Onion can be propagated three different ways: by direct seeding, from transplants, and through sets (small dry onion bulbs raised specifically for planting) (Serra and Currah, 2002). Although sets require more investments than direct seeding, sets are easier to grow successfully and are widely used by small-scale producers (Brewster, 2008; O’Connor, 2006). The use of onion sets is an important practice in Europe, The Netherlands, Poland, and Hungary being the main producers of onion sets (O’Connor, 2006). Good-quality onions from sets can be grown by small-scale producers under almost any conditions, and some professional growers also use sets when irrigation is unavailable (O’Connor, 2006). For example, in Hungary, growing onions from sets without irrigation is still often the practice (Barnóczki, 2004). However, omitting irrigation can result in very low yields in extremely dry years, endangering profitability of onion production (Barnóczki, 2004).
Storage cultivars with high dry matter content are often produced from sets (Serra and Currah, 2002). Dry matter content in onions ranges from 7% to 22%, with typical values of 10% to 12%. From 60% to 80% of the dry matter is made up of non-structural carbohydrates (Brewster, 2008). Onions contain several valuable phytonutrients, including organo-sulfur and -selenium compounds, vitamins, steroid saponins, and phenols (Keusgen, 2002). Onion bulbs are especially rich in flavonoids belonging to the flavonol, dihydroflavonol, and anthocyanin subgroups (Pérez-Gregorio et al., 2014; Slimestad et al., 2007). As a result of these compounds, onions have medicinal properties, for example antibiotic, antithrombotic, antiasthmatic, and anticancer effects (Griffith et al., 2002).
Although onions can survive long periods with a shortage of water, their photosynthetic activity and growth rate are very sensitive to water stress (Brewster, 2008). Accordingly, onions react well to irrigation under arid conditions, producing higher yields and good bulb quality with increased watering and irrigation frequency regardless of the type of irrigation system used (Enciso et al., 2009; Kumar et al., 2007; Leskovar et al., 2012; Pejic et al., 2011; Shock et al., 2000). For onions, the irrigation criterion for soil water tension varies from 8.5 to 45 kPa, depending on the environmental conditions and irrigation system (Shock and Wang, 2011). However, excessive watering can actually reduce yields in regions with adequate rainfall (Brewster, 2008), and occasionally irrigation can reduce dry matter content in onions (Pejic et al., 2011).
Water supply effects on the nutritive value of onions raised from sets have not been investigated in detail. Therefore, the objective of this study was to examine the influence of irrigation on dry matter concentration, storage carbohydrates, vitamin C, total flavonols and polyphenols of onions raised from sets compared with a rain-fed control.
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