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  • Author or Editor: Gale McGranahan x
  • Journal of the American Society for Horticultural Science x
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A first backcross population of walnuts {[Juglans hindsii (Jeps.) Jeps. × Juglans regia L.] × J. regia} was used to evaluate the correlation between morphological (statistical) and genetic distance during introgression. Five traits based on leaf morphology were identified to quantitate the morphology of the parental species, their F1 hybrids, and the backcrosses to each parent. These traits were used to evaluate the morphological similarity of first backcrosses to J. regia using Mahalanobis' distance. The amount of genomic introgression of each backcross was estimated using 59 randomly amplified polymorphic DNA (RAPD) and 41 restriction fragment-length polymorphism (RFLP) genetic markers that identify polymorphisms between J. regia and J. hindsii. A smaller scaffold set of markers was also identified using published linkage data. The correlation between the measures of morphological and genomic introgression for the first backcrosses was low (0.23) but significant. The results suggest that selection based on morphology during backcrossing will not be an effective way to recover recurrent parent genome.

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Abstract

Data on 15 traits collected from 30 walnut selections were analyzed for changes in relation to both clone and rootstock age. Data collection began at first flowering (age 3 or 4) and continued annually for up to 28 years on each clone. Significant correlations were found between seasonal timing of the expression of phenological traits and clone age. The general trend was towards earlier leafing, bloom, and time of nut maturity as the clone aged. Correlations with rootstock age were lower than with clone age for phenological traits. Shell and kernel trait expression was more highly correlated with rootstock age than clone age, suggesting that changes may be due to vigor and other effects of grafting rather than aging per se. Estimates of the age of stabilization for phenological traits ranged between 9 and 18 years from germination. It is suggested that changes in leafing, bloom, and nut maturity dates be considered prior to commercial release of walnut cultivars.

Open Access

Twenty-five random decamer primers were used to evaluate the level of polymorphism between Persian walnut and the Northern California black walnut. Sixty-six randomly amplified polymorphic DNA (RAPD) markers were identified using an interspecific walnut backcross population [(Juglans hindsii × J. regia) × J. regia]. Segregation data from these polymorphisms were joined to a previously published set of restriction fragment-length polymorphism (RFLP) marker data to expand the genetic map of walnut to 107 markers in 15 linkage groups.

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The utility of intersimple sequence repeat (ISSR) markers for identification of English or Persian walnut (Juglans regia L.) cultivars was explored. Four cultivars were screened with 47 ISSR primers; eight of these primers, which generated reproducible and informative data, were selected for further study. Two individuals from each of 48 cultivars, including many currently important in the California walnut industry as well as accessions from Europe and Asia, were then examined with the eight ISSR primers. Polymerase chain reaction (PCR) products were separated on agarose gels and stained with ethidium bromide. Fifty-four bands were scored as present or absent in each cultivar; of these, 31 (57%) were polymorphic among the 48 cultivars. Combined data from the eight ISSR primers provided a unique fingerprint for each of the cultivars tested. Fifteen of the cultivars could be distinguished from all others with just one primer, 31 with a minimum of two primers, and two required three primers. Pairwise genetic distances between the cultivars were calculated and a dendrogram was generated using the neighbor-joining algorithm. Some of the groupings in the dendrogram corresponded to groups which, based on known pedigrees, are genealogically closely related. Others included accessions from diverse genetic and/or geographic origins. These results can be attributed to a combination of the limitations of the ISSR method for inferring genetic relationships, on the one hand, and the complex history of walnut cultivar development involving extensive exchange and breeding of germplasm from different geographic regions, on the other.

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Walnut blight of English walnut (Juglans regia L.), incited by Xanthomonas campestris pv. juglandis (Pierce) Dowson, causes significant crop loss in California. To assess levels of resistance in walnut germplasm, leaves and nuts of mature walnut genotypes were inoculated with X. campestris pv. juglandis. Significant differences were found among cultivars in size and frequency of lesions on leaves and in frequency of abscission of diseased leaves. Cultivars also varied in frequency of abscission of nuts following infection and in marketability of infected nuts. Afthough there was considerable variation in disease levels over 2 years, leaves of PI 159568 consistently received significantly higher disease ratings than leaves of `Chandler' or `Adams'. Nuts of `Adams', `Payne', PI 18256, and `Sinensis 5' abscised less frequently following inoculation than nuts of other cultivars. In addition, the quality of infected nuts that did not abscise was consistently better for PI 18256 and `Sinensis 5'. The rank of cultivars for levels of disease in inoculated leaves was not significantly correlated with the rank of cultivars for frequency of infestation of dormant buds associated with infected foliage. The apparent resistance of walnut germplasm may be affected by the abscission or necrosis of infected tissues.

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Abstract

Genetic variation in vigor, phenology, and branching in juvenile seedlings of Juglans californica and J. hindsii was investigated. Significant differences between species were detected in traits that reflect vigor (height, diameter, volume), in phenological traits (dates of leafing out and leaf drop), and in branching. J. californica was generally more vigorous, more branched, leafed out earlier, and dropped leaves later than J. hindsii. When species were analyzed separately, seed source was a highly significant source of variation for phenological traits and branching in J. hindsii. Upper estimates of heritability for phenological traits ranged from 0.47 to 0.88. The results of this study suggest that J. hindsii may have had a history of differentiation and adaptation to latitude, elevation, or other climatic characters. Conservation of germplasm resources in J. hindsii will be essential to maintain the purity of the species and to provide resources for studying the species and breeding rootstock for J. regia, the Persian walnut.

Open Access

Abstract

A system was developed to transform walnut cultivars using the natural gene transfer system of Agrobacterium tumefaciens. We report the infection of English walnut (Juglans regia L.), northern California black walnut (Juglans hindsii), and their F1 hybrid ‘Paradox’ with A. tumefaciens carrying various recombinant derivatives of the tumor-inducing (Ti) plasmids, pTiA6 and pTiB6S3. The three walnut species, each represented by a single micropropagated clone, were found to be equally susceptible to Agrobacterium-induced tumor formation in vitro. Stable lines were established from tumors induced on each clone, and, unlike normal stem callus, these tumor cells grew rapidly in culture media without exogenous plant hormones. High-voltage paper electrophoretic analysis revealed the presence of opines in the walnut tumor tissue. The presence of a foreign gene was demonstrated by expression of a chimeric bacterial gene that encodes resistance to the antibiotic kanamycin, and also by the presence of foreign DNA sequences in genomic DNA isolated from tumors.

Open Access

The role of pollen in abscission of pistillate flowers of Persian walnut (Juglans regia L.) cv. Serr was investigated over a 4-year period by controlled pollinations and pollen counts. Self-pollen, pollen from other walnut selections or cultivars, or dead pollen was applied at high and low doses to pistillate flowers enclosed in pollination bags. Unbagged, open-pollinated flowers and bagged, nonpollinated flowers served as controls. In all cases, presence of pollen significantly increased the probability of pistillate flower abscission (PFA). Dead pollen resulted in as much PFA as live pollen. Counts of pollen grains confirmed that PFA-type flowers had significantly more pollen than normal flowers. In the fourth year `Serr' pollen was applied to unbagged flowers of `Serr' and ten other Persian walnut cultivars, and the amount of PFA on the artificially pollinated flowers was significantly higher than on the open-pollinated flowers, while the control flowers dusted with talc or pine pollen had almost no PFA. These results clearly indicate that excess pollen is involved in pistillate flower abscission in `Serr' walnut and suggests that other cultivars may also be sensitive to pollen load. This phenomenon may have implications in the biology of selfing and evolution.

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One hundred and forty-seven primer pairs originally designed to amplify microsatellites, also known as simple sequence repeats (SSR), in black walnut (Juglans nigra L.) were screened for utility in persian walnut (J. regia L.). Based on scorability and number of informative polymorphisms, the best 14 loci were selected to analyze a diverse group of 47 persian walnut accessions and one J. hindsii (Jepson) Jepson ex R.E. Sm × J. regia hybrid (Paradox) rootstock. Among the 48 accessions, there were 44 unique multi-locus profiles; the accessions with identical profiles appeared to be synonyms. The pairwise genetic distance based on proportion of shared alleles was calculated for all accessions and a UPGMA (unweighted pair group method with arithmetic mean) dendrogram constructed. The results agree well with what is known about the pedigree and/or origins of the genotypes. The SSR markers distinguished pairs of closely related cultivars and should be able to uniquely characterize all walnut cultivars with the exception of budsports. They provide a more powerful and reliable system for the molecular characterization of walnut germplasm than those previously tested. These markers have numerous applications for the walnut industry, including cultivar identification, verification of pedigrees for cultivar and rootstock breeding programs, paternity analysis, and understanding the genetic diversity of germplasm collections.

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Walnuts (Juglans spp.) are difficult-to-root woody plants. The rolABC genes (rolA + rolB + rolC), derived from the bacteria Agrobacterium rhizogenes, have been shown to increase the rooting potential of other difficult-to-root woody plants. We inserted the rolABC genes into somatic embryos of a `Paradox' hybrid (J. hindsii × J. regia) clone PX1 using the A. tumefaciens gene transfer system. A transgenic sub-clone, designated PX1 rolABC 2-2 was selected and compared to the untransformed clone for a variety of phenotypic characteristics, including rooting potential. Transformed and untransformed shoots were budded onto seedling J. regia rootstock in the greenhouse and established in the field. Transformed trees displayed reduced internode length, an increase in lateral branching, and wrinkled leaves. In another test, a commercial persian walnut cultivar J. regia `Chandler' was grafted onto rooted cuttings of both the untransformed and transformed plants. The presence of the rolABC genes in the rootstock had no visible effects on the grafted scion. Several of these trees were excavated from the field and the root systems of each genotype were examined for root number, diameter, and biomass. Trees with the rolABC rootstock had significantly more small diameter roots compared to the controls and less recovered biomass. Tests of the rooting potential of leafy semi-hardwood cuttings for two years resulted in 14% to 59% rooting of the transformed cuttings compared to 51% to 81% rooting of the control. Both transformed hardwood cuttings and microshoots in tissue culture also rooted significantly less (52% and 29% respectively) than untransformed hardwood cuttings and tissue cultured shoots (82% and 54% respectively). Thus, although the rolABC genes induced a shorter internode length and a more fibrous root system (typical of rol-tranformed plants), they were not useful for increasing the rooting potential, and as rootstock they did not affect the phenotype of the scion.

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