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w i n e G R O WIN G 24 100% Sunlight N1 N5 N10 Solube solids (ºBrix) 22 mean = 20.4 20 20% Sunlight N1 N5 N10 2% Sunlight N1 N5 N10 18 mean = 298 16 50 150 250 350 450 550 Yield (g/vine) Figure III: Nitrogen supply and light conditions can influence the relationship between grapevine yield and fruit quality. Nitrogen was applied during bloom (N1 = 0.34 g N/vine; N5 = 1.7 g; N10 = 3.4 g), and light conditions were varied during veraison (pot-grown Cabernet Sauvignon, modified from Keller et al. 1998). of nitrogen availability, because of lack of light for photosynthesis. Yield and quality are maximized with sufficient nitrogen during the critical bloom period and a clear sky during veraison. Excess nitrogen can lead to poor fruit quality as discussed above. It can, moreover, increase the vine's susceptibility to low-light stress later in the season (Figure IV). Nitrogen impacts color Pigmentation of red grapes during vérai son is maximized at moderately high temperatures and low to intermediate bloom-time nitrogen availability; it is minimized when heavy nitrogen supply is followed by overcast conditions. Not only total grape color is affected by nitrogen and weather factors, but also the distribution of individual anthocyanins. Conditions favoring color accumulation (such as low nitrogen, high light and moderate temperature) may also lead to the most balanced distribution of pigments. Because the formation of malvidinglucoside appears to be more tolerant of unfavorable environmental conditions than other anthocyanins, it becomes dominant in grapes grown in poor light or excessive heat, particularly in combination with excess nitrogen.4,11 Vine nitrogen status, therefore, has a direct influence on production of individ42 p racti c al w i ne ry & v i n e yard JANUARY 20 14 ual pigments in grape skins in addition to the indirect effect brought about by modifications of vigor and fruit set.4,7 This has significant implications for winemaking, because the anthocyanin profile in grape skins determines the color potential of a wine. The type and quantities of the different pigments influence both hue and color stability of a red wine. We can expect wine made from grapes that ripened in full sunlight on vines with relatively low nitrogen status to be deeply colored showing a well-balanced, crimson to purple hue. On the other hand, a decrease in total pigment content and a color shift toward red might be expected in wine made from grapes that experienced excessive nitrogen and poor light conditions. Nevertheless, soil nitrogen availability should not be deficient during bloom. This period is critical because of rapid shoot and root growth, which can compete with developing flower clusters for available carbohydrates.6 A second peak in root development appears to be the post-harvest period, especially in warmer climates.1 When to apply nitrogen? Both bloom and post-harvest are therefore suitable periods for nitrogen augmentation, whether from mineralization of organic matter or from fertilizer appli- cation. Which one of the two periods is preferable depends on vine nitrogen status, grapegrowing region, weather conditions, and labor availability. For example, post-harvest nitrogen application is an option only where leaves remain active on the vine long enough after harvest to enable nitrogen uptake. Regardless of the timing of nitrogen supply, nutrient availability is invariably linked to water supply, because nutrients can only enter the roots if they are dissolved in the soil water. Thus, water management, whether it involves rainfall, irrigation or cover crop management, is an important aspect of nitrogen management.8 Indeed water supply can sometimes be more important than nitrogen supply, especially in warm, dry climates. A comparison of irrigation strategies in conjunction with nitrogen application shows once again that the relationship between yield and quality is not a simple one.12 It seems that berry color is linked more closely to berry size than to actual yield, and berry size can be manipulated by irrigation. While standard, calendarbased (drip) irrigation in a warm climate may lead to large but poorly colored berries, deficit irrigation results in medium-size to small berries of good color, regardless of nitrogen supply. It is clear, then, that the effect of nitrogen on yield and fruit quality depends on the grapegrowing region (climate), soil type and composition, and prevailing weather conditions. In other words, the vineyard site influences our ability to manipulate the yield quality relationship in grapes, using nutrient and water management or other cultural practices. If our goal is to produce top-quality grapes, it is unwise, and in fact impossible, to use a cookbook approach with a specified amount of fertilizer applied at a specific point in time to each cultivar in every year. The approach to nitrogen nutrition has to be flexible or adaptive, and it depends on the current vine nitrogen status and seasonal conditions. As vignerons, we should always keep in mind that high yields of excellent quality come from balanced vines with an open canopy and ideal microclimate. Summary Effects of nitrogen on yield and quality can be both direct and indirect. The bloom and post-harvest periods are critical for nitrogen supply. A shortage during bloom will result in a loss of potential yield, while a surplus will reduce grape quality.