Potentiel d'entreposage à long terme de la carotte après une introduction de résistance naturelle aux maladies par la la lumière UV-C

Authors: Kouassi, Niankan
Advisor: Arul, JosephTweddell, Russell
Abstract: As there is no fungicide currently registered for postharvest application on carrot, the long-term storage of this crop is limited because of the postharvest decay, which shortens the storability. UV-C hormesis is the stimulation of beneficial effects at low doses of this potentially harmful stressor at high doses, and is considered as one of the most promising alternatives to fungicides for preservation and long-term storage of carrots. The overall objective of this study was to develop a prestorage treatment with UV-C to induce disease resistance in carrots and increase their storage life. The methodology included the determination of the hormetic (optimum) dose to induce disease resistance in carrots against Botrytis cinerea, Sclerotinia sclerotiorum, Rhizoctonia carotae and Pythium sulcatum; the evaluation of time delay after harvest before UV-C treatment and storage temperature on carrot resistance; and the evaluation of the persistence of UV-C induced resistance in carrots during the long-term storage. The induced resistance was evaluated by challenge inoculation with pathogens and natural infections of the treated carrots, and defenswe mechanisms such as the accumulation of the phytoalexin, 6-methoxymellein (6-MM), the polyacetylenes falcarinol and falcarindiol, myristicin and the induction of defense proteins, PR-2 (β-1,3-glucanases) and PR-3 (chitinases). The progress of senescence was evaluated by the severity of sprouting and rooting. There was a significant quadratic dose-response relationship between UV-C dose and resistance to B. cinerea or S. sclerotiorum, R. carotae, and P. sulcatum. Carrot resistance to diseases progressively increased compared with control roots, up to a dose of about 5.4 kJ.m-2. At the hormetic dose, a maximum of about 55% reduction in growth of the challenged organisms was observed and beyond that dose, disease resistance decreased. Storage temperature had also an impact on the inhibition of the B. cinerea growth on carrots, and the inhibition was greater on the treated carrots and stored at 4 °C during 70 days. In addition, during 7 months of the long-term storage, the naturally occurring infections, as well as the senescence indicators such as sprouting and rooting were delayed by about two months on treated carrots compared with the non-treated roots. Prestorage treatment with the hormetic UV-C dose elicited the production the phytoalexin 6-MM in carrot peel with maximum accumulation level of about 120 µg.g-1 FW with the hormetic UV-C dose. Regardless of the storage temperature, the level of 6-MM reached a certain maximum, followed by a decrease, and thereafter, it remained steady. At 4°C, 6-MM reached the highest maximum value (120 µg.g-1 FW), and decreased gradualy with storage time to reach a level of about 45 µg.g-1 FW after seven months. It is likely that this carrot phytoalexin plays an important role in the roots long-term induced resistance, since it is known that the ED50 of 6-MM inhibitory for the growth of decay agents is 20 µg.g-1. At the storage temperature of 24 °C, it appears that 6-MM accumulates at the fastest rate, but also degrades at the fastest rate resulting in lowest maximum as well as steady state level, presumably due to high sensitivity of carrot tissue at higher storage temperatures The polyacetylenes falcarinol and falcarindiol were present in untreated carrots and they are likely to be preformed antimicrobial compounds. The prestorage UV-C treatment did not have any impact on falcarindiol and falcarinol at any storage temperature. UV-C treatment does not seem to affect falcarinol and falcarindiol accumulation; hence those compounds tend to be preformed. There was an accumulation of myristicin in carrots treated by UV-C. However, the role of this compound in disease resistance and its fungistatic effects remain unclear. Immunoblotting analysis revealed two constitutive bands at 38 kDa and 45 kDa for both PR-2 (β-1,3-glucanases) and PR-3 (chitinases) proteins. There was a progressive and significant enhancement of 38 kDa of PR-2 (3 to 4 folds the initial level) in carrots treated with UV-C hormetic dose and stored at low temperature during long term storage, while the other PR-proteins detected did not exhibit any changes during storage at 4 °C. However, all PR-proteins degraded at various rate in roots stored at higher temperatures (14 °C and 24 °C), which suggests that PR-proteins are an integral part of long-term resistance in UV-C hormesis treated carrots. Our data suggest, overall, that UV-C treatment could be a beneficial technology in the integrated management of diseases for long-term storage of carrots.
Document Type: Thèse de doctorat
Issue Date: 2016
Open Access Date: 23 April 2018
Permalink: http://hdl.handle.net/20.500.11794/26624
Grantor: Université Laval
Collection:Thèses et mémoires

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