CSIRO PUBLISHING
Crop & Pasture Science
http://dx.doi.org/10.1071/CP16279
Response of Digitaria insularis seed germination
to environmental factors
F. H. Oreja A,B, E. B. de la Fuente A, and M. E. Fernandez-Duvivier A
A
Department of Vegetal Production, Faculty of Agronomy, University of Buenos Aires, Avenida San Martín 4453,
C1417DSE, Buenos Aires, Argentina.
B
Corresponding author. Email: [email protected]
Abstract. Digitaria insularis (sourgrass) is a weed problem emerging in importance in agricultural fields from the north
of Argentina and has recently been reported as resistant to glyphosate. Understanding the germination of local biotypes of
D. insularis could help to reduce invasion and improve the long-term management strategies for this weed. The objective
of this work was to study the effect of environmental factors on germination of D. insularis seeds from two different
populations of Argentina. Three experiments were performed in germination chambers by using recently dispersed seeds.
Seeds with or without pre-chilling treatments had 95% germination, suggesting the absence of dormancy in freshly
harvested seed. Germination at constant temperature of 258C was ~55% lower than germination at fluctuating temperature
of 208358C. At constant 258C, germination was higher for seeds from Santiago del Estero than seeds from Córdoba, and
as the number of hydration–dehydration cycles increased. Germination was reduced with exposure to far-red light for 1 h.
Any crop management decision that reduces soil thermal fluctuations and/or far-red : red ratio (such as stubble or cover
crops) could reduce seedling field emergence for this species.
Additional keywords: environmental factors, grass weed, management decisions, seedling emergence, seed origin.
Received 2 August 2016, accepted 5 January 2017, published online 31 January 2017
Introduction
Digitaria insularis (L) Mez ex Ekman (sourgrass) is a native
species of tropical and subtropical America, distributed from the
southern United States to the centre of Argentina (Pyon et al.
1977). It has been reported as a glyphosate-resistant weed in
soybean (Glycine max L.) and maize (Zea mays L.) crop fields
from Brazil, Paraguay and Argentina (Heap 2016). In Argentina,
D. insularis was reported as a problem in agricultural fields
since 2013 (Papa and Tuesca 2015). The mechanism involved
in glyphosate resistance of the Brazilian biotype is related to a
limited herbicide translocation, rapid degradation of glyphosate,
and changes by mutation of two amino acids in the enzyme
EPSPS (de Carvalho et al. 2012). This weed has a very high
infestation potential because it is able to reproduce by both seeds
and rhizomes. As in many perennial grass species, seeds are
the principal means of introduction into new areas, whereas
rhizomes may be the primary means of dispersal within the
field (Mitskas et al. 2003). In particular, the high production
of small, easily dispersed seeds with a high germination
percentage allows the colonisation of new sites (de Carvalho
et al. 2011). In tropical climates, the seedbank of D. insularis
is renewed continuously, owing to plants producing seeds
throughout the year and temperatures allowing seedling
emergence at any time (Lacerda 2003). There is limited
information about this species in temperate climates where low
temperatures during winter could affect seed production and
Journal compilation CSIRO 2017
seedling emergence. Therefore, a better understanding of the
germination ecology of local biotypes of D. insularis will help
to improve the long-term management of this weed in temperate
croplands favouring the reduction of dispersal and invasion.
Germination timing and extent is related to seed dormancy.
Dormancy release dictates the time of seedling emergence
and can influence the number of plants that establish. Seed
dormancy is a common characteristic of most grassy weeds,
acquired during evolution by selection for the ability to
survive in unfavourable environments. However, seeds of
some species never experience a deep dormancy, or the range
of environmental conditions permissive of germination is very
wide (Benech-Arnold et al. 2000). The number of plant species
with seed dormancy tends to increase with geographical distance
from the equator (Bewley et al. 2013). Nevertheless, even in
tropical environments, especially those with cyclic wet and dry
seasons, seed dormancy is widespread (Garwood 1989).
Environmental factors affecting seed dormancy could be
divided into dormancy-level-regulating factors (i.e. temperature
and moisture) and dormancy-terminating factors or germinationinitiating factors (i.e. fluctuating temperatures, light flux density
and quality, nitrate concentration, etc.) (Benech-Arnold et al.
2000; Forcella et al. 2000). Dormancy-level-regulating factors
are related to seed germination during the favourable season for
plant growth and reproduction (Bewley et al. 2013), whereas
dormancy-terminating factors or germination-initiating factors
www.publish.csiro.au/journals/cp
B
Crop & Pasture Science
F. H. Oreja et al.
are signals of canopy gaps and depth of seed burial, thus avoiding
futile germination (Fenner 1980; Thompson and Grime 1983;
Batlla et al. 2000; Kruk et al. 2006). The presence of a crop
canopy or abundant stubble could reduce terminating factors such
as fluctuating temperatures and hydration–dehydration cycles of
soil surface, and the red : far-red (R : FR) ratio of the light reaching
the soil surface. Soil water potential in the first few centimetres of
soil is higher under a crop canopy than without crop canopy
(Norsworthy and Oliveira 2007). Therefore, before crop-canopy
coverage, seeds on the soil surface would be exposed to
more hydration–dehydration cycles and more susceptible to
germination in the field. After crop canopy closure,
germination is lower for those seeds requiring fluctuating
temperatures (Benech-Arnold et al. 1988; Huarte and BenechArnold 2003), red-enriched light (Bewley et al. 2013) or
fluctuations of the soil-water content (Downs and Cavers
2000) to terminate seed dormancy. This, in turn, can affect the
dormancy status of the buried seeds (Batlla and Benech-Arnold
2006), reducing the germination percentage (Martínez-Ghersa
et al. 1997) and rate (Lush et al. 1984; Kagaya et al. 2005).
Accordingly, management decisions could be decided in order
to modify soil factors and reduce or delay seed germination
and, thus, seedling emergence on the field.
Information about seed-germination response to environmental
factors is scarce for D. insularis. Most studies come from Brazil
and show that seeds germinate better at fluctuating temperatures
of 208308C, 208358C and 158358C (cycle 16 h–8 h) than
at constant temperatures (Mondo et al. 2010; de Mendonça
et al. 2014). Some authors reported that light is not required
for germination of D. insularis (Mondo et al. 2010), whereas
others reported the opposite (de Mendonça et al. 2014). Beside the
differences among authors, the effect of hydration–dehydration
cycles and light quality on seed germination has not been
studied. Moreover, local information on this species is lacking
in Argentina, and it is known that different biotypes of a species
can differ with regard to germination rate (Kaya Altop and
Mennan 2011). These differences could imply differences
in seedling emergence in the field and, therefore, different
management decisions to be considered.
The objective of this work was to determine the effect
of temperature, light–darkness, light quality and hydration–
dehydration cycles on the germination of D. insularis seeds
from different origins. To achieve this objective, experiments
on germination chambers were made.
Three experiments were carried out in germination chambers
at the Faculty of Agronomy of Buenos Aires University,
Argentina. For all of them, the experimental units consisted of
36 seeds per replicate placed in a 9-cm-diameter Petri dish with
two filter papers (No. 1; Double Rings, Buenos Aires, Argentina),
moistened with 4 mL of deionised water, sealed with Parafilm
to maintain moisture and then placed in a germination chamber.
Germination (>2 mm radicle protrusion through the seed coat)
was recorded at regular intervals until no further seeds
germinated. At the end of each experimental incubation
period, viability of non-germinated seeds was tested with a 1%
tetrazolium (2.3.5-triphenyl-2H-tetrazolium chloride) solution
(International Seed Testing Association 1999).
Materials and methods
Seed collection and germination test
The dispersal unit of D. insularis is usually called a ‘seed’ but it
is actually the spikelet composed of the caryopsis enclosed
within the lemma and palea and all of these enclosed by the
glumes. Hereafter, ‘spikelet’ will be referred to as seed unless
otherwise specified. Mature seeds were collected during April
2014, the season of natural dispersal, from two sites located in
northern central Argentina, Villa María del Río Seco, Córdoba
province (298550 S, 638450 W), and Zanjón, Santiago del Estero
province (278550 S, 648150 W). After collection, seeds were bulked
and stored in paper bags at room temperature (208258C) until
they were used in the experiments during May 2014.
Experiment 3: temperature and light quality
To test the effect of temperature and light quality on germination,
a completely randomised factorial experiment with five
replicates was performed. Seeds from the two sites (Santiago
del Estero and Córdoba) were placed under two temperature
regimes (constant 258C and fluctuating 208358C for 8 h–16 h)
and four light regimes (60R, 60 min red light; 60FR, 60 min
far-red light; 30FR, 30 min far-red light; R-FR, a cycle of
60 min red light–30 min darkness–30 min far-red light). After
light treatments, seeds were incubated at 208358C (cycle
8 h–6 h) in darkness for 20 days. Petri dishes were prepared
initially in light but light was excluded by wrapping them in
black polypropylene bags at 258C, in the dark, for 24 h until
Experiment 1: pre-chilling
In order to evaluate the dormancy level of D. insularis, a
completely randomised experiment with five replicates was
performed with recently dispersed seeds from both populations.
Treatments were: (i) seeds pre-chilled at 58C in darkness for
15 days and then transferred to fluctuating temperature of
208358C (cycle 8 h–16 h) with light; and (ii) seeds without
pre-chilling placed at fluctuating temperature of 208358C
(cycle of 8 h–16 h) with light.
Experiment 2: temperature, light or darkness,
and hydration–dehydration cycles
To test the effects of temperature and light or darkness and
hydration–dehydration cycles on germination, a completely
randomised factorial experiment with five replicates was
performed. Seeds from the two sites (Santiago del Estero and
Córdoba) were placed under two temperature regimes (258C
and fluctuating 208358C for 8 h–16 h), two light conditions
(continuous light and continuous darkness) and three hydration
regimes (continuous hydration, one cycle of 3 h hydration–8 h
dehydration, or two cycles). For treatments in darkness, Petri
dishes were prepared initially in light, but light was excluded
by wrapping the dishes in black polypropylene bags before
placing them in the germination chambers. Hydration time
was estimated by hydrating a group of 100 seeds at 258C until
reaching constant weight. Dehydration time was estimated
by removing the lid of the Petri dishes and allowing the seeds
to dry to their original weight, in natural light at 258C (Vincent
and Cavers 1978). Once seeds were exposed to different
hydration–dehydration cycles, Petri dishes were transferred to
the germination chambers.
Digitaria insularis germination
exposure to the different light regimes, when the Petri dishes were
unwrapped, exposed to the light, wrapped again and placed in the
germination chambers. Red light (calculated proportion of
phytochrome as Pfr (Pfr : Pr) 87%, 35 mmol m–2 s–1) was
provided by red fluorescent tubes (Philips 40/15; Philips,
Amsterdam). Far-red light (calculated proportion of
phytochrome as Pfr (Pfr : Pr) 2,7%, 42 mmol m–2 s–1) was
provided by a 150-W incandescent reflector lamp (Phillips
R95) in combination with an 8-cm water filter and an RG9
filter (Schott, Mainz, Germany) (Scopel et al. 1991).
Statistical analyses
Analysis of variance (ANOVA) followed by Tukey’s multiple
comparison test was done using a general linear model procedure
of STATISTICA version 8 (StatSoft, Tulsa, OK, USA) to
determine significant differences at P = 0.05. When factorial
analysis showed significant interactions between main effects,
factors were analysed separately. The assumptions of the
ANOVA (random, homogenous and normal distribution of
residuals) were tested. If the assumptions of variance were not
met, cumulative germination percentages were square-rootarcsine transformed (Bartlett 1947). Adjusted means were
back-transformed for graphical presentation.
Crop & Pasture Science
Table 1. Summary of analysis of variance for germination results of
D. insularis seeds from two sites, two temperature regimes, two light
conditions and three hydration–dehydration cycles
Factor
d.f.
F
P-value
Site (S)
Temperature (T)
Light (L)
Hydration–dehydration (H)
ST
SL
SH
TL
TH
LH
STL
STH
SLH
TLH
STLH
Error
1
1
1
2
1
1
2
1
2
2
1
2
2
2
2
72
215.4
822.4
17.04
41.08
18.82
3.02
0.11
1.07
51.38
1.38
7.51
0.21
0.47
0.4
0.06
<0.0001
<0.0001
0.0001
<0.0001
<0.0001
0.0866
0.8993
0.3048
<0.0001
0.2572
0.0078
0.8077
0.6264
0.6751
0.9423
Santiago
del Estero
100
Santiago
del Estero
Córdoba
ab
bc
80
Germination (%)
No significant differences (P > 0.05) were observed between
seeds with and without pre-chilling. The final germination
percentage reached in all the treatments was >95% and the rest
of seeds that did not germinate showed 100% viability with the
tetrazolium test (data not shown).
a
Córdoba
cd
Results
Experiment 1: pre-chilling
C
d
60
e
40
f
f
20
0
20–35°C
Experiment 2: temperature, light or darkness,
and hydration–dehydration cycles
Statistically
significant
site temperature light
and
temperature hydration–dehydration cycle interactions were
observed (Table 1). Therefore, data were analysed separately.
Independent of the origin of the seeds and light treatments,
germination at constant temperature 258C was lower than
germination at fluctuating temperatures 208358C. The average
germination at 208358C was 97% for the seeds from Santiago
del Estero and 84% for the seeds from Cordoba, without
differences between light treatments. At 258C, germination
of the seeds from Santiago del Estero averaged 53% and was
significantly higher than germination of the seeds from Córdoba
at 17%. The only combination with differences among light
treatments was seeds from Santiago del Estero at 258C,
showing lower germination in light than in darkness (Fig. 1).
Independent of the seed origin, at 208358C germination
was >90% without significant differences among hydration–
dehydration cycles. By contrast, at constant 258C germination
was different among hydration–dehydration cycles, being on
average 14%, 34% and 58%, for continuous hydration, one
hydration–dehydration cycle and two cycles, respectively
(Fig. 2).
25°C
Darkness
Light
Fig. 1. Effect of temperature (fluctuating 208358C and constant 258C) and
light (light and darkness) on Digitaria insularis seed germination (%), from
two sites (Santiago del Estero and Córdoba). Values are the means and vertical
bars are standard error. Pairs of columns with the same letters are not
significantly different according to Tukey’s test (at P = 0.05).
Experiment 3: temperature and light quality
There was a significant temperature light quality interaction
(P < 0.01) (Table 2); therefore, data were analysed separated.
Independent of the seed origin, germination was higher
(P < 0.01) at alternating temperatures 208358C than at constant
258C in all light-quality treatments (Fig. 3). At constant
temperature, seeds in the 60FR treatment had significantly
reduced germination (~20% less) compared with seeds in the
60R and 30FR treatments (Fig. 3).
Discussion
The high germination percentage observed in both populations
of D. insularis under fluctuating temperatures compared with
D
Crop & Pasture Science
100
a
F. H. Oreja et al.
a
100
a
a
a
a
80
b
60
c
40
b
60
b
bc
c
40
20
d
20
Germination (%)
80
Germination (%)
a
0
60R
0
20–35°C
H
1C
60FR 30FR R-FR
20–35°C
25°C
2C
Fig. 2. Effect of temperature (fluctuating 208358C and constant 258C) and
hydration–dehydration cycles (continuous hydration, H; one cycle, 1C; and
two cycles, 2C) on Digitaria insularis seed germination (%). Values are the
means and vertical bars are standard error. Pairs of columns with the same
letters are not significantly different according to Tukey’s test (at P = 0.05).
Table 2. Summary of analysis of variance for germination results
of D. insularis seeds from two sites, two temperature regimes and four
light-quality regimes
Factor
d.f.
F
P-value
Site (S)
Temperature (T)
Light quality (LQ)
ST
S LQ
T LQ
S T LQ
Error
1
1
3
1
3
3
3
48
266.52
756.14
3.29
206.24
0.76
6.71
3.08
<0.0001
<0.0001
0.0283
<0.0001
0.5249
0.0007
0.0362
the same populations under constant temperatures suggests
that recently dispersed seeds have a low dormancy level. As is
true for many summer arable weeds, high dormancy levels are
reduced by exposure to low temperatures (Benech-Arnold et al.
2000). The ability to respond to fluctuating temperatures is
important for species where the seeds need to germinate close
to the soil surface. This is one factor aiding seeds of such species
in avoiding germination at depth (Forcella et al. 2000; Fenner
and Thompson 2005). Such species are likely to become more
prevalent under no-tillage where seeds remain at the soil
surface; however, the presence of abundant stubble can reduce
the size of temperature fluctuations at the soil surface (Teasdale
and Mohler 1993; Dahiya et al. 2007).
The ideal conditions for seed germination of D. insularis
were fluctuating temperatures 208358C (cycle 8 h–16 h).
Under these temperature conditions, light was not necessary to
stimulate seed germination. These results agree with those of
Mondo et al. (2010), which showed the highest germination
for D. insularis seeds from Brazil at 208358C and 158358C,
independent of light effect. Under fluctuating temperatures, no
differences were observed among the other factors evaluated
60R
60FR 30FR R-FR
25°C
Fig. 3. Effect of temperature (fluctuating 208358C and constant 258C)
and light-quality regime (60R, 60 min red light; 60FR, 60 min far-red light;
30FR, 30 min of FR light; R–FR, cycle of 60 min R light–30 min darkness–
30 min FR light) on Digitaria insularis seed germination (%). Values are
means and vertical bars are standard errors. Means with the same letters are
not significantly different according to Tukey’s test (at P = 0.05).
for germination (light and darkness, hydration–dehydration
cycles and light quality). It is known that changes in the
degree of dormancy comprise changes not only in temperature
requirements for germination, but also in sensitivity to the
effects of dormancy-terminating factors (Forcella et al. 2000).
Exposure of D. insularis to fluctuating temperatures not only
removed the constraints for germination, but also reduced the
sensitivity to light v. darkness, hydration–dehydration cycles
and light quality. These results explain the lack of differences
among different light or hydration–dehydration treatments for
germination under fluctuating temperatures.
Germination of seeds from Santiago del Estero was higher
at constant temperature (258C) with darkness than with light
(Fig. 1). These results contrast with those reported by de
Mendonça et al. (2014), who found a higher germination for
D. insularis seeds with light at 258C. However, no significant
differences among light treatments were observed in seeds
from Córdoba (Fig. 1), and these results agree with Mondo
et al. (2010). In addition, seeds from Santiago del Estero had
greater germination than seeds from Córdoba, independent of
the germination treatment. These differences could be due to
the growing conditions of the parent plants during seed
development, known as maternal effects (Fenner 1991), or
could be due to genetic differences between biotypes; de
Mendonça et al. (2014) found differences among D. insularis
biotypes from the same region in germination response to
different temperatures.
At constant 258C, germination increased as the number of
hydration–dehydration cycles increased (14% always hydrated,
34% with one hydration–dehydration cycle and 58% with two
cycles, Fig. 2). These results agree with Martínez-Ghersa et al.
(1997), who found that hydration–dehydration cycles increase
germination of some seeds. Hydration–dehydration cycles
could have an effect similar to other dormancy-termination
factors, and the sensitivity of seeds to this factor is diminished
by exposure to fluctuating temperatures. Most grain crops in
Argentina are cultivated by using a no-till seeding system,
Digitaria insularis germination
where weed seeds remain on the soil surface and are subject to
repeated hydration–dehydration fluctuations as a result of
evaporation cycles in the upper 5–10 cm of the soil when
stubble is scarce (Ritchie and Johnson 1990). Considering that
immediately after winter the temperature of the soil is lower
than the 208358C cycle, seedling emergence of D. insularis by
the end of winter could be strongly determined by hydration–
dehydration cycles. This could be especially important under
a no-till system with limited stubble at the beginning of spring,
when soil thermal amplitude is lower than that registered in
summer.
On the other hand, at constant temperatures (258C) the
exposure of the seeds to far-red light for 60 min reduced the
germination percentage compared with seeds exposed to red
light for 60 min or far-red light for 30 min. The inhibitory
effect of far-red light on seed germination was reported for
several weed species (Ballaré et al. 1992) such as Lolium
multiflorum (Deregibus et al. 1994), Silene gallica and
Brassica campestris (Batlla et al. 2000), and Amaranthus
palmeri (Jha and Norsworthy 2009). Results observed in the
present study are evidence of the inhibitory effect of low R : FR
ratio on the germination of D. insularis seeds. Far-red light
increases and the ratio R : FR (red, ~645 nm) decreases as
plant canopies develop after the summer solstice. As a result,
emergence of sensitive species could be inhibited as the crop
canopy develops or as the summer progresses (Forcella et al.
2000). Furthermore, the crop canopy reduces not only the R : FR
ratio but also the soil thermal amplitude (Huarte and BenechArnold 2003; Kruk et al. 2006; Jha and Norsworthy 2009).
Many summer grass species usually show a deep seed
dormancy, which is naturally removed by exposure to low
temperatures (usually called pre-chilling) during the cool
season (Vegis 1964; Benech-Arnold et al. 2000; Adkins et al.
2002). This mechanism allows seed germination under ideal
conditions in the following warm season. The lack of a deep
dormancy in D. insularis could be related to the tropical and
subtropical origin of this species, where cold temperatures
are rare.
Implications for management
From these results, management decisions that affect the
environment of the seed could be useful to reduce or delay
seed germination and seedling emergence in the field. Such
management decisions could include sowing crops or cover
crops that generate a dense crop canopy or maintaining abundant
stubble on soil surface to reduce soil thermal amplitude, R : FR
ratio and hydration–dehydration cycles. These practices, in turn,
could help to reduce the colonisation of this weed into new
areas (Guglielmini and Ferraro 2016).
On the other hand, the ambiguous germination response to
the light according to seed origin, suggests that seeds burial to
reduce seedling emergence could be an ineffective option for
D. insularis.
Acknowledgements
This research has been supported by ANPyCT-MINCyT (PICT 2620) and
the UBA (UBACYT 21020160500006BA).
Crop & Pasture Science
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