School of Agriculture,
Policy and Development
Some implications of weed seed
ecology for weed management
Alistair Murdoch
EWRS conference , Santorini, September 2009
Novel and sustainable weed management in arid and semi-arid agro-ecosystems
... Weeds are opportunists !!!
Photo by Dannie Romney, International Livestock Research Institute, Kenya
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.
Watch out!
We’ve got to stay alive
http://www.filebuzz.com/software_screenshot/full/75438-Cartoon_Character_Screensaver.jpg
Preserving viability
Seed 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
Loss of viability in dry storage
Phelipanche aegyptiaca
50°C, 80%rh
Note seed to seed variation
within population
Kebreab & Murdoch (1999)
Weed Research 10, 447–457
60°C, 75%rh
Ph. aegyptiaca
Kebreab & Murdoch (1999)
Weed Research 10, 447–457
O. minor
20°C
30°C
Germination of dry stored seeds, %
Survival of
Phelipanche/
Orobanche
seeds after
dry storage
for up to
c. 400 days
at various
temperatures and
moistures
O. crenata
40°C
50°C
60°C
Storage period, days
100
Phelipanche aegyptiaca
Orobanche cernua 
90
O. crenata
Germination (%)
80
70
Survival of
seeds after
wet storage
for up to
210 days at
30°C
60
50
40
30
20
10
0
0
30
60
90
120
150
Conditioning period (days)
180
210
Kebreab & Murdoch (1999)
J Exp Bot 50: 211-219
How long can you
live in soil?
http://www.filebuzz.com/software_screenshot/full/75438-Cartoon_Character_Screensaver.jpg
Modelling seed depletion in soil
Negative 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
Predicting seed depletion
A depletion factor of 106 is like
reducing the seed bank
from 1000 seeds/m2 to 10 seeds/ha.
Elimination is therefore an unrealistic
outcome for invasive plant
management unless the
depletion rate is very high.
Say > 90% per annum
Depletion
factors
Don’t germinate all
together
http://www.filebuzz.com/software_screenshot/full/75438-Cartoon_Character_Screensaver.jpg
Germination prevention
• Dormancy
• Quiescence
Germination percentage (probability scale)
Germination percentage (probability scale)
Seed
dormancy
varies with
maturation
conditions
Cenchrus
ciliaris
Spikelets
Open symbols: 32/27ºC, 13h/11h day/night
Closed symbols: 27/20ºC , 13h/11h day/night
93
Water
Nutrient
84
69
Drought
50
31
16
7
Note seed to seed variation
within population
2
0
10
20
30
40
50
60
After-ripening
(days) at 40ºC, 43% rh
After-ripening period
period (days)
Sharif Zadeh & Murdoch Seed Science Research (2000) 10, 447–457
Dormancy varies with post-harvest
dry storage (afterripening), Striga hermonthica
100
60 C
Germination, %
90
50 C
80
70
30 C
20 C
40 C
60
50
40
30
20
10
0
0
10
Sonko & Murdoch, unpublished
20
30
40
50
60
70
80
90
100
110
120
130
Afterripening period, days
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.
To germinate
or
not to germinate
that is the
question
-
a balancing act
for the
individual seed –
How does this
balance vary?
•
•
•
•
•
•
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
•
•
•
•
•
•
•
•
•
•
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
Collaborators
and co-authors
Jose M. Urbano,
Sevilla, Spain
Viability or non-dormancy,
angles
General correlation of viability and
“non-dormancy” of Chenopodium album
with latitude of origin
90
80
Viability (NS)
70
60
50
40
Non-dormancy (P<0.05)
30
20
10
0
35
40
45
50
55
Latitude, degrees North
Murdoch et al. unpublished
60
65
Optimum temperature, deg C
Optimum constant temperature
for germination of Chenopodium album
is correlated with “non-dormancy”
Spearmann Rank
Correlation Coefficient:
-0.752 (P=0.005)
30
26
22
18
14
10
0.0
20.0
40.0
60.0
80.0
Maximum germination on plate, %
100.0
Optimum temperature, deg C
Optimum constant temperature
for germination of Chenopodium album
is NOT correlated with latitude of origin
30
26
22
18
Spearmann Rank Correlation
Coefficient: -0.164 (P=0.56, NS)
14
10
35
40
45
50
55
Latitude, degrees North
60
65
Germination, angles
Responses of common seed lot of
Chenopodium album to 0.01 mol/L
potassium nitrate in dark and light
Murdoch et al. unpublished
90
80
70
60
50
40
30
20
10
0
Common Seedlot
Light
LSD
Dark
Water
Nitrate
Murdoch et al. unpublished
Germination (angles) responses to KNO3 in darkness and light
Interaction, angles
Chenopodium album :
Northern seed lots: additive effects of nitrate and light
Southern seed lots: positive interaction (synergism)
40
35
30
25
Interaction of light
and nitrate (P<0.05)
20
15
10
5
0
-5 35
Canada
40
45
50
55
Latitude, degrees North
Murdoch et al. unpublished
60
65
Germination (º) after chilling for various periods
Spain 37º
Negative slope
70
60
50
Portugal 38º
Slope not significant
70
60
50
USA-IL 40º
Slope not significant
70
60
50
60
50
40
40
40
40
30
30
30
30
20
20
20
10
10
10
20
LSD (P =0.05)
10
0
0
0
10
20
30
40
50
Canada 45º
Slope not significant
70
60
0
0
10
20
30
40
50
USA-MN 45º
Positive slope
70
60
0
0
10
20
30
40
50
Czech Rep. 48º
Slope not significant
70
60
0
50
40
40
40
40
30
30
30
30
20
20
20
20
10
10
10
10
0
10
20
30
40
50
0
0
10
20
30
40
50
50
10
20
30
40
50
0
70
70
60
60
60
50
50
40
40
40
30
30
20
20
10
Common seed 55º
Slope not significant
40
30
Denmark 55º
Slope not significant
30
20
20
10
10
10
0
0
0
0
10
20
30
40
50
0
10
20
30
40
50
30
40
50
0
0
70
50
20
UK 52º
Negative slope
60
50
0
10
70
50
0
Italy 43º
Negative slope
70
10
20
30
40
50
70
Norway 59º
Slope not significant
60
50
Finland 60º
Negative slope
0
0
10
Chilling period, days
20
30
40
50
0
10
20
30
Chilled at 3.1ºC; germinated at 10º/20º C (12h/12h) with light;
means of 4 replicates of 50 seeds; SED = 4.4, 249 df
40
50
For the population of seeds:
quantitative variation in behaviour
in populations tends to be normally distributed.
But there can be qualitative differences …
More light
please
Chill out? I’m going back to
sleep
Nondormant
Photo by Dannie Romney, International Livestock Research Institute, Kenya
Light AND nitrate
AND alternating
temperature
Nitrate
please
Don’t get suppressed
by a crop
http://www.filebuzz.com/software_screenshot/full/75438-Cartoon_Character_Screensaver.jpg
100
Medium
High
80
70
60
90
Sed df 29
Recommended
Vigour
Double
Spring wheat cv Axona. 2002 growing season.
S.
nig
ru
m
ice
P.
sp
ec
lbu
m
C.
a
ve
ns
is
ar
V.
co
A.
rv
e
100
tul
a
50
C.
a
weed
ividual
)
low
90
ns
is
Suppression of weed
biomass of individual
species (%)
Suppression of biomass of different weeds (%)
uctive
specie
100
ar v
Spring wheat cv Axona. 2002 growing season. Doukali unpublished
Recommended
Double
m
ice
Medium
S.
nig
ru
P.
spe
c
low
lbu
m
en
sis
100
C.
a
V.
tul
a
ns
is
co
rv e
A.
C.
a
Suppression of reproductive
units of individual weed
species (%)
ar v
lb
en
S.
nig
r
P.
spe
c
C.
a
V.
t
n
co
rv e
A.
C.
a
Suppression of reproductive units (%)
Rate
High
90
80
70
60
50
Vigour
Weed biomass suppression.
Weeds suppressed similarly by high and low
vigour crop
emerged
Preseed
HV lots if weeds
F.AtHV
At HV
synchronously with crop. Effect of vigour
PostLV
F.At LV
At LV
eliminated at a given crop density.
50
Weed dry matter, g per pot
(g/pot)
40
30
20
F. Syn
() Low vigour
Asynchronous
High () and low
() vigour
synchronous
10
() High vigour
Asynchronous
0
0
100
250
400
550
2
Croppopulation
density, plantsplant/m
/m 2
Crop
Spring wheat cv Paragon. Unpublished results, Doukali and Murdoch 2006
Weed seed suppression.
Seed production suppressed similarly by
high and low vigour crop seed lots if weeds
Pre v1
F.AtHV
emerged synchronously
with
crop. Effect At
of HV
PostLV at a given
F.AtLV
vigour eliminated
crop density.AtLV
F.Syn
25
() Low vigour
Asynchronous
(g/pot)
Weed seeds, g per pot
20
15
High () and low
() vigour
synchronous
10
() High vigour
Asynchronous
5
0
0
100
250
Crop density, plants / m2
400
550
Crop poulation plant/m2
Spring wheat cv Paragon. Unpublished results, Doukali and Murdoch 2006
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
Watch out for new weed species
Take your eyes off the ball and before you know it, a new
patch will have appeared.
Acknowledgements
u
Several research students contributed to this work
–
–
–
–
–
u
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
Aren’t these strange flowers
pretty? I’m going to take some
home!
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US fed highways administration