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Jernej Jakse, William Martin, John McCallum, and Michael J. Havey

The commercial production of onion (Allium cepa L.) inbreds, hybrids, and open-pollinated (OP) cultivars would benefit from a robust set of molecular markers that confidently distinguish among elite germplasms. Large-scale DNA sequencing has revealed that single nucleotide polymorphisms (SNPs), short insertion-deletion (indel) events, and simple sequence repeats (SSRs) are relatively abundant classes of codominant DNA markers. We identified 398 SNPs, indels, and SSRs among 35 elite onion ulations and observed that all populations could be distinguished. Phylogenetic analyses of simple-matching and Jaccard's coefficients for SSRs produced essentially identical trees and relationships were consistent with known pedigrees and previous marker evaluations. The SSRs revealed that elite germplasms from specific companies or breeding programs were often closely related. In contrast, phylogenetic analyses of SNPs and indels did not reveal clear relationships among elite onion populations and there was no agreement among trees generated using SNPs and indels vs. SSRs. This discrepancy was likely due to SNPs and indels occurring among amplicons from duplicated regions (paralogs) of the onion genome. Nevertheless, these PCR-based markers will be useful in the quality control of inbred, hybrid, and OP onion seed lots.

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Jordan O'N Caldwell, Malcolm E. Sumner, and Charles S. Vavrina

The Diagnosis and Recommendation Integrated System (DRIS) was developed to incorporate the importance of nutrient balance into plant analysis. Yield and plant analysis data from five fertilizer trials conducted in the field during 2 years, using `Granex 33' onions (Allium cepa L.), were entered into a data bank. The trials consisting of a N4 × P4 × K4 × S4, a N4 × P4 × K4 × plant density4, two N4 × P4 × K4, and a 4N × 6S factorial were conducted on sandy Ultisols in Georgia. Significant yield responses resulted from the addition of P and N. Leaf samples were analyzed for N, P, K, S, Ca, Mg, Mn, Zn, Cu, and B. Nutrient data were expressed in ratio form, and the population with yields >45 Mg·ha–1 were used to calculate the DRIS norms. The proposed norms for N, P, K, Mg, and Cu were tested using published data from independently conducted field and greenhouse studies. By accurately diagnosing the most limiting nutrients, these norms successfully predicted yield responses to treatment. Preliminary norms for S, Ca, Mn, Zn, and B were determined but not tested.

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Haim D. Rabinowitch, Batya Friedlander, and Ross Peters

Recently, a dwarf scape mutant was found in `Autumn Beit-Alpha' onion (Allium cepa L.). The development of dwarf scape in onion, the genetic control of this attribute, and its response to external application of ethephon and GA3 were studied. Data from segregating populations conclusively showed that a single recessive gene, designated dw1, controls scape dwarfness in onions. Its expression is slightly modified by minor genes. Relatively slow growth and early cessation of cell elongation are the characteristics associated with scape dwarfness. A similar developmental pattern characterized emerging normal flower stalks treated with ethephon. GA3 application at 50 ppm had no effect on scape elongation of dwarf plants. In each of 3 years, dwarf genotypes always produced scapes about half the length of normal ones. The marked expression stability of the dw 1 gene will facilitate its introduction into onion cultivars. Providing there is no negative pleiotropic effect, the dwarfness gene is expected to reduce lodging and, thus, improve mechanical harvest of onion seed. Chemical names used: 2-chloroethyl phosphoric acid (ethephon), gibberellic acid (GA3).

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Shinichi Masuzaki, Naoyuki Araki, Naoki Yamauchi, Naoko Yamane, Tadayuki Wako, Akio Kojima, and Masayoshi Shigyo

Bulb onion (Allium cepa L.) has a very large genome composed of a high proportion of repetitive DNAs. Genetic analyses of repetitive sequences may reveal microsatellites in order to increase the number of genetic markers in onion. Thirty microsatellites were previously isolated from an onion genomic library (Fischer and Bachmann, 2000). A complete set of Japanese bunching onion (A. fistulosum) – shallot (A. cepa Aggregatum group) monosomic addition lines were used to assign these microsatellites to the chromosomes of A. cepa. Simplified PCR conditions for each microsatellite were determined and 28 of the 30 primer pairs amplified DNA fragments, of which 21 microsatellite markers were assigned to chromosomes of A. cepa. Subsequent mapping of these microsatellites will enable us to establish the chromosomal distribution of these markers.

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John B. Masiunas

Experiments determined the effectiveness of the bipyridinium herbicides paraquat and diquat and of the diphenyl ether herbicide lactofen to desiccate onion (Allium cepa L.) shoots without affecting bulb quality and storage life. Paraquat, applied once, desiccated 80% of onion shoots within 3 days. Diquat desiccated ≈ 60% of onion shoots within 10 days of treatment. Lactofen caused slight necrosis but did not adequately desiccate onion shoots. Diquat and paraquat reduced sprouting of `Red Wethersfield' more than of `White Portugal'. Chemical names used: 6,7-dihydrodipyrido[l,2 2',1'-c] pyrazinediium ion (diquat); (±)2-ethoxy-l-methyl-2-oxoethy1 5-[2chloro-4-(trifluoromethyl)phenoxyl] -2-nitrobenzoate (lactofen); 1,1'-dimethyl-4,4'-bipyridinium ion (paraquat).

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L.E. Francois

This study, conducted in large, outdoor sand cultures, was initiated to determine the effects of excess B on garlic (Allium sativum L.) and onion (Allium cepa L.) and to establish their B tolerances as measured by yield and quality of the marketable product. Boron treatments were imposed by irrigation with culture solutions that contained 0.5, 1.0, 5.0, 10.0, 15.0, or 20.0 mg B/liter. Relative yields of garlic and onion were reduced 2.7% and 1.9% with each unit (mg·liter-1) increase in soil solution B (BSW) above 4.3 and 8.9 mg B/liter, respectively. Increasing BSW reduced garlic bulb weight and diameter but did not significantly affect onion bulb weight or diameter. Boron concentration in leaves and bulbs was directly correlated to BSW.

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Borut Bohanec, Marijana Jakse, and Michael J. Havey

The production of doubled haploid plants is desirable as an alternative to sexual inbreeding of longer-generation crops. Onion (Allium cepa L.) is a biennial plant and amenable to the production of gynogenic haploids. Although a strong population effect has been observed for gynogenic haploid production, there is no report describing the genetic basis of greater haploid production in onion. We evaluated over years the frequency of haploid production among onion inbreds and identified lines showing significantly (P < 0.01) greater production of haploids. The onion inbreds, B0223B and B2923B, produced the highest mean frequencies of haploids so far reported. Hybrid families from crosses of B2923B with inbreds having relatively low haploid production showed significantly higher haploid production than the low-producing parent and significantly lower haploid production than B2923B. Plants from B0223B and B2923B with established rates of haploid production were testcrossed and/or self-pollinated. The F1 family from B1717A-1 × B2923B-3 showed rates of haploid production slightly greater than the low parent (B1717A-1) and significantly less than the high parent (B2923B-3). Self-pollination of plants from B2923B showing relatively high rates of haploid production generated S1 progenies also producing relatively high frequencies of haploids. Selfed progenies from plant B2923B-6 showed a high mean rate of haploid production (56.8% ± 14.5%) and, more importantly, the highest level of haploid production (82.2%) reported for any single onion plant. These results indicate that relatively high haploid production, at least for B2923B, was quantitatively inherited with dominance towards low production. We suggest S1 family selection as an effective method to increase gynogenic haploid production of onion populations.

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R.D. Gitaitis, R.R. Walcott, H.F. Sanders, L. Zolobowska, and J.C. Diaz-Perez

Mulch (black plastic, wheat straw, or bare ground) and irrigation (drip or overhead sprinkler) treatments were evaluated for their effect on center rot of onion (Allium cepa L.), caused by the bacterium Pantoea ananatis, over the course of two seasons. Irrigation type had no effect on center rot incidence or severity in either year. In contrast, center rot development was delayed by 7 to 14 days on onions grown in straw mulch or bare ground compared to those in black plastic. Straw mulch resulted in later harvest dates and was associated with reduced levels of center rot. In contrast, black plastic increased disease incidence and hastened the onset of the epidemic. The spatial distribution of disease incidence in both years indicated the presence of a primary disease gradient. At harvest, infected plants were segregated by treatment and by duration of infection [based on disease ratings taken from the time of first symptom expression (beginning at 110 to 120 days after transplanting and then every 5 to 10 days until harvest)]. Early-vs. late-infected plants had no significant effect on yield (bulb weight). However, symptom expression in terms of the number of days after planting was significantly correlated with a disease severity index. Amount of rot in bulbs from plants displaying their first symptoms only 1 to 2 days before harvest (late-season infection) was not significant from rot levels in control bulbs at harvest. However, at 4 weeks after harvest, onions from plants with late-season infections exhibited significantly more rot in storage compared to the control.

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Steven A. Sargent, Peter J. Stoffella, and Donald N. Maynard

Short-day onions (Allium cepa L.) grown under humid, subtropical conditions at two locations were evaluated for bulb size and yield at five harvest dates (H1 to H5) ranging from 94 to 132 days after transplanting (DAT) for `Granex 33' and from 115 to 153 DAT for `Texas Grano 1015Y'. Maximum yields were attained by H4 for both cultivars and were attributed to increased bulb size rather than differences in plant (bulb) population. Nondried, large bulbs (>7.6 cm diameter) from each harvest were trimmed and stored at 1 or 10 °C and 80% relative humidity (RH) for 2 weeks plus 2 weeks at 20 °C and 80% RH to simulate commercial storage and handling. Initial respiration rates of bulbs of both cultivars decreased >60% between H1 and H4. Bulbs also retained higher fresh weight during storage as harvest was delayed. Storage for 2 weeks at 1 °C suppressed sprouting of immature (H1) `Texas Grano 1015Y' bulbs, but not of `Granex 33' bulbs from one location. Storage at 10 °C did not suppress sprouting of either cultivar. Decay became more prevalent with delayed harvest, but `Granex 33' was more resistant to decay than was `Texas Grano 1015Y', which developed up to 40% decay after 2 weeks at 20 °C. Harvest at 115 and 132 DAT resulted in acceptable yields for `Granex 33' and `Texas Grano 1015', respectively, and satisfactory postharvest quality of nondried bulbs following 2 weeks of storage at 1 °C and 80% RH plus 2 weeks at 20 °C.

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W.M. Randle, D.A. Kopsell, D.E. Kopsell, R.L. Snyder, and R. Torrance

The marketing of onions (Allium cepa L.) based on bulb pungency as a measure of overall flavor intensity is being considered by the onion industry. Pungency is highly variable within and among fields due to genetic and environmental factors. Therefore, a study was undertaken to develop a sampling procedure to estimate onion pungency means and variances from field-grown onions with predetermined degrees of accuracy and confidence. Two shortday onion cultivars, commonly grown in the Vidalia, Ga., area, were each randomly sampled from four different fields. The sampled bulbs were analyzed for enzymatically formed pyruvic acid (EPY) and soluble solids content (SSC) to assess pungency and sugars, respectively. EPY concentration and SSC varied between the two cultivars, among the four fields within cultivars, and among the fifty samples within each field. In a combined analysis of all eight fields, at least 1.3 ten-bulb samples would be needed per acre to come within ±0.5 μmol EPY of a field's true EPY mean with 95% confidence. If the accuracy of the estimation was lowered to ±1.0 μmol EPY of a field's true mean, then at least 0.4 ten-bulb samples would be needed per acre. Because SSC was less variable than EPY, the number of ten-bulb samples needed per acre to estimate a field's true mean was lower than the number required to estimate EPY. Establishing a sampling method to estimate an onion field's EPY and SSC will provide the mechanism to standardize onion flavor in the market place and instill greater consumer confidence in purchasing onions.