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.
W.M. Randle and M.L. Bussard
Sixteen short-day onion (Allium cepa L.) cultivars with high (4.0 meq·liter-1) and low S (0.1 meq·liter-1) fertility were evaluated for several characteristics associated with bulb flavor. Sulfur levels interacted with cultivars in influencing bulb pungency and concentrations of S and individual sugars, except for fructose. Enzymatically formed pyruvic acid correlated poorly with bulb S concentration, which suggests differential partitioning of S into flavor and nonflavor compounds among cultivars. Bulb percent dry weight correlated negatively with bulb S concentration. Since poor correlations were found between enzymatically formed pyruvic acid and water-soluble carbohydrates, we postulate that pungency and sweetness function independently in bulbs of fresh-market short-day onion cultivars.
Timothy W. Coolong and William M. Randle
Sandy soils in the onion (Allium cepa L.) growing region of southeastern Georgia are generally low in calcium (Ca). Bulbs grown in these soils are often soft and susceptible to postharvest diseases. Preliminary greenhouse studies have indicated that supplemental calcium chloride (CaCl2) can improve bulb firmness. The effects of supplemental CaCl2 on the quality of field-grown onions were therefore investigated. Other preliminary studies indicated that CaCl2 may inhibit sulfur (S) uptake in onion and decrease bulb pungency. Thus, ammonium sulfate (NH4)2SO4 and CaCl2 levels were varied to determine if CaCl2 could improve flavor at different levels of nitrogen (N) and S fertility. Onions, cv Georgia Boy, were grown with 0, 250, and 500 kg·ha−1 (NH4)2SO4 and 0, 115, and 230 kg·ha−1 CaCl2 in a factorial combination in 2005 and 2006. Total bulb yield increased with increasing (NH4)2SO4, but was unaffected by CaCl2. The percentage of diseased bulbs increased during storage in both years, and was affected by (NH4)2SO4 fertility in 2006. Bulb scale firmness increased with supplemental CaCl2 fertility and decreased significantly during storage in both years. Fertility treatments had little effect on bulb pectin composition, although total pectin concentrations fell during storage in 2005 and 2006. In addition, bulb pungency decreased with additional CaCl2 in 2006. However, CaCl2 had a limited effect on flavor precursor concentrations. There were no interactions between fertility treatments, but there were CaCl2 and (NH4)2SO4 by storage duration interactions affecting firmness and disease incidence, respectively. With the exception of yield, differences among years in the parameters measured were generally small.
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.
D.F. Warnock, W.M. Randle, and O.M. Lindstrom Jr.
Proper acclimation of onion (Allium cepa L.) seedlings can enhance winter freeze survival; therefore, the effects of photoperiod-temperature combinations, photoperiod, and plant age on the cold hardiness of short-day onions were investigated. Following acclimation at various photoperiod-temperature regimes, different-aged plants were frozen to various subzero temperatures in an ethylene glycol bath and evaluated for cold hardiness. Older plants were more cold hardy than younger plants. An 11-hour photoperiod-decreasing temperature (20/15 to 10/5C day/night) treatment improved plant cold hardiness over other photoperiod-temperature regimes. Various photoperiods (8-, 11-, 14-, and 24-hour) applied during a 14-day, 3C acclimation treatment before freezing had little effect on plant cold hardiness. However, day 7 foliar and day 14 root evaluations indicated that 81-day-old plants given an 8- or 11-hour photoperiod during the 3C acclimation treatment were less cold hardy than older plants (91 or 112 days) given the same acclimation photoperiod.
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.
Timothy W. Coolong, Ronald R. Walcott, and William M. Randle
A real-time polymerase chain reaction (PCR) assay has been developed for the detection and quantification of Botrytis aclada (Fresenius), a causal agent of neck rot in onion (Allium cepa L.) bulbs. The assay uses TaqMan probe-based chemistry to detect an amplicon from the L45-550 region of B. aclada while using a DNA sequence from the onion serine acetyl transferase gene (SAT1) as a control. The assay detection limits for B. aclada and onion were 10 pg·μL−1 of genomic DNA. The detection limit for lyophilized B. aclada mycelium was 1 μg. The presence of onion tissue in the samples did not affect the performance of the real-time PCR assay. The assay distinguished among different amounts of B. aclada mycelium growing on onion disks that were inoculated with 0, 102, or 104 B. aclada conidia. Visual observations during the incubation period corresponded with changes in real-time PCR results. This assay could be used to determine the amount of B. aclada mycelium in bulbs during growth, harvest, and storage, thus giving researchers an objective and efficient tool by which to quantify the growth rate and virulence of B. aclada strains in vivo.