heterozygosity at a specific locus ranged from 0.563 to 0.741. Additionally, population differentiation (Fst) ranged between 0.176 and 0.261. These findings highlight the importance of considering germplasm diversity in walnut breeding programs and conservation efforts aimed at enhancing walnut cultivation in the region. Overall, this study contributes to our understanding of walnut genetic diversity in the Northwestern Himalayan region of Jammu and Kashmir and informs future breeding and conservation strategies.

Diversity of specific indicators of walnut fruits during selection using various methods

Walnut (Juglans regia L.) is a particularly valuable plant for humans. This is due to its multifaceted beneficial properties and qualities as a food, medicinal, technical, recreational and environmental tree species. The main goal of nut breeding is to obtain high-quality fruits. In the Russian Federation for the selection of individuals that provide such products, two methods are mainly used: method 1 – «Program and method for breeding walnuts» (Sukhorukikh et al., 2007) and method 2 – «Program and method for the study of fruit, berry and nut crops» (1999). The purpose of the study is to study the diversity of promising forms of walnut identified on the basis of methods 1 and 2 in terms of taste, kernel extractability and the overall score of the breeding value (quality) of the fruit. Other characteristics assessed identically or similarly are not considered. The object of the research was the literature data of the authors assessing the quality of nuts using methods 1, 2. For statistical processing, the Stadia 8.0/prof. program was used, and Microsoft Excel was used for graphical construction. Diversity was assessed using coefficients of variation and indices of distributed rank diversity (IRRR) based on known methods. It was established that the coefficients of variation of the studied indicators, assessed using various methods, did not have a significant statistical difference, and the selection gradations of taste and kernel extractability were equally distributed. When assessed using method 1, the statistical distribution of the total score of the selection value of fruits had a normal distribution, when using method 2 it differed from normal. When assessing diversity using IRRR values, method 1, compared to method 2, turned out to be more sensitive and provided greater diversity of the allocated gene pool in terms of the overall score of the breeding value of walnut fruits. The results can be used in walnut breeding.

Morphological and pomological assessments of seedling-originated walnut (Juglans regia L.) trees to select the promising late-leafing genotypes.

In many parts of the world, including Iran, walnut (Juglans regia L.) production is limited by late-spring frosts. Therefore, the use of late-leafing walnuts in areas with late-spring frost is the most important method to improve yield. In the present study, the phenotypic diversity of 141 seedling genotypes of walnut available in the Senejan area, Arak region, Markazi province, Iran was studied based on morphological traits to obtain superior late-leafing genotypes in the cropping seasons of 2022 and 2023. Based on the results of the analysis of variance, the studied genotypes showed a significant variation in terms of most of the studied morphological and pomological traits. Therefore, it is possible to choose genotypes for different values ​​of a trait. Kernel weight showed positive and significant correlations with leaf length (r = 0.32), leaf width (r = 0.33), petiole length (r = 0.26), terminal leaflet length (r = 0.34), terminal leaflet width (r = 0.21), nut length (r = 0.48), nut width (r = 0.73), nut weight (r = 0.83), kernel length (r = 0.64), and kernel width (r = 0.89). The 46 out of 141 studied genotypes were late-leafing and were analyzed separately. Among late-leafing genotypes, the length of the nut was in the range of 29.33-48.50mm, the width of the nut was in the range of 27.51-39.89mm, and nut weight was in the range of 8.18-16.06g. The thickness of shell was in the range of 1.11-2.60mm. Also, kernel length ranged from 21.97-34.84mm, kernel width ranged from 21.10-31.09mm, and kernel weight ranged from 3.10-7.97g. Based on important and commercial traits in walnut breeding programs, such as nut weight, kernel weight, kernel percentage, kernel color, and ease of kernel removal from nuts, 15 genotypes, including no.92, 91, 31, 38, 33, 18, 93, 3, 58, 108, 16, 70, 15, 82, and 32 were superior and could be used in walnut breeding programs in line with the introduction of new cultivars and the revival of traditional walnut orchards to commercialize them.

Unravelling the genetic diversity and population structure of common walnut in the Iranian Plateau

BackgroundCommon walnut (Juglans regia L.) has a long cultivation history, given its highly valuable wood and rich nutritious nuts. The Iranian Plateau has been considered as one of the last glaciation refugia and a centre of origin and domestication for the common walnut. However, a prerequisite to conserve or utilize the genetic resources of J. regia in the plateau is a comprehensive evaluation of the genetic diversity that is conspicuously lacking. In this regard, we used 31 polymorphic simple sequence repeat (SSR) markers to delineate the genetic variation and population structure of 508 J. regia individuals among 27 populations from the Iranian Plateau.ResultsThe SSR markers expressed a high level of genetic diversity (HO = 0.438, and HE = 0.437). Genetic differentiation among the populations was moderate (FST = 0.124), and genetic variation within the populations (79%) significantly surpassed among populations (21%). The gene flow (Nm = 1.840) may have remarkably influenced the population genetic structure of J. regia, which can be attributed to anthropological activities and wind dispersal of pollen. The STRUCTURE analysis divided the 27 populations into two main clusters. Comparing the neighbor-joining and principal coordinate analysis dendrograms and the Bayesian STRUCTURE analysis revealed the general agreement between the population subdivisions and the genetic relationships among the populations. However, a few geographically close populations dispersed into different clusters. Further, the low genetic diversity of the Sulaymaniyah (SMR) population of Iraq necessitates urgent conservation by propagation and seedling management or tissue culture methods; additionally, we recommend the indispensable preservation of the Gonabad (RGR) and Arak (AKR) populations in Iran.ConclusionsThese results reflected consistent high geographical affinity of the accession across the plateau. Our findings suggest that gene flow is a driving factor influencing the genetic structure of J. regia populations, whereas ecological and geological variables did not act as strong barriers. Moreover, the data reported herein provide new insights into the population structure of J. regia germplasm, which will help conserve genetic resources for the future, hence improving walnut breeding programs’ efficiency.

A superior genetic source for late leafing in walnut 'Ahir Nut'

One of the main objectives of walnut crossbreeding programmes is to produce cultivars with late leafing dates. Sixteen years ago, a walnut crossbreeding programme was initiated by Prof. Mehmet Sütyemez, and now a new walnut genotype, namely ‘Ahir Nut’, has been generated with a leafing date that starts later than all commercially-famous walnut cultivars in the world. The present study describes this walnut genotype. For a detailed analysis, ‘Ahir Nut’ was compared with two French-origin walnut cultivars, i.e., ‘Franquette’ and ‘Ronde de Montignac’, which have had the latest leafing dates until now. In an experimental orchard in Turkey, the leafing date in ‘Ahir Nut’ was 14 May, whereas the leafing date in ‘Franquette’ and ‘Ronde de Montignac’ were 23 April and 25 April, respectively. This approximately 3-week difference between ‘Franquette’ and ‘Ronde de Montignac’ to ‘Ahir Nut’ reveals how valuable this genotype is for walnut breeding studies. The harvest dates for ‘Ahir Nut’, ‘Franquette’ and ‘Ronde de Montignac’ were 4 October, 3 October, and 7 October, respectively. The defoliation date for ‘Ahir Nut’ was 4 December. The nut weight and kernel percentage of ‘Ahir Nut’ were calculated as 11.61 g and 49.84%, respectively. Remarkable phenological traits were found in ‘Ahir Nut’, making it a promising option for future walnut cultivation. In the present study, the late leafing date of ‘Ahir Nut’ prompted further research into the molecular and walnut crossbreeding potentials. This new genotype has been conserved as an important genetic resource for future walnut breeding programmes around the world.

Domestication and selection footprints in Persian walnuts (Juglans regia).

Walnut (Juglans) species are economically important hardwood trees cultivated worldwide for both edible nuts and high-quality wood. Broad-scale assessments of species diversity, evolutionary history, and domestication are needed to improve walnut breeding. In this study, we sequenced 309 walnut accessions from around the world, including 55 Juglans relatives, 98 wild Persian walnuts (J. regia), 70 J. regia landraces, and 86 J. regia cultivars. The phylogenetic tree indicated that J. regia samples (section Dioscaryon) were monophyletic within Juglans. The core areas of genetic diversity of J. regia germplasm were southwestern China and southern Asia near the Qinghai-Tibet Plateau and the Himalayas, and the uplift of the Himalayas was speculated to be the main factor leading to the current population dynamics of Persian walnut. The pattern of genomic variation in terms of nucleotide diversity, linkage disequilibrium, single nucleotide polymorphisms, and insertions/deletions revealed the domestication and selection footprints in Persian walnut. Selective sweep analysis, GWAS, and expression analysis further identified two transcription factors, JrbHLH and JrMYB6, that influence the thickness of the nut diaphragm as loci under selection during domestication. Our results elucidate the domestication and selection footprints in Persian walnuts and provide a valuable resource for the genomics-assisted breeding of this important crop.

Tracing Superior Late-Leafing Genotypes of Persian Walnut for Managing Late-Spring Frost in Walnut Orchards

Evaluating genetic diversity in walnut (Juglans regia L.) populations is a rapid approach used by walnut breeding programs to distinguish superior genotypes. The present study was conducted on the walnut population of Hamedan Province, one of the richest and most genetically diverse regions in Iran, during 2018–2019. After the initial screening, 47 genotypes were selected for further evaluation of pomological and phenological traits based on International Plant Genetic Resources Institute (IPGRI) descriptors. Nut and kernel weights among the selected genotypes ranged from 7.15 to 21.05 g and 3.0 to 10.8 g, respectively. Principal component analysis (PCA) categorized the genotypes into three distinct groups. Whereas the cluster analysis (CA) revealed the similarities and dissimilarities among the genotypes by identifying four major clusters. Spearman correlation analysis showed a positive correlation (p < 0.01) between nut weight (NWT), nut size, and kernel weight (KW), while a negative correlation (p < 0.01) between shell thickness (STH) and packing tissue thickness (PTT) with kernel percentage (KP) was observed. Lastly, 10 of 47 genotypes (TAL8, TAL9, TAL10, TAL14, TAL19, TAL22, TB2, TB4, TB6, and RDGH5) were considered superior. Superior genotypes were late-leafing (25–40 days after the standard) and displayed a lateral bearing (LB) habit with heavy nuts (12.52–16.82 g) and kernels (6.53–8.15 g), thin shells (1.06–1.25 mm), and lightly colored kernels.

Comparison of methods for selecting a promising walnut gene pool according to fruit quality

Background. Walnut (Juglans regia L.) is considered a particularly valuable plant for humans. A number of methods have been developed to select the species’ gene pool that produces high-quality fruits. Breeders need to know which of them ensures identification of the best ancestors. The objective of this work was to compare the effectiveness of the most frequently used techniques for selecting a valuable gene pool according to taste, extractability, weight, kernel yield, and overall fruit score. They were presented in the Program and Methods for Walnut Breeding (method 1) and the Program and Methodology of Variety Studies for Fruit, Berry and Nut Crops (method 2).Materials and methods. Statistical data processing was performed using the licensed Stadia-8 software.Results. Comparing the results with the program requirements for new cultivars, all the ancestors selected according to method 1 had better taste and higher kernel weight, while according to method 2 12.5% of the forms showed lower values. The recommended kernel yield of 50% or more turned out to be lower than the threshold value in 14.29% (method 1) and 62.5% (method 2) of the selected ramets. Evaluation by to method 1 showed that the group of the “superior quality” breeding category had higher average values in taste (by 3.63%), weight (7.86%), extractability (4.25%), and kernel yield (9.9%). When selecting the first-rank forms by method 1, the values were higher in extractability (by 0.25%), weight (5.26%), taste (3.09%), kernel yield (7.55%), and overall score (7.61%). In the average values for the first-rank and second-rank forms, the excess was by 4.44, 21.91, 4.75, 9.67, and 9.86%, respectively.Conclusions. When selecting a promising walnut gene pool, method 1 provides for stricter selection and higher food quality indicators (taste, extractability, weight, kernel yield, and overall fruit score) compared to method 2.

Phenotypic Differences and Physiological Responses of Salt Resistance of Walnut with Four Rootstock Types.

Walnut is one of the world’s four largest nuts. Currently, the bottleneck in walnut breeding is the production of resistant variants. Soil salinization is a global problem, and the use of salt-tolerant rootstocks is a basic strategy to overcome the challenge of sustained walnut production. Providing a scientific basis for the selection of walnut salt-tolerant rootstocks is possible by studying the physiological and biochemical response characteristics and salt tolerance variations of different walnut genotypes under salt stress. In the present study, seedlings of four genotypes of walnut rootstocks, including J1 (Juglans hindsii), J2 (J. mandshurica), J3 (J. regia × J. mandshurica), and J4 (J. regia × J. hindsii), were employed as test materials to conduct a 28-day pot experiment under NaCl stress with five NaCl concentrations (0, 50, 100, 200, and 300 mmol/L). Under different NaCl treatment concentrations, seedling morphology, growth indices, chlorophyll content, photosynthetic parameters, relative electrical conductivity (REC), malondialdehyde (MDA), proline (Pro), soluble sugar (SS), and the activity of superoxide dismutase (SOD) and peroxidase (POD) in the leaves were examined. Salt stress altered the morphological characteristics and growth indices of seedlings from four genotypes to varying degrees. In addition, according to the analysis of physiological and biochemical data, salt stress had a considerable impact on both the physiological and biochemical processes of seedlings. Salt stress decreased the chlorophyll content and photosynthetic parameters of four genotypes, the REC, MDA content, Pro content, and SS content of each genotype increased by different degrees, and the enzymatic activities showed different trends. The salt tolerance of rootstocks was evaluated thoroughly using principal component analysis and membership function analysis based on the 16 parameters. The results of a comprehensive evaluation of salt tolerance showed that the order of salt tolerance of the four genotypes was J4 > J1 > J3 > J2, which corresponded to the order of the morphological symptoms of salt injury. In conclusion, J4 has strong salt tolerance and is an important germplasm resource for walnut salt-tolerant rootstock breeding.

Morphometric traits and iPBS based molecular characterizations of walnut (Juglans regia L.) genotypes

In this study, walnut genotypes that were selected during two growing seasons among thousands of seedlings were analyzed in terms of detailed morphometric, phenological, and chemical traits. A multivariate analysis was conducted with valuable traits for breeding and selection such as morphometric traits, chemical composition, and phenological characteristics. Also, genotypes were characterized by a retrotransposon-based iPBS marker system. The correlation analysis showed significant positive and negative correlations between agro-morphological characters. The principal component analysis explained 71.44% of the total variance into five main components. Principal component and hierarchical cluster analysis divided genotypes into three groups and identified subgroups based on both agro-morphological characters and iPBS marker systems. A high level of polymorphism ratio was observed for tested markers. Mantel’s test demonstrated relatively low correlations between molecular and morphological treats (r = 0.04). The genetic similarities among all individuals ranged from 0.39 (between 018 and 015 or 045 genotypes) to 0.98 (between 090 and 094 genotypes) with a mean similarity of 0.67. Remarkable phenotypic and molecular variations were observed among the genotypes. The features of some investigated genotypes were above the acceptable thresholds for walnut selection in breeding programs, and our study indicated that iPBS markers can be beneficial in walnut breeding programs, allowing the evaluation of the genetic relationship between genotypes, helping to differentiate and select the best genotypes to improve agronomic properties.

Genetic Identification of Hybrid Walnuts (Juglans × intermedia Carr.) in Hungary, the Hidden Potential for Future Breeding

The question of the hybrid walnut (Juglans × intermedia Carr.) is still under debate in the Central European region. There is not simply just an underutilization, rather, even the existence of these hybrid forms is not broadly accepted. On the contrary, there is an intensive cross-breeding activity in the western part of Europe resulting in commercially available hybrid clones. In Hungary, several individuals have been reported with intermediate morphology from different old black walnut plantations. Due to the lacking information, a preliminary study was conducted in order to prove the difference of these selected trees and to test the hybrid state. For this purpose, DNA fingerprinting was used by applying 13 simple sequence repeat (SSR) markers for the identification of 22 hybrid genotypes selected from one study plot. A comparative analysis with a reference sample set of the ‘parental’ species and other known hybrids was performed as well. The genetic analysis resulted distinct, unique genotypes for all of the samples. Based on the genetic pattern, the analyzed hybrid group was clearly distinguishable from the other two walnut groups. The result of this study also highlights the hidden potential in walnut breeding in the Central European region. Future concepts concerning hybrid walnut utilization in plantation forestry, agroforestry or as breeding material are also discussed.

Identification of TIFY gene family in walnut and analysis of its expression under abiotic stresses

BackgroundWalnuts (Juglans regia L.) are known for their nutrient-rich nuts and are one of the important economic tree species in the world. However, due to global warming and soil salinization, walnuts suffer from various abiotic stresses. TIFY (TIF[F/Y]XG) proteins play an essential role in the growth and development of plants, signal transduction, and stress response in plants. At present, although the TIFY gene family of a number of plants has been identified and studied, how TIFY takes part in stress tolerance remains obscure and many functions of TIFY require further investigation.ResultIn this study, twenty-one TIFY transcription factors were identified in the walnut genome database, and they were divided into four subfamilies (TIFY, JAZ, ZML, and PPD) by bioinformatics analysis. Chromosome location revealed tandem duplication of some genes. Phylogenetic tree analysis showed JrTIFYs were closely related to the TIFY gene family of Arabidopsis thaliana (A. thaliana). qRT-PCR (quantitative real-time PCR) analysis revealed the TIFY genes have different expression patterns in ‘Qingxiang’ and ‘Xiangling’ walnut varieties under drought, heat, and salt stress. JAZ subfamily was more expressed in different abiotic stress than other subfamilies. The expressions of JrTIFY14 under heat and salt stress were significantly higher than those under drought stress. However, the expression of JrTIFYs was not significant in ‘Xiangling’.ConclusionThis study reveals the TIFY gene family plays an important role in walnuts facing abiotic stresses and provides a theoretical basis for walnut breeding.

Walnut breeding in California

Walnut breeding in California

Composite core set construction and diversity analysis of Iranian walnut germplasm using molecular markers and phenotypic traits.

Iran is a center of origin and diversity for walnuts (Juglans regia L.) with very good potential for breeding purposes. The rich germplasm available, creates an opportunity for study and selection of the diverse walnut genotypes. In this study, the population structure of 104 Persian walnut accessions was assessed using AFLP markers in combination with phenotypic variability of 17 and 18 qualitative and quantitative traits respetively. The primers E-TG/M-CAG, with high values of number of polymorphic bands, polymorphic information content, marker index and Shannon's diversity index, were the most effective in detecting genetic variation within the walnut germplasm. Multivariate analysis of variance indicated 93.98% of the genetic variability was between individuals, while 6.32% of variation was among populations. A relatively new technique, an advanced maximization strategy with a heuristic approach, was deployed to develop the core collection. Initially, three independent core collections (CC1-CC3) were created using phenotypic data and molecular markers. The three core collections (CC1-CC3) were then merged to generate a composite core collection (CC4). The mean difference percentage, variance difference percentage, variable rate of coefficient of variance percentage, coincidence rate of range percentage, Shannon's diversity index, and Nei's gene diversity were employed for comparative analysis. The CC4 with 46 accessions represented the complete range of phenotypic and genetic variability. This study is the first report describing development of a core collection in walnut using molecular marker data in combination with phenotypic values. The construction of core collection could facilitate the work for identification of genetic determinants of trait variability and aid effective utilization of diversity caused by outcrossing, in walnut breeding programs.

Eastern Black Walnut (Juglans nigra L.) Originating From Native Range Varies in Their Response to Inoculation With Geosmithia morbida

Thousand cankers disease (TCD) is caused by the walnut twig beetle (Pityophthorus juglandis) vectoring the fungal canker pathogen Geosmithia morbida, which can result in severe dieback and eventual death to species of walnut (Juglans spp.) and wingnut (Pterocarya spp.). This disease is most devastating to the highly valued species J. nigra (black walnut). This species is primarily grown and harvested for timber production in the Central Hardwood Region of the United States, which comprises part of its native range. Management options for TCD are limited; therefore, finding resistant genotypes is needed. Initial studies on black walnut susceptibility to G. morbida documented some genetic variation and suggested potential resistance. Furthermore, G. morbida is thought to be native to the United States, which may have allowed for co-evolution. To capture the representative genetic diversity and screen for resistance to G. morbida, J. nigra families were collected from across the native range. These wild trees, in conjunction with seedlings developed in a black walnut timber improvement program, were planted in a common garden in Fort Collins, Colorado and repeatedly inoculated with G. morbida over the course of four years and three growing seasons. Improved seedlings exhibited larger cankered areas than wild J. nigra of the same provenance. Cankers induced by G. morbida in wild germplasm were smaller on J. nigra collected from the western and central portions of the native range compared to those collected from the eastern portion. Although trees from the western and central part of the range still incurred cankers, our findings indicate that variation in genetic resistance to G. morbida is present in black walnut. This study was performed with G. morbida independent of the walnut twig beetle, but our results suggest the limited G. morbida resistance observed in J. nigra will prevent the full compromise of black walnut to TCD. Results from this study should be taken into consideration in future black walnut breeding programs.

The Persian (English) walnut (Juglans regia L.) assortment of Hungary: Nut characteristics and origin

Persian walnut growing has a long-term tradition in Hungary. The walnut breeding started with the domestication of French varieties propagated by seeds in the 1910s, which was not successful. From the 1950s the Hungarian breeding research used selections from the local population, cross breeding and back crosses as breeding methods. The aim of this paper is to examine the origin and the nut characteristics of the Hungarian bred varieties registered on the National Variety List. The grafted trees for sample collection were planted in spring of 1990 at the Experimental Fields. Strong relationships between nut height and dried nut weight, dried nut weight and kernel weight, nut width and dried nut weight, nut diameter and nut width, dried nut weight and nut diameter, kernel weight and nut diameter as described in the literature was confirmed. Another relation (nut width and kernel weight) showed a strong relation too. All of the Hungarian bred varieties have a unique genetic background. The relation is not apparent between the introduced and the local varieties in all cases. It was concluded from the genetic pattern that the French seedling varieties might have not influenced the Hungarian varieties in the extent it was supposed formerly.

Diversity assessment and selection of superior Persian walnut (Juglans regia L.) trees of seedling origin from North-Western Himalayan region

North-Western Himalayan region of Jammu and Kashmir has significant Persian walnut (Juglans regia L.) trees, most of which are of seedling origin. The region have rich morphological, phenological, and carpological walnut diversity because of the large variability of native walnut, resulting from cross-pollination and seedling propagation. We studied 84 walnut accessions from this region to assess this diversity and identified trees with superior fruit traits. Studying the growth, leaf, phenological, inflorescence, and fruit characteristics show high variability. Nut weight (NW) ranged from 10.04 to 22.66 g, whereas kernel weight (KW) varied from 4.75 to 14.00 g. In this study, we recorded kernel percentage in a broader range from 30.64% to 61.78%, and shell thickness (ST) from 0.98 to 2.83 mm. The principal component analysis revealed that traits related to KW, kernel percentage, ST, NW, nut length (NL), kernel length (KL), and kernel width (KW) accounted for a large proportion of phenotypic variability. Cluster analysis divided accessions into four cluster groups. We identified three promising genotypes, such as JWSP-06, JWSG-43, and JWSD-59, after considering the important traits in a selection index. These promising genotypes will be used to the benefit of conservation research, cultivation, and breeding. Based on the observed differentiation of variation, it is concluded that the magnitude of phenotypic variation in the population under study is high, and the implication of the results will play a vital role in future walnut breeding programmes.

Exploring low-chill genotypes of Persian walnut (Juglans regia L.) in west of Iran

Climate change may cause a significant decline in winter chill over the coming decades and is becoming an important issue for walnut (Juglans regia L.) breeding programs to address. The current study was conducted to identify superior walnut genotypes with low-chilling requirements in western Iran (Ilam province). In the first step, approximately 200 walnut trees of seed-origin were pre-selected based on a questionnaire. Based on this primary evaluation for phenotypic and horticultural traits, 55 of the 200 genotypes were selected for further study. Morphological and phenological traits of these 55 were evaluated for two consecutive years and 11 genotypes were selected as superior. These superior genotypes were moderate to late-leafing with high yield, lateral fruitfulness, thin shells, and light to extra light kernel color. Nut weight, kernel weight and kernel percentage of the 11 superior genotypes ranged between 12.6–15.9 g, 6.9–8.2 g, and 49.7–62.0%, respectively. These superior genotypes had high levels of protein (13.3–18.2%), oil (55.3–65.9%) and PUFAs content. Assessment of chilling requirement of these superior genotypes showed that eight (IrIlZg1, IrIlZg13, IrIlZg7, IrIlZg12, IrIlAa92, IrIlAa47, IrIlAa48 and IrIlAa27) had significantly lower chilling requirements than the cultivar ‘Chandler’. Although having low-chill requirements, these genotypes were moderate to late-leafing. This may be due to their high heat requirement, since chilling requirement together with heat accumulation determines the leafing and flowering date. In conclusion, the selected low chilling requirement genotypes having moderate to late leafing could be used in walnut breeding programs to release commercial cultivars which are suitable for areas with spring frost concerns and expected future loss of winter chill due to climate change.