Walnut Root Research Articles

Transcriptomic Evidence of a Link between Cell Wall Biogenesis, Pathogenesis, and Vigor in Walnut Root and Trunk Diseases.

Crown gall disease (Agrobacterium tumefaciens), crown/root rot disease (Phytophthora spp.), root lesion disease (Pratylenchus vulnus) and tree vigor are key traits affecting the productivity and quality of walnuts in California. Unchallenged hybrid rootstocks were analyzed by RNA-seq to examine pre-formed factors affecting these traits. Enrichment analysis of the differentially expressed genes revealed that the increased expression of cell wall biogenesis-related genes plays a key role in susceptibility to A. tumefaciens, susceptibility to Phytophthora spp. and increased vigor. Analysis of the predicted subcellular loci of the encoded proteins revealed that many gene products associated with vigor and susceptibility were targeted to the plasma membrane and extracellular space, connecting these traits to sustaining barrier function. We observed that RNA processing and splicing, along with predicted nuclear targeting, were associated with resistance to A. tumefaciens, resistance to Phytophthora spp. and low vigor. Four genes within the J. microcarpa QTL region for resistance to A. tumefaciens and Phytophthora spp. were represented among our transcripts, with two of the genes being differentially expressed in association with resistance to A. tumefaciens and decreased vigor. No differential expression related to Phytophthora spp. or P. vulnus resistance was observed in this region. Additionally, the J. microcarpa haplotype expressed more transcripts associated with resistance to A. tumefaciens, Phytophthora spp. and low vigor, but not P. vulnus, than the J. regia haplotype. We also report unique and shared hormone and defense responses associated with each trait. This research suggests a link between cell wall biogenesis, vigor and critical root diseases of walnut.

A Property-Response Approach to Evaluate Acute Toxicity Profile and Pharmacological Quality of Hydro-Alcoholic Extract of Walnut Root Bark Juglans Regia Linn. in Streptozotocin-Induced Diabetic Mode

Juglans regia L., commonly known as walnut, is a valuable medicinal herb with the potency to treat various conditions and illnesses including diabetes, rheumatism, fever and skin illnesses. This study assessed the acute toxicity, anti-diabetic and anti-atherogenic activities of the hydro-alcoholic extract of the walnut root bark of Juglans regia L. (Juglandaceae), in vivo, using Wistar rats. The toxicological effects of the hydro-alcoholic extract of walnut bark Juglans regia L: are still elusive. In order to evaluate the toxicity profile of this plant, rats were orally treated with a single concentration of 2000 mg/kg and observed during a period of two weeks. For the anti-diabetic study, thirty male wistar rats (130–170 g) were randomized into 5 groups (n = 6/group). Groups I and II served as negative and normal controls, respectively. Diabetes was induced in test groups (II–V) using 200 mg/kg of body weight (BW) streptozotocin. Concerning the clincial outcomes, no mortality, morbidity, or abnormal hematological, biochemical and histopathological alterations were observed. Accordingly, J. regia L is considered a non-toxic plant. The extract was found to limit weight loss and reduce blood glucose levels by −32.30% after 14 days of treatment for the anti-diabetic and anti-atherogenic study. The extract also reduced dyslipidaemia. Our data indicated that J. regia L contains bio-compounds that may alleviate chronic hyperglycemia while preventing cardiovascular complications by improving dyslipidaemia. This could be a potential herb for future studies to develop more effective drugs for improving glycemic and cholesterol control.

Two different strategies of Diversispora spurca-inoculated walnut seedlings to improve leaf P acquisition at low and moderate P levels.

Walnut (Juglans regia) is an important nut tree species in the world, whereas walnut trees often face inadequate phosphorus (P) levels of soil, negatively limiting its growth and yield. Arbuscular mycorrhizal fungi (AMF) can colonize walnut roots, but whether and how AMF promotes walnut growth, physiological activities, and P acquisition is unclear. The present study aimed to evaluate the effects of Diversispora spurca on plant growth, chlorophyll component concentrations, leaf gas exchange, sugar and P concentrations, and expression of purple acid phosphatase (PAP) and phosphate transporter (PT) genes in leaves of J. regia var. Liaohe 1 seedling under moderate (100 μmol/L P) and low P (1 μmol/L P) levels conditions. Three months after inoculation, the root mycorrhizal colonization rate and soil hyphal length were 45.6-53.2% and 18.7-39.9 cm/g soil, respectively, and low P treatment significantly increased both root mycorrhizal colonization rate and soil hyphal length. Low P levels inhibited plant growth (height, stem diameter, and total biomass) and leaf gas exchange (photosynthetic rate, transpiration rate and stomatal conductance), while AMF colonization significantly increased these variables at moderate and low P levels. Low P treatment limited the level of chlorophyll a, but AMF colonization did not significantly affect the level of chlorophyll components, independent on soil P levels. AMF colonization also increased leaf glucose at appropriate P levels and leaf fructose at low P levels than non-AMF treatment. AMF colonization significantly increased leaf P concentration by 21.0-26.2% than non-AMF colonization at low and moderate P levels. Low P treatment reduced the expression of leaf JrPAP10, JrPAP12, and JrPT3;2 in the inoculated plants, whereas AMF colonization up-regulated the expression of leaf JrPAP10, JrPAP12, and JrPT3;2 at moderate P levels, although AMF did not significantly alter the expression of JrPAPs and JrPTs at low P levels. It is concluded that AMF improved plant growth, leaf gas exchange, and P acquisition of walnut seedlings at different P levels, where mycorrhizal promotion of P acquisition was dominated by direct mycorrhizal involvement in P uptake at low P levels, while up-regulation of host PAPs and PTs expressions at moderate P levels.

Isolation, Identification, and Characterization of Endophytic Bacillus from Walnut (Juglans sigillata) Root and Its Biocontrol Effects on Walnut Anthracnose

Anthracnose is a major disease of walnut, which seriously reduces the yield and quality of walnut in Yunnan province. Therefore, it is necessary to explore and find a biological control agent for the prevention and control of anthracnose disease. In this study, an endophytic Bacillus WB1, with broad-spectrum antibacterial activity was isolated and screened from healthy walnut roots. The strain WB1 was identified as Bacillus siamensis WB1 based on morphological characteristics, physiological and biochemical tests, and 16S rRNA gene sequence analysis. Bacillus siamensis WB1 produces siderophores and indole-3-acetic acid and solubilizes inorganic phosphate. The strain WB1 not only showed a significant inhibition effect on fourteen phytopathogens, but also showed obvious inhibition on the spore germination of Colletotrichum acutatum. Meanwhile, strain WB1 can code genes for the production of antifungal lipopeptides and generate extracellular hydrolytic enzymes (protease, β-1, 3-glucanase, cellulase, and amylase). In addition, WB1 activated the systemic resistance of the host plant by enhancing the activity of defense enzymes, including phenylalanine ammonia lyase (PAL), peroxidase (POD), and polyphenol oxidase (PPO). The results of greenhouse assays also revealed that B. siamensis WB1 can effectively reduce the occurrence and severity of walnut anthracnose disease. These results also indicated that B. siamensis WB1 is a potential biocontrol agent for walnut anthracnose.

Foliar Application of Selenium Reduces Cadmium Accumulation in Walnut Seedlings

Cadmium (Cd) and selenium (Se) could jointly affect plant growth. To investigate the affect of Se on the Cd accumulation in Juglans regia and the physiological mechanism by which Se regulates Cd-induced oxidative stress, in this study, the effects of different foliar application doses of Se (0 (Se0), 20 (Se20), and 200 (Se200) μM) on J. regia (variety Xinfeng) seedlings under Cd stress (5 mM) were determined. The results show that exogenous application of Se (Se20 and Se200) increased walnut biomass compared with Se0 under Cd stress. Under Cd stress, exogenous application of 20 μM Se increased the catalase (CAT), peroxidase (POD), and ascorbate oxidase (AAO) activities in walnut roots and the CAT and AAO activities in walnut leaves, and exogenous application of 200 μM Se increased the CAT, POD, and AAO activities in walnut roots. Furthermore, under Cd stress, exogenous application of 20 and 200 μM Se both decreased the contents of superoxide (O2•−), hydrogen peroxide (H2O2), and malondialdehyde (MDA) in walnut roots and the content of MDA in walnut leaves. Moreover, application of 20 and 200 μM Se both reduced the accumulation of Cd in the root, wood, bark, and leaves of walnuts, and application of 200 μM Se enhanced Se concentration in the root, wood, bark, and leaves. Overall, exogenous application of Se, especially 200 μM Se, could reduce Cd accumulation and enhance CAT, POD, and AAO activities in Cd-stressed walnut roots, thus alleviating Cd stress. This study provides technical guidance for reducing the effects of Cd stress on walnut growth.

Understanding Arbuscular Mycorrhizal Colonization in Walnut Plantations: The Contribution of Cover Crops and Soil Microbial Communities

Soil microorganisms play a central role in biological soil functioning. One of the beneficial microbiota that has a symbiotic association with most of the plants is arbuscular mycorrhizal fungi (AMF). Nevertheless, little is known about the impact of cover crops—widely used in conservation agriculture or organic farming—on native mycorrhizal fungi. This study was conducted in Southern France, in 20-year-old walnut orchards, where faba bean (Vicia faba Roth) was intercropped. To find whether the native AM fungal community associated with walnut trees was influenced by cover crops and soil microbial communities, analyses of soil physicochemical and microbiological indicators were carried out with roots and soil samples collected from four modalities (walnut in conventional farming with and without cover crops, and walnut in organic farming with and without cover crops). Our results showed that the presence of cover crops mainly influenced the soil microbial abundance and activities in conventional plots. In contrast, cover crops stimulated AM fungal colonization of walnut roots in organic plots, reaching 35% and 54% for arbuscule abundance and mycorrhizal intensity, respectively. In conventional plots, ergosterol and mineral nitrogen contents were mainly correlated with mycorrhizal colonization, while only acid phosphatase activity in soil was positively correlated with mycorrhizal colonization in organic plots. The use of the faba bean showed the great role played by cover crops in the enhancement of walnut trees’ mycorrhizal colonization. Identification of the functional traits of AM fungi sensitive to walnut trees is required to inform decisions in specific agricultural practices.

Antifungal Activity of Bacillus velezensis CE 100 against Anthracnose Disease (Colletotrichum gloeosporioides) and Growth Promotion of Walnut (Juglans regia L.) Trees

Walnut anthracnose caused by Colletotrichum gloeosporioides is a deleterious disease that severely affects the production of walnut (Juglans regia L.). The aim of this study was to assess the antifungal and growth promotion activities of Bacillus velezensis CE 100 as an alternative to chemical use in walnut production. The crude enzyme from B. velezensis CE 100 exhibited chitinase, protease, and β-l,3-glucanase activity and degraded the cell wall of C. gloeosporioides, causing the inhibition of spore germination and mycelial growth by 99.3% and 33.6% at 100 µL/mL, respectively. The field application of B. velezensis CE 100 culture broth resulted in a 1.3-fold and 6.9-fold decrease in anthracnose disease severity compared to the conventional and control groups, respectively. Moreover, B. velezensis CE 100 produced indole-3-acetic acid (up to 1.4 µg/mL) and exhibited the potential for ammonium production and phosphate solubilization to enhance the availability of essential nutrients. Thus, field inoculation of B. velezensis CE 100 improved walnut root development, increased nutrient uptake, enhanced chlorophyll content, and consequently improved total biomass by 1.5-fold and 2.0-fold compared to the conventional and control groups, respectively. These results demonstrate that B. velezensis CE 100 is an effective biocontrol agent against anthracnose disease and a potential plant growth-promoting bacteria in walnut tree production.

Tetracarpidium conophorum (African walnut) Seed Phenolic Extract Modulates the Activities of Selected Enzymes of Cardiovascular and Neurological Relevance

Background: African walnut (Tetracarpidium conophorum) root, leaf and seed are used to treat variety of ailments in ethno-medicine. The objective of this study was to evaluate effects of phenolic extract of T. conophorum seed on some enzymes of cardiovascular and neurological significance. Materials and Methods: Pulverized seed (400g) of T. conophorum was extracted with methanol-acetone using standard extraction procedures. Total phenolic content (TPC) and Gas Chromatography-Mass Spectrophotometry analysis of free phenolic of T. conophorum seed (FPTC) extract were determined. Fifteen albino rats were sacrificed by cervical dislocation. Lung, heart and muscle, and brain tissues were freshly harvested, homogenized using standard laboratory procedures and used as enzyme extracts for angiotensin-1 converting enzyme (ACE), lactate dehydrogenase (LDH), sodium-potassium (Na+/K)+-ATPase, and acetylcholinesterase, AChE assays respectively. Effects of FPTC on ACE, LDH, Na+/K-+ATPase and AChE activities were assessed exvivo. Results and Conclusion: TPC of FPTC was 86.99 mg/GAE/g dry weight. FPTC

Using isotopic labeling to investigate root water uptake in an alley cropping system within Taklimakan Desert Oasis, China

Intercropped trees are becoming popular as an ecological protection screen for a desert oasis from wind-blown sand and sand erosion. However, trees also consume a large amount of water and thus could compete for water resources with other crops. Here, we evaluated the water uptake in a typical alley cropping system within an oasis of the South Taklimakan desert. The system consisted of a poplar tree (Populus L.) used for sand protection and a walnut (Juglans regia L.)-corn (Zea mays Linn.) intercropping system. We evaluated root distribution by isotopically-labeled water (δD) and water uptake (δ18O) via SIAR models as well as the direct inference approach for different periods after tracer injection. We found that the different plants used water from different soil layers. While corn used water mostly from the 0–40 cm soil layer during the jointing (61%) and silking–filling periods (58%), respectively, walnut used water mostly from deeper than 40-cm depth (65%), and poplar used water from both shallow (about 30%, 0–40 cm) and deep soil layers (about 40%, 140–200 cm). Furthermore, while walnut roots can reach 200-cm depth, poplar roots can reach down to 400-cm depth. Due to those root differences, we infer that poplar trees do not necessarily compete for water resources with walnut and corn, and, therefore, can provide a valuable ‘safety-net’ service in this alley cropping system.

Molecular characterization of the Pratylenchus vulnus populations on cereals in Turkey.

Pratylenchus vulnus (walnut root lesion nematode) is one of the most damaging root lesion nematode species worldwide. In this study, 17 populations of P. vulnus obtained from wheat and barley cultivated fields in 2016 to 2017 in Turkey (Karaman and Konya provinces) were identified using real-time PCR and melting curve analysis. Samples provided a single peak at 87.3˚C with real-time PCR. D2 to D3 expansion segments of the 28S rRNA of one population from Cihanbeyli district in Konya province was sequenced and recorded in GenBank (Accession number: MT320536.1). Alignments of the population was identical 98.66% to the populations of P. vulnus available in GenBank (Accs. nos: LT985479.1 and LT965052.1) and 98.65% (Accs. nos: KY424305.1 and KY424304.1).

The origin and biosynthesis of the naphthalenoid moiety of juglone in black walnut

Several members of the Juglandaceae family produce juglone, a specialized 1,4-naphthoquinone (1,4-NQ) natural product that is responsible for the notorious allelopathic effects of black walnut (Juglans nigra). Despite its documented ecological roles and potential for being developed as a novel natural product-based herbicide, none of the genes involved in synthesizing juglone have been identified. Based on classical labeling studies, we hypothesized that biosynthesis of juglone’s naphthalenoid moiety is shared with biochemical steps of the phylloquinone pathway. Here, using comparative transcriptomics in combination with targeted metabolic profiling of 1,4-NQs in various black walnut organs, we provide evidence that phylloquinone pathway genes involved in 1,4-dihydroxynaphthoic acid (DHNA) formation are expressed in roots for synthesis of a compound other than phylloquinone. Feeding experiments using axenic black walnut root cultures revealed that stable isotopically labeled l-glutamate incorporates into juglone resulting in the same mass shift as that expected for labeling of the quinone ring in phylloquinone. Taken together, these results indicate that in planta, an intermediate from the phylloquinone pathway provides the naphthalenoid moiety of juglone. Moreover, this work shows that juglone can be de novo synthesized in roots without the contribution of immediate precursors translocated from aerial tissues. The present study illuminates all genes involved in synthesizing the juglone naphthoquinone ring and provides RNA-sequencing datasets that can be used with functional screening studies to elucidate the remaining juglone pathway genes. Translation of the generated knowledge is expected to inform future metabolic engineering strategies for harnessing juglone as a novel natural product-based herbicide.

Root order-based traits of Manchurian walnut & larch and their plasticity under interspecific competition

Manchurian walnut and larch are key timber species of northeast China but information on (fine) root traits of both species is scarce. Plasticity of root traits in mixed plantations has been studied rarely although this could give important insights into mechanisms of root competition. This study examined root traits by branching order in 30-yr-old monocultures and their plasticity in mixed plantations. In monocultures, Manchurian walnut and larch differed in key fine root traits. Larch roots hold more absorptive root orders, larger diameter and lower specific root length/area. Walnut root orders featured greater cortex:stele ratios, N-concentrations and respiration rates. Under interspecific competition, the proportion of walnut root tips increased, the biomass/length of larch root orders 1–3 decreased. Larch possessed a greater morphological and anatomical plasticity of terminal root orders than walnut. Mycorrhizal colonization rates of walnut were reduced. Both species differed fundamentally in their fine root properties. Absorptive fine root orders reacted plastic under interspecific competition while traits of higher root orders remained unchanged. In mixture, larch roots possessed a greater plasticity in traits related to resource uptake (efficiency) than walnut roots whose reaction norm is suggested to be predominantly based on interference competition via juglone exudation.

Determining tree water acquisition zones with stable isotopes in a temperate tree-based intercropping system

Tree-based intercropping (TBI) systems induce microclimate modifications that can enhance crop performance through water use efficiency. However, the extent of this efficiency is dependent on water acquisition strategies of trees in such agroforestry systems. Charting water uptake zones at the tree-crop root interface in TBI systems is essential for managing water resources, particularly with increasing water stress from a changing climate. In this study, we identify the water uptake zones of Juglans nigra (walnut) and Populus sp. (hybrid of Populus deltoids X Populus nigra clone DN-177) (poplar) intercropped with winter barley (Hordeum vulgare) in a temperate TBI system. Water isotopic analysis using δ18O was conducted on paired soil water and tree xylem water samples in early season (pre-crop) and late season (with crop) collected from the Agroforestry Research Station in Guelph, Ontario, Canada. Two approaches for identifying the water uptake zones were used: direct inference and multiple source mass-balance analysis. Both approaches showed that poplar exhibits a dominant soil water uptake zone at 20 cm in the soil profile in the early season, while the dominant walnut uptake zone was higher in the soil profile at 10 cm. Depth of water uptake was lower in the soil profile (40–70 cm) in the late season for both species. This study indicates: (i) tree species dependent water acquisition zones, and (ii) a shift in tree water acquisition to deeper in the soil profile over the growing season. Using isotopic techniques, we suggest that poplar and walnut root activity stratify below the crop root zone later in the growing season, potentially reducing competition for water resources.

Stacking resistance to crown gall and nematodes in walnut rootstocks

BackgroundCrown gall (CG) (Agrobacterium tumefaciens) and the root lesion nematodes (RLNs) (Pratylenchus vulnus) are major challenges faced by the California walnut industry, reducing productivity and increasing the cost of establishing and maintaining orchards. Current nematode control strategies include nematicides, crop rotation, and tolerant cultivars, but these methods have limits. Developing genetic resistance through novel approaches like RNA interference (RNAi) can address these problems. RNAi-mediated silencing of CG disease in walnut (Juglans regia L.) has been achieved previously. We sought to place both CG and nematode resistance into a single walnut rootstock genotype using co-transformation to stack the resistance genes. A. tumefaciens, carrying self-complimentary iaaM and ipt transgenes, and Agrobacterium rhizogenes, carrying a self-complimentary Pv010 gene from P. vulnus, were used as co-transformation vectors. RolABC genes were introduced by the resident T-DNA in the A. rhizogenes Ri-plasmid used as a vector for plant transformation. Pv010 and Pv194 (transgenic control) genes were also transferred separately using A. tumefaciens. To test for resistance, transformed walnut roots were challenged with P. vulnus and microshoots were challenged with a virulent strain of A. tumefaciens.ResultsCombining the two bacterial strains at a 1:1 rather than 1:3 ratio increased the co-transformation efficiency. Although complete immunity to nematode infection was not observed, transgenic lines yielded up to 79% fewer nematodes per root following in vitro co-culture than untransformed controls. Transgenic line 33-3-1 exhibited complete crown gall control and 32% fewer nematodes. The transgenic plants had thicker, longer roots than untransformed controls possibly due to insertion of rolABC genes. When the Pv010 gene was present in roots with or without rolABC genes there was partial or complete control of RLNs. Transformation using only one vector showed 100% control in some lines.ConclusionsCG and nematode resistance gene stacking controlled CG and RLNs simultaneously in walnuts. Silencing genes encoding iaaM, ipt, and Pv010 decrease CG formation and RLNs populations in walnut. Beneficial plant genotype and phenotype changes are caused by co-transformation using A. tumefaciens and A. rhizogenes strains. Viable resistance against root lesion nematodes in walnut plants may be accomplished in the future using this gene stacking technology.

EFFECTS OF VARIOUS ROOTS ON 'CHANDLER' WALNUT CATKIN AND PISTILLATE BLOOM, TREE GROWTH, YIELD, AND NUT QUALITY

'Chandler' is the leading English walnut cultivar currently being planted in California. Rootstock selection can have significant effects on the performance of the scion in terms of catkin abundance, tree growth, yield efficiency, and the nut quality attributes of percent edible kernel and kernel color. This rootstock trial was planted in March 1999 in a randomized complete block design with six replicates of two trees per treatment at the California State University Farm in Chico. Six treatments are evaluated; three on English walnut roots and three on paradox walnut roots. Rootstock treatments include: own rooted 'Chandler', grafted and not grafted, English 'Waterloo', common paradox, 'Trinta' paradox, and 'Px1' clonal paradox. Own rooted trees have lower catkin abundance and yield efficiency but good nut quality and value. English walnut rootstocks are performing similarly to one another. `Px1' paradox is similar to common paradox in nearly all respects and both stocks produce larger tree size than the 'Trinta' paradox rootstock. Own rooted 'Chandler' trees could provide an advantage in areas with a high incidence of blackline caused by cherry leaf roll virus since without a Northern California black or paradox rootstock English walnut trees can tolerate the disease.

Effect of larch (Larix gmelini Rupr.) root exudates on Manchurian walnut (Juglans mandshurica Maxim.) growth and soil juglone in a mixed-species plantation

The establishment and productivity of a Manchurian walnut (Juglans mandshurica Maxim.) plantation can be improved by inter-planting with larch (Larix gmelini Rupr.) in Northeast China, but the potential mechanism remains obscure. We carried out a series of experiments in a 20-year-old mixed-species plantation, as well as in Manchurian walnut and larch plantations. Manchurian walnut seedlings had difficulty surviving in the Manchurian walnut plantation because their growth was inhibited by their own soil and root exudates. In sharp contrast, Manchurian walnut seedlings grew well in larch and mixed-species plantations. Larch soil and root exudates greatly stimulated the growth of Manchurian walnut seedlings in controlled conditions. In particular, larch root exudates can increase the soil microbial populations, including bacteria, actinomycetes, azotobacter and cellulose-decomposing microorganisms; larch root exudates can also increase the enzyme activities of saccharase, urease, proteinase and polyphenol oxidase. Significant results led to a rapid degradation of the root-exuded phytotoxic juglone from Manchurian walnut. Manchurian walnut root exudates contained juglone at a high concentration of 121.3 ± 6.6 mg g−1, while juglone concentrations in the soil beneath Manchurian walnut trees ranged from 2.9–6.2 µg g−1 soil. It appears from the results that juglone may be released from Manchurian walnut roots into the soil in a sufficient quantity but rapidly degrades due to interactions with soil factors. Furthermore, juglone was more resistant toward degradation in the Manchurian walnut soil (t 1/2 = 7.36 ± 0.63 h) when compared to the larch soil (t 1/2 = 4.66 ± 0.82 h). The results suggest that larch may improve the establishment and productivity of Manchurian walnut in a mixed-species plantation through the release of root exudates.

Competition for water between walnut seedlings (Juglans regia) and rye grass (Lolium perenne) assessed by carbon isotope discrimination and delta18O enrichment.

Container-grown walnut seedlings (Juglans regia L.) were subjected to competition with rye grass (Lolium perenne L.) and to a 2-week soil drying cycle. One and 2 weeks after the beginning of the drought treatment, H2 18O (delta approximately equals +100%) was added to the bottom layer of soil in the plant containers to create a vertical H2 18O gradient. Rye grass competition reduced aboveground and belowground biomass of the walnut seedlings by 60%, whereas drought had no effect. The presence of rye grass reduced the dry weight of walnut roots in the upper soil layer and caused a 50% reduction in lateral root length. Rye grass competition combined with the drought treatment reduced walnut leaf CO2 assimilation rate (A) and leaf conductance (gw) by 20 and 39%, respectively. Transpiration rates in rye grass, both at the leaf level and at the plant or tiller level, were higher than in walnut seedlings. Leaf intrinsic water-use efficiency (A/gw) of walnut seedlings increased in response to drought and no differences were observed between the single-species and mixed-species treatments, as confirmed by leaf carbon isotope discrimination measurements. Measurement of delta18O in soil and in plant xylem sap indicated that the presence of rye grass did not affect the vertical profile of soil water uptake by walnut seedlings. Walnut seedlings and rye grass withdrew water from the top and middle soil layers in well-watered conditions, whereas during the drought treatment, walnut seedlings obtained water from all soil layers, but rye grass took up water from the bottom soil layer only.

Defining competition vectors in a temperate alley cropping system in the midwestern USA; 3. Competition for nitrogen and litter decomposition dynamics.

An experiment was conducted in an 11-year-old black walnut (Juglans nigra L.), red oak (Quercus rubra L.), maize (Zea mays L.) alley cropping system in the midwestern USA to examine the extent of tree-crop competition for nitrogen and decomposition dynamics of tree leaves and fine roots. A below-ground polyethylene root barrier (1.2 m deep) isolated black walnut roots from maize alleys in half the number of plots providing two treatments viz. ‘barrier’ and ‘no barrier’. The percentage of N derived from fertilizer (%NDF) and fertilizer use efficiency (%UFN) were determined using 15N enriched fertilizer. Further, maize grain and stover biomass, tree leaf biomass, tissue N concentration, and N content were quantified in both treatments. The ‘barrier’ treatment resulted in a significantly greater grain (67.3% more) and stover (37.2% more) biomass than the ‘no barrier’ treatment. The %NDF in both grain and stover was higher in the ‘no barrier’ treatment as a result of competition from tree roots for water and mineralized N in soil. Maize plants growing in the ‘no barrier’ treatment had a lower %UFN than those in the ‘barrier’ treatment due to their smaller size and inability to take up fertilizer. Analysis of tree leaf and fine root decomposition patterns revealed faster release of N (39% over 15 days for black walnut and 17.7% for red oak) and P (30% over 15 days for both species) from roots compared to the leaves of both species. Following an early release of P (11.3% over 45 days), red oak leaves exhibited significant immobilization for the rest of the incubation period. The data indicate that competition for N from fertilizer is minimal since nutrient acquisition is not simultaneous among black walnut and maize. However, competition for mineralized N in soil can exist between black walnut and maize depending on water availability and competition. Tree leaves and fine roots can enhance soil nutrient pools through the addition of soil carbon and nutrients. Tree fine roots seem to play a more significant role in nutrient cycling within the alley cropping system because of their faster release of both N and P as compared to leaves. Selection of tree species and their phenology will impact the magnitude and rate of nutrient cycling.

Allelopathy in black walnut ( Juglans nigra L.) alley cropping. I. Spatio-temporal variation in soil juglone in a black walnut–corn ( Zea mays L.) alley cropping system in the midwestern USA

A study was conducted to quantify the spatial and temporal variation in soil juglone (5-hydroxy-1,4-naphthoquinone) in a 10-year-old black walnut (Juglans nigra L.)–corn (Zea mays L.) alley cropping system. Two treatments (‘root barrier’ and ‘no barrier’) were applied to determine if soil juglone in the alley can be minimized by preventing black walnut root growth into the alley. Although no significant seasonal variation in soil juglone existed, a distinct spatial pattern was observed. Juglone concentration decreased as much as 80% as the distance increased to 4.25 m from the tree row. Installation of polyethylene root barriers minimized juglone concentration to trace levels in the alley. However, this treatment increased juglone levels within the tree row as compared to the ‘no barrier’ treatment, probably as a result of increased rooting density within a limited volume of soil.

THE EFFECT OF HYDROXYL‐ION CONCENTRATION ON THE GROWTH OF WALNUT ROOTS

American Journal of BotanyVolume 11, Issue 2 p. 78-84 Article THE EFFECT OF HYDROXYL-ION CONCENTRATION ON THE GROWTH OF WALNUT ROOTS† H. S. Reed, H. S. ReedSearch for more papers by this authorA. R. C. Haas, A. R. C. HaasSearch for more papers by this author H. S. Reed, H. S. ReedSearch for more papers by this authorA. R. C. Haas, A. R. C. HaasSearch for more papers by this author First published: 01 February 1924 https://doi.org/10.1002/j.1537-2197.1924.tb05762.xCitations: 1 †Paper No. 93, University of California, Graduate School of Tropical Agriculture and Citrus Experiment Station, Riverside, California. AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Citing Literature Volume11, Issue2February 1924Pages 78-84 RelatedInformation