Total sulfur and sulfate accumulation in onion is affected by sulfur fertility

Because sulfate (SO4-2) accumulation patterns were unknown in onion, a study was conducted to determine how three onion varieties accumulated total bulb-sulfur (S) and bulb-SO4-2 in response to increasing S-fertility. Plants were greenhouse grown in pots containing washed river sand and fertilized with five different nutrient solutions possessing SO4-2 concentrations of 5, 25, 50, 75, or 150 mg liter(-1) until the plants were mature. Bulbs were harvested and analyzed for total-S, SO4-2, and pungency, as measured by enzymatically developed pyruvic acid (EPY). Bulbs accumulated significant levels of SO4-2 and S. The amount of bulb-SO4-2 and bulb-S increased linearly as S-fertility increased. The three varieties differed in total bulb-S, bulb-SO4-2, the percent of total bulb-S accumulated as SO4-2, and EPY. Bulb-SO4-2 ranged from 0.047 to 0.318% dry mass in response to S-fertility level and variety, while total bulb-S ranged from 0.154 to 0.535% dry mass. Diverse S and SO4-2 accumulation and utilization patterns denoted complex S metabolism for onion in relation to flavor development.