Wahington State University
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
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
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 vines 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
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
vines 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 vines 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
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 vines 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
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
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
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
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.
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.
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