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Taiwania 69(1): 50‒56, 2024
DOI: 10.6165/tai.2024.69.50
50
Similar germination but dissimilar flood tolerance behaviour of seeds of
two weed species (Ludwigia) inhabiting rice fields in Rajgir, India
K.M.G. Gehan JAYASURIYA1,2, 3,*, Shyam S. PHARTYAL4
1. India Science and Research Fellow, School of Ecology and Environment Studies, Nalanda University, Rajgir, India ORCID:
0000-0001-6518-7951. 2. Department of Botany, University of Peradeniya, Peradeniya, Sri Lanka. 3. Postgraduate Institute of
Science, University of Peradeniya, Peradeniya, Sri Lanka. 4. School of Ecology and Environment Studies, Nalanda University,
Rajgir, India ORCID: 0000-0003-3266-6619 *Corresponding author’s email: gejaya@sci.pdn.ac.lk
(Manuscript received 3 November 2023; Accepted 29 December 2023; Online published 10 January 2024)
ABSTRACT: We aimed to determine seed germination responses and flood tolerance of Ludwigia hyssopifolia and L. perennis that
grow in rice fields in Rajgir, India. Freshly-matured seeds were incubated in 12 hr / 12 hr light / dark and complete darkness at
constant 25 oC and natural daily fluctuating temperatures. Seeds exposed to different light durations were then incubated in complete
darkness. Seeds exposed to different flooding durations were incubated in continuous flooded or non-flooded environments. Seeds
of both species germinated within four days in light/dark but failed to germinate in complete darkness, revealing their nondormant
and positive photoblastic behavior. Some seeds of both species (10 – 20 %) germinated in complete darkness after exposure to light
for 24h. Seeds failed to produce normal seedlings in a continuously flooded environment. Seeds of the two studied species tolerate
at least one week of flood. Seeds of L. perennis have a higher tolerance to flooding than those of L. hyssopifolia, which survived
four weeks in a flooded environment. The two species have the same germination behaviour but differ in ability to tolerate flooding.
Since seeds of both species are nondormant, positively photoblastic, and have different degrees of flood tolerance, a flooding regime
of rice fields will not be sufficient to control these weeds.
KEY WORDS: flooding tolerance, hypoxic conditions, nondormant seeds, photoblastic seeds, weed control.
INTRODUCTION
Ludwigia (Onagraceae) is an aquatic plant genus
native to Central and South America (Mabberley, 2017)
and is now distributed not only in many other tropical
countries but also in some temperate countries (Dandelot
et al., 2005; Hussner, 2010; Nehring and Kolthoff, 2011).
Many Ludwigia spp. are invasive in aquatic ecosystems
and also occur as weeds in many croplands, especially in
rice fields (Moody, 1989; Holm et al., 1997; Chauhan et
al., 2011). Invasive Ludwigia spp. adversely affect
aquatic ecosystems by replacing the native diverse
vegetation (Dandelot et al., 2008; Pivari et al., 2008;
Stiers et al., 2011; Grewell et al., 2016), blocking the
water runaways (Thouvenot et al., 2013; Gallardo et al.,
2016; Sarat et al. 2019) and enhancing sedimentation
(Brusati, 2009; Gallardo et al., 2016). Some Ludwigia spp.
have been reported to produce allelopathic chemicals,
which suppress the growth of other plants (Dandelot et al.,
2008; Sakpere et al., 2010; Roy et al., 2011; Mukherjee
and Barik, 2013). As a weed in rice fields, Ludwigia
causes significant decreases in yield through competition
(Dandelot et al., 2008; Chauhan and Johnson, 2010a;
Chauhan et al., 2011; Mukherjee and Barik, 2013), and
high economic costs are involved in controlling Ludwigia
infestations (EPPO, 2011).
Among the weedy Ludwigia species, L. hyssopifolia
and L. perennis are widely distributed weeds with high
invasive potential (Holm et al., 1997). Ludwigia
hyssopifolia is native to tropical America and Northern
Australia (POWO, 2023), while it occurs as a rice field
weed in many rice-growing countries, including India
(Panda et al., 2019), Sri Lanka (Chandrasena, 1987),
Malaysia (Begum et al., 2008), Thailand, and Indonesia
(Holm et al., 1997). On the contrary, L. perennis is a
native species in tropical Africa, South and Southeast
Asia, and Northern Australia (POWO, 2023). In India,
these two species have been recorded in direct seedings
(Panda et al., 2019; Jannu and Narender, 2023) and
transplanted rice fields (Sreemadhavan, 1966). Mutakin
et al. (2021) reported that even under the new rice
growing technology 'System of Rice Intensification
(SRI)', L. hyssopifolia became the dominant weed in rice
fields in Indonesia.
Rice is the staple food in Asia, including India, and
much effort has been put into improving the efficiency of
rice farming systems and production. Weeds are one of
the significant factors causing the yield reduction in rice
farming systems. Thus, many approaches have been used
to control them. Using herbicides is the easiest and most
effective method among weed control techniques
(Chauhan et al. 2014). However, emerging evidence of
the harmful effects of herbicides on human health and
environmental risks is promoting a trend toward using
eco-friendly, less hazardous weed control methods
(Zimdahl, 2018).
Furthermore, evidence on the development of
herbicide-resistant weed varieties (Kaur et al., 2022) has
also discouraged the use of herbicides alone for weed
control (Kayeke et al., 2017). Integrated weed
2024 Jayasuriya & Phartyal: Seed germination of two Ludwigia species
51
management is the current trend in rice cultivation
(Sangramsingh et al., 2022; Jannu and Marender, 2023);
however, much scientific information is required to plan
integrated weed management properly (Janu and Marender,
2023). The biology and physiology of the crop species,
hydrology and soil conditions of the cropland, and biology
and physiology of weed species are of utmost importance
in an adequately integrated weed management plan
(Mortensen et al., 2000; Cherry and Gough, 2006; Rao and
Nagamani, 2010; Chauhan and Johnson, 2010b; Ibáñez et
al., 2014; Grewell et al., 2016). Although seedling and
adult plant biology and physiology are often considered,
seed biological information about weeds is seldom used in
planning integrated weed management (Mortensen et al.,
2000; Chauhan and Johnson, 2010b; Chauhan, 2012a).
Lacking the necessary seed biological information on
weeds is one of the reasons why this information is not
considered in proper planning (Chauhan, 2012a; Thapa and
Bhatt, 2014; Grewell et al., 2016).
Seed germination requirements, specifically, light and
temperature and the ability of seeds to germinate under
hypoxic (flooded) conditions, should be considered in
planning proper weed management in rice farming
systems. In particular, flooding is used as a weed control
strategy in rice fields (Yamauchi, 1996). However, the
prior exposure of seeds of mudflat plants (also common
in rice fields) to flooded environments strongly interferes
with their germination process both positively and
negatively once the non-flooded environment is regained
(Phartyal et al., 2020 and reference therein). Therefore, it
is essential to consider the ability of rice field weed seeds
to germinate and survive under flooded (hypoxic)
conditions. Although some information is available in the
scientific literature on the seed germination of some
Ludwigia species, no information is available on L.
hyssopifolia or L. perennis. Available information suggests
that seeds of Ludwigia spp. have either no dormancy
(Oziegbe et al., 2010; Sumudunie and Jayasuriya, 2019) or
physiological dormancy (Wulff and Briceno, 1976; Chul
and Moody, 1989; Wogu and Ugborogho, 2000;
Sumudunie and Jayasuriya, 2019). Further, germination
studies suggested that Ludwigia seeds are photoblastic, and
only a few seeds germinate under complete darkness
(Salisbury, 1972; Wulff and Briceno, 1976; Gillard et al.,
2017; Sumudunie and Jayasuriya, 2019). However, no
information is available on the effect of flooding on the
germinability of Ludwigia species.
Therefore, research was conducted to evaluate the
germination behaviour of two weedy Ludwigia species,
Ludwigia hyssopifolia (G. Don) Exell. and Ludwigia
perennis L. widely inhabit rice fields in Rajgir, Bihar,
India. Special attention was given to evaluating how
different durations of prior exposure of their seeds to light
and flooding affect seed germination in complete
darkness and non-flooding conditions. The findings of
our study will be valuable information for better
understanding the seed germination behaviour of these
two weed species and planning effective measures for
integrated weed management in rice fields.
MATERIALS AND METHODS
Fruit collection and seed extraction
The fruits were collected from more than five
individuals of Ludwigia hyssopifolia and L. perennis
plants grown on a lower bank of a water canal around a
rice field in Rajgir (25.019964N, 85.405461E), Bihar,
India. Care was taken to collect only mature dried fruits
directly from the plants. The fruit collection was carried
out in February 2019. The fruits were collected in brown
paper bags, brought to the Seed Biology Laboratory of
Nalanda University, Rajgir, India, and stored for two days
until they were used for laboratory experiments. The
seeds were extracted by crushing the fruits and cleaning
out the debris.
Seed germination
Four replicates of about 250 seeds each of both species
were placed separately in 9-cm diameter Petri dishes on
filter papers moistened with distilled water. Ungerminated
seeds at the termination of the experiment were counted for
each replicate to determine the total number of seeds
shown for calculating the final germination percentage.
Seed samples were incubated at constant 25 °C under both
12 hr / 12 hr light / dark and complete dark conditions in a
temperature-controlled incubator. A cool white fluorescent
lamp provided the light (1000 lumens). The darkness was
provided by wrapping Petri dishes with three layers of
aluminium foil. The same experiment was repeated by
keeping Petri dishes on a laboratory bench to expose seeds
to natural, fluctuating temperature conditions. Seeds
incubated in light / dark were observed for germination at
2-day intervals for 14 days or until all the seeds germinated.
Seeds kept in the dark were observed for germination after
14 days at the end of the experiment. The emergence of the
visible shoot was considered the criterion for germination.
Effect of prior light exposure on seed germination in
darkness
Six sets containing four replicates of approximately
250 seeds each of both species were incubated on filter
papers moistened with distilled water at 25 °C under light
conditions for 1, 2, 4, 8, 12, and 24 hours (for prior light
exposure treatments). After that, each set of seeds was
moved for incubation under completely dark conditions
at the same incubation temperature for 14 days, and the
seeds were observed for germination. The emergence of
the visible shoot was the criterion for germination.
Ungerminated seeds at the termination of the experiment
were counted for each replicate to determine the total
number of seeds shown for calculating the final
germination percentage.
Taiwania Vol. 69, No. 1
52
Effect of hypoxic (flooded) environments on seed
germination
In the first set of experiments, four replicates of about
250 seeds of both species were placed between two filter
papers on the bottom of eight 250 ml beakers to avoid
floatation of seeds and ensure they were always under
flooded conditions throughout the experimental period.
Subsequently, the beakers were filled with 200 ml of
water (to maintain a water depth of approximately 10 cm
and create hypoxic environments) and incubated at 25 oC
in the 12 hr / 12 hr light / dark regime. The light was
provided from the side to ensure that all the seeds
received enough light. Filter papers were taken out from
the beaker, and seeds were observed for germination at 2-
day intervals for 14 days. After observations, which took
about 10 min per replicate, filter papers with seeds were
returned to the bottom of the beaker. This experiment
aimed to determine whether continuously flooded seeds
had normal germination (root and shoot emergence).
In the second set of experiments, as described above,
three subsets of four replicates of about 250 seeds for each
species were exposed to hypoxic (flooded) environments
under full dark conditions. However, after 2, 3, or 4 weeks
of flooding, one set of seeds for each species was
retrieved and incubated on filter papers moistened with
distilled water at 25 °C in light / dark (12 hr / 12 hr) under
oxic (unflooded) environments. Seeds were observed for
germination in 2-day intervals for 14 days or until all the
seeds germinated. The emergence of the visible shoot was
the criterion for germination. This experiment aimed to
evaluate the effect of prior exposure to flooding on seed
germination under non-flooding conditions. As described
above, the total number of seeds shown and the final
germination percentage were counted.
Statistical analysis
All the germination percentage data were arcsine
transformed. Transformed data were analyzed with a two-
way ANOVA procedure in PAST 4.1 statistical software.
The analysis used species and germination treatments as
the two factors. When ANOVA was significant, Dunnet's
Mean separation procedure was conducted to determine the
treatment combinations that differed significantly from
others. Non-transformed data were used in all graphs.
RESULTS
Seed germination
Seeds of two Ludwigia species germinated to 80 – 90
% under light / dark conditions within 14 days regardless
of the temperature (constant 25 oC vs. natural daily
fluctuating outside temperature) (Figure 1). However,
only an insignificant number of seeds germinated when
incubated in complete darkness. There were no
significant differences in germination between species (F
= 0.169, P = 0.69) or between temperature conditions (F
= 0.005, P = 0.9413). Seed germination under light / dark
conditions was significantly higher than under complete
darkness (F = 113.4, P < 0.001).
Fig. 1. Germination of Ludwigia hyssopifolia and L. perennis
seeds in 12hr / 12hr light / dark and complete dark conditions at
constant 25 oC and at natural daily temperature fluctuation of
outside environmental conditions. Error bars are + SE. Different
lowercase letters indicate significant differences between
treatments within a species. There are no significant differences
in germination between species.
Effect of prior light exposure on seed germination in
darkness
The germination percentage increased with increased
prior light exposure time (Figure 2, F = 839, P < 0.001).
Even after 24 hrs. prior light exposure, seeds of L.
hyssopifolia and L. perennis germinated only to 11.3 ±
1.9 and 20.6 ± 2.4 %, respectively. The germination
percentage of L. perennis was significantly higher than
that of L. hyssopifolia after 12 and 24 hours of prior light
exposure (F = 16.42, P < 0.001). However, they
germinated to 80 – 90 % in light / dark (12 hr / 12 hr)
within 4 days at their standard germination test.
Effect of hypoxic (flooded) environments on seed
germination
In the first experiment, 80 – 90 % of the seeds of both
species had radicle emergence under continuously
flooded conditions (data not shown). However, no shoot
emergence occurred from radicle-emerged seeds in a
flooded environment; instead, they all rotted and died.
In the second experiment, the germination percentage
of L. hyssopifolia incubated in the non-flooded
environment (seed kept on filter papers moistened with
distilled water after flooded treatment) was significantly
reduced with increased duration of flooding (Figure 3, F
= 350.7, P < 0.001). However, all radicle-emergent seeds
had produced a normal and healthy shoot. In contrast, the
germination percentage of L. perennis seeds incubated in
a non-flooded environment after being moved from
flooding was approximately 75 % for all the tested
flooding durations. The germination percentage of L.
2024 Jayasuriya & Phartyal: Seed germination of two Ludwigia species
53
perennis seeds was not significantly different among
exposure treatments or from the control (Figure 3, F=
3.49, P = 0.048). Like L. hyssopifolia, all radicle-
emergent seeds of L. perennis also produced a normal and
healthy shoot.
Fig. 2. Germination of Ludwigia hyssopifolia and L. perennis
seeds at 25 oC in complete darkness after prior exposure to light
for different periods (0, 1, 2, 4, 8, 12 and 24 h). Error bars are +
SE. Different uppercase letters indicate significant differences
between species within the same exposure time. Different
lowercase letters indicate significant differences between
different exposure times within the same species.
Fig. 3. Germination of Ludwigia hyssopifolia and L. perennis
seeds after prior expoure for different duration (0, 2, 3 and 4
weeks) under flooded (hypoxic) environments in complete
darkness and then moved to non-flooded environments (on filter
papers moistened with distilled water at 25 oC in 12 hr / 12 hr light
/ dark conditions). Error bars are ± SE. Different lowercase letters
indicate significant differences between different immersion times
within the same species.
DISCUSSION
The seeds of the two study species were nondormant,
as 80 – 90 % germinated within four days. This
conclusion agrees with the observations of Oziegbe et al.
(2010) and Sumudunie and Jayasuriya (2019), who
reported that seeds of L. abyssinica, L. adscendens, L.
erecta, L. leptocarpa and L. octovalvis and L. peruviana
are nondormant. However, some species of Ludwigia
have been reported to produce seeds with physiological
dormancy (Salisbury, 1972; Wulff and Briceno, 1976;
Gillard et al., 2017; Sumudunie and Jayasuriya, 2019).
Interestingly, Sumudunie and Jayasuriya (2019) reported
that L. peruviana and L. decurrens in the same habitat
produce nondormant and physiologically dormant seeds,
respectively. Moreover, our experiments showed that the
seeds of L. hyssopifolia and L. perennis are photoblastic
as observed for many other Ludwigia species (Salisbury,
1972; Wulff and Briceno, 1976; Gillard et al., 2017;
Sumudunie and Jayasuriya, 2019). Thus, L. hyssopifolia
and L. perennis seeds can also form a soil seed bank.
Seeds of the two species are tiny (diameter < 0.5 mm) and
thus have a high potential to be incorporated into the soil.
Tiny seeds can always be remarkably incorporated into
the soil seed bank and move into deeper soil layers
(Thompson et al., 1993; Metzner et al., 2017; and
Shiferaw et al., 2018). Further, when they were
incorporated into the soil, seeds would not receive the
required light conditions for germination and, thus, stay
in the soil seed bank until brought to the soil surface,
where they would be exposed to the light conditions
required for germination. Since L. perennis seeds
(0.00001 mg) were reported to be 14 times smaller than
those of L. hyssopifolia seeds (0.00014 mg) (SNR, INSR
and RBGK, 2023), L. perennis seeds have a higher
potential to be incorporated into the deeper layers of the
soil seed bank than L. hyssopifolia. Furthermore, both
species are known to produce orthodox seeds (SNR,
INSR and RBGK, 2023), an additional trait that plays a
crucial role in seed persistence in the soil.
The ability to form a soil seed bank is an essential
characteristic of a successful weed (Holzner, 1982;
Grundy and Jones, 2002). Further, it is a problematic
character for controlling the weeds. Thus, taking
necessary precautions to deplete the soil seed banks is
crucial when planning to eradicate these two species.
Weed plants are recommended to be removed before they
reach the stage of seed production and dispersal maturity.
A pre-tillage (before crop sowing tillage) can expose
buried weed seeds in the soil seed bank to light. When
nondormant seeds of both species are exposed to light,
and a sufficient amount of moisture is present, they would
germinate. The emerging seedlings could be removed
using physical or chemical eradication techniques for
weed control.
Prior exposure to light causes the seeds of both
species to germinate in completely dark conditions.
However, only < 25 % of the seeds germinated in
complete darkness, even after 24 hours of exposure to
prior light. Thus, most seeds require > 48 hrs exposure to
light for germination to progress. However, a gradual
increase of germination with an increased exposure time
reveals that seeds of L. hosopifolia and L. perrenis have a
varying degree of sensitivity to light exposure, i.e., 12 hrs
Taiwania Vol. 69, No. 1
54
exposure is enough to trigger the germination of 10 %
of the seeds, while another 15 % of the seeds require 24
hrs exposure to light. Different sensitivities to light by the
seeds could be a bet-hedging adaptation of these seeds
(Venable, 2007). Soon after dispersal, some portion of the
seeds potentially can germinate before they are
incorporated into the soil seed bank.
As shown in our experiment, if seeds of both species
experience continuously flooded environments and are
exposed to light, they germinate (radicle emerge).
However, no shoot emergence occurs, and germinated
seeds fail to produce normal mature plants in a flooded
environment. On the other hand, our experiments showed
that if the flooded seeds were not exposed to light, they
would not germinate. Further, 60 % of the L. perennis
seeds and > 75 % of the L. hyssopifolia seeds germinated
when they were moved from 2 weeks of exposure to a
flooded environment to a non-flooded environment (on
filter papers moistened with distilled water). That is, most
of the seeds do not lose viability when they remain
immersed in water for two weeks. Furthermore, even after
being immersed in water for four weeks, > 75 % of the L.
perennis seeds germinated, demonstrating that these seeds
withstand hypoxic (flooded) conditions and overhydration
stress and regain the germination ability soon after water
withdrawal. Phartyal et al. (2020) noted similar seed
germination behaviour in several species of mudflat plants.
Ludwigia perennis produces dust seeds, while L.
hyssopifolia produces seeds that are about 0.5 mm in
diameter. Thus, the seeds of L. hyssopifolia are many
times larger than those of L. perennis. Since large seeds
are said to be more vigorous than small ones (Roy et al.,
1996; Cookson et al., 2001) and more tolerant of stress
conditions (Donaldson, 1996; Canak et al., 2020), it could
be assumed that large seeds of L. hyssopifolia survive
better under hypoxic conditions compared to smaller
seeds of L. perennis. However, the opposite was observed,
where smaller seeds of L. perennis tolerated hypoxic
conditions better than L. hyssopifolia seeds. This may be
due to the adaptation of these two species to their habitat
conditions. Both species disperse seeds during the dry
period when the mudflats and streams are dry. Since L.
perennis seeds are tiny, they have a higher ability to be
incorporated into the deep layers in the soil seed bank
than those of L. hyssopifolia (Thompson et al., 1993;
Metzner et al., 2017; Shiferaw et al., 2018), where they
are susceptible to more extended periods of flood and
hypoxic conditions than seeds on the surface. As a result,
L. perennis seeds must tolerate hypoxic conditions longer
than L. hyssopifolia seeds to survive in habitats with these
conditions. Thus, although these two species are in the
same habitat, they have contrasting seed germination
responses to similar flooding (hypoxic) environmental
conditions. Ludwigia hyssopifolia produces large seeds
that stay in the top layers of the soil and germinate as soon
as conditions are favourable for germination. Since L.
hyssopifolia seedlings are produced from somewhat large
seeds, their competitiveness will probably be higher than
that of L. perennis (Coomes and Grubb, 2003; Wu and
Du, 2008). However, L. perennis produces many tiny
seeds capable of being incorporated into the deep layers
of the soil seed bank, and seeds have developed a greater
tolerance to flooding conditions than those of L.
hyssopifolia. Therefore, they seem to have developed a
greater tolerance to flooding conditions.
Flooding before rice seed sowing is often
recommended as a method to control weeds, especially in
integrated weed management planning (Hill et al., 1994,
2001; Yan et al., 2007; Chauhan, 2012b). Flooding the
field is supposed to kill the existing weed community in
the field (Hill et al., 1994, 2001; Yan et al., 2007;
Chauhan, 2012b) as well as kill many seeds in the soil
seed bank (Chauhan, 2012b). However, our results
indicated that more than two weeks of flooding is
required to kill L. hyssopifolia seeds, while flooding may
not kill L. perennis seeds in the soil seed bank. Thus, to
eradicate L. perennis, it is vital to promote most of the
seeds to germinate before practising flooding. This can be
quickly done because L. perennis seeds are nondormant
but photoblastic; therefore, it is only necessary to expose
the buried seeds to light. Thus, presowing tillage can be
practised several times to expose the buried seeds. This
method will expose both L. perennis and L. hyssopifolia
seeds to light and promote germination. Then, subsequent
flooding can eliminate the seedlings that were produced.
CONCLUSIONS
Seeds of L. perennis and L. hyssopifolia are
nondormant but photoblastic. In addition, seeds of both
species can tolerate flood for one week, while the
flooding tolerance of L. perennis is more than four weeks.
Although the two study species share the same habitat and
have a phylogenetically close relationship, L. perennis
and L. hyssopifolia have adopted two contrasting
germination strategies with regard to flooding
environmental conditions of their habitat. To control
these two troublesome weeds in rice fields, we
recommend that fields be given several rounds of
presowing tillage and then flooded for one week.
ACKNOWLEDGMENTS
K.M.G.Gehan Jayasuriya is grateful to the Centre for Co-
operation in Science and Technology among Developing
Societies (CCSTDS), DST-GoI, New Delhi, India, for awarding
him the India Science and Research Fellowship.
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