Effects of water stress at specific growth stages on onion bulb yield and quality

One onion (Allium cepa L.) quality parameter, the percentage of single-center bulbs, has become important to meet demands of both processing and fresh market buyers, who generally require a minimum of 70% single-center bulbs. Field studies on commercial drip-irrigated farms were conducted in 1999, 2000, and 2002 to determine the effects of soil-water stress imposed at different plant growth stages on long-day onion yield and quality including the percentage of single-center bulbs. Irrigation was withheld at four onion growth stages (3, 5, 7, and 9-leaf) or not at all in the control. In addition, one treatment had water stress imposed at both the 3 and 7-leaf stages. When soil-matric potential was ≤−70 kPa (approximately 40–50% depletion of available water for the soils at the sites) at 0.15 m soil depth, irrigation was restored to farmer’s normal levels for the duration of the growing season. Analysis of the three-year data, showed that total yield was reduced by soil-water stress imposed at any growth stage but the greatest effect was at the 5-leaf, 7-leaf, and 3- and 7-leaf stages. Soil-water stress caused by withholding irrigation at both the 3- and 7-leaf stages reduced yields by 26% compared with the control. The percentage of single-center onion bulbs was lower when the soil-water stress occurred earlier in the growing season than when the stress occurred later. Based on the three-year data analysis, only the control and 9-leaf treatments met the industry standards for percent single-center bulbs. Compared to the control, the percentage of single-center bulbs was reduced by 40, 32, and 18% when soil-water stress was imposed at the 3- and 7-leaf, 3-leaf, and 5-leaf stages, respectively.