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A Technical Guide for Wine Producers
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The effect of light quantity and quality on grape composition and wine quality
Shading
Archer & Strauss (1989) conducted trials to determine the effect of natural shading. The shoots of adjacent vines were used to obtain various degrees of shade. It was found that an increase in shading caused an increase in the potassium concentration, pH and titratable acids. The increase in titratable acids was ascribed to an increase in malic acid, however, while the more stable tartaric acid decreased. The skin colour of the Cabernet Sauvignon was also considerably reduced, which is in accordance with the findings of wine tasters that the wine quality was reduced in proportion to the degree of shading. A reduction in light intensity therefore caused a big reduction in quality in the grapes and wine.
The specific organs that are shaded may have different effects. Morrison & Noble (1990) found that shade on the bunches only had no effect on the sugar, potassium and acid concentrations, but that there was indeed a reduction in the phenol and anthocyanin concentrations. Shading of the leaves caused a delay in berry growth and sugar accumulation; while the pH, potassium and malic acid contents increased.
Phytochrome
Reactions to shading cannot be explained simply by taking the amount of light that penetrates the vine into effect. The quality of the light also plays an important role. Within the vine photon receptors occur, namely phytochrome, which can control and regulate a large amount of enzymes and reactions. Phytochrome acts as a biological switch and has an "on" and "off" position which is determined by the molecular orientation in which it occurs. The phytochrome molecules are also interchangeable from the active to the inactive form. The ratio of active to inactive phytochrome is determined by the amount of red and far-red light that penetrates the vine. Vine leaves absorb a lot more red than far-red light, which transforms more phytochrome to the inactive form. The denser the canopy, therefore, the less phytochrome in the active form.
Phytochrome controls various reactions and enzymes. It has been proven that the enzymes PEP-carboxylase (involved in malic acid synthesis), malic acid dehydrogenase (involved in the breakdown of malic acid), phenylalanine ammonia liase (FAL) and nitrate reductase are controlled by phytochrome.
FAL is the enzyme responsible for diverting phenylalanine from protein synthesis to be used for phenol and anthocyanin synthesis. Nitrate reductase impels the reaction during which nitrate is converted to nitrite within the vine. The reaction requires potassium which is readily taken up by the vine. If the enzyme is not sufficiently "switched on", however, the reaction cannot occur quickly enough and the potassium begins to build up in the shoots and later in the grapes, with an accompanying increase in the pH.
Conclusions
An excessively vigorous vineyard with a dense canopy is indeed very harmful to quality grapes and wines. Already in the planning stage of vineyard establishment decisions and adjustments must be made to prevent this from happening. The choice of terrain, cultivar, rootstock, soil preparation, vine spacing, trellis system and irrigation and fertilising practices are determining factors when it comes to canopy density. Fortunately there are short term practices such as suckering, shoot positioning, tipping and leaf removal which may be used to improve the level of canopy density. Unfortunately there is no recipe for these practices, since climatic differences cause widely divergent results. In warm regions, for example, grapes hanging in direct sunlight become up to ten degrees Celcius warmer than the ambient temperature. It has been noted that colour improves with greater light exposure, but due to the inherent dangers of the heat the grapes cannot be exposed to the same degree as in the cooler regions. Vineyards should therefore be judged on an individual basis so that the correct practices may be applied to obtain the best results for the specific climate.
References
ARCHER, E. & STRAUSS, H.C., 1989. Effect of shading on the performance of Vitis vinifera L. cv. Cabernet Sauvignon. S. Afr. J. Enol. Vitic. 10,74-77.
MORRISON, J.C. & NOBLE, A.C., 1990. The effects of leaf and cluster shading on the composition of Cabernet Sauvignon grapes and on fruit and wine sensory properties. Am. J. Enol. Vitic. 41, 193 - 200.
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The canopy density of a vine is determined by the amount of shoots and leaves on the vine and the way in which these are arranged. Canopy density has an enormous influence on the amount of light that penetrates the foliage and can therefore influence the grape composition and wine quality. In South Africa it often happens that vines are excessively vigorous, resulting in too dense canopies which in turn has a negative effect on the quantity and quality of grapes produced.
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