Investigation into the agronomic and biological factors affecting post-harvest bruising in Pastinaca Sativa

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Booth, David (2021). Investigation into the agronomic and biological factors affecting post-harvest bruising in Pastinaca Sativa. University of Birmingham. Ph.D.

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Abstract

The aim of this study was to provide novel insight into bruising susceptibility in Pastinaca sativa (parsnip) roots to facilitate industry-focused solutions. Bruising damage contributes significantly towards pack-house losses, which regularly surpass 50%, and so identifying the causes of post harvest damage is required to maximise quality and minimise economic losses.

Throughout this study, three varieties of parsnip (V1, V2 and V3) were employed to investigate varietal differences in bruising susceptibility. A bruise replication protocol was developed and bruise severity was calculated, with bruising frequency also contributing towards overall bruising susceptibility. In field trials, variety and harvest date were found to significantly affect bruising severity, whilst irrigation protocol influenced the frequency of bruises, but not bruise severity. Fully irrigated roots harvested later in the year elicited greater bruising than earlier harvested parsnips; V3 elicited the greatest bruise severity (3.09 ± 0.43) across the whole trial during the final harvest, whilst the most bruise resistant variety (V1) elicited a bruise severity of 1.72 ± 0.24. It was found that irrigation scheme did not significantly affect the severity of bruises; however, droughting roots caused the bruising frequency to decrease in all varieties during the first and second harvests.

Post-harvest factors including impact force (g), storage temperature (oC) and storage duration were found to significantly affect bruising susceptibility in parsnips. Bruising severity increased as impact magnitude increased, with the most severe bruising being witnessed in the highest impact group after 72 hours at 20 oC storage (10.58 ± 3.14). In comparison, the lowest impact group elicited a bruising severity of 0.20 ± 0.10 under the same storage conditions. Storing roots post-harvest at 6 oC, as is industry practice, reduced the severity of bruises present compared to higher storage temperatures.

Quantification of impact forces exerted onto roots throughout processing was achieved via employment of a tri-axial accelerometer housed into a 3D-printed shell (“electronic parsnip”). Analysis of impact forces via the electronic parsnip facilitated the identification of destructive processes, comparison of processes across industry, and testing of modifications to find the least destructive working practises. Across packhouse B, polishing exerted the greatest total impact force (g) (197.13 ± 18.03), whilst plastic packing exerted the lowest (64.85 ± 12.30 g). The most destructive process across the entire study was polisher I (734.03 ± 26.89 g), however a significant 55% reduction in force exerted was achieved via modifications to the barrel and brush settings of this polisher.

Scanning electron micrographs indicated that cell rupture and membrane leakage in bruised parsnip tissue were associated with tissue browning. The inherent discolouration potential of parsnip tissue did not vary across varieties, but the bruising response did. V3 was the only variety where damage initiated a significant increase in solute leakage, and across all harvests V3 exhibited greater % tissue solute leakage than the bruise resistant varieties. V3 also had the highest tissue relative water content across all harvest dates during both field trials. This suggests that bruising susceptibility in parsnips is determined not by phenolic and enzymatic activity, but rather solute leakage, concurring with previous research.

Following RNA sequencing and the denovo construction of a parsnip transcriptome, a number of damage inducible genes were found to be overexpressed. The phenylpropanoid pathway was significantly upregulated by V1 and V2 following damage, however it was not upregulated by V3. A total of 6 phenylalanine ammonia lyase (PAL) genes were upregulated by V1, V2 upregulated 5 PAL genes whilst V3 did not upregulate any. V3 also did not upregulate any polyphenol oxidase (PPO) genes, whereas V1 upregulated 3 PPO genes, 2 of which were homologous with V2. All 3 varieties significantly upregulated tyrosine decarboxylase (TYDC) as a response to damage. V1 and V2 also upregulated 12 peroxidase (POD) genes, whilst V3 upregulated one POD gene (PNC1). This indicates differences in the transcriptional response to damage between bruise resistant and susceptible varieties. The molecular mechanism for parsnip tissue browning is thus suggested via PPO, POD, TYDC and PAL following cell rupture and membrane leakage, as a result of mechanical impacts during harvesting and processing.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):