Short-day Onion Cultivars Research Articles

Short-day onion cultivars differ in bulb selenium and sulfur accumulation which can affect bulb pungency

The close chemical similarity of S and Se allow for antagonistic behavior between the two elements. To determine variation for Se accumulation within onions, sixteen short-day cultivars were grown under high level (2.0 mg Na2SeO4 per litre) and no Se treatments. Selenium accumulation in bulb tissues was cultivar dependent, ranging from 60 to 113 µg Se per g dry wt. High Se fertility enhanced bulb S accumulation. Pungency was reduced in some cultivars grown under Se fertility when compared to the no Se treatment indicating that although S uptake was enhanced, S metabolism in the flavour precursor biosynthetic pathway was affected. The existing variability in onion Se uptake and accumulation have important implications for Se intake in mammalian diets.

Control of Bolting in Autumn-sown Sweet Onions through Undercutting

The efficacy of undercutting as a technique to control bolting of two short-day onion cultivars was studied in controlled-environment chambers. `Buffalo' and `Granex 33' onions were grown to the third, fifth, and seventh visible leaf stages in a 10-hour photoperiod at 22/18 °C (day/night) and then exposed to 30, 40, 50, 60, or 70 days of vernalizing temperatures (10/10 °C). Half of the plants were undercut at the initiation of the vernalizing treatment. After vernalizing treatments, plants were returned to 14-hour photoperiods at 22/18 °C. `Buffalo', which is resistant to bolting, did not flower significantly under any of these conditions. The flowering response of `Granex 33' increased with leaf number at vernalization and as the duration of vernalization increased. Undercutting `Granex 33' increased the days of vernalization required for flowering and reduced the proportion of flowering relative to controls. Overall dry-matter accumulation was unaffected by leaf number at vernalization or the duration of vernalization but was reduced ≈30% by undercutting. In both cultivars, fresh mass per bulb decreased with increasing leaf stage of vernalization and number of vernalizing days. Undercutting also decreased fresh mass per bulb, but through its effect on bolting, undercutting increased marketable yield for plants vernalized and undercut at the fifth and seventh leaf stages.

Postharvest Losses from Delayed Harvest and during Common Storage of Short-day Onions

A 2-year field study was conducted to evaluate the effects of maturity and storage on fresh-market onion (Allium cepa L.) quality. Four short-day onion cultivars (`NuMex BR1', `NuMex Sunlite', `NuMex Starlite', and `Buffalo') were seeded in early October each year. Bulbs were harvested at five times; the first and second harvests were when 20% and 80 %, respectively, of the bulbs in a plot had mature necks; the third, fourth, and fifth harvests were at 5,10, and 15 days after the second harvest date, respectively. After curing for 3 days, bulb firmness, weight, and incidence of disease were evaluated for all harvests. Bulbs were re-evaluated after 10 and 20 days storage in a shed under ambient conditions. `Buffalo' and `NuMex Surdite' bulbs had the lowest incidence of disease before storage. For all cultivars, average bulb weight increased and firmness decreased with delayed harvest. Percent diseased bulbs increased for all cultivars as harvest was delayed in 1991 but not in 1992. The optimum harvest time was at 80% maturity. In storage, average bulb weight and firmness decreased, and the incidence of bulb diseases increased.

443 PB 250 PUNGENCY EVALUATION OF FIFTEEN SHORTDAY ONION CULTIVARS

Fifteen shortday onion cultivars grown at two production locations (GB and ST) in the Lower Rio Grande Valley, Texas were evaluated for pungency levels using gas chromatography (GC) and pyruvic acid tests. Significant differences (P=0.05) were observed between cultivars in the pyruvic acid and GC tests within each location. Pyruvic acid content ranged from 3.0 to 5.1 μmol·g-1 fresh wt. The amount of total sulfur volatiles measured by the GC method ranged from 28 × 103 to 58 × 103EU. The correlation coefficients between GC and pyruvic acid were 0.10*** and 0.18*** at the GB and ST location, respectively. When the two locations were combined, no significant differences (P=0.05) were observed between cultivars or locations using the GC test. However, the pyruvic acid test showed significant differences between locations. This result indicated that each cultivar had a different response in pungency as influenced by production location or environment.

Pungency and Sugars of Short-day Onions as Affected by Sulfur Nutrition

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.

Ontogeny and Sulfur Fertility Affect Leaf Sulfur in Short-day Onions

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.

SULFUR UPTAKE AND UTILIZATION IN SHORT-DAY ONION CULTIVARS

Sulfur fertility influences flavor in onions. To determine how sulfur is utilized by onions over the course of growth and development, five short-day onion (Allium cepa L.) cultivars were grown at high (4.0 mequiv.liter-1) and low (0.1 mequiv. liter-1) sulfur fertility regimes under greenhouse conditions. Plants were measured for foliar sulfur content at 4 week intervals during the course of the growing season and for bulb sulfur and pyruvic acid development at plant maturity. Patterns of sulfur uptake and utilization were similar for all five cultivars but cultivars differed in their magnitude of sulfur accumulation. Foliar sulfur accumulation correlated poorly with bulb sulfur and pyruvic acid development at all sampling dates.