Agriculture Reference
In-Depth Information
rainfall, normal scheduling may aim to provide
% of daily water needs leav-
ing roots outside the irrigated zone to supply the rest (Anon.,
). More may
be applied on sandy soils and in drought years.
Trickle irrigation has a very high water use efficiency, typically
%,
because of the enclosed nature of the water delivery system and the restriction
of the water supply to the soil zone with the highest density of fruit tree roots.
-
Deficit irrigation
Deficit irrigation (Behboudian and Mills,
)isasystem of managing soil
water supply to impose periods of plant water deficit in such a way as to be
economically beneficial. It involves applying less than the calculated water
need. Initially it was used in Australia to control tree vigour in high-density
plantings by imposing a water deficit at a period of rapid shoot and slow
fruit growth (Chalmers
et al.
,
). It has subsequently evolved into a
technology for reducing water use while obtaining some benefits from control
of canopy growth, e.g. less shade on fruits within the canopy, possibly less
water demand because of the check to leaf growth, and reduced pruning
costs.
Two types of deficit irrigation are under test. The term regulated deficit
irrigation (RDI) is commonly used to describe deficit irrigation early in the
season when shoot growth is rapid but before rapid fruit growth. Late-season
RDI has also been used sometimes in an attempt to improve fruit quality.
The key to its successful use is a difference in the times during the growing
season when fruit development and shoot growth are sensitive to water stress.
Fruit set and flower bud differentiation are very sensitive to water stress
(Powell,
,
) and apple fruit growth is most sensitive to water stress late
in the season. Regulated deficit irrigation is therefore typically applied to
apple over the period from
,
days after full bloom. Even this may have
an adverse effect on fruit growth during the deficit period and, conversely,
late deficit irrigation may not have the expected adverse effects (Mills
et al.
,
to
).
On Asian pears (
Pyrus serotina
) deficit irrigation can reduce consumption of
water without affecting either yield or fruit size, although when the RDI treat-
ment was imposed from
days after full bloom, fruit bud formation (return
bloom) was reduced (Caspari
et al.
,
). With 'Bartlett' pear (
P. communis
),
RDI not only checked shoot growth but after normal irrigation was resumed
fruit growth and size and consequently yield, were increased (Chalmers
et al.
,
) osmotic adjustment of the fruits while under
deficit irrigation and accelerated growth when this was replaced by normal
irrigation, and (
). This was attributed to (
) less negative leaf water potentials in the RDI trees once this
was replaced by normal irrigation than in trees irrigated normally throughout.
