Dormancy cycling of Brassicaceae species in the field: impact of thermal gradients
and nitrate on seeds during annual cycles.
.
Steven Footitt, Heather Clay, Katherine Dent, Andrew Mead, and Bill Finch-Savage
Warwick HRI, University of Warwick, Wellesbourne, Warwick CV35 9EF. UK
Introduction
We are investigating and modeling seed dormancy cycling under the same field conditions in members of the Brassicacea that are
closely related to the model species Arabidopsis. Seeds of the three species chosen have contrasting dormancy behaviour and
include both winter and summer annual cycling strategies. Seeds of winter annuals (WA) lose dormancy in the summer prior to
autumn germination and over winter in the vegetative state prior to spring flowering. Seeds of summer annuals (SA) lose dormancy
during the winter prior to spring germination and flowering. The species employed are Alliaria petiolata (SA), Sinapis arvensis (WA),
and Thlaspi arvense (WA).
Alliaria petiolata dormancy cycling
•In 2007/08, percentage seed germination in the soil was recorded. In 2008/09,
buried seeds were exhumed in the dark and germinated at a range of
constant and alternating temperatures in the dark.
•Maximum germination occurred at 5°C, and at alternating temperatures with a
low mean temperature and an alternation of 5°C (data not shown).
•Light inhibits germination.
•In the two field trials germination was widely dispersed across the spring,
occurring earlier in the colder winter/spring of 2008/09 (Fig. 1).
•When plotted against accumulated thermal chilling time i.e. degree days when
the temperature was below 5°C, seeds from both years respond the same (Fig. 2).
•Results indicate that dormancy is broken in this summer annual by the
accumulated effects of low temperature..
Fig. 3: Sensitivity of Sinapis arvensis to 10 mM nitrate at 20°C.
Fig. 4: Temperature map of thermogradient table
15
40
30
20
30 C
Hot
Cold
10
10
20
5
0
Nov/07 Feb/08 May/08 Aug/08 Nov/08 Feb/09 May/09 Aug/09 Nov/09
0
0
30 C
0C
Cold
Exhumation date
Fig. 5: Germination response to alternating temperature of seeds exhumed from the field after 3, 12, 15 and 24 months
08harvest 3 predicted
08 harvest 3observed
08 harvest 12 observed
0.8
0.6
0.4
0.2
5
10
0.8
0.6
0.4
0.2
0.0
30
25
20
20
25
10
15
20
25
1.0
0.8
0.6
0.2
0.0
0.4
0.2
0.0
30
5
10
8
6
4
2
2
4
30
10
8
6
6
8
10
4
2
2
4
6
8
10
0.2
15
15
10 5
0.8
1.0
10
15
0.6
0.8
5
20
0.4
0.6
10 5
25
0.0
0.4
15
1.0
1.0
20
Observed germination 24 months
25
30
1.0
07 harvest 24 predicted
30
10
0.8
0.8
8
0.6
0.6
6
0.4
0.4
4
0.2
0.2
2
0.0
0.0
2
1.0
4
0.8
6
0.6
8
0.4
10
0.2
10 5
30
25
20
15
10 5
5
10
15
20
25
30
Predicted germination 15 months
15
07 harvest 24 observed
20
07 harvest 15 predicted
0.0
Predicted germination 12 months
25
07 harvest 15 observed
10
8
6
4
2
2
4
6
8
10
Observed germination 15 months
08 harvest 12 predicted
Observed germination 12 months
1.0
Predicted germination 3 months
30
Observed germination 3 months
80
Germination in the soil 2008
Germination at 5°C in 2009
G ermination in the soil 2008
G ermination at 5°C in 2009
60
Germination (%)
Germination(%)
60
40
40
20
20
0
0
Oct
Nov
Dec
Jan
Feb
M ar
Apr
M ay
0
40
20
0
Nov/07
Jan/08
Mar/08
May/08
Jul/08
Sep/08
80
100
120
140
160
180
200
Predicted germination 24 months
Fig. 7: Sensitivity of Thlaspi arvense to 10 mM nitrate at 20°C.
Nov/08
25
100
20°C Control
20°C + Nitrate
20
80
Germination (%)
Germination (%)
60
60
Sinapis arvensis dormancy cycling
• Over two years in the field, sensitivity to nitrate at constant temperature occurred
predominantly from late summer to early winter. (Fig. 3). In the second year an increase in
sensitivity was also seen in the spring. This change in sensitivity is influenced by increasing
soil temperature and moisture.
• Sinapis is more responsive to alternating than to constant temperatures in the absence of
nitrate.
• We have used a thermogradient table to map this alternating thermal response. The table
has a temperature range of 0-30°C and amplitude of up to 20°C, in a 12 hour diurnal cycle
under continuous light (Fig. 4).
• Seeds were buried in the field for up to 2 years. They were exhumed at monthly intervals
and their germination response to alternating temperature determined.
• The results are being analyzed to model the thermal response of Sinapis in the field in
conjunction with comprehensive soil moisture and temperature data.
• Initial analysis of the total proportion germinated across the range of temperature
combinations (Fig. 4) for each field retrieval used a Generalised Linear Model assuming a
binomial error distribution and logit link function.
• Models including the additive effects of mean temperature and temperature amplitude were
better able to describe the observed responses than those including the additive effects of the
temperature in each 12-hour period.
• Figure 5 shows comparisons of the observed and predicted thermal response of seeds
exhumed after 3, 12, 15 & 24 months in the field. Further analysis will assess how fitted
model parameters might be related to field conditions prior to each retrieval, as well as the
effects of temperature combinations on the rate of germination.
• Over a yearly cycle, the germination of exhumed seeds decreased progressively at both
lower temperatures and amplitudes in the summer months. However, in late autumn to spring
exhumed seeds germinated in a widening range of both temperature and amplitude. This
response was also higher in exhumed seed experiencing a second year in the field.
Thlapsi arvense dormancy cycling
• There was little or no germination at 5-10°C (Fig. 6). Sensitivity abovethis temperature
declined rapidly in winter (07/08) before increasing in early summer.
• At 20°C, nitrate (10 mM) increased germination above the control (Fig. 7).
• Soil moisture and temperature indicate minimal after-ripening occurred in a wet summer.
• T. arvense exhibits winter annual behavior with dormancy declining from mid -summer
probably due to after-ripening, the degree of which is dictated by the soil environment.
• The influence of nitrate increases the range of field temperatures at which germination
will occur.
5°C
10°C
15°C
20°C
25°C
80
40
Chilling degree days (°C days -1 )
Fig 6: Thermal response to constant temperature of exhumed Thlapsi arvense
100
20
Exhumation date
15
60
30
20
40
10
20
5
10
0
0
0
Nov/07
Jan/08
Mar/08
Exhumation date
Discussion
•These closely related species have adapted to respond very differently to the same
soil environment, resulting in different seedling emergence patterns. Finch-Savage
and Leubner-Metzger (2006) suggest that adaptation has taken place on a theme
rather than via fundamentally different paths. One longer-term aim for this work is
to investigate this hypothesis.
•Changes in nitrate sensitivity suggest soil nitrate status to be an under appreciated
driving force in expanding the window for seedling emergence in disturbed soils.
Especially, when soil conditions reduce the effective after-ripening time.
•Dormancy cycling is also seen to be highly sensitive to changing soil moisture and
temperature, which are seen to have dramatic effects on dormancy. Indicating that
dormant seeds are adept at sensing and responding to the soil environmental.
40
May/08
Jul/08
Sep/08
Soil MC at 5 cm (%)
Germination (%)
20
60
Hot
Fig. 2: Alliaria germination in relation to the accumulation of thermal chilling time
80
Soil temperature at 5 cm (°C)
25
80
30 C
40
Soil MC at 5 cm (%)
20°C Control
20°C + Nitrate
Soil temperature 5 cm (C)
30
100
Fig. 1: Alliaria germination in the soil (2008) and at 5°C in the dark follow ing
exhum ation (2009)
Nov/08
Exhumation date
Future work
• A thermal response model is being developed for S. arvensis and further
models are being developed for Alliaria and Thlapsi for incorporation into our
seed germination and seedling emergence simulation
(http://www2.warwick.ac.uk/fac/sci/whri/research/seedscience/simulation/).
• Further field germination/emergence experiments under simulated climate
change (global warming) conditions are underway. These will asses the effect
of cold and warm winters, timing of soil disturbance and fertilizer application at
agriculturally relevant times in the farming year. The responses will be
incorporated into our simulation.
Reference
Finch-Savage and Leubner-Metzger (2006) New Phytologist 171: 501-523