Four cultivars of onion (Allium cepa L. `Primavera', `Granex 33', `Pegasus', and `Sweet Success') were grown to maturity in modified nutrient solutions with or without 2.0 mg·L-1 Na2 SeO4 (1.51 mg·L-1 SeO4 -2). Selenium did not affect total flavor precursor content (ACSO) in `Granex 33', `Pegasus', and `Sweet Success'. However, Se affected several individual ACSOs and precursor intermediates. Selenium decreased γ-L-glutamyl-S-(1-propenyl)-L-cysteine sulfoxide and trans(+)-S-(1-propenyl)-L-cysteine sulfoxide content in all four cultivars. (+)-S-Methyl-L-cysteine sulfoxide content was higher while (+)-S-propyl-L-cysteine sulfoxide content was lower with the added Se for two cultivars. Selenium lowered total bulb S content in all cultivars, and increased the percentage of total S accumulated as SO4 -2 in three cultivars. The effect of Se on the flavor pathway was similar to that found when onions were grown under low S-concentrations. Flavor changes can be expected when onions are grown in a high Se environment, however, specific changes may be cultivar dependent.
Dean A. Kopsell and William M. Randle
David E. Kopsell, William M. Randle, and Mark A. Eiteman
Onion (Allium cepa L.) pungency changes during storage. To better understand these flavor changes, seven onion cultivars representing different storage duration, photoperiodic requirement, and flavor intensity were greenhouse grown and the bulbs stored for 3 or 6 months at 5±3 °C, 0.8 to 1.1 kPa vapor pressure deficit. Bulbs were evaluated using high-pressure liquid chromatography quantification for changes in S-alk(en)yl cysteine sulfoxide (ACSO) flavor precursors and γ-glutamyl peptide (γ-GP) biosynthetic intermediates before storage and monthly thereafter. Before and during storage, cultivars differed in total ACSO, (+) S-methyl-L-cysteine sulfoxide (MCSO), trans-(+)-S-(1-propenyl)-L-cysteine sulfoxide (PRENCSO), (+) propyl-L-cysteine sulfoxide (PCSO), S-2 carboxypropyl glutathione (2-CARB), and γ-L-glutamyl-S-(1-propenyl)-L-cysteine sulfoxide (γGPECSO) concentration. During storage MCSO generally decreased while PRENCSO increased in concentration for most cultivars. The linear increase in PRENCSO concentration during storage was accompanied by a linear decrease in γGPECSO concentration. While not measured in this study, these trends indicate γ-glutamyl transpeptidase activity throughout bulb storage. γ-Glutamyl transpeptidase was previously reported to be active only in the later stages of bulb storage or during bulb sprouting. Changes in ACSO and γ-GP compounds during storage did not follow previously reported changes during storage for enzymatically formed pyruvic acid (EPY) for these cultivars. To better understand what causes flavor changes in onions during storage, future investigations should include analysis of the enzymes involved in flavor development and ACSO hydrolysis products.
John A. McCallum, Meeghan Pither-Joyce, and Martin Shaw
Genetic and environmental factors affect onion (Allium cepa L.) pungency but the molecular basis for this variation is not understood. To initiate molecular analysis of onion sulfur metabolism we isolated cDNAs from onion associated with sulfur assimilation and compared gene expression and sulfur metabolism of mild and pungent onion cultivars. We isolated cDNAs encoding homologues of 5'adenosine-phosphosulfate (APS) reductase, γ-glutamylcysteine synthetase and serine acetyl transferase using a homology-based RT-PCR approach. Homologues of high-affinity sulfate transporters and sulfite reductase were isolated from an onion root differential cDNA library enriched for genes up-regulated by 48 hours sulfur deprivation. The influence of genotype and sulfur nutrition on root expression of selected genes was measured in an experiment in which a low pungency onion cultivar (`Houston Grano') and a high pungency cultivar (`Canterbury Longkeeper') were grown hydroponically in low (0.1 meq·L-1) or high (4.0 meq·L-1) sulfate medium and harvested before bulbing. `Canterbury Longkeeper' contained higher concentrations of (+)-S-methyl-L-cysteine sulfoxide in leaf and root than `Houston Grano' but cultivars did not differ in leaf trans-(+)-S-(1-propenyl)-L-cysteine sulfoxide concentrations. `Houston Grano' accumulated significantly higher concentrations of total N, nitrate, and basic amino acids in leaves and roots, suggesting these cultivars differ markedly in maintenance of S/N homeostasis. Steady-state transcript levels of APS reductase and high-affinity sulfate transporter in roots were significantly higher (2- to 3-fold) at low sulfate. By contrast, steady state levels of ATP sulfurylase transcript were significantly higher at high sulfate levels and in `Canterbury Longkeeper'. We conclude that differences in regulation of the sulfur assimilation pathway may underlie genetic differences in pungency.
Timothy W. Coolong and William M. Randle
The effects of temperature and developmental age on flavor intensity and quality were tested by growing `Granex 33' onions (Allium cepa L.) at 16.5, 22.1, 26.7, and 32.2 (±0.4) °C for 50 days and to maturity. Plants were harvested and evaluated for growth characteristics. Bulbs were then analyzed for sulfur (S) assimilation and flavor development parameters. Total bulb S increased linearly with temperature regardless of bulb age. Bulb sulfate changed little over temperatures, indicating that organically bound S increased with temperature. Total pyruvic acid content (pungency), total S-alkenyl cysteine sulfoxide (ACSO) content and individual ACSOs increased linearly in response to temperature when measured at the two developmental stages. Though trans-(+)-S-(1-propenyl)-L-cysteine sulfoxide was the predominant ACSO at most temperatures, (+)-S-methyl-L-cysteine sulfoxide accumulation was greatest among the individual ACSOs in mature bulbs grown at 32.2 °C. Additionally, (+)-S-propyl cysteine sulfoxide was present in the least amount at all treatment levels and developmental stages. Gamma glutamyl propenyl cysteine sulfoxide and 2-carboxypropyl glutathione peptides in the flavor biosynthetic pathway also increased linearly with temperature. When ACSOs were assessed in onion macerate as a measure of alliinase activity, levels of degraded ACSOs increased linearly with growing temperature. The relative percentage of most ACSOs hydrolyzed, however, did not change in response to growing temperature. This suggested that the activity of alliinase was proportional to the amounts of flavor precursors synthesized. Growing temperature, therefore, should be considered when evaluating and interpreting yearly and regional variability in onion flavor.
Timothy W. Coolong and William M. Randle
To determine the extent to which sulfur (S) and nitrogen (N) fertility interact to influence the flavor biosynthetic pathway in onion (Allium cepa L.), `Granex 33' onions were grown in hydroponic solution culture with varying levels of S and N availability. Plants were grown at 5, 45, or 125 mg·L-1 sulfate (SO4 2-), and 10, 50, 90, or 130 mg·L-1 N, in a factorial combination. Total bulb S, total and individual flavor precursors and their peptide intermediates in intact onion tissue were measured. To measure the effect of S and N on alliinase activity, flavor precursors were also measured in onion macerates. Sulfur and N availability in the hydroponics solution interacted to influence all flavor compounds except S-methyl-L-cysteine sulfoxide. Levels of S-methyl-L-cysteine sulfoxide were influenced by N and S levels in the solutions; however, no interaction was present. At the lowest SO 4 2- or N levels, most precursors and peptides measured were present in very low concentrations. When SO 4 2- or N availability was adequate, differences among flavor compounds were small. Results indicated that S fertility had a greater influence on trans-S-1-propenyl-L-cysteine sulfoxide (1-PRENCSO) accumulation, while N availability had a greater influence on S-methyl-L-cysteine sulfoxide levels. Flavor precursors remaining in the onion macerates revealed that the percentage of intact precursors hydrolyzed by alliinase were not significantly influenced by either SO 4 2- or N levels in the solutions, except for 1-PRENCSO, which was affected by N levels. Nitrogen and S fertility interacted to influence the flavor biosynthetic pathway and may need to be considered together when manipulating onion flavor compounds.
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.