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Open access

Luis A. Rivera-Burgos, Emily Silverman, Nebahat Sari, and Todd C. Wehner

Gummy stem blight (GSB), a major disease caused by Stagonosporopsis cucurbitacearum (syn. Didymella bryoniae), has caused significant losses of watermelon in the United States. The lack of progress in the development of resistant cultivars is the result of complex inheritance of resistance and breeding strategies that rely on single-plant selection. Because the sources of resistance are wild watermelon relatives, good fruit quality has been difficult to maintain during the selection process. Three hundred recombinant inbred line (RILs) in a population that carries resistance genes to GSB as well as good fruit quality were produced. This was accomplished by crossing and intercrossing resistant plant introductions, crossing the resulting progenies with elite cultivars, intercrossing those progenies, and, finally, self-pollinating to the S3 generation. The 300 RILs were evaluated for disease severity and fruit morphological and quality traits under greenhouse and field conditions in a randomized complete block design with 10 replications and 3 years. The means and correlations for disease severity ratings and fruit quality traits were estimated. Approximately 186 RILs had disease severity ratings below the mean value of the disease assessment scale (4.5), indicating that they possibly carry one or more genes for resistance to GSB. All disease severity ratings were correlated to each other (r = 0.67–0.98; P < 0.001), but they were not correlated with fruit quality traits. Most importantly, several resistant RILs showed good to excellent fruit quality. Our results provide evidence of improved germplasm with high resistance and good fruit quality.

Open access

Xiuli Shen and Myeong-Je Cho

Mature sugar pine (Pinus lambertiana Dougl.) trees produce large amounts of viable seeds but have seed dormancy. In this study, we used three sugar pine genotypes, 8877, 9306, and 9375, to test seed germination response. Seed germination from local sources varied greatly, and germination percentages were poor. There was a large variation in seed size and seed weight among the genotypes. Seeds of 9375 and 9306 were significantly larger and heavier (30.7 and 28.8 g/100 seeds, respectively) than 8877 (23.6 g/100 seeds). Three types of seeds—intact seeds, hulled seeds, and naked embryos—were examined for germination. Intact seeds failed to germinate due to the physical restraint and water impermeability of the seed. Chemical scarification with 5 m hydrochloric acid and 5 m sodium hydroxide did not soften the hard seedcoat and also failed to induce any germination of intact seeds. Hulled seeds resulted in an extremely low germination percentage (≤5%) with abnormal seedling development even though the endosperm was water permeable. Germination of the hulled seeds was not increased by adding 1 mg·L−1 gibberellic acid to the culture medium. Artificial opening of the hulled seeds created by longitudinal or horizontal cuts on the endosperm after removal of the seedcoat to avoid physical restraint and allow air exchange also failed to improve germination, indicating that inhibitors related to germination were present in the endosperm. However, naked embryos of all three genotypes germinated rapidly and uniformly with 70% to 95% germination percentage regardless of cold stratification treatment. Our data indicate that sugar pine seeds from the current source did not have physiological dormancy of embryos themselves, but dormancy was imposed by the seedcoat and endosperm. Using the naked embryos as donor explants, we have successfully established an efficient in vitro culture system. The protocol described here can be applied for the tissue culture and genetic transformation of sugar pine.

Open access

Mark K. Ehlenfeldt and James L. Luteyn

Vaccinium meridionale (section Pyxothamnus), a tetraploid species native to higher altitude locations in Jamaica, Colombia, and Venezuela, is of considerable interest to blueberry breeders for its profuse, concentrated flowering and monopodial plant structure, both of which may be useful in breeding for mechanical harvest. In this study, tetraploid V. meridionale was successfully hybridized as a male with 4x V. corymbosum (section Cyanococcus, highbush blueberry). The first-generation hybrids with highbush blueberry selections were intermediate in morphology and notably vigorous. The 4x F1 hybrids displayed variable branching structure, dormancy, prolificacy, fruit wax, etc.; however, most appear to be deciduous to semi-evergreen, with small, dark-colored fruit. The F1 hybrids displayed good fertility as females in backcrosses to 4x highbush and these crosses have produced numerous offspring morphologically indistinguishable from 4x highbush at the seedling stage. Evaluations of male fertility found variation for pollen production and quality but, significantly, found some clones with very good shed, high stainability, and almost complete tetrad production. The fertility suggests that these hybrids, despite being derived from intersectional crosses, might be conventionally used without significant difficulty. These hybrids also have potential value for the nascent V. meridionale breeding efforts occurring in Colombia, South America.

Open access

Michael J. Havey, Derek J. Hunsaker, and Eduardo D. Munaiz

The amounts and types of epicuticular waxes on onion (Allium cepa) leaves affect the severity of feeding damage by onion thrips (Thrips tabaci), a serious insect pest of onion. Onion plants with light green leaves are referred to as “glossy” and accumulate less epicuticular wax relative to the blue–green (“waxy”) foliage of wild-type onion. The onion cultivar Odourless Greenleaf (OGL) has visually glossy foliage, shows resistance to thrips feeding damage, and has the unique profile of accumulating waxes with 28 or fewer carbons. Plants of glossy OGL were crossed with the glossy inbred B9885 and waxy inbred lines DH2107, DH066619, and B8667. Hybrid progenies from glossy OGL by waxy plants had waxy foliage, indicating recessiveness of the glossy OGL phenotype relative to the waxy phenotype. Hybrids from the cross of glossy OGL with glossy B9885 were also waxy, revealing different genetic bases for the glossy phenotype in these two onions. Hybrid plants were self-pollinated and segregations in F2 families from OGL × waxy crosses fit the expected 3:1 ratio for the single locus at which the homozygous recessive genotype conditions glossy foliage. Segregations in F2 families from crosses of glossy 9885 × glossy OGL fit the 9:7 ratio, supporting two independently segregating loci, where the recessive genotype at either locus conditions the glossy phenotype. Amounts and types of epicuticular waxes on leaves of F2 progenies from crosses of OGL × waxy B8667 and glossy B9885 × OGL were determined using gas chromatography-mass spectrometry. Single-nucleotide polymorphisms were genotyped and genetic maps were constructed. The visually glossy phenotype from OGL and its unique profile of epicuticular waxes were conditioned by one locus on chromosome 6, for which we propose the name gl ogl. Onion populations such as OGL with unique epicuticular wax profiles will be important germplasms for the development of onion cultivars that suffer less feeding damage from onion thrips compared with waxy onion.

Open access

Prashant Bhandari and Tong Geon Lee

Genetic maps saturated with genetic markers are useful for genetic research and crop breeding; however, the genetic map for the large-fruited fresh-market tomato (Solanum lycopersicum) has never been constructed, and the recombination frequency between DNA fragments is only partly understood for fresh-market tomato. We constructed a novel fresh-market tomato genetic map by using 3614 single nucleotide polymorphism (SNP) markers and a 93 F2 segregating progeny derived from a cross between two United States large-fruited fresh-market tomato lines. The average distance between markers was less than 1 cM, and substantial recombination densities between markers were observed across the approximate centromere locations. A linkage panel for large-fruited fresh-market tomato was also established using the combined dataset of the genetic map and 58 SNP-genotyped core tomato lines. The allelic information in the linkage panel will be a significant resource for both tomato genetics and future breeding approaches.

Open access

S. Brooks Parrish, Renjuan Qian, and Zhanao Deng

Lantana species are an important component of the U.S. environmental horticulture industry. The most commonly produced and used species are L. camara and, on a smaller scale, L. montevidensis. Both were introduced to the United States from Central and/or South America. Lantana species native to the continental United States include L. canescens, L. depressa, L. involucrata, etc. and most of them have not been well exploited. This study was conducted to obtain information about somatic chromosome numbers, karyotypes, and genome size of these five species. Nuclear DNA content in these species ranged from 2.74 pg/2C (L. involucrata) to 6.29 pg/2C (L. depressa var. depressa). Four chromosome numbers were observed: 2n = 2x = 22 in L. camara ‘Lola’ and ‘Denholm White’, 2n = 4x = 44 in L. depressa var. depressa, 2n = 2x = 24 in L. canescens and L. involucrata, and 2n = 3x = 36 in L. montevidensis. Two basic chromosome numbers were observed: x = 11 in L. camara and L. depressa var. depressa, and x = 12 in L. canescens, L. involucrata, and L. montevidensis. Analysis of somatic metaphases resulted in formulas of 20m + 2sm for L. camara ‘Lola’ and ‘Denholm White’, 12m + 12sm for L. canescens, 44m for L. depressa var. depressa, 10m + 14sm for L. involucrata, and 32m + 4sm for L. montevidensis. Satellites were identified in all five species, but were associated with a different chromosome group in different species. L. depressa var. depressa had the longest total chromatin length (146.78 µm) with a range of 1.88 to 4.41 µm for individual chromosomes. The maximum arm ratio was observed in L. canescens, with a ratio of 2.5 in chromosome group 3. L. depressa var. depressa was the only species that had all of its centromeres located in the median region of the chromosome. The results show significant differences in nuclear DNA content, chromosome number, and karyotype among three native and two introduced lantana species and will help to identify, preserve, protect, and use native lantana species. The information will be helpful in assessing the ploidy levels in the genus by flow cytometry.

Open access

Rongpei Yu, Ying Cheng, Yanfei Pu, Fan Li, and Shugang Lu

The resurrection plant Selaginella pulvinata (Hook. & Grev.) Maxim is used as an ornamental and medicinal plant. It is also a good candidate for exploring the desiccation tolerance of resurrection plants. However, there is not an efficient propagation method for S. pulvinata. In the present study, we evaluated the establishment of in vitro propagation of S. pulvinata using frond tips as explants. The original shoot induction, adventitious shoot proliferation and plantlet growth media, and substrate type of plantlet acclimatization were investigated. The highest induction rate of original shoots (61.77 ± 5.17%) was obtained on half-strength (1/2) MS medium supplemented with 0.1 mg·L−1 N6-benzylaminopurine (BAP). The 1/2 MS with 1.0 mg·L−1 BAP was the most effective medium for the adventitious shoot proliferation. The quarter-strength (1/4) MS containing 0.1% (w/v) active charcoal (AC) was optimum for plantlets proliferated from adventitious shoots and plantlet growth. Approximately 98 plantlets could be obtained from one single original shoot via one-time shoot proliferation cultivation and plantlet cultivation. The acclimated plants on a 5:1 (v/v) mixture of peat and perlite had the highest survival rate (92.13 ± 1.67%). The acclimated plants maintained excellent resurrection ability.

Open access

Bernadine C. Strik and Amanda J. Davis

A 4-year trial was established in Oct. 2016 in western Oregon to evaluate the effects of various in-row mulch treatments on yield, fruit quality, and costs of installation and maintenance during establishment of northern highbush blueberry (Vaccinium corymbosum L. ‘Duke’). The treatments included douglas fir [Pseudotsuga menziesii (Mirb.) Franco] sawdust, black weed mat (woven polypropylene groundcover), green weed mat, and sawdust covered with black or green weed mat. Fruit were harvested during 2018–20 (second through fourth growing seasons). Weed mat color had no effect on yield or fruit quality. In 2018, yield was higher with black weed mat over sawdust mulch than with black weed mat alone, whereas mulch had no effects during 2019 and 2020, or on cumulative yield. Percent total soluble solids in the berries was highest with sawdust and weed mat alone compared with weed mat over sawdust mulches, whereas berry weight, diameter, and firmness were unaffected by mulch. Sawdust was the most expensive mulch over the lifespan of the planting because it required replenishment after 2 years. Black weed mat over sawdust resulted in the highest net profit when fruit sales and cost of materials and labor were considered.

Open access

Kevin Laskowski and Emily Merewitz

Annual bluegrass (Poa annua var. reptans), when grown as a putting green species, is sensitive to winter injury such as ice cover. Inhibiting plant ethylene production could be a way to improve annual bluegrass tolerance of ice encasement. The goals of this study were to determine how winter conditions and ethylene regulatory treatments affect the antioxidant system, fatty acid composition, and apoplastic proteins of annual bluegrass plant tissues. Ethylene-promotive (1-aminocyclopropane-1-carboxylic acid or ethephon) and ethylene inhibition treatments [aminoethoxyvinylglycine (AVG)] were applied to plants in the field during acclimation. Plant plugs were taken and subjected to low temperature (−4 °C) and ice-encasement treatments in growth chamber conditions. Antioxidant activities of ascorbate peroxidase (APX), peroxidase (POD), catalase (CAT), and superoxide dismutase (SOD) were measured along with malondialdehyde content (MDA) and apoplastic protein content in leaf and crown tissue. Saturated and unsaturated fatty acid contents were measured in leaf, crown, and root tissue. Higher unsaturated fatty acids are often associated with greater low-temperature tolerance. Compared with the untreated controls, ethephon-treated annual bluegrass had greater MDA contents, lower POD and SOD activity, and greater saturated and decreased unsaturated fatty acids. Ethylene inhibition treatments caused annual bluegrass to have less saturated fatty acid content and greater unsaturated fatty acid content, a greater content of apoplast proteins, and higher CAT activity when compared with the untreated controls. The activity of APX was greater in AVG-treated annual bluegrass than in controls. Ethylene may reduce physiological health overwinter, and inhibitory treatments may promote winter tolerance by promoting antioxidant activity, apoplast proteins, and the content of unsaturated fatty acids in plant tissues.

Open access

Xuan Liu and Donald L. Suarez

Soil salinization is a widespread problem severely impacting crop production. Understanding how salt stress affects growth-controlling photosynthetic performance is essential for improving crop salt tolerance and alleviating the salt impact. Lima bean (Phaseolus lunatus) is an important crop, but little information is available on its growth and leaf gas exchange in relation to a wide range of salinity. In this study, the responses of leaf gas exchange and whole plant growth of lima bean (cv. Fordhook 242) to six salinities with electrical conductivity (EC) of 2.9 (control), 5.7, 7.8, 10.0, 13.0, and 15.5 dS·m−1 in irrigation waters were assessed. Significant linear reduction by increasing salinity was observed on plant biomass, bean yield, and leaf net carbon assimilation rate (A). As EC increased from the control to 15.5 dS·m−1, plant biomass and A decreased by 87% and 69%, respectively, at the vegetative growth stage, and by 96% and 83%, respectively, at the pod growth stage, and bean yield decreased by 98%. Judged by the linear relations, the reduction in A accounted for a large portion of the growth reduction and bean yield loss. Salinity also had a significantly negative and linear effect on leaf stomatal conductance (g S). Leaf intercellular CO2 concentration (Ci) and leaf C13 isotope discrimination (Δ13) declined in parallel significantly with increasing salinity. The A-Ci curve analysis revealed that stomatal limitation [L g (percent)] to A increased significantly and linearly, from 18% to 78% and from 22% to 87% at the vegetative and pod-filling stages, respectively, as EC increased from the control to the highest level. Thus, relatively nonstomatal or biochemical limitation [L m (percent), L m = 100 − L g] to A responded negatively to increasing salinity. This result is coincident with the observed Δ13 salt-response trend. Furthermore, leaf carboxylation efficiency and CO2-saturated photosynthetic capacity [maximum A (Amax)] were unaffected by increasing salinity. Our results strongly indicate that the reduction in lima bean A by salt stress was mainly due to stomatal limitation and biochemical properties for photosynthesis might not be impaired. Because stomatal limitation reduces A exactly from lowering CO2 availability to leaves, increasing CO2 supply with an elevated CO2 concentration may raise A of the salt-stressed lima bean leaves and alleviate the salt impact. This is supported by our finding that the external CO2 concentration for 50% of Amax increased significantly and linearly with increasing salinity at the both growth stages. Leaf water use efficiency showed an increasing trend and no evident decline in leaf chlorophyll soil plant analysis development (SPAD) readings was observed as salinity increased.