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
Crowded conditions in ‘Serr’ walnut orchards predispose trees to initiate numerous catkins which reduce crop potential in 3 ways: a) catkins replace potentially fruitful buds at several nodes; b) they occasionally induce abscission of the terminal bud; and c) they utilize much reserve food, requisite for pistillate flower set, especially, because the cultivar is protandrous.
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.
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.
In vitro initiation and development of shoot-buds of walnuts (Juglans regia L.) was obtained on a defined medium using seedlings as a primary explants. Benzylamino purine (BA) at 40 μM induced multiple shoot formation derived from an abnormal conical shoot. Periodic subculture on a fresh media with 0.4 μM of both indolebutyric acid (IBA) and BA resulted in shoots multiplication.
A rapid method was developed for extraction and purification of free abscisic acid (ABA) from leaves of waterlogged seedlings of Juglans. There were significant increases (6-15 fold) in leaf ABA contents of treated plants reaching maximum levels after the first 12 to 18 hours of waterlogging. With the exception of J. nigra, the leaves of which were desiccated at 24 hr after treatment, ABA content rapidly declined to original levels by 30 hours after waterlogging. Changes in ABA concentration in walnut is likely a secondary expression to the stress induced by waterlogging.
Optimum conditions for in vivo activity of nitrate reductase (NR) from grape leaves (Vitis vinifera L.) were pH 7.5, 100 mm NO3 substrate, and incubation at 40°C. Pretreatment with low concentrations of NO3 (0.05m) at room temperature did not increase activity substantially.
Induction of NR by NO3 substrate was investigated. Enzyme activity was maximum at the highest NO3 levels.
Intact-tissue assay detected NR in mature leaves of grapevines, walnut (Junglans regia L.), plum (Prunus domestica L.), pears (Pyrus communis L.), and sweet cherry (Prunus avium L.) grown under field conditions. In addition, a particulate NR was demonstrated in walnuts and grapevine leaves by in vitro assay.
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.
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.
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.