Seed Science Research (1998) 8, 29–38
29
Variation in seed dormancy among mother plants, populations
and years of seed collection
L. Andersson* and P. Milberg
Department of Crop Production Science, SLU, Box 7043, S-750 07 Uppsala, Sweden
Abstract
Variation in dormancy level was tested in seeds of four
species, each collected from three populations in 1994
and 1995 (experiment 1). Germination was tested in light
and darkness on recently-harvested seeds and on those
after-ripened in dry storage for one year. In addition,
seeds from each of eight individual plants within each of
eight populations were tested for germination when
recently harvested and after warm stratification or cold
stratification followed by a drying period (experiment 2).
Seeds from the two years differed in dormancy level in
Silene noctiflora, Sinapis arvensis and Spergula
arvensis. Germination percentage differed significantly
among populations in Sinapis arvensis and Spergula
arvensis in both experiments and in Thlaspi arvense in
experiment 2. Furthermore, dormancy level in seeds
from different mother plants also varied in the three
species tested in experiment 2. Variations at the three
levels tested (year, population and mother plant) indicate
that these species have a random pattern of variation in
dormancy level. It is concluded that variation in seed
dormancy among mother plants, populations and years
must be taken into account when testing the germination
characteristics of a species and also when attempting to
model weed seed bank dynamics.
Keywords: dormancy variation, population, Silene
noctiflora, Sinapis arvensis, Spergula arvensis, Thlaspi
arvense, year.
Introduction
Primary seed dormancy is a characteristic of plant
species that postpones germination until periods
favourable for seedling growth. However, the
intensity of dormancy is far from fixed, and it shows
high degrees of intraspecific variation at several
*Correspondence
Fax: +46-18 67 2909
E-mail: Lars.Andersson@vo.slu.se
levels. Variation in dormancy level among
populations is a well-known phenomenon (Frost and
Cavers, 1975; Miller et al., 1976; Paterson et al., 1976;
Naylor and Abdalla, 1982; Drennan and Bain, 1987;
Evans and Cabin, 1995; Milberg et al., 1996b; Schütz
and Milberg, 1997; Andersson et al., 1997).
Furthermore, differences in dormancy level between
individual plants have been shown in several species
(Strand, 1989; Pérez-García, 1993; Philippi, 1993;
Milberg and Andersson, 1994; Evans and Cabin, 1995;
Meyer et al., 1995). Germination also differs between
seeds collected in different years (Beckstead et al.,
1996).
The level of dormancy in seeds is determined by
several factors apart from genetic origin, such as
maternal environment during maturation, age of the
mother plant during maturation and position of the
seed on the plant (Fenner, 1991; Gutterman, 1992). In
addition, the different conditions during natural
stratification cause a more or less constant change in
dormancy level (Vleeshouwers et al., 1995). These
factors often account for a great deal of the variation
in germination percentage among populations, and
also for the differences in germination between seeds
harvested in different years. This variation has
important
consequences
for
the
ecological
interpretations of germination studies (especially
those conducted on single populations), for the
interpretation of dormancy in populations as a
genetic adaptation to local environments, and for
attempts to model seed dynamics.
The aim of this study was to document the
variability in seed dormancy at different levels in
wild populations of some annual weed species. Two
experiments were carried out:
(1) Germination percentages of recently harvested
seeds of four species collected from three sites
in two consecutive years were compared. We
also determined if differences remained after
an extended period of after-ripening of the
seeds.
(2) For three species, we sampled seeds from each
of eight individual plants at each of eight sites
30
L. Andersson and P. Milberg
Table 1. Collection site, habitat and collection date for seeds in experiment 1. All sites were in the province of Östergötland
except Ultuna (Uppland), Virå (Södermanland) and Alnarp (Skåne)
Collection date
Population
No.
Province of
collection site
Habitat type
1*
2
3
Marstad
Hogstad
Kvarnstugan
Field edge
Field edge
Horticultural field
1
2
3
Renstad
Linköping
Ultuna
1
2
3
1
2
3
1994
1995
Silene noctiflora
3 Aug.
5 Aug.
8 Aug.
15 Aug.
15 Aug.
16 Aug.
Fallow
Field edge
Field edge
8 Aug.
9 Aug.
12 Aug.
16 Aug.
14 Aug.
22 Aug.
Virå
Åby
Alnarp
Fallow
Horticultural field
Agricultural field
1 Aug.
1 Aug.
30 July
14 Aug.
18 Aug.
8 Aug.
Åby
Boxholm
Heda
Horticultural field
Horticultural field
Fallow
1 Aug.
2 Aug.
2 Aug.
14 Aug.
16 Aug.
16 Aug.
Sinapis arvensis
Spergula arvensis
Thlaspi arvense
* The two seed batches were not collected from the same field, but from nearby fields separated by 800 m.
to test for variation in seed dormancy within
and between sites.
The species were chosen because they are known to
exhibit large variations in germination (Milberg and
Andersson, 1994; Milberg et al., 1996b; Andersson et
al., 1997).
Materials and methods
Experiment 1. Variation between populations and
years
Seeds of Sinapis arvensis L., Silene noctiflora L., Spergula
arvensis L. and Thlaspi arvense L. were collected from
each of three sites in southern Sweden in the periods
30 July–12 August 1994 and 8–22 August 1995 (Table
1). The sites were separated by >8 km. Seed lots from
the different sites are hereafter referred to as different
populations.
Seeds were dried at room temperature (c. 22°C) for
c. 10 days, then counted into four batches of c. 100
each. Seeds were kept in open plastic containers at
room temperature and 30–50% RH until the start of
the germination tests on 29–30 September 1994 and 2
October 1995. The 1995 test also included seeds
collected in 1994 and dry-stored at c. 22°C.
Each batch of c. 100 seeds was placed on filter
paper (two sheets of Munktell 1003, 90 mm diameter)
and wetted with deionized water (4.0 ml) in a Petri
dish (90 mm diameter). Two of each group of four
dishes for each seed population were sealed with
Parafilm, while the other two were wrapped
immediately in aluminium foil. Petri dishes were
placed in two piles, with the dark treatment at the
bottom, and kept in a room with a diurnally
fluctuating temperature regime of 4 ± 2°C for 9 h and
18.5 ± 2°C for 11 h, with a 4-h transition between
them. Light exposure (14 h) coincided with the
period with the higher temperature and was
provided by cool white fluorescent tubes (OSRAM
L65W/20R, PAR 10 ± 2 mmol m–2 s–1; ratio R/FR = 8).
Seeds germinated in the light treatment were
counted and removed every second to fourth day
during the first two weeks and thereafter once a
week. Dishes wrapped in aluminium foil were not
opened until after 23–28 days (dishes for the same
species were opened on the same day), when the
experiment was terminated. When terminating the
light treatment, the remaining seeds were identified
as “dead” or “alive”. For dishes wrapped in
aluminium foil, seedlings were counted and the
remaining seeds inspected as above.
Experiment 2. Variation among seeds from different
individuals and from different populations
Seeds of Sinapis arvensis, Spergula arvensis and Thlaspi
arvense were collected from each of eight individual
plants at each of eight sites on 14–18 August 1995
(Table 2). They were treated as above, and divided
into six similar-sized groups, which were allocated to
Petri dishes as above. Three dishes (replicates) with
seeds from each seed batch, i.e. from individual
plants, were sealed with Parafilm and immediately
used in a germination experiment.
Variation in seed dormancy
31
Table 2. Collection site and habitat type for seeds in experiment 2, which were collected
in early August 1995. All sites were in the province of Östergötland except Sandhem, S.
Stråken and Mullsjö (Västergötland), and Virå (Södermanland)
Population
No.
Sinapis arvensis
1
2
3
4
5
6
7
8
Spergula arvensis
1
2
3
4
5
6
7
8
Thlaspi arvense
1
2
3
4
5
6
7
8
Collection site
Habitat type
Säby, Dagsberg
Beatelund
Vårdsberg
Domaregården
Viby
Lottstad, Normlösa
Vallerstad, Solhem
Karleby, Väderstad
Oilseed crop
Waste land
Oilseed crop
Oilseed crop
Oilseed crop
Oilseed crop
Oilseed crop
Oilseed crop
Åby
Hattorp
Skrukeby
Långtorp
Sandhem
S.Stråken
Mullsjö
Virå
Horticultural field
Fallow
Fallow
Horticultural field
Field edge
Waste land
Field edge
Fallow
Säby, Dagsberg
Beatelund
Viby
Boxholm, Dyrshagen
Renstad, NO
Ask, Åsnestad
Skrukeby, Hulje
Vida Vättern
Oilseed crop
Waste land
Oilseed crop
Horticultural field
Oilseed crop
Oilseed crop
Oilseed crop
Waste land
A set of three other sealed dishes per batch were
moist-stratified in darkness. Thlaspi arvense and
Sinapis arvensis were stored at 4.2 ± 0.2°C and
Spergula arvensis at 27.5 ± 1°C for 14 days, after
which a germination experiment began. After 10
days the germination experiment was terminated
and the remaining seeds of Thlaspi arvense and
Sinapis arvensis were again moist-cold-stratified (7
weeks), followed by a second germination period of
10 days. Thereafter, ungerminated seeds were dried
(dish lids removed for 15 days; ambient RH was
15–30%). Then the dishes were re-wetted (with 4.0
ml water) and a third germination period was
begun, lasting for c. 10 days. The number of seeds
germinated in each dish was summed over these
three experiments.
Germination experiments were conducted in a
room with fluctuating temperature: 8 ± 3°C for 8 h
and 19 ± 2°C for 15 h, with a 1-h transition between
the high and low temperatures. Light exposure (14 h)
coincided with the higher temperature period and
was provided by cool white fluorescent tubes
(OSRAM L65W/20R, PAR 6–9 and 3–4 mmol m–2 s–1
for Thlaspi arvense/Spergula arvensis and Sinapis
arvensis, respectively; R/FR ratio = 9–12). Germinated
seeds were counted and removed every third day.
Germination tests were not designed to achieve
maximum germination as this would disguise
possible differences in germinability among seed
batches. The aim of the pretreatments in experiment 2
was to weaken dormancy enough to make
comparisons possible. In previous work (Milberg and
Andersson, unpublished data) we found that a cold
moist stratification increased germination in T. arvense
and Sinapis arvensis and decreased germination in
Spergula arvensis. For this reason the two former
species were cold-stratified while Spergula arvensis
was warm-stratified. Our findings were not
unexpected since T. arvense mainly occurs in spring
crops in Sweden (Hallgren, 1996) and Spergula
arvensis is difficult to classify as a winter or summer
annual (Bouwmeester and Karssen, 1993). The
method of successive pretreatments of the seeds was
chosen because it enabled us to adjust the level of
pretreatment to the dormancy level of the species.
Thus, it was in most cases possible to achieve the
germination percentages at which differences were
revealed.
32
L. Andersson and P. Milberg
Figure 1. Germination (proportion) of seeds collected at the same site in 1994 (94:1 and 94:2) and
in 1995. Recently harvested (94:1 and 95) and one-year-old dry-stored (94:2) seeds were tested for
germination in light (stippled columns) and darkness (striped columns).
Statistics
In both experiments, differences in germination
percentage were tested by categorical data analysis,
for each species separately, of the number of viable
seeds germinating, using the CATMOD procedure
(SAS, 1989). This model is commonly used for
analysing categorical data, as in our study, for (i) a
design with one response variable and three factors
and (ii) a nested design with one response variable
and three factors. In experiment 1, differences
between treatments were tested by modelling the
number of germinated seeds as a function of year,
light conditions and population and the interaction
between these three factors (see Freeman, 1987 for
reference). In addition, the effect of after-ripening for
seeds collected in 1994 was tested by using the
function of storage, light conditions, populations and
interactions. For experiment 2, the number of
germinated seeds was modelled as a function of
individuals
within
populations,
populations,
stratification and the interaction between these
factors. Due to limitations of the model, values of zero
or full germination were set to 0.001 and 0.999,
respectively.
Results
Experiment 1
Silene noctiflora germinated to a very high percentage
in light in eight of the nine possible cases (3 tests 3 3
populations). There were, however, large variations in
Variation in seed dormancy
Table 3. Maximum-likelihood ANOVA table for germination of seeds from three populations and two years, tested in
light and darkness
Silene noctiflora
A: Population
B: Year
C: Light treatment
A3B
B3C
A3C
Sinapis arvensis
A: Population
B: Year
C: Light treatment
A3B
B3C
A3C
Spergula arvensis
A: Population
B: Year
C: Light treatment
A3B
B3C
A3C
Thlaspi arvense
A: Population
B: Year
C: Light treatment
A3B
B3C
A3C
DF
Chi-square
Prob
2
1
1
2
1
2
1.85
11.97
48.45
13.93
0.57
4.18
0.3959
0.0005
0.0000
0.0009
0.4516
0.1234
2
1
1
2
1
2
95.32
40.86
0.02
112.37
4.04
1.68
0.0000
0.0000
0.8931
0.0000
0.0444
0.4325
2
1
1
2
1
2
27.51
27.72
6.35
428.77
0.04
1.85
0.0000
0.0000
0.0117
0.0000
0.8475
0.3960
2
1
1
2
1
2
0.40
3.62
1.32
0.40
1.32
0.25
0.8179
0.0570
0.2501
0.8179
0.2501
0.8826
33
Table 4. Maximum-likelihood ANOVA table for germination of recently harvested and one-year-after-ripened seeds
from three populations tested in light and darkness
Silene noctiflora
A: Population
B: Storage
C: Light treatment
A3B
B3C
A3C
Sinapis arvensis
A: Population
B: Storage
C: Light treatment
A3B
B3C
A3C
Spergula arvensis
A: Population
B: Storage
C: Light treatment
A3B
B3C
A3C
Thlaspi arvense
A: Population
B: Storage
C: Light treatment
A3B
B3C
A3C
DF
Chi-square
Prob
2
1
1
2
1
2
0.61
2.93
19.38
2.73
4.53
4.12
0.7357
0.0868
0.0000
0.2549
0.0333
0.1274
2
1
1
2
1
2
248.78
88.75
85.73
3.26
5.84
12.19
0.0000
0.0000
0.0000
0.1957
0.0156
0.0023
2
1
1
2
1
2
280.18
15.73
7.13
7.47
0.47
11.91
0.0000
0.0001
0.0076
0.0238
0.4924
0.0026
2
1
1
2
1
2
0.30
0.49
0.49
0.30
0.49
0.30
0.8598
0.4859
0.4859
0.8598
0.4859
0.8598
DF = degrees of freedom.
DF = degrees of freedom.
dark germination, which accounted for most of the
significant differences between years (Fig. 1; Table 3).
Seeds of Sinapis arvensis collected in 1994
germinated to a higher percentage than those
collected in 1995, with significant differences between
populations but not between light treatments.
Germination response to the different light conditions
was somewhat inconsistent, as indicated by the
significant interactions of light 3 population and of
light 3 year (Fig. 1; Table 3).
Seeds of one of the populations of Spergula arvensis
germinated to a higher percentage in light than in
darkness. Also, there were significant differences in
germination
between
seeds
from
different
populations as well as between seeds collected in
different years. The latter difference was, however, far
from consistent, as indicated by the population 3
year interaction (Fig. 1; Table 3).
Thlaspi arvense germinated to a very low
percentage in all seed batches under both light
regimes and showed no differences in germination
percentage between seeds from different populations
and years (Fig. 1; Table 3).
After one year of dry storage, germination of
Spergula arvensis and Sinapis arvensis had increased
significantly, while that of Silene noctiflora and Thlaspi
arvense had not (Fig. 1, Table 4).
Experiment 2
For all three species, germination percentage differed
significantly between seeds from individual plants
within populations as well as between seeds from
different populations. Cold-stratified seeds of Sinapis
arvensis and Thlaspi arvense germinated to higher
percentages than recently harvested seeds of these
two species. In addition, as indicated by the
interactions, stratification affected the differences
between seeds from different individual plants and
populations (Figs 2, 3, 4; Table 5).
Discussion
Differences between populations have sometimes
been explained on a genetic basis, with the level of
34
L. Andersson and P. Milberg
Figure 2. Germination (proportion) of Sinapis arvensis seeds from eight individual plants
from each of eight populations. Dotted lines indicate mean germination within
populations.
dormancy being related to the mean precipitation in
the habitat (Philippi, 1993), “false breaks” of dry
periods (Paterson et al., 1976), altitude (Dorne, 1981)
or winter temperature (Meyer and Monsen, 1992;
Meyer and Kitchen, 1994; Meyer et al., 1995). The two
experiments presented here revealed large variations
in seed dormancy at all levels, i.e. between
individuals, between populations and between seeds
harvested in different years. Hence, the variation
cannot be explained explicitly as a result of genetic
adaptation to a local environment; in which case,
variation between years and within a population
would have been small. Since seeds were collected
within a restricted area with small local variation in
climate, it is reasonable to assume that variations
between populations are not an expression of
different ecotypes. In most cases this assumption is
also supported by the significant interactions between
years and populations. It is likely that the impact of
the mother plant environment (e.g. Gutterman, 1990;
Fenner, 1991; Milberg and Andersson, 1994), accounts
for most of the variation in dormancy level. Factors
such as precipitation (Strand, 1989; Philippi, 1993),
soil moisture (Peters, 1982) and temperature (Peters,
1982; Strand, 1989) during seed maturation, or the
nutritional status of the mother plant (Fawcett and
Variation in seed dormancy
35
Spergula arvensis, fresh
Spergula arvensis, stratified
Figure 3. Germination (proportion) of Spergula arvensis seeds from eight individual
plants from each of eight populations. Dotted lines indicate mean germination within
populations.
Slife, 1978; Watson and Watson, 1982) are known to
affect dormancy. Also, germination characteristics
differ due to seed mass (Milberg et al., 1996a) and size
of the mother plant (Philippi, 1993), both of which are
at least in part determined by the above factors.
Even though plants growing side by side are
subjected to the same environmental factors, it is still
possible that weather conditions contributed to the
large differences in germination percentage between
seeds from individual plants (in some cases ranging
from <3% to >90%). Small differences in development
time might cause seeds to mature at different
temperatures or under different humidity conditions.
Also, access to water and nutrients might vary even
within a small area due, for example, to competition,
causing differences in dormancy between seeds from
individual plants.
Variation in dormancy between years is
sometimes described as an adaption of populations
to unpredictable environments. Beckstead et al.
(1996) suggested that populations from more
favourable but somewhat unpredictable environments show more variation between years in
germination characteristics than those from more
extreme yet predictable environments. Whether this
is to be attributed to an ecological adaptation or
36
L. Andersson and P. Milberg
Thlaspi arvense, fresh
Thlaspi arvense, stratified
Figure 4. Germination (proportion) of Thlaspi arvense seeds from eight individual plants
from each of eight populations. Dotted lines indicate mean germination within
populations.
differences in weather is unclear. In either case, for
the weed species tested here variation in seed
dormancy means a better chance for survival of
populations in an environment that is both harsh
and unpredictable.
Variation among individuals will reduce the risk
of all seedlings being subjected to poor growing
conditions and help avoid sib competition. Also,
the age distribution of the seeds will increase with
an increased variation in dormancy level, thus
enhancing the genetic variation within the
population. This might partly explain the large
genetic variation in populations from highly
disturbed soils proposed by Bosbach et al.
(1982).
It may be argued that germination tests on
recently harvested seeds are of little value when
discussing field germination. Most seeds are likely to
endure at least a couple of months in soil before
germination is induced, and the effect of stratification
might in that case obviate the differences in dormancy
level. However, in this experiment seeds not only
differed in level of primary dormancy, but differences
seemed to remain, and were in some cases larger, after
stratification or after-ripening. This agrees with
observations from earlier work (unpublished data)
Variation in seed dormancy
Table 5. Maximum-likelihood ANOVA table for germination between individual plants within populations, between
populations, between pretreatments, and interactions for
pretreatment 3 individuals within populations and
pretreatment 3 populations
37
should be drawn unless tests include seeds from
different years and/or populations. In addition,
when modelling dynamics of weed species it is
vital to recognize that many of the factors that
determine
dormancy
level
are
largely
unpredictable. Thus, these factors have to be
considered to make reliable predictions of seed
bank development and seedling emergence
possible.
DF
Chi-square
Prob
Sinapis arvensis
A: Individuals
B: Populations
C: Stratification
A3C
B3C
56
7
1
56
7
250.86
322.20
846.80
114.38
61.44
0.0000
0.0000
0.0000
0.0000
0.0000
Spergula arvensis
A: Individuals
B: Populations
C: Stratification
A3C
B3C
56
7
1
56
7
786.84
1431.16
0.02
89.53
23.49
0.0000
0.0000
0.8998
0.0029
0.0014
This study was supported by the Swedish Council for
Forestry and Agricultural Research. We are grateful to
Ângela Noronha and Ya Schang for technical
assistance.
Thlaspi arvense
A: Individuals
B: Populations
C: Stratification
A3C
B3C
56
7
1
56
7
241.42
307.70
97.12
127.38
165.13
0.0000
0.0000
0.0000
0.0000
0.0000
References
Acknowledgements
DF = degrees of freedom.
with seeds stratified on filter paper or in soil. That
fraction of the seeds which maintains its strong
dormancy, or has dormancy induced, will increase the
likelihood of at least some offspring surviving even
during a year that turns out to be unsuitable for seed
production.
The test conditions of the two experiments were
designed to achieve germination percentages within a
range where differences between seed batches became
visible. Had pretreatments and/or test conditions
been optimal or more sub-optimal, i.e. had
germination percentages of all seed batches been close
to 100% or 0%, it is very likely that the differences
described above would have been smaller or
negligible. Testing germination under several different
conditions would have added interesting information.
It might, for example, have revealed significant
differences in those cases where germination
percentages were close to 100% or 0% (Silene noctiflora
and Thlaspi arvense, respectively, in experiment 1). It
would not, however, have altered our conclusion that
there were large variations in dormancy level among
mother plants, populations and years of seed
collection in several cases.
The very large variation revealed in the
experiments strongly emphasizes the difficulties
involved when interpreting the results of
germination tests. We suggest that no conclusions
regarding germination characteristics of a species
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Received 25 May 1997, accepted after revision
5 September 1997
© CAB INTERNATIONAL, 1998