Biomarkers for onion storage

Dr. Leon A. Terry, Head of Plant Science Laboratory Cranfield University, UK, (l.a.terry@cranfield.ac.uk), March 2009.

The aim of onion bulb storage is to meet consumer demand for extended availability of onions whilst maintaining product quality. The principal biological factors leading to onion bulb deterioration are respiration, resumption of growth and pathogen attack. In onion bulbs a dormant period, when sprouting and rooting can not be induced, is followed by a period of internal changes that prepare the bulb for breaking of dormancy and subsequent growth. Out of storage, the bulb then proceeds towards flowering and seed production. Sprouting occurs when the leaf primordia that are produced in stored onion bulbs develop green leaves, rather than scale leaves, which elongate and eventually protrude from the neck of the bulb. The growth rate of the sprout inside the bulb varies according to cultivar and storage regime, and is a major factor in determining the storage life of onions. Temperature has a profound effect on the dormancy period and storage life of onion bulbs. In general, sprouting is inhibited both by low and by high temperatures, and encouraged at intermediate temperatures. Cultivars respond differentially to temperature. Typically, the optimum temperature range for sprouting in dry storage is 10-20ºC. Moisture loss is greater at temperature ranges <10ºC and >27ºC.

Many biochemical characteristics change during storage. These include changes in water content, the concentrations of flavour-related compounds, organic acids, carbohydrates, plant growth regulators and phenolics. Onions are eaten for their unique taste and the supposed health giving properties of their sulphur containing flavour compounds, S-alk(en)yl-L-cysteine sulphoxides (ACSOs). Total ACSO content is positively correlated with enzymatically produced pyruvate, which in turn is positively correlated with pungency. Water-soluble carbohydrates in onion bulbs include fructose, glucose, and sucrose, and a series of oligosaccharides called fructans (Davis et al., 2007), and can constitute some 60-80% of the dry weight. Fructan concentration in onion bulbs tends to decrease during cold storage. Biochemical changes during storage are likely to be linked with respiration. All nutrients required for growth of the sprout must come from within the bulb; therefore, changes in certain key characteristics might be used to predict the onset of sprouting. Maximal and minimal concentrations of certain substances are known to coincide with sprouting, but there is currently no biochemical assay that anticipates sprouting.

During storage a gradual change in the relative composition of plant growth regulators occurs as the concentration of growth promoters and/or growth inhibitors rise or fall, respectively. Abscisic acid (ABA) is a naturally occurring phytohormone. Endogenous ABA is found in all onion tissues. The absolute concentration of ABA varies according to plant age and tissue type, and is also influenced by environmental challenges such as drought stress prior to the falling of green leaves at bulb maturity, and has been associated with dormancy in onions. In a recent study at Cranfield University, onion bulbs of cultivars with long, medium and short storage lives, viz. Renate, Ailsa Craig and SS1, respectively, were stored in controlled atmosphere (CA) conditions (3.03 kPa CO2; 5.05 kPa O2; 2ºC). In all cultivars, bulb ABA concentration declined exponentially during storage. Onion bulb ABA concentration at harvest (measured on a fresh weight basis) may prove to be a better indicator of storage life.

It has been postulated that carbohydrate content is correlated with storage life. In a recent study, the effect of the transition between CA and air (and vice versa) on onion cvs. Renate, Carlos and SS1 in terms of the respiration rate, ABA concentration and non-structural carbohydrate composition was assessed. Removal of bulbs from CA storage resulted in an immediate increase in the respiration rate (measured in air), which then reverted to a lower rate following subsequent storage under air conditions for 21 days. In some cultivars, this could be sufficient to trigger the onset of sprouting and thus account for the detrimental effect of CA storage on shelf life. Delaying the start of CA storage of onions cv. SS1 for 21 days was as effective in suppressing sprout growth as CA storage for 42 days. There was a significant decrease in the ABA concentration between the time of harvest and the beginning of storage. This is likely to be due to the effects of curing and suggests that curing is having a detrimental effect on storage potential (Chope et al., 2007).

A newly commissioned project under the auspices of a Defra HortLink project (HL0182* Sustaining UK fresh onion supply by improving consumer acceptability, quality and availability) aims to create a more competitive and expanded UK onion industry based on energy efficient and residue-free long-term storage.