Although it is well known that onion yields are reduced by moisture stress, the impact of water shortages on quality is less clear. Onions exposed to at least mild moisture stress, especially late in the growing season, tend to be more pungent with a higher dry matter content than onions grown with consistently abundant supplies of moisture. Stronger flavored onions with a high dry matter content tend to maintain their quality during long term storage. When onions are being grown for the onion ring market, a strong flavor is not overly desirable, but a high proportion of bulbs with a single center is very important. Production contracts often stipulate the minimum % of single centers (typically > 75%). The tendency to produce single versus multiple centers is influenced by both genetics and production conditions. In a 2007 trial evaluating Spanish type onions, the U of S found that most available cultivars had > 90% multiple centers when grown under typical production conditions in Saskatchewan. The sole exception was Teton Hybrid from Norseco – 100% of its bulbs had single centers. The excessively high proportion of bulbs producing multiple centers in this trial suggests that the production conditions may not have been ideal. Any factor that interrupts bulb development will lead to an increased proportion of bulbs with multiple centers. Shock et al (2007) found that a single short period of moderate drought stress caused a significant increase in double centers – often without having any impact on bulb yields. The onion plants appeared most susceptible to this stress when they were in the 4-6 leaf stage. Although yields from the U of S trial were excellent, the high proportion of bulbs with multiple centers suggests that even more careful moisture management of the crop is required.
Source : University of Saskatchewan Vegetable Cultivar Trials for 2007 (http :
Shock, Feibert and Saunders (2007). HortScience 42: 1450-1455.