WATER IN GRAPE BERRY AND WINE
Water is by far the most abundant constituent in must, defined
as the juice of freshly pressed grapes (75-85%), and
in wine (65-94%, depending on wine sugar content), acting
as a solvent of volatile and fixed chemical compounds. Up
to 99% of the water content in the must and in wine is absorbed
by the roots from the soil. The quantity of water
present on the surface of fruits and leaves when grapes are
harvested and that deriving from enologic treatments can
contribute up to 5% of the total water content in wine (Blouin
and Cruège 2003). Vine growth and grape berry development
are closely related to water availability in the soil. Berry
volume per vine, which sets the limit of crop production,
depends on berry number and water volume per berry. Sink
size is by far the most variable factor in commercial vineyards
(Delrot et al. 2001). Water represents 90% of the harvest
weight in a crop primarily valued at a fresh weight basis,
and carries most of the remaining 10% of crop weight
into fruit, determining thereby fruit quality (Lang and Thorpe
1989).
Fruit volumetric growth is primarily the result of water
accumulation, and hence the maintenance of fruit growth requires
coordination between water and solute transport, both
through the vascular tissue and at the level of individual
cells. One hypothesized coordinating principle is that for
fruit growth to occur there must be a favourable gradient in
total water potential between the fruit and the rest of the
plant. This gradient is generated both by transpiration and
by an osmotic gradient (reviewed by Matthews and Shackel
2005).
Water deficit generally leads to smaller berries since it
inhibits both cell division and, especially, cell expansion. It
is widely acknowledged that berry size is a determining factor
in wine grape quality. The implicit mechanism of this
concept is that surface area/volume ratio of the approximately
spherical berries decreases with the increase of berry
size. Then, since anthocyanins and other phenolic compounds
accumulate in the skin (Coombe et al. 1987), smalller
berries have a relatively greater solute to solvent ratio
than larger berries. However, there are effects of vine water
status on fruit composition that arise independently of the
resultant differences in fruit size. Indeed, the effect of vine
water status on the concentration of skin tannins and anthocyanins
is greater than the effect of fruit size per se on those
same variables (Roby et al. 2004). The primary mechanism
responsible for that is probably the differential growth responses
of skin and inner mesocarp tissue to water deficit
(Roby and Matthews 2003), although there may also be a
direct stimulation of phenolic biosynthesis. Water stress
may also affect the chemical breakdown or formation of important
berry acids and flavours, and promote the conversion
of carotenoid to aroma compounds. In addition, the
concentration of arginine, a yeast-assimilable amino acid, is
lowered. Indirectly, water stress may reduce the shading of
fruit (Goodwin 2002; Keller 2005). Rojas-Lara and Morrison
(1989) selectively shaded grape vines to separate the
effects of shading fruit from the effects of shading foliage
on berry development and on the accumulation patterns of
sugar, anthocyanins, malate, tartrate and potassium in the
fruit. Results showed that shading fruit and shading foliage
differentially affected berry development and ripening.
However, in general, shading decreases fruit colour, and the
concentration of tartrate and soluble solids, and increases
pH and the concentration of malate and potassium.
The timing of the water deficit is clearly important in
order to determine fruit and wine composition, and irrigation
can provide a means to manipulate wine sensory characteristics
in the vineyard (Matthews et al. 1990). Although
water deficit has a less pronounced effect on sugar accumulation
than on berry growth, when it occurs at post-véraison,
fruit sugar is often reduced rather than being improved.
Conversely, malate decrease occurs primarily when water
deficit occurs before véraison. Just before harvest excess
water should be avoided since it can increase berry size and
cause a “dilution” of solutes (sugars, acids, anthocyanins,
tannins, etc.) or cracking of berries (Keller 2005; Keller et
al. 2006).
Water influx into fruits occurs via both the xylem and
phloem and most of the berry volume gain before véraison
is due to water import from the xylem, whereas most of the
post-véraison gain is due to water import from the phloem
(Ollat et al. 2002; Keller et al. 2006). Moreover, the xylem
and phloem are interconnected along the entire length and
can readily exchange water and solutes (Zwieniecki et al.
2004). Indeed, despite the presence of intact and apparently
functional xylem conduits in the berry, it seems clear that,
at véraison, there is a substantial reduction in the proportion
of water that is transported to the berry through the xylem
compared to the phloem, which perhaps depends more on
source and/or sink behaviour than on physical loss in xylem
conductance (Bondada et al. 2005). At the same time, there
is also a shift of phloem unloading, from the symplasmic to
apoplasmic pathway that allows and is linked to the
property of fruit flesh to accumulate high levels of soluble
sugars (Patrick 1997; Zhang et al. 2006). The active removal
of unloaded sugars from the mesocarp apoplast, which
happens at a faster rate than water influx into mesocarp
cells, explains how sugar accumulation in sink organs is associated
to the shift from xylem to phloem water transport
at the transient onset of grape berry ripening. Keller et al.
(2006) have demonstrated that berry-xylem conduits retain
their capacity for water and solute transport during ripening,
and propose that apoplastic phloem unloading coupled with
solute accumulation in the berry apoplast may be responsible
for the decline in xylem water influx into ripening
grape berries. In addition, the involvement of cell compartmented
solutes, the rising solute concentration inside the
vacuoles, make it more and more difficult for leaves to extract
water from ripening berries. Xylem may serve to recycle
excess phloem water back to the shoot. It is generally
believed that at any given turgor, the growth rate is determined
by the weakening rate of the cell wall structure (Cosgrove
2000). These two processes (turgor increase vs. wall
loosening) are used to explain an increased fruit growth at
véraison (Mathews and Shackel 2005). The shift from
xylem to phloem function at véraison would prevent a substantial
water or solute backflow from the fruit to the plant,
which is increased by fruit transpiration (Delrot et al. 2001).
As most of the water coming into the berry after véraison
is imported by the phloem, it has to cross biological
membranes (Lang and Thorpe 1989). The hydraulic conductivity
of the biological membranes is controlled mainly
by the presence of specialized proteins that belong to the
major intrinsic proteins (MIP) family. The aquaporin
VvPIP1A (Vitis vinifera plasma membrane intrinsic protein)
mediates water transport and is mainly expressed in the berries
after véraison (Picaud et al. 2003). The co-expression
of some aquaporins at the same time as sugars transporters,
suggests a functional link between sugar and water fluxes
during the processes of unloading and sugar accumulation
in the vacuoles of the flesh cells (Delrot et al. 2001). In our
lab, the aquaporin VvSIP1 (Vitis vinifera small intrinsic protein,
DQ086835) was cloned and its functionality is being
studied by a heterologous expression in Xenopus oocytes.
Baiges et al. (2001) obtained the nucleotide sequences of
eight cDNAs encoding putative Vitis aquaporins (PIPs and
TIPs) and found that most of them had higher expression in
roots, eventually enhancing and regulating root water permeability.
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