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Mariano F. Galla, Bradley D. Hanson and Kassim Al-Khatib

orchard at the University of California (UC) Davis Plant Science Field Station, near Davis, CA. Walnut trees (cultivar Chandler) were 3 years old and grafted on Paradox ( Juglans hindsii × Juglans regia ) rootstock. The soil was classified as Yolo silt

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John E. Preece and Gale McGranahan

Luther Burbank began making controlled crosses between walnut species in the late 19th century after hearing about a “supposed natural European hybrid walnut.” He crossed Juglans hindsii (northern California black walnut) × J. regia (Persian walnut) and produced progeny that he named ‘Paradox’ because of its extremely fast growth and other “anomalies.” He also crossed two American species, J. hindsii × J. nigra (eastern black walnut), producing ‘Royal’ walnut progeny that were fast-growing and prolific nut producers. A third interspecific hybrid was a cross between J. ailantifolia (Japanese walnut) × J. regia that resulted in extremely vigorous progeny but was not named. He observed segregation in the F2 populations and described giants and dwarfs as reversions to ancestral forms. Luther Burbank also made selections for walnut scion cultivars and was especially interested in thin-shelled nuts. He collected seeds from a J. regia growing in San Francisco because it produced regularly and had very high-quality nuts with relatively thin but poorly sealed shells. He selected one of its seedlings as ‘Santa Rosa Soft-Shell’ and described it as bearing large crops of nuts that were nearly white with thin shells and delicious white meat. Burbank’s contributions to the walnut industry endure to this day, especially through the widespread use of seedling and clonal ‘Paradox’ walnut rootstocks.

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William H. Olson, D.E. Ramos, K. Ryugo and R.G. Snyder

Annual pruning was compared with nonpruning for 8 years and to two biennial pruning treatments for 4 years in a mature full-canopied `Ashley' walnut (Juglans regia L.) orchard. Light penetration and nut distribution through the canopy was improved by pruning. Nut size and percent edible kernel was consistently lower in nonpruned trees than in trees pruned annually or biennially. Yield from annually pruned trees was not significantly different from that of the nonpruned trees because of the removal of fruitful spurs. Yield of biennially pruned trees was similar to annually pruned or nonpruned trees in the year following pruning, but yield was usually greater during years in which trees were not pruned.

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P.B. Catlin and E.A. Olsson

Distillate flower abscission (PFA) was measured for four cultivars of walnut (Juglans regia L.): `Serr', `Sunland', `Howard', and `Chandler'. Mean PFA for `Serr' over 7 years was greater than for the other cultivars and there were no differences among any of the latter. The high PFA potential of `Serr', shown here and earlier, was not expressed in `Sunland', even though both cultivars have one common parent. There was no association of PFA with either of two rootstock. Flowering index did not differ among cultivars and was not related to PFA.

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Chuck A. Ingels, Gale H. McGranahan and Ann C. Noble

To determine if flavor differences could be detected among several Persian walnut (Juglans regia L.) cultivars, difference tests with eight cultivars were conducted using the duo-trio method. No differences were found when `Hartley' was compared to `Vina', `Scharsch Franquette', and `Mayette'. However, `Chandler', `Chico', `Howard', and `Sunland' were significantly different, and paired comparisons were then used to test these cultivars against `Hartley' in terms of several flavor characteristics. No differences in astringency and “walnut flavor” were detected; however, `Chandler' was judged to be sweeter than `Hartley', which was sweeter than `Howard'. `Chico' was found to be the firmest cultivar.

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Ramon Dolcet-Sanjuan, Elisabet Claveria, Robert Gruselle, Adreas Meier-Dinkel, Christian Jay-Allemand and Thomas Gaspar

Various factors were found to influence the in vitro induction and elongation of adventitious roots from walnut shoot microcuttings. Diverse walnut genotypes (Juglans regia, J. nigra × J. regia hybrids) and selected elite J. regia clones were micropropagated throughout the establishment of in vitro shoot-tip cultures. New evidence is presented here that demonstrates the importance of the genotype and juvenility of the plant material on the in vitro rooting ability. Selection of the best adapted genotypes to multiplication and rooting, and rejuvenation of mature clones through repetitive subcultures or micrografting were examined. Adult J. regia clones were rejuvenated through subsequent subcultures and their rooting was consequently improved. The same results were not accomplished by micrografting on juvenile shoots. A differential response to auxin type and concentration was observed for Juglans regia or J. nigra × J. regia clones. A short prerooting culture in multiplication medium, lowering the sucrose concentration in the root elongation medium and increasing the atmospheric carbon dioxide during the root elongation phase affected the number of shoots forming roots as well as the quality of plantlets and roots.

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Keith Woeste, Douglas Shaw, Gale McGranahan and Robert Bernatzky

We characterized a population of hybrids between English walnut and Northern California black walnut (Juglans regia X J. hindsii) and their backcrosses (BC) using both genomic markers and morphological traits. ANOVA and regression methods were used on three years' data to identify a subset of five variables that describe the morphological variability among backcross populations and their parents (R2 = 0.89). Genomic markers were identified using randomly amplified polymorphic DNA (RAPD). A subset of 60 markers specific to the donor species (J. hindsii) were scored in 50 backcrosses to estimate the percent recipient genome in each evaluated BC. The backcrosses were ranked using each method of evaluation; correlation between the ranks was 0.423 and highly significant. Each evaluation method has advantages but neither was able to reliably identify elite progeny.

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L. Carl Greve, Gale McGranahan, Janine Hasey, Ronald Snyder, Kathy Kelly, David Goldhamer and John M. Labavitch

The variation in polyunsaturated fatty acid content of walnut (Juglans regia L.) oils was determined by analysis of samples isolated from specimens growing in four germplasm collections [California (55 cultivars), Washington (64 seedlings), China (12 cultivars), and France (20 cultivars)]. In addition, the impact of within-state geographic differences on oil composition was examined by comparing samples from three California cultivars (`Ashley', `Hartley', and `Franquette') grown in three locations. Local environmental effects on oil composition of `Chico' were also examined by comparing 1) samples collected from shaded and sun-exposed locations of the same trees and 2) samples collected from trees subjected to three irrigation regimes. Polyunsaturated fatty acid content, as a percentage of total fatty acids, ranged from 47.2% in nuts from PI 142323 from France to 81.0% in `Ashley' from California. However, our data indicate that environment, genotype, nut maturity, and their interactions all contribute significantly to variation in the degree of unsaturation of walnut oil.

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X. Deng, S.A. Weinbaum, T.M. DeJong and T.T. Muraoka

Abortion of distillate flowers (PFA) in a protandrous cultivar of walnut (Juglans regia L. cv. Serr) was increased by N deficiency. Starch and N concentrations in wood of 2-year-old twigs decreased to minimal levels during abortion of distillate flowers. Nitrogen reserves in woody tissues were reduced by foliar N deficiency, as were concentrations of sugars and N in vacuum-extracted xylem sap. Abortive distillate flowers ceased growth before spur leaves reached 50% of full expansion. PFA may result from transient deficiencies of C and N during the spring flush of growth. Depletion of storage C and N was accentuated before maturation of distillate flowers in this cultivar by the metabolic demands of many catkins, spur growth, and leaf expansion.

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Keith Woeste, Gale McGranahan and Robert Bernatzky

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.