- Title
- Maintaining the quality of citrus fruits for long term storage
- Creator
- Alhassan, Nasiru
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2022
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Postharvest senescence of fruits and vegetables is characterized by changes in a range of physiochemical factors limiting shelf life. The plant hormone ethylene is often responsible for accelerating senescence in postharvest produce and its effects have been the subject of scientific investigation for a number of years. Recently, hydrogen sulphide (H2S) has been shown to act as plant signalling agent, affecting all aspects of the plant life cycle, including senescence in horticultural produce. Synthetic auxins have been shown to affect cell division and growth and stimulate ethylene production in plants. This thesis examines the effects of H2S and a range of synthetic auxins in delaying the emergence of senescence characteristics in citrus fruits. Study 1 of the thesis evaluated the effects of continuous ethylene exposure at different storage temperatures on the condition of the citrus fruit calyx (button) and other quality parameters during long-term storage of Navel oranges and ‘Afourer’ mandarins. ‘Afourer’ mandarins were exposed at 1, 0.1, 0.01 µL L−1 ethylene and ≤0.001 µL L−1 ethylene (i.e., without ethylene) in storage at 5, 10 and 20 °C. Navel oranges were exposed to 1.0, 0.1 and ≤0.001 µL L−1 ethylene at either 1 or 10 °C. Changes in internal and external postharvest quality parameters were evaluated for up to 8 weeks storage in ‘Afourer’ mandarins and 10 weeks storage for Navel oranges. The results showed that higher ethylene levels at all storage temperatures increased calyx senescence, weight loss, and respiration rate and fruit firmness. Study 2 assessed the impact of hydrogen sulphide (H2S) fumigation on the postharvest storage life of Navel and Valencia oranges and ‘Afourer’ mandarins stored at 20 °C storage for five weeks. Fumigation was conducted using different H2S concentrations (0, 100, 250 and 500 µL L−1). The treated fruit was then stored in the presence of a 0.1 µL L−1 ethylene atmosphere to simulate commercial storage conditions. H2S fumigation successfully reduced the incidence of calyx browning, calyx abscission, fungal decay, endogenous ethylene production and ethanol formation, with the greatest benefit found to occur with the 100 µL L−1 H2S treatment. Study 3 assessed the effects of the synthetic auxin, 3,5,6-trichloro-2-pyridyloxyacetic acid (3,5,6-TPA) on calyx browning, calyx abscission and fruit decay, as well as citrus internal quality factors during long-term storage. Two TPA treatment concentrations ranges: 0, 2, 4, 8, 16 and 32 μM and 0, 15, 30, 60 and 120 μM were evaluated, with treated produce stored in air at ambient temperature. TPA was selected for its more favourable toxicological properties relative to 2,4-D, the current standard postharvest treatment in Australia and the United States for delaying postharvest senescence. Postharvest TPA treatment exhibited a concentration-dependent effect on fruit storage quality, with higher concentrations (60 or 120 μM) leading to a greater reduction of calyx senescence (deterioration) and decay incidence, lower ethylene production, respiration rate and ethanol accumulation, and delayed changes in TSS and TA content during the 32 and 28-day storage period. Study 4 assessed the relative efficacy of a range of postharvest citrus treatments for delaying postharvest senescence in citrus. The synthetic auxins; fluroxypyr ([(4-Amino-3,5-dichloro-6-fluoropyridin-2-yl)oxy]acetic acid), dicamba (3,6-dichloro-2-methoxybenzoic acid) and MCPA (2-methyl-4-chlorophenoxyacetic acid), were assessed against the current commercial treatment (2,4-D) and hydrogen sulphide using Valencia oranges from two growing locations. Two auxin concentrations were studied (0.2 and 1 mM), while hydrogen sulphide fumigation was performed at 100 µL L-1 (as optimised from Study 2). Post-treatment, the fruit was stored at ambient temperature for 4 weeks. Fluroxypyr produced the greatest reduction in calyx abscission, calyx browning and fruit rot and favourably impacted internal quality parameters (endogenous ethylene production, respiration rate and ethanol formation) compared to the other auxin treatments. Treatment with 1.0 mM fluroxypyr was found to be more effective than the 0.2 mM treatment. Both treatment concentrations were found to be superior to both 0.02 and 1 mM 2,4-D concentrations. MCPA treatment showed only slight benefits, while dicamba was ineffective at any concentration. Hydrogen sulphide fumigation successfully reduced calyx deterioration and lost internal quality factors but was less effective than fluroxypyr or 2,4-D. This thesis investigated the potential of synthetic auxins including TPA, dicamba, MCPA and fluroxypyr through dipping and H2S exposure as a substitute to commercial 2,4-D to inhibit calyx senescence, which its application in the citrus industry is limited in Europe and other countries due to health and environmental concerns. H2S fumigation and MCPA dipping provided a relatively small benefit of maintaining calyx quality and other postharvest citrus parameters and was considered unsuitable to use as a replacement for 2,4-D. Dicamba did not have a positive impact on citrus fruit quality during storage. Fluroxypyr and TPA results indicated they are viable substitutes for 2,4-D for limiting calyx changes and maintaining citrus fruit quality parameters. A comparative study demonstrated lower calyx senescence in citrus fruit for fluroxypyr when compared with those treated with 2,4-D and the other auxins and H2S. As discussed in study 4, fluroxypyr is less toxic than 2,4-D, and the chemical properties and physiological effects of fluroxypyr on the quality may be similar to those of 2,4-D. However, more studies are needed to reveal the feasibility and practicality of fluroxypyr and TPA auxins use in large scales on a range of citrus species to replace 2,4-D is warranted. Such understanding may give way for the commercial postharvest application of the auxins in the citrus industry.
- Subject
- auxins; citrus fruit; calyx senescence; hydrogen sulphide; maintaining quality; storage; treatment; thesis by publication
- Identifier
- http://hdl.handle.net/1959.13/1513165
- Identifier
- uon:56696
- Rights
- Copyright 2022 Nasiru Alhassan
- Language
- eng
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