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K. Francis Salifu, Douglass F. Jacobs, Guillermo Pardillo, and Mary Schott

We examined growth and nitrogen (N), phosphorus (P), potassium (K), and microelement nutrition of grafted black walnut (Juglans nigra L.) seedlings exposed to increasing nutrient supply and grown in the greenhouse for 18 week. Plants were potted and grafted within the first 4 week, then fertigated once each week for a 7-week period with a varying nutrient solution of 20N–4.4P–16.6K that delivered 0, 1160, 2320, and 4620 mg N/plant. Plants were harvested at week 18. There was a positive mean growth response to increased fertilization, although trends were statistically similar across treatments. Leaf nutrient concentration ranged from 22 to 31 g · kg–1 N, 5 to 14 g · kg–1 P, and 19 to 25 g · kg–1 K. The 2320 mg N/plant treatment increased leaf nutrient content 18% to 86% for N, 33% to 303% for P, and 23% to 58% for K compared with the control. Nitrogen efficiency decreased with increased N supply. Increased nutrient retention in the growing medium at higher fertility suggests root plugs could serve as immediate critical nutrient sources for grafted black walnut seedlings after outplanting. Study results suggests nursery fertilization can be used to improve the nutritional quality of grafted black walnut as well as store nutrients in root plugs for later utilization to benefit early establishment success.

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J. W. Van Sambeek, Lisa J. Lambus, and John E. Preece

At monthly intervals for 1 year, one branch was removed from the lower crown of three 30-year-old trees of black walnut (Juglans nigra L.). The basal 1.3 m of each branch was cut into four 32-cm-long segments that were placed horizontally in shallow plastic trays filled with perlite and watered daily with tap water. Branch segments cut early in the dormant season (29 Sept., 31 Oct., or 1 Dec.) or shortly after flushing (6 June) produced few, if any, epicormic sprouts. Approximately half the branch segments cut on 3 Jan. or 3 Feb. produced one sprout that elongated slowly. Most branch segments cut in the late dormant season (2 Mar., 30 Mar., 3 May) or growing season (5 July, 4 Aug., 6 Sept.) produced one or two sprouts >20 mm long. To prepare explants for in vitro culture, the terminal 2.5 cm was harvested when sprouts exceeded 3.0 cm, trimmed of all leaves, and disinfested. Explants were placed vertically in liquid Long & Preece (LP) medium supplemented with 3% sucrose, 0.3 μM TDZ, 0.05 μM IBA, and 1 μM BA. When shoots began to elongate (4 to 6 weeks), they were then placed horizontally on agar-solidified LP medium with liquid LP overlays to induce axillary shoot proliferation. Advantages of forcing epicormic sprouts on large branch segments are: 1) they can be a source of in vitro explant material for 6 to 7 months a year, 2) aseptic cultures can be easily obtained, 3) shoots from the base of branches may show more juvenility than shoots forced from branch tips, 4) softwood shoot wilting is not a problem as with forcing shoots from branch tips, 5) the procedure does not require preparing and changing forcing solutions, and 6) branch segments should have more stored food than dormant branch tips for forcing softwood growth.

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Scott A. Brawner and Michele R. Warmund

In the United States, an average of 11,340 t of hulled Eastern black walnuts ( Juglans nigra L.) is sold at buying stations annually ( Reid et al., 2004 ). The supply of black walnuts does not meet the current demand for kernels (B. Hammons, pers

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Lynn M. Long, John E. Preece, Gerald R. Gaffney, and J. W. Van Sambeek

Cotyledon explants were harvested from immature walnut fruits during July and August 1991. Media consisted of either WPM with 0.1 μM 2,4-D, 5.0 μM TDZ and 1.0 g/liter casein hydrolysate or DKW with 4.4 μM BA, 0.05 μM IBA, 9.3 μM Kinetin and 250 mg/liter l-glutamine. Treatments were arranged factorially with 2 gelling agents, 7 g/liter Sigma agar or 2 g/liter Gelrite and were incubated in light or in darkness. After 4 weeks, all explants were placed on basal DKW with no growth regulators and were cultured in darkness. The best treatment tested was from seeds collected 14 weeks post-anthesis on WPM, agar, and incubation in light (22 embryos/explant, 78% embryogenesis). Use of DKW and gelrite in darkness resulted in 1 embryo/explant and 38% embryogenesis. Up to 90% shoot organogenesis also occurred on cotyledon explants from seeds collected 16 weeks post-anthesis and placed on WPM. Shoots elongated on stationary liquid DKW with 10 μM BA.

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Michele Renee Warmund

Eastern black walnuts ( Juglans nigra ) are prized for their unique kernel flavor when consumed fresh or in confectionary products. Approximately 90% of the crop is currently produced from native seedling trees and hulling of nuts is often delayed

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Michele R. Warmund and J.W. Van Sambeek

Black walnut ( Juglans nigra L.) is native to much of the eastern United States and is highly valued for its nuts and timber. Black walnut trees are monoecious with one to four pistillate flowers in a cluster at distal ends of the current season

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Scott A. Brawner, Michele R. Warmund, and Mark V. Coggeshall

A study was conducted to: 1) evaluate the use of a durometer for determining husk softening and the date of black walnut harvest and 2) elucidate the relationship between husk hardness, kernel color and weight, and date of harvest. Thirty nuts were randomly collected weekly from mature `Sparrow', `Emma K', `Kwik Krop', and `Football' trees from 1 Sept. to 13 Oct. 2004. Husk denting, hardness, and color measurements were recorded immediately after harvest. Husk denting is the method commonly used by growers to determine the optimum time of harvest. Nuts were then hulled within 48 hours and the in-shell fresh weights were recorded. After drying under natural conditions for 5 weeks, kernel color and weights were assessed. On 15 Sept., 99% of `Sparrow' husks dented with a mean durometer value of 54. On 29 Sept., ≥99% of `Emma K' and `Kwik Krop' husks dented with mean durometer values of 63 and 68, respectively. By 13 Oct., 80% of `Football' husks dented with a mean durometer value of 74. From the first harvest date to the time of maximum denting, kernel weight and color (L*, hue values) of `Sparrow' and `Emma K' generally increased. The L* and hue values of `Kwik Krop' were inversely related to increased kernel weight over time. Kernel color of `Football' remained relatively constant as kernel weight increased over time.

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Susan Pearson, John E. Preece, and J.W. Van Sambeek

During Apr. 1999, the lower branches of mature black walnut trees were removed and cut into sections 48 cm long and placed horizontally in plastic flats filled with perlite in a shaded polyethylene-covered greenhouse. Water was applied by drip emitters and care was taken to avoid overhead water contact with the stem sections. Within 2 months, elongating, green, leafy shoots were excised, brought into the laboratory, surface disinfested and placed in vitro onto agar-solidifed Long and Preece (LP) medium with 0.3 μM thidiazuron (TDZ), 0.5 μM indolebutyric acid (IBA), and either 0.1, 1.0, or 10.0 μM benzyladenine (BA). Explants were transferred to fresh medium after 1, 3, and 5 days in vitro and every 2 weeks thereafter. After 3 months in vitro, callus was excised and and explants were all placed on LP medium with 10 μM BA and 0.5 μM IBA for 4 weeks. They were then transferred to LP with 0.3 μM TDZ, 1.0 μM BA, and 0.5 μM IBA for 2 weeks. This 4-2-week alternation of media has continued for more than 6 months. After 4 months in vitro, shoot clusters were subdivided, and individual microshoots recultured. Of the original 260 explants, 30 survived and have been subdivided into 111 cultures. These explants have produced 132 axillary shoots that are also multiplying. Adult black walnut will acclimate and proliferate in vitro, but only with careful attention to detail and regular transfers to fresh medium.

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Sagib B. Khan, John E. Preece, Bradley H. Taylor, and J.W. Van Sambeek

Branch tips (30 to 40 cm long) of adult black walnut were forced in a half-strength solution of Long and Preece medium (LP) salts (minus iron) plus 1 mM 8-hydroxyquinoline citrate (8-HQC). The resulting softwood shoots were surface-disinfested and cut into 1.5-cm-long nodal segments. Explants were placed on two media: Driver and Kuniyuki Walnut medium (DKW) or LP with four plant growth regulator combinations: 5 μM BA with 0.05 μM IBA, 10 μM BA, 1 nM TDZ, or 10 nM TDZ in a factorial arrangement. Gelrite was used as the gelling agent. Explants were transferred to fresh medium on days 1, 3, 5, and 7 after initiation, then weekly. Data recorded 60 days after culture initiation showed more and longer shoots and leaves, greater explant diameter, more green (living) tissue, and less exudation per explant on LP than on DKW. Greatest explant and shoot length were observed when the medium contained 10 nM TDZ. BA (10 μM) and LP were best for long-term maintenance of cultures

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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.