Doctoral Degrees (Horticulture)

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    Role of cell membrane integrity and oxidation stress in development of external browning of white seedless table grapes (Vitis vinifera L.)
    (Stellenbosch : Stellenbosch University, 2023-03) Botes, Wilhelmina Jacoba; Jooste, Mariana; Crouch, Elke; Stellenbosch University. Faculty of Agrisciences. Dept. of Horticulture.
    ENGLISH ABSTRACT: Berry skin browning is an important quality problem in white seedless table grapes - it negatively affects the cosmetic appearance, causes reduced market value, financial losses and restricted market access. In a high browning incidence season the accumulation of saturated fatty acids (SFAs) and high levels of total sterols were the driving factors for high browning incidence at harvest in ‘Regal Seedless’ (highly susceptible to external browning). The higher unsaturated:saturated phospholipid fatty acid (FA) ratio, monounsaturated:polyunsaturated fatty acid (MUFA:PUFA) ratio and higher ascorbic acid levels explained the lower browning levels in ‘Regal Seedless’ in the low browning incidence season. ‘Thompson Seedless’ is less susceptible to external browning than ‘Regal Seedless’ due to its high glutathione levels. The higher levels of MUFAs, total sterols and antioxidants resulted in lower browning incidence for ‘Thompson Seedless’ between seasons. Variables determined in grapes, harvested at two maturities and stored at an intermittent warming (IW) regime, at -0.5 °C (control) and 4 °C could explain the postharvest variance in external browning between a high and low browning incidence season in ‘Regal Seedless’. The IW regime effected the lowest browning incidence in both harvest maturities after 6 weeks of cold storage, because it caused higher levels of antioxidants and MUFAs than PUFAs in the berry skins. Storing grapes at 4 °C for 6 weeks resulted in high external browning incidence, especially in more mature grapes in the high browning incidence season, because it could not maintain the antioxidant levels and caused a low MUFA:PUFA ratio in the membranes. The -0.5 °C regime (control) caused high browning levels in especially the less mature grapes in the low incidence season (due to the ratios of the SFAs, MUFAs, PUFAs being less optimal in the less mature compared to the more mature grapes), but in both harvest maturities in the high incidence season (due to the lower MUFA levels and MUFA:PUFA ratio, and higher SFA concentrations in the cell membranes). The variables determined in this study could not discriminate between the different browning types, but could successfully separate the green and brown berry skins of the two harvest maturities. Therefore, although the external browning types look visually different on the berry skin, there is not a difference in the mechanism of how each develops on a cellular level. The variables that discriminated between green and brown berry skins differed between the low and high browning season. In a low browning season, berries of both harvest maturities developed the same level of external browning after storage. The brown berry skins had higher levels of total sterols and total PUFAs, and lower ascorbic acid than the green berry skins. In the high browning season both maturities had the same percentage of external browning after 6 weeks of storage at -0.5 °C. The brown berries had higher oxidized glutathione and malondialdehyde levels, higher total sterol:total phospholipid ratio and %SFAs than the green berries. This result indicates that berries on the same bunch differ in maturity and that this might be the reason for their differing susceptibilities to develop browning pre- and postharvest. It is also confirmed that external browning is initiated pre- and not postharvest. Future work should, therefore, focus on preharvest factors and vineyard manipulations that will ensure grape bunches of more uniform maturity. Postharvest, different IW regimes and/or preconditioning at higher temperatures before cold storage could be tested in combination with lower dose SO2 sheets, to prevent the development of external browning during cold storage.
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    Sensitivity of apple rootstocks to water stress
    (Stellenbosch : Stellenbosch University, 2023-03) Muchena, Lindsay; Midgley, S. J. E.; Dzikiti, Sebonasi; Lotze, Elmi; Stellenbosch University. Faculty of Agrisciences. Dept. of Horticulture.
    ENGLISH ABSTRACT: The widespread occurrence of water deficits is increasingly one of the most prevalent limiting factors in apple production, which will likely worsen due to population growth and increased rate of climate change. Since apple production in South Africa is done under irrigation, increased periods of water deficits require, amongst other technologies, the use of apple rootstock selections that support high orchard water productivity under limited water supply. This would create resilience specifically in areas at risk of having their water allocation cut because of insufficient irrigation water availability. The aim of this study was to quantify whole tree water use, gas exchange and growth dynamics of mature, bearing (in-field) and non- bearing (potted) ‘Rosy Glow’ apple trees on a range of rootstocks, which varied from semi-vigorous to dwarfing, before, during and after periods of water deficit during the 2017/2018 and 2018/2019 growing seasons. In the field trial, mean basal crop coefficient (Kcb), orchard transpiration per unit ground area (Ec) and transpiration per unit leaf area (EL) were higher in well-watered trees on the more vigorous rootstocks compared to more dwarfing rootstocks. An analysis of the tree hydraulic characteristics showed that well- watered trees on more dwarfing rootstocks had higher stem (Rx), canopy (Rc), and soil to stem (Rsx) hydraulic resistances compared to trees on the more vigorous rootstocks, which could possibly explain their lower water use. The Penman-Monteith equation accurately predicted water use of trees on different rootstocks, despite differences in tree vigour and hydraulic resistances. Measured and projected water productivity (i.e., calculated using the South African standard industry spacings for each rootstock) under well-watered conditions were both found to be higher in trees on more dwarfing MM.109/M.9 (MM.109 with an M.9 EMLA interstem) and M.9 RN.29 rootstocks compared to trees on more vigorous rootstocks. This means that these rootstocks could be a viable option to maximise productivity in high density planting systems. In the field trial EL was not reduced in water deficit relative to well-watered trees for any rootstock during both water deficit cycles. Trees on M.793 and M.9 RN.29 displayed more water use, relative to pre- stress values, indicating improved water uptake after re-watering. Trees on more vigorous rootstocks had significantly higher net CO2 assimilation rate (A), stomatal conductance (gs) and transpiration rate (E), and significantly lower intrinsic water use efficiency (WUEi) compared to trees on more dwarfing rootstocks on day 18 of water deficit cycle 1. These responses were closely associated with higher maximum rate of carboxylation (Vcmax) and higher leaf chlorophyll a (chla), chlorophyll b (chlb) and carotenoids concentrations. In the pot trial with non-bearing trees, there was a significant reduction in water use in the water deficit treatment relative to the well-watered treatment during both water deficit cycles. Like the field trial, trees on M.793 and M.9 RN.29 displayed enhanced water use after the two water deficit cycles indicating enhanced water uptake. Except for trees on G.222, trees on more dwarfing rootstocks displayed higher Rx, Rc and Rsx than more vigorous rootstocks, as observed in the field trial. Water deficit treatment decreased A, gs, E, chla, chlb, and chla/chlb ratio and improved WUEi, for all rootstocks. Trees on G.222, M.7 and M.9 RN.29 had higher Vcmax, Jmax and triose phosphate utilization (TPU) than trees on M.793 and G.202 during water deficit cycle 2, but this did not lead to significant differences in A amongst the different rootstocks. Partitioning of biomass towards root development differed significantly amongst trees on different rootstocks, with trees on more vigorous rootstocks having significantly higher increase in root biomass compared to trees on more dwarfing rootstocks. This study sheds light on the water requirements and stress responses of apple trees on rootstocks with different vigour-inducing capacities, which is instrumental in selecting suitable rootstock options to maximise water productivity. Trees on the dwarfing M.9 RN.29 rootstock were not more sensitive to water deficit in the field and in pot trials relative to more vigorous rootstocks. The use of M.9 RN.29 in high-density plantings could result in a significant saving in irrigation water.
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    Physiological and molecular mechanisms underlying shoot manipulation strategies with the plant growth regulator ethephon for the improvement of flowering in litchi (Litchi chinensis Sonn.) cv. ‘Mauritius’
    (Stellenbosch : Stellenbosch University, 2023-03) Cronje, Regina Barbara; Hoffman, E. W. ; Theron, K. I.; Huang, X. M.; Stellenbosch University. Faculty of Agrisciences. Dept. of Horticulture.
    ENGLISH ABSTRACT: Erratic flowering is a major challenge in most litchi (Litchi chinensis Sonn.) producing countries, including South Africa. In recent decades, above-average temperatures in autumn and early winter have increased the incidence of vegetative shoot growth prior to floral induction, further exacerbating irregular flowering and rendering the conventional ethephon shoot control measures less effective. Therefore, this study aimed to investigate the effect of ethephon applied to mature terminal shoots for its potential to prevent shoot growth prior to floral induction, to delay panicle emergence to a period with consistently low temperature and to promote carbohydrate accumulation for improved flowering and yield compared with conventional spot spray applications to emerging, immature vegetative shoots. In a four-year orchard-based study, ethephon was applied to ‘Mauritius’ trees, either as single or double whole-canopy spray at 500, 750 and 1000 mg⋅L⁻¹, each at three different dates between autumn and early winter, or as a combination of whole-canopy spray at first signs of shoot growth and spot sprays on developing shoots, in two climatically different production areas of South Africa. All whole-canopy treatments were compared with the conventional spot spray applications at 1000 mg⋅L⁻¹ and untreated trees. Results revealed that whole-canopy ethephon applications to mature terminal shoots successfully inhibited pre-induction vegetative shoot growth in a dose-dependent manner, subsequently delaying panicle emergence to a cooler period, which promoted carbohydrate accumulation, improved inflorescence quality, flowering rate and yield without delaying fruit maturity, compared with spot-sprayed and untreated trees. The date of application played an important role in the efficacy of ethephon applications, with higher ethephon concentrations being more beneficial under warmer conditions, such as in early autumn or generally in warm areas, while lower concentrations were sufficient for applications later in autumn and early winter. In a second study, ethylene evolution as well as phenological, physiological and molecular changes underlying ethephon application and its associations with bud dormancy and flowering were investigated, by comparing a single whole-canopy ethephon application at 1000 mg⋅L⁻¹ to mature terminal shoots with untreated trees. Ethylene evolution peaked in leaves on the day of application with a rapid decline thereafter, but persisted in buds for seven days before gradually declining. Ethephon application significantly increased relative expression of LcEIN3 and LcFLC in terminal buds one day after application, while LcFT2 expression in leaves and LcAP1 expression in terminal buds were significantly increased at the bud break stage. The ethephon treatment also significantly increased soluble sugar concentrations in leaves and shoots at the bud break or floral initiation stage. The current study provided evidence that ethephon application plays an important role in the physiological and molecular regulation of bud dormancy and subsequent floral regulation of litchi. By preventing pre-flowering shoot growth and influencing the time of bud break, ethephon application to mature terminal shoots proved to be a more effective than conventional spot spray applications and can be a powerful tool to manage panicle emergence under less inductive conditions. Moreover, the modified use of ethephon contributes to the sustainability and expansion of the South African litchi industry.
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    Alternative postharvest technologies for the maintenance of ‘Parthenon’ broccoli quality during export and domestic marketing.
    (Stellenbosch : Stellenbosch University, 2022-04) De Beer, Tarryn; Crouch, E. M.; Crouch, Ian; Theron, K. I.; Stellenbosch University. Faculty of AgriSciences. Dept. of Horticulture.
    ENGLISH ABSTRACT: Broccoli is prized for its nutritional quality. The green colour and a compact head symbolise freshness. Export and domestic marketing of broccoli is limited by its high perishability. The aim of this study was to investigate the use of two 1-methylcyclopropene (1-MCP) application methods in combination with packing to maximise broccoli storage life under different temperature regimes typically found in the export and domestic handling chains. Under simulation of domestic handling chains, optimal temperature storage (0 °C) of broccoli (cv. Parthenon) in combination with 1-MCP and packaging [Xtend™ modified atmosphere packaging (MAP) or macro-perforated high-density polyethylene (HDPE) liners] resulted in a maximum storage duration of 30 days. In a second season, optimal temperature storage with MAP (RipeLock™) allowed for 42 days storage. Storage at suboptimal temperatures (viz. 7.5 °C or 12 °C), in mixed loads with ethylene is, however, common during domestic handling. Under such environments 1-MCP prevented broccoli (cv. Parthenon) yellowing that occurred with increasing storage temperature from 0 °C-7.5 °C and 7.5 °C-12 °C. Furthermore, 1-MCP alone or in the presence of ethylene extended shelf-life of broccoli (cv. Parthenon) compared to controls, at both temperatures. This research showed how an in-box treatment system of 1-MCP can be used as a viable alternative to the traditional, bulk, in-room application method, having a similar effect in reducing yellowing compared to controls. Sensory quality of broccoli was maintained in MAP (RipeLock™) at 0 °C and 7.5 °C. At 12 °C, where sensory quality was the poorest, 1-MCP reduced the perception of ‘sulphur odour’ compared to untreated and ethylene exposed broccoli. Dimethyl trisulphide (DMTS), an off-odour related volatile, was the most abundant volatile measured at all storage temperatures. The oxidative state of broccoli increased with an increase in storage temperature from 0 °C to 7.5 °C to 12 °C. Ascorbic acid and glutathione were oxidized with an increase in storge temperature. Glutathione, however, had a consistently higher percentage oxidation compared to ascorbic acid. Under simulated export conditions, broccoli was stored at 0 °C in various MAP types with or without 1-MCP. 1-MCP was either applied in-room before packaging or in-box during packaging. Results showed that extended storage without packaging is not recommended due to excessive weight loss. Across two seasons variations in shelf-life were noted. In the first season, a maximum storage duration of 25 days was realised with broccoli stored in Xtend™, or a generic HDPE MAP film with 1-MCP. In the second season, the maximum storage duration was 35 days, irrespective of MAP type or 1-MCP treatment. Here, the limiting factor for shelf-life was sensory quality. At optimal temperature volatiles related to off-odour were negligible, indicating that sensory deterioration was not related to fermentation in the MAP environment. In the second season it was reported that if good quality broccoli is stored at 0 °C, RipeLock™ or Xtend™ MAP in combination with 1-MCP should yield no benefit over untreated broccoli in non-specific, generic liners bags (15 μm HDPE).
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    Quantity and quality losses of 'Crimson Seedless' grape and 'Packham's Triumph' pear along the supply chain and associated impacts
    (Stellenbosch : Stellenbosch University, 2022-04) Blanckenberg, Anelle; Opara, Umezuruike Linus; Fawole, Olaniyi Amos; Stellenbosch University. Faculty of AgriSciences. Dept. of Horticulture.
    ENGLISH ABSTRACT: Postharvest loss and waste (often referred to as wastage) is a global problem affecting all produce along the supply chain from farm to plate. These losses not only decrease food supply but also mean that huge amounts of the resources and effort used in the production of horticultural crops are squandered. From an economic perspective, addressing postharvest losses is not only helpful to producers aiming to sell more, but also to consumers who could save money as the available food becomes more affordable. To date, there is little scientific data available on the incidence and magnitude of postharvest losses of fruit and other food crops in South Africa. The aims of this study were to assess postharvest losses in quantity and quality of ‘Crimson Seedless’ table grape and ‘Packham’s Triumph’ pear along the supply chain and quantify the associated economic, environmental and resource impacts in order to inform mitigating actions. The base measurement for table grape losses occurred in the packhouses of four farms in the Western Cape during the commercial harvest. The highest quantity (%) of physical losses in the supply chain was found to occur at this level when compared to the cold storage (2 or 4 weeks at -0.3°C ± 0.7°C and 81.3% ± 4.1% RH), retail (10 days at 5.4°C ± 0.6°C and 83.7% ± 2.9% RH) and consumer/home (ambient) storage (25.1 ± 1.3°C and 46.6 ± 6.0% RH) stages. There were large differences between the 2017 and 2018 seasons, with the 2018 season’s losses being half that of the first. The main reason for losses at the packhouse level was mechanical damage (7.1% in 2017 and 3.09% in 2018) due to rough handling of crates and could be improved by making workers more aware of the necessity to handle crates with care. Harvest timing is also essential, as delayed harvesting reduces shelf life and increases postharvest losses, as evidenced by this research. The farm that sustained the highest losses in 2017 (23.3%) harvested later than was optimal, and therefore, the bunches stayed on the vines too long. In 2018 the harvest occurred two weeks earlier than in 2017, and the grapes were in better condition leading to fewer losses on farm level (5.85%). Among all supply chain scenarios, the main quality problem was rachis and stem browning at temperatures higher than -0.5ºC. This caused berries to drop faster and bunches to look less fresh, as well as causing bunches to weigh less when sold. While 500 g and 1 kg punnets are routinely kept at around 5ºC at the retail level, during peak season 4.5–10 kg cartons are often stacked on the floor under ambient conditions. Therefore, the table grapes would have a maximum shelf life of seven days before the stems have browned and too many berries per bunch are decayed to sell. Therefore, it is advisable to keep cartons at -0.5ºC and high RH and only place bunches in punnets in 5ºC display fridges as the stock sells. The base measurement for losses of pear occurred in the orchard of two farms in the Western Cape during commercial harvest. It was found that 18% of the harvest on the one farm and 19% of the harvest on the other, did not reach the minimum quality standards. The main reasons were deformed fruit and too small size. The only decay, among all supply chain scenarios, occurred when pears were kept under ambient conditions (25.1 ± 1.3°C, 46.6 ± 6.0% RH) where 3.3% were decayed after seven days and 6.6% after 10 days. The majority of physical losses were due to weight loss with a 3.9%, 3.6 and 3.7% decrease in weight for supply chain Scenario B (to local retail markets), supply chain Scenario C (to export retail markets) and supply chain Scenario D (simulated ‘abusive’ treatment of fruit within the export chain) respectively. . Of the data gaps in the existing knowledge on global food loss and waste, the largest gap is the lack of available data on postharvest losses, retail and household level (shelf-life) food waste data. Therefore, this study contributed to the advancement of new knowledge by generating primary data on postharvest quantity and quality losses along the supply chain to manage the food loss and waste problem better.