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Charles S. Vavrina and Doyle A. Smittle

Six onion (Allium cepa L.) cultivars were grown during 2 years to evaluate the effects of environment on bulb quality as measured by sugar and pyruvate (pungency) concentrations. Within each year, bulb fresh weight was not affected by cultivar; however, bulb fresh weights were 36% higher in a year when most of the rain fell during maximum bulb expansion. Total bulb sugar concentration and pungency varied among cultivars and years. Pungency was higher and the sugar: pungency ratio was lower in `Texas 1015Y' and `Sweet Georgia' than in `Dessex', `Rio Bravo', 'Hybrid Yellow Granex', and `Granex 33'. Under low S nutrition, market acceptance of “sweet” onion cultivars that vary slightly in nonstructural water-soluble carbohydrates may be assessed more precisely by the sugar: pungency ratio than by sugar or pungency assessments.

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A.M. Salama, J.R. Hicks, and J.F. Nock

Maleic hydrazide (MH)-treated and untreated (control) onion (Allium cepa L.) bulbs were stored for up to 20 weeks at 0, 15, or 30C with relative humidities (RH) of 40% or 60%. MH and RH had minimal effect on sugars and organic acids in inner or outer scale leaves that were analyzed at S-week intervals. Concentrations of fructose, glucose, and total sugars were higher in inner than outer leaves of the bulb, while the reverse was true for sucrose. Total sugars, glucose, and fructose decreased and sucrose increased with higher storage temperature. Total sugars and glucose decreased with increased storage duration. Malic acid concentration was greater in the outer leaves while citric acid levels were higher in inner leaves. Malic acid increased in onion bulbs during storage while citric acid levels were not influenced by storage duration. Total acids showed little difference across temperatures, due to the concurrent increase in citric acid and decrease in malic acid at 30C.

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Jun Song, Lihua Fan, Paul D. Hildebrand, and Charles F. Forney

The biological effect of corona discharge on onions (Allium cepa L.) in a commercial storage was investigated. Surface discoloration and mold were modestly but significantly reduced by the corona discharge when onions were stored for 2 or 4 weeks with or without an additional 2 weeks of shelf life under high humidity. Corona discharge treatment also reduced airborne mold spores in the storage room. No significant changes in internal decay, firmness, sprouting, or rooting, in treated onions were found.

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J.E.P. Debaene, I.L. Goldman, and B.S. Yandell

Two mild and two pungent onion (Allium cepa L.) selections (hereafter referred to as cultitypes), W420B, W424B, MSU8155B, and Exhibition, were grown at two locations in two states (Wisconsin and Oregon) during 1994 and 1995. Onion bulbs were harvested, stored at 4 °C and sampled for antiplatelet activity, pungency, and soluble solids 10 days after harvest and every 40 days during a 210-day postharvest storage period. Significant cultitype × state and cultitype × year interactions were detected. However, these were primarily due to the change in rank of cultitypes within the mild or pungent group. Averaged over all environments, antiplatelet activity was significantly greater in 1994 compared to 1995 for all cultitypes. Significantly greater antiplatelet activity was measured for three out of four cultitypes grown in Oregon compared to Wisconsin. During postharvest storage, antiplatelet activity increased 61% and 56% across all cultitypes and across both states during 1994, and across all cultitypes in Wisconsin during 1995, respectively. Although pungency determination can be a good indicator for relative rankings of different cultitypes for antiplatelet activity, changes in pungency were not correlated with changes in antiplatelet activity during postharvest storage. Results demonstrate cultitype, environment, duration of postharvest storage and genotype × environment interactions influence pungency, soluble solids, and antiplatelet activity, which should be considered when assessing onion-induced antiplatelet activity.

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Charles L. Biles, Mel Holland, Mauricio Ulloa-Godinez, Dennis Clason, and Joe Corgan

Experiments were conducted to determine variation of in vitro microsclerotia production, pigmentation, and growth of five Pyrenochaeta terrestris (Hans.) Gorenz, Walker, and Larson isolates from Texas and New Mexico. Isolates of P. terrestris, the causal agent of pink root rot, were placed on agar with lo-mm-long sections of sterile onion (Allium cepa L.) roots. Microsclerotia were present after 20 days at 20, 25, 27, or 32C, with the number of microsclerotia dependent on the isolate. Microsclerotia were absent at 35C. One isolate produced few microsclerotia at all temperatures. Optimum temperatures for growth on potato dextrose agar for the isolates tested were 25 and 27C. Pigment production in roots on agar varied depending on isolate and temperature. Three isolates produced high levels of pigment in onion roots at 15, 20, 25, and 27C. A New Mexico isolate produced significantly less visible pigment than the other New Mexico and Texas isolates. One isolate produced very little pigment at all temperatures tested. Constant fluorescent light stimulated pycnidia production in one isolate and reduced microsclerotia production in all other isolates. Isolates varied significantly in microsclerotia production and pigment synthesis.

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George H. Clough

Field trials were conducted at Hermiston, Ore., from 1995 through 1998, to determine impact of stand loss and plant damage at different growth stages on yield of onions (Allium cepa) grown for dehydration. Stand reduction (0%, 20%, 40%, 60%, 80%) and foliage damage (0%, 25%, 50%, 75%, 100%) treatments were applied at three-, six-, nine-, and twelve-leaf onion growth stages. Although average bulb weight increased as stand was reduced, marketable, cull, and total yields decreased as stand reduction increased (plant population decreased) at all plant growth stages. Bulb weight was not changed by up to 100% foliage removal at the three-leaf stage. At the six- and twelve-leaf stages, weight was reduced when ≥50% of the foliage was removed. The most severe response occurred at the nine-leaf stage. At the three-leaf stage, yield was not affected by foliage damage. At the six-leaf growth stage, yield was reduced by 75% or more foliage loss, but at the nine- and twelve-leaf stages, ≥50% foliage removal reduced expected yields. As with bulb weight, the impact of foliage removal on yield was most severe at the nine-leaf growth stage.

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C.S. Vavrina and F.M. Roka

In 4 years of research comparing production of short-day onions (Allium cepa L.) on plastic mulch versus bare ground in southern Florida, greater marketable yields were obtained when onions were grown on plastic mulch. Results showed that in a semitropical environment, white-on-black plastic mulch provided the greatest yield enhancement from increased weight and bulb size. Yield loss due to splitting, while apparent, was not sufficient to reduce the impact of mulch on the increase in individual bulb weight. Adopting plastic mulch for sweet onion production will add between $400 and $500/acre ($988 and $1,235/ha) of additional operating expenses. While this may increase cash-flow burdens and heighten overall financial risks, the added value from increased yields by weight and greater percentages of jumbo sized bulbs suggest that plastic mulch has an excellent chance to increase a grower's overall net return. Using conservative yield and market price assumptions, an economic analysis showed an increase in grower's net return of more than $120/acre ($296/ha).

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W.M. Randle, M.L. Bussard, and D.F. Warnock

Five short-day onion (Allium cepa L.) cultivars grown with high (4.0 meq S/liter) and low (0.1 meq S/liter) S fertility were evaluated for ontogenetic changes in leaf S concentration and the association between leaf S concentration and final bulb pungency as reflected by pyruvate formation. Cultivars differed in leaf S concentration at each of eight sampling dates during growth and development, but the pattern of leaf S accumulation among cultivars was similar. Leaf S concentration increased during early plant development while in a nonbulbing photoperiod, but decreased as bulbing progressed to maturity with high and low S fertility. The decline in leaf S concentration during bulbing was more severe with low than with high S fertility. Leaves that were left to dry on the mature bulb lost most of their S, especially with 0.1 meq S/liter, a result that could have implications in final bulb flavor intensity. Correlations generally were poor between leaf and final bulb S concentration or pungency.

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Joseph J. King, James M. Bradeen, and Michael J. Havey

Nuclear RFLPs were used to estimate relationships among 14 elite commercial inbreds of bulb onion (Allium cepa) from Holland, Japan, and the United States. Variability for known alleles at 75 RFLP loci and 194 polymorphic fragments revealed by 69 anonymous cDNA probes and a clone of alliinase were scored to yield genetically characterized and uncharacterized data sets, respectively. The inbred onion populations possessed more than two alleles at 20 of 43 (46%) codominant RFLP loci. Relationships among the inbreds were estimated by cluster analysis of simple-matching (genetically characterized data) and Jaccard (genetically uncharacterized data) coefficients using the unweighted pair group method and agreed with known pedigrees. RFLPs confidently distinguished among elite inbreds within and between specific market classes. RFLP profiles for virtual hybrids were computer-generated by combining gametic arrays among inbreds of the same market class and analyzed as described above. Allelic and genetically uncharacterized RFLPs confidently distinguished among these hybrids, even though heterozygosity for many markers produced a majority of monomorphic fragments. We randomly sampled decreasing numbers of RFLPs from the complete data sets and calculated simple-matching and Jaccard distances, noting the numbers of probes that were unable to distinguish any two inbreds or hybrids. As few as 10 polymorphic probe-enzyme combinations distinguished among all the inbreds and samples of 20 genetically characterized or 10 genetically uncharacterized clones distinguished all the virtual hybrids. This study demonstrated that the previously reported few RFLPs observed among open-pollinated (OP) onion populations were due to the highly heterozygous nature of the OP population.

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Ali Fuat Gökçe and Michael J. Havey

Cytoplasmic male sterility (CMS) in onion (Allium cepa L.) is conditioned by the interaction of the male-sterile (S) cytoplasm with recessive alleles at a single nuclear male-fertility restoration locus (Ms). In order to seed propagate male-sterile plants (S msms), onion breeders must identify maintainer lines possessing normal (N) male-fertile cytoplasm and homozygous recessive at the Ms locus (N msms). Molecular markers have been identified distinguishing N and S cytoplasms and closely linked to the nuclear Ms locus. In this study, we evaluated testcross progenies from randomly selected N-cytoplasmic plants from three open-pollinated populations for nuclear restoration of male fertility over at least three environments. The Ms locus and linked restriction fragment length polymorphisms (0.9 and 1.7 cM) were at linkage equilibrium in all three open-pollinated onion populations, indicating that these linked markers cannot be used to identify maintaining genotypes in open-pollinated onion populations. However, cytoplasmic evaluations were effective in reducing the number of testcrosses required to identify CMS-maintaining genotypes.