Agriculture Reference
In-Depth Information
readily available, especially during the period of rapid shoot growth from pre-
flowering to veraison (see fig. 5.6), excess vigor may result.
However, if water stress occurs, shoot growth slows, internodes shorten, and
the tip becomes a dull grey-green. The leaves become flaccid. With severe stress,
growth stops and the tips and tendrils may die. Late in the season, water-stressed
lower leaves become yellow and may develop necrotic areas at the edges; leaf drop
begins at the basal leaves and progresses toward the tip.
Severe stress at flowering reduces fruit set. From fruit set to veraison, berry
size is decreased by moderate to severe stress (an effect on cell division). During
ripening, mild stresses can enhance the accumulation of soluble solids by sup-
pressing vegetative growth. Under more severe stress, soluble solids increase
through a reduction in berry size or shriveling (an effect on cell expansion). In
this case, sugar accumulation and flavor development may be delayed because of
reduced photosynthesis and premature leaf drop.
In dryland vineyards, several factors affect the soil water supply to the vines:
• Slope of the land and whether it is contoured or terraced (section 7.5.4)
• Infiltration rate of the surface, which depends on soil type, the presence of
a cover crop, and the extent of compaction from wheeled traffic (section
7.1.1)
• Type of cover crop (annual or perennial, and its depth of rooting) and
weeds in the vine rows, all of which compete with the vines for water
(section 7.3.1)
• Mulching with straw or cover crop residues in the vine row to reduce soil
evaporation (section 7.3.2)
• Installation of subsoil drainage to remove surplus water and control the
height of the water table (section 7.2.4)
All these factors can also operate in an irrigated vineyard; but the availability
of irrigation water, and manipulation of its amount and timing, enable a higher
degree of control of the soil water supply, especially in environments with low
rainfall. There are several approaches to managing irrigation to meet specific pro-
duction objectives.
Using Soil Water Potentials to Manage Irrigation
Based on the soil water retention curve (fig. 6.3b), the
available water capacity
AWC
can be subdivided into readily available water (
RAW
), deficit available wa-
ter (
DAW
), and the remainder (
AWC
RAW
DAW
). With a
FC
corre-
sponding to
6.5.1
10 kPa, the lower limit of
RAW
is set between
40 and
60
kPa. Grapevines can obtain this water without any stress. The lower limit of
DAW
is set in the range
60 to
400 kPa, which defines the
stress limit
for the vines;
the vines can extract water held at lower potentials, but the rate of supply is too
slow for adequate physiological functioning.
The stress limit varies with the age of vines, soil type, root distribution, and
weather conditions. For example, for young vines (
3 years) growing in a sandy
loam soil under hot summer conditions, the stress limit should be set at
60 kPa;
for older vines under the same conditions, the stress limit can be set at
100 kPa.
On the other hand, for old, deep-rooted vines growing in a clay loam soil, or a
soil at least 1 m deep over permeable limestone under cooler conditions, the stress
m
