Agriculture Reference
In-Depth Information
isallowedtodryout.Ahormone(abscisicacid)inducedinrootsinthedrysoil
“tricks” the vine into responding, as if it were suffering water stress, by partial
closure of its leaf stomata, which in turn decreases transpiration and slows shoot
growth.Itisrecommendedthatthewetanddryzonesbealternatedtoavoidthe
possibility of the vine adjusting physiologically by limiting root growth on the dry
side and expanding it on the wet side.
Alternation is achieved by having an emitter with its own water supply on
each side of individual vines in a row. Modern PRD systems consist of two lines
molded together, yet working independently, to allow an alternating cycle of
watering. Free-draining soils give the best results because the nonirrigated soil
driesquicklyandthewetanddryzonesareeasytokeepseparate.Inhot,dry
weather, the cycle may be as short as three to five days, whereas in mild weather
itcouldbeaslongas14days.Soilwaterorsuctionsensorsshouldbeinstalledto
monitor soil moisture.
In Australia, PRD potentially works best in hot inland regions where little
rain falls during the growing season and vines have to rely on irrigation for most
of their water. Although yields may be slightly decreased, berry size is usually
decreased too and, for red varieties, the concentration of anthocyanins and phe-
nolics in the juice increased. However, the main benefit of PRD is a saving of
waterupto50%offullirrigation,whichieldexperimentsinAustraliaindicateis
comparabletotheefectofSDI(Chalmersetal.,2005).
Interactions among Cover Crops, Mulches, and Soil Water
Cover Crops and Soil Water
Cover crops growing in the mid-rows have a significant effect on the soil water
supply to vines. A winter cover crop takes up soil water and, in drying the soil,
improvesitsstrengthandtraicability(see“SoilStrength,”thischapter).Italso
protectsthesoilfromerosionbywinterrains.Figure4.19showsacovercropof
wintercerealintheWesternCapeProvinceregion,SouthAfrica.
In drying the soil, a cover crop helps to control excess vigor in vines early
in the season, which is important on high-potential sites. However, after an
extensivereviewofcovercroppinginAustralianvineyards,Proittetal.(2013)
concluded that for a cover crop to be effective in vigor control, vine roots
neededtobeexploringthemid-rowsoil.Wherevinerootsareconinedmainly
to the soil in-row because of drip irrigation in dry regions or wheel track com-
paction, a cover crop has little effect on vine vigor. It also follows that a grower
can modify competition between the vines and a cover crop by managing the
width of the mid-row sward. A complete sward of a vigorous grass such as
