Germination ecology of turnip weed (Rapistrum rugosum (L.) All.) in the northern regions of Australia
Germination ecology of turnip weed (Rapistrum rugosum (L.) All.) in the northern regions of Australia
Sudheesh Manalil 0 1
Hafiz Haider Ali 1
Bhagirath Singh Chauhan 0 1
0 The Centre for Crop Science, Queensland Alliance for Agriculture and Food Innovation (QAAFI), The University of Queensland , Gatton, Queensland , Australia , 2 School of Plant Biology, Institute of Agriculture, The University of Western Australia , Perth, Crawley , Australia , 3 Amrita University , Coimbatore , India , 4 Department of Agronomy, University College of Agriculture, University of Sargodha , Sargodha, Punjab , Pakistan
1 Editor: Craig Eliot Coleman, Brigham Young University , UNITED STATES
In Australia, turnip weed has been rapidly emerging as one of the major weeds in conservation agricultural systems. Germination and emergence of turnip weed were examined for two populations collected from Gatton and St George regions of Australia; two locations with high and low rainfall, respectively. The seeds of turnip weed germinated at all the tested temperatures, but germination was the lowest at 15/5ÊC, intermediate at 20/10ÊC and highest at 25/ 15ÊC and 30/20ÊC. The results indicated a high adaptability of turnip weed to warm environmental conditions, although it is a major problem in the winter season. Germination was higher in dark than light/dark regimes except at 30/20ÊC. Three was a concomitant reduction in germination as the osmotic potential values decreased from 0 to -1.0 MPa. There was 2 and 4% germination at -0.8 MPa for Gatton and St George populations, respectively, and no germination occurred at an osmotic potential of -1.0 MPa. There was a reduction in germination when the sodium chloride (NaCl) concentration was increased from 0 to 150 mM, and no germination was observed at 200 and 250 mM of NaCl. Turnip weed germinated over a broad range of pH (4 to 10). Seedling emergence was higher at 1 cm depth compared to 0.5 cm or at the soil surface. There was 28 and 33% emergence at the surface for the Gatton and St George populations, respectively, compared to 48 and 56% emergence from 1 cm depth for the Gatton and St George populations, respectively and no emergence was observed from 6 cm depth. The results indicated that tillage leading to shallow burial would promote the emergence of turnip weed; on the contrary, tillage that could bury seeds deep into the soil profile might minimise the emergence. Under ideal conditions and lack of integrated weed management programmes, this weed will emerge, set seeds and enrich the soil seed bank and thereby continue to be a problem in the northern grain region of Australia.
Data Availability Statement: All relevant data are
within the Supporting Information files.
Funding: This work was supported by a grant from
Grains Research Development Corporation
(GRDC), Australia under project UA00156.
Competing interests: The authors have declared
that no competing interests exist.
Turnip weed (Rapistrum rugosum (L.) All.) is a major agricultural weed from the family of
Brassicaceae that is rapidly increasing in prevalence in Australia, Iran, USA and Russia [1±4].
In Australia, major patches of turnip weed are observed in wheat (Triticum aestivum L.),
chickpea (Cicer arietinum L.) and other winter crops [
]. Turnip weed is a highly competitive weed;
in addition to the cropping areas, this weed is prevalent in the fallow regions, railway tracks,
and road side [
1, 4, 6
]. Weeds of Brassicaceae are rapidly emerging under conservation
agricultural systems in Australia as these weeds could adapt to varying environmental conditions and
prevailing crop management practices [
Turnip weed can produce up to 77,000 seeds per plant [
]. Previous reports indicated that
this weed could germinate under varying soil and physical environments [
]. When tested
in the laboratory environments, turnip weed germinated over a wide range of pH (4 to 10) and
there was germination even at a medium level of salinity (160 mM of sodium chloride (NaCl))
. Generally, germination of Brassicaceae weeds and in particular turnip weed is not limited
by dark conditions [
6, 9, 10
]. In addition, the presence of seed coat ensures a physical barrier
leading to periodicity in germination and will expose to different environments that are highly
conducive for emergence, growth, and reproduction [
Turnip weed is a highly competitive weed and around 10 plants m-2 could reduce the
chickpea yield by 40% . A study conducted in Gatton, Queensland, indicated that a weed density
of 18 plants m-2 could cause a yield reduction of 50% (Manalil and Chauhan; unpublished
data). Being a broadleaf weed, management of this weed is difficult when present in chickpea.
Resistance against acetolactate synthase inhibiting herbicides was observed in many
Brassicaceae weeds [
2, 11, 12
]. In Australia, Adkins et al. [
] identified chlorsulfuron resistant turnip
weed populations way back in the 1990s. In Iran, turnip weed endowing multiple resistance
mechanisms against acetolactate synthase inhibiting herbicides were identified [
Knowledge on germination ecology of weeds would help to frame the most appropriate
weed management options [
]. Exposure of weed seeds to varying environments and
prevailing agronomic management would significantly affect the germination and emergence of
weeds [15±17]. Drought conditions, inherent salinity, and soil pH may affect weed
germination and emergence differentially [14±16]. Chauhan et al. [
] explored the germination biology
of turnip weed in a South Australian population. However, those results may not be fully
applicable to turnip weed populations of Queensland owing to the difference in weather, soil type
and crops. Mediterranean type weather with rainy winter and dry summer prevails in South
]. On the contrary, the weather is quite varying across Queensland with hot
humid summer (wet season) and mild to warm winter. In addition, in Queensland,
considerable variation exists in day time temperature even during peak winter time [
Environmental conditions during the seed development may affect the germination characteristics of
], as there would be differential supply of nutrients and hormones to developing
embryo with varying growth environments [
]. In Queensland, considerable variation
exists in terms of cumulative rainfall and its distribution between locations , therefore, it is
likely that weed populations vary in their response to biotic and abiotic factors. With all these
backgrounds, a study was conducted to examine the effect of light, temperature, salt, osmotic
stress, pH, and burial depth of weed seeds on germination and emergence of two populations
of turnip weed collected from two locations in Queensland with contrasting rainfall patterns.
Materials and methods
Seed description and details of sites
Seeds were collected from St George (RRS) and Gatton (RRG) in November 2015, low (500
mm) and high rainfall (770 mm) areas in Queensland, respectively. St George is 325 km (aerial
distance) away from Gatton and at an elevation of 200 m above mean sea level (AMSL);
whereas, elevation of Gatton is 89 m (AMSL) [
]. Soil of St George is redsodosl with bulk
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density and pH of 1.40 g cm-3 and 8.4, respectively; soil of Gatton is Grey Vertosol with a bulk
density and pH of 1.32 g cm-3 and 7.2, respectively [
]. Although difference in mean annual
rainfall is only around 200 mm, Gatton receives well distributed and assured summer rains
compared to St George [
]. In 2015, Gatton received an annual rainfall of 681 mm in 99 rainy
days and St George received a total rainfall of 426 mm in 66 rainy days. In 2014±15 period,
Gatton received 341 mm of summer rainfall compared to 90 mm of rainfall for St George.
Although both the seed collection sites practice conservation tillage and totally depend on
rainfall, cropping in these sites differs as only winter crop is raised at St George (chickpea or
wheat), whereas, both winter (chickpea or wheat) and summer (sorghum (Sorghum bicolor (L.)
Moench)) crops are raised in Gatton due to adequate summer showers. The RRS population
was collected from a chickpea field (S28Ê11.104', E 148Ê 38.054') and RRG populations from a
wheat field (S27Ê 33.552', E 152Ê 19.443'). Fully matured seeds (from plants that were
completely senesced) were collected by gently tapping the inflorescence into a basin. Populations
were collected from around 50 plants distributed in an area of around 5 ha. Collected seeds
were kept in paper bags and stored in a fully ventilated rain out facility at the Gatton research
facility of the University of Queensland until used in the experiments (May to August 2016).
Experiments on temperature and light
Naked seeds were used to examine the effect of temperature and light as there was no
germination of seeds with silique intact. The assessment was carried out by placing 30 seeds evenly in a
9 cm diameter Petri dish with two Whatman No.1 filter papers and moistened with 5 ml of
distilled water. Petri dishes were covered with zip lock plastic bags to minimise moisture loss and
placed in an incubator set at day/night alternating temperature (15/5, 20/10, 25/15 and 30/
20ÊC) with photoperiod coinciding high temperature. Germination was assessed both under
the light and dark regimes after three weeks. Darkness was simulated by covering Petri dishes
with two layers of aluminium foil immediately after placing seeds. Initial germination was
continued up to 3 weeks and visible protrusion of radicle was counted at a weekly interval.
Effect of osmotic stress, salt stress and pH on germination
The effect of water stress was assessed by preparing solutions with osmotic potential 0.0, -1,
-0.2, -0.4, -0.6,-0.8 and -1.0 MPa by dissolving 0.0, 93.6, 132.4, 187.2, 229.2, 264.7 and 295.9 g
of polyethylene glycol 8000 in 1 L of distilled water, respectively [
]. Germination was
assessed under sodium chloride (NaCl) stress, osmotic stress and pH at the day/night
alternating temperature of 25/15ÊC as there was the highest germination at this temperature regime in
the temperature and light experiment. The effect of salt stress was studied by NaCl solutions of
0, 25, 50, 100, 150, 200 and 250 mM. To examine the effect of pH, buffer solutions were
prepared by following the procedures of Chauhan et al. [
]. A 2-mM solution of MES
[2-(N-morpholino) ethanesulfonic acid] was adjusted to pH 5 or 6 with 1 N hydrogen chloride (HCl) or
sodium hydroxide (NaOH). A 2-mM solution of HEPES [N-(2-hydroxymethyl)
piperazine-N(2-ethanesulfonic acid)] was adjusted to pH 7 or 8 with 1 N NaOH. A pH 9 or 10 buffer was
prepared with 2-mM tricine [N-Tris (hydroxymethyl) methylglycine] and adjusted with 1 N
NaOH. Unbuffered deionized water (pH 6.7) was used as a control.
Effect of burial depth on emergence
The effect of seed burial depth was studied by placing 30 seeds at 0, 1, 2, 3, 4 and 6 cm depths.
The soil used in this experiment was collected from the Gatton Research Farm of the
University of Queensland. The soil of the experimental site had a pH of 7.2, organic matter of 2.7%,
nitrogen of 33 mg kg-1, phosphorus of 215 mg kg-1 and potash of 412 mg kg-1. The soil was
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filtered through a 4 mm sieve and then filled in pots of 12 cm diameter (four replications) and
maintained under the rainout shelter facility at the University of Queensland under an
A randomised complete block design was used in all the experiments with three replicates for
Petri dish assays and four replicates for the pot studies. In the laboratory study, blocking was
done by placing Petri dishes on different shelves of the incubator and in the pot study by
grouping the pots of the same replicate together. All experiments were repeated twice. Data
were pooled for the analysis as there was no time by treatment interaction. Analysis of variance
was performed on the data from the light and temperature experiment, effect of pH and burial
depth experiments. Non-linear regression analysis was performed on osmotic potential and
salinity experiments. Germination percentage was fitted to a functional three parameter
sigmoid model using Sigmaplot software. The model fitted was
where G (%) is the percentage of germination, Gmax is the maximum germination as per the
fitted model, x is treatment level or concentration, x0 is the treatment level or concentration
corresponds to 50% germination or emergence, and b is the slope [
Effect of temperature and light on germination
No germination was observed when freshly harvested seeds were tested with silique intact
(data not shown); however, germination improved significantly when naked seeds were used
in the experiment (Fig 1). When tested at varying temperature regimes, germination of turnip
weed was affected by the tested temperature and light, although population difference was not
observed except at 30/20ÊC for the dark treatment (Fig 1). Germination was less than 29% at
15/5ÊC day/night temperature for RRG and RRS populations. At 20/10ÊC, germination was
higher than 15/5ÊC but was lower than 25/15 and 30/20ÊC. Germination was more than 85%
for both the populations under the dark environment in all the temperature ranges except for
30/20ÊC where germination was less than 45%, indicating sensitivity to darkness at high
temperature. In addition, unlike other temperature regimes, difference between populations was
observed for their response to dark at 30/20ÊC.
Effect of osmotic stress, salinity and pH on germination
A three-parameter sigmoid model fitted to the germination data (%) corresponds to the
osmotic potential values (Fig 2). There was a concomitant reduction in germination as osmotic
potential values decreased from 0 to -1.0 MPa. Germination was only 2 and 4% at -0.8 MPa
for RRG and RRS populations, respectively, and no germination was recorded at -1.0 MPa.
Osmotic potentials that can cause 50% reduction in germination based on the regression
models were -0.50 and -0.51 MPa, for RRG and RRS populations, respectively.
A three-parameter sigmoid model was fitted to the germination data obtained at different
concentrations of NaCl (Fig 3). There was a reduction in germination when NaCl
concentration was increased from 0 to 150 mM (Fig 3) and no germination was observed beyond this
concentration. Germination was 6 and 12% at 150 mM for RRG and RRS populations,
respectively. The concentration for 50% inhibition of the maximum germination, estimated from the
fitted model, was 77 and 81 mM NaCl for the RRG and RRS populations, respectively (Fig 3).
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Fig 1. Effect of alternating day/night temperatures and light regimes on seed germination of turnip weed seeds from
Gatton (RRG) and St George (RRS) incubated at 15/5, 20/10, 25/15 and 30/20ÊC light/dark and dark in a 12-h
photoperiod for 21 days. Error bars are LSD (p 0.05, n = 6).
Turnip weed could germinate over a broad range of pH. Turnip weed germination was more
than 75% over a pH range of 6 to 7. However, a reduction in germination was observed at pH
lower than 6 and higher than 7 (Fig 4).
Effect of seed burial depth on emergence
The seed burial depth experiment indicated that the emergence was lower at the soil surface
compared to 0.5 cm and 1 cm depths (Fig 5). There was 28 and 33% emergence at the surface
for the RRG and RRS populations, respectively, compared to 48 and 56% emergence at 1 cm
depth for the RRG and RRS populations, respectively. Emergence was only 9 and 13% at 4 cm
depths for the RRG and RRS populations, respectively, and no germination was observed at 6
The results of this study vary from that carried out in South Australia where germination was
not affected by the varying temperature regimes under the light environment [
]; however, in
this study, germination was higher under warmer environmental conditions than cooler
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Fig 2. Effect of osmotic potential on the germination of two populations of turnip weed from St George (RRS) and Gatton (RRG)
incubated at 25/15ÊC day/night temperatures in a 12-h photoperiod for 21 days. Lines represent the functional three- parameter sigmoid
model fitted to the data. Error bars are standard error of mean (n = 6).
temperature conditions, although it is a major problem weed in the winter season. High
germination in complete darkness was observed in this study. Photo inhibition in turnip weed was
observed in an earlier study carried out in New South Wales in the 1990s [
]. There was
enhanced germination in turnip weed populations studied in Iran under dark conditions
when naked seeds were stored (prior to experiment) at constant temperature of 3 and 25ÊC
]. However, in the current study, seeds were stored under ambient conditions and high
germination in darkness was observed. However, in the study carried out in South Australia,
germination was improved by exposure to light over complete darkness [
]. Annual ground
cherry (Physalis divaricata L.) populations from Iran exhibited variation in seed dormancy and
this was related to the temperature conditions at seed maturity; seeds developed at warmer
temperature exhibited less dormancy compared to cooler temperature [
]. In a study,
adjacent populations of ripgut brome (Bromus diandrus Roth.) from crop field and fence line
exhibited a variation in dormancy characteristics, difference in crop management practices
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Fig 3. Effect of sodium chloride (NaCl) on the emergence of two populations of turnip weed from St George (RRS) and Gatton (RRG) incubated at 25/
15ÊC day/night temperatures in a 12-h photoperiod for 21 days. Lines represent the functional three-parameter sigmoid model fitted to the data. Error
bars are standard error of mean (n = 6).
were hypothesised to be the reason behind this difference . This difference between
populations from South Australia and Queensland in their germination response to dark could be
ascribed to the difference in weather, difference in cropping systems [
], and changes that
have been occurring in the agronomic management over time. In South Australia,
Mediterranean weather prevails with predominant cropping in the winter season; contrary to warmer
subtropical or tropical weather in Queensland with both summer and winter dominant
Other researchers have reported that seed germination of Brassicaceae weeds could be
affected by light and temperature conditions. For example, germination of African mustard
(Brassica tournefortii Gouan.) was significantly inhibited by the lower temperature (15/9ÊC)
compared to higher temperature [
]. In another study, Kleemann et al.  observed a
reduced germination of perennial wall rocket (Diplotaxis tenuifolia (L.) DC.) at lower
temperatures (10 to 20ÊC) compared to higher temperature. Germination of Oriental mustard
(Sisymbrium orientale L.) was significantly higher at 25/15ÊC compared to low temperatures (15/5 or
20/10ÊC) . Germination of naked seeds was greater than the seed in intact silique. Under
field conditions seeds release dormancy rapidly and substantial seeds germinated in field
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Fig 4. Effect of buffered pH solutions on the germination of two populations of turnip weed from St George (RRS) and Gatton (RRG) incubated at 25/
15ÊC day/night temperatures in a 12-h photoperiod for 21 days. Error bars are LSD (p 0.05, n = 6).
within a period of three months (Manalil and Chauhan; unpublished data). However, intact
seed coat allows this weed to extend the periodicity in germination as dormancy (due to seed
coat) release will not be abrupt making the management difficult [
]. In a nut shell, the ability
to germinate under varying temperature conditions, darkness and dormancy of freshly
harvested seeds favour turnip weed to adapt to diversified environments.
The osmotic potential study indicated that turnip weed has adaptability to water stress
environments. The results are in agreement with the earlier observations of Chauhan et al. [
illustrates the prevalence of this weed in roadsides, railway tracks and fallow areas [
substantial portion of soils in the northern regions of Australia is vertosol where surface layers dry
rapidly ; however, turnip weed may emerge under water-limiting environments.
Turnip weed exhibited a moderate level of tolerance to different salinity levels (NaCl
concentrations) (Fig 3). The present study is in agreement with the earlier study carried out on a
South Australian populations of turnip weed [
]. The results are important as salinity is a
major production constraint of Australian soils and can limit plant growth . Another
Brassicaceae weed, African mustard had shown some level of salt tolerance when tested under
laboratory environment [
]. The results of the pH experiment indicated that pH might not be
a limiting factor for the germination and emergence of turnip weed.—Similarly, there was
more than 45% germination of musk weed (Myagrum perfoliatum L.) over a pH range of 4±10
. Seeds of African mustard germinated over a broad range of pH from 4±10 [
response of turnip weed to salinity and pH indicates that the weed can thrive extreme soil
conditions. The results are important as salinity and alkalinity are often associated and is a major
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Fig 5. Effect of burial depth on seedling emergence of two populations of turnip weed from St George (RRS) and Gatton (RRG) in a
pot study for 21 days. Error bars are standard error of mean (n = 8).
production constraint of Australian soils ; however, turnip weed could cope with extreme
levels of salinity and pH and continue to spread even under conditions that limit crop
The results of burial depth study indicate that shallow burial could increase the emergence
of turnip weed (Fig 5). This may be due to better soil, moisture, and seed contact and darkness
may not limit germination [
]. Seed germination decreased substantially with increasing soil
depths and this pattern is observed in many weeds [33±36]. The results indicated that
conducting occasional shallow tillage may not reduce the emergence of turnip weed; on the contrary,
deep inversion tillage may reduce the weed emergence. Tillage is a recommended option to
manage heavy weed infestations . Different burial depths examined in this study has
relevance as crops like cotton (Gossypium hirsutum L.) requires intensive tillage and this crop
could be rotated with cereal crops . In this study, seeds were unable to emerge from deeper
soil layers indicating inversion tillage can be a weed management strategy under heavy
infestation. Small seeded weeds like turnip weed fails to emerge as the carbohydrate reserve may not
support the seedling growth through the soil profile [
]. Results are in agreement with the
similar studies carried out in other broadleaf weeds [
Generally, environmental conditions and water availability during seed maturity (maternal
environments) would strongly influence the germination and dormancy rates .
Contrasting rainfall patterns exist in St George and Gatton locations and variations in emergence
pattern between the populations were expected. Considerable variation exists between sites in
terms of soil properties, total amount of rainfall and its distribution. Field visits in different
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parts of the region and discussion with agronomists indicated that a proportion of turnip weed
could germinate during summer season producing seeds depending on rainfall. Gatton offers
more favourable environment for turnip weed to establish in summer as the region receives
more summer rainfall and variability between populations were expected. However,
germination characteristics were not inferior for population sourced from the low rainfall area (St
George) compared to the high rainfall area (Gatton). Overall, turnip weed is highly adapted to
agricultural areas of Queensland where a considerable variation in weather, soil and crop
management exists. The biological potential and ecological adaptability of turnip weed to
conservation agricultural systems would favour this weed to increase in prevalence under the current
agronomic and weed management system.
Potential of turnip weed to germinate under varying temperature and light regimes point to
the adaptability of this weed to infest the cropping regions and fallows of the northern regions
of Australia. The results show the potential of turnip weed to thrive occasional water stress and
cope up with the inherent soil variability due to salinity, soil pH, and soil moisture retention.
Management options should target summer, winter and fallow phase of cropping seasons as
turnip weed has the potential to emerge under diversified environments and enrich soil seed
bank. The results of the seed burial study indicated that tillage leading to shallow burial would
promote the emergence of turnip weed; on the contrary, tillage that could bury seeds deep into
the soil profile might minimise the emergence. This indicates the potential of soil inversion
tillage to reduce the weed infestation level. The results indicted a high adaptability of turnip weed
to the prevailing agronomic management under the conservation systems. Under ideal
conditions and lack of integrated weed management programmes, this weed may continue to be a
problem in the northern grain regions of Australia.
S1 File. Data set used in the analysis.
This work was supported by a grant from Grains Research Development Corporation
(GRDC), Australia under project UA00156.
Conceptualization: Sudheesh Manalil.
Data curation: Sudheesh Manalil, Hafiz Haider Ali.
Formal analysis: Sudheesh Manalil.
Writing ± original draft: Sudheesh Manalil.
Writing ± review & editing: Hafiz Haider Ali, Bhagirath Singh Chauhan.
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