16
121
Grapevine Water Relations
L a r r y E . W i ll i a m s
Water is important to all living organisms. It is an
essential constituent of cells: 80 to 90 percent of the
fresh weight of living cells is water. It is a solvent in
which gases, salts, and other solutes are able to move
in and out of cells and from organ to organ. Water is
a reagent in photosynthesis and in a number of other
biochemical or biophysical processes. Lastly, water
is essential for the maintenance of turgor (a certain
degree of turgor is required for cell growth, the operation of stomata, and the maintenance of form of nonlignified structures).
Water movement occurs along gradients of decreasing free energy or molecular activity, often described
as differences in water potential. If some external agent
produces the difference in water potential, the movement of water is called mass flow. One example of mass
flow in a plant is the upward movement of water in
the vine (in the xylem tissue) caused by evaporation
from the leaves. If the movement of water results from
the random motion of molecules, as in evaporation,
the process is called diffusion. Osmosis is an example of
diffusion caused by a difference in water potential on
two sides of a membrane, usually caused by differences
in the concentration of solutes.
Water Movement in the
Soil-Vine-Atmosphere Continuum
Stomata are microscopic pores on the lower surface of
grape leaves where water vapor is lost and carbon dioxide (CO2) is taken into the leaf. Stomata are closed in
darkness, so little water is lost from the vine once the
sun sets in the evening. When the sun comes up in the
morning, stomata will gradually open. They are fully
open at approximately one-third of full sunlight. The
opening of stomata results in the loss of water vapor
from the leaf due to the large gradient in water potential between the atmosphere and inside the leaf (which
is considered to be at 100 percent relative humidity).
As the leaf loses water vapor, water moves from the
cells surrounding the substomatal cavity into that cavity, continuing the process.
The movement of water out of a cell lowers its
water potential (a decrease in free energy), so water
will move into that cell from another cell with a higher
water potential. More and more cells will continue to
lose water until water is lost from cells located next to
the leafs vascular tissue. The vascular tissue contains
specialized cells (vessels) that transport water from the
roots to the leaves. The loss of water from these cells
in the leaf creates a tension within the xylem, which is
transmitted down the length of the vascular tissue in
the vine. This tension effectively pulls water up through
the vine as a result of the strong cohesive properties of
water molecules in these small water-conducting cells,
which are analogous to microscopic water pipes. Once
water begins to move upward within the xylem of the
root system, more water will move from the cortical
cells in the roots to the xylem. As within the leaf, water
movement out of a root cortical cell lowers its water
potential and induces more water to move into that
cell. Finally, this process initiates water uptake into the
root from the soil profile.
Field capacity (FC) is the amount of water retained
in the soil after gravitational water has drained from
the soil. The water that remains is held in soil pores
by capillary action and as a thin film surrounding individual soil particles. Soil pores up to about 10 m in
diameter will hold water by capillary action, whereas
larger pores (over 60 m) will allow water to rapidly pass through. The component of the soil water
potential that is of major importance here is the matric potential; capillary action and adsorption hold the
water to colloids such as clay and organic matter.
As the soil dries out, the water in the larger soil
pores is depleted first, so that only the smaller pores
retain water. At a certain point, soil water is no longer
available to the plant due to the strong capillary action
of the finer pores. This is the permanent wilting point
(PWP). When you subtract the PWP from the FC, the
difference is the available water content (AWC). One-
Crop
half of the AWC is readily available to a plant; below
this point, water is harder and harder to extract from
the soil. As with water movement within the vine, the
movement of water in the soil depends upon the existence of gradients of decreasing free energy or decreasing water potential. Water will move in the soil from
areas of higher water potential to areas of lower water
potential.
Daily and Seasonal Vine Water
Relations and Water Use
Pure water has a water potential of 0 megapascals
(MPa). The introduction of any solute into water will
decrease its water potential. The predawn leaf water
potential of a grapevine is high (i.e., it can approach
0 MPa). This may be due in part to root pressures that
develop in grapevines. Root pressures as great at 0.4
MPa (4 bars) have been reported. The sap exudate from
cut surfaces of the vine prior to budbreak is probably
a result of root pressure. The leaf water potential of
grapevines undergoes daily fluctuations, with the lowest value of the day measured sometime between 1:00
pm and 3:00 pm daily. Increasing evaporative demand
and decreasing availability of soil water generally cause
midday values for leaf water potential to decline as the
season progresses.
Midday leaf water potential for Thompson Seedless grapevines, however, generally does not fall lower
(i.e., more negative) than 0.8 to 1.0 MPa (8 to 10
bars) throughout the growing season if the vines are
well watered and not water stressed. Both predawn and
midday values of leaf water potential are more negative for water-stressed vines than for those that are not
stressed. Late in the season, midday leaf water potential of non-irrigated vines in the San Joaquin Valley
may fall as low as 1.4 MPa (14 bars). The leaf water
potential of all vines rebounds in the afternoon and
well into the evening, with the highest (least negative)
value recorded before dawn. Seasonal values for midday leaf water potentials of Thompson Seedless grapevines are linearly related to soil water content: as the
soil dries out, leaf water potential decreases.
The main driving force for vineyard water use (or
evapotranspiration [ET]) is net radiation. Net radiation provides the energy to convert water in the liquid state (inside the leaf) to the vapor state (lost via
the stomatal pore) outside the leaf. As you can see in
Figure 16.1, vine water use is more highly correlated
with net radiation than with ambient temperature.
Other environmental factors influencing ET include
wind speed and vapor pressure deficit (as the relative
humidity decreases, vapor pressure deficit increases).
600
100
Net radiation
500
Ambient
temperature
90
400
300
80
200
70
100
Ambient Temperature (F)
the
Net Radiation (W/m2)
Producing
0
0
8
12
16
Time of Day (hour)
20
24
8
12
16
Time of Day (hour)
20
24
60
1.5
1.0
ET vine (gal/hr)
122
0.5
0.0
0
Figure 16.1 The diurnal time course of vine water use, net radiation, and ambient temperature on June 2, 1996. Vine water use was
obtained from Thompson Seedless grapevines grown in a weighing
lysimeter at the UC Kearney Agricultural Center. The trellis system
for the vines was a 2-foot crossarm at the top of a 7-foot stake, with
fruiting wires at either end of the crossarm. Vines within the lysimeter
were irrigated whenever 2 mm (approximately 2.11 gallons) of water
had been used, so the vines were not stressed for water at any time.
Net radiation and ambient temperatures were obtained from a CIMIS
(California Irrigation Management Information System) weather station located approximately 1 km (0.6 mile) from the lysimeter. For
a complete description of the weighing lysimeter, see Phene et al.,
Automated Lysimeter for Irrigation and Drainage Con-
Vapor pressure deficit is highly dependent upon ambient temperature. Therefore, the increase in water use
at higher temperatures is due generally to higher net
radiation and lower relative humidity during those
periods. In addition to providing the energy to drive
C h a p t e r 16: G r a p e v i n e W at e r R e l at i o n s
ET, light influences the degree to which the stomata
open. Wind and vapor pressure deficit also influence
the degree of grapevine stomatal opening. In this way,
the vine is able to regulate the amount of water it uses
via changes in stomatal conductance.
The level of vineyard water use depends upon a
number of factors. During establishment a vineyard
uses less water than a mature vineyard (Table 16.1).
Results from a study at the UC Kearney Agricultural
Center that used a weighing lysimeter (a very sensitive device to measure plant water use) indicate that
during the first two years of growth vines only use
approximately 50 percent as much water as a mature
vine. During the first year, much of the water went to
evaporation from the soil surface. Mature vines used
an average of 1,650 gallons from budbreak to the end
of October from 1990 to 1996. Mature vines at full
canopy covered approximately 60 to 65 percent of the
surface area allotted to each vine, with approximately
9 m2 of total canopy surface area per vine. Third-leaf
vines used approximately 70 percent of the water used
by mature vines. Other studies have demonstrated that
trellis type and vine size have a significant effect on
vine water use. Vines grown on trellis systems that
spread the canopy and vines with more leaf area will
123
use more water. The pruning pattern you use may or
may not have an effect on vine water use (Table 16.1).
Vine water use also varies throughout the growing season. Water use is low early in the season, from
budbreak until one month later, as the vine has little
leaf area during that time (Figure 16.2). Once there is
appreciable leaf area and evaporative demand increases, vine water use increases in an almost linear fashion. Vine water use becomes constant at full canopy.
Maximum water use in 1996 was about 13.5 gallons
of water per day for a period of approximately 60 days.
The decrease in vine water use from day 150 to day 175
(approximately 1 June to 27 June) reflects a decrease
in evaporative demand during that period. Vine water
use decreases as the season progresses because leaves
start to senesce and fall off the vine. During the course
of the Kearney Agricultural Center study, researchers
found that leaf damage from variegated leafhoppers
(Erythroneura variabilis Beamer) significantly reduced
vine water use late in the season as compared to water
use in years when leafhopper populations had been
controlled with pesticides.
Effects of Irrigation
Table 16.1 Water use of Thompson Seedless grapevines during
and after vineyard establishment, determined with a weighing lysimeter (vine water use [ETc] or potential ET [ETo] amounts obtained by
summing data from date of budbreak [or the day vines were planted,
in 1987] until the end of October each year)
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
ETc
ETc
ETo
(inches) (gal per vine) (inches)*
14
16
23
29
34
32
34
33
29
34
700
800
1,160
1,400
1,720
1,590
1,704
1,678
1,430
1,717
46
45
47
47
47
47
44
44
42
45
Cultural/pruning
practice
Planted April 9
Trained up the stake
Two 12-bud canes
Four 15-bud canes
Six 15-bud canes
Six 15-bud canes
Eight 15-bud canes
Eight 15-bud canes
Eight 15-bud canes
Eight 15-bud canes
*Potential ET (ETo) data were obtained from a CIMIS weather station at the
Kearney Agricultural Center. ETo is the amount of water used by a short, green
crop completely shading the ground. It is a measure of the evaporative demand
of a particular region throughout the year.
The number of canes to be left on the vines from 1991 to 1996 was determined by dissecting buds the previous winter to determine bud fruitfulness and
subsequently devising a pruning pattern.
Vines in the lysimeter were furrow irrigated the first growing season. From
1988 to 1996, vines within the lysimeter were drip irrigated. Daily irrigations
took place whenever vines used 2 mm (2.11 gallons) of water. Thus the vines
in the lysimeter may have been irrigated five to six times a day during the portion of the season with the greatest evaporative demand. The irrigation season
generally commenced the first week of May and continued until the end of
October.
The vines in the lysimeter were trunk girdled in 1994 and 1995 as the vines
were used to produce table grapes. The vines were not girdled in 1996.
Daily Water Use (gal/vine)
Year
15
1996 Growing Season
10
canes
cut
harvest
5
bloom
0
50
100
150
200
250
300
Day of Year
Figure 16.2 Seasonal water use of Thompson Seedless grapevines
growing in a weighing lysimeter at the Kearney Agricultural Center
during 1996. Budbreak, bloom, and harvest occurred on March 10,
May 12, and September 2, respectively. To calculate daily water use,
we summed weekly water use and divided that number by 7. Canes of
vines growing within the lysimeter were manually cut approximately
18 inches from the ground on August 5 to simulate the mechanical
cane cutting performed on vines surrounding the lysimeter. Other
information as described in Figure 16.1 and Table 16.1.
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Producing
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on Vine Growth
12
Vegetative Growth
Most studies on grapevine irrigation have demonstrated that water deficits affect vegetative growth to
a greater degree than they affect reproductive growth.
A reduction in shoot growth is one of the first visible
symptoms of vine water stress. When water stress is
severe or when its onset is too rapid, the stress can kill
the shoot tip. Moderate water deficits will decrease
the rate of shoot elongation, along with internode
length and radial expansion. Leaf area per vine is less
under water deficit conditions due to reduced shoot
length and smaller leaves. The growth of lateral shoots
is reduced more by water stress than that of primary
shoots. This may be because soil water deficits do not
develop until later in the growing season, after the primary shoots have had considerable growth and before
most lateral shoots initiate their growth. The weight
of pruned canes taken during the dormant portion of
the growing season is used as a measure of the previous seasons shoot growth. There is an almost linear
increase in pruning weights of Thompson Seedless
grapevines using a single-wire trellis as the amount of
applied water increases from 0 to 120 percent of the
water used by vines growing in a weighing lysimeter
(Figure 16.3). A further increase in water application
amount (to 140 percent) decreased pruning weights
for that trellis treatment. Vines using the crossarm trellis system exhibited a sigmoidal increase in pruning
weights with an increase in applied water, leveling off
at the higher level of applied water.
Few studies have examined the effects of water deficits on the growth of the grapevines permanent structures (root system, trunk, and cordons, if any). Potted
vine studies indicate that root growth is less sensitive to
water deficits than is shoot growth. Trunk biomass and
diameter are both reduced by water deficits. It should
be pointed out that the concentration of storage sugars
in the root system and trunk are not affected by water
deficits, but since biomass is reduced, total sugars are
less in water-stressed vines than in non-stressed vines.
Reproductive Growth
Reproductive growth of grapevines is generally less
sensitive to water stress than vegetative growth. However, the stage at which berry growth is most sensitive
to water deficits is stage I (see chapter 5, Grape Berry
Growth and Development, for definitions of the stages
of berry growth). Water deficits during stage I decrease
both the division and the elongation of the cells in the
berry. Berry growth is reduced more by a water stress
episode during stage I than by a similar episode during
Pruning Weight (lb./vine)
10
Trellis type
Single wire trellis
Trellis w/crossarm
0.2
0.4 0.6 0.8 1.0 1.2
Applied Water
(fraction of lysimeter water use)
1.4
Figure 16.3 Pruning weights of Thompson Seedless grapevines
irrigated at various fractions of full vine ET (as determined with a
weighing lysimeter). Whenever vines in the lysimeter were irrigated
daily and throughout the growing season, vines in the eight irrigation
treatments were also irrigated at the fraction indicated on the x-axis
of the graph. Each irrigation treatment was replicated eight times.
The data are the means of four growing seasons (1990 to 1993). The
single-wire trellis system consisted of a wire run atop a 7-foot stake.
The crossarm trellis consisted of a 2-foot crossarm atop a 7-foot stake,
with fruiting wires at either end of the crossarm. Bars represent one
standard error.
stage II or III. In addition, reductions in berry growth
caused by stage I water stress cannot be reversed by
supplemental irrigation during stages II or III. An irrigation study at the Kearney Agricultural Center demonstrated a linear increase in berry weights correlating
to applied water levels of 0 to 80 percent of full ET,
when irrigating was maintained at the same level for
the full growing season (Figure 16.4). This study also
showed that applying water in excess of 80 percent
of full ET all season long did not result in larger berries. Maximum berry size for Thompson Seedless can
therefore be obtained under mild water deficits. More
recent studies have also demonstrated that deficit-irrigation (50 percent of full ET) after veraison has no detrimental effect on berry size: cutting off water to raisin
vineyards in order to prepare the soil for fruit drying
does not adversely affect berry size.
Vine water status will affect the solute concentration (mainly sugars) throughout berry development.
The accumulation of sugar in the fruit appears to be
less affected by water deficits than is berry growth.
Many vines that are water stressed have fruit with a
higher concentration of sugar than on vines that are
given more water (Figure 16.4). This may be due to at
least three factors. First, berries may lose water under
C h a p t e r 16: G r a p e v i n e W at e r R e l at i o n s
125
25
200
150
Yield (tons/acre)
Berry Weight (g per 100 berries)
20
100
0.2
0.4
0.6 0.8 1.0 1.2
Applied Water
(fraction of lysimeter water use)
1.4
20
Soluble Solids (Brix)
Trellis type
Single wire trellis
Trellis w/crossarm
0.2
0.4
0.6 0.8 1.0 1.2
Applied Water
(fraction of lysimeter water use)
1.4
Figure 16.5 Yield (fresh weight) of Thompson Seedless grapevines
irrigated at various fractions of full vine ET. Data points represent the
means of five growing seasons (1990 to 1993 and 1996). Other information as described in the legend for Figure 16.3.
25
15
10
10
50
15
0.2
0.4
0.6 0.8 1.0 1.2
Applied Water
(fraction of lysimeter water use)
1.4
Figure 16.4 Berry weight and soluble solids of Thompson Seedless
grapevines irrigated at various fractions of full vine ET. Data represent
the means of data collected in 1990, 1991, and 1992, and are averaged
across the two trellis treatments. The legends for figures 16.1 through
16.3 explain how irrigations for each treatment were scheduled.
water deficit conditions such that the sugar in the
fruit becomes more concentrated. Second, vines that
are water stressed generally have lower yields whereas
vines given more water have more sinks (higher yield)
competing for carbohydrates. And third, it has been
demonstrated that fruit growing in the shade, as they
might be on irrigated vines with excessive vegetative
growth, have lower rates of sugar accumulation.
The yield of Thompson Seedless grapevines as a
function of applied water and trellis type is shown in
Figure 16.5. There is an almost linear increase in yield
for vines grown with a crossarm trellis as water applications increase from 0 to 80 percent of full ET. Beyond
80 percent of ET, yield decreases slightly before leveling off. The optimum water application amount for a
single-wire trellis is 60 to 80 percent of full ET, with
reductions in yield on either side of those levels. It
would appear that under severe soil water deficits the
single-wire trellis would be an advantage, whereas it
would be a disadvantage under conditions of too much
water. The optimization of yield for Thompson Seedless vines in this study at water applications of 60 to
80 percent of full ET indicates that the smaller canopy
that develops under mild water deficits (see Figure
16.3 for pruning weight data) does not hinder berry
size or final yield.
From the data in Figures 16.3, 16.4, and 16.5, one
can draw some useful conclusions regarding an irrigation strategy for Thompson Seedless grapevines
grown for raisins. The major yield component determining final yield for Thompson Seedless grapevines
is the number of clusters per vine. One reason yield is
maximized at water applications of 60 to 80 percent of
full ET is that those treatments have the greatest bud
fruitfulness year after year (see chapter 4, Bud Development and Fruitfulness of Grapevines). Overirriga-
126
Producing
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Crop
tion results in fewer clusters per vine due to lower bud
fruitfulness and increased bud necrosis. The reduction
in yield under greater water deficits is mostly the result
of decreased berry growth and fruit dehydration, especially for vines irrigated at 0 and 20 percent of full ET.
Another important consideration when producing raisins is to ensure that the berries mature (accumulate
sugar) early enough to allow time for them to be laid
out to dry. Even though yields were highest for 60 and
80 percent of full ET across all treatments, the sugar
accumulation rate in those fruit was not the lowest.
Therefore, you may be able to maximize yields without
significantly delaying the harvest date.
The data presented here support numerous studies that indicate beneficial effects from regulated deficit
irrigation (RDI) for woody perennial crops. Following
this practice, growers irrigate plants at a deficit during
specific phenological stages of growth. In sustained deficit irrigation, as is describe above for Thompson Seedless grapevines, growers irrigate at a fraction of full ET
throughout the growing season. This is clearly a useful
way to save water while maximizing production.
R e f e r e n c e s
Grimes D. W., and L. E. Williams. 1990. Irrigation effects
on plant water relations and productivity of Thompson
Seedless grapevines. Crop Sci. 30:25560.
Phene, C. J., G. J. Hoffman, T. A. Howell, D. A. Clark, R.
M. Mead, R. S. Johnson, and L. E. Williams. 1991. Automated lysimeter for irrigation and drainage control. In
Proceedings of lysimeters for evapotranspiration and
environmental measurements. St. Joseph, MO: ASCE.
2836
Williams, L. E. 1996. Grape. In E. Zamski and A. A. Schaffer,
eds., Photoassimilate distribution in plants and crops.
Source-sink relationships. New York: Marcel Dekker, Inc.
85181.
Williams, L. E., N. K. Dokoozlian, and R. Wample. 1994.
Grape. In B. Shaffer and P. C. Anderson, eds., Handbook
of environmental physiology of fruit crops. Orlando:
CRC Press. 83133.
Williams, L. E., and M. A. Matthews. 1990. Grapevine. In
B. A. Stewart and D. R. Nielsen, eds. Irrigation of agricultural crops. Agronomy monograph No. 30. Madison:
ASA-CSSA-SSSA. 1,01955.
Williams, L. E., D. W. Williams, and C. J. Phene. 1993. Modeling grapevine water use. In C. S. Stockley, R. S. Johnstone, P. A. Leske, and T. H. Lee. Proc. eighth Australian
wine ind. tech. conf. Adelaide: Winetitles. 2933.
