Edible alliums are important crops worldwide. Approximately 46% of the international trade value in alliums is for dry bulb onions (Brewster, 2008). World dry bulb onion production in 2009 was 72 million t cultivated on 202,446 ha (FAO, 2010). The major producing countries in 2008 were China, India, and the United States with 21, 14, and 3 million t, respectively (FAO, 2010). Bulb onions are produced from the subarctic regions to the humid tropics, although they are best adapted to production in subtropical and temperate areas (Brewster, 2008).
Improvements in water use efficiency in agriculture are needed as a result of scarcity of fresh water, increasing costs, and increasing world population growth (Bessembinder et al., 2005). Agriculture uses 70% of the developed water supplies in the United States (FAO, 2010); therefore, water conservation programs are expected to play a pivotal role to improve water use efficiency. Significant water savings can be achieved by deliberately stressing plants to a certain profitable level. This management technique is generally known as deficit irrigation. If irrigation rates are reduced at predetermined developmental stages where deficits would not severely impact productivity, it is called regulated deficit irrigation, which was first proposed by Chalmers et al. (1981) for peach [Prunus persica (L.) Batsch] trees. Because of increasing water scarcity, it is expected that deficit irrigation would be adopted for a wide variety of crops and in more regions, especially in arid and semiarid climates of the world (FAO, 2002). Furthermore, the application of growth stage-specific crop coefficients (KC) for irrigation management in onion can provide precise water applications to meet crop water demand, resulting in greater yields, crop quality, and enhanced water use efficiency (Piccinni et al., 2009).
Subsurface drip irrigation allows the application of small amounts of water to the soil through drippers placed directly in the root zone, thereby improving irrigation water use efficiency, nitrogen use efficiency, and profitability (Halvorson et al., 2008; Shock et al., 2007). Patel and Rajput (2008, 2009) reported higher yields in onion cultivated with subsurface drip when compared with surface drip irrigation. High irrigation efficiencies in onions can also be achieved with a center pivot using drops converted to low-energy precision application heads placed at ≈0.3 m above the ground (Piccinni et al., 2009).
It is questionable whether conventional plant spacing results in maximum yield, particularly for root and tuber crops. In accordance with Caliskan et al. (2009), the optimization of plant density is of great importance in maximizing yield, because increasing plant density per unit area normally results in competition between plants for growth resources such as solar radiation, water, and nutrients, whereas suboptimal densities lead to waste of these inputs.
Onions are primarily consumed for their flavor and ability to enhance the flavor of other foods (Kopsell and Randle, 1997). Flavor intensity in onion is dominated by organosulphur compounds arising from the enzymatic decomposition of S-alk(en)yl-L-cysteine S-oxide when the cells are mechanically ruptured. This reaction produces thiopropanol S-oxide, pyruvic acid, ammonia, and many sulfur volatiles (Whitaker, 1976). The determination of pyruvate as an indicator of pungency is the most commonly established method for pungency assessment in onion. Pungency level and total soluble solids are important quality attributes of onion bulbs for processing and storage. Pungency contributes to postharvest life and processing quality (Dhumal et al., 2007), and it is an indicator of the onion flavor intensity (Abayomi et al., 2006).
Onion has significant nutritional and medicinal properties such as anticarcinogenic, antiplatelet activity, antithrombotic activity, and antiasthmatic and antibiotic effects (Griffiths et al., 2002). They are good sources of polyphenolic flavonoids such as quercetin (aglycone and its glycosides), which comprise a large group of natural antioxidants (Caridi et al., 2007; Mogren et al., 2007). The content of these phytonutrients in onions is highly dependent on genetic (genotype, cultivar, etc.) as well as environmental factors. For instance, yellow, red, and pink onions have been shown to contain higher quercetin concentrations, up to 286 μg·g−1 fresh weight, than white onions (Patil et al., 1995). The effect of irrigation strategies on quercetin content of onion remains largely unknown.
Previous investigations have not evaluated the interactive effects of PD and deficit irrigation rates using specific growth-stage crop coefficients as a tool for evapotranspiration-based irrigation management of short-day onions. Selecting improved irrigation practices for onions is crucial for production in water-limited areas of the world. The aim of this 2-year study was to determine the optimal combination of PD and subsurface drip irrigation level that would lead to water savings while maintaining yield and quality of short-day onion.
Abayomi, L.A., Terry, L.A., White, S.F. & Warner, P.J. 2006 Development of a disposable pyruvate biosensor to determine pungency in onions (Allium cepa L.) Biosens. Bioelectron. 21 2176 2179
Bekele, S. & Tilahum, K. 2007 Regulated deficit irrigation scheduling of onion in a semiarid region of Ethiopia Agr. Water Mgt. 89 148 152
Bessembinder, J.J.E., Leffelaar, P.A., Dhindmal, A.S. & Ponsioen, T.C. 2005 Which crop and which drop, and the scope for improvement of water productivity Agr. Water Mgt. 73 113 130
Boyhan, G.E., Purvis, A.C., Randle, W.M., Torrance, R.L., Jeffernson Cook, M. IV, Hardisson, G., Backley, R.H., Paradise, H., Hill, R.C. & Panlk, J.T. 2005 Harvest and postharvest quality of short-day onions in variety trials in Georgia, 2000–03 HortTechnology 15 694 706
Brewster, J.L. 2008 The classification, origins, distribution and economic importance of the major vegetable crops in onions and other vegetable alliums 2nd Ed CABI Wallingford, UK
Brewster, J.L. & Salter, P.J. 1980 The effect of plant spacing on the yield and bolting of two cultivars of overwintered bulb onions J. Hort. Sci. 5 97 102
Caliskan, M.E., Kusman, N. & Caliskan, S. 2009 Effects of plant density and yield components of true potato seed (TPS) hybrids in early and main crop potato production systems Field Crops Res. 114 223 232
Caridi, D., Trenerry, V.C., Rochfort, S., Duong, S., Laugher, D. & Jones, R. 2007 Profiling and quantifying quercetin glucosides in onion (Allium cepa L.) varieties using capillary zone electrophoresis and high performance liquid chromatography Food Chem. 105 691 699
Cecílio Filho, A.B., May, A., Porto, D.R.Q. & Barbosa, J.C. 2009 Crescimento da cebola em função de doses de nitrógeno, potasio e da população de plantas em semeadura directa Hort. Brasileira 27 49 54
Chalmers, D.J., Mitchel, P.D. & van Heek, L. 1981 Control of peach tree growth and productivity by regulated water supply, tree density and summer pruning J. Amer. Soc. Hort. Sci. 106 307 312
Dellacecca, V. & Lovato, A.F.S. 2000 Effects of different plant densities and planting systems on onion (Allium cepa L.) bulb quality and yield Acta Hort. 533 197 203
Dhumal, K., Datir, S. & Pandey, R. 2007 Assessment of bulb pungency level in different Indian cultivars of onion (Allium cepa L.) Food Chem. 100 1328 1330
Ells, J.E., McSay, A.E., Soltanpour, P.N., Schweissing, F.C., Bartolo, M.E. & Kruse, E.G. 1993 Onion irrigation and nitrogen leaching in the Arkansas Valley of Colorado 1990–1991 HortTechnology 3 184 187
Enciso, J., Wiedenfeld, B., Jifon, J. & Nelson, S. 2009 Onion yield and quality response to two irrigation scheduling strategies Sci. Hort. 120 301 305
Halvorson, A.D., Bartolo, M.E., Reule, C.A. & Berrada, A. 2008 Nitrogen effects on onion yield under drip and furrow irrigation Agron. J. 100 1062 1069
Jiménez, M., De Juan, J.A., Tarjuelo, J.M. & Ortega, J.F. 2010 Effect of irrigation uniformity on evapotranspiration and onion yield J. Agr. Sci. 148 139 157
Kalra, A., Sood, D.R. & Pandey, U.C. 1995 Studies on growth, pungency and flavour characteristics of different varieties of onions during bulb development J. Food Sci. Technol. 32 189 192
Kanton, R.A.L., Abbey, L., Hilla, R.G., Tabil, M.A. & Jan, N.D. 2002 Density affects plant development and yield of bulb onion (Allium cepa L.) in Northen Ghana J. Veg. Crop Production 8 5 25
Kumar, S., Imtiyaz, M. & Kumar, A. 2007a Effect of differential soil moisture and nutrient regimes on postharvest attributes of onion (Allium cepa L.) Sci. Hort. 112 121 129
Kumar, S., Imtiyaz, M., Kumar, A. & Singh, R. 2007b Response of onion (Allium cepa L.) to different levels of irrigation water Agr. Water Mgt. 89 161 166
Lee, Y., Howard, L.R. & Villalón, B. 1995 Flavonoids and antioxidant activity of fresh pepper (Capsicum annuum) cultivars J. Food Sci. 60 473 476
Leskovar, D.I., Kolenda, K., Yoo, K.S. & Pike, L.M. 2004 Containerized transplants and subsurface drip improved yield and quality of short-day onions Acta Hort. 631 155 160
Mogren, L., Olsson, M.E. & Gertsson, U.E. 2007 Effects of cultivar, lifting time and nitrogen fertiliser level on quercetin content in onion (Allium cepa L.) at lifting J. Sci. Food Agr. 87 470 476
Patel, N. & Rajput, T.B.S. 2008 Dynamics and modelling of soil water under subsurface drip irrigated onion Agr. Water Mgt. 95 1335 1349
Patil, B.S., Pike, L.M. & Yoo, K.S. 1995 Variation in the quercetin content in different colored onions (Allium cepa L.) J. Amer. Soc. Hort. Sci. 120 909 913
Piccinni, G., Ko, J., Marek, T. & Leskovar, D.I. 2009 Crop coefficients specific to multiple phenological stages for evapotranspiration-based irrigation management of onion and spinach HortScience 44 421 425
Saghir, A.R., Mann, L.K. & Yamaguchi, M. 1965 Composition of volatiles in allium as related to habitat, stage of maturity and plant part Plant Physiol. 40 681 685
Shock, C.C., Feibert, E.B. & Saunders, L.D. 1998 Onion yield and quality affected by soil water potential as irrigation threshold HortScience 33 1188 1191
Shock, C.C., Feibert, E.B. & Saunders, L.D. 2004 Plant population and nitrogen fertilization for subsurface drip-irrigated onion HortScience 39 1722 1727
Shock, C.C., Feibert, E.B. & Saunders, L.D. 2007 Short-duration water stress decreases onion single centers without causing translucent scale HortScience 42 1450 1455