Agriculture Reference
In-Depth Information
The reduction in berry size is often beneficial for wine styles requiring a high skin/
pulp ratio as is the case for some red wines (and to a much lesser extent for some
white wines) when high colour and tannin levels are preferred. For most white culti-
vars as well as table grape cultivars, a reduction in berry size is however unwanted.
Hardie and Martin (
1989
) recommended soil based water potentials for the imple-
mentation of RDI (Table
7.2
).
Goodwin & Jerie (
1992
) added the following recommendations to accommodate
the responses of different soils to the wetting/drying cycle in cool and warm cli-
mates for the veraison to harvest growth stage:
• In coarse textured soils, water should be withheld in the mid-zone of the grape-
vine root system until the soil water potential (measured using Gypsum blocks)
reaches − 100 kPa.
• Fine textured soils should permit drying of the mid-zone of the grapevine root
system until soil water potential reaches − 200 kPa.
• In either case, water is added to replace 25 % of the computed pan evaporation
for the period of deficit irrigation. It was recommended that care should be taken
to rewet a similar volume of soil each time irrigation is carried out to ensure that
the wetting/drying cycle is applied to the same population of roots. Vineyard
water savings were approximately 30 %.
Partial Rootzone Drying
Partial Rootzone Drying (PRD) is a recent advance in irrigation management based
upon the physiological response of grapevines to drought (Dry et al.
2000a
). In PRD
the root system is water fed using two independent delivery networks, virtually
splitting the root system in two zones. Each zone is watered alternatively thus creat-
ing wet/dry cycles, thought to induce drought like response from the plant. Several
authors reported a lower juice pH, an increase in berry anthocyanins, an increase
in berry phenolics, a reduction in shoot growth, increased water use efficiency,
equivalent yields to fully irrigated or RDI irrigation strategies (Dry et al.
2000a
,
b
;
Kriedemann and Goodwin
2004
; Loveys
2000
; dos Santos et al.
2003
), even in very
high yield situations (37 t ha
− 1
, Riesling) (Kriedemann and Goodwin
2004
).
PRD vines are able to maintain a pre dawn leaf water potential similar to that of
fully hydrated vines combined with stomatal behaviour similar to that of droughted
vines. Under PRD, shoot growth and water loss are restrained by hormonal signals
from the droughted roots while photoassimilation capacity remains relatively un-
impaired. This physiological response was found to be temporary and cultivar de-
pendent. After partial droughting of the roots for between 5 and 14 days, stomatal
control returns to normal, i.e. stomatal conductance of water increases (Kriedemann
and Goodwin
2004
). The determination of the most appropriate wet/dry cycle length,
perhaps as a function of evaporative demand, the ability to induce sufficient dryness,
and the high cost of establishment therefore seem critical issues in regard to broader
uptake of PRD by the grape and wine industry, both in Australia and abroad.
