Browsing by Author "Toolo, Keboneilwe Boitumelo"

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    Effect of post-harvest summer pruning on carbohydrate reserve status, bud break and fertility of Sultanina H5 in the Lower Orange River region.
    (Stellenbosch : Stellenbosch University, 2022-12) Toolo, Keboneilwe Boitumelo; Avenant, Eunice; Avenant, J. H.; Stellenbosch University. Faculty of AgriSciences. Dept. of Viticulture and Oenology.
    ENGLISH ABSTRACT: Carbohydrate reserves produced from photosynthesis are stored in perennial tissues of the grapevine in the form of starch and free-sugar fractions or soluble sugars, mostly sucrose, glucose, and fructose. These reserves are highly affected by viticultural practices altering the source-sink relationship in the grapevine. Post-harvest pruning is a practice widely applied by several table grape producers in South Africa aiming to channel carbohydrate reserve accumulation to the remaining shoots. Due to the high input costs of table grape production, any manipulation, including post-harvest summer pruning, should be applied only if it is scientifically proven to have practical and economic benefits. This study, comprising of two trials, aimed to determine whether post-harvest pruning results in increased carbohydrate reserve status, improved bud break and fertility, as well as to establish a base for quantifying and practically assessing the carbohydrate reserve status of grapevines. The first trial focused on establishing the seasonal dynamics of non-structural carbohydrate (NSC) reserves of Vitis vinifera L. cv. Sultanina H5 in the semi-arid Lower Orange River (LOR) and the Mediterranean Hex River Valley (HRV). Root, trunk, cane and/or shoot tissues were sampled monthly and analysed for NSC. The Anthrone method was used to analyse soluble sugars and starch, while enzymatic analysis was used to quantify specific sugars (sucrose, d-fructose and d- glucose). Starch and sucrose were the most abundant forms of NSC in all tissues in both regions. In both regions, soluble sugars in permanent tissues (roots, trunks, canes) reached their highest concentration during dormancy (June-July). The starch concentration was low in all tissues in winter (July), during grapevine dormancy, whereafter it increased to a peak occurring in August (before bud break). A steep decrease in starch concentration was recorded from dormancy to flowering in both regions, indicating a dependency of the vine on carbohydrate reserves during that period. Accumulation of NSC reserves began after flowering to the post-harvest period, reaching their second peaks in autumn. The overall higher soluble sugars and starch (roots and canes) concentrations in the tissues of the Mediterranean region is ascribed to the earlier accumulation of reserves, lower crop load and a shorter post-harvest period characteristic of this region. A basis was established for sampling grapevine tissues for qualitative assessment of grapevine NSC reserve status, linking sampling time to occurrence of peaks in soluble sugars and starch concentrations. It is recommended that sampling for qualitative assessment of soluble sugars should be done after leaf fall, during dormancy (June-July under the conditions of this study). Starch concentrations should be assessed before bud break (August under the conditions of this study). Based on significant positive correlations between NSC concentrations of different tissue types, tissue types that could be sampled for indication of the overall NSC status of the grapevine were identified. Canes and shoots could be sampled and analysed to indicate the overall NSC reserve status of the vine. These tissues are considered the most practical tissue types to sample for determination of grapevine NSC reserve status. The second trial investigated the effect of post-harvest summer pruning applied in the semi-arid Lower Orange River region on NSC reserve status, bud break and fertility of Sultanina H5 grapevines. Five post-harvest pruning treatments were applied, namely an early 33% and a 66% shoot removal pruning treatment one day after harvest (33_1dAH and 66_1dAH respectively), a late 33% and a 66% shoot removal pruning treatment 45 days after harvest (33_45dAH and 66_45dAH respectively) and a control (Ctr), in which no post-harvest summer pruning was applied. To quantify pruning severity, the number and length of removed shoots, as well as the number of leaves and leaf area removed were determined at the time of the post-harvest summer pruning treatment application. After winter pruning was applied, the removed canes and shoots were measured to calculate the overall shoot length and leaf area removed per vine. The day after pruning treatments were applied, photosynthetically active radiation (PAR), as well as photosynthetic activity and related physiological parameters were measured. Cane and/or shoot, stem and root tissue, were sampled on 4 dates for assessing the effect of pruning treatments on NSC reserve status. Bud break and fertility were assessed through forced bud break and bud dissection for potential bud break and fertility, while actual bud break and fertility were assessed in the vineyard. Post-harvest pruning proved to be beneficial for light penetration, but it did not improve the photosynthetic rate of the leaves. A few significant differences were recorded on the impact of the treatments on TNC. These however, do not show a clear trend. Post-harvest summer pruning did not have a significant effect on final bud break and potential fertility of grapevines in the season following the treatment. Based on this one season’s results, post-harvest pruning did not have overall practical benefits. Repeating the treatments for two more seasons on the same data vines, would indicate whether there is a carry-over effect of the practice on NSC, bud break and fertility. It is recommended that in a further phase of this project, available rapid and accurate methods to quantify carbohydrate reserves should be used and/or evaluated for use in grapevine studies, including Near-Infrared spectroscopy, as well as the starch iodine test (already commercially used in the apple and forestry industries).