Agriculture Reference
In-Depth Information
A pressure chamber for measuring leaf water potential. Photograph by the author.
Figure 6.7
limit is more likely to be
400 kPa. These stress limits have been identified by
experiments in which
m
values at different depths in the root zone have been
compared with the leaf water potential (
l
value is measured
on individual leaves plucked from the vine at a particular time of day and placed
in a pressure chamber (fig. 6.7). The
l
) in the vines. The
l
of a nonstressed vine, measured at mid-
day, should not be
1 MPa (
1000 kPa), whereas that of a stressed vine can
fall as low as
1.5 MPa (Williams 1996).
Because measurements are made at a point (see box 6.3),
will vary with
depth and distance from each vine, depending on the root distribution and the
balance between rainfall and
ET
. Values of
m
are also influenced by soil struc-
ture and texture, which vary spatially. Tensiometers are commonly used under
young vines that are kept well watered (
m
60 kPa); otherwise, gypsum blocks
are used. These instruments should be installed at a minimum of two depths (the
midroot zone at 25-50-cm depth, and near the bottom at ca. 1-m depth). Be-
cause
as measured will change with depth as a result of the change in
g
, the
measured values must be corrected to obtain the
m
value (see equation 6.4). Ten-
siometers or gypsum blocks should be placed in the row (so as not to obstruct be-
tween-row operations) and between vines, at a suitable distance from any drip ir-
rigation emitter or minisprinkler (10-15 cm in a sandy soil and 20-25 cm in a
clay soil).
Growers do not want their vines to suffer water stress before flowering and
at the time of flowering (
m
in the midroot zone should not fall below
40 kPa
in sandy soils and
60 kPa in clay soils). Between flowering and veraison, the
grower may want to impose some stress to moderate shoot growth and slow down
cell division in the berries (to control berry size). Hence,
m
(midroot zone) should
not fall below about
100 kPa in sandy soils,
200 kPa in loamy soils, and
400
