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School of Agriculture,
Policy and Development
EWRS conference , Santorini, September 2009 Novel and sustainable weed management in arid and semi-arid agro-ecosystems
Some implications of weed seed ecology for weed management
Alistair Murdoch
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... Weeds are opportunists !!!
Photo by Dannie Romney, International Livestock Research Institute, Kenya
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Prerequisites for invasive weeds
• Propagule dispersal to a new site during which…– Viability must be preserved
– Growth must generally be prevented
– Predation must be avoided
• After dispersal, invasion occurs if propagules can– Maintain viability
– Prevent growth and
– Avoid predation
until conditions are suitable for growth
• Propagules must then be in the right physiological state to grow, reproduce and disperse progeny.
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http://www.filebuzz.com/software_screenshot/full/75438-Cartoon_Character_Screensaver.jpg
Watch out!We’ve got to stay alive
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Preserving viabilitySeed storage behaviour
• Most weeds have ‘orthodox’ seed storage behaviour:– Seeds survive better when dried to very low moisture
contents and seeds can be frozen when dry
• Recalcitrant seeds:– Seeds cannot be dried without damage and some are
damaged by chilling below 15ºC
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Loss of viability in dry storage
50°C, 80%rh
60°C, 75%rh
Phelipanche aegyptiaca
Kebre
ab &
Murd
och
(1
99
9)
Weed R
ese
arc
h 1
0,
44
7–4
57
Note seed to seed variationwithin population
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Survival of Phelipanche/Orobanch
e seeds after dry
storage for up to
c. 400 days at various temper-
atures and moistures
Ph. aegyptiaca O. crenata O. minor
20°C
30°C
40°C
50°C
60°C
Germ
inati
on
of
dry
sto
red
seed
s,
%
Storage period, days
Kebreab & Murdoch (1999) Weed Research 10, 447–457
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0
10
20
30
40
50
60
70
80
90
100
0 30 60 90 120 150 180 210
Conditioning period (days)
Ger
min
atio
n (%
)Phelipanche
aegyptiaca Orobanche cernua
O. crenata Survival of seeds after
wet storage for
up to 210 days at
30°C
Kebreab & Murdoch (1999) J Exp Bot 50: 211-219
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How long can you
live in soil?
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Modelling seed depletion in soilNegative exponential decay curve
S = S0 e-gt or loge S = loge S0 - gt
where S is the number of surviving seeds from an initial population S0 after t years. g is the annual rate of depletion.
This model is only valid on– a year-to-year basis, and – in the absence of seed influx.
If true, implications include that seed banks have:– a constant half life, and – a constant annual probability of depletion
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Predicting seed depletion Depletion factors
A depletion factor of 106 is likereducing the seed bank
from 1000 seeds/m2 to 10 seeds/ha.
Elimination is therefore an unrealistic outcome for invasive plantmanagement unless the depletion rate is very high. Say > 90% per annum
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Don’t germinate all together
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Germination prevention
• Dormancy
• Quiescence
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After-ripening period (days) at 40ºC, 43% rh
Open symbols: 32/27ºC, 13h/11h day/nightClosed symbols: 27/20ºC , 13h/11h day/night
Note seed to seed variationwithin population
Ger
min
atio
n pe
rcen
tage
(pro
babi
lity s
cale
)Seeddormancyvaries withmaturationconditionsCenchrusciliaris Sharif Zadeh & Murdoch Seed Science Research (2000) 10, 447–457
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0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40 50 60 70 80 90 100 110 120 130
Ger
min
atio
n,
%
Afterripening period, days
20 C30 C
40 C50 C
60 C
Dormancy varies with post-harvest dry storage (afterripening), Striga hermonthica
After dry storage, seeds were pre-conditioned for 14 d in water at 30ºC and then germinated at 30ºC in 1ppm GR24. Seeds were collected in The Gambia from an infestation of millet.
Sonko & Murdoch, unpublished
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To germinateor
not to germinate
that is the question
-
a balancing actfor the
individual seed –
How does this balance vary?
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Collaborators and co-authors
• Laila Karlsson , Linköping, Sweden • Per Milberg, Linköping, Sweden • Paul Neve, Warwick, UK• Ilse A. Rasmussen, Flakkeberg, Denmark• Jukka Salonen, Jokioinen, Finland • Bozena Sera, Czech Republic• Edite Sousa, Tapada da Ajuda, Portugal • Francesco Tei, Perugia, Italy• Kirsten Tørresen, Ås, Norway • Jose M. Urbano, Sevilla, Spain
• Jose L. Gonzalez Andujar, Sevilla, Spain• Diane Benoit, Québec, Canada• Adam Davis, Urbana, IL, USA • Frank Forcella, Morris, MN, USA• Federica Graziani, Perugia, ITALY• Andrea Grundy, Warwick, UK
Regional variation of germination and dormancy of Chenopodium album seedlots tested at Reading as part of EWRS WG joint experiment. Seeds were obtained from most of those listed
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0
10
20
30
40
50
60
70
80
90
35 40 45 50 55 60 65
Latitude, degrees North
Via
bili
ty o
r n
on
-do
rman
cy,
ang
les
Viability (NS)
Non-dormancy (P<0.05)
General correlation of viability and “non-dormancy” of Chenopodium album with latitude of origin
Murdoch et al. unpublished
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Optimum constant temperaturefor germination of Chenopodium albumis correlated with “non-dormancy”
10
14
18
22
26
30
0.0 20.0 40.0 60.0 80.0 100.0
Maximum germination on plate, %
Op
tim
um
te
mp
era
ture
, de
g C Spearmann Rank
Correlation Coefficient: -0.752 (P=0.005)
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Optimum constant temperature for germination of Chenopodium album is NOT correlated with latitude of origin
10
14
18
22
26
30
35 40 45 50 55 60 65
Latitude, degrees North
Op
tim
um
te
mp
era
ture
, de
g C
Spearmann Rank Correlation Coefficient: -0.164 (P=0.56, NS)
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Responses of common seed lot ofChenopodium album to 0.01 mol/Lpotassium nitrate in dark and light
0102030405060708090
Ger
min
atio
n,
ang
les
Common Seedlot Light
Dark
LSD
Water NitrateMurdoch et al. unpublished
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Germination (angles) responses to KNO3 in darkness and light
Murd
och
et
al. u
npu
blis
hed
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Chenopodium album : Northern seed lots: additive effects of nitrate and lightSouthern seed lots: positive interaction (synergism)
-5
0
5
10
15
20
25
30
35
40
35 40 45 50 55 60 65
Latitude, degrees North
Inte
rac
tio
n, a
ng
les
Interaction of lightand nitrate (P<0.05)
Canada
Murdoch et al. unpublished
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Germination (º) after chilling for various periods
0
10
20
30
40
50
60
70
0 10 20 30 40 50
Spain 37ºNegative slope
LSD (P =0.05)
0
10
20
30
40
50
60
70
0 10 20 30 40 50
Portugal 38ºSlope not significant
0
10
20
30
40
50
60
70
0 10 20 30 40 50
USA-IL 40ºSlope not significant
0
10
20
30
40
50
60
70
0 10 20 30 40 50
Italy 43ºNegative slope
0
10
20
30
40
50
60
70
0 10 20 30 40 50
Canada 45ºSlope not significant
0
10
20
30
40
50
60
70
0 10 20 30 40 50
USA-MN 45ºPositive slope
0
10
20
30
40
50
60
70
0 10 20 30 40 50
Czech Rep. 48ºSlope not significant
0
10
20
30
40
50
60
70
0 10 20 30 40 50
UK 52ºNegative slope
0
10
20
30
40
50
60
70
0 10 20 30 40 50
Common seed 55ºSlope not significant
0
10
20
30
40
50
60
70
0 10 20 30 40 50
Denmark 55ºSlope not significant
0
10
20
30
40
50
60
70
0 10 20 30 40 50
Norway 59ºSlope not significant
0
10
20
30
40
50
60
70
0 10 20 30 40 50
Finland 60ºNegative slope
Chilling period, daysChilled at 3.1ºC; germinated at 10º/20º C (12h/12h) with light;means of 4 replicates of 50 seeds; SED = 4.4, 249 df
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For the population of seeds: quantitative variation in behaviourin populations tends to be normally distributed.
Photo by Dannie Romney, International Livestock Research Institute, Kenya
Non-dorman
t
More light
please
Light AND nitrate AND
alternating temperatur
e
Chill out? I’m going back to
sleep
But there can be qualitative differences …
Nitrate please
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Don’t get suppressedby a crop
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Suppression of biomass of different weeds (%)
Spring wheat cv Axona. 2002 growing season.
50
60
70
80
90
100
Vigour
Supp
ress
ion
of w
eed
biom
ass o
f ind
ivid
ual
spec
ies (
%)
low Medium High
Convo
lvul
us a
rven
sis
LSD 7
.8 Anthe
mis
cotu
laVio
la a
rven
sis
LSD 3
.8
Cheno
podi
um
albu
m L
SD
3.1
Polygo
num
spp
.
LSD
4.5
Sola
num
nig
rum
LSD 3
.3
Sed df 29
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50
60
70
80
90
100
50
60
70
80
90
100
Supp
ress
ion
of re
prod
uctiv
e un
its o
f ind
ivid
ual w
eed
spec
ies (%
)
low Medium High
Suppression of reproductive units (%)
Spring wheat cv Axona. 2002 growing season. Doukali unpublished
Convo
lvul
us a
rven
sis
LSD 6
.3 Anthe
mis
cotu
laVio
la a
rven
sis
LSD 2
.8
Cheno
podi
um
albu
m L
SD
3.3
Polygo
num
spp
.
LSD
3.8
Sola
num
nig
rum
LSD 4
.0
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0
10
20
30
40
50
0 100 250 400 550
Weed biomass suppression.Weeds suppressed similarly by high and low vigour crop seed lots if weeds emerged synchronously with crop. Effect of vigour eliminated at a given crop density.
() High vigour Asynchronous
() Low vigour Asynchronous
High () and low () vigour
synchronous
Crop density, plants / m2
Weed d
ry m
att
er,
g
per
pot
Spring wheat cv Paragon. Unpublished results, Doukali and Murdoch 2006
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0
5
10
15
20
25
0 100 250 400 550
Weed seed suppression.Seed production suppressed similarly by high and low vigour crop seed lots if weeds emerged synchronously with crop. Effect of vigour eliminated at a given crop density.
Spring wheat cv Paragon. Unpublished results, Doukali and Murdoch 2006
() High vigour Asynchronous
() Low vigour AsynchronousHigh () and
low () vigour
synchronous
Crop density, plants / m2
Weed s
eeds,
g p
er
pot
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Some concluding outcomes for weed management
• Predicting population dynamics:– Quantify the life cycle– Quantify seed-to-seed variation in population
• Mitigate risk of sudden expansion of infestation– Explore options for control at various stages of the life cycle– Manage the crop to suppress weeds– Consider the overall farming system– Rarely find magic bullets!– Integrate diverse options is a best
• Have rational expectations of control of invasions– Understand factors affecting seed germination, dormancy and longevity
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Watch out for new weed speciesTake your eyes off the ball and before you know it, a new
patch will have appeared.
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Acknowledgements
Several research students contributed to this work– Farzad Sharifzadeh (Iran)– Landing Sonko (The Gambia)– Ermias Kebreab (Eritrea)– Musa Doukali (Libya)– Sophie Allen (UK)
EWRS Germination and Early Growth WG for seeds of Chenopodium album
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Aren’t these strange flowers pretty? I’m going to take some home!
http://www.fhwa.dot.gov/environment/greenerroadsides/cartoon.gifUS fed highways administration