Furthermore, the leaf area required
to ripen one gram of fruit also depends
on the trellis and training system; it
appears to be considerably higher in
a typical vertically shoot-positioned
canopy than in vines that are sprawltrained
or trained to divided canopies.
Overcropping ordinarily delays fruit
maturation and therefore decreases
grape sugar and color if harvest cannot
be delayed.1,36,38,40 However, the
effect of crop load on berry composition
depends on how a difference in
crop load is achieved. An outbreak of
pests or diseases, or a hail storm, can
reduce photosynthetically-active leaf
area after the yield potential has been
established, which results in reduced
If an increase in yield is accompanied
by deterioration in the canopy
microclimate, then the changes in fruit
and wine composition are negative.29
In other words, so-called overcropping
effects are often actually shade effects
caused by poor pruning practices or
other vineyard management errors.
For example, if pruning is too light
(too many buds left), then there may
be too many shoots, which leads to a
dense canopy. Alternatively, if pruning
is too severe (too few buds left), then
the few remaining shoots may grow too
vigorously and produce too many laterals,
which leads to shade in the fruiting
zone. In contrast, where an increase in
yield is accompanied by an improvement
in the canopy microclimate, berry
composition may also be improved.
For example, the concentration of
undesirable methoxypyrazines in
grapes grown on high-yielding minimally
pruned vines can be dramatically
reduced compared with lower-yielding
spur-pruned vines. Indeed, when
yields are increased by decreasing the
pruning level (leaving more buds),
the resulting wine is often fruitier and
less vegetative, perhaps due to the
reduced shoot vigor (also see Chapter
6.1).4,5 If, on the other hand, yields are
stimulated by abundant water supply,
the result is usually the opposite.
Therefore, balancing shoot growth
and fruit production is an important
viticultural goal. For an individual
vine, the crop-load/fruit-quality relationship
generally follows an optimum
curve with increasing quality
as crop load is increased from a very
low level, followed by a plateau, and
finally a reduction in quality when
crop load is further increased. Under
changing external conditions (including
cultural practices), this curve can
be shifted upward or downward.
Rather than setting a specific, inflexible
target yield, economically-minded
vineyard managers aim to achieve the
highest possible yield without compromising
quality. Moreover, where
weather conditions permit, delayed
ripening can sometimes be compensated
by delayed harvest.
On overcropped vines, cluster thinning
is typically employed to reduce the
crop load and enhance ripening. This
can be especially beneficial in cultivars
that are prone to overcropping due
to their large clusters, such as Syrah,
Mourvèdre, Grenache, or Zinfandel.
One form of cluster thinning is cutting
away the distal one-third or half
of the flower clusters during bloom,
which not only decreases yield but also
often leads to less compact clusters
due to the compensatory stretching
of the rachis. It is possible that the
advanced maturity observed following
such early thinning may be attributed
less to the smaller crop than to the
tendency for berries in the proximal
portion of a cluster to ripen more rapidly
than those in the distal portion.37
In table grapes, where large berries are
desirable, cluster thinning is sometimes
supplemented by berry thinning.
The size of individual berries is often
more important than the crop level or
even crop load in determining fruit composition.
For instance, large increases in
berry size after veraison may be coupled
with concomitant increases in berry
sugar and K due to phloem import.30
The concentration of K in the berries
decreases as the crop load increases.15
However, because tartrate synthesis
inside the berry ceases at veraison,
an increase in berry size can lead to a
substantial decrease in tartrate due to a
“dilution effect.” Of course, more K and
less tartrate in the berries will result in a
corresponding increase in juice and wine
pH. In addition, larger berries have a relatively
smaller skin:pulp ratio, which has
implications for red wine composition
and quality due to the importance of the
extraction of skin-derived compounds
(mainly anthocyanins, tannins, and flavonols)
during fermentation. In contrast
to grapes used for red winemaking, large
size and crispness are important quality
traits of table grapes.
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