Onion (Allium cepa L.) is one of the most important commercial crops grown worldwide. Based on the minimum daylength required to bulb formation, onions are classified into short-, intermediate-, and long-day types. Short-day onions can begin bulb development when daylength exceeds 11–12 h, intermediate onions between 13 and 14 h, and long-day types more than 16 h (Brewster, 1994). In the Southwestern United States, most short-day onion cultivars are direct-seeded between early September and early November and grown to maturity from May to June. During early development, onions seedlings are highly susceptible to soil-borne diseases, extreme temperatures, and weeds, often reducing plant stands (Leskovar et al., 2004a). To compensate for these losses in plant population, growers typically increase the seeding density (Leskovar et al., 2012).
In some growing regions of the world, especially Mexico, onions are planted using containerized transplants grown previously in greenhouses. It is well known that a high-quality onion transplant can produce higher yield and more uniform bulbs (Leskovar and Vavrina, 1999). However, the use of transplants is not a common practice followed in the United States because of the considerably higher cost of production when using multiple-cell tray containers (Leskovar et al., 2004b) and the lack of adaptable transplanting machines. In Georgia, the estimated transplant cost of production and setting is ≈$1370 per ha (Boyhan et al., 2009), whereas it is more than $3500 per ha in Florida and Texas in 2018 (Speedling, Inc., personal communication). To offset this cost, multiseeding in individual cells has been tested as an alternative to provide a greater number of seedlings per unit area at lower cost per plant (Leskovar et al., 2004a; Rubatzky and Yamaguchi, 1997). An early field study by Leskovar et al. (2004a) aimed at producing medium bulbs for export to Brazil, compared the effect of different establishment techniques in onion yield production, including bare-root and containerized transplants grown from one, two, and three seedlings per cell in 228 cell tray, which later were individually separated at transplanting or planting with two or three plants without separation. They found that total yields were not statistically different among different densities in containerized and bare-root transplants, and that two and three seeds per cell were best to obtain the target 50–70 mm bulb diameter category. Therefore, planting multiple seeds in individual cells can be an alternative and cost-efficient system for onion production compared with direct seeding and onion sets.
Onion transplants grown under low light conditions [photosynthetically active radiation (PAR) < 800 mmol·m−2·s−1] and low temperatures in the United Kingdom showed a negative correlation between transplant size and time to onset of bulbing and maturity, and positive correlation to bulb weight. Conversely, transplants grown under high light intensities (PAR > 1500 mmol·m−2·s−1) and high temperatures in Sri Lanka showed a positive correlation between transplant size, onset of bulbing, and maturity (Mettananda and Fordham, 1999). Therefore, manipulation of the transplant size during the nursery stage could optimize the time of bulbing and final bulb size. With favorable light and temperature conditions, small transplants should bulb earlier, whereas it has been reported that large onion transplants should mature later and increase bulb size (Brewster, 1990b).
The interaction of planting date and daylength influences plant survival, growth, bulbing, and ultimately bulb size. Previous research showed that late plantings during autumn in the Northern Hemisphere usually result in a reduction in plant survival and yield (Richwine, 1990). Gaskell (1998) observed that planting dates are critical for superior foliage development and bulb size. An experiment conducted with short- and intermediate-day onions in Queensland, Australia (lat. 27°35′S, long. 152°23′E) in the Southern Hemisphere showed April plantings producing the greatest yield as compared with February, March, May, and June plantings (Jackson et al., 1997).
In New Mexico, early September plantings resulted in high levels of bolting and low yield; mid to late September reduced bolting and increased yields; whereas mid-October produced no bolting, but reduced yields (Corgan and Kedar, 1990). In Georgia, onions can be transplanted starting the first week of November until the last week of December, and still obtain reasonable yield and quality. For example, onions transplanted in December had a lower yield compared with those transplanted in November, but showed no differences from the onions transplanted in January (Boyhan et al., 2009). Similar results were observed by El-Rehim et al. (1997) in Egypt, where higher yields resulted from transplanting in November rather than February. Thus, determining optimal sowing and transplanting dates are critical to maximizing yield and bulb quality.
In 2016, onion represented the fifth most valuable vegetable in the United States with a fresh market value of $925 million (USDA, 2016). Its value is attributed to its unique pungency and the ability to enhance other food flavors. Onion pungency is produced by the release of the enzyme allinase reacting with the flavor precursors S-alk(en)yl-cysteine sulfoxide when cells are disrupted (Lancaster and Boland, 1990). In addition, onions have numerous health benefits because they are a rich source of various phytonutrients such as anthocyanins, phenolic acids, flavonoids, and thiosulfates. Flavonoids continue to attract attention because of their potential implication in cardiovascular diseases, inflammation, and cancer (Middleton et al., 2000). Onions also have significant medicinal properties such as anticarcinogenic, antiasthmatic, antibiotic effects, antithrombotic activity, and antiplatelet effects (Griffiths et al., 2002). Antioxidants can inhibit oxidative reactions and help in functional performance of enzyme systems for self-defense mechanisms in cells (Lee et al., 2009). It is well known that plant chemical properties could vary according to weather conditions such as temperature and rainfall (Patil et al., 2012). To our knowledge, there are no studies emphasizing the influence and interactions of three management strategies (seedling density, planting date, and cultivar) on growth, yield, and onion quality. Thus, this study aims to evaluate the effects of three sequential planting dates and two seedling densities in multicelled trays on growth, yield components, bulb quality, and phytonutrient content of three short-day onion cultivars differing in scale color.
Arce, J.P. 1995 Effect of planting date, nitrogen rate, and in-row plant spacing on the maturity, diasease infection severity, pungency, bolting and doubling incidence, and yield of selected short-day onions. Texas A&M Univ., College Station, TX, PhD Thesis
Boyhan, G.E., Torrance, R.L., Cook, J., Riner, C. & Hill, C.R. 2009 Sowing date, transplanting date, and variety effect on transplanted short-day onion production HortTechnology 19 66 71
Brewster, J. 1994 Onion and other vegetable Allium. International Cambridge, Wellington, UK
Brewster, J. 1997 Environmental physiology of the onion: Towards quantitative models for the effects of photoperiod, temperature and irradiance on bulbing, flowering and growth Acta Hort. 433 347 374
Brewster, J., Lawes, W. & Whitlock, A. 1987 The phenology of onion bulb development at different sites and its relevance to incomplete bulbing (‘thick-necking’) J. Hort. Sci. 62 371 378
Brewster, J.L. 1990a Onion and allied crops: Botany, physiology and genetics. CRC Press, Boca Raton, FL
Brewster, J.L. 1990b Onions and allied crops: Agronomy, biotic interactions, pathology and crop Protection. In: D. Rabinowitch and J.L. Brewster (eds.). Cultural system and agronomic practices in temperate climates. CRC Press, Boca Raton, FL
Caruso, G., Conti, S., Villari, G., Borrelli, C., Melchionna, G., Minutolo, M., Russo, G. & Amalfitano, C. 2014 Effects of transplanting time and plant density on yield, quality and antioxidant content of onion (Allium cepa L.) in southern Italy Scientia Hort. 166 111 120
Corgan, J.N. & Kedar, N. 1990 Onion cultivation in subtropical climates, p. 31–47. In: H.D. Rabinowitch and J.L. Brewster (eds.). Onions and allied crops. CRC Press, Boca Raton, FL
Currah, L. & Proctor, F.J. 1990 Onions in tropical regions. Natural Resources Institute, Overseas Development Administration, Kent, UK
El-Rehim, G., Ahmed, F., Shalaby, G. & Waly, A. 1997 Effect of transplanting date and planting density on bulb yield and quality of Giza 20 onion in upper Egypt. Egyptian J. Agr. Res. 75 681 695
Finch-Savage, W. & Phelps, K. 1993 Onion (Allium cepa L.) seedling emergence patterns can be explained by the influence of soil temperature and water potential on seed germination J. Expt. Bot. 44 407 414
Gaskell, M. 1998 Effect of transplant date and transplant size on production, quality and pungency of sweet onions. Newsletter Articles, Small Farms and Specialty Crops
Hamilton, B.K., Yoo, K.S. & Pike, L.M. 1998 Changes in pungency of onions by soil type, sulphur nutrition and bulb maturity Scientia Hort. 74 249 256
Jackson, K., Duff, A.A. & O’Donnell, W.E. 1997 Development of a suite of onion varieties for the subtropical region of the Lockyer Valley in SE Queensland, Australia Acta Hort. 555 239 242
Kandil, A.A., Sharief, A.E. & Fathalla, F.H. 2013 Effect of transplanting dates of some onion cultivars on vegetative growth, bulb yield and its quality Crop Production 2 72 82
Khokhar, K., Kaska, N., Hussain, S., Qureshi, K. & Mahmood, T. 1990 Effect of different sowing dates, direct seeding and transplanting of seedling on maturation, bulb weight and yield in onion (Allium cepa) cultivars Indian J. Agr. Sci. 60 668 671
Lancaster, J.E. & Boland, M.J. 1990 Flavor Biochemistry, p. 33–72. In: J.L. Brewster and H.D. Rabinowitch (eds.). Onion and allied crop. CRC Press, Boca Raton, FL
Lee, E.J., Yoo, K.S., Jifon, J. & Patil, B.S. 2009 Characterization of shortday onion cultivars of 3 pungency levels with flavor precursor, free amino acid, sulfur and sugar contents J. Food Sci. 74 475 480
Lee, J.E., Patil, B.S. & Yoo, K.S. 2015 Antioxidant of 15 onions white, yellow, red colors and their relationship with pungency and quercetin Food Sci. Technol. 63 108 114
Leskovar, D., Cantamutto, M., Marinangelli, P. & Gaido, E. 2004a Comparison of direct-seeded, bareroot, and various tray seedling densities on growth dynamics and yield of long-day onion Agronomie 24 35 40
Leskovar, D.I., Kolenda, K., Yoo, K.S. & Pike, L.M. 2004b Containerized transplants and subsurface drip improved yield and quality of short-day onions Acta Hort. 631 155 160
Leskovar, D.I., Agehara, S., Yoo, K. & Pascual-Seva, N. 2012 Crop coefficient-based deficit irrigation and planting density for onion: Growth, yield, and bulb quality HortScience 47 31 37
Mettananda, S.A. & Fordham, Sr. 1999 The effects of plant size and leaf number on the bulbing of tropical short-day onion cultivars (Allium cepa L.) under controlled environments in the United Kingdom and tropical field conditions in Sri Lanka J. Hort. Sci. Biotechnol. 74 622 631
Middleton, E., Kandaswami, C. & Theoharides, T.C. 2000 The effects of plant flavonoids on mammalian cells: Implications for inflammation, heart disease, and cancer Pharmacol. Rev. 52 673 751
Patil, D., Dhake, A., Sane, P. & Subramaniam, V. 2012 Studies on different genotypes and transplanting dates on bulb yield of high solid white onion (Allium cepa L.) under short-day conditions Acta Hort. 969 143 148
Peffley, E.B., Ortiz, M. & Corgan, J.N. 1981 A technique for onion cold hardiness evaluation: Effects of plant age and size on hardiness HortScience 16 773
Pérez-Gregorio, R.M., García-Falcón, M.S., Simal-Gándara, J., Rodrigues, A.S. & Almeida, D.P.F. 2010 Identification and quantification of flavonoids in traditional cultivars of red and white onions at harvest J. Food Compos. Anal. 23 592 598
Rubatzky, V.E. & Yamaguchi, M. 1997 Vegetables consisting of succulent roots, bulbs, leaves and fruits, p. 279–304. In: Science (ed.). World vegetables: Principles, production and nutrient values. AVI, Wesport, CT
Tesfay, S.Z., Bertling, I., Odindo, A.O., Greenfield, P.L. & Workneh, T.S. 2011 Growth responses of tropical onion cultivars to photoperiod and temperature based on growing degree days Afr. J. Biotechnol. 10 15875 15882
USDA 2016 Vegetables and pulses production yearbook data (online). In: Service., U.N.A.S. (ed.)
Yoo, K.S., Lee, E.J. & Patil, B.S. 2011 Underestimation of pyruvic acid concentrations by fructose and cysteine in 2,4-dinitrophenylhydrazine-mediated onion pungency test J. Food Sci. 76 1136 1142