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Hironobu Tsuchida, Cheng Dan-Hong, Kazuko Inoue, Nobuyuki Kozukue, and Susumu Mizuno

This study investigated the mechanism of alanine accumulation observed as a common factor in chilling-sensitive crops [cucumber (Cucumis sativus L.) and eggplant (Solanum melongena L.)] during cold storage. Pyruvic acid in acids increased rapidly during the storage of these crops at 1C. However, in sensitive crops at 20C or in a chilling-resistant crop [carrot (Daucus carota L.)] at 1C, the increase was not found. No significant changes in glutamate-pyruvate transaminase (GPT) activity closely related to the biosynthesis of alanine were found in chilling-sensitive or chilling-resistant crops during storage at 1, 5, or 20C. We suggest that the accumulated alanine found in chilling-sensitive crops may be produced by GPT from accumulated pyruvic acid during cold storage.

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Mau-Wei Lin, James F. Watson, and James R. Baggett

Analysis of parents and progeny generations of bulb onion (Allium cepa L.) crosses among parents with differing content of soluble solids (SS) and pyruvic acid (PA) showed that SS and PA are expressed and inherited in a quantitative manner. Distribution of SS and PA in both parents and progenies covered a range of values. Generation means, frequency distributions, deviation from midparent value, and estimates of gene effects all indicated that inheritance of SS and PA was additive, except for small deviations from the additive hypothesis in several individual backcrosses. Estimates of broad-sense heritability ranged from 48% to 53% for PA and 8 % to 56 % for SS. Phenotypic correlations between PA and SS estimated from the F2 generations of two crosses, were moderate and positive (r = 0.50 and 0.42).

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W.M. Randle and M.L. Bussard

Abbreviations: BPY, background pyruvic acid concentration; DM, dry matter; EPY, enzymatically formed pyruvic acid concentration; FT, total fructan concentration; FW, fresh weight; SSC, soluble solids concentration; TPY, total pyruvic acid

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Kil Sun Yoo, Leonard M. Pike, and Brian K. Hamilton

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William M. Randle

Twenty bulbs from each of 10 onion (AIlium cepa L.) cultivars and one mass population were harvested from two locations and evaluated for three traits associated with flavor quality. Variance components for soluble solids content (SSC), pyruvic acid concentration (PAC), and percent S were calculated, and sampling schemes required to detect specific differences among treatment means were determined. In general, a five-bulb sample and four replications were sufficient to detect desired differences for SSC and PAC, whereas percent S required a larger sample size and more replications.

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William M. Randle, David E. Kopsell, and Dean A. Kopsell

A major decision in producing onions with mild flavor on low sulfur soils is determining when to stop applying SO4 -2 to the crop. Sulfate (SO4 -2) is necessary for good early growth, but high levels of available SO4 -2 late in the season increase bulb pungency. The objective of this research was to determine how sequentially reducing the availability of SO4 -2 during onion growth and development would affect flavor intensity and quality of Granex-type onions. Starting 77 days before harvest, SO4 -2 concentrations were lowered from 1 mm to 0.05 mm on different blocks of onions in a greenhouse experiment at bi-weekly intervals. Total leaf and bulb S were measured at harvest to monitor S accumulation as SO4 -2 fertility was sequentially reduced. Bulbs were harvested and analyzed for flavor precursors and their biosynthetic intermediates, gross flavor intensity as measured by enzymatically developed pyruvic acid (EPY), and soluble solids content. As SO4 -2 fertility reductions were delayed during the experiment, total leaf and bulb S increased linearly. In addition, bulb EPY concentrations increased linearly as SO4 -2 reduction was delayed, indicating increases in overall flavor intensity. While the total concentration of flavor precursors did not significantly change in response to lowering SO4 -2 fertility during the experiment, the concentrations of MCSO to 1-PRENCSO did. MCSO concentration decreased and then increased in a quadratic manner. MCSO produces fresh onion and cabbage like flavors. 1-PRENCSO, on the other hand, increased linearly as the high SO4 -2 fertility level was extended through bulb maturation. Increasing concentrations of 1-PRENCSO causes onions to have significantly more heat and mouth burn when eaten. Reducing available SO4 -2 49 days prior to harvest coincided with a reduction in EPY and a change in the flavor biosynthetic pathway that appeared to be associated with the metabolic changes occurring with the onset of bulbing. Chemical names used: enzymatically developed pyruvic acid (EPY); methyl cysteine sulfoxide (MCSO); 1-propenyl cysteine sulfoxide (1-PRENCSO).

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W.M. Randle

To test the effects of high nitrogen (N) fertilization levels on onion quality and bulb flavor, `Granex 33' onions (Allium cepa L.) were greenhouse grown in hydroponic solution culture with increasing N concentrations. Nitrogen was adjusted in the solutions with NH4NO3 and increased incrementally from 0.22 g·L-1 to 0.97 g·L-1 over five treatments. Plants were harvested at maturity and subjected to quality, flavor, and mineral analysis. As solution N increased, bulb fresh weight and bulb firmness decreased linearly. Gross flavor intensity, as measured by enzymatically developed pyruvic acid (EPY) increased linearly for N concentrations between 0.22 and 0.78 g·L-1, but EPY was reduced slightly in bulbs grown at the highest N level (0.97 g·L-1). Soluble solids content was unaffected by solution N concentration. Solution N had an affect on flavor quality. Methyl cysteine sulfoxide, which gives rise to cabbage (Brassica L. sp.) and fresh onion flavors upon eating, generally increased in concentration as solution N increased. 1-Propenyl cysteine sulfoxide, which imparts heat, mouth burn, pungency, and raw onion flavors increased between the two lowest N concentrations, and then decreased as solution N increased. Propyl cysteine sulfoxide, which imparts fresh onion and sulfur flavors upon eating, generally increased with increasing solution N concentration. Several minerals were also affected by solution N concentration. Total bulb N and NO3 - increased linearly while B, Ca, and Mg decreased linearly. Total bulb S and K increased and then decreased quadratically in response to increasing solution N. Nitrogen fertility can have a pronounced affect on onion flavor and as a consequence, needs to be considered when growing onions for specific flavor quality and nutritional attributes.

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David E. Kopsell and William M. Randle

Pungency and bulb quality changes during storage were evaluated using onion (Allium cepa L.) cultivars representing different storage abilities, pungency, and soluble solids content. Bulbs were harvested from greenhouse-grown plants, cured, and stored for 3 or 6 months at 5 ± 3 °C, 80% ± 5% relative humidity (0.8 to 1.1 kPa vapor pressure deficit). Prior to storage, and after each month of storage, bulbs were evaluated for pungency by measuring enzymatically formed pyruvic acid (EPY), soluble solids content (SSC), percent loss in mass (%ML), and loss of dormancy. Pungency differed among cultivars prior to and during storage. Among short-day (SD) cultivars, EPY either decreased or increased linearly with increased storage duration. Among intermediate (ID)- and long-day (LD) cultivars, EPY decreased linearly or quadratically during storage. Short-day cultivar SSC increased, then decreased quadratically during storage, while ID and LD cultivar SSC decreased linearly over time. Percent loss in mass increased linearly during storage among all cultivars, although SD cultivars exhibited greater %ML than did ID or LD cultivars.

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William M. Randle, Jane E. Lancaster, Martin L. Shaw, Kevin H. Sutton, Rob L. Hay, and Mark L. Bussard

Three onion (Allium cepa L.) cultivars were grown to maturity at five S fertility levels and analyzed for S-alk(en)yl-L-cysteine sulfoxide (ACSO) flavor precursors, γ-glutamyl peptide (γ-GP) intermediates, bulb S, pyruvic acid, and soluble solids content. ACSO concentration and composition changed with S fertility, and the response was cultivar dependent. At S treatments that induced S deficiency symptoms during active bulbing, (+)S-methyl-L-cysteine sulfoxide was the dominant flavor precursor, and the flavor pathway was a strong sink for available S. As S fertility increased to luxuriant levels, trans(+)-S-(1-propenyl)-L-cysteine sulfoxide (PRENCSO) became the dominant ACSO. (+)S-propyl-L-cysteine sulfoxide was found in low concentration relative to total ACSO at all S fertility treatments. With low S fertility, S rapidly was metabolized and low γ-GP concentrations were detected. As S fertility increased, γ-GP increased, especially γ-L-glutamyl-S-(1-propenyl)-L-cysteine sulfoxide, the penultimate compound leading to ACSO synthesis. Nearly 95% of the total bulb S could be accounted for in the measured S compounds at low S fertility. However, at the highest S treatment, only 40 % of the total bulb S could be attributed to the ACSO and γ-GP, indicating that other S compounds were significant S reservoirs in onions. Concentrations of enzymatically produced pyruvic acid (EPY) were most closely related to PRENCSO concentrations. Understanding the dynamics of flavor accumulation in onion and other vegetable Alliums will become increasing important as the food and phytomedicinal industries move toward greater product standardization and characterization.

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William M. Randle and Mark L. Bussard