the quest for acid-tolerant lucerne · fig. 2. the distribution of length of seedling tap roots...
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The quest for acid-tolerant
lucerne
Graham Centre Sheep Forum
Richard Hayes, Research Scientist
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Why do we want Acid-tolerant (AT)
lucerne?
Soils of SNSW are generally acidic
– 74% of ~ 4700 soils in SW NSW had pHCa ≤ 5.0 (Scott et al. 2007)
Lucerne is the only perennial legume broadly adapted across this region
Lucerne is sensitive to acid soils
NSW represents ~ 50% of domestic lucerne sales
Acid soil tolerance has proven to be useful in other forage and crop species
More than a dozen scientific articles exist internationally describing the need for AT lucerne
Still, no AT lucerne cultivar exists today
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Lucerne biomass (kg/ha) on an acid soil with
and without surface applied lime (L)
0
1000
2000
3000
4000
5000
6000
Spr 0
4
Aut 0
5
Win
05
Spr1
05
Spr2
05
Aut 0
6
Win
06
Spr 0
6
Sum
07
Aut 0
7
Lucerne
Luc + L
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Challenges in developing acid soil
tolerance Soil is highly variable
– Up to 1.5 pH unit difference in 1 m2 soil
A range of acid soil-related factors
– H+ toxicity
– Al toxicity
– Mn toxicity
– Δ nutrient availability (eg Mo deficiency)
Other factors affecting plant response eg topography, soil depth, water holding capacity etc
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Challenges in developing AT lucerne
We are starting off a low
base
– Devine et al. (1976)
estimated that Al
tolerance existed in only
2% of a lucerne
population
Lucerne is sensitive to a
range of acid soil factors
– Eg Mn & Al toxicity
Photo: B. Scott
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Challenges in developing AT lucerne
Lucerne genetics
– Is an autotetraploid
– Is almost an obligate
out-crosser
– Need to maintain
genetic diversity to avoid
in-breeding suppression
Therefore, rate of genetic
improvement is slower
Photo: B. Dear
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Challenges in developing AT lucerne
Lucerne is a legume
Root nodule bacteria (RNB)
is commonly more sensitive
to acidity than the plant
Therefore, for genuine acid
soil tolerance you need:
– AT plant
– AT RNB
– The AT plant to be
compatible with the RNB
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Summary – why doesn’t AT lucerne
exist? A range of acid soil factors to consider
Sensitivity to multiple acid soil factors
Starting from a low base
Need to select a large number of elite individuals to develop
a robust population
Need to consider the RNB
Need to ensure the elite RNB is compatible with the elite
plant germplasm
In a context where public plant breeding programs are not
fashionable
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Recent Australian research
Screening in high Al solution culture; (Scott et al. 2008)
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Recent Australian research
Screening in high Al solution culture; (Scott et al. 2008)
Sensitive Tolerant
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Source; (Scott et al. 2008)
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Po
pu
latio
n e
qu
al to
or
gre
ate
r th
an
no
min
ate
d
roo
t le
ng
th (
%)
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Po
pu
latio
n e
qu
al to
or
gre
ate
r th
an
no
min
ate
d
roo
t le
ng
th (
%)
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Source; (Scott et al. 2008)
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Recent Australian research
Seedling validation in high Al soil;
(Hayes et al. 2011)
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Recent Australian research P
roport
ion o
f pop
ula
tion w
ith t
ap r
oot
length
equal to
or
gre
ate
r th
an
nom
inate
d length
(%
)
0
20
40
60
80
100
Tap root length (mm)0 50 100 150 200
0
20
40
60
80
100
0 50 100 150 200
a) Experiment 2; pH 4.34
d) Experiment 3; pH 5.36
c) Experiment 3; pH 4.48
b) Experiment 2; pH 5.26
Fig. 2. The distribution of length of seedling tap roots exhibiting
evidence of increased Al tolerance (●) and 3 populations exhibiting
evidence of increased seedling vigour (○) compared with individuals
from 6 control populations (▲).
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Pro
port
ion o
f pop
ula
tion w
ith t
ap r
oot
length
equal to
or
gre
ate
r th
an
nom
inate
d length
(%
)
0
20
40
60
80
100
Tap root length (mm)0 50 100 150 200
0
20
40
60
80
100
0 50 100 150 200
a) Experiment 2; pH 4.34
d) Experiment 3; pH 5.36
c) Experiment 3; pH 4.48
b) Experiment 2; pH 5.26
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Pro
port
ion o
f pop
ula
tion w
ith t
ap r
oot
length
equal to
or
gre
ate
r th
an
nom
inate
d length
(%
)
0
20
40
60
80
100
Tap root length (mm)0 50 100 150 200
0
20
40
60
80
100
0 50 100 150 200
a) Experiment 2; pH 4.34
d) Experiment 3; pH 5.36
c) Experiment 3; pH 4.48
b) Experiment 2; pH 5.26
pH 4.34Ca
Al tolerant populations
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Pro
port
ion o
f pop
ula
tion w
ith t
ap r
oot
length
equal to
or
gre
ate
r th
an
nom
inate
d length
(%
)
0
20
40
60
80
100
Tap root length (mm)0 50 100 150 200
0
20
40
60
80
100
0 50 100 150 200
a) Experiment 2; pH 4.34
d) Experiment 3; pH 5.36
c) Experiment 3; pH 4.48
b) Experiment 2; pH 5.26
pH 4.34Ca
Al tolerant populations
Increased seedling vigour
pH 5.26Ca
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Population pH 4.34 pH 5.26
Sardi 7 58.7 110.6
CRCSA 34-
36 81.9 107.6
Table 1. Mean seedling root length (mm) for a pair
comparison under low and high lime rates
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Population pH 4.34 pH 5.26
Sardi 7 58.7 110.6
CRCSA 34-
36 81.9 107.6
Table 1. Mean seedling root length (mm) for a pair
comparison under low and high lime rates
40% increase
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Rhizobia research
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Rhizobia research Photo: R. Ballard
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Rhizobia research Photo: R. Ballard
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0
20
40
60
80
100
NO RHIZOBIA RRI 128 SRDI 672 SRDI 722 SRDI736
Pla
nts
with n
od
ule
s (
%)
Fig. 3. Effect of inoculation treatment on the percentage of
SARDI ten lucerne seedlings forming nodules in solution culture
(pH 4.8), at 11 days after inoculation.
Courtesy: R. Ballard, SARDI, Adelaide
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0
20
40
60
80
100
NO RHIZOBIA RRI 128 SRDI 672 SRDI 722 SRDI736
Pla
nts
with n
od
ule
s (
%)
Fig. 3. Effect of inoculation treatment on the percentage of
SARDI ten lucerne seedlings forming nodules in solution culture
(pH 4.8), at 11 days after inoculation.
Courtesy: R. Ballard, SARDI, Adelaide
From Book Book, NSW
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Progress to date:
Developed a methodology for mass recurrent
selection of seedlings in high Al solution culture
Demonstrated up to 40 % increased tap root
growth at pH 4.3
Have selected a replacement RNB strain for
lucerne adapted for acid soils
Have made further plant selections to develop an
elite lucerne/RNB combo, for commercial release
from 2015
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Challenges ahead
Mn toxicity
No selection for Mn toxicity
has yet taken place within
elite Al lucerne
Populations selected for
enhanced growth under
high Al are unlikely to be
tolerant of Mn toxicity
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Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan
Mn c
oncentr
atio
n (
mg/k
g)
of
lucern
e h
erb
age
0
500
1000
1500
2000
2500
Plot B2
Plot B3
Plot B4
Mean
Critical level
Plot G1
Plot G2
Plot G3
Plot G4 2
0.2
2.5
7.1
14
.3 5.4
4.4 11
.8
6.3
22
.3
22
.7
26
.1
53
.6
47.7
50.4
44.1
12
.7
38
.7
10
.5
4.9
19
.7
5.7
13
.3
13
.3
23
.2
11
.8 33
.8
37
.3
51.3
29.9
42.9
34.3
(Hayes et al. 2012)
Binalong
Gerogery
Critical threshold
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Mn toxicity
Reduced shoot growth by 20%
Reduced lucerne seedling survival by up to 35%
Reduced root length by 40%
Therefore, has the potential to offset all the
existing gains delivered through enhanced Al
tolerance
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Opportunities
1. Select for Mn tolerance in lucerne populations
2. Greater use of gene markers to further enhance
selection techniques
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Conclusions
Al tolerant lucerne is coming
Likely to be accompanied by RNB with enhanced
performance on acid soils
New products unlikely to be tolerant of Mn toxicity
When those products become available, be sure to
use them in conjunction with lime