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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
60°C, 75%rh
Weed Research 10, 447–457
Kebreab & Murdoch (1999)
Note seed to seed variation
within population
Ph. aegyptiaca O. crenata O. minor
Survival of
Phelipanche/
20°C
Orobanche 30°C
seeds after
dry storage Germination of dry stored seeds, %
for up to
40°C
c. 400 days
at various 50°C
temper-
atures and
60°C
moistures
Kebreab & Murdoch (1999)
Weed Research 10, 447–457
Storage period, days
100
Phelipanche aegyptiaca
90 Orobanche cernua
80 O. crenata
Germination (%)
70
60
Survival of
50
seeds after
40 wet storage
30 for up to
20 210 days at
10 30°C
0
0 30 60 90 120 150 180 210
Conditioning period (days) 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
Depletion
Predicting seed depletion factors
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
Don’t germinate all
together
http://www.filebuzz.com/software_screenshot/full/75438-Cartoon_Character_Screensaver.jpg
Germination prevention
• Dormancy
• Quiescence
Open symbols: 32/27ºC, 13h/11h day/night
Spikelets
Closed symbols: 27/20ºC , 13h/11h day/night
Germination percentage (probability scale)
Germination percentage (probability scale)
93
Water Nutrient
84
69
Drought
50
31
16
7
Seed Note seed to seed variation
dormancy
2 within population
varies with 0 10 20 30 40 50 60
maturation
After-ripening period (days) at 40ºC, 43% rh
conditions After-ripening period (days)
Cenchrus
ciliaris Sharif Zadeh & Murdoch Seed Science Research (2000) 10, 447–457
Dormancy varies with post-harvest
dry storage (afterripening), Striga hermonthica
100
60 C
90
30 C 20 C
Germination, %
80 50 C
40 C
70
60
50
40
30
20
10
0
0 10 20 30 40 50 60 70 80 90 100 110 120 130
Sonko & Murdoch, unpublished
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
• Collaborators and co-authors
Kirsten Tørresen, Ås, Norway
• Jose M. Urbano, Sevilla, Spain
General correlation of viability and
“non-dormancy” of Chenopodium album
with latitude of origin
Viability or non-dormancy,
90
80 Viability (NS)
70
60
angles
50
40
Non-dormancy (P<0.05)
30
20
10
0
35 40 45 50 55 60 65
Latitude, degrees North
Murdoch et al. unpublished
Optimum constant temperature
for germination of Chenopodium album
is correlated with “non-dormancy”
Spearmann Rank
Optimum temperature, deg C
30
Correlation Coefficient:
26 -0.752 (P=0.005)
22
18
14
10
0.0 20.0 40.0 60.0 80.0 100.0
Maximum germination on plate, %
Optimum constant temperature
for germination of Chenopodium album
is NOT correlated with latitude of origin
Optimum temperature, deg C
30
26
22
18
14 Spearmann Rank Correlation
Coefficient: -0.164 (P=0.56, NS)
10
35 40 45 50 55 60 65
Latitude, degrees North
Responses of common seed lot of
Chenopodium album to 0.01 mol/L
potassium nitrate in dark and light
Germination, angles
90
80 Common Seedlot Light
70 LSD
60
50 Dark
40
30
20
10
0
Murdoch et al. unpublished
Water Nitrate
Germination (angles) responses to KNO3 in darkness and light
Murdoch et al. unpublished
Chenopodium album :
Northern seed lots: additive effects of nitrate and light
Southern seed lots: positive interaction (synergism)
40
Interaction of light
35
and nitrate (P<0.05)
Interaction, angles
30
25
20
15
10
5 Canada
0
-5 35 40 45 50 55 60 65
Latitude, degrees North
Murdoch et al. unpublished
Germination (º) after chilling for various periods
70 Spain 37º 70 Portugal 38º 70 USA-IL 40º 70 Italy 43º
60
Negative slope 60
Slope not significant 60
Slope not significant 60 Negative slope
50 50 50 50
40 40 40 40
30 30 30 30
20 20 20 20
LSD (P =0.05)
10 10 10 10
0 0 0 0
0 10 20 30 40 50 0 10 20 30 40 50 0 10 20 30 40 50 0 10 20 30 40 50
70 Canada 45º 70 USA-MN 45º 70 Czech Rep. 48º 70 UK 52º
60 Slope not significant 60 Positive slope 60
Slope not significant 60
Negative slope
50 50 50 50
40 40 40 40
30 30 30 30
20 20 20 20
10 10 10 10
0 0 0 0
0 10 20 30 40 50 0 10 20 30 40 50 0 10 20 30 40 50 0 10 20 30 40 50
70 70 70 70
60 60 60 Norway 59º 60
50 50 50 Slope not significant 50
40 Common seed 55º 40 Denmark 55º 40 40
30 Slope not significant 30
Slope not significant
30 30
20 20 20 20 Finland 60º
10 10 10 10 Negative slope
0 0 0 0
0 10 20 30 40 50 0 10 20 30 40 50 0 10 20 30 40 50 0 10 20 30 40 50
Chilling period, days 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
For the population of seeds:
quantitative variation in behaviour
in populations tends to be normally distributed.
But there can be qualitative differences …
Chill out? I’m going back to Light AND nitrate
More light sleep
please AND alternating
temperature
Non-dormant
Nitrate please
Photo by Dannie Romney, International Livestock Research Institute, Kenya
Don’t get suppressed
by a crop
http://www.filebuzz.com/software_screenshot/full/75438-Cartoon_Character_Screensaver.jpg
Suppression of biomass of different weeds (%)
biomass of individual
100
Suppression of weed
low Medium High
90
species (%)
80
70
60
50
m
m
is
is
a
ice
tul
ns
ru
lbu
ns
ec
ve
nig
co
e
a
rv
sp
ar
C.
A.
a
S.
P.
V.
C.
Vigour
Sed df 29
ividual
weed
100 Recommended Double
90 Spring wheat cv Axona. 2002 growing season.
)
uctive Suppression of reproductive
specie units of individual weed
species (%)
C. C.
a
100
a
50
60
70
80
90
100
rv e rv e
ns
is
A. A.
co co
tul
a t
low
V. V.
ar v ar v
en
sis e
Recommended
C. C.
Rate
Vigour a lbu a lb
m
Medium
Suppression of reproductive units (%)
Double P. P.
spe spe
c ice c
High
S. S.
nig nig
ru m
Spring wheat cv Axona. 2002 growing season. Doukali unpublished
Weed biomass suppression.
Weeds suppressed similarly by high and low
vigour crop seed lots if weeds emerged
Pre HV F.AtHV At HV F. Syn
synchronously with crop. Effect of vigour
PostLV F.At LV At LV
eliminated at a given crop density.
50
40 () Low vigour
Asynchronous
(g/pot)
30
Weed dry matter, g per pot
High () and low
() vigour
20 synchronous
10 () High vigour
Asynchronous
0
0 100 250 400 550
density, plants / m2
Croppopulation plant/m2
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
of
emerged synchronously with crop. Effect At HV
Pre v1 F.AtHV F.Syn
vigour eliminated at a given crop density.AtLV
PostLV F.AtLV
25
20 () Low vigour
Asynchronous
High () and low
Weed seeds, g per pot
(g/pot)
15 () vigour
synchronous
10
() High vigour
5 Asynchronous
0
0 100 250 400 550
Crop density, plants / m2
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
– Farzad Sharifzadeh (Iran)
– Landing Sonko (The Gambia)
– Ermias Kebreab (Eritrea)
– Musa Doukali (Libya)
– Sophie Allen (UK)
u 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
