Comparison of irrigation scheduling strategies for achieving water use efficiency in highbush blueberry


Keen, B; Slavich, P

New Zealand Journal of Crop and Horticultural Science 40(1): 3-20

2012


New
Zealand
Journal
of
Crop
and
Horticultural
Science
Vol.
40,
No.
1,
March
2012,
3-20
CI)
Taylor
&Francis
Taylor
&Francis
Group
Comparison
of
irrigation
scheduling
strategies
for
achieving
water
use
efficiency
in
highbush
blueberry
B
Keen*
and
P
Slavich
Wollongbar
Primary
Industries
Institute,
NSW
Department
of
Primary
Industries,
Wollongbar,
NSW,
Australia
(Received
29
March
2011;
final
version
received
16
June
2011)
A
glasshouse
experiment
was
carried
out
to
assess
the
feasibility
of
applying
partial
rootzone
drying
(PRD)
to
highbush
blueberries
(Vaccinium
corymbosum).
A
subsequent
field
experiment
was
established
to
assess
four
irrigation
strategies
with
the
aim
of
improving
water
use
efficiency
in
blueberry
production.
Applying
PRD
to
plants
during
a
glasshouse
experiment
reduced
stomatal
conductance
without
reducing
plant
water
potential.
Hindered
by
high
rainfall,
a
physiological
response
to
PRD
was
not
repeated
in
field
grown
plants.
However,
irrigation
scheduled
using
a
IC,
(crop
coefficient)
curve
constructed
from
Food
and
Agriculture
Organization
(FAO)
56
guidelines
and
post-harvest
regulated
deficit
irrigation
(RDI)
delivered
annual
water
savings
of
0.8
ML
ha
—1
and
1.3
ML
ha
—1
,
respectively,
compared
with
a
total
3.6
ML
ha
—1
applied
using
a
'rule-of-thumb'
approach
commonly
adopted
by
Australian
blueberry
growers.
These
savings
were
achieved
without
reducing
berry
yield
or
quality.
This
study
is
the
first
to
report
on
the
feasibility
of
applying
FAO
56
guidelines,
RDI
and
PRD
as
strategies
to
maximize
water
use
efficiency
in
highbush
blueberry
production.
Keywords:
Vaccinium
corymbosum;
blueberry;
irrigation;
regulated
deficit
irrigation;
partial
rootzone
drying
Introduction
The
majority
of
Australia's
blueberries
are
produced
within
the
higher
rainfall
districts
of
subtropical
north-eastern
New
South
Wales
(NSW)
with
the
remainder
produced
in
temperate
southern
Australia.
Rainfall
patterns
across
subtropical
and
temperate
Australia
can
vary
considerably
seasonally
and
from
year
to
year.
In
subtropical
and
temperate
eastern
Australia,
the
greatest
risk
of
water
deficits
occurs
through
the
spring
and
summer
months
(September
to
February)
with
the
highest
risk
of
extended
droughts
occurring
under
the
influence
of
El
Nitio
events.
Over
the
coming
decades,
the
impacts
of
climate
change
are
forecast
to
exacerbate
climate
variability
and
water
security
issues
across
eastern
and
southern
Australia
(Australian
Department
of
Climate
Change
2009).
Consequently,
as
the
blueberry
industry
continues
to
operate
and
expand
in
Australia,
the
need
to
better
define
and
implement
'best
practice'
irrigation
will
be
coupled
with
a
need
for
strategies
that
enable
growers
to
adapt
to
potentially
increased
frequency
and
duration
of
water
shortages.
Current
irrigation
practices
applied
to
south-
ern
highbush
blueberry
(
Vaccinium
corymbosum
L.)
cultivars
grown
in
Australia
are
almost
entirely
based
on
local
grower
experience
with
some
influence
from
a
limited
composite
of
studies
undertaken
in
the
US
(Wilk
et
al.
2009).
Baseline
estimates
for
southern
highbush
blueberry
water
requirements
are
25
mm
during
*Corresponding
author.
Email:
ISSN
0114-0671
print/ISSN
1175-8783
online
©
2012
The
Royal
Society
of
New
Zealand
http://dx.doi.org/10.1080/01140671.2011.599398
http://www.tandfonline.com
4
B
Keen
and
P
Slavich
low-demand
periods
to
38
mm
during
high-
demand
periods
applied
weekly
to
the
cropped
area
(Bell
1982).
From
these
estimates
many
growers
have
adopted
a
standard
c.
4
L
water
per
mature
plant
per
day
'rule-of-thumb'
approach
to
irrigation.
However,
anecdotal
observations
by
local
professionals
working
with
the
blueberry
industry
indicate
that
inputs
vary
among
growers,
with
a
tendency
towards
over
irrigation.
The
Penman-Monteith
reference
evapo-
transpiration
(ET.)
equation
(Monteith
and
Unsworth
1990;
Allen
et
al.
1998)
provides
an
effective
tool
for
managing
irrigation
inputs.
To
estimate
plant
water
requirements,
ET
o
is
calculated
using
climatic
or
pan
evaporation
data
and
crop
evapotranspiration
(ET,)
is
esti-
mated
by
multiplying
ET
o
by
an
appropriate
crop
coefficient
(K
r
).
Greater
adoption
of
this
approach
by
Australian
blueberry
growers
has
the
potential
to
significantly
improve
water
use
efficiency
across
the
industry.
However,
one
of
the
barriers
to
adoption
is
an
uncertainty
among
growers
of
which
K,
values
to
use.
This
uncertainty
arises
as
there
have
been
relatively
few
IC,
curves
published
for
southern
high-
bush
blueberries
and
these
tend
to
vary
for
specific
cultivars
grown
under
different
cultural
conditions
(Byers
&
Moore
1987;
Storlie
&
Eck
1996;
Haman
et
al.
1997a,
b;
Yang
et
al.
2005).
In
addition,
no
research
has
been
undertaken
to
determine
IC,
curves
for
southern
highbush
blueberry
cultivars grown
in
Australia.
A
generic
mid-season
IC,
value
for
berry
crops,
presented
in
the
Food
and
Agriculture
Organization's
Irrigation
and
Drainage
Paper
56
(FAO
56)
(Allen
et
al.
1998),
potentially
offers
a
satis-
factory
starting
point
for
Australian
blueberry
growers,
but
its
suitability
requires
evaluation.
To
date,
blueberry
irrigation
research
has
mostly
focused
on
utilizing
irrigation
to
optimize
yield
performance.
However,
under
conditions
of
limited
water
availability,
such
optimums
may
not
always
be
achievable
and
maximizing
water
use
efficiency
to
maintain
production
may,
at
times,
become
the
overriding
priority.
Deficit
irrigation
strategies
have
been
successfully
applied
in
a
range
of
horticultural
crops
with
no
to
little
negative
impact
(Fereres
&
Soriano
2007).
For
some
crops,
such
as
grapes,
increased
yields
have
been
achieved
(de
la
Hera
et
al.
2007).
There
are
two
main
approaches
to
implementing
deficit
irrigation:
first,
regulated
deficit
irrigation
(RDI)
(FAO
2002;
Kriedmann
&
Goodwin
2005);
and
second,
partial
root-
zone
drying
(PRD)
(FAO
2002;
Kriedmann
&
Goodwin
2005).
Deficit
irrigation
occurs
when
the
water
applied
to
a
crop
is
below
ET,
requirements.
RDI
is
implemented
by
imposing
prescribed
limits
on
soil
moisture
depletion
and
limits
to
irrigation
inputs
at
specific
phenological
stages
of
the
crop
cycle.
PRD
involves
irrigation
inputs
that
are
below
ET,
requirements
but
irrigation
is
alternated
between
plant
roots
separated
into
drying
and
wetted
zones.
When
successfully
applied,
plants
respond
to
PRD
by
reducing
transpiration
through
partially
closing
their
stomata
without
loss
of
plant
turgor
(Kriedmann
&
Goodwin
2005).
Although
there
are
a
handful
of
studies
in
which
negative
results
have
been
reported
after
applying
PRD
(Gu
et
al.
2004;
O'Connell
&
Goodwin
2004,
2007;
Neuhaus
et
al.
2007),
there
are
a
greater
number
of
studies
that
demonstrate
successful
application
of
the
technique
to
grapes
(Dry
et
al.
1996;
Loveys
et
al.
1998;
Stoll
et
al.
2000;
Chalmers
et
al.
2004)
and
many
other
horticultural
crops
including
apple
(Leib
et
al.
2006),
pear
(Kang
et
al.
2002),
mango
(Spreer
et
al.
2009),
olive
(Fernandez
et
al.
2006),
tomato
(Tahi
et
al.
2007)
and
raspberry
(Grant
et
al.
2004).
Despite
successes
in
other
crops,
there
are
no
reports
of
PRD
having
been
trialled
with
high-
bush
blueberries.
We
report
on
the
outcomes
of
a
pilot
PRD
pot
trial
carried
out
with
the
specific
aim
of
observing
whether
the
desired
physio-
logical
response
of
PRD
could
be
initiated
in
southern
highbush
blueberry
plants.
We
also
report
on
the
outcomes
of
a
field
experiment
in
which
a
participating
blueberry
grower's
'rule-
of-thumb'
irrigation
management
strategy
was
compared
with
irrigation
scheduled
using
ET,
estimated
by
applying
a
IC,
curve
constructed
using
FAO
56
guidelines
(Allen
et
al.
1998).
Both
Irrigation
strategies
for
blueberries
5
RDI
and
PRD
were
also
evaluated
in
the
field
trial.
Materials
and
methods
PRD
pilot
pot
trial
A
PRD
pot
trial
was
conducted
at
the
Centre
for
Tropical
Horticulture,
Alstonville,
NSW,
Australia.
Prior
to
establishing
the
experiment,
12-month-old
highbush
blueberry
cultivar
`Star'
plants
were
transferred
to
and
grown
in
20
cm
(6
L)
pots.
Each
pot
was
divided
into
two
chambers
by
an
impermeable
barrier
and
dur-
able
plastic
bags
were
inserted
into
each
chamber
(plastic
bags
were
punctured
to
permit
drainage).
The
root
ball
of
each
plant
was
washed
to
remove
adhering
potting
mix
and
split
without
damaging
the
crown.
The
two
sides
of
the
separated
root
ball
were
positioned
on
either
side
of
the
divider.
Plants
were
grown
in
a
glasshouse
for
a
further
13
months
(December
2007
to
February
2008)
in
washed
river
sand
mixed
with
10
g
complete
slow
release
fertilizer
pellets.
During
establishment,
5
g
slow
release
fertilizer
pellets
were
applied
at
8-week
intervals.
Irrigation
was
controlled
by
automatic
timers
set
to
run
for
5
min
at
midday
and
early
evening
each
day
(early
evening
only
during
winter
months)
with
water
delivered
via
two
1
L
h
—I
drip
emitters
with
one
emitter
installed
on
each
side
of
the
split
pot.
Four
treatments
were
applied
throughout
the
experiment:
1.
full
irrigation
(FI
=
plants
watered
on
both
sides
of
the
pot)
2.
alternating
PRD
(PRD
=
irrigation
and
drying
alternated
between
each
side
of
the
pot)
3.
fixed
PRD
(FPRD
=
irrigation
and
drying
each
fixed
to
one
side
of
the
pot)
4.
stressed
(plants
watered
on
both
sides
of
the
pot
but
exposed
to
repeated
stress
and
recovery
cycles
imposed
by
withholding
water
until
plants
began
to
wilt
at
which
point
irrigation
was
resumed).
Treatments
were
replicated
four
times
in
a
randomized
complete
block
and
run
over
a
5-week
period.
During
this
period,
the
moisture
content
of
the
potted
sand
was
measured
daily
using
a
single
portable
time-domain
reflectometer
(TDR)
sensor
with
12
cm
probes
(Campbell
Scientific,
Australia).
Plants
were
irrigated
as
described
above
but
with
emitters
installed
on
the
irrigated
sides
of
pots
only.
Mid-
morning
stomatal
conductance
and
midday
leaf
water
potential
were
measured
for
all
plants
at
the
start
and
completion
of
each
stress
and
recovery
cycle
(as
applied
to
the
stressed
treatment
plants).
Field
irrigation
experiment
Trial
site
A
field
experiment
was
established
in
a
leading
commercial
blueberry
orchard
near
the
town
of
Wollongbar,
north-eastern
NSW,
Australia
(28.80°S,
153.38°E,
142
m
ASL).
The
region
is
influenced
by
a
subtropical
climate
with
the
nearby
Alstonville
weather
station
(11
km
from
study
location)
recording
mean
annual
rainfall
of
1817
mm
(median
1683
mm)
and
mean
annual
pan
evaporation
of
1570
mm
(1963-2010)
(Australian
Bureau
of
Meteoro-
logy
2010).
The
lowest
rainfall
typically
occurs
from
July
to
January
and
the
wettest
months
occur
from
February
to
April
(Fig.
1).
The
formation
of
rainfall
deficits
is
more
common
through
winter,
spring
and
early
summer.
Rainfall
deficits
tend
to
be
more
severe
during
El
Nifio
years
of
which
11
events
have
occurred
since
1963
(Australian
Bureau
of
Meteorology
2010).
The
trial
site
is
characterized
by
basaltic
clay-loam
red
Ferrosol
soil
(Isbelle
2002)
of
the
Wollongbar
series
(Morand
1994).
Soil
bulk
density
measured
from
36
soil
cores
taken
from
within
soil
mounds
averaged
1.0
g
cm
—3
(between
10
cm
and
40
cm
depth)
and
1.1
g
cm
-3
under
soil
mounds
(between
40
cm
and
100
cm
depth).
Applying
the
soil
moisture
characteristic
model
of
Williams
et
al.
(1983);
6
B
Keen
and
P
Slavich
decile
1
rainfall
median
rainfall
decile
9
rainfall
evaporation
1
0
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Figure
1
Rainfall
and
evaporation
statistics
recorded
at
Alstonville
Tropical
Fruit
Research
Station
(28.85°S,
153.46°E,
140
m
ASL)
for
the
period
between
1963
and
2010.
Source:
Australian
Bureau
of
Meteorology
(2010).
500
450
-
400
-
350
-
E
300
-
E
250
-
200
-
150
-
100
-
50
-
soil
group
7),
the
theoretical
drained
upper
limit
was
estimated
at
0.4
m
3
m
—3
,
plant
available
water
content
(AWC),
i.e.
water
available
between
0.1
MPa
and
15
MPa,
was
estimated
at
0.12
m
3
m
-3
and
readily
available
water
content
(RAW),
i.e.
50%
of
AWC,
was
estimated
at
0.06
m
3
m
—3
.
During
this
study,
core
samples
collected
48
h
after
saturating
and
draining
the
soil
averaged
0.36
m
3
m
—3
.
Field
capacity
was
therefore
adjusted
to
0.36
m
3
m
—3
and
the
theoretical
AWC
and
RAW
values
were
subtracted
to
give
an
estimated
permanent
wilting
point
(PWP)
of
0.24
m
3
m
-3
and
refill
point
of
0.30
m
3
m
—3
.
These
estimates
were
within
the
range
of
previous
measurements
recorded
for
Ferrosol
soils
at
nearby
locations
(Marsh
&
Rixon
1991;
Batten
et
al.
1994;
Bell
et
al.
2005).
Experimental
treatments
Five-year-old
highbush
blueberry
'Star'
cultivar
plants
were
used
for
the
study.
Plants
were
established
under
the
farmer's
standard
pruning,
irrigation
and
fertilizer
management
practices
and
planted
at
3
m
row
x
0.8
m
in-row
spacings
in
mounded
soil
rows
with
dimensions
of
1
m
base
width
x
0.4
m
high.
When
the
soil
mounds
were
formed
they
were
covered
with
permanent
woven
plastic
weedmat
with
the
weedmat
folded
under
the
base-edges
of
the
soil
mound
thus
semi-isolating
the
mound
from
the
adjacent
grassed
inter-row.
During
the
field
experiment,
four
irrigation
treatments
were
applied:
1.
farmer
practice
2.
ET,
(100%
replacement
of
crop
water
use)
3.
RDI
(50%
ET,
applied
to
both
sides
of
plants)
4.
PRD
(50%
ET,
applied
to
alternating
sides
of
plants).
Treatments
were
replicated
four
times
in
a
randomized
complete
block
with
each
block
formed
within
a
single
row
and
five
buffer
rows
(buffer
rows
contained
different
cultivars)
separating
each
block.
Each
plot
consisted
of
10
plants
with
two
or
more
buffer
plants
on
the
periphery
of
each
plot.
The
experiment
was
run
from
November
2008
to
end
of
harvest,
November
2009.
RDI
and
PRD
treatments
were
applied
over
late
spring-summer
between
early-November
2008
and
end-February
2009.
Irrigation
strategies
for
blueberries
7
From
March
2009
onwards,
RDI
and
PRD
treatment
plots
reverted
to
irrigation
inputs
based
on
100%
ET
c
.
Irrigation
water
was
delivered
to
plants
via
2
L
hr
I
drip
emitters
spaced
at
40
cm
intervals
(standard
farmer
practice).
During
preliminary
observational
studies,
soil
moisture
measure-
ments
indicated
that
this
emitter
spacing
main-
tained
relatively
even
wetting
of
soil
mounds.
A
number
of
excavations
were
also
made
to
visually
assess
the
effective
rooting
depth
of
plants.
The
highest
root
density
was
observed
within
the
top
0.4
m
of
the
soil
with
fewer
roots
observed
to
a
maximum
0.6
m
depth.
Plant
roots
were
also
found
to
occupy
space
to
the
edges
of
the
soil
mound.
Based
on
the
area
of
soil
occupied
by
each
plant,
the
total
irrigated
cropped
surface
area
was
estimated
at
0.8
m
2
plant
I
(c.
3000
m
2
cropped
area
ha
I
).
The
farmer's
existing
irrigation
regime
was
applied
to
the
farmer
practice
treatment
and
involved
a
target
of
applying
4
L
water
per
plant
at
each
irrigation
event
with
5
L
water
per
plant
applied
once
each
week
as
fertigation.
From
post-harvest
(November),
through
the
summer
to
early-autumn
(March)
vegetative
growth
stage,
irrigation
was
scheduled
four
times
weekly
(equivalent
to
c.
25
mm
per
week).
During
the
cooler
mid-autumn
and
winter
months,
irriga-
tion
was
applied
twice
weekly
(equivalent
to
c.
15
mm
per
week).
From
flowering
in
mid-August
through
to
end
of
harvest,
target
irrigation
volumes
were
increased
to
5
L
per
plant
at
each
irrigation
event
and
irrigation
was
scheduled
four
times
each
week
(equivalent
to
c.
30
mm
per
week).
During
the
final
6
weeks
of
the
crop
cycle
(fruit
expansion
and
harvesting),
irrigation
frequency
was
increased
to
five
times
each
week
(equivalent
to
c.
40
mm
per
week).
This
regime
was
not
rigorously
applied,
however.
The
irrigation
manager
continued
to
apply
their
experientially
derived
perceptions
to
guide
irrigation
inputs
on
any
single
day
as
influenced
by
rainfall
and
physical
examination
of
soil
moisture.
During
the
trial,
the
on-farm
irrigation
manager
recorded
the
date,
time
and
duration
of
each
irrigation
event.
These
data
were
used
to
calculate
irrigation
inputs.
Irrigation
in
the
ET
c
,
RDI
and
PRD
plots
was
initiated
at
the
same
time
as
each
irrigation
event
in
the
farmer
practice
plots
and
input
volumes
were
controlled
by
varying
irrigation
runtimes
using
automatic
timers
activated
by
a
pressure
switch.
The
timers
were
adjusted
weekly
to
accommodate
the
expected
number
of
irrigation
events
and
estimated
plant
water
requirements
for
the
proceeding
week.
An
automated
weather
station
(AWS)
(Environ-
mental
Information
Systems,
Australia)
in-
stalled
central
to
the
experimental
block
was
used
to
monitor
climatic
variables,
rainfall
and
calculate
daily
ET
c
,
(AWS
uses
the
standard
FAO
56
Penman-Monteith
equation
to
calculate
ET.).
Plant
water
requirements
were
estimated
by
applying
a
K
c
curve
constructed
using
FAO
56
guidelines
(Allen
et
al.
1998).
The
crop
cycle
was
defined
in
three
main
stages
(Fig.
2):
IC,
ph
=
post-harvest
through
the
vegetative
growth
stage
(21
weeks;
November
to
March);
=
semi-dormancy
stage
with
partial
leaf
fall
(16
weeks;
May
to
August);
K
c
h
=
peak
water
demand
period
from
start
of
ripening
through
to
end
of
harvest
(5
weeks;
October
to
November).
A
generic
berry
mid-season
IC,
value
(IC,
mid
)
of
1.05
was
used
as
the
initial
reference
for
each
stage
with
adjustments
made
to
account
for
crop
and
management
factors.
Equation
1
was
used
to
construct
the
adjusted
K
c
adj
)
curve.
K
c
adj
= K
c
mid
Mc
Pc
Acin
(1)
where:
mulch
adjustment
coefficient,
M
c
=
K
c
mid
x
0.2
post-harvest
adjustment
coefficient,
P
c
=
(K
c
mid
M
c
)
x
0.3
0.
ground
cover
coefficient,
A
im
=
1
c
1
5
.
[f
c
dense=0.9
9
where
fc
=
ground
cover
fraction
The
plastic
weedmat
was
assumed
to
reduce
IC,
by
20%
(Allen
et
al.
1998)
and
was
applied
throughout
each
stage
of
the
crop
cycle.
Observations
have
been
made
of
plant
water
8
B
Keen
and
P
Slavich
K
ch
=
0.8
K
c
=
0.6
21
wk
\
0.5
=
0.4
16
wk
November
April
August
October
Figure
2
curve
applied
during
the
field
experiment.
0.6
0.7
5
wk
0.5
requirements
in
highbush
blueberries
rapidly
declining
by
c.
30%
post-harvest
(Bryla
&
Strik
2007)
and
local
growers
also
reduce
their
post-harvest
irrigation
inputs
by
around
30%.
Consequently,
during
the
post-harvest
vegeta-
tive
growth
stage
(lc
ph)
a
30%
post-harvest
adjustment
(P
a
)
was
applied
from
November
to
March.
Under
local
conditions,
during
the
cooler
months,
'Star'
cultivar
is
semi-dormant
with
partial
leaf
fall
occurring.
To
compensate
for
this,
adjustments
for
changes
in
the
ground
cover
fraction
(f
c
)
were
made
during
the
16-
week
semi-dormant
period
(IC,
,),
when
the
ground
cover
fraction
declined
from
an
esti-
mated
90%
to
30%
due
to
leaf
senescence
and
partial
leaf
fall.
Adjustments
were
also
made
for
rainfall
infiltrating
into
the
soil
under
the
plastic
weedmat.
To
simplify
water
budget
calcula-
tions,
20%
infiltration
of
the
gross
incident
rainfall
was
assumed.
This
assumption
was
based
on
course
estimates
for
rainfall entering
the
soil
via
stem
flow
(the
portion
of
incident
rainfall
that
infiltrates
the
soil
via
the
trunk)
(Levia
&
Frost
2003),
holes
cut
into
the
plastic
weedmat
to
accommodate
plants
and
seepage
through
pores
occurring
at
strand
intersections
within
the
woven
plastic
weedmat.
Soil
moisture
measurements
A
Micro-gopher©
mobile
capacitance
probe
(Odyssey,
New
Zealand)
was
used
to
manually
record
soil
moisture.
Prior
to
using
the
Micro-
gopher
©
,
to
record
soil
moisture
values
during
the
field
trial,
an
insitu
calibration
was
carried
out.
This
was
undertaken
as
recommended
by
Geesing
et
al.
(2004)
with
comparisons
made
between
soil
moisture
readings
recorded
with
the
Micro-gopher©
and
36
soil
core
samples
taken
adjacent
to
probe
reading
positions
at
depths
between
10
cm
and
100
cm
across
the
study
area.
A
regression
analysis
of
Micro-
gopher
©
generated
soil
moisture
values
against
the
same
obtained
from
soil
cores
returned
an
r
2
=0.93
(P
<
0.01).
The
margin
of
error
across
36
observations
averaged
±0.1
mm
with
a
maximum
error
of
+2.5
mm.
During
the
field
trial,
moisture
was
measured
on
a
weekly
cycle
with
most
measurements
recorded
on
the
same
day
near
to
the
same
time
once
each
week.
Measurements
were
re-
corded
at
10
cm
intervals
to
a
depth
of
50
cm
in
each
plot
with
both
left
and
right
sides
measured
in
PRD
plots.
Soil
moisture
data
are
presented
as
means
of
measurements
recorded
across
the
four
replicate
plots
for
each
treatment.
Midday
leaf
water
potential
Midday
leaf
water
potential
was
measured
using
a
plant
water
status
console
(Model
3005,
Soil
Moisture
Equipment
Corp,
US).
Sunlit
mature
leaves
were
selected
for
measure-
ments.
Stem
tips
with
two
to
three
leaves
attached
were
covered
in
a
plastic
bag
(to
slow
transpiration)
and
excised
from
the
plant.
The
excised
stem
with
leaves
attached
was
then
placed
immediately
inside
the
pressure
chamber.
Compressed
N2
gas
was
released
into
the
chamber
until
water
appeared
at
the
cut
stem
surface,
at
which
point
the
inlet
valve
Irrigation
strategies
for
blueberries
9
was
closed
and
the
pressure
inside
the
chamber
recorded.
During
the
pot
trial,
duplicate
values
were
recorded
for
each
plant
with
stems
excised
from
opposing
canes.
For
the
field
experiment,
single
values
were
recorded
for
each
plot.
Stomatal
conductance
Stomatal
conductance
was
measured
using
a
leaf
porometer
(Model
SC-1,
Decagon,
US).
Fully
opened
sunlit
leaves
were
selected
and
measurements
were
carried
out
on
abaxial
sur-
face
of
leaves
between
9:00
am
and
10:30
am.
Intervals
between
measurement
events
were
dictated
by
the
absence
of
rainfall
and
cloud
cover.
Berry
yield
Blueberries
were
manually
harvested
by
profes-
sional
fruit
pickers.
Three
repeat
harvests
of
ripe
berries
were
carried
out
over
a
4-week
period
commencing
6
October
2009.
Berries
were
separated
into
premium
and
second
grade
classes
and
the
weights
of
both
recorded
for
each
plant.
The
weights
of
both
were
combined
to
give
a
total
yield
and
the
weight
of
the
second
grade
berries
were
expressed
as
a
percentage
of
the
total
yield.
The
individual
weights
of
a
subsample
of
50
berries
from
each
plot
at
peak
harvest
(second
harvest)
were
also
recorded
and
averaged
for
each
plot.
Statistical
analysis
Where
appropriate,
data
were
fitted
to
a
general
linear
model
and
means
were
separated
by
Tukey's
honestly
significant
difference
(HSD)
(P
=
0.05).
All
analyses
were
carried
out
in
the
R
environment
(R
Development
Core
Team
2009).
withheld
from
pots
(Fig.
3A—D).
This
facili-
tated
frequent
drying
cycles
on
alternating
sides
of
PRD
plants
and
permitted
stress
and
recov-
ery
cycles
to
be
completed
on
a
weekly
basis
in
stressed
treatment
plants.
Depletion
of
water
in
non-irrigated
soil
was
negligible
once
sand
dried
to
5%
(Fig.
3C).
At
the
commencement
of
the
experiment,
stomatal
conductance
was
similar
in
all
plants
(Fig.
4).
Within
4
days,
stomatal
conductance
fell
to
an
average
18.4
mmol
m
—2
s
—1
in
stressed
treatment
plants.
This
pattern
was
repeated
at
the
completion
of
each
stress
cycle.
At
264
to
300
mmol
m
-2
s
—1
,
stomatal
con-
ductance
in
the
PRD
and
FPRD
plants
(respec-
tively)
was
well
below
the
fully
irrigated
plants
which
scored
a
mean
value
of
370
mmol
m
—2
—1
s
.
During
the
second
drying
cycle,
the
gap
in
stomatal
conductance,
measured
between
fully
irrigated
plants
and
plants
exposed
to
PRD
and
FPRD,
widened
significantly
(P
<0.05)
to
around
half
that
observed
for
fully
irrigated
plants.
Stomatal
conductance
partially
recov-
ered
in
the
stressed
plants
within
3
days
of
resuming
irrigation
with
stomatal
conductance
rising
to
match
values
observed
for
PRD
and
FPRD
plants.
Similar
patterns
were
observed
during
subsequent
drying
cycles
with
the
only
deviation
being
for
FPRD
plants
whereby
at
the
completion
of
the
third
cycle,
stomatal
conduc-
tance
for
FPRD
plants
was
higher
than
for
PRD
plants
but
similar
to
fully
irrigated
plants.
This
pattern
was
repeated
at
the
commence-
ment
of
the
fourth
cycle
but
was
significantly
different
(P
<0.05)
between
all
treatments
at
the
completion
of
the
experiment.
A
significant
(P
<0.05)
decline
in
leaf
water
potential
was
observed
in
stressed
treatment
plants
during
the
deficit
stage
of
each
stress
and
recovering
cycle
(Fig.
5)
but
differences
did
not
occur
between
the
other
three
treatments.
Results
PRD
pilot
pot
trial
Drying
of
the
potted
sand
was
easily
achieved
within
a
few
days
when
irrigation water
was
Field
experiment
Over
the
duration
of
the
field
experiment,
the
region
experienced
frequent
and
above
average
rainfall
(annual
total
2235
mm),
with
most
of
v
o
l.
so
il
mo
is
tu
re
(
%)
16
14
12
10
8
6
4
2
16
14
2
12
--
(1)
10
8
0
(7,
4
2
16
10
B
Keen
and
P
Slavich
A
-
alternating
left
alternating
right
.
.
14
-
2
12
-
.D
10
-
0
8
-
=
6
-
(7)
4
-
2
C
-
irrigated
side
dry
side
..........
...........
D
0
5
10
15
20
25
30
Day
Figure
3
Volumetric
soil
moisture
measured
daily
over
the
duration
of
the
blueberry
PRD
pot
experiment.
Data
points
represent
means
(n
=4).
A,
Fully
irrigated.
B,
Partial
root
zone
drying.
C,
Fixed
partial
rootzone
drying.
D,
Plants
exposed
to
repeated
stress
and
recovery
cycles.
16
14
2
12
.cn
10
0
8
6
0
4
2
0.0
0.5
-
1.0
-
1.5
-
-
2.0-
2.5
-
111—
full
irrigation
PRD
-
3.0
-
fixed
PRD
0
stressed
bry bo,btrb
‘F
4'
el
4'
0
C
‹b
a?
,
-§;
0
4,
0
Us
ti,
-§;
0
a
oR
6
4
'
Irrigation
strategies
for
blueberries
11
450
-
full
irrigation
--•—
PRD
400
-
---a
--
fixed
PRD
350
-
stressed
300
-
250
-
200
-
150
-
0
100-
50
-
0
t•••a
ry
b
0
.4
.
ri
O
R
Figure
4
Abaxial
stomatal
conductance
(g0
mea-
sured
in
potted
blueberry
plants
exposed
to
fixed
irrigation
(FI),
partial
root
zone
drying
(PRD)
and
fixed
partial
root
zone
drying
(FPRD)
and
stressed
treatments.
Data
points
represent
means
(n
=4)
and
error
bars
indicate
standard
errors
of
the
mean.
this
rain
falling
between
November
2008
and
July
2009.
While
rainfall
exceeded
ET
°
and
estimated
ET
c
,
after
accounting
for
20%
rainfall
infiltration
into
soil
beneath
the
plastic
Figure
5
Midday
leaf
water
potential
(Tl
ind
)
mea-
sured
in
potted
blueberry
plants
exposed
to
fixed
irrigation
(FI),
partial
root
zone
drying
(PRD)
and
fixed
partial
root
zone
drying
(FPRD)
and
stressed
treatments.
Data
points
represent
means
(n
=4).
Error
bars
are
not
shown
as
the
standard
errors
of
the
mean
were
each
<0.05.
weedmat,
both
cumulative
ET
0
and
ET
c
exceeded
water
entering
the
soil
as
rainfall
(Fig.
6).
There
were
also
several
periods
through
late
spring
to
early
autumn
during
which
ET
°
and
ET
c
exceeded
rainfall
for
that
period.
An
extended
period
with
very
little
rainfall
also
occurred
from
mid-winter
through
to
mid-spring.
Irrigation
inputs
for
the
12-month
season
totalled
3.60
ML
ha
—1
(equivalent
to
1174
mm
applied
to
the
cropped
area)
under
the
farmer's
regime
and
2.80
ML
ha
—1
(913
mm
cropped
area
equivalent)
for
the
ET
c
treatment
and
2.42
ML
ha
—1
(790
mm
cropped
area
equivalent)
for
both
RDI
and
PRD
treatments
(Table
1).
During
the
peak
ET
0
period
(November
to
March),
estimated
plant
water
requirements
totalled
536
mm
of
which
an
estimated
143
mm
was
met
by
rainfall
infiltrating
beneath
the
plastic
weedmat.
During
this
period,
the
farmer's
irrigation
inputs
totalled
405
mm,
inputs
to
ET
c
plots
totalled
355
mm,
and
RDI
and
PRD
plots
totalled
232
mm.
This
contrasts
with
the
lowest
evapotranspiration
period
(April
to
August)
during
which
estimated
plant
water
requirements
totalled
172
mm,
rainfall
infiltration
was
estimated
at
237
mm
and
the
farmer's
inputs
totalled
263
mm
with
ET
c
,
RDI
and
PRD
plots
receiving
180
mm,
mostly
from
mandatory
once
weekly
fertigation
events.
Between
flowering
and
final
harvest
(mid-
August
to
early
November),
the
farmer
applied
506
mm
in
addition
to
estimated
rainfall
infiltration
of
44
mm.
Estimated
plant
water
requirement
for
the
same
period
was
347
mm
with
ET
c
,
RDI
and
PRD
plots
each
receiving
378
mm.
Throughout
the
duration
of
the
experiment,
soil
moisture
in
the
farmer's
plots
remained
relatively
constant
within
a
range
at
or
near
field
capacity
(Fig.
7).
A
similar
pattern
was
observed
for
ET
c
plots.
Soil
moisture
in
both
these
treatment
plots
demonstrated
a
slow
declining
trend
from
November
through
to
late
January.
The
decline
in
soil
moisture
was
much
more
rapid
in
the
RDI
and
PRD
plots
during
this
same
period.
Increased
rainfall
from
7
0
E
E
Sep
Nov
2400
2000
-
E
1600
-
E
=
1200
-
co
`°-
800
-
400
-
rainfall
----
20%
rainfall
infiltration
ET
0
ET
c
as.
Mar
May
Jul
...----
1
--
'
..-''
------- ----
..
.-
0
.
Nov
Jan
12
B
Keen
and
P
Slavich
2400
2000
1600
-.
E
.
'
1200
f...
i—
w
800
400
0
Figure
6
Cumulative
rainfall,
estimated
rainfall
infiltration,
ET.
and
ET
e
over
the
duration
of
the
blueberry
field
trial.
early
February
and
a
resumption
of
irrigation
inputs
based
on
ET,
estimates
from
March
resulted
in
a
steady
increase
in
soil
moisture
through
the
autumn
and
winter
months.
Between
November
and
March,
alternating
wetting
and
drying
patterns
in
the
PRD
plots
were
observed.
However,
frequent
rainfall
interfered
with
the
clarity
of
this
pattern.
At
no
point
during
the
PRD
and
RDI
application
phase
were
significant
differences
in
stomatal
conductance
and
leaf
water
poten-
tial
observed
between
treatments
(data
not
shown).
Mean
berry
yield
ranged
between
1.59
kg
plant
I
in
the
PRD
plots
to
2.02
kg
plant
in
the
RDI
plots
but
differences
between
Table
1
Irrigation
volumes
for
each
treatment
during
the
field
trial.
Stage
of
crop
cycle
Treatment
Total
(mm)
Total
(ML
ha
-1
)
Daily
average
(mm)
Post-harvest
vegetative
growth
November—March
Farmer
405
1.23
2.7
147
days;
lc
0.6;
ET
e
536
mm;
20%
rainfall
infiltration
ET
e
355
1.08
2.4
143
mm
RDI/
232
0.70
1.6
PRD
Bud
develop
semi-dormanncy
April—August
Farmer
263
0.83
1.9
142
days;
April
IC,
0.5,
May
to
August
IC,
0.4;
ET
e
ET
e
180
0.57
1.3
172
mm;
20%
rainfall
infiltration
237
mm
RDI/
180
0.57
1.3
PRD
Flowering
end
harvest
mid-August—November
Farmer
506
1.54
5.4
77
days;
mid-August
lc
0.5
to
lc
0.8
in
final
four
weeks;
ET
e
378
1.15
4.1
ET
e
347
mm;
20%
rainfall
infiltration
44
mm
RDI/
378
1.15
4.1
PRD
Totals
Farmer
1174
3.60
ET
e
913
2.80
RDI/
790
2.42
PRD
C
FC
Refill
0.40
0.36
0.32
0.28
0.24
-
0.20
0.40
D
0.36
0.32
0.28
0.24
-
alternating
left
alternating
right
FC
Refill
so
il
mo
is
ture
(m
3
m
-
3
)
so
il
mo
is
ture
(m
3
m
-
3)
Irrigation
strategies
for
blueberries
13
0.40
0.36
E
c
''
)
E
ar
c
0.32
0.28
N
0.24
-
0.20
A
FC
Refill
0.40
E
0.36
0.32
.L0
0.28
-
0
E
7
0.24
B
FC
Refill
0.20
0.20
-
Nov
Jan
Mar
May
Jul
Sep
Nov
Figure
7
Soil
moisture
measured
at
weekly
intervals
during
the
field
experiment.
A,
farmer
practice.
B,
ET
e
treatment.
C,
RDI
treatment.
D,
PRD
treatment.
Values
represent
mean
(n
=4)
soil
moisture
summed
to
40
cm
depth
(sum
of
soil
moisture
measured
at
10,
20,
30
and
40
cm
depth).
FC
=
field
capacity;
Refill
=
soil
moisture
depletion
threshold
(0.3
m
3
m
-3
)
at
which
point
the
soil
profile
should
be
refilled.
14
B
Keen
and
P
Slavich
treatment
means
were
not
statistically
signifi-
cant
(Table
2).
The
percentage
of
the
total
berry
yield
graded
as
seconds
ranged
between
17.6%
for
the
farmer
practice
plots
and
19.4%
for
the
PRD
plots.
Average
berry
weights
ranged
between
2.7
g
for
the
farmer
practice
plots
and
2.3
g
for
ET
c
plots.
Differences
between
means
for
both
these
quality
parameters
were
not
significant.
Under
the
farmer's
regime,
WUE
was
2.1
kg
kL
-1
and
a
significant
(P
<
0.05)
improvement
in
WUE
was
achieved
under
the
RDI
treatment
(3.1
kg
kL
-1
).
The
ET
c
and
PRD
treatments
delivered
a
mean
WUE
value
of
2.5
kg
kL
-1
.
Discussion
During
our
field
experiment,
FAO
56
guide-
lines
(Allen
et
al.
1998)
for
estimating
plant
water
requirements
(ET
c
)
were
evaluated
against
the
farmer's
'rule-of-thumb'
approach.
The
field
experiment
commenced
in
November
2008
with
a
IC,
ph
value
of
0.6
through
to
March
2009.
During
this
period,
the
farmer's
inputs
were
slightly
above
ET
c
until
early
January
when
inputs
were
then
slightly
below
ET
c
through
to
the
end
of
March.
By
necessity,
Table
2
Blueberry
yield,
quality
and
water
use
efficiency
(WUE).
No
significant
differences
were
observed
for
yield,
percent
seconds
or
berry
weight
(n
=4).
Treatment
Yield
(kg/
plant)
Seconds'
(%
of
total)
Berry
weight
b
(g)
WUE
C
(kg
kL
-I
)
Farmer
1.97
17.6
2.7
2.1
ET
e
1.84
15.1
2.3
2.5
RDI
2.02
14.7
2.5
3.1
PRD
1.59
19.4
2.4
2.5
Tukey
HSD
0.52
5.80
0.4
0.8
(P
<0.05)
'Percentage
of
berries
graded
as
seconds.
b
Mean
weight
of
50
berries
per
plot
sampled
at
peak
harvest.
`Water
use
efficiency
calculated
as
plant
yield/total
volume
of
water
applied
to
each
plant
(10
plants
per
plot).
WUE
values
shown
are
treatment
means
(n
=4).
irrigation
water
for
each
plot
was
drawn
from
a
single
water
main.
Consequently,
the
maximum
water
input
for
all
treatments
was
limited
by
the
frequency
and
duration
of
the
farmer-
initiated
irrigation
events.
This
resulted
in
a
minor
deficit
of
38
mm
developing
in
the
ET
c
treatment
plots
and
was
accompanied
by
an
equivalent
decline
in
soil
moisture.
Regardless,
irrigation
inputs
in
both
treatments
were
close
to
estimated
ET
c
,
the
soil
moisture
data
indi-
cated
that
water
inputs
were
adequate
to
maintain
RAW
and
leaf
water
potential
and
stomatal
conductance
measurements
did
not
indicate
that
plants
were
water
stressed.
As
autumn
temperatures
cooled,
a
IC,
value
of
0.5
was
applied
in
the
month
of
April.
With
further
cooling
and
changes
in
leaf
colour
progressing
to
partial
leaf
fall
(signalling
the
onset
of
semi-dormancy),
the
K
c
value
was
reduced
to
0.4
through
May
to
mid-August.
Estimated
rainfall
infiltration
during
this
period
exceeded
ET
c
by
65
mm
and
the
farmer
applied
263
mm
irrigation
water.
Irrigation
inputs
totalling
180
mm
in
the
ET
c
,
RDI
and
PRD
plots
were
also
well
in
excess
of
estimated
plant
water
requirements.
Most
of
this
excess
water
was
added
via
mandatory
fertigation
events
which,
as
a
standard
practice,
were
initiated
once
each
week
throughout
the
trial.
It
was
necessary
to
permit
fertigation
in
all
plots
to
ensure
that
each
experimental
treat-
ment
received
nutrient
inputs
equivalent
to
the
farmer
practice
plots.
Had
these
fertigation
events
not
occurred,
total
irrigation
inputs
could
have
been
reduced
by
0.33
ML
ha
-1
.
This
indicates
that
there
is
scope
for
growers
to
reconsider
the
need
for
frequent
fertigation
events
through
the
winter.
From
the
commencement
of
bloom
in mid-
August,
IC,
values
were
increased
in
0.1
incre-
ments
at
fortnightly
intervals
until
reaching
the
ripening
stage
when
an
adjusted
IC,
value
of
0.8
was
applied
through
to
the
end
of
harvest.
Estimated
plant
water
requirements
for
this
period
totalled
347
mm
with
378
mm
water
applied
to
experimental
treatment
plots
and
506
mm
applied
to
plots
irrigated
under
the
Irrigation
strategies
for
blueberries
15
farmer's
regime.
Rainfall
was
relatively
low
through
most
of
this
period
with
only
6
mm
estimated
rainfall
infiltration
up
to
the
final
2
weeks
of
the
harvest
period
when
186
mm
rain
fell,
taking
the
estimated
total
rainfall
infiltra-
tion
to
44
mm.
Low
rainfall
during
most
of
the
period
was
probably
one
factor
that
contribu-
ted
to
the
farmer's
perception
of
irrigation
requirements.
Another
contributing
factor
is
that
preventing
fluctuations
in
soil
moisture
during
ripening
is
considered
critical
to
avoid
fruit
splitting.
As
such,
the
standard
practice
is
to
increase
the
frequency
and
volume
of
irriga-
tion
to
maintain
consistent
soil
moisture
during
fruit
expansion
and
ripening
(Strik
et
al.
2003).
Without
making
adjustments
for
changes
in
actual
plant
water
use,
however,
there
is
a
risk
of
over-watering
as
was
the
case
in
this
instance.
A
second
objective
for
the
field
trial
was
to
evaluate
the
feasibility
of
applying
RDI
as
a
strategy
to
conserve
water
in
blueberry
produc-
tion
under
a
water
shortage
scenario.
When
applied
to
'Star'
cultivar
between
the
November
to
February
summer
vegetative
growth
stage,
RDI
reduced
total
irrigation
inputs
by
1.18
ML
ha
—1
and
0.38
ML
ha
—1
compared
with
the
farmer
and
ET,
treatments,
respec-
tively.
With
water
inputs
set
at
50%
ET
C
,
soil
moisture
progressively
declined
between
November
and
January,
finally
falling
outside
the
RAW
range
in
the
final
week
of
January.
A
plant
physiological
response
to
the
water
deficit
was
expected
by
this
stage
but
was
not
observed.
Bryla
&
Strik
(2006)
found
that,
independent
of
cultivar,
midday
stomatal
con-
ductance
in
highbush
blueberry
plants
de-
creased
as
midday
plant
water
potential
approached
—0.6
to
0.8
MPa.
Davies
&
Johnson
(1982)
observed
that
the
critical
mid-
day
water
potential
for
stomatal
closure
in
the
more
drought-tolerant
rabbiteye
blueberry
(V.
ashen)
was
2.2
MPa.
In
the
present
study,
midday
leaf
water
potential
never
fell
below
0.66
MPa
and
mid-morning
stomatal
con-
ductance
consistently
moderated
between
the
lowest
measurement
of
260
mmol
m
S
-1
and
the
highest
of
396
mmol
m
—2
s
—1
and
no
significant
differences
were
observed
between
treatments
for
leaf
water
potential
or
stomatal
conductance.
The
apparent
absence
of
a
phy-
siological
response
to
the
water
deficit
may
be
explained
by
the
possibility
that
actual
RAW
was
greater
than
that
estimated
using
the
model
of
Williams
et
al.
(1983).
It
is
also
possible
that
plants
were
accessing
water
deeper
than
the
estimated
40
cm
effective
rooting
depth.
Even
so,
during
this
study
an
irrigation
deficit
was
applied
within
prescribed
limits
without
any
apparent
water
stress
and
without
negatively
impacting
on
berry
yield
or
quality.
Prior
to
this
study,
the
concept
of
applying
RDI
to
highbush
blueberries
may
have
been
overlooked
due
to
several
studies
having
de-
monstrated
the
sensitivity
of
highbush
blue-
berries
to
soil
water
deficits.
For
example,
Haman
et
al.
(1997a,
b)
reported
that
growth
and
yield
of
the
highbush
cultivar
`Sharpblue'
were
significantly
reduced
when
the
W
soil
(soil
water
potential)
was
maintained
below
10
kPa.
Matric
potentials
in
soils
that
are
at
field
capacity
range
between
—10
kPa
for
coarse
textured
soils
and
33
kPa
W
soil
for
fine
textured
soils
(Cassel
&
Nielson
1986).
As
such,
at
first
glance,
the
results
of
Haman
et
al.
(1997a,
b)
seem
to
indicate
that
the
perfor-
mance
of
highbush
blueberries
declines
when
soil
moisture
is
permitted
to
fall,
even
margin-
ally,
below
field
capacity.
It
should
be
noted,
however,
that
soil
moisture
in
the
Haman
et
al.
(1997a,
b)
study
was
maintained
at
either
10
kPa,
—15
kPa
or
20
kPa
throughout
the
duration
of
the
experiment,
which
ran
for
3
years.
This
contrasts
with
the
current
study
in
which
we
propose
that
RDI
could
be
applied
as
a
short-term
strategy
when
water
resources
become
limited.
In
eastern
Australia,
the
high-
est
risk
of
water
shortages
occurs
from
late
spring
through
summer,
which
coincides
with
the
post-harvest
vegetative
growth
stage
of
early
and
mid-season
southern
highbush
blue-
berry
cultivars
such
as
'Star'.
Bloom
and
fruit
expansion
are
the
phenological
stages
most
sensitive
to
water
stress
(Ameglio
et
al.
1999;
16
B
Keen
and
P
Slavich
Mingeau
et
al.
2001).
As
such,
RDI
may
have
greater
potential
to
cause
negative
impacts
on
yield
if
imposed
at
these
stages.
However,
further
research
is
required
to
determine
whether
varying
prescribed
levels
of
deficit
within
and
outside
the
vegetative
growth
stage
has
application
to
maximise
water
use
effi-
ciency.
Our
final
objective
was
to
evaluate
PRD
as
a
water
conservation
strategy
for
highbush
blue-
berry
production.
The
primary
criterion
for
declaring
a
successful
plant
response
to
PRD
is
an
observation
of
stomatal
closure
without
loss
of
turgidity
(Kriedmann
&
Goodwin
2005).
This
criterion
defined
the
objective
for
our
pilot
glasshouse
pot
trial
during
which
such
a
re-
sponse
was
observed,
as
indicated
by
a
signifi-
cant
reduction
in
stomatal
conductance
without
a
corresponding
fall
in
leaf
water
potential.
This
result
supports
previous
observations
of
iso-
hydric
behaviour
in
highbush
blueberries
made
by
Ameglio
et
al.
(1999).
For
such
a
response
to
have
occurred,
the
plants
used
in
the
trial
needed
to
have
at
least
some
capacity
to
translocate
water
from
the
irrigated
roots
to
other
parts
of
the
plant.
This
insight
is
impor-
tant
in
that
it
challenges
the
standing
hypothesis
which
states
that
highbush
blueberries
do
not
have
the
capacity
to
translocate
water
laterally
(Abbott
&
Gough
1986;
Strik
et
al.
2003).
This
hypothesis
originates
from
studies
by
Gough
(1984)
and
Abbott
&
Gough
(1986).
The
critical
difference
between
our
split-pot
experiment
and
theirs
was
that
we
did
not
damage
the
crown
when
separating
the
roots.
The
current
under-
standing
of
lateral
hydraulic
translocation
in
woody
plants
is
that
it
mostly
occurs
overnight
with
water
from
roots
in
wet
soil
moving
in
an
axial
and
circumferential
flow
through
the
lower
stem,
to
opposing
roots
with
lower
water
potentials.
This
translocated
water
is
then
lifted
to
the
canopy
when
transpiration
resumes
(Smart
et
al.
2004;
Burgess
&
Bleby
2006).
By
splitting
stems
5
cm
vertically
through
the
basal
crown,
Gough
(1984)
and
Abbott
&
Gough
(1986)
may
have
removed
the
plant's
ability
to
translocate
water
and
nutrients
from
roots
on
the
irrigated
side
to
roots
on
the
non-irrigated
side.
Gough
(1984)
and
Abbott
&
Gough
(1986)
also
fixed
irrigation
to
one
side
of
the
plant
and,
after
6
months,
observed
that
roots
on
the
dry
side
were
severely
damaged.
Others
have
also
observed
that
highbush
bluberry
plants
perform
poorly
with
incidental
fixed
partial
watering
of
roots
(Shelton
&
Freeman
1989;
Strik
et
al.
2003).
These
are
reasonable
obser-
vations
considering
that
extended
exposure
of
roots
to
dry
soil
would
likely
cause
vascular
damage
which
in
turn
would
remove
the
roots'
ability
to
transport
water
and
nutrients.
Unlike
fixed
partial
wetting,
however,
PRD
involves
alternations
of
wetting
and
drying
to
each
side
of
the
root
system
with
alternations
initiated
before
the
soil
has
dried
to
the
point
where
vascular
damage
might
occur.
Root
damage
by
desiccation
would
also
remove
the
ability
of
roots
to
produce
and
transport
abscisic
acid
(ABA),
a
plant
hormone
that
acts
as
the
primary
mechanism
driving
the
plant's
physio-
logical
response
to
PRD
(Stoll
et
al.
2000;
Davies
et
al.
2002;
de
Souza
et
al.
2005;
Dodd
et
al.
2006;
Liu
et
al.
2006).
With
each
PRD
cycle,
ABA
is
produced
in
roots
on
the
drying
side
of
the
plant
and
when
irrigation
is
resumed
to
these
roots,
the
water
that
they
transport
into
the
plant
carries
a
pulse
of
ABA
to
the
leaves.
Upon
arrival
in
the
leaves,
the
ABA
signals
the
stomata
to
reduce
their
aperture.
During
our
pot
trial,
FPRD
plants
exhibited
a
physiological
response
similar
to
the
PRD
plants.
This
response
indicates
that,
initially,
the
roots
probably
continued
to
receive
water
translocated
overnight
from
the
irrigated
roots,
produce
ABA
and
transport
both
into
the
above-ground
parts
of
the
plant
when
tran-
spiration
resumed
at
daylight.
During
the
final
2
weeks
of
the
experiment,
however,
the
stomatal
response
in
FPRD
plants
diminished
which
may
have
been
an
early
symptom
indicating
the
onset
of
root
damage
due
to
desiccation.
While
results
from
the
pilot
glasshouse
experiment
demonstrated
a
stomatal
response
Irrigation
strategies
for
blueberries
17
to
PRD
in
southern
highbush
blueberry
plants,
we
were
unable
to
stimulate
the
same
response
to
PRD
in
field
grown
plants.
This
result
may
be
attributed
to
unfavourable
climatic
and
soil
conditions.
Rainfall
throughout
the
duration
of
the
PRD
application
period
(November
to
February)
was
relatively
frequent.
While
most
of
this
rainfall
would
have
been
deflected
by
the
plastic
woven
weedmat,
some
also
infiltrated
the
soil
as
indicated
by
rises
in
soil
moisture
coinciding
with
rainfall
events.
In
between
frequent
rainfall
events
there
was
also
persistent
cloud
cover
which
reduced
ET
o
and
the
rate
of
soil
drying.
These
conditions
made
it
difficult
to
maintain
continuity
in
soil
drying
cycles
in
the
PRD
treatment
plots.
No
rain
fell
during
a
3-
week
period
in
January
at
which
time
ET
o
also
accelerated.
Soil
moisture
data
from
this
period
indicated
that
only
in
the
final
week
of
a
3-week
cycle
did
soil
moisture
begin
to
demonstrate
a
pattern
consistent
for
what
would
be
expected
with
alternating
irrigations
under
PRD.
This
period
was
the
most
likely
time
during
the
trial
in
which
to
observe
a
plant
response
to
PRD.
Soil
moisture
on
the
drying
side
fell
within
a
band
at
which
a
stomatal
response
to
PRD
was
expected
but
no
such
response
was
observed.
Clay-loam
Ferrosol
soils
characteristically
have
good
drainage
but
they
also
have
a
high
water-holding
capacity
comparable
with
hea-
vier
clays
(McKenzie
et
al.
1999).
As
such,
soil
water
is
available
to
plants
for
a
longer
duration
than
in,
for
example,
sandy
soils
(McKenzie
et
al.
1999).
Kriedmann
&
Goodwin
(2005)
provide
several
examples
where
small
or
nil
responses
to
PRD
occurred
when
PRD
was
applied
to
plants
grown
in
clay
soils.
They
speculate
that
the
most
likely
explanation
for
these
poor
responses
is
due
to
soil
moisture
depletion
on
the
drying
side
of
plants
being
too
sluggish
to
generate
distinctive
and
repeated
pulses
of
ABA.
This
provides
a
plausible
explanation
for
both
the
successful
application
of
PRD
in
the
glasshouse
experiment,
where
wetting
and
drying
cycles
were
alternated
every
3-4
days,
and
the
lack
of
stomatal
response
to
PRD
in
the
field
experiment,
where
soil
drying
may
have
been
too
slow
to
permit
alternating
wetting
and
drying
cycles
of
sufficient
fre-
quency.
The
conflicting
results
between
the
pot
and
field
experiment
highlight
an
opportu-
nity
for
re-evaluating
PRD
with
highbush
blue-
berries
grown
in
soil
with
low
water
holding
capacity
(e.g.
a
sandy
soil)
and
under
more
favourable
or
controlled
climatic
conditions
than
could
be
achieved
during
our
field
study.
Conclusion
The
results
from
this
study
indicate
that
using
FAO
56
guidelines
to
construct
a
K.
curve,
commencing
with
a
generic
berry
Ic
mid
of
1.05,
can
be
recommended
for
adoption
as
a
strategy
to
improve
water
use
efficiency
among
Australian
blueberry
growers.
We
summarized
these
guidelines
into
an
equation
to
save
growers
and
extentionsists
the
trouble
of
having
to
read
and
interpret
FAO
56.
Using
this
equation
to
adjust
for
crop
and
site
conditions
specific
to
our
field
trial,
we
arrived
at
an
adjusted
K.
curve
for
the
southern
highbush
blueberry
cultivar
'Star'
of
0.4
I(
w
,
0.8
K
e
h,
0.6
lc
ph.
The
suitability
of
this
Ic
curve
was
validated
by
observations
recorded
during
the
field
trial.
We
stress,
however,
that
rather
than
applying
these
exact
Ic
values,
growers
should
use
the
equation
defined
in
this
study
to
construct
a
Ic
curve
adjusted
to
suit
their
situation.
We
found
that
under
the
farmer's
`rule-of-
thumb'
regime
plant
water
requirements
were
over-estimated
during
the
cooler
autumn
and
winter
months
and
during
spring
flowering
through
to
harvest.
During
winter,
most
water
inputs
were
via
weekly
fertigation
and
so
it
is
clear
that
this
practice
needs
to
be
re-evaluated.
Reducing
irrigation
in
winter
would
also
leave
more
water
in
storage
in
preparation
for
unforeseen
water
shortages,
the
highest
risk
of
which
occurs
during
the
east-Australian
spring
and
summer.
Should
a
blueberry
farmer
be
faced
with
water
shortages
under
such
condi-
tions,
our
results
indicate
that
RDI
could
be
applied
to
southern
highbush
blueberries
18
B
Keen
and
P
Slavich
during
the
post-harvest
vegetative
growth
stage.
However,
a
grower
should
apply
RDI
within
the
prescribed
limits
of
replacing
water
used
by
the
crop
at
a
rate
of
50%
ET
c
with
additional
water
applied
when
required
to
prevent
soil
moisture
falling
to
a
critical
stress
point
(i.e.
past
full
depletion
of
the
RAW).
Acknowledgements
This
work
was
funded
by
the
Australia
Centre
for
International
Agricultural
Research
(ACIAR),
Southern
Cross
University
and
Industry
and
Invest-
ment
NSW
under
ACIAR
project
SMCN2003/035.
Lisa
MacFadyen
is
acknowledged
for
contributions
as
an
adviser,
Stephen
Morris
is
acknowledged
for
biometric
services
and
Samuel
North
and
Brian
Dunn
are
acknowledged
for
reviewing
the
manu-
script.
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