Department of Agronomy

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Browsing Department of Agronomy by browse.metadata.advisor "Archer, E."

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    The effect of shoot heterogeneity on the physiology and grape composition of Shiraz/Richter 99 grapevines
    (Stellenbosch : Stellenbosch University, 2004-12) Cloete, Hanle; Archer, E.; Hunter, J. J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
    ENGLISH ABSTRACT: The effect of shoot heterogeneity on vegetative and reproductive growth parameters, vine physiology and grape composition was investigated in a ShirazlRichter 99 vineyard. Comparisons between underdeveloped (typically shorter and less ripened at véraison) and normally developed shoots in both shaded (non-manipulated) and well-exposed (manipulated) canopies were made. Compared to underdeveloped shoots, normal shoots had a larger total leaf area, due to the higher occurrence of secondary shoots as well as larger leaves on primary and secondary shoots. Since the photosynthetic activity of the leaves from normal shoots was higher than those from underdeveloped shoots, higher levels of carbohydrates were produced and stored in the former. Starch was more evenly distributed over the length of the whole shoot in normally developed shoots compared to underdeveloped shoots. Normally developed shoots were longer and thicker in diameter than underdeveloped shoots. The larger clusters of the normally developed shoots are evidence of their more favourable total leaf area per gram berry mass. Berries from the normally developed shoots were smaller at five weeks after véraison than those from underdeveloped shoots, displaying a higher skin to pulp ratio and therefore higher anthocyanin and total phenolic extraction potential for winemaking. The smaller clusters and fewer berries per cluster found for the underdeveloped shoots indicate an imbalance between vegetative and reproductive growth initiated during the vegetative growth phase and continued during the ripening period. The peculiar absence of statistically significant differences in grape composition between normally and underdeveloped shoots indicates that assimilates needed for berry ripening of the latter originated in organs other than the leaves [e.g. from adjacent normal shoots and the rest of the permanent structure of the vine (cordon, trunk, roots)]. The larger differences in berry size that occurred between shoot types in the shaded compared to the well-exposed canopies may be evidence for this. The photosynthetic activity of shoots was lower in shaded than in exposed canopies. The total carbohydrate production of the normal shoots in shaded canopies seemed insufficient to supply in the ripening needs of the shoot itself, their own clusters as well as the ripening of stem tissue and clusters of the underdeveloped shoots in the canopy. This is illustrated by the lower levels of starch that accumulated in the normal shoots from shaded compared to that of exposed canopies. Vine shoot heterogeneity clearly leads to visible and physiological imbalances that would impact negatively on grape and wine quality as well as production costs and should therefore be avoided on any terroir.