POTASSIUM IN GRAPE BERRY AND WINE
Mineral soil composition (and density) has an essential influence
on grape quality and on the organoleptic properties
of wine. Grape berries are very rich in potassium which is
an essential macronutrient for grapevine and grape berry
growth and development. Thus, potassium is the main
cation in must and wine (~ 900 mg/L; Blouin and Cruège
2003). As in other plants, potassium plays a key role in the
grapevine physiology: (1) enzyme activation, (2) major
control on the transmembrane potential difference of the
plasma membrane, which to a large extent determines the
uptake of many different cations, anions and sugars, and (3)
regulation of osmotic potential, thus controlling plant water
relations, turgor maintenance and growth. Although other
cations may replace potassium in some of these functions,
potassium plays the major role given its high concentration
in plant tissues and the high permeability of plant membranes
to potassium (reviewed by Mpelasoka et al. 2003).
Potassium is absorbed by the roots and distributed to all
parts of the vine. Early in the season, when the growth rate
is high, much of the potassium accumulates in the leaves.
After véraison, a sharp increase in berry potassium is observed
as a result of potassium redistribution from leaves to
berries (Blouin and Cruège 2003). Potassium uptake by
Cabernet Sauvignon berries is slow before véraison and
strongly increases when ripening starts in the same proportion
as sink strength and phloem water influx (Ollat and
Gaudillere 1996).
Despite its importance to grapevine, excessive levels of
potassium in berries at harvest may reduce the quality of
fruit and have a negative impact on wine quality, particularly
on red wines (Davies et al. 2006). Grape juice pH is a
critical determinant of wine quality. The stoichiometry exchange
of tartaric acid protons with potassium cations results
in the formation of largely insoluble potassium bitartrate.
This leads to a decrease in free acid and tartrate:malate
ratio (Gawel et al. 2000), resulting in an increase of overall
pH. Grape juice with a high pH often results in unstable
musts and wines that are more susceptible to oxidative and
microbiological spoilage. These wines have a high pH, low
acidity and generally a flat taste (Somers 1977; reviewed by
Mpelasoka et al. 2003). Also, the presence of higher levels
of malate to tartrate enhances malolactic fermentation (Rühl
2000). High potassium levels in the berry may decrease the
rate of malate degradation by impairing malate transport
from the storage pools in the vacuole to the cytoplasm.
The high pH of grape juice and wine also leads to a
decrease of the colour quality and stability of red wines as a
result of reduced anthocyanins ionisation at higher pH. Anthocyanins
are located in the berry skin where potassium
concentration is generally higher than in the pulp. Grape
juice contains from 0.5 to 3 grams of potassium per liter.
Grape skin contains about 9 grams of potassium per liter.
Therefore, berry potassium levels are often more important
to red than to white wines, because during red wine fermentation
the skin is left for some period after crushing for the
extraction of anthocyanins. During this period, even more
potassium leaches out of the skins into the juice (reviewed
by Mpelasoka et al. 2003).
Given the importance of berry potassium levels for wine
quality and its possible deleterious consequences, understanding
the dynamics and mechanisms of grape berry potassium
accumulation is of extreme importance. Potassium
uptake into cells occurs by multiple mechanisms. Potassium
channels mediate passive low affinity potassium transport
across membranes, whereas potassium carriers mediate
energized high and low affinity uptake (Davies et al. 2006).
External potassium levels are thought to determine which
mechanism is used by the cell, in a way that a sufficiently
high external potassium concentration will allow transport
through channels; but carrier-mediated uptake becomes critical
at low external potassium levels, when the gradient
may not be favourable to passive transport (Maathuis and
Sanders 1996; Davies et al. 2006). During berry development,
potassium may play different roles depending on the
development stage. During an initial rapid phase of berry
growth, cells are divided and expanded at a high rate, during
which time potassium may play an important role as osmoregulator.
After véraison, grape berries continue to enlarge
due to cell expansion driven by the increase of sugar content
in the cell vacuole. Potassium may play an important secondary
role in the accumulation of sugars during this phase.
Recently, two cDNAs encoding putative potassium transporters
from grape berries have been isolated (Davies et al.
2006). VvKUP1 and VvKUP2 function as potassium transporters
as demonstrated by heterologous expression in an
Escherichia coli mutant deficient in potassium uptake. The
two transporters were shown to be highly expressed in the
berry skin during the first phase of berry development, suggesting
that, at this time, these transporters play a role either
in potassium uptake into the berries or in its compartmen-