Onion (Allium cepa L.) is an important crop cultivated worldwide and is used mostly for various culinary purposes (Brewster, 2008). They are typically employed for food uses though other non-food uses have been reported globally. The onion is a rich source of phosphorous, calcium sodium and fibre with no fat and is an important component of folk medicine (Marwat et al., 2011). Extracts from onions have been utilized as insecticides for control of field pests, skin healing for post-surgical needs and as medical agents for various bodily disorders. Furthermore, onions have been studied through diverse in vitro and in vivo methodologies as being functional for health. Scientific investigations have reported them to bear antidiabetic, antimicrobial, anti-inflammatory, anti-obesity and antioxidant potentials and these functionalities have been attributed to the phytochemicals they possess.
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Onion bulbs have high water content and display an active life after harvesting. Onions are characterized by three major periods: periods: rest, dormancy and re-growth (sprouting). After harvest, onions go through a rest period for 4-6 weeks and during this rest period, they do not respond to rapid environmental change and there is no visible cellular activity (Jones and Mann, 1963). The activity of endogenous hormones (cytokinins, gibberellins and auxin) is very low and inhibit inhibitor activity is high. After completion of the rest period the onion enters its period of dormancy. During the maturating period of bulb onions, i.e., the final growth period before top fall-down, growth inhibitory substances are believed to be translocated from the leaves to the bulbs (Komochi, 1990), which then enter a phase of dormancy. During dormancy, onion bulbs respond to environmental changes. The effect of this environmental change is slower in the early stage of dormancy and more rapid in the later stage until dormancy is finally lost. Even though the onion bulb is in its dormant stage, the source-to-sink transition takes place at a greatly reduced level that keeps the bulb metabolically active, but the change is unnoticeable (Lang et al., 1987). It was found that during storage both carbohydrates and nitrogen compounds from the outer thickened leaf base were decomposed into available forms, translocated to the inner thickened leaf base and accumulated there in succession. (Kato 1966)
Afterwards there is a gradual loss of dormancy and younger inner leaves continue to elongate within the bulb (Abdalla and Mann, 1963). Sprouting is physiological phenomenon that comes after dormancy breakage and it is a major problem during the storage of onion. The timing of rooting and subsequent sprouting was investigated and conclude that roots appeared first, followed by sprouts (Gvinianidze et al., 2020). Sprouting occurs when these leaves become visible. Sprouting begins with the re-allocation of water and metabolites from scales to a base plate. The sprouts originate from this base plate and this reallocation of Phyto-nutrients is responsible for the formation of new cells and cell elongation. Before harvest and during sprout growth, there is an increase in mitotic activity in onion meristem.
On another hand, onion sprouting is typically considered as a quality loss occurrence which causes such onions to be regarded as waste and discarded. However, studies have shown that in similarity with other vegetables, sprouting also enhances the nutritional content of onions. Flavonoids and other phenolics have been reported to be considerably improved due to onion sprouting. Dormancy of bulbs has major impact on their storage. One of the most popular approaches in the study of dormancy has been to study its physiological basis in sprouts during subsequent exposure to rest-breaking treatments at low temperature (Benkeblia & Selselet-Attou, 1999). Gradual changes in the biochemistry of the bulbs throughout the dormancy period were observed and low temperatures caused compositional and physiological changes in tissues. (Benkeblia et al., 2002)
Rotting and re-growth increase the rate of respiration, heat generation and consequently enhance moisture loss and reduce the shelf life are the major factors of deterioration in onion bulbs during storage (Doug, 2004). Cold storage of onion is rare and the growers generally store onions with traditional techniques (Tariq et al., 2005). The quality of onion bulbs is better retained at low temperature (0°C which inhibit sprouting and decay, thus ensure longer storage life. By contrast storage at high temperature (18-21°C) results in increased total storage losses (Krawiec, 2002). To prevent sprouting during long-term storage, it has for many years been normal practice to apply maleic hydrazide to the onion crop shortly before harvest. To prevent sprouting during long-term storage, it has for many years been normal practice to apply maleic hydrazide to the onion crop shortly before harvest. Ethylene supplementation has previously been shown to delay and suppress sprouting in stored onion bulbs.
Changes during sprouting
Onion’s bulbs have evolved as a storage organ to allow the plant to over-winter. During the transition from dormancy (endodormancy; dormancy reliant on conditions or factors within the bulb) to sprout suppression (eco-dormancy; dormancy reliant on external environmental factors) and subsequent growth, the bulb undergoes the transition from sink organ to source to sustain cell division in the meristematic tissue. Many physiological and biochemical characteristics change during onion bulb storage, including water content and the concentration of flavour compounds, carbohydrates, minerals and plant growth regulators (PGRs).
Changes in these characteristics are likely to be linked to respiration and remobilization of carbohydrates to provide energy for the growing sprout. All nutrients required for growth of the sprout must come from within the bulb; therefore, changes in the concentrations of key metabolites could be used to predict the onset of sprouting. Fluctuations in certain metabolites are known to coincide with sprouting, but there is currently no biochemical or molecular assay that accurately predicts sprouting. Dry weight, NSCs, pyruvate and flavonols can be used to assess onion bulb quality. These characteristics differ both between cultivars and during storage. Onion bulbs with high dry weight are more suitable for longer term storage and tend also to contain higher concentrations of fructans.
Soluble sugars are required to provide energy for sprout growth and so the concentration of soluble sugars decreases when sprouting occurs. Quercetin 4′ -glucoside and quercetin 3,4′-diglucoside are the dominant flavonol compounds in onion flesh and have been shown to increase during curing. Storage of onion bulbs can affect chemical composition during storage, since many changes have been reported, e.g., changes in glucose and pyruvate content (Abayomi and Terry 2009), changes in flavonol and sugar contents (Sharma et al., 2014), soluble solids content, bulb fimness, increased pungency, pH and flavonols (Coolong et al. 2008) and antixidant activity. (Gennaro et al. 2002)
Mitigation of sprouting
Storage can be extended by forcing the onion bulbs into an Endo dormant state through exposure to threshold low temperatures or from the preharvest application of chemical inhibitors. Pre-harvest sprays have been widely applied without impairing the keeping quality of onion. Chemicals tried so far for mitigating sprouting and enhancing shelf life in onion includes maleic hydrazide (MH), abscisic acid (ABA), gibberellin (GA3), auxin, cytokinin (CK), cycocel (CCC), ethrel and paraquat (Anbukkarasi et al, 2013). Gamma irradiation and use of fungicides are other mitigation strategies used for the inhibition of sprouting. To prevent sprouting during long-term storage, it has for many years been normal practice to apply maleic hydrazide to the onion crop shortly before harvest. After translocation to the shoot apex, maleic hydrazide disrupts cell division and prevents development of the leaves, which in untreated onions elongate into sprouts. On the other hand, the ethylene supplementation has previously been shown to delay and suppress sprouting in stored onion bulbs. Onion bulbs are low endogenous ethylene producers (Cools et al., 2011); nonetheless, continuous exogenous ethylene supplementation at 10 μL/L during storage suppresses sprout growth (Chope et al., 2012). Exposure of onion to ethylene during storage was reported to reduce sprouting (Bufler, 2009). Qadir et al (2007) revealed that application of ethanol delayed rooting, sprouting and reduced decay in onion cv. Tazan for a period of one-month storage.
Cytokinins play a role in sprouting by stimulating cell division. Removal of the primordial roots gives less chance for the accumulation of cytokinins and that inhibits sprouting. Abscisic acid (ABA) has been identified as part of the inhibitory complex present in onion bulbs and has been demonstrated to play a functional role in maintenance of dormancy in Allium wakegi through the application of exogenous ABA and fluridone, an inhibitor of ABA biosynthesis. Various other chemicals such as streptocyclin, bavistin, copper sulphate, salicylic acid, sodium thiosulphinate, etc. were sprayed fifteen days before harvesting to control rotting and sprouting. Combined application of carbendazim and maleic hydrazide (1000 and 2000 ppm respectively) reduced the percentage of rotting, fungal infection and sprouting as compared to the individual treatments in onion crop. (Sable and Kalebere, 2004)
Elaborate work has been done with gamma irradiation as a means of extending the storage life of common onion. Nuttall et al. (1961) tried various doses, viz., 3.2, 4.7, 7.9 and 9.5 krad and in most of the treatments, sprouting inhibited up to 300 days in storage of onion bulbs. Langerak (1975) recommended 75 Krads as the optimal dose for onions and higher doses intensified discolouration and caused off-flavour. An environmentally friendly alternative to the use of maleic hydrazide and other chemical inhibitors would be to prolong the dormancy of bulb onions by altering the pre-harvest growing conditions.
References:
1. Abayomi LA, Terry LA (2009) Implicatins of spatial and temporal changes in concentration of pyruvate and glucose in onion (Allium cepa L.) bulbs during controlled atmosphere storage. J Sci Food Agr 89: 683 –687
2. Abdalla, A. A. and Mann, L. K. (1963). Bulb development in the onion (Allium cepa L.) and the effect of storage temperature on bulb rest. Hilgardia, 35, 85±112.
3. Anbukkarasi, V., Paramaguru, P., Pugalendhi, L., Ragupathi, N., & Jeyakumar, P. (2013). Studies on pre- and post-harvest treatments for extending shelf life in onion–A review. Agricultural Reviews, 34(4), 256-268.
4. Benkeblia, N.; Selselet-attou, G. Changes in oligosaccharides, phenolics and peroxidase activity in inner bud of onion bulbs during break of dormancy by low temperatures. Acta Agriculture Scandinavica, v.49, p.98-102, 1999.
5. Benkeblia, N.; Varoquaux, P.; Shiomi. N.; Sakai, H. Effect of irradiation, MH and CIP treatments on respiration rate and oligosaccharides variations in onion bulbs in store. International Journal of Food Science and Technology, v.37, p.169-176, 2002.
6. Brewster JL. 2008. Onions and other vegetable alliums. 2nd ed. Wallingford: CAB Int; p. 433.
7. Briddon, A., Sbeu, B., 2006. The use of ethylene for sprout control. Br. Potato Counc. Res. Rev No. R279 29.
8. Bufler, G., 2009. Exogenous ethylene inhibits sprout growth in onion bulbs. Ann. Bot. 103, 23–28. https://doi.org/10.1093/aob/mcn203.
9. Chope, G.A., Cools, K., Hammond, J.P., Thompson, A.J., Terry, L.A., 2012. Physiological, biochemical and transcriptional analysis of onion bulbs during storage. Ann. Bot. 109, 819–831. https://doi.org/10.1093/aob/mcr318
10. Coolong TW, Randle WM, Wicker L (2008) Structural and chemical diffrences in the cell wall regions in relatin to scale fimness of three onion (Allium cepa L.) selections at harvest and during storage. J Sci Food Agr 88:1277–1286
11. Cools, K., Chope, G.A., Hammond, J.P., Thompson, A.J., Terry, L.A., 2011. Ethylene and 1-methylcyclopropene differentially regulate gene expression during onion sprout suppression. Plant Physiol. 156, 1639–1652. https://doi.org/10.1104/pp.111. 174979.
12. Doug, W. 2004. Food and Rural Revitalization. Plant Sci. Department University of Saskatchewan, Canada. Onions Production in Saskatchewan.htm
13. Gennaro L, Leonardi C, Esposito F, Salucci M, Maiani G, Quaglia G, Fogliano V (2002) Flavonoid and carbohydrate contents in tropea red onions: effects of homelike peeling and storage. J Agr Food Chem 50:1904–1910
14. Gvinianidze TN, Margvelashvili EG, Kopaliani RS. Biologically Active Compounds and Antioxidant Activity of Zeibel 5455 Grape- Seed Super fluid Extracts. Adv Biotech & Micro.
2020;15(3):555915.DOI:10.19080/AIBM.2019.14.555915(http://dx.doi.org/10.19080/AIBM.2019.14.555915)
15. Jones H A and L KMann (1963) Onion and their allies. Inter-science Pub., New York
16. Kato T (1966) Physiological studies on the bulbing and dormancy of onion plant. V111. Relation between dormancy and organic constituents of bulbs. Journal of the Japanese Society for Horticultural Science 35(2): 142-151.
17. Komochi, S. (1990). Bulb dormancy and storage physiology. In: Onions and allied crops, Vol. I, (Rabinowitch, H. D. and Brewster, J. L., Eds). CRC Press, Inc., Boca Raton, Florida, USA, 89, 111.
18. Krawiec, M. 2002. Evaluation of storage losses of onion sets depending on the storage temperature Folia Horticulturae, (Poland). 14(1): 13-20.
19. Lang GA, Early JD, Martin GC, Darnell RL (1987) Endo para and Eco dormancy: physiological terminology and classification for dormancy research. Hort Science 22: 371-377.
20. Langerak, D.I. (1975). The influence of in-adiation and packaging on the keeping quality of prepacked cut endive, chicory and onions. Hort. Abstr.. 46(1): 314.
21. Marwat, S.K., F. Rehman, M.A. Khan, M. Ahmad, M. Zafar and S. Ghulam. 2011. Medicinal folk recipes used as traditional phytotherapies in District Dera Ismail Khan. KPK. Pakistan. Pak. J. Bot., 43(3): 1453-1462.
22. Nuttall, V.W., L.H. Lyall and K.E Me Queen. (1961). Some effects of gamma irradiation on stored onions. Hort. Abstr., 32(2): 3065.
23. Qadir A, F.Hashinaga and M.R. Karim (2007). Effects of pre storage treatment with ethanol and CO2 on onion dormancy. J.Bio Sci. 15:55-62
24. Sharma K, Assefa AD, Kima S, Koa E, Parka SW (2014) Change in chemical composition of onion (Allium cepa L. cv. Sunpower) during post-storage under ambiente conditions. New Zeal J Crop Hort Sci 42:87–98
25. Tariq, A., B. Abdul, and M. Khan. 2005. Assessment of post-harvest losses of onion bulbs during storage at room condition. Sarhad J. Agric., 21(2): 189-191.
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