BY Marcus Keller,
Wahington State University
Prosser, WA

Yield is determined by the amount of carbohydrates (sugar) partitioned to the fruit rather than to other organs of grapevines. The classical view of the relationship between grape yield and quality is that of a linear decrease in quality as yield increases. Moreover, many grapegrowers believe that the more they stress their vines, the better the resulting fruit quality will be.

However, reality is not that simple. A more scientific view is that of an optimum curve with initially increasing quality, followed by a plateau, and then a reduction in quality when yield is further increased. Under changing external conditions, this curve can be shifted upward or downward.

Rather than setting a specific, inflexible target yield, it should be the goal of every economically thinking vineyard manager to achieve the highest yield possible without compromising quality.

Nitrogen is only one of a whole range of tools available to vignerons to manage the balance between yield and quality. Other tools include vineyard design (row orientation, planting distance, trellis design, training system), pruning (node number and position), canopy management, and soil and water management.

However, of all mineral nutrients, nitrogen is the most potent in terms of influencing vine growth, morphology, and tissue composition. This is primarily because nitrogen is a chemical component of many critically important plant constituents. Nucleic acids contain it, and they in turn make up the genetic information contained in the vine’s DNA.

Nitrogen is also in amino acids, which when linked together, make up the proteins and enzymes that drive all biochemical reactions. It is an integral part of chlorophyll, responsible for intercepting and capturing sunlight, of hormones used for communication between different plant organs, and of certain secondary metabolites, some of which also contribute to wine flavor.

Nitrogen is present in the soil solution in the form of nitrate (NO3^–) and ammonium (NH4⁺) ions. Nitrate is the vine roots’ nitrogen uptake form of choice. It is reduced to ammonium in the roots and metabolized (assimilated) into the amino acids glutamine and glutamic acid.

Glutamine is the main nitrogen transport form in the transpiration stream (xylem), but with increasing nitrogen supply in the root zone, nitrate is increasingly transported to the shoots in addition to glutamine.^2,5 This is because nitrogen uptake and assimilation in the roots are expensive in terms of carbohydrate requirements (both to fuel energy generation and as backbones for amino acids). Therefore, if supply exceeds demand, the excess nitrate must then be assimilated in the leaves and fruit, which for them, constitutes a conflict of interests.

Nitrogen (in chlorophyll and enzymes) stimulates photosynthesis in the leaves. Photosynthesis is the process by which the energy from sunlight is transformed into biochemical energy (ATP) and used to fix carbon dioxide (CO2) and water (H2O) to produce sugar (glucose). This glucose is stored as starch or made into sucrose, which is then exported in the phloem to various plant parts.

Increasing nitrogen supply enhances photosynthesis, which means that more sugar is available for growth and fruit ripening. However, when excess nitrate arrives in the leaves, it competes for carbohydrates and may result in a shift of the vine’s priorities from fruit ripening to shoot growth.⁸

Excessive vegetative growth (vigor) can lead to shaded canopies and, along with 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.

Sugar is not the only grape component that is adversely affected by shade.⁸ 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 the popular canopy management technique of 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 (see photo).

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 and eliminates young, photosynthetically active leaves while leaving old, inefficient leaves behind.

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. This is illustrated in Figure III; the relationship between grapevine yield and fruit quality can be influenced by nitrogen supply and 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 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).

Pigmentation of red grapes during veraison 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. It is not only total grape color that 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 malvidin-glucoside 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.^4,9 Vine nitrogen status, therefore, has a direct influence on production of individual 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 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 significant 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

Both bloom and post-harvest are therefore suitable periods for nitrogen augmentation, whether from mineralization of organic matter or from fertilizer application. Which one of the two periods is preferable depends on vine nitrogen status, grapegrowing region, weather conditions, and labor availability.

For instance, 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.

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.¹⁰

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, calendar-based (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 site terroir 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.

Color is among the quality attributes most easily influenced by nitrogen availability, but it is also linked to water supply. Both timing and amount of nitrogen supply depend on vine nitrogen status, climate and weather, and soil properties. The goal is to achieve balanced vines with an open canopy and ideal microclimate for high yields and top quality grapes.

The relationship between grape yield and quality is complex and can be influenced by a variety of cultural practices. Nitrogen supply is one of these practices that, along with water supply, is crucial for vine growth, yield formation, and fruit quality. Errors made in nitrogen management cannot be corrected by other cultural practices, such as canopy management.

Edited from Proceedings of the 13th International Enology Symposium, at the Institut National de la Recherche Agronomique (INRA) de Montpellier, France sponsored by International Association of Enology, Management, and Wine Marketing, www.oenology.de.

References