Physiology of onion bulbs destined for export markets (VN12000)

What was it all about?

This project, which ran from 2015 to 2018, investigated the factors underpinning the robustness of onion bulbs, to help maintain the quality advantage of onions being exported to Northern Hemisphere countries, which involves six to eight weeks of transit through the tropics. Robust bulbs are those that have not broken dormancy and that have retained both firmness and skins during shipping under ambient conditions.

The research investigated the role that both pre-harvest growth and development factors and post-harvest mechanical impacts play in maintaining this robustness.

It found that, in general, bulbs that received mechanical impacts during grading and sorting had higher respiration rates, which were linked to greater weight loss and reduced storage life. The extent to which mechanical impacts reduce storage life was further explored by controlling the number, magnitude and position of mechanical impacts to bulbs in laboratory studies. While respiration rate and sprout growth during storage was increased from greater impacts, the study provided new evidence that bulbs were more sensitive to impacts to the base plate than to impacts to the equator or neck.

This was the first study to report the high degree of sensitivity of the onion baseplate to mechanical impact.

Furthermore, bulbs from different crops varied in storage life potential. The project found that bulbs with low respiration rate prior to grading had longer storage life, indicating that pre-harvest factors can play an important role in determining bulb sensitivity to impacts and storage life. This crop-to-crop variation in bulb respiration rate prior to grading provided, in part, an explanation to the differences among crops in robustness and storage life reported by industry.

Rate of bulb growth and timing of interruption to its development (lifting for in-field curing) were investigated as possible explanations for the crop-to-crop variation in bulb robustness and storage life. While growth rate had only a minor influence on bulb storage life, the timing of interruption was found to be important — lifting bulbs from the soil for curing when the foliage of between 80 and 100 per cent plants in a crop had collapsed resulted in a longer storage life than lifting earlier or later in the stage of development.