National Plant Germplasm System Research Articles

Genetic characterization of the brown lint phenotype for desi cotton (Gossypium arboreum) accession PI 408765 (cv. ‘Sanguineum‐1’)

AbstractCotton fibre can show a range in brown colouration, and these coloured lint fibres can be used for textile production reducing the environmental and human health hazards associated with synthetic dyes; however, coloured cotton cultivars have lower yield potential and reduced lint quality. A greater understanding of the genetics of lint colouration would be useful for the development of improved cultivars. Gossypium arboreum L. accession PI 408765 (cv. ‘Sanguineum‐1’) having the brown lint phenotype was identified from the United States Department Agriculture, National Plant Germplasm System cotton collection and crossed with G. arboreum accessions PI 529729 (cv. ‘Garo Hill’) having white lint and with PI 615733 (cv. ‘Zi‐Hua Guang Zi’) having brown lint. Lint colour data from the F2 populations support a single incomplete dominant gene model for the brown lint phenotype. Additionally, the brown lint phenotypes for PI 408765 and PI 615733 are controlled by two independent genes. Plants in these populations showed a range of brown lint colouration, and these data would be useful to further characterize the mechanisms controlling brown lint development.

Genetic characterization of melon accessions in the U.S. National Plant Germplasm System and construction of a melon core collection

Melon (C. melo L.) is an economically important vegetable crop cultivated worldwide. The melon collection in the U.S. National Plant Germplasm System (NPGS) is a valuable resource to conserve natural genetic diversity and provide novel traits for melon breeding. Here we use the genotyping-by-sequencing (GBS) technology to characterize 2083 melon accessions in the NPGS collected from major melon production areas as well as regions where primitive melons exist. Population structure and genetic diversity analyses suggested that C. melo ssp. melo was firstly introduced from the centers of origin, Indian and Pakistan, to Central and West Asia, and then brought to Europe and Americas. C. melo ssp. melo from East Asia was likely derived from C. melo ssp. agrestis in India and Pakistan and displayed a distinct genetic background compared to the rest of ssp. melo accessions from other geographic regions. We developed a core collection of 383 accessions capturing more than 98% of genetic variation in the germplasm, providing a publicly accessible collection for future research and genomics-assisted breeding of melon. Thirty-five morphological characters investigated in the core collection indicated high variability of these characters across accessions in the collection. Genome-wide association studies using the core collection panel identified potentially associated genome regions related to fruit quality and other horticultural traits. This study provides insights into melon origin and domestication, and the constructed core collection and identified genome loci potentially associated with important traits provide valuable resources for future melon research and breeding.

DNA Variation in a Diversity Panel of Tomato Genetic Resources

A diversity panel of 190 National Plant Germplasm System (NPGS) tomato (Solanum lycopersicum) accessions was genotyped using genotyping by sequencing. These originated from 31 countries and included fresh market, ornamental, processing, breeders’ lines, landraces, and home gardening types, as well as six different accessions of the economically valuable cultivar San Marzano. Most of the 34,531 discovered single nucleotide polymorphisms were rare and therefore excluded from downstream analyses. A total of 3713 high-quality, mapped single nucleotide polymorphisms that were present in at least two accessions were used to estimate genetic distances and population structure. Results showed that these phenotypically and geographically diverse NPGS tomato accessions were closely related to each other. However, a subset of divergent genotypes was identified that included landraces from primary centers of diversity (South America), secondary centers of diversity (Italy, Taiwan, and France), and genotypes that originated from wild species through 20th century breeding for disease resistance (e.g., ‘VFNT Cherry’). Extreme variant accessions produce cultivated fruit traits in a background that contains many wild or primitive genes. These accessions are promising sources of novel genes for continued crop improvement.

KASP Markers Reveal Established and Novel Sources of Resistance to Pea Seedborne Mosaic Virus in Pea Genetic Resources.

Pea seed-borne mosaic virus (PSbMV) is both seedborne and aphid-transmitted and can cause economic losses for pea (Pisum sativum L.) production by reducing yield through decreased seed weight and number. The P1 pathotype is especially virulent, affecting this important vegetable crop across the United States and internationally in regions of West Asia, North Africa, Europe, and Australia. Previously, two kompetitive allele-specific PCR (KASP) genotyping markers (eIF4E resistant 1 and 2) were developed and validated on P. sativum accessions identifying two PSbMV pathotype P1 resistance alleles in the eukaryotic translation initiation factor gene, eIF4E. The current study utilized these novel markers to rapidly evaluate 318 genetic resource accessions maintained as part of the United States Department of Agriculture National Plant Germplasm System's Pea Single Plant Collection (PSPC). The evaluations also included 58 commercial and other plant introduction (PI) lines that were assessed for the two eIF4E resistance alleles. All genotyping results were validated in greenhouse assays by confirmation of observable disease symptoms after inoculations and by enzyme-linked immunosorbent assays. The eIF4E resistant 1 and 2 alleles were found in 18 accessions from the PSPC, five commercial lines, and 14 other PI accessions. A single PSPC accession showed resistance to PSbMV pathotype P1 that is believed to be a novel source of resistance based on sequencing analysis of eIF4E. Sources of resistance were identified in the PSPC and in commercial cultivars that can be introgressed into breeding lines using traditional techniques to reduce the time and cost required to generate germplasm with superior disease-resistant traits.

Screening of USDA Onion Germplasm for Fusarium Basal Rot Resistance

Fusarium basal rot (FBR) caused by Fusarium oxysporum f. sp. cepae (FOC) is a major threat to onion (Allium cepa L.) production and marketing worldwide. Finding new sources of FBR-resistance to develop synthetic cultivars is a priority for onion breeders. As there are no FBR-resistant short-day onion cultivars, 85 U.S. National Plant Germplasm System plant introduction onion accessions that originated from 23 different countries were screened for their FBR susceptibility. To compare FBR susceptibility of these accessions at their seedling and mature bulb stages, a susceptible check cultivar, NuMex Crimson, a partially resistant check cultivar, Serrana and its selected population, Serrana-sel, were included in the study. The seedling screening was performed after inoculating silica-sand media with a virulent FOC isolate ‘CSC-515’ at 1.0 × 104 macroconidia g−1 of sand. Each entry was evaluated twice in growth chambers, and percent survival was adjusted to the number of seedlings that germinated in the uninoculated trays. Mature bulbs produced in the field were screened by inoculating transversely cut basal plates with potato dextrose agar plugs containing spores of the same isolate at 3.0 × 104 macroconidia mL−1. FBR severity and incidence were then calculated after 20 days of incubation. Significant variation was found among the accessions for FBR-susceptibility (p < 0.001) at both the seedling and the mature bulb stages. Two sets of 18 accessions were identified either for their higher seedling survival or higher mature bulb FBR-resistance compared to the checks. Among them, PI 256326 (‘Baia Periforme’, the originator cultivar of ‘Serrana’) had a higher seedling survival than both the checks, and a lower mature bulb severity than the susceptible check. Another accession, PI 656956 (‘S015’), exhibited higher seedling survival than the susceptible check and a low FBR severity (4.3 on a 1 to 9 scale) and incidence (41.7%). These two accessions, which were known previously for their high intra-population heterogeneity and root or bulb resistance for FBR, respectively, show promise for incorporating FBR-resistance into short-day onion cultivars. The cultivar rankings could vary in future studies with a range of FOC isolates due to a high cultivar × isolate interaction as observed in past studies.

Genetic variation in a radish (Raphanus sativus L.) geodiversity collection

A radish panel of 152 accessions with diverse root shapes was assembled from the National Plant Germplasm System (NPGS) collection based on weighted geographical sampling. Accessions represented 35 countries and were categorized into eight geographical regions—North America, Europe, Africa, West Asia, Central Asia, South Asia, East Asia, and Southeast Asia. Multiple plants per accession were assayed using genotyping by sequencing (GBS), and 8539 high-quality polymorphisms were discovered. Average observed heterozygosity (Ho) of the markers consistently ranged from 5 to 7% in all geographical regions. Central Asia was most diverse in terms of average expected heterozygosity (He = 0.107), while Southeast Asia was the least diverse (He = 0.050). Europe and Southeast Asia were the most divergent from each other (pairwise FST = 0.199), while Europe and North America were the least divergent (pairwise FST = 0.022). The clustering of genotypes based on principal components analysis (PCA) illustrated an east/west geographical pattern. Large numbers of private alleles and a substantial proportion (3%) of markers showing signals of selection based on allele frequency differences indicate that allelic variation is available for cultivar development. European and North American gene pools could be expanded by crossing with East Asian and Southeast Asian germplasm, and vice-versa. Both could potentially benefit from South Asian sources, while the addition of West Asian germplasm could increase East Asian and Southeast Asian diversity. This resource for molecular marker development combined with an understanding of global genetic relationships will inform the continued germplasm conservation and genetic improvement of radish.

Identifying sources of resistance in chickpea to seed rot and seedling damping‐off caused by metalaxyl‐resistant Pythium ultimum

AbstractChickpea (Cicer arietinum L.) has been a globally important source of nutrition for human and animal diets for several thousand years. In the United States, chickpeas are integral components of dryland agriculture systems throughout the Pacific Northwest and Northern Plains, but seed rot and preemergent damping‐off of chickpea caused by metalaxyl‐resistant isolates of Pythium ultimum Trow have emerged as diseases of increasing importance in the U.S. Pacific Northwest. The objective of this study was to identify sources of resistance to metalaxyl‐resistant P. ultimum in chickpea. Resistance was evaluated based on the emergence of chickpea seedlings from soil artificially infested with the pathogen. Significant differences in emergence were consistently detected between the desi chickpea cultivar ‘Myles’ and the kabuli chickpea cultivar ‘Sierra’. These two cultivars were used as checks to screen two different chickpea mini‐core collections, including a 39‐accession mini‐core collection from the U.S. National Plant Germplasm System and 209 accessions from a chickpea mini‐core collection developed by International Crops Research Institute for the Semi‐Arid Tropics (ICRISAT). A total of 194 desi accessions and 41 kabuli accessions were significantly more resistant than Sierra, whereas 85 desi accessions identified from the ICRISAT collection were highly resistant to both P. ultimum isolates. Accessions with pigmented seed coats were significantly more resistant than accessions with beige seed coats, which is a characteristic of kabuli chickpeas. Two accessions, W6 25882 and W6 25884, may be especially promising parental materials for improving kabuli cultivars based on their consistent resistant reactions to P. ultimum isolates and higher 100‐seed weight.

Genetic fingerprinting identifies apple (Malus domestica Borkh.) fruit cultivars in historic orchards on public lands

The National Historic Preservation Act of 1966 mandates that historic resources, including orchards, on federal lands be evaluated for potential significance. Evaluations include assessments and documentation of an orchard’s layout and location, as well as the architecture, ages, conditions, and cultivars of trees. These factors may reveal an orchard’s history and uniqueness and determine an orchard’s historic importance. Fruit tree cultivar identities are a key piece of this puzzle. They reveal information about an orchard’s use, origin, and uniqueness. In the past, historic preservationists relied on pomologists to identify cultivars based primarily on the phenotypic traits of the tree. DNA fingerprinting technologies, in combination with high-quality reference cultivar collections, have dramatically improved the accuracy of heritage cultivar identification. We demonstrate the utility of using DNA technologies for cultivar identifications on federal lands using examples from trees in orchards of Redwood National and State Parks and the Eldorado National Forest. A total of 87 historic apple trees from these locations were genotyped and cultivar names were determined for 35 trees, using a reference set of 1319 apple cultivars from the USDA National Plant Germplasm System, Washington State University, Seed Savers Exchange, and Temperate Orchard Conservancy. Some of the trees that were not identified may be cultivars that are not included in the reference sets and others may be unique trees derived from seeds. The information derived from genetic fingerprinting analyses will aid in the determination of the historic significance of orchards on public lands.

Evaluation of 21 Thailand Maize Germplasms for Resistance to Leaf Feeding Spodoptera frugiperda (Lepidoptera: Noctuidae)

Native to the Americas, the fall armyworm, Spodoptera frugiperda (J.E. Smith) (Lepidoptera:Noctuidae), is a recent invasive pest of Africa and Asia. This insect causes economic damage to maize (Zea mays L.) primarily through leaf and ear tissue feeding. The pest is resistant to several classes of insecticide and several Bt-maize toxins. Native resistance in maize to fall armyworm could be used with existing control tactics to enhance control of this insect. The objective for this study was to evaluate maize germplasm, developed from Thailand and maintained by the U.S. National Plant Germplasm System, for resistance to leaf feeding fall armyworm. Plants were grown in the field and artificially infested at the seven-leaf stage with fall armyworms. Visual rating scores for leaf feeding damage were recorded at 7 and 14 days post infestation. Scores for maize germplasm Ames 26254 were not different than the resistant check, GT-FAWCC(C5), and germplasm PI 506347 and PI 690324 were not significantly different from Ames 26254. In total, five Thailand maize germplasms received 14-day damage scores of ≤ 6.0 across both years of testing and were considered moderately resistant to fall armyworm leaf feeding. These maize germplasms are tropical and require breeding to adapt progeny for research in temperate areas.

Natural Genetic Diversity of Nutritive Value Traits in the Genus Cynodon

The Cynodon spp. collection maintained by United States Department of Agriculture National Plant Germplasm System (USDA-NPGS) has limited information on nutritive value (NV) traits. In this study, crude protein (CP), phosphorous concentration (P), in vitro digestible organic matter (IVDOM), and neutral detergent fiber (NDF) were determined to (i) estimate genetic parameters for NV, (ii) obtain genetic values for the whole population across two harvests, (iii) estimate genotype by harvest interaction (GHI) for NV traits, and (iv) select accessions exhibiting improved NV traits compared to ‘Tifton 85′. The experiment was setup as a row-column design with two replicates and augmented representation of controls: Tifton 85, ‘Jiggs’, and ‘Coastal’. The whole-population was harvested twice, and data were analyzed using linear mixed models with repeated measures. In addition, a selected population of 15 genotypes were evaluated across 11 harvests to determine the extent of GHI. Genetic parameters revealed the presence of significant genetic variability, indicating potential improvements for NV through breeding. Specifically, P and IVDOM presented large variation, while NDF had lower diversity but some accessions exhibited lower NDF than Tifton 85. Low GHI, except for IVDOM, indicated genotypic stability and potential for selecting improved accessions under fewer harvests. Breeding line 240, PI-316510, and PI-3166536 presented superior NV than Tifton 85.

Optimization of in vitro germination and cryopreservation conditions for preserving date palm pollen in the USDA National Plant Germplasm System

Date palm (Phoenix dactylifera L.) is a staple crop across arid and semi-arid regions around the world. Both genebanks and date palm breeders benefit by having clearly defined methods for the long-term preservation of pollen. Methods were developed to cryopreserve pollen from date palm trees in the USDA-ARS National Plant Germplasm System. First, in vitro pollen germination conditions were optimized using five cultivars. Then, date palm pollen equilibrated over saturated salt solutions of Ca(NO3)2 or MgCl2 at 23 °C (46% and 33% RH, respectively) or in a room set at 23% RH and 5 °C. Pollen was kept at these conditions for one to 56 days and then placed into liquid nitrogen vapor (LNV). Viability was assessed for fresh, moisture-equilibrated (non-LNV and LNV-exposed), and pollen stored in LNV for 9 months. The highest level of in vitro pollen germination was observed for date palm cultivar Thoory BC3 (60 ± 2%) when it was rehydrated for 2 h and then incubated at 23 °C on Marquard medium with 15% sucrose. Viability remained stable for pollen that was stored in LNV for 9 months. In addition, pollen also remained viable if it was cooled to and warmed from LNV for up to 60 cycles. These data suggest that date palm pollen can be successfully placed in genebank cryostorage. Using optimized in vitro germination conditions, date palm pollen can survive at least 9 months in LNV and can be cooled to and warmed from LNV at least 60 times.

Genetic data inform Yosemite National Park's apple orchard management guidelines

Societal Impact StatementYosemite National Park, California, USA has hundreds of apple trees in orchards that were planted in the 1800s by local settlers. These orchards played a cultural role in the early history of the land that was to become a national park. Most of Yosemite’s orchards are well over one hundred years old and have reached the end of their lifespan. Genetic data has facilitated the assignment of cultivar names to historic apple trees within Yosemite and this information will be used to develop an Orchard Management Plan. Yosemite will use this Plan to balance orchard conservation with other necessities such as reducing human‐wildlife conflict from bears that enter developed areas to consume fruit from the trees.Summary Yosemite National Park, California, USA has hundreds of historic apple trees that were planted within orchards in the 1800s, prior to the establishment of the park. These apple trees have been a concern within Yosemite for decades because their fruit attracts bears and other wildlife into visitor recreation areas, causing undesirable wildlife–human interactions. Herein, 361 of Yosemite's apple trees were genotyped to determine cultivar identities. The cultivar names of 117 apple trees were identified by matching genotypic data with genetic data available for an apple cultivar reference dataset for the United States Department of Agriculture‐Agricultural Research Service (USDA‐ARS) National Plant Germplasm System as well as collections maintained by other sources. An additional 92 trees were determined to be likely cultivars because they matched genotypes in a reference set of trees whose identities are not known. A total of 152 trees had unique genotypes that did not match reference datasets. Cultivar information, made possible by having digital genetic information available, will allow for the preparation of a more comprehensive Orchard Management Plan. As such, park managers will be able use the Plan to more effectively make decisions about balancing orchard conservation with other park necessities, including eliminating the human–wildlife conflict that results from bears entering developed areas to feed on the fruit trees.

Enhancing the searchability, breeding utility, and efficient management of germplasm accessions in the USDA−ARS rice collection

AbstractGenebanks conserve worldwide crop genetic diversity in systematically assembled and maintained ex situ collections for use by plant breeders and geneticists to improve the productivity, value, and sustainability of agriculture. Challenges faced in genebank management include providing sufficient and accurate trait information to facilitate searching the collection; controlling redundant accessions, seed mixtures, and mislabeled accessions; and identifying gaps in diversity. To help address these issues, a system that employs genotyping using 24 trait‐specific markers (TSMs), fingerprint markers (FPMs), or markers that are unique to subspecies was implemented for the USDA–ARS National Plant Germplasm System (NPGS), National Small Grains Collection (NSGC) for rice (Oryza sativa L.). Trait‐specific markers were used to validate phenotypic data for fragrance, pericarp color, apparent amylose content, starch pasting properties, gelatinization temperature, resistance to rice blast disease, plant pubescence, and plant height. Discrepancies between genotypic and phenotypic data are useful for quality control during curation or may present opportunities for identifying novel alleles, particularly for TSMs. Over 2,000 accessions were classified by species, O. sativa or O. glaberrima Steud.; subspecies, Indica or Japonica; and subpopulation, aromatic, indica, aus, temperate japonica, or tropical japonica using the subspecies marker and FPMs. This small panel of TSMs and FPMs was also adequate for differentiating important U.S. cultivars, which are primarily of tropical japonica background. As a result of this study, TSM and FPM descriptors will be added to the rice NSGC database, redundancies reduced, and mislabeled accessions corrected, thus increasing the value of the rice NSGC for breeding programs and providing new opportunities for gene discovery.

Phenotypic and Nodule Microbial Diversity among Crimson Clover (Trifolium incarnatum L.) Accessions

Crimson clover (Trifolium incarnatum L.) is the most common legume cover crop in the United States. Previous research found limited genetic variation for crimson clover within the National Plant Germplasm System (NPGS) collection. The aim of this study was to assess the phenotypic and nodule microbial diversity within the NPGS crimson clover collection, focusing on traits important for cover crop performance. Experiments were conducted at the Beltsville Agricultural Research Center (Maryland, USA) across three growing seasons (2012–2013, 2013–2014, 2014–2015) to evaluate 37 crimson clover accessions for six phenotypic traits: fall emergence, winter survival, flowering time, biomass per plant, nitrogen (N) content in aboveground biomass, and proportion of plant N from biological nitrogen fixation (BNF). Accession effect was significant across all six traits. Fall emergence of plant introductions (PIs) ranged from 16.0% to 70.5%, winter survival ranged from 52.8% to 82.0%, and growing degree days (GDD) to 25% maturity ranged from 1470 GDD to 1910 GDD. Biomass per plant ranged from 1.52 to 6.51 g, N content ranged from 1.87% to 2.24%, and proportion of plant N from BNF ranged from 50.2% to 85.6%. Accessions showed particularly clear differences for fall emergence and flowering time, indicating greater diversity and potential for selection in cover crop breeding programs. Fall emergence and winter survival were positively correlated, and both were negatively correlated with biomass per plant and plant N from BNF. A few promising lines performed well across multiple key traits, and are of particular interest as parents in future breeding efforts, including PIs 369045, 418900, 561943, 561944, and 655006. In 2014–2015, accessions were also assessed for nodule microbiome diversity, and 11 genera were identified across the sampled nodules. There was large variation among accessions in terms of species diversity, but this diversity was not associated with observed plant traits, and the functional implications of nodule microbiome diversity remain unclear.

A comprehensive phenotypic and genomic characterization of Ethiopian sorghum germplasm defines core collection and reveals rich genetic potential in adaptive traits.

Understanding population genetic structure and diversity of a crop is essential in designing selection strategies in plant breeding. About 2010 Ethiopian sorghum accessions were phenotyped for different traits at multiple locations. A subset of the collection, 1628 accessions, predominantly landraces, some improved varieties, and inbred lines were genotyped by sequencing. Phenotypic data revealed association of important traits with different sorghum growing agro-climatic regions, high genetic diversity and the presence of rare natural variation in the Ethiopian sorghum germplasm. Subsequent genotypic analysis determined optimum number of sub-populations, distinct cluster groups and ancestries of each sorghum accessions. To improve utilization of germplasm, a core subset of 387 lines were selected following posteriorigroupingof genotypes based on cluster groups obtained through GBS analysis followed by stratified random sampling using quantitative traits. In order to evaluate how well this new sorghum and millet innovation lab (SMIL) collection from Ethiopia is represented within the largest world sorghum collection at United States Department of Agriculture - National Plant Germplasm System (USDA-NPGS) and the sorghum association panel (SAP), comparisons were conducted based on SNP data. The SMIL collection displayed high genetic diversity with some redundancy with the USDA-NPGS germplasm but SAP showed clear distinction. Furthermore, genome-environment association analysis identified candidate genes associated with adaptation to abiotic factors, that will be important for exploitation of adaptive potential to different environments.In summary, our results described the diversity and relationship of sorghum collections, representativeness of developed core and provide novel insights into candidate genes associated to abiotic stress tolerance.

Genotyping by sequencing for SNP marker development in onion.

Onion (Allium cepa) is not highly tractable for development of molecular markers due to its large (16 gigabases per 1C) nuclear genome. Single nucleotide polymorphisms (SNPs) are useful for genetic characterization and marker-aided selection of onion because of codominance and common occurrence in elite germplasm. We completed genotyping by sequencing (GBS) to identify SNPs in onion using 46 F2 plants, parents of the F2 plants (Ailsa Craig 43 and Brigham Yellow Globe 15-23), two doubled haploid (DH) lines (DH2107 and DH2110), and plants from 94 accessions in the USDA National Plant Germplasm System (NPGS). SNPs were called using the TASSEL 3.0 Universal Network Enabled Analysis (UNEAK) bioinformatics pipeline. Sequences from the F2 and DH plants were used to construct a pseudo-reference genome against which genotypes from all accessions were scored. Quality filters were used to identify a set of 284 high quality SNPs, which were placed onto an existing genetic map for the F2 family. Accessions showed a moderate level of diversity (mean He = 0.341) and evidence of inbreeding (mean F = 0.592). GBS is promising for SNP discovery in onion, although lack of a reference genome required extensive custom scripts for bioinformatics analyses to identify high quality markers.

Crop Wild Relatives as Germplasm Resource for Cultivar Improvement in Mint (Mentha L.).

Mentha is a strongly scented herb of the Lamiaceae (formerly Labiatae) and includes about 30 species and hybrid species that are distributed or introduced throughout the globe. These fragrant plants have been selected throughout millennia for use by humans as herbs, spices, and pharmaceutical needs. The distilling of essential oils from mint began in Japan and England but has become a significant industrial product for the US, China, India, and other countries. The US Department of Agriculture (USDA), Agricultural Research Service, National Clonal Germplasm Repository (NCGR) maintains a mint genebank in Corvallis, Oregon. This facility preserves and distributes about 450 clones representing 34 taxa, hybrid species, advanced breeder selections, and F1 hybrids. Mint crop wild relatives are included in this unique resource. The majority of mint accessions and hybrids in this collection were initially donated in the 1970s by the A.M. Todd Company, located in Kalamazoo, Michigan. Other representatives of diverse mint taxa and crop wild relatives have since been obtained from collaborators in Australia, New Zealand, Europe, and Vietnam. These mints have been evaluated for cytology, oil components, verticillium wilt resistance, and key morphological characters. Pressed voucher specimens have been prepared for morphological identity verification. An initial set of microsatellite markers has been developed to determine clonal identity and assess genetic diversity. Plant breeders at private and public institutions are using molecular analysis to determine identity and diversity of the USDA mint collection. Evaluation and characterization includes essential oil content, disease resistance, male sterility, and other traits for potential breeding use. These accessions can be a source for parental genes for enhancement efforts to produce hybrids, or for breeding new cultivars for agricultural production. Propagules of Mentha are available for distribution to international researchers as stem cuttings, rhizome cuttings, or seed, which can be requested through the GRIN-Global database of the US National Plant Germplasm System, subject to international treaty and quarantine regulations.

DNA content and ploidy estimation of Festuca ovina accessions by flow cytometry

AbstractFestuca ovina L. is a fine fescue that is used as a low‐input turfgrass. The ploidy levels of F. ovina accessions held by the USDA National Plant Germplasm System (NPGS) are unknown, limiting the use of the germplasm in breeding programs. The objective of this study was to determine DNA content and estimate ploidy of these accessions. Among 127 accessions, we identified a wide range of ploidy levels from diploid to octoploid. We also found the accessions with higher ploidy levels usually had larger seed size. These results will be informative to plant breeders and researchers using germplasm from the F. ovina collection and point to challenges in maintaining polyploid, outcrossing germplasm seed stocks in common nurseries.

Salix dormant bud cryotolerance varies by taxon, harvest year, and stem‐segment length

AbstractWillow (Salix L.) species have good qualities for becoming a biomass crop for production of biofuels. They grow on marginal soils, produce high yields of wood in a short period of time, and are easily propagated. Salix is one of the few genera that may be cryopreserved as dormant winter buds (DBs) instead of using tissue culture techniques. The objective of our study was to evaluate selected factors that affect cryotolerance of nine Salix germplasm accessions in the USDA‐ARS National Plant Germplasm System collection. One‐year‐old branches harvested in January over 3 yr were cut into either 6‐ to 7‐ or 10‐cm segments containing at least two DBs. Segments were treated with a slow‐cooling procedure and stored in vapor‐phase liquid N (LNV). Control segments were stored at −5 °C until planting. The LNV‐exposed and the −5 °C‐stored nodal segments were warmed and directly planted in a cold greenhouse. Six weeks later, the material was evaluated for shoot and root development. Segments that developed a shoot were considered viable. Average viability varied among years (0–35.1%) even for the same accession, species (4.2–51.4%), and segment length. Dormant buds on 10‐cm segments had higher average viability (82.2%) than did DBs on 6‐ to 7‐cm segments (43.9%), suggesting higher suitability for cryopreservation. In the material studied, Salix DB viability was correlated with branch diameter, DB density, and the ability to develop shoots and roots prior to cryopreservation.

Genetic characterisation of the petal spot phenotype for Gossypium arboreum accession PI 408798

The occurrence of floral petal spots is common among flowering plants and plays a major role in attracting pollinators. Cotton genotypes having large red petal spots are frequent in the United States Department of Agriculture, National Plant Germplasm System Gossypium arboreum (L.) collection. One accession, PI 408798, showed a unique faint red petal spot phenotype. To genetically characterise this trait, a F<sub>2</sub> population of 226 plants was developed by crossing PI 408798 with G. arboreum accession PI 529714 that lacked pigmented petal spots. From the population, 161 plants showed the presence of faint red petal spots; whereas, 65 plants lacked pigmented petal spots. These data supported the single dominant gene model for the presence of floral petal spots. Accession PI 408798 will provide an important genetic resource to characterise the genes involved in the pathway controlling floral pigmentation.