tation into the skin cells. The transcript levels of both transporters
are lower at post-véraison than at pre-véraison, but
significant levels of expression were detected. It is suggested
that VvKUP1 and VvKUP2 may therefore continue
to contribute to potassium homeostasis throughout berry
development. Other mechanisms may also be involved after
véraison (Davies et al. 2006).
The levels of potassium in grape berries may be
affected by numerous factors including potassium level in
the soil, grape variety, and viticultural practices (Mpelasoka
et al. 2003; Davies et al. 2006). A detailed knowledge of
the mechanisms involved in potassium transport from the
soil, xylem and phloem translocation through the vine and
its accumulation in the berry, is crucial in order to develop
strategies which may reduce its excessive accumulation in
grape berries and thereby improve fruit and wine quality.
Other minerals, such as calcium and magnesium,
together with minimal amounts of sodium and iron, are also
present in grape berries. Calcium concentration is at its maximum
at véraison, and remains stable or decreases during
maturation. Calcium has a significant role in the resistance
against Botrytis cinerea infection. In must, its content (80
mg/L) can slightly decrease because of the increase of calcium
tartrate resulting from alcohol formation. Magnesium,
a component of chlorophyll, is present in grapes and in
must at levels similar to calcium. Grapes are poor in iron
and copper, their concentrations in must and wine being approximately
10 and 1 mg/L, respectively. However, an unusual
increase of these compounds in must can occur as a result
of viticulture and vinification practices. Copper affects
wine flavour by oxidizing thiol-containing compounds
(Blouin and Cruège 2003).
SUGAR IN GRAPE BERRY AND WINE
Sugar transport and accumulation in berry
Efficient assimilation and use of nutrients by plants is of
prime importance for the optimization of crop productivity.
Grape berries, as typical “sink organs”, rely on the use of
available carbohydrate resources produced by photosynthesis
to support their growth and development. The transport
and allocation of sugars between the photosynthetic “source
tissues” and the heterotrophic “sink tissues” is known as assimilate
partitioning and is a major determinant of plant
growth and productivity (Kingston-Smith 2001) (Fig. 3).
Our understanding of the mechanisms of carbon partitioning
in grapevine has made considerable progress over
the past 10 years. Sucrose produced through photosynthesis
in the mesophyll of mature leaves is the main carbohydrate
used for long distance transport (Swanson and Elshishiny
1958). Sucrose is loaded into the phloem by either a symplastic
(via plasmodesmata) or apoplastic mechanism (reviewed
by Boss and Davies 2001). According to Munch’s
mass flow hypothesis, sucrose, as the major osmotically active
constituent in the phloem, also provides the driving
force for translocating all other compounds in the phloem
sap. Upon its arrival in the phloem of the berry, sucrose may
be unloaded by the different pathways described above (reviewed
by Boss and Davies 2001). It has been suggested
that symplastic connections via plasmodesmata between the
sieve tubes and the mesocarp cells remain for quite a long
period during berry development. However, at the phloem/
storage parenchyma interface, sufficient plasma membrane
surface area is available to support exchange with the apo