Doctoral Degrees (Viticulture and Oenology)

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Browsing Doctoral Degrees (Viticulture and Oenology) by browse.metadata.advisor "De Villiers, O. T."

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    Physiological implications of partial defoliation of the grapevine (Vitis vinifera L. cv. Cabernet Sauvignon)
    (Stellenbosch : Stellenbosch University, 1991-03) Hunter, Jacobus Johannes; De Villiers, O. T.; Stellenbosch University. Faculty of AgricSciences. Dept. of Viticulture & Oenology & Institute for Wine Biotechnology.
    ENGLISH ABSTRACT: The effect of partial defoliation as canopy management practice on metabolism and grape composition of the grapevine, Vitis vinifera L cv. Cabernet Sauvignon, was investigated. The establishment of physiologically sound principles on how to overcome deleterious effects associated with vigorous and dense-canopy vines is emphasized. Experimental vines were defoliated 33 % and 66 % evenly over the whole canopy from different developmental stages in a field study. Effects on canopy microclimate, photosynthesis, photosynthate translocation, vegetative growth, reproductive growth, root development and distribution, as well as grape and wine quality, were determined. A method for the simultaneous extraction of sugars and organic acids from freeze-dried berries at different developmental stages is described. Partial defoliation of vines improved canopy microclimate and photosynthetic efficiency of remaining leaves. Normal translocation and distribution patterns of photosynthates were apparently unaffected by partial defoliation. Translocation to and accumulation of photosynthetic products in the leaves and bunches of partially defoliated vines were, however, improved. Remaining leaves of partially defoliated vines were in comparison photosynthetically more active. Apart from a less favourable canopy microclimate, it seemed that the sink capacity of non-defoliated vines did not comply to the source capacity, inducing a reduced rate of photosynthesis. Normal sigmoidal growth patterns of vines were not affected by partial defoliation as applied in this study. This is important for the longevity, healthiness and productivity of vines. Vegetative growth was differentially affected by partial defoliation. No compensatory leaf growth occurred in reaction to partial defoliation from different developmental stages. Main shoot length, however, decreased slightly. Lateral shoot length and number of laterals increased, whereas cane mass decreased when vines were partially defoliated, particularly the earlier and more severe the defoliation. Reproductive growth in terms of yield was deleteriously affected by 33 % defoliation prior to pea size and 66 % defoliation prior to veraison. Budding percentage was, however, improved by 33 % and 66 % defoliation, whereas bud fertility was only improved by 33 % defoliation. Partial defoliation changed the canopy microclimate to conditions favourable for pest and disease control and higher grape quality. Subterranean growth was favourably affected by partial defoliation, particularly when applied from pea size stage. These changes included higher root densities, development of higher numbers of fine and medium diameter roots and occurrence of higher total root numbers in all soil layers. Generally, defoliations from pea size and veraison were more efficient regarding root development than defoliations from just after bud break and from berry set. Partially defoliated vines reacted by forming new roots, creating a more efficient nutrient absorption capacity and utilization of soil and available water and that, together with higher photosynthetic activities of leaves, provided an efficient mechanism for continued high performance. Grape quality was not affected markedly by partial defoliation. Total soluble solids in berries of defoliated vines were comparable to and even significantly higher than those of non-defoliated vines in some cases, in spite of much lower leaf areas. Generally, total titratable acidity of musts was also slightly higher for partially defoliated vines. Glucose and fructose concentrations in berries were unaffected by partial defoliation, while tartaric acid concentrations were slightly increased and malic acid concentrations slightly decreased. Partial defoliation generally increased the anthocyanin concentration of berry skins. These changes in grape composition suggest higher grape quality and seemed to result from improved light conditions in the canopy interior. Berry volume decreased with partial defoliation, which lowered the pulp:skin ratio. These berries are more desirable for quality wines. Regardless of severity or the developmental stage defoliation was commenced, wine cultivar character and overall wine quality were significantly improved. Partial defoliation changed the general metabolism of vines, mainly in terms of more favourable source:sink ratios, resulting in more efficient photosynthesis, subterranean performance and canopy microclimate. In general, the results suggest that an even removal of 33 % of leaves opposite and below bunches during the period from flowering or berry set to pea size stage may be applied. lt is further suggested that existing vigorous and dense-canopy vines be 33 % defoliated evenly on the lower half of the shoot (canopy) from pea size or veraison. This hypothesis proved effective in improving canopy microclimate, photosynthetic activity and yield, while vegetative growth was inhibited. Grape and wine quality were higher. On the whole, partial defoliation as applied in this study, is recommended as canopy management practice in order to facilitate the abolishment of deleterious effects of excessive vegetative growth and canopy density on balanced metabolic activity, fruit and wine quantity and quality, as well as longevity and healthiness of grapevines.