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.
Sweet potato [Ipomoea batatas (L.) Lam.] cv. Travis storage roots were bedded on 8 and 27 Mar. 1985 according to standard cultural practices. The following row cover and mulch treatments were tested for their effects on early transplant production: a) bare soil (control); b) black polyethylene mulch; c) nonwoven polyester row cover; d) slitted, clear polyethylene row cover; or e) slitted, white polyethylene row cover. All row cover and mulch treatments increased soil temperature (at a 5-cm depth) relative to the bare soil. Compared to black polyethylene mulch, slitted, clear polyethylene row covers increased soil temperatures, whereas soil temperature was reduced by slitted white polyethylene. Mulch and row covers increased weight and number of large transplants as compared to bare soil. The primary effect of row covers as compared to mulch was to increase the percentage of large (>15 cm long) transplants. The use of slitted, clear polyethylene and slitted, white polyethylene row covers appeared to reduce the incidence of sclerotial blight (Scleratium rolfsii) in plant beds. Row cover and mulch treatments did not alter yield of storage roots grown from transplants produced under the various bed treatments.
Field experimentation with sweet potatoes [Ipomoea batatas (L.) Lam.] is difficult because extraneous sources of error often require extensive replication and multiple experiments (1). These problems are compounded when cultivars of different growth habits are compared in adjacent plots. Because sweet potato plants produce extensive prostrate vines, there has been considerable controversy among researchers as to whether cultivars planted in adjacent rows compete with one another such that cultivar evaluation is compromised. This controversy was recently asserted by Bouwkamp (2), who used turning point tests and showed that the competitive effects of cultivars do not affect sweet potato yield trial results enough to justify the use of multi-row plots (2).
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.
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.
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 (SO42-), 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 42- or N levels, most precursors and peptides measured were present in very low concentrations. When SO 42- 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 42- 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.
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.
Ethylene production by wounded sweet potato (Ipomoea batatas L.) roots varied with storage time and method of wounding. Preliminary experiments showed that wounded cured roots produced more ethylene than freshly harvested roots and roots held in 15°C storage for up to 22 weeks. The longer the sweet potato roots were in storage, the greater the ethylene production from wounded roots. On average, broken roots produced 30% more ethylene than cut roots. Maximum ethylene production occurred 2 to 4 days after wounding. Since ethylene was not detectable 6 hr after injury, it should be referred to as stress-induced rather than wound ethylene.
Cured storage roots of sweet potato [Ipomoea batatas (L.) Lam. cv. Centennial] were wounded and recured for 12 days with or without 2,5-norbornadiene, AOA, or CoCl2 treatments. Ethylene production, wound lignification, and wound periderm formation were measured 0, 3, 6, 9, and 12 days after wounding. Ethylene production preceded wound lignification and wound periderm formation by 24 to 48 hr, respectively. Blocking ethylene action with 2,5-norbornadiene increased ethylene production, blocked wound periderm formation for up to 12 days, and strongly suppressed and delayed lignification. Blocking ethylene synthesis with AOA or CoCl2 decreased ethylene production to 10% of the control. Lignification and wound periderm formation were also suppressed and initiation delayed. These results suggest that ethylene is involved in lignification and periderm formation in wounded sweet potato roots. Chemical names used: bicyclo[2.2.l]hepta-2,5-diene (2,5-norbornadiene); (aminooxy) acetic acid (AOA).
The influence of common leaf rust (Puccinia sorghi Schw.) on 2 sweet corn (Zea mays L.) hybrids was compared in rusted and nonrusted plots for several maturity and ear quality characters. Differences were found for time of silking, ear length, ear diameter, and percentage of moisture between rusted and nonrusted plots. The percentage of Brix ranged from 4% to 25%, with the rusted plots always having reduced mean values. Correlations (P = 0.01) were found between ear diameter and percentage of moisture, percentage of Brix and percentage of moisture, and between ear length and ear diameter.