Masters Degrees (Horticulture)

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Browsing Masters Degrees (Horticulture) by Author "Barasu, Prince Dogbeda"

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    Acclimation of apple peel to light and temperature and the effect thereof on red colour development and tolerance to sunburn
    (Stellenbosch : Stellenbosch University, 2017-12) Barasu, Prince Dogbeda; Midgley, S. J. E.; Schmeisser, M.; Steyn, Willem Jakobus; Stellenbosch University. Faculty of AgriSciences. Dept. of Horticulture.
    ENGLISH SUMMARY: Sunburn is the biggest abiotic quality defect affecting apple orchards in South Africa. In blushed cultivars, inadequate red colour development at harvest is of similar importance as sunburn as quality defect. The presence of these two quality defects negatively affects profitability of South African apple orchards. This study was undertaken to assess the response of apple peels from trees on different rootstocks of differing vigour to photothermal stress. Rootstocks are purported to produce different microclimatic conditions to developing fruit depending on their vigour. The objective was to identify whether previous acclimation to light and temperature affected the sensitivity of ‘Rosy Glow’ (RG) and ‘Golden Delicious’ (GD) apple peel from a range of rootstocks, to damage (photosystem and visible peel damage) under induced natural photothermal stress and whether such acclimation affected the ability of ‘Rosy Glow’ apples to colour under different temperature conditions. We found that damage to peel photosystems occurred at all exposure periods in both cultivars, with peels under one hour exposure showing general indications of progressive recovery over the five-day period. Duration of exposure to the stress condition, the recovery period, and canopy position were identified as the dominant influences on damage and recovery of RG photosystems with duration of exposure and recovery period being the dominant influences on GD photosystems. Likewise, duration of exposure, the length of the evaluation period together with canopy position were the dominant influences on visible peel damage observed on both RG and GD apples. However, rootstock plays a role in the visible peel damage observed on RG apples in 2016. Fruit from the different canopy positions acclimated differently which showed in their response to the photothermal stress. Slightly lower peel sensitivity occurred in fruit from trees on the rootstock G3007 and a higher sensitivity in M793. To ascertain the effect of rootstock on the colouring potential of RG apples, fruit peel discs were subjected to six temperature treatments. The effect on red colour development of RG apples under lab conditions is rootstock related and not related to vigour. Although fruit colour development varied between different rootstocks under different temperatures, results indicate different optimum temperature ranges for different rootstocks in the red colour development of RG apples. Geneva rootstocks G222 and G3007 rootstocks showed the highest potential for good colour development following a cold front under warm late-season conditions on par with the current industry standard M793. In our final experiment, the effect of fruit cooling (as a means of modifying fruit microclimate) on the red colour response of ‘Cripps’ Pink’ (CP) apples at harvest was evaluated. The cooling treatments applied showed different responses on change in hue of CP apples at harvest, but all cooling treatments were beneficial to red colour development. Late cooling treatment from mid-February to mid-March was more effective in decreasing hue of CP apples at the end of the trial.