Agriculture Reference
In-Depth Information
Water stress at lowering can reduce fruit set. From fruit set to veraison,
moderatetoseverestressreducesberrysizethroughitsefectoncelldivisionand
enlargement. During ripening, mild water stress enhances the accumulation of
soluble solids in the berries by suppressing vegetative growth, but more severe
stressdecreasesberrysizethroughitsefectoncellexpansion(commonlyseen
as“berryshrivel”).Inthiscase,sugaraccumulationandlavordevelopmentare
delayed as a result of decreased photosynthesis and premature leaf fall.
In cool, humid climates such as the Burgundy region, France, and humid,
maritimeclimatessuchastheMédocregion,France,dry-grownvinesdonot
usually experience water stress from flowering to veraison because the supply
ofsoilwaterisadequate.Duringtheinalripeningperiod,however,agradual
increase in stress enhances the intensity of flavors in the berries so that out-
standing vintages are produced in years of dry and hot mid- to late summers.
This natural response to a changing water supply seldom occurs in hot inland
regions where irrigation is essential to grow grapes. Instead, winegrowers rely on
manipulating the amount and timing of irrigation to control vigor and enhance
grapequality,asdiscussedlaterin“ControllingtheSoilWaterDeicitthrough
Irrigation.”
HowaSoilWaterDeicitDevelops
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Evaporation and Transpiration
In winter-rainfall Mediterranean and cool, humid climates, the soil profile under
dormant vines is normally at
FC
by the end of winter. During spring, evaporation
rates increase as a result of more sunshine and rising temperatures. Evaporation is
theprocessofliquidwaterconversiontowatervapor:itisdrivenprimarilybythe
absorption of radiant energy from the sun.
A fraction of the solar radiation received is directly reflected, depending on
surfacerelectanceoralbedo(literally,its“whiteness”).Someoftheshort-wave
radiation absorbed by the soil and vegetation is reradiated to the atmosphere
aslong-waveradiation.hegreatestpartoftheremaining“netradiation”(i.e.,
incoming minus relected radiation) is dissipated through the evaporation of
water(fromsoilandvines),andtherestispartitionedbetweenheattransmittedto
the air above ground and heat penetrating deeper into the soil. Heat transferred to
the air aids grape ripening whereas heat transferred into the ground raises the soil
temperature. Figure 4.16 shows a summary of these energy transfers.
Waterevaporatesfrommoistbaresoilatamaximumratedeterminedbythe
evaporative demand of the atmosphere until the surface 1 to 2 cm begins to dry.
Similarly,afterbudburstoccursandleavesexpand,thevinetranspireswaterat
a rate determined by the air's evaporative demand. The combined effect of these
“atmosphere-driven” processes, not limited by soil water supply, produces potential
