The role of abscisic acid and ethylene in onion bulb dormancy and
sprout suppression

 

 

Gemma A Chope and Leon A Terry^*

Plant Science Laboratory, Cranfield University, Bedfordshire, UK

 

 

 

Abstract

Purpose of review: Research to extend onion storage life has concentrated on plant breeding, husbandry and the storage environment, with relatively little attention being paid to the mechanisms involved in onion bulb dormancy. This article highlights the roles of abscisic acid (ABA) and ethylene in onion bulb dormancy with relation to recent results from the authors and previous work by others.

Recent findings: Temporal variation in ABA concentration was measured in onion cultivars with different storage potentials. Bulb ABA concentration declined during storage, and the pattern of decline was similar for all cultivars, although the initial ABA concentration (on a dry weight [DW] basis) in short-storing bulbs was 2.5-fold less than in long-storing bulbs. The storage potential of bulbs of different onion cultivars was inversely related to the time at which they reached a minimal ABA concentration (ca. 50–120 ng/g DW). Continuous exposure to ethylene in store has recently been introduced as a method of increasing onion storage life, but sprout growth was also reduced in short storage onions treated with 1 µL/L of the ethylene binding inhibitor, 1-methylcyclopropene (1-MCP), for
24 h at 20°C and subsequently stored at 4 or 12°C, but not when stored at 20°C. ABA concentration before storage has previously been shown to be correlated with storage life; however, there were no differences in the ABA concentration between 1-MCP-treated and -untreated onion cv. SS1 bulbs. Greater (ca. 2-fold) concentrations of sucrose, glucose and fructose were measured in 1-MCP-treated bulbs stored at 12°C as compared with untreated bulbs, suggesting that 1-MCP reduced the rate of carbon utilisation during storage. The contrast between the use of ethylene to extend storage commercially, and the use of an inhibitor of ethylene perception to extend storage demonstrates that there may be a dichotomy in the role that ethylene has to play in onion dormancy. The role that ethylene and ABA have on carbon utilisation and sprouting in onions is discussed.

Directions for future research: In the future, the challenge of extending the storage life of onion bulbs without the use of sprout suppressants could be tackled using a genetic or molecular biological approach. ABA has clearly been identified as playing an important role in onion storage. The pathways involved in ABA biosynthesis and catabolism have been elucidated, but in order to understand the molecular mechanisms, the genes and enzymes involved in ABA biosynthesis and catabolism must be studied in more detail. Further investigation of the genes involved in ethylene metabolism would also be useful.

 

Keywords: Allium cepa L.; 1-methylcyclopropene; storage; sprouting

 

Stewart Postharvest Review 2008, 2:5

Published online 01 April 2008

doi: 10.2212/spr.2008.2.5