Genome-Wide Analysis of Transcription Factor R2R3-MYB Gene Family and Gene Expression Profiles during Anthocyanin Synthesis in Common Walnut (Juglans regia L.)

The R2R3-MYB gene family, encoding plant transcriptional regulators, participates in many metabolic pathways of plant physiology and development, including flavonoid metabolism and anthocyanin synthesis. This study proceeded as follows: the JrR2R3-MYB gene family was analyzed genome-wide, and the family members were identified and characterized using the high-quality walnut reference genome "Chandler 2.0". All 204 JrR2R3-MYBs were established and categorized into 30 subgroups via phylogenetic analysis. JrR2R3-MYBs were unevenly distributed over 16 chromosomes. Most JrR2R3-MYBs had similar structures and conservative motifs. The cis-acting elements exhibit multiple functions of JrR2R3-MYBs such as light response, metabolite response, and stress response. We found that the expansion of JrR2R3-MYBs was mainly caused by WGD or segmental duplication events. Ka/Ks analysis indicated that these genes were in a state of negative purifying selection. Transcriptome results suggested that JrR2R3-MYBs were widely entangled in the process of walnut organ development and differentially expressed in different colored varieties of walnuts. Subsequently, we identified 17 differentially expressed JrR2R3-MYBs, 9 of which may regulate anthocyanin biosynthesis based on the results of a phylogenetic analysis. These genes were present in greater expression levels in 'Zijing' leaves than in 'Lvling' leaves, as revealed by the results of qRT-PCR experiments. These results contributed to the elucidation of the functions of JrR2R3-MYBs in walnut coloration. Collectively, this work provides a foundation for exploring the functional characteristics of the JrR2R3-MYBs in walnuts and improving the nutritional value and appearance quality of walnuts.

Is winter coming? Minor effect of the onset of chilling accumulation on the prediction of endodormancy release and budbreak

The buds of perennial plants become dormant in autumn and must integrate the information related to chilling and forcing temperatures to resume their growth in spring. In many studies, the initial date for chilling accumulation (D_CA ) is set arbitrarily using various rules resulting in high variability across studies and sites. To test the relevancy of different rules to set D_CA , sequential models (taking into account or not the negative effect of warm temperature) were optimized by minimizing the sums of squares between observed and predicted values for 34 endodormancy release and 77 budbreak dates for the walnut Juglans regia L. cv Franquette across France. Optimization of these different models highlighted that many of the D_CA rules, incorporating a photoperiod signal on endodormancy induction, were effective (predicted root mean square standard error less than 10 and 8 days for endodormancy onset and bud break, respectively). Furthermore, the use of functions that compute negative chilling accumulation did not improve the performance of the models. Among the different rules, the projections of the best models were explored under different climates (current climate and Representative Concentration Pathways RCP scenarios). The projections revealed a tipping point at a mean annual temperature between 13 and 15°C, beyond which the advance in ontogenic development during ecodormancy does not compensate for the delay in endodormancy release. Although the physiological mechanisms driving the onset of endodormancy may be profoundly altered by global change, they appear to have minimal impact on the way current models predict dormancy and budbreak dates in walnut.

Screening and functional prediction of differentially expressed genes in walnut endocarp during hardening period based on deep neural network under agricultural internet of things

The deep neural network is used to establish a neural network model to solve the problems of low accuracy and poor accuracy of traditional algorithms in screening differentially expressed genes and function prediction during the walnut endocarp hardening stage. The paper walnut is used as the research object to analyze the biological information of paper walnut. The changes of lignin deposition during endocarp hardening from 50 days to 90 days are observed by microscope. Then, the Convolutional Neural Network (CNN) and Long and Short-term Memory (LSTM) network model are adopted to construct an expression gene screening and function prediction model. Then, the transcriptome and proteome sequencing and biological information of walnut endocarp samples at 50, 57, 78, and 90 days after flowering are analyzed and taken as the training data set of the CNN + LSTM model. The experimental results demonstrate that the endocarp of paper walnut began to harden at 57 days, and the endocarp tissue on the hardened inner side also began to stain. This indicates that the endocarp hardened laterally from outside to inside. The screening and prediction results show that the CNN + LSTM model’s highest accuracy can reach 0.9264. The Accuracy, Precision, Recall, and F1-score of the CNN + LSTM model are better than the traditional machine learning algorithm. Moreover, the Receiver Operating Curve (ROC) area enclosed by the CNN + LSTM model and coordinate axis is the largest, and the Area Under Curve (AUC) value is 0.9796. The comparison of ROC and AUC proves that the CNN + LSTM model is better than the traditional algorithm for screening differentially expressed genes and function prediction in the walnut endocarp hardening stage. Using deep learning to predict expressed genes’ function accurately can reduce the breeding cost and significantly improve the yield and quality of crops. This research provides scientific guidance for the scientific breeding of paper walnut.

Identification and Characterization of Wall-Associated Kinase (WAK) and WAK-like (WAKL) Gene Family in Juglans regia and Its Wild Related Species Juglans mandshurica

Wall-associated kinase (WAK) and WAK-like kinase (WAKL) are receptor-like kinases (RLKs), which play important roles in signal transduction between the cell wall and the cytoplasm in plants. WAK/WAKLs have been studied in many plants, but were rarely studied in the important economic walnut tree. In this study, 27 and 14 WAK/WAKL genes were identified in Juglans regia and its wild related species Juglans mandshurica, respectively. We found tandem duplication might play a critical role in the expansion of WAK/WAKL gene family in J. regia, and most of the WAK/WAKL homologous pairs underwent purified selection during evolution. All WAK/WAKL proteins have the extracellular WAK domain and the cytoplasmic protein kinase domain, and the latter was more conserved than the former. Cis-acting elements analysis showed that WAK/WAKL might be involved in plant growth and development, plant response to abiotic stress and hormones. Gene expression pattern analysis further indicated that most WAK/WAKL genes in J. regia might play a role in the development of leaves and be involved in plant response to biotic stress. Our study provides a new perspective for the evolutionary analysis of gene families in tree species and also provides potential candidate genes for studying WAK/WAKL gene function in walnuts.

Transcriptome profiling based on Illumina- and SMRT-based RNA-seq reveals circadian regulation of key pathways in flower bud development in walnut

Flower bud development is a defining feature of walnut, which contributes to the kernel yield, yield stability, fruit quality and commodity value. However, little is known about the mechanism of the flower bud development in walnut. Here, the stages of walnut female flower bud development were divided into five period (P01-05) by using histological observation. They were further studied through PacBio Iso-Seq and RNA-seq analysis. Accordingly, we obtained 52,875 full-length transcripts, where 4,579 were new transcripts, 3,065 were novel genes, 1,437 were consensus lncRNAs and 20,813 were alternatively spliced isoforms. These transcripts greatly improved the current genome annotation and enhanced our understanding of the walnut transcriptome. Next, RNA sequencing of female flower buds at five periods revealed that circadian rhythm-plant was commonly enriched along with the flower bud developmental gradient. A total of 14 differentially expressed genes (DEGs) were identified, and six of them were confirmed by real-time quantitative analysis. Additionally, six and two differentially expressed clock genes were detected to be regulated by AS events and lncRNAs, respectively. All these detected plant circadian genes form a complex interconnected network to regulate the flower bud development. Thus, investigation of key genes associated with the circadian clock could clarify the process of flower bud development in walnut.

Genome-Wide Identification of NAC Transcription Factor Family in Juglans mandshurica and Their Expression Analysis during the Fruit Development and Ripening

The NAC (NAM, ATAF and CUC) gene family plays a crucial role in the transcriptional regulation of various biological processes and has been identified and characterized in multiple plant species. However, genome-wide identification of this gene family has not been implemented in Juglans mandshurica, and specific functions of these genes in the development of fruits remain unknown. In this study, we performed genome-wide identification and functional analysis of the NAC gene family during fruit development and identified a total of 114 JmNAC genes in the J. mandshurica genome. Chromosomal location analysis revealed that JmNAC genes were unevenly distributed in 16 chromosomes; the highest numbers were found in chromosomes 2 and 4. Furthermore, according to the homologues of JmNAC genes in Arabidopsis thaliana, a phylogenetic tree was constructed, and the results demonstrated 114 JmNAC genes, which were divided into eight subgroups. Four JmNAC gene pairs were identified as the result of tandem duplicates. Tissue-specific analysis of JmNAC genes during different developmental stages revealed that 39 and 25 JmNAC genes exhibited upregulation during the mature stage in walnut exocarp and embryos, indicating that they may serve key functions in fruit development. Furthermore, 12 upregulated JmNAC genes were common in fruit ripening stage in walnut exocarp and embryos, which demonstrated that these genes were positively correlated with fruit development in J. mandshurica. This study provides new insights into the regulatory functions of JmNAC genes during fruit development in J. mandshurica, thereby improving the understanding of characteristics and evolution of the JmNAC gene family.

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.

The role of JrPPOs in the browning of walnut explants

BACKGROUND

Tissue culture is an effective method for the rapid breeding of seedlings and improving production efficiency, but explant browning is a key limiting factor of walnut tissue culture. Specifically, the polymerization of PPO-derived quinones that cause explant browning of walnut is not well understood. This study investigated explants of 'Zanmei' walnut shoot apices cultured in agar (A) or vermiculite (V) media, and the survival percentage, changes in phenolic content, POD and PPO activity, and JrPPO expression in explants were studied to determine the role of PPO in the browning of walnut explants.

RESULTS

The results showed that the V media greatly reduced the death rate of explants, and 89.9 and 38.7% of the explants cultured in V media and A media survived, respectively. Compared with that of explants at 0 h, the PPO of explants cultured in A was highly active throughout the culture, but activity in those cultured in V remained low. The phenolic level of explants cultured in A increased significantly at 72 h but subsequently declined, and the content in the explants cultured in V increased to a high level only at 144 h. The POD in explants cultured in V showed high activity that did not cause browning. Gene expression assays showed that the expression of JrPPO1 was downregulated in explants cultured in both A and V. However, the expression of JrPPO2 was upregulated in explants cultured in A throughout the culture and upregulated in V at 144 h. JrPPO expression analyses in different tissues showed that JrPPO1 was highly expressed in stems, young leaves, mature leaves, catkins, pistils, and hulls, and JrPPO2 was highly expressed in mature leaves and pistils. Moreover, browning assays showed that both explants in A and leaf tissue exhibited high JrPPO2 activity.

CONCLUSION

The rapid increase in phenolic content caused the browning and death of explants. V media delayed the rapid accumulation of phenolic compounds in walnut explants in the short term, which significantly decreased explants mortality. The results suggest that JrPPO2 plays a key role in the oxidation of phenols in explants after branch injury.

Evaluation of risk levels of trace elements in walnuts from China and their influence factors: Planting area and cultivar

The potential health risk of trace elements in nut foods has been widely concerned. The accumulations of trace elements by plants in the environment are disturbed by multiple factors. The objective of this work was to investigate the risk levels of trace elements in walnuts and their influence factors (planting area and cultivar). A total of 228 walnut samples were collected from four major walnut producing areas of China. The contents of essential elements for Fe, Cu, Zn, Mo and Se were 35.8, 21.9, 14.8, 0.3 and 0.04 mg/kg, respectively. The contribution of Cu for dietary reference intake was as high as 82.22%. The risk levels of potential toxicological elements and toxic elements within the acceptable limits. Significantly, the hazard quotients (HQs) of Ba and Co were up to 26.14% and 25.31%, respectively. The effect of planting area on trace elements was determined from the aspects of regional distribution and urbanization. Significant differences of essential elements were found between northeast and northwest areas. The urbanization directly influenced toxic elements, which could cause variation up to 85.0% (Pb) and 42.9% (As). Finally, cultivar effect was confirmed that all walnut cultivars were divided into four categories with different trace element characteristics.

Comparative transcriptome analyses provide novel insights into etiolated shoot development of walnut (Juglans regia L.)

In general, genes promoting IAA, CTK GA and ethylene biosynthesis were upregulated, while genes participating in ABA, chlorophyll and starch biosynthesis pathways performed opposite tendency during etiolation. Etiolation as a method for rejuvenation plays an important role in the vegetative propagation of woody plants. However, the molecular mechanism of etiolated shoot development remains unclear. In this study, we investigated changes at different etiolation stages of Juglans regia. The histology and transcriptome of J. regia were analysed using etiolated stems, which were treated in darkness for 30, 60, 90 days. The results showed that the ratios of pith (Pi) diameter/stem diameter (D), cortex (Co) width/D, and phloem (Ph) width/D increased, while the ratio of xylem (Xy) width/D decreased after etiolation, and the difference in these ratios between etiolated stems and the control was more significant at 60 days than 90 days. Differentially expressed genes (DEGs) were significantly enriched in pathways such as plant hormone biosynthesis and signal transduction, chlorophyll biosynthesis and degradation, and starch and sucrose metabolism. The difference in the contents of indole-3-acetic acid (IAA), abscisic acid (ABA), sugar and chlorophyll between etiolated stems and the control increased with increasing treatment duration; in contrast, the concentrations of gibberellin (GA), zeatin (ZT), and starch, as well as the difference between the etiolated stems and control were lowest at 60 days among the three stages. On the whole, the positive effect of etiolation on the rejuvenation of walnut stems changed as the treatment period increased. The present investigation lays a foundation for future studies on the effect of etiolation on rejuvenation and for promoting the efficiency of vegetative propagation.

Animal wastes as fertilizers enhance growth of young walnut trees under soil drought conditions

BACKGROUND

Using nutrient-rich animal wastes as organic fertilizers in agricultural practices is a sustainable method for soil amendment and avoiding environmental pollution. In order to evaluate their practical effect, we applied different proportions of animal waste as fertilizers to wet or dry soils that were either planted or not planted with young walnut trees.

RESULTS

The results showed that animal waste could increase soil C accumulation and carbon to nitrogen (C/N) ratio and reduce soil organic nitrogen and total nitrogen contents as well as the nitrogen to phosphorus (N/P) ratio in the planted group soil. This framework of soil C and N composition (a high C/N ratio) resulted in high N and Mg contents as well as high Cu and Zn contents in the leaves of the young trees as well as a high dry matter weight/leaf N ratio, causing increased leaf photosynthesis, reduced transpiration and relatively high water use efficiency under soil drought conditions. Also, animal wastes as fertilizers caused the branching of walnut to switch from elongation growth to thickening growth under soil drought conditions.

CONCLUSIONS

Principal component analysis and redundancy analysis demonstrated the mechanism by which the soil C/N ratio mediates the flux of available nutrients from the soil to the plant and thereby regulates plant dry matter accumulation and branching architecture under soil drought conditions. The results of this study provide new insights into the improvement of hilly soils using animal waste. © 2020 Society of Chemical Industry.

Flowering in Persian walnut: patterns of gene expression during flower development

BACKGROUND

Flower development and sufficient fruit set are important parameters with respect to walnut yield. Knowledge about flowering genes of fruit trees can help to conduct better molecular breeding programs. Therefore, this study was carried out to investigate the expression pattern of some flowering genes (FT, SOC1, CAL, LFY and TFL1) in Persian walnut (cv. Chandler) during the growing season and winter dormancy.

RESULTS

The results showed that walnut flower induction and initiation in Shahmirzad, Iran occurred in early June and late September, respectively. After meeting chilling and heat requirement, flower differentiation and anthesis occurred in late-March and mid-April to early-May, respectively. Study of flowering gene expression showed that the expression of the FT gene increased in three stages including before breaking of bud dormancy, from late March to late April (coincided with flower differentiation and anthesis) and from late May to mid-June (coincided with flower induction). Like FT, the expression of SOC1 gene increased during flower induction and initiation (mid-May to early-August) as well as flower anthesis (mid-April to early-May). LFY and CAL genes as floral meristem identity genes are activated by FT and SOC1 genes. In contrast with flowering stimulus genes, TFL1 showed overexpression during winter dormancy which prevented flowering.

CONCLUSION

The expression of FT gene activated downstream floral meristem identity genes including SOC1, CAL and LFY which consequently led to release bud dormancy as well as flower anthesis and induction. Also, TFL1 as a flowering inhibitor gene in walnut showed overexpression during the bud dormancy. Chilling accumulation reduced TFL1 gene expression and increased the expression of flowering genes which ultimately led to overcome dormancy.

The walnut genetic resources of INRA: chronological phenotypic data and ontology

OBJECTIVES

Persian walnut (Juglans regia L.), the walnut species cultivated for nut production, is grown worldwide in temperate areas. In this work, chronological phenotypic data have been collected regarding a part of the walnut genetic resources of the French National Institute for Agricultural Research (INRA) of Bordeaux. Using a well described ontology, these data have been collected in order to assess the phenotypic variations among the accessions, and to better manage the germplasm collection. These data can also be helpful for any breeding program as they provide a clear phenotypic characterization of the main cultivars.

DATA DESCRIPTION

This paper introduces a dataset collected for 150 J. regia accessions for a period from 1965 to 2016, and for 3 observation sites, released as comma separated value spreadsheet. It includes observations about phenological traits (e.g. flowering dates), traits related to in-shell walnut (e.g. weight and size), and traits related to kernel (e.g. color). It can be used by other researchers particularly for multi-site phenological studies in the context of climate change since climate data files are also available. In addition, a complete walnut ontology was deposited in this repository and can assist to standardize the management of any walnut germplasm collection.

Applying the AOGCM-AR5 models to the assessments of land suitability for walnut cultivation in response to climate change: A case study of Iran

Due to higher temperatures and lower water availability, climate change is likely to have a major impact on walnut production in the near future. Climate change will alter the land suitability for walnut cultivation around the world, especially in arid and semi-arid regions like Iran. Here, land suitability for the cultivation of walnut (Juglans regia L.) in Iran was determined using the GIS for present and future conditions (2020-2049) with an approach to climate change. Accordingly, data from 375 synoptic stations throughout Iran were gathered for climatic factors including average, minimum and maximum temperatures, relative humidity and chilling requirement. Also, ASTER sensors (Advanced Spaceborne Thermal Emission and Reflection Radiometer) and their data provided this research with cells that make a precision of 150 m (5 s), and the data were used for gauging geological parameters such as altitude and land slope. The electrical conductivity (EC) of soil and water were informed by the data bank of the Iranian Water Resources Management. The results of temperature simulations for the future (2020-2049) were analyzed by 21 AOGCM-AR5 models under the RCP4.5 emission scenario. In the first phase of evaluations, the maps of land suitability were constructed for present conditions by considering a network of the above-mentioned parameters. By combining these layers of information, the final map of land suitability was illustrated for walnut cultivation. In the second phase, the NEX-GDDP was used in order to determine land suitability for the future (2020-2049). The results showed that Iran currently has 582844 km2 of land suitable for walnut cultivation. However, the future will see less suitable lands: the current area will be reduced by 6.19%, from 582844 km2 to 546710 km2. In general, the northern, northwestern and western margins of Iran are currently suitable for walnut cultivation. By approximation, these lands will also be major areas for prospective cultivations of walnut in the future (2020-2049), even though their current stretch will be reduced.

Changes in phenolic profiles of red-colored pellicle walnut and hazelnut kernel during ripening

In studies of secondary metabolites in nuts, many constituents in the kernel remain unidentified due to a high content of phenolic compounds in the pellicle. In the present study, we focused on the investigation of the phenolic and dicarboxylic acid profiles of walnut and hazelnut pellicle-less kernels. High-performance liquid chromatography with diode array and mass spectrometric detection (HPLC-DAD-MSn) was used to carry out the determination of individual phenolics and dicarboxylic acids in brown and red-pellicle walnut and hazelnut. Results show that hexahydroxydiphenic acid (HHDP) di-galloyl hexose isomer, vanillic acid hexoside, quinic acid derivative and catechin are the main constituents of the phenolic profile of walnut, while galloylquinic derivative, caffeoyl hexoside and catechin are the main constituents of the hazelnut kernel. Even though both walnut and hazelnut kernels have a considerably lower content of phenolic compounds and dicarboxylic acids in comparison to the pellicles, when calculated as a percentage of the total mass, the kernel makes a significant contribution to the total phenolic content of the whole nut.

Localized gene expression changes during adventitious root formation in black walnut (Juglans nigra L.)

Cutting propagation plays a large role in the forestry and horticulture industries where superior genotypes need to be clonally multiplied. Integral to this process is the ability of cuttings to form adventitious roots. Recalcitrance to adventitious root development is a serious hurdle for many woody plant propagation systems including black walnut (Juglans nigra L.), an economically valuable species. The inability of black walnut to reliably form adventitious roots limits propagation of superior genotypes. Adventitious roots originate from different locations, and root induction is controlled by many environmental and endogenous factors. At the molecular level, however, the regulation of adventitious root formation is still poorly understood. In order to elucidate the transcriptional changes during adventitious root development in black walnut, we used quantitative real-time polymerase chain reaction to measure the expression of nine key genes regulating root formation in other species. Using our previously developed spatially explicit timeline of adventitious root development in black walnut softwood cuttings, we optimized a laser capture microdissection protocol to isolate RNA from cortical, phloem fiber and phloem parenchyma cells throughout adventitious root formation. Laser capture microdissection permitted high-resolution, site-specific analysis of gene expression that differentiated between participatory and non-participatory root progenitor cells. Results indicated mRNA abundance was altered in all nine rooting-related genes in response to auxin treatment in both juvenile and mature cuttings. SCARECROW LIKE-1 (SCL) had the greatest change in expression in juvenile rooting-competent cells at days 16 and 18, with a 24- and 23-fold increase relative to day 0, respectively. Tissues not linked to root organogenesis had little change in SCL expression at similar time points. AUXIN RESPONSE FACTOR (ARF)6 and ARF8 as well as SHORTROOT expression also increased 2- to 4-fold in rooting-competent tissue. The greatest transcript abundance in rooting-competent cuttings was restricted to root progenitor cells, while recalcitrant cuttings had a diffuse mRNA signal among tissue types.

Identification and Expression of miRNAs Related to Female Flower Induction in Walnut (Juglans regia L.)

Flower induction is an essential stage in walnut (Juglans regia L.) trees, directly affecting yield, yield stability, fruit quality and commodity value. The objective of this study was to identify miRNAs related to female flower induction via high-throughput sequencing and bioinformatics analysis. A total of 123 miRNAs were identified including 51 known miRNAs and 72 novel miRNAs. Differential expression was observed in 19 of the known miRNAs and 34 of the novel miRNAs. Twelve miRNAs were confirmed by RT-qPCR. A total of 1339 target genes were predicted for the differentially expressed miRNAs. The functions of 616 of those target genes had been previously annotated. The target genes of the differentially expressed miRNAs included: (i) floral homeotic protein APETALA 2 (AP2) and ethylene-responsive transcription factor RAP2-7 which were targeted by jre-miRn69; (ii) squamosa promoter-binding protein 1 (SPB1) and various SPLs (squamosa promoter-binding-like protein) which were targeted by jre-miR157a-5p; (iii) various hormone response factors which were targeted by jre-miR160a-5p (ARF18) and jre-miR167a-5p (ARF8) and (iv) transcription factor SCL6 which was targeted by jre-miR171b-3p, jre-miRn46 and jre-miRn49. The KEGG pathway analysis of the target genes indicated that the differentially expressed miRNAs were mainly enriched to ubiquitin mediated proteolysis, RNA degradation and various carbohydrate metabolism pathways. Many miRNAs were detected in J. regia during female flower induction. Some miRNAs (jre-miR157a-5p, jre-miR160a-5p, jre-miR167a-5p, miR171b-3p jre-miRn69 and jre-miRn49) were involved in female flower induction. The results of this experiment will contribute valuable information for further research about the function of miRNAs in flower induction of J. regia and other fruit trees.

Red Walnut: Characterization of the Phenolic Profiles, Activities and Gene Expression of Selected Enzymes Related to the Phenylpropanoid Pathway in Pellicle during Walnut Development

A rare walnut variant with a red seed coat (pellicle) was examined for alterations in its phenolic profile during development. The red-walnut (RW) pellicle was compared with two commonly colored walnut varieties: 'Lara' (brown) and 'Fernor' (light brown). Furthermore, the activities of selected enzymes of the phenylpropanoid- and flavonoid-related pathways and the relative expressions of the structural genes phenylalanine ammonia lyase ( PAL) and anthocyanidin synthase ( ANS) were examined in the pellicles of the three varieties. In the pellicles of the RWs, phenylalanine ammonia lyase (PAL) activity and related PAL expression was most pronounced in August, about one month before commercial maturity, suggesting a high synthesis rate of phenolic compounds at this development stage. The most pronounced differences between the red and light- and dark-brown varieties were the increased PAL activity, PAL expression, and ANS expression in RWs in August. The vibrant color of the RW pellicle is based on the presence of four derivatives of cyanidin- and delphinidin-hexosides.

Comparative Transcriptome Analysis of Genes Involved in Anthocyanin Biosynthesis in Red and Green Walnut (Juglans regia L.)

Fruit color is an important economic trait. The color of red walnut cultivars is mainly attributed to anthocyanins. The aim of this study was to explore the differences in the molecular mechanism of leaf and peel color change between red and green walnut. A reference transcriptome of walnut was sequenced and annotated to identify genes related to fruit color at the ripening stage. More than 290 million high-quality reads were assembled into 39,411 genes using a combined assembly strategy. Using Illumina digital gene expression profiling, we identified 4568 differentially expressed genes (DEGs) between red and green walnut leaf and 3038 DEGs between red and green walnut peel at the ripening stage. We also identified some transcription factor families (MYB, bHLH, and WD40) involved in the control of anthocyanin biosynthesis. The trends in the expression levels of several genes encoding anthocyanin biosynthetic enzymes and transcription factors in the leaf and peel of red and green walnut were verified by quantitative real-time PCR. Together, our results identified the genes involved in anthocyanin accumulation in red walnut. These data provide a valuable resource for understanding the coloration of red walnut.

Spring bud growth depends on sugar delivery by xylem and water recirculation by phloem Münch flow in Juglans regia

During spring, bud growth relies on long-distance transport of remotely stored carbohydrates. A new hypothesis suggests this transport is achieved by the interplay of xylem and phloem. During the spring, carbohydrate demand of developing buds often exceeds locally available storage, thus requiring the translocation of sugars from distant locations like limbs, stems and roots. Both the phloem and xylem have the capacity for such long-distance transport, but their functional contribution is unclear. To address this ambiguity, the spatial and temporal dynamics of carbohydrate availability in extension shoots of Juglans regia L. were analyzed. A significant loss of extension shoot carbohydrates in remote locations was observed while carbohydrate availability near the buds remained unaffected. This pattern of depletion of carbohydrate reserves supports the notion of long-distance translocation. Girdling and dye perfusion experiments were performed to assess the role of phloem and xylem in the transport of carbohydrate and water towards the buds. Girdling caused a decrease in non-structural carbohydrate concentration above the point of girdling and an unexpected concurrent increase in water content associated with impeded xylem transport. Based on experimental observations and modeling, we propose a novel mechanism for maintenance of spring carbohydrate translocation in trees where xylem transports carbohydrates and this transport is maintained with the recirculation of water by phloem Münch flow. Phloem Münch flow acts as a pump for generating water flux in xylem and allows for transport and mobilization of sugars from distal locations prior to leaves photosynthetic independence and in the absence of transpiration.

Addition of Pear Ester With Sex Pheromone Enhances Disruption of Mating by Female Codling Moth (Lepidoptera: Tortricidae) in Walnut Orchards Treated with Meso Dispensers

We evaluated the low-density application of 50 dispensers per hectare, in contrast to the traditional >800 dispensers per hectare in apple orchards, to achieve disruption of communication of adult codling moth, Cydia pomonella (L.), in walnuts, Juglans regia (L.), using several methods. These methods included cumulative catches of male moths in traps baited with sex pheromone (Ph) or codlemone, (E,E)-8,10-dodecadien-1-ol, or a combination of codlemone, pear ester (PE), ethyl (E,Z)-2,4-decadienoate, and acetic acid, and by examining the mating status of females. These data were collected from 2011-2014 in nontreated plots and in similar plots treated with Meso dispensers loaded with codlemone (Ph Meso) or codlemone and PE (Ph + PE Meso). Male moth captures in both the Ph and combination lure traps reduced by 88-96% and 72 to 77%, respectively, compared with traps in the nontreated plots. A significantly higher proportion of female moths were nonmated in plots treated with Ph + PE Meso dispensers (33%) than in plots treated with Ph Meso (18-26%), or left nontreated (13%). In addition, significantly fewer multiple-mated females were trapped in the Ph + PE Meso-treated plots (6%) than in either Ph Meso-treated (13-18%) or nontreated plots (23%). These data suggest that the addition of PE can effectively improve Ph-based disruption of C. pomonella in walnut orchards. In addition, these data suggest that the use of low-density hand-applied dispensers can be an effective and lower-cost approach to manage this pest in the large canopy presented by walnut orchards.

Nitrogen-Mediated Interaction: A Walnut-Aphid-Parasitoid System

The effects of plant quality on natural enemies are often overlooked in planning and executing biological control programs for insect pests in agriculture. Plant quality, however, could help to explain some of the observed variation in effectiveness of biological control, as it can indirectly influence natural enemy populations. In this study, we used the walnut aphid Chromaphis juglandicola (Kaltenbach) to address the effect of increased nitrogen availability to the host plant on parasitism by the specialist parasitoid Trioxys pallidus (Haliday). In laboratory experiments with walnut seedlings, a higher chlorophyll content index of the foliage in response to added nitrogen was correlated with a decrease in the number of mummies produced by female parasitoids over a 24-h period but an increase in the proportion and the size of female offspring. In field sampling of walnut orchards, there was no relationship between the percent parasitism of walnut aphids by T. pallidus and the chlorophyll content index of the trees. Nitrogen fertilizer and plant quality can clearly affect biological control and should be given greater consideration in integrated pest management.

Morphometric Differentiation Among Anastrepha fraterculus (Diptera: Tephritidae) Exploiting Sympatric Alternate Hosts

Anastrepha fraterculus (Wiedemann) is currently considered a complex of cryptic species infesting fruits from Mexico to Argentina and represents an interesting biological model for evolutionary studies. Moreover, detecting and quantifying behavioral, morphological, and genetic differentiation among populations is also relevant to the application of environment-friendly control programs. Here, phenotypic differentiation among individuals coexisting in the wild in a Northern region of Argentina was unveiled and associated with host choice. Six morphometric traits were measured in sympatric flies exploiting three different host species. Phenotypic variation was shown to be host-dependent regardless of geographical or temporal overlap. Flies collected from synchronous alternate hosts (peach and walnut) differed from each other despite the lack of geographical isolation. By contrast, flies emerging from guavas that ripen about two months later than peach and walnut showed no significant differentiation in comparison to flies collected from walnuts, but they differ significantly from flies originating from peaches. This result is consistent with the hypothesis that the same population of flies shifts from walnuts to guavas throughout the year, whereas the population of flies that uses peaches as a host is probably exploiting other alternate hosts when peach availability decreases. Further research is needed to study the underlying mechanism. Results are consistent with previous molecular markers (inter-simple sequence repeat-ISSR) research on flies stemming from the same hosts and the same area, suggesting that differentiation among flies emerging from alternative hosts occurs at both genetic and phenotypic levels. The contribution of host preference in long-term genetic differentiation is discussed.

Changes in soil organic carbon and total nitrogen in croplands converted to walnut-based agroforestry systems and orchards in southeastern Loess Plateau of China

Limited information is available on the effects of agroforestry system practices on soil properties in the Loess Plateau of China. Over the last decade, a vegetation restoration project has been conducted in this area by converting cropland into tree-based agroforestry systems and orchards to combat soil erosion and degradation. The objective of the present study was to determine the effects of land use conversion on soil organic carbon and total nitrogen in southeastern Loess Plateau. The experiment included three treatments: walnut intercropping system (AF), walnut orchard (WO), and traditional cropland (CR). After 7 years of continual management, soil samples were collected at 0-10, 10-30, and 30-50-cm depths for three treatments, and soil organic carbon (SOC) and total nitrogen (TN) were measured. Results showed that compared with the CR and AF treatments, WO treatment decreased both SOC and TN concentrations in the 0-50-cm soil profile. However, similar patterns of SOC and TN concentrations were observed in the AF and CR treatments across the entire profile. The SOC stocks at 0-50-cm depth were 5.42, 5.52, and 4.67 kg m(-2) for CR, AF, and WO treatments, respectively. The calculated TN stocks at 0-50-cm depth were 0.63, 0.62, and 0.57 kg m(-2) for CR, AF, and WO treatments, respectively. This result demonstrated that the stocks of SOC and TN in WO were clearly lower than those of AF and CR and that the walnut-based agroforestry system was more beneficial than walnut monoculture in terms of SOC and TN sequestration. Owing to the short-term intercropping practice, the changes in SOC and TN stocks were slight in AF compared with those in CR. However, a significant decrease in SOC and TN stocks was observed during the conversion of cropland to walnut orchard after 7 years of management. We also found that land use types had no significant effect on soil C/N ratio. These findings demonstrated that intercropping between walnut rows can potentially maintain more SOC and TN stocks than walnut monoculture and that agroforestry is a sustainable management pattern for vegetation restoration in the Loess Plateau area.

Construction of a high-density genetic map using specific length amplified fragment markers and identification of a quantitative trait locus for anthracnose resistance in walnut (Juglans regia L.)

BACKGROUND

Walnut (Juglans regia, 2n = 32, approximately 606 Mb per 1C genome) is an economically important tree crop. Resistance to anthracnose, caused by Colletotrichum gloeosporioides, is a major objective of walnut genetic improvement in China. The recently developed specific length amplified fragment sequencing (SLAF-seq) is an efficient strategy that can obtain large numbers of markers with sufficient sequence information to construct high-density genetic maps and permits detection of quantitative trait loci (QTLs) for molecular breeding.

RESULTS

SLAF-seq generated 161.64 M paired-end reads. 153,820 SLAF markers were obtained, of which 49,174 were polymorphic. 13,635 polymorphic markers were sorted into five segregation types and 2,577 markers of them were used to construct genetic linkage maps: 2,395 of these fell into 16 linkage groups (LGs) for the female map, 448 markers for the male map, and 2,577 markers for the integrated map. Taking into account the size of all LGs, the marker coverage was 2,664.36 cM for the female map, 1,305.58 cM for the male map, and 2,457.82 cM for the integrated map. The average intervals between two adjacent mapped markers were 1.11 cM, 2.91 cM and 0.95 cM for three maps, respectively. 'SNP_only' markers accounted for 89.25% of the markers on the integrated map. Mapping markers contained 5,043 single nucleotide polymorphisms (SNPs) loci, which corresponded to two SNP loci per SLAF marker. According to the integrated map, we used interval mapping (Logarithm of odds, LOD > 3.0) to detect our quantitative trait. One QTL was detected for anthracnose resistance. The interval of this QTL ranged from 165.51 cM to 176.33 cM on LG14, and ten markers in this interval that were above the threshold value were considered to be linked markers to the anthracnose resistance trait. The phenotypic variance explained by each marker ranged from 16.2 to 19.9%, and their LOD scores varied from 3.22 to 4.04.

CONCLUSIONS

High-density genetic maps for walnut containing 16 LGs were constructed using the SLAF-seq method with an F1 population. One QTL for walnut anthracnose resistance was identified based on the map. The results will aid molecular marker-assisted breeding and walnut resistance genes identification.

Root growth dynamics linked to above-ground growth in walnut (Juglans regia)

BACKGROUND AND AIMS

Examination of plant growth below ground is relatively scant compared with that above ground, and is needed to understand whole-plant responses to the environment. This study examines whether the seasonal timing of fine root growth and the spatial distribution of this growth through the soil profile varies in response to canopy manipulation and soil temperature.

METHODS

Plasticity in the seasonal timing and vertical distribution of root production in response to canopy and soil water manipulation was analysed in field-grown walnut (Juglans regia 'Chandler') using minirhizotron techniques.

KEY RESULTS

Root production in walnuts followed a unimodal curve, with one marked flush of root growth starting in mid-May, with a peak in mid-June. Root production declined later in the season, corresponding to increased soil temperature, as well as to the period of major carbohydrate allocation to reproduction. Canopy and soil moisture manipulation did not influence the timing of root production, but did influence the vertical distribution of roots through the soil profile. Water deficit appeared to promote root production in deeper soil layers for mining soil water. Canopy removal appeared to promote shallow root production.

CONCLUSIONS

The findings of this study add to growing evidence that root growth in many ecosystems follows a unimodal curve with one marked flush of root growth in coordination with the initial leaf flush of the season. Root vertical distribution appeared to have greater plasticity than timing of root production in this system, with temperature and/or carbohydrate competition constraining the timing of root growth. Effects on root distribution can have serious impacts on trees, with shallow rooting having negative impacts in years with limited soil water or positive impacts in years with wet springs, and deep rooting having positive impacts on soil water mining from deeper soil layers but negative impacts in years with wet springs.

Photosynthetic Response of Soybean to Microclimate in 26-Year-Old Tree-Based Intercropping Systems in Southern Ontario, Canada

In order to study the effect of light competition and microclimatic modifications on the net assimilation (NA), growth and yield of soybean (Glycine max L.) as an understory crop, three 26-year-old soybean-tree (Acer saccharinum Marsh., Populus deltoides X nigra, Juglans nigra L.) intercropping systems were examined. Tree competition reduced photosynthetically active radiation (PAR) incident on soybeans and reduced net assimilation, growth and yield of soybean. Soil moisture of 20 cm depth close (< 3 m) to the tree rows was also reduced. Correlation analysis showed that NA and soil water content were highly correlated with growth and yield of soybean. When compared with the monoculture soybean system, the relative humidity (RH) of the poplar-soybean, silver maple-soybean, and black walnut-soybean intercropped systems was increased by 7.1%, 8.0% and 5.9%, soil water content was reduced by 37.8%, 26.3% and 30.9%, ambient temperature was reduced by 1.3°C, 1.4°C and 1.0°C, PAR was reduced by 53.6%, 57.9% and 39.9%, and air CO2 concentration was reduced by 3.7μmol·mol(-1), 4.2μmol·mol(-1) and 2.8μmol·mol(-1), respectively. Compared to the monoculture, the average NA of soybean in poplar, maple and walnut treatments was also reduced by 53.1%, 67.5% and 46.5%, respectively. Multivariate stepwise regression analysis showed that PAR, ambient temperature and CO2 concentration were the dominant factors influencing net photosynthetic rate.

Allelopathic activity and chemical constituents of walnut (Juglans regia) leaf litter in walnut-winter vegetable agroforestry system

Walnut agroforestry systems have many ecological and economic benefits when intercropped with cool-season species. However, decomposing leaf litter is one of the main sources of allelochemicals in such systems. In this study, lettuce (Lactuca sativa var. angustata) was grown in the soil incorporated with walnut leaf litter to assess its allelopathic activity. Lettuce growth and physiological processes were inhibited by walnut leaf litter, especially during early growth stage (1-2 euphylla period) or with large amount of litter addition. The plants treated by small amount of leaf litter recovered their growth afterwards, while the inhibition for 180 g leaf litter persisted until harvest. Twenty-eight compounds were identified in the leaf litter, and several of them were reported to be phytotoxic, which may be responsible for the stress induced by walnut leaf litter. Thus, for highest economic value of vegetables such as lettuce, excessive incorporation of leaf litter should be discouraged.

Seasonal variation of the main individual phenolics and juglone in walnut (Juglans regia) leaves

CONTEXT

Walnut [Juglans regia L. (Juglandaceae)] is a rich source of phenolic compounds, including phenolic acids, naphtoquinones and flavonoids. The increasing interest in the powerful biological activities of plant phenolics has outlined the necessity of determining their content in leaves of different walnut cultivars.

OBJECTIVE

In this study, walnut leaves from walnut cultivars, originating from the same orchard and from the same year of production, were analyzed for their content in ellagic acid, rutin, myricetin and juglone. In addition, the seasonal variation of these major individual phenolics from June to August was determined.

MATERIALS AND METHODS

An HPLC method was used for identification and quantification of ellagic acid, rutin, myricetin and juglone contained in the methanol extract of walnut leaves in nine different cultivars grown under the same agricultural, geographical and climatic conditions.

RESULTS

Cultivars and sampling date had statistically significant influence on the phenolics contents in walnut leaves. The results showed that ellagic acid, rutin, myricetin and juglone were more abundant in July 15th samples (average content is 84.62 mg/100 g FW, 98.9 mg/100 g FW, 178.09 mg/100 g FW and 73.81 mg/100 g FW, respectively). Their contents increases similarly in all the cultivars; therefore, the walnut leaves should preferentially be collected until early August, when phenolics content is higher.

DISCUSSION AND CONCLUSION

The results reported here show that genotype and its interaction with the environment could make significant differences in leaf polyphenols. Walnut leaves may become a noticeable source of compounds with health protective potential.

[Relationship between changes of herbaceous plants and Oncomelania hupensis snail distribution under walnut forest of inhibition of snails in mountainous regions of Yunnan Province]

OBJECTIVE

To explore the relationship between the changes of herbaceous plants and Oncomelania hupensis snail distribution under the walnut forest of inhibition of snails in mountainous regions of Yunnan Province.

METHODS

The experimental field was established at Sanying Village of Eryuan County, Yunnan Province, where the "Flourishing Forest and Controlling Snails Project" was implemented. The different stand ages (2, 4, 6, 8, 10 years)of walnut forest in experimental groups were selected based on the method of space replacing time, and the non-stocked land was served as a control group. The growth of forest, change of snails, number, biomass, overcast, height of the herbaceous plant and the soil moisture were investigated.

RESULTS

The crown closure of 6-year-old walnut forest of inhibition of snails was 0.65. There were 11 species of herbaceous plant belonging to 11 genera, 6 families in 10-year-old forest and its crown closure was 0.77. Compared with the control group, the numbers of families, genera, and species of the 10-year-old forest were decreased by 64.71%, 69.44%, and 77.08%, and the biomass, overcast, and height of it decreased by 12.63%, 19%, and 22.18%, respectively. The soil moisture content (0-20 cm) monthly changes were increased obviously with the increase of stand age. There were no snails besides the control group and 2-year-old walnut forest. Compare with the control group, the occurrence rate of frames with living snails in the 2-year-old walnut forest was decreased by 50%, which was 1.25%. The density of living snails was decreased by 60.16%.

CONCLUSIONS

The construction of walnut forest of inhibition of snails in mountainous regions of Yunnan Province are suitable for controlling the growth of herbaceous plants and altering the environment of snails. If the coalescence intercropped with crops is carried out, it is not only beneficial to the construction of good ecological environment, but also improves the utilization efficiencies of land, light, and thermal resource, and the income of peasants.

Roles of JnRAP2.6-like from the transition zone of black walnut in hormone signaling

An EST sequence, designated JnRAP2-like, was isolated from tissue at the heartwood/sapwood transition zone (TZ) in black walnut (Juglans nigra L). The deduced amino acid sequence of JnRAP2-like protein consists of a single AP2-containing domain with significant similarity to conserved AP2/ERF DNA-binding domains in other species. Based on multiple sequence alignment, JnRAP2-like appears to be an ortholog of RAP2.6L (At5g13330), which encodes an ethylene response element binding protein in Arabidopsis thaliana. Real-time PCR revealed that the JnRAP2-like was expressed most abundantly in TZ of trees harvested in fall when compared with other xylem tissues harvested in the fall or summer. Independent transgenic lines over-expressing JnRAP2-like in Arabidopsis developed dramatic ethylene-related phenotypes when treated with 50 µM methyl jasmonate (MeJA). Taken together, these results indicated that JnRAP2-like may participate in the integration of ethylene and jasmonate signals in the xylem and other tissues. Given the role of ethylene in heartwood formation, it is possible JnRAP2-like expression in the transition zone is part of the signal transduction pathway leading to heartwood formation in black walnut.

Genetic differentiation and spatial structure of Geosmithia morbida, the causal agent of thousand cankers disease in black walnut (Juglans nigra)

The main objectives of this study were to evaluate genetic composition of Geosmithia morbida populations in the native range of black walnut and provide a better understanding regarding demography of the pathogen. The fungus G. morbida, and the walnut twig beetle, Pityophthorus juglandis, have been associated with a disease complex of black walnut (Juglans nigra) known as thousand cankers disease (TCD). The disease is manifested as branch dieback and canopy loss, eventually resulting in tree death. In 2010, the disease was detected in black walnut in Tennessee, and subsequently in Virginia and Pennsylvania in 2011 and North Carolina in 2012. These were the first incidences of TCD east of Colorado, where the disease has been established for more than a decade on indigenous walnut species. A genetic diversity and population structure study of 62 G. morbida isolates from Tennessee, Pennsylvania, North Carolina and Oregon was completed using 15 polymorphic microsatellite loci. The results revealed high haploid genetic diversity among seven G. morbida populations with evidence of gene flow, and significant differentiation among two identified genetic clusters. There was a significant correlation between geographic and genetic distance. Understanding the genetic composition and demography of G. morbida can provide valuable insight into recognizing factors affecting the persistence and spread of an invasive pathogen, disease progression, and future infestation predictions. Overall, these data support the hypotheses of two separate, highly diverse pathogen introductions into the native range of black walnut.

The effect of temperature on survival of Pityophthorus juglandis (Coleoptera: Curculionidae)

The walnut twig beetle (Pityophthorus juglandis Blackman) vectors Geosmithia morbida, the causal agent of thousand cankers disease in Juglans, and is particularly damaging to Juglans nigra L. (black walnut). Native hosts of P. juglandis are distributed in the southwestern United States where winter temperatures tend to be higher than those found within the native range of black walnut. To better understand temperature effects on survival of P. juglandis, we initiated studies to determine: 1) seasonal variations in cold tolerance, as measured by the supercooling point (SCP), and 2) upper and lower lethal temperatures (LT). In the lower LT study, Xyleborinus saxeseni (Ratzeberg) was tested for comparison. Insects were either exposed to increasing or decreasing temperatures and then checked for survival. Upper and lower LTs were estimated using a logistic model. For the SCP study, data were grouped into seasons. Seasonal mean SCPs were highest in summer (-15.4°C) and lowest in fall (-18.1°C). The upper lethal limit estimations required to kill 99% of the population (LT99) for adults and larvae were 52.7 and 48.1°C, respectively, and lower limit LT99 estimations for adults and larvae were -18.1 and -18.7°C, respectively. The lower median LT (LT50) of X. saxeseni was -24.7°C. These studies, as well as beetle survival in infested Colorado trees where temperatures reached -29°C in February 2011, suggest P. juglandis could survive the winter in much of the native range of black walnut, but may be limited in trees where temperatures regularly exceed the lower LT.

Detecting nonlinear response of spring phenology to climate change by Bayesian analysis

The impact of climate change on the advancement of plant phenological events has been heavily studied in the last decade. Although the majority of spring plant phenological events have been trending earlier, this is not universally true. Recent work has suggested that species that are not advancing in their spring phenological behavior are responding more to lack of winter chill than increased spring heat. One way to test this hypothesis is by evaluating the behavior of a species known to have a moderate to high chilling requirement and examining how it is responding to increased warming. This study used a 60-year data set for timing of leaf-out and male flowering of walnut (Juglans regia) cultivar 'Payne' to examine this issue. The spring phenological behavior of 'Payne' walnut differed depending on bud type. The vegetative buds, which have a higher chilling requirement, trended toward earlier leaf-out until about 1994, when they shifted to later leaf-out. The date of male bud pollen shedding advanced over the course of the whole record. Our findings suggest that many species which have exhibited earlier bud break are responding to warmer spring temperatures, but may shift into responding more to winter temperatures (lack of adequate chilling) as warming continues.

Are budburst dates, dormancy and cold acclimation in walnut trees (Juglans regia L.) under mainly genotypic or environmental control?

As observed for most stresses, tree frost resistance can be split into two main processes: avoidance and tolerance. Avoidance of freezing is achieved by introducing species only in the climatic context in which the probability of freezing events is very low for the sensitive stages of buds or stems; i.e., when good synchronism exists between the annual cycle and the critical climatic periods. Buds become able to grow only after chilling requirements have been satisfied (endodormancy released) during winter; they subsequently break after heat requirements have been completed (end of ecodormancy) in early spring. Actually, this period is often subject to more or less severe freezing events. Trees are also able to adjust their freezing tolerance by increasing their capacity of extracellular freezing and decreasing the possibility of intracellular freezing through the process of frost acclimation. Both freezing resistance processes (avoidance and tolerance) are environmentally driven (by photoperiod and temperature), but there are also genotypic effects among species or cultivars. Here, we evaluated the degree to which differences in dormancy release and frost acclimation were related to environmental and genetic influences by comparing trees growing in common garden conditions. This investigation was carried out for two winters in lowland and mountain locations on different walnut genotypes differing significantly for budburst dates. Chilling requirement for endodormancy release and heat requirement during ecodormancy were evaluated in all situations. In addition, frost acclimation was assessed by the electrolyte leakage method on stems from the same trees before leaf fall through budburst. No significant differences were observed in chilling requirements among genotypes. Moreover, frost acclimation dynamics were similar between genotypes or locations when expressed depending on chilling units accumulated since 15 September as a time basis instead of Julian day. The only exception was for maximal frost hardiness observed during winter with the timber-oriented being significantly more resistant than fruit-oriented genotypes. Heat requirement was significantly different among genotypes. Thus, growth was significantly faster in fruit-oriented than in wood-oriented genotypes. Furthermore, among wood-oriented genotypes, differences in growth rate were observed only at cold temperatures. Frost acclimation changes differed significantly between fruit- and wood- walnuts from January through budburst. In conclusion, from September through January, the acclimation dynamic was driven mainly by environmental factors whereas from January through budburst a significant genotype effect was identified in both frost tolerance and avoidance processes.

Deciphering the developmental plasticity of walnut saplings in relation to climatic factors and light environment

Developmental plasticity, the acclimation of plants to their local environment, is known to be crucial for the fitness of perennial organisms such as trees. However, deciphering the many possible developmental and environmental influences involved in such plasticity in natural conditions requires dedicated statistical models integrating developmental phases, environmental factors, and interindividual heterogeneity. These models should be able to analyse retrospective data (number of leaves or length of annual shoots along the main stem in the present case). In this study Markov switching linear mixed models were applied to the analysis of the developmental plasticity of walnut saplings during the establishment phase in a mixed Mediterranean forest. In the Markov switching linear mixed models estimated from walnut data sets, the underlying Markov chain represents both the succession and lengths of growth phases, while the linear mixed models represent both the influence of climatic factors and interindividual heterogeneity within each growth phase. On the basis of these integrative statistical models, it is shown that walnut saplings have an opportunistic mode of development that is primarily driven by the changing light environment. In particular, light availability explains the ability of a tree to reach a phase of strong growth where the first branches can appear. It is also shown that growth fluctuation amplitudes in response to climatic factors increased while interindividual heterogeneity decreased during tree development.

[Cold-tolerance of walnut cultivars: a comprehensive evaluation]

In order to speed up the cold-tolerance breeding and improve the cultivation efficiency of walnut to further expand its planting area, six walnut cultivars (lines) including Luguo No. 8, N13-1, Luguo No. 12, N17-24, Taile, and Xiangling were taken as test materials and treated at -15 degrees C, -20 degrees C, -25 degrees C, -30 degrees C, and -35 degrees C, with their tissue's water content, membrane permeability, lipid peroxidation, protective enzyme activities, and osmotic adjustment substances determined to detect the cold resistance of the cultivars (lines). There existed greater differences in the ratio of free water to bound water in the branches of the cultivars (lines). Under low temperature treatment, the branches membrane permeability and malondialdehyde content increased, superoxide dismutase activity presented an ascend-descend-ascend-descend trend, catalase activity decreased after an initial increase, and the soluble protein and proline contents increased, but the variation degrees of these parameters differed with cultivars (lines). After the recovery from low temperature, N17-24 and Luguo No. 12 had a significantly higher germination rate than the other cultivars (lines). By using fuzzy mathematics method, the cold-resistance of test walnut cultivars (lines) was evaluated as N17-24 > Luguo No. 12 > N13-1 > Luguo No. 8 > Taile > Xiangling.

Contrasting the morphology, anatomy and fungal colonization of new pioneer and fibrous roots

Not all roots born as first-order branches are the same and this has important consequences for overall function. We hypothesized that, compared with fibrous roots, pioneer roots are built to live longer at the expense of absorptive capacity. We tested this hypothesis by investigating pioneer and fibrous roots in their first 14 d of life in the arbuscular mycorrhizal tree species: Acer negundo, Acer saccharum, Juglans nigra, Liriodendron tulipifera and Populus tremuloides. Root observations were made with root-access boxes that allowed roots to be sampled at known ages in field-grown trees. Compared to fibrous roots, pioneer roots had larger diameter, lower specific root length, greater average length and a lack of mycorrhizal or nonmycorrhizal fungal colonization. Pioneer roots < 14 d old had more layers of hypodermis with a lower percentage of putative passage cells and more protoxylem groups than similar age fibrous roots. Our results suggest that pioneer roots are constructed for defense against biotic and abiotic challenges, exploration of soil distal to the stem, high fibrous root branching and secondary development with high axial hydraulic conductivity at the expense of mycorrhizal colonization and high absorptive capacity for water and nutrients.

[Soil heavy metals concentration in different type plantations in Harbin City]

In order to understand the soil heavy metals characteristics in different type plantations in Harbin City, 0-10 cm and 10-30 cm soil samples were collected from eight mono-cultured plantations (Larix gmelinii, Pinus sylvestris var. mongolica, Pinus tabulaeformis var. mukdensis, Phellodendron amurense, Juglans mandshurica, Fraxinus mandshurica, Betula platyphylla, and Quercus mongolica) and one mixed plantation (P. sylvestris var. mongolica + F. mandshurica + Pinus koraiensis + P. amurense + B. platyphylla) established in the Urban Forestry Demonstration Research Base of Northeast Forestry University in the 1950s to analyze the concentrations of Cu, Zn, Cd, Ni, Pb, and As, and the comprehensive pollution index was used to assess the soil heavy metals pollution in the plantations, based on the background values of black soil in the City. There existed significant differences in the heavy metals concentration in the same soil layers among different type plantations. In the test plantations, the concentrations of soil heavy metals except As and Ni tended to decrease with soil depth. In the same soil layers, Zn had the highest concentration (62.29-126.35 mg x kg(-1)), while Cd had the lowest one (0.06-0.47 mg x kg(-1)). The abundance and accumulation tendency of test heavy metals in the plantations was in the order of Zn>Pb>Cu(Ni)> Ni(Cu)>As>Cd, and Cd>Pb>Zn>Cu>Ni>As, respectively. There existed positive correlations among soil Cu, Zn, Pb, and Cd concentrations in the plantations except F. mandshurica, and the concentrations of these four heavy metals in the plantations except F. mandshurica and Q. mongolica were positively correlated with soil organic matter, N, P, and K. The concentrations of soil Cd, Pb, Zn, and Cu were higher than the background values, suggesting the anthropogenic inputs; while those of Ni and As were similar to or lower than the background values, suggesting the origin from soil materials. The soil comprehensive pollution index of the plantations was decreased in the order of F. mandshurica > P. amurense > mixed plantation > J. mandshurica > P. sylvestris var. mongolica > P. tabulaeformis var. mukdensis > L. gmelinii >B. platyphylla > Q. mongolica.

The role of tyloses in crown hydraulic failure of mature walnut trees afflicted by apoplexy disorder

In the Central Valley of California, mature walnut trees afflicted with apoplexy disorder exhibit rapid and complete crown defoliation within a few weeks of symptom initiation. Symptoms are typically found throughout the entire crown and are initially expressed as wilting and chlorosis followed by scorching of leaves. Since the cause of apoplexy disorder is unknown, we set out to elucidate the water relations physiology underlying this condition. Stem water potential (Psi(s)) of healthy, asymptomatic trees remained high throughout the 2007 growing season, while that of apoplexy-afflicted trees decreased significantly with the onset of symptoms for both healthy-appearing and symptomatic portions of these trees. Psi(s)s were significantly reduced by at least 0.7 MPa in the lower, middle and upper portions of the symptomatic canopies compared with those from asymptomatic trees. Heat pulse velocities measured in the main trunk at three radial depths consistently decreased prior to the onset of symptoms and eventually reached zero with complete crown defoliation. Comparison of sap flow with predicted water use based on walnut evapotranspiration suggests that stomata of symptomatic trees were closing at higher evaporative demand prior to symptom formation. Specific hydraulic conductivity (K(s)) of symptomatic stems was significantly lower than that of asymptomatic stems, and no detectable K(s) could be measured on several of the symptomatic stem samples. However, shallow root K(s) did not significantly differ between symptomatic and asymptomatic trees, suggesting that hydraulic failure was isolated to the crown of these grafted trees. Light and scanning electron microscopy of stem and trunk sapwood revealed extensive tylose development in vessels throughout the crown of symptomatic trees. Analysis of the formation of tyloses on multiple dates revealed rapid development of these vessel occlusions in conjunction with visual symptom formation and dramatic decreases in sap flow. In 2008, tylose development was associated with elevated ethylene production in the active sapwood of symptomatic trees. The cause of elevated ethylene associated with tylose production and symptom formation is yet to be determined.

A KNAT3-like homeobox gene from Juglans nigra L., JnKNAT3-like, highly expressed during heartwood formation

The value of black walnut (Juglans nigra L.) is affected by the quality and quantity of darkly colored heartwood in its stem. We are exploring the regulation of heartwood production by identifying genes associated with the transition from sapwood to heartwood. Previous microarray data indicated that heartwood formation may be related to programmed cell death (PCD). To test this hypothesis, we analyzed the region of heartwood formation in walnut stems (i.e., the transition zone, TZ) for the expression of 80 ESTs putatively associated with PCD. Semi-quantitative RT-PCR and real-time PCR was performed to detect the expression changes in candidate genes in the TZ and sapwood of trees harvested in summer and fall. The results revealed that the transcript of a clone that encodes a presumed homeobox protein knotted-1-like 3 (KNAT3) was highly expressed in the TZ when compared with other tissues. Analysis of the full-length coding sequence revealed that the black walnut gene contains regions with 67% similarity to Knox1 and Knox2 domains from the Arabidopsis thaliana KNAT3, as well as a putative homeodomain known to be a transcription factor in other plants. JnKNAT3-like transcript was detected in the pith meristem, roots, embryogenic callus, vascular cambium, female flowers, male flowers, green leaves, and partially and fully senescent leaves of black walnut, although transcript abundance varied considerably among tissues. These analyses may provide insight into the mechanism regulating heartwood formation in walnut and other hardwood trees.

[Light competition and productivity of agroforestry system in loess area of Weibei in Shaanxi]

Agroforestry is the most effective way for the restoration of disturbed land on Loess Plateau and the development of poorly local economy. Taking the tree-based intercropping systems of walnut or plum with soybean or pepper in the loess area of Weibei as test objects, the photosynthesis, growth, and yield of soybean (Qindou 8) and pepper (Shanjiao 981) in the systems were studied. The results showed that the photosynthetic active radiation (PAR), net photosynthetic rate (Pn), growth, and yield of individual soybean or pepper plants were significantly decreased, with the effects increased with decreasing distance from tree rows. Leaf water potential was not significantly or poorly correlated with the Pn, growth, and yield of the two crops. However, there were significant positive correlations between the soil moisture content in 10-20 cm layer and the biomass and yield of soybean, and the above-ground biomass of pepper. PAR was highly correlated with the yield of both crops, which indicated that light competition was one of the key factors leading to the decrease of crop yield.

Morphological and molecular methods to identify butternut (Juglans cinerea) and butternut hybrids: relevance to butternut conservation

Butternut (Juglans cinerea L.) is a native, cold-tolerant, hard-mast species formerly valued for its nuts and wood, which is now endangered. The most immediate threat to butternut restoration is the spread of butternut canker disease, caused by the exotic fungus Sirococcus clavigignenti-juglandacearum Nair, Kostichka & Kuntz. Other threats include the hybridization of butternut with the exotic Japanese walnut (Juglans ailantifolia Carr.) and poor regeneration. The hybrids, known as buartnuts, are vegetatively vigorous, highly fecund, more resistant than butternut to butternut canker disease and difficult to identify. We review the vegetative and reproductive morphological traits that distinguish butternut from hybrids and identify those that can be used by field biologists to separate the taxa. No single trait was sufficient to separate butternut from hybrids, but pith color, lenticel size, shape and abundance, and the presence or absence of a notch in the upper margin of leaf scars, can be used in combination with other traits to identify butternuts and exclude most hybrids. In at least one butternut population, reduced symptoms of butternut canker disease were significantly associated with a dark barked phenotype. We also describe two randomly amplified polymorphic DNA (RAPD) markers that differentiate butternuts from hybrids based on DNA polymorphism. Together, these results should assist in the identification and testing of non-hybrid butternut for breeding and reintroduction of the species to its former habitats.

Sucrose (JrSUT1) and hexose (JrHT1 and JrHT2) transporters in walnut xylem parenchyma cells: their potential role in early events of growth resumption

In temperate woody species, the vegetative growth period is characterized by active physiological events (e.g., bud break), which require an adequate supply of soluble sugars imported in the xylem sap stream. One-year-old shoots of walnut (Juglans regia L. cv. 'Franquette') trees, which have an acrotonic branching pattern (only apical and distal vegetative buds burst), were used to study the regulation of xylem sugar transporters in relation to bud break. At the end of April (beginning of bud break), a higher xylem sap sucrose concentration and a higher active sucrose uptake by xylem parenchyma cells were found in the apical portion (bearing buds able to burst) than in the basal portion (bearing buds unable to burst) of the sample shoots. At the same time, xylem parenchyma cells of the apical portion of the shoots exhibited greater amounts of both transcripts and proteins of JrSUT1 (Juglans regia putative sucrose transporter 1) than those of the basal stem segment. Conversely, no pronounced difference was found for putative hexose transporters JrHT1 and JrHT2 (Juglans regia hexose transporters 1 and 2). These findings demonstrate the high capacity of bursting vegetative buds to import sucrose. Immunological analysis revealed that sucrose transporters were localized in all parenchyma cells of the xylem, including vessel-associated cells, which are highly specialized in nutrient exchange. Taken together, our results indicate that xylem parenchyma sucrose transporters make a greater contribution than hexose transporters to the imported carbon supply of bursting vegetative buds.

Walnut (Juglans)

Walnut species are important nut and timber producers in temperate regions of Europe, Asia, South America, and North America. Trees can be impacted by Phytophthora, crown gall, nematodes, and cherry leaf roll virus; nuts can be severely damaged by codling moth and Xanthomonas blight. The long-generation time of walnuts and an absence of identified natural resistance for most of these problems suggest biotechnological approaches to crop improvement. Described here is a somatic embryo based transformation protocol that has been used to successfully insert horticulturally useful traits into walnut. Selection is based on the combined use of the selectable neomycin phosphotransferase (nptII) gene and the scorable uidA gene. Transformed embryos can be germinated or micropropagated and rooted for plant production. The method described has been used to establish field trials of mature trees.

Flowering phenology and wind-pollination efficacy of heterodichogamous Juglans mandshurica (Juglandaceae)

BACKGROUND AND AIMS

Heterodichogamy differs from normal dichogamy, in that it involves two mating types (protogyny and protandry) that occur at a 1 : 1 ratio in a population. Flowering phases of the two mating types are synchronized and reciprocal, which was considered to ensure between-type outcrossing. This study aims to quantify the flowering pattern and pollination efficacy in Juglans mandshurica, a wind-pollinated heterodichogamous tree.

METHODS

The pattern of flowering phenology was monitored within individual trees and pollen traps were used to measure air-borne pollen loads during the spring in 2003 and 2004. Pollen longevity was determined by staining technique. Also a pollen supplementation experiment was performed in 2004 to assess pollen limitation of fruit production.

KEY RESULTS

There was no overlap between sexual functions within individual trees. Flowering periods of the two mating types were reciprocal and synchronous in both 2003 and 2004. Air-borne pollen loads were large, and protogynous and protandrous individuals each produced a high pollination peak, consistent with the two blooming periods. Maximum pollen longevity was about 4 h for protandrous individuals, and 3 h for protogynous individuals. Pollen supplementation did not increase fruit production in either protogynous or protandrous individuals.

CONCLUSIONS

Heterodichogamous flowering in Juglans mandshurica effectively avoids selfing, promotes between-type outcrossing, and leads to efficient pollination in a natural population.

Physiological effects of kaolin applications in well-irrigated and water-stressed walnut and almond trees

BACKGROUND AND AIMS

Kaolin applications have been used to mitigate the negative effects of water and heat stress on plant physiology and productivity with variable results, ranging from increased to decreased yields and photosynthetic rates. The mechanisms of action of kaolin applications are not clear: although the increased albedo reduces leaf temperature and the consequent heat stress, it also reduces the light available for photosynthesis, possibly offsetting benefits of lower temperature. The objective of this study was to investigate which of these effects are prevalent and under which conditions.

METHODS

A 6% kaolin suspension was applied on well-irrigated and water-stressed walnut (Juglans regia) and almond (Prunus dulcis) trees. Water status (i.e. stem water potential, psi(s)), gas exchange (i.e. light-saturated CO2 assimilation rate, Amax; stomatal conductance, g(s)), leaf temperature (T(l)) and physiological relationships in treated and control trees were then measured and compared.

KEY RESULTS

In both species, kaolin did not affect the daily course of psi(s) whereas it reduced Amax by 1-4 micromol CO2 m(-2) s(-1) throughout the day in all combinations of species and irrigation treatments. Kaolin did not reduce g(s) in any situation. Consequently, intercellular CO2 concentration (C(i)) was always greater in treated trees than in controls, suggesting that the reduction of Amax with kaolin was not due to stomatal limitations. Kaolin reduced leaf temperature (T(l)) by about 1-3 degrees C and leaf-to-air vapour pressure difference (VPD(l)) by about 0.1-0.7 kPa. Amax was lower at all values of g(s), T(l) and VPD(l) in kaolin-treated trees. Kaolin affected the photosynthetic response to the photosynthetically active radiation (PAR) in almond leaves: kaolin-coated leaves had similar dark respiration rates and light-saturated photosynthesis, but a higher light compensation point and lower apparent quantum yield, while the photosynthetic light-response curve saturated at higher PAR. When these parameters were used to model the photosynthetic response curve to PAR, it was estimated that the kaolin film allowed 63% of the incident PAR to reach the leaf.

CONCLUSIONS

The main effect of kaolin application was the reduction, albeit minor, of photosynthesis, which appeared to be related to the shading of the leaves. The reduction in T(l) and VPD(l) with kaolin did not suffice to mitigate the adverse effects of heat and water stress on Amax.

[Effects of Larix gmelini aqueous extracts on seed germination and seedling growth of Juglans mandshurica]

In this study, Juglans mandshurica seeds were drenched in different concentrations (12.5, 25, 50 and 100 g x kg(-1)) aqueous extracts from adult Larix gmelini root, bark, branch and leaf for one week, and cultured in greenhouse in mid-April, 2004. The germination rate and germination index were calculated 18 days later, and the seedling's height and collar diameter were measured on August 1st. The results suggested that 25 g x kg(-1) root and 12.5 g x kg(-1) bark and branch aqueous extracts of L. gmelini could significantly increase the seed germination rate and germination index of J. mandshurica (P < 0.05), and 12.5 g x kg(-1) root and bark aqueous extracts could significantly increase the seedling height and collar diameter (P < 0.05). High concentrations of L. gmelini leaf aqueous extract could significantly promote the seed germination and seedling growth of J. mandshurica, but the promotion effect decreased gradually with decreasing concentration of leaf aqueous extract. The promotion of L. gmelini aqueous extracts to J. mandshurica seed germination and seedling growth might be related to the allelopathy between these two tree species.

Nitrogen availability, local light regime and leaf rank effects on the amount and sources of N allocated within the foliage of young walnut (Juglans nigra x regia) trees

Early season leaf growth depends largely on nitrogen (N) provided by remobilization from storage, and many studies have tested the effect of N availability to roots on the amount of N provided for new leaf development by remobilization. Although it is well known that the light regime experienced by a leaf influences the amount of N per unit leaf area (LA), the effect of the local light regime on the amount of N derived either directly from root uptake or from remobilization for early season leaf growth has never been tested at an intra- canopy scale. The objective of this study was to quantify the relative importance of (1) N availability to roots, (2) local light regime experienced by the foliage (at the shoot scale) and (3) leaf rank along the shoot, on the total amount of N allocated to leaves and on the proportions of N provided by remobilization and root uptake. To quantify the importance of N uptake and remobilization as sources of leaf N, potted hybrid walnut trees (Juglans nigra L. x regia L.) were grown outdoors in sand and fed with a labeled ((15)N) nutrient solution. By removing the apical bud, the trees were manipulated to produce only two shoots. The experimental design had two factors: (1) high (HN; 8 mol N m(-3)) and low (LN; 2 mol N m(-3)) N availability; and (2) high (HL; 90% of incident photosynthetically active photon flux (PPF)) and low (LL; 10% of incident PPF) light. Total leaf N per tree was unaffected by either N availability or irradiance. The HN treatment increased the amount of leaf N derived from root uptake at the whole-tree scale (typically around 8 and 2% in the HN and LN treatments, respectively). Nitrogen allocation within foliage of individual trees was controlled by the local light regime, which strongly affected individual leaf characteristics as leaf mass per unit LA and area- based amount of leaf (N(a)). Decreasing the light availability to a branch decreased the amount of N allocated to it, benefiting the less shaded branches. In contrast, shading of the lower branch did not affect the fraction of total leaf N remobilized for either the lower, shaded branch or the upper, unshaded branch. The relevance of these findings for tree growth modeling is discussed.

Experimental analysis of the role of water and carbon in tree stem diameter variations

The variations of stem diameter as they can be accurately measured by Linear Variable Differential Transformer (LVDT) reflect the addition of four components: irreversible radial growth, reversible living-cell dehydration/rehydration, thermal expansion and contraction, and expansion of dead conducting elements due to the increase and relaxation of internal tensions. The correct interpretation of LVDT signals, with respect to the practical applications, should make an exact distinction between these four components. This paper describes a set of two experiments with potted hybrid walnut trees. Double girdling, water stress, and duration of the day versus night periods were used in the phytotron as experimental factors to induce variations of the carbon and water status of plant tissues. The latter were assessed, respectively, by water potential and transpiration, and by local stem respiration and carbohydrate content. The results are interpreted in terms of carbon or water limitation effects on stem diameter variations where radial growth and tissue elasticity could be distinguished. Moreover, they suggest no or very low involvement of CO2 originating from a distance, i.e. carried by the transpirational flux of xylem sap, in the total stem CO2 efflux rate.