Fructans redistribution prior to sprouting in stored onion bulbs is a potential marker for dormancy break

I.C Ohanenye,M.C Alamar,A.J Thompson,L.A Terry

doi:10.1016/j.postharvbio.2018.12.002

Abstract

Continuous supply of high quality onion bulbs to meet year-round demand is dependent on maintaining dormancy and bulb quality during storage. Sprouting impacts negatively on the storage quality of onion bulbs. Ethylene supplementation has previously been revealed to inhibit sprout growth in stored onion bulbs. Fructans content, especially those at higher degree of polymerisation (DP), are reported to positively correlate with delayed sprouting. However, little is known about the impact of pre-harvest irrigation regimes on fructans accumulation and redistribution in relation to onion bulb dormancy and quality in store. Across two seasons, onion plants of cultivars ‘Red Baron’ and ‘Sherpa’ were subjected to full irrigation (FI) (100% replenishment of crop evapotranspiration) or deficit irrigation (DI) (50% of FI treatment) from bulb initiation to harvest. Bulbs were harvested at full maturity and stored at 1 °C for five months. Bulbs were treated with or without 1-MCP (1 μL L−1) for 24 h before storage under continuous ethylene supplementation (10 μL L−1) or air. DI had no effect on dormancy-break, sprout emergence, total fructans content and total sugar content. In contrast, ethylene delayed sprout emergence and suppressed sprout growth; added 1-MCP enhanced this effect. The concentration of DP3-8 fructans were higher in top and bottom sections compared to the baseplate. Before sprout emergence, fructans of DPs 7–8 were no longer present in the top and bottom wedges, while they accumulated in the baseplate; irrespective of pre- or postharvest treatments. This redistribution of fructans within the bulb suggested a transition in dormancy state and could be used as a predictive marker for sprouting in stored onion bulbs.

Highlights

Despite being a seasonal crop, the demand for onion bulbs is allyear-round, the onion industry relies on innate dormancy and storage treatments to extend availability
Bulb storage weight-loss was continuous throughout the storage period, there were no significant differences between pre- or postharvest treatments (Appendix C – Fig. B1 [2015] and D [2016])
Deficit irrigation had no effect on sprout emergence, or the accumulation or distribution of total fructans content within the onion bulbs under cold storage

Summary

Introduction

Despite being a seasonal crop, the demand for onion bulbs is allyear-round, the onion industry relies on innate dormancy and storage treatments to extend availability. 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; Chope et al, 2012); continuous exogenous ethylene supplementation at 10 μL L−1 during storage suppresses sprout growth (Briddon and Sbeu, 2006; Bufler, 2009; Cools et al, 2011; Chope et al, 2012). Further evidence of ethylene sprout suppression in other crops was revealed by Foukaraki et al (2014) (potatoes) and Amoah et al (2016) (sweet potatoes).

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