Optimising onion (Allium cepa L.) bulb quality for counter-seasonal export markets

Onion bulbs cultivated in the Southern Hemisphere for counter-season, Northern Hemisphere markets must be of high quality at the point of presentation to the consumer. Consistently achieving this requires an in-depth understanding of the interaction between pre- and post-harvest factors that affect bulb quality and storage life. This industry-focussed study specifically investigated key factors that affect storage life (sprouting) in Tasmanian, Australia, grown bulbs. These factors were identified in consultation with industry and a review of the scientific literature. Onion bulb production in Tasmania, a cool-temperate region, is a low cost production system that takes advantage of the region's long dry summers that enable bulbs to be lifted out of the ground (termed lifting‚ÄövÑvp) prior to complete canopy senescence and field curing in windrows. The timing of lifting based on plant growth and development events associated with lodged crop canopies (termed tops-down‚ÄövÑvp) was one of the pre-harvest factors investigated in this study; 80% tops-down is used commercially as an indicator of optimal bulb development for lifting. Internal sprout growth and respiration rate during long term storage were generally lowest in bulbs lifted at ca 90% tops-down and increased with early or late lifting treatments. These data support the recommendation that to optimise bulbs for long term storage, lifting should be changed from 80% to ca 90% tops-down. While 80% tops-down is used by industry as a marker to determining time of lifting, forecasting this event to schedule lifting operations is difficult, because it is not known how to predict the time at which foliage lodging begins, or the length of time to 80% tops-down. Changes in the mechanical properties of pseudostem bases were evaluated using a custom-built tool, which measured stem ovalisation under transverse load as a proxy for susceptibility to structural failure via Brazier buckling. Softening of tissues at the pseudostem base was found to precede and govern canopy collapse within and among cultivars. Monitoring of this softening provided a pre-lodging indicator of the development progression to pseudostem failure (tops-down) independent of genotype and environment. Post-harvest factors investigated include the effect of mechanical impacts during handling on bulb respiration rate, weight loss and sprout growth. Cured bulbs from different crops sampled during commercial grading were found to have different respiration rates prior to handling, demonstrating the interaction between pre- and post-harvest factors. In subsequent controlled impact experiments, higher respiration rates and faster sprout growth were recorded when bulbs were impacted on the base plate, and also with greater impact magnitudes. Minimising the force of impacts, and avoiding impacts to the base plate are thus recommended as best practice for the handling of onions intended for long term storage and the sensitivity of bulbs to impacts in future studies should control impact location. Lifting the crop at ca 90% tops-down reduces sprout growth and respiration rates during long term storage. To enable prediction of this important stage, this study demonstrates that foliage collapse is both preceded and governed by softening of the neck tissues, which leads to structural failure via Brazier buckling. Measuring deformation at a constant load may be used to estimate a crops susceptibility to lodging, and may lead to the development of predictive models. Finally, storage life can also be increased by minimising mechanical impacts, particularly to the base plate, which increases respiration rate and weight loss, and accelerates sprout growth.