Viability of walnut, Juglans regia L., pollen was not diminished by storage at subfreezing temperature, as previously indicated. Pollen stored 20 days at −19°C effected high percentages of fruit set in the orchard in 1969. Fruit set of the bagged flowers was relatively low in 1970, but the set effected by pollen stored a year at −19°C was not significantly different from that effected by fresh pollen. Laboratory tests indicated less than 1% germination for both freshly dehisced and stored pollen, and were unreliable for indicating the ability of walnut pollen to effect fertilization.
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.
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.
Exposure to photosynthetically active radiation and the consequent effect on leaf mass per unit leaf area (SLW) and nitrogen (percent dry weight and μg·mm-2) allocation within tree canopies was investigated in walnut (Juglans regia `Serr' and `Hartley') trees. Percent contribution of discrete light flux densities below light saturation (100-700 μmol·s-1·m-2) to the total light exposure of individual spurs, exposed up to 9 hour·day-1 to saturating light (>700 μmol·s-1·m-2), was minimal (<1 hour), indicating that individual spurs were either exposed or shaded most of the day. SLW and N content per unit leaf area of individual spurs were highly correlated (second-order polynomial curve fit) with light exposure within the tree canopy, indicating uneven allocation of available N for optimal utilization. Nitrogen expressed as percent dry weight was not correlated with light exposure and SLW. Leaf N content per leaf area was highly correlated (linear fit) with SLW.
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.
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.
The ability to remove the pistillate flowers and young nuts from precocious lateral-bearing English walnut (Juglans regia L.) cultivars during the first several years following planting would be useful both in reducing competition with vegetative growth and eliminating potential infection sites for blackline disease (cherry leafroll virus). Applications of ethephon shortly after full bloom were shown to effectively remove all or most pistillate flowers depending upon spray timing and ethephon concentration. Moderate phytotoxicity and reduced seasonal growth limit the usefulness of this technique in the field. Removal of staminate flowers (catkins) prior to pollen release may reduce the excessive pistillate flower abscission of the `Serr' cultivar. Applications of ethephon shortly before the onset of pollen shedding were shown to be ineffective in catkin removal.
Somatic embryos derived from walnut (Juglans regia L.) ovule tissues were evaluated to determine whether they were of zygotic or maternal origin. Molecular markers were used to permit evaluation at an early stage, before whole plant development. Somatic embryos developed from potentially apomictic `Sunland' and `Cisco' ovule tissue isolated from bagged putatively unpollinated flowers. Phosphoglucomutase (PGM) isozyme analysis showed that all of these embryos, except one from each cultivar, carry the same zymotype as the maternal tissue. However, restriction fragment length polymorphism (RPLP) analysis combined with isozyme evaluation demonstrated that the tested embryos originated from zygotic rather than maternal tissues. This study demonstrates the application of molecular marker analyses, particularly RFLPs, evaluation of somatic embryo origin.
Black walnut toxicity to crop plants was found to be due to the juglone (5-hydroxy-1, 4-naphthoquinone) in the tree (5). Cook (3) and Massey (9) suggested that toxic material came from the roots of the walnut tree. Bode (2) believed that the toxin came from the leaves. However, the quantity of juglone in the different parts of the walnut tree has not been fully established. Daglish (4) conducted experiments on Juglans regia, in which he suggested that juglone existed in the plant as glucoside of 1,4,5-trihydroxynaphthalene. On hydrolysis it yielded glocose and alpha-hydrojuglone.. This non-toxic hydrojuglone is oxidized to its toxic juglone from exposure to the air or some oxidizing substance from roots of other plants (6). Recent experimental data (Wang, unpublished) showed that 10 ppm commercially purified juglone reduced tomato seedling growth by 50 per cent when roots were immersed in the solution. A 100 ppm application killed the seedlings.
The discovery in Anatolia (western Turkey) of carbonized remains of apples (Malus P.) and at Swiss prehistoric sites of apples, walnuts (Juglans regia L.), bullace (Prunus insititia L.), sweet cherry (Prunus avium L.), and European grapes (Vitis vinifera L.) dating back to 6500 bc shows that man had been collecting and using wild fruits as food for many thousands of years (ref. 1, p. 2 and 3). In Britain, the pollen, wood, and fruits of hazel nut (Corylus avellana L.) frequently have been identified in postglacial deposits (ref. 1, p. 227). Hazel nut is indigenous throughout Europe, western Asia, North Africa, and the Caucasus; it has been suggested that the nuts provided a source of food later replaced by cereals (ref. 1, p. 226).