Understanding the mechanisms behind onion bulb dormancy in relation to the potential for improved onion storage

Onion (Allium cepa L.) is the most economically important Allium crop. Onion cultivars adapted for growth in temperate regions require long days for bulb initiation, so the summer crop is stored over-winter. Deterioration in store is largely due to resumption of growth. Extended storage of onions is currently dependent on the synthetic sprout suppressant maleic hydrazide, whose future use is uncertain due to pressure to reduce residues in foods. Abscisic acid (ABA) has previously been linked with dormancy in a relative of onion (A. wakegi). Bulb ABA concentration in three onion cultivars (viz. Renate, Ailsa Craig and SS1) with contrasting storage lives declined exponentially during controlled atmosphere (CA – 3% CO2, 5% O2; 2°C) storage at the same rate in each cultivar. Sprouting occurred at a minimal ABA concentration (ca. 50-120 ng g-1 DW). It was proposed that extended periods of high concentrations of ABA may delay sprouting. An ABA analogue (PBI-365) and exogenous ABA, were applied as preharvest foliar sprays (cvs. Renate, Carlos, Dinaro, Hysam, Red Baron and SS1), or as postharvest bulb soaks (cv. Hysam) with the aim of increasing endogenous ABA concentration and, thereby, extending the storage period. Endogenous bulb ABA concentration was not affected. Bulb ABA concentration again decreased during storage at a range of temperatures (4, 12 and 20°C) and sprouting occurred at minimal ABA concentration (ca. 75-150 ng g-1 DW). After the onset of sprouting, ABA concentration increased again, probably due to synthesis by the sprout. The concentration of certain carbohydrates has been linked with storage potential. No straightforward relationship between ABA and non-structural carbohydrate (NSC; fructose, glucose, sucrose and fructans) metabolism could be determined. It was therefore postulated that ABA is more likely to play a role in mediating cell elongation rather than cell division, or that minimal ABA concentration could be a trigger for remobilisation of carbohydrates. Controlled atmosphere (CA) is used to extend storage life of onions; however, shelf-life can be compromised. The effects of the transition between CA (3 % CO2, 5 % O2; 2°C) and air storage on ABA concentration, quality characteristics, respiration rate(RR) and NSCs in three onion cultivars (viz. Renate, Carlos and SS1) were investigated. The RR of the short storing cultivar, SS1, was greatest by ca. 0.5-fold. The RR increased on removal from CA storage, with no accompanying decrease in carbohydrate concentration, indicating that the increased RR may have been a transient stress response. Storage of onions cv. SS1 for three weeks in air, followed by three weeks CA was as effective in suppressing sprout growth as six weeks continuous CA storage. Bulb ABA concentration decreased significantly between the time of harvest and after curing. Therefore the current practice of curing onions for extended periods at high temperatures could be reducing bulb ABA concentration, and therefore storage life. The literature concerning the role of ethylene in onion storage is limited and conflicting. The effect of 1-methylcyclopropene (1-MCP; an ethylene perception inhibitor) on sprout growth in onions cv. SS1 stored at a range of temperatures (4, 12 and 20°C) was investigated, along with quality characteristics, NSCs and ABA. Sprout growth was reduced in onions treated with 1-MCP and stored at 4 or 12°C, but not at 20°C. Approximately 2-fold greater concentrations of sucrose, glucose and fructose were maintained in 1-MCP-treated bulbs stored at 12°C as compared with untreated bulbs. It appeared that 1-MCP reduced carbohydrate metabolism. Both ethylene and ABA are highly likely to impact significantly on onion storage life. The results are discussed in relation to the potential to influence storage life by changes in horticultural practices, including the recommendation to re-evaluate the curing and drying protocol, and to investigate the possibility of delaying the start of CA storage.