Doctoral Degrees (Horticulture)
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Browsing Doctoral Degrees (Horticulture) by Subject "Apples"
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- ItemBrowning and watercore disorders in 'Fuji' apples explored by means of X-ray computed tomography (CT)(Stellenbosch : Stellenbosch University, 2020-12) Chigwaya, Kenias; Crouch, E. M.; Crouch, Ian; Stellenbosch University. Faculty of AgriSciences. Dept. of Horticulture.ENGLISH ABSTRACT: The Fuji apple cultivar occupies 9% of land under apple production in South Africa. To ensure all year-round fruit availability, ‘Fuji’ apples are stored for extended durations. However, ‘Fuji’ apples are prone to internal browning (IB) during storage. IB is characterized by patches of brown flesh in the apple tissue, which makes fruit unmarketable and causes financial losses. Browning symptoms that have been identified in apple cultivars include radial browning, diffuse browning, combination browning, CO2 damage and core-flush. Techniques such as X-ray computed tomography (CT) that can evaluate IB disorders non-destructively are important. This study aimed to explore IB types and watercore in ‘Fuji’ apples quantitatively and qualitatively using X-ray CT. Exposure of fruit to high CO2 conditions for 3 days after harvest at 21 °C induced IB in the core region of fruit. The construction of porosity maps for intact fruit enabled characterization of tissue structure before and after disorder development. Porosity distribution of ‘Fuji’ was higher in the cortex region compared to the core region. High-resolution X-ray CT scans performed on IB affected and unaffected fruit tissue showed differences in microstructural properties such as porosity, pore size distribution and pore connectivity. Fruit size had a significant effect on the susceptibility of ‘Fuji’ apples to CO2 stress-induced IB. Radial porosity profiles did not differ significantly between fruit that developed IB and fruit that did not develop IB. However, porosity along the axial profile was generally higher for fruit that did not develop IB, particularly in the region between the calyx end and the core region. This was the first X-ray CT study carried out on South African ‘Fuji’ apples to evaluate how fruit microstructural properties relate to the IB types identified under different storage conditions. A further study was done to determine microstructural properties of watercore affected fruit tissue and the effects on storability of ‘Fuji’ apples. X-ray CT scans showed that fruit tissue with watercore had a significantly low porosity and connectivity of pores. This may have a negative impact on respiratory gas diffusion in the fruit and could increase susceptibility to IB during storage. Furthermore, watercore affected tissue had significantly smaller cells due to plasmolysis. For long-term storage experiments, it was found that regular atmosphere (RA) stored fruit had a significantly higher incidence of core-flush compared to fruit from controlled atmosphere (CA) and delayed controlled atmosphere (delayed CA) storage. Although CA and delayed CA were effective in reducing core-flush incidence, they both resulted in a significantly higher incidence of radial browning. Fruit with CO2 damage and cavities were also evaluated in this study. CO2 damage was associated with cell damage and increased pore sphericity. All IB types evaluated resulted in an increased tissue porosity and altering of pore sphericity, anisotropy and pore size distribution. IB after short-term exposure to CO2 stress occurred only in the core region while IB types observed after long-term storage occurred in all fruit tissue regions. This study provided unique insights into the microstructural properties of different IB types occurring in ‘Fuji’ apples.
- ItemInfluence of crop based water and nutrient strategies on physiological aspects of apple trees ‘Brookfield Gala’(Stellenbosch : Stellenbosch University, 2008-12) Lebese, Thabiso C.; Stassen, P. J. C.; Midgley, S. J. E.; Stellenbosch University. Faculty of AgriSciences. Dept. of Horticulture.It is a common practise in the Western Cape to use micro sprinklers as the standard irrigation system for apple trees. Over the past forty years much effort has been put into the optimisation of the tree canopy. Less attention has been given to root proliferation, and the question as to whether root stimulation and proliferation, through intensive water and nutrient management, can contribute towards improved tree efficiency and more efficient water use. This is addressed in this study. ‘Brookfield Gala’ apple trees were studied in the Genadendal area near Greyton, in the Western Cape Province, South Africa. The trees were planted in Dundee soil (well aerated sandy loam soil) during winter 2003. Both horticultural aspects (tree growth, shoot growth, fruit yield and quality, trunk circumference and root growth) and gas exchange were studied from 2004/5 until 2007/8 under three different water application strategies, namely micro sprinkler irrigation, daily drip and pulsing drip irrigation and using two different rootstocks: M793 and M7. Irrigation under micro sprinkler irrigation was applied once to three times weekly, daily drip irrigation once daily/twice daily, and pulsing drip irrigation one to six times daily. Water use for bearing apple trees was calculated using long-term evaporation data (for Villiersdorp and Caledon) and existing crop factors for apples. Annual nutrient requirements were adapted from literature and divided percentage-wise into the requirements for five different phenological stages. Soil sensors were used to keep plant available soil water between 100% and 50%. A computer software program was used to incorporate all the above mentioned information and calculate the exact amounts of water and nutrients, and the application times. In general, drip irrigation systems used ±26% less water than micro sprinkler irrigation system. Significantly higher fruit yields were obtained with trees under daily or pulsing drip irrigation than those under micro irrigation during 2005/6 and 2007/8. During 2006/7 the crop load was low due to unfavourable weather conditions during flowering, resulting in poor fruit set and no differences in yield. There was a significantly higher number of thin plus medium roots (3mm and less in diameter) in the 0─400mm rooting zone and total root mass at 0─800mm rooting zone under drip irrigation systems than under micro sprinkler irrigation. Brookfield Gala’ apple trees grown under daily drip irrigation and pulsing drip irrigation performed better compared to those grown under micro sprinkler irrigation with respect to CO2 assimilation rate (A), stomatal conductance (gs), water use efficiency (WUE) and leaf water potential. None of the three irrigation systems affected the biochemical efficiency of the leaf significantly, except on a few occasions during the pre-harvest period. This implied that the changes in leaf biochemical efficiency were as a result of both stomatal and non-stomatal effects (temperature and vapour pressure deficit). The removal of fruit at harvest had a great influence on leaf photosynthetic capacity under micro irrigation but less so under drip irrigation systems. Higher chlorophyll a and chlorophyll b concentrations were observed under drip irrigation systems than under micro sprinkler irrigation, implying efficient biochemical efficiency under these systems compared to micro sprinkler irrigation during the post-harvest period. Use of daily drip irrigation and pulsing drip irrigation delayed the process of leaf ageing. This study demonstrated the benefits of more intensive water and nutrient application for apple trees. Improved root proliferation, increased fruit yield and photosynthetic efficiency have been found under drip irrigation system than under micro sprinkler irrigation.
- ItemSensitivity 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.