Understanding the biochemical, molecular and physiological effects of postharvest treatments to improve onion storage

Abstract

In the UK, consumers and retailers demand that there be a continuous supply of onions all year round. To meet this demand, prehavest and postharvest methods are used to improve onion quality and storage life. Onions are currently cured at 28°C for 3-6 weeks to seal the neck of the bulbs thereby reducing disease incidence and esh moisture loss. Onions are then transferred to long term storage in ambient conditions or cold storage with controlled atmosphere 'storage or continuous ethylene depending on their intended storage duration. This study aimed to review and improve current UK curing and storage practises by investigating the biochemical, physiological and molecular effects these postharvest regimes have on the esh and skins of onion bulbs. Reducing curing temperatures from 28°C to 20°C was found to reduce the oxidation and degradation of avonols and anthocyanins, respectively, resulting in paler brown skins and more intense red skins. Following curing, onions intended for long term storage can be stored in continuous ethylene. This said, both ethylene and the ethylene binding inhibitor, l-methylcyclopropene (1-MCP), applied for just 24 h before or after curing reduced orion sprout growth for up to four months storage. However, onion sprout suppression beyond four months cold storage would still require continuous ethylene treatment. To elucidate the mechanisms by which ethylene and l-MCP reduce sprout growth, the rst onion microarray was developed. A cluster ofprobes were found to be down regulated in response to ethylene alone or in combination with 1-MCP. This, along with biochemical and physiological results, suggested ethylene and 1-MCP may both reduce sprout growth via different mechanisms by binding different ethylene receptors.