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w i n e G R O WIN G It's not only total grape color that is affected by nitrogen and weather factors, but also distribution of individual anthocyanins (red pigments). Sugar is not the only grape component that is adversely affected by shade.9,10 Shaded grapes normally have less tartaric and more malic acid, which may result in an increase or decrease of pH. Potassium content is often higher with follow-on effects on juice pH, while phenolic compounds, such as tannins and anthocyanins (red pigments), are reduced along with flavor compounds. This obviously affects the sensory properties of wine produced from these grapes; such wine often tastes thin and herbaceous. To make matters worse, many fungal pathogens thrive in a shaded and humid microclimate, leading to an increase in disease incidence. This shade problem cannot be overcome by leaf removal in the cluster zone in an attempt to improve fruit exposure to sunlight, because the high nitrogen and low phenol content in the berries makes them susceptible to sunburn (Figure I).9 Another common Band-Aid action is to trim (often several times during the season) excess shoot tips, but again, this only makes matters worse, as shown in Figure II.7 It is ecologically and economically unwise to stimulate vigor by applying large amounts of nitrogen early in the season and then remove the surplus growth by summer pruning. Hedging wastes the vine's resources by stimulating lateral shoot growth and eliminating young, photosynthetically active leaves while leaving old, inefficient leaves behind. 200 Total anthocyanins (mg/L) the potential sugar shortage, reduced fruit quality. It is important, therefore, to develop a sufficiently large (roughly 10 to 15 square centimeters of leaf area per gram of fruit), sun-exposed canopy as early in the season as possible and then to stop shoot growth. ST/no N RT/no N ST/90 kg N/ha RT/90 kg N/ha 150 100 50 0 Day 0 Day 2 Day 4 Day 6 After press After MLF After cold st. Figure II: The adverse effect on wine quality of excessive nitrogen supply during bloom cannot be alleviated by shoot topping (ST = single topping; RT = repeated topping) during the season. The development of wine color (anthocyanins) is shown from inoculation to cold stabilization (field-grown Pinot Noir, modified from Keller et al. 1999). light conditions.2,4 Nitrogen deficiency during bloom results in poor fruit set and reduces a vine's yield potential. Despite low yield, however, fruit sugar concentration is below average, because there is insufficient nitrogen available for efficient photosynthesis. On the other hand, when clouds prevent sunlight from reaching the leaves, fruit quality is low regardless How much is too much? But how much nitrogen is good for the vine? The short answer is —it depends. In a warm, dry, sunny growing season, the optimum nitrogen supply may be higher than under cool, humid and cloudy conditions. See Figure III: the relationship between grapevine yield and fruit quality can be influenced by nitrogen supply and pr actica l win ery & vin eya rd JANUARY 20 14 41