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Aquatic Botany 88 (2008) 311–316
www.elsevier.com/locate/aquabot
Allelopathic potential of two invasive alien Ludwigia spp.
Sophie Dandelot a,*, Christine Robles b, Nicolas Pech c,
Arlette Cazaubon a, Régine Verlaque b
a
Laboratoire d’Ecologie des Eaux Continentales Méditerranéennes (Case C 31), Institut Méditerranéen d’Ecologie et de Paléoécologie,
UMR 6116, Faculté des Sciences et Techniques de Saint-Jérôme, Université Paul Cézanne, 13397 Marseille Cedex 20, France
b
Laboratoire de Biosystématique et d’Ecologie Méditerranéenne (Case 4), Institut Méditerranéen d’Ecologie et de Paléoécologie,
UMR 6116, Université de Provence, Centre Saint Charles, Place Victor Hugo, 13331 Marseille Cedex 3, France
c
Laboratoire Evolution Génome Environnement (Case 36), Institut Méditerranéen d’Ecologie et de Paléoécologie,
UMR 6116, Université de Provence, Centre Saint Charles, Place Victor Hugo, 13331 Marseille Cedex 3, France
Received 29 January 2007; received in revised form 28 November 2007; accepted 3 December 2007
Available online 8 December 2007
Abstract
The allelopathic potential of two invasive alien Ludwigia [Onagraceae: L. peploides (Kunth) Raven and L. grandiflora (Michaux) Greuter and
Burdet], that have developed quasi-monotypic stands in many aquatic ecosystems in France, was investigated. Since allelopathy involves the
release of compounds into the environment, the water of monospecific experimental cultures was directly tested against two target species: Lactuca
sativa L., the standard cultivar for bioassays, and Nasturtium officinale R. Brown, a resistant and widespread native hydrophyte. The treatment was
carried out at the three main phases of development of both Ludwigia in February, May and August. For each experiment, the germination,
mortality and culture yield percentages, the seedling growth (radicle and hypocotyl elongation) and the health of 15-day-old-seedlings were
measured. The water of each Ludwigia tank induced: (1) a decrease in germination for watercress in August (control: 68.6%, L. peploides: 48.6%,
L. grandiflora: 61.1%); (2) an increase in mortality in May only for watercress (control: 3.4%, L. peploides: 13.5%, L. grandiflora: 12%) and in
August for both target species (up to 22.3% vs. 3% for lettuce and 27% vs. 12.5% for watercress); (3) a disturbance of seedling elongation for
lettuce in all seasons; and (4) a seedling chlorosis of both target species, particularly in May and August. This study showed that L. peploides and L.
grandiflora possess an allelopathic activity that influences the water quality throughout the year. Combined with the various competitive attributes,
allelopathy may contribute to the great success of these two invasive Ludwigia in Europe. In threatened wetland communities of the Mediterranean
area, in particular, allelopathy might have an important impact by diminishing the seedling survival of the most vulnerable species.
# 2008 Elsevier B.V. All rights reserved.
Keywords: Ludwigia peploides; L. grandiflora; Allelopathy; Invasion; Lettuce; Watercress; Southern France; Wetland
1. Introduction
In France, two alien amphibious Ludwigia (Onagraceae)
have for the past 20 years caused major ecological and
economic problems. These water primroses are morphologically very similar and are often confused: L. peploides (Kunth)
Raven and L. grandiflora (Michaux) Greuter and Burdet.
Introduced at Montpellier (Lez River: Hérault) in 1830,
probably from America, these taxa have become the most
aggressive weeds in all shallow-water habitats in France and
* Corresponding author.
E-mail addresses: [email protected] (S. Dandelot),
[email protected] (R. Verlaque).
0304-3770/$ – see front matter # 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.aquabot.2007.12.004
neighbouring countries. Their inexorable spread has been
achieved because of their intensive growth and easy propagation by cuttings (Dandelot et al., 2005b).
In the South of France, both Ludwigia exhibit a seasonal
pattern of development: in early spring leaves appear at the
water surface; by June up to 50 cm of stems emerge; they flower
from July to October, producing a strong total biomass (2–3 kg
dry weight m 2); in November all aerial stems fall and in winter
persistent organs form with sediments a dense blackish mat
(Dandelot, 2004). The major effects of these invasions are:
reduction of the flow and hyper-sedimentation leading to silting
up, in particular in ponds, the drastic decline of the local
biodiversity (dense monotypic stands) and in summer water
hypoxia and major alterations in bacterial communities
(Dutartre et al., 1997; Dandelot et al., 2005a). These vigorous
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S. Dandelot et al. / Aquatic Botany 88 (2008) 311–316
ornamental plants can survive and flower under the worst
conditions (pollution, salinity, drought, etc.). In addition, they
are avoided by herbivores and pathogens (Dandelot, 2004),
owing to the production of numerous noxious or repellent
compounds (Ghani, 1998). All these features suggest that both
Ludwigia might possess a hitherto unstudied allelopathic
potential.
Allelopathy is defined as any effect, direct or indirect, positive
or negative, of one species on the growth of another through the
release of chemical compounds into the environment (Rice,
1984). Allelopathic interactions can play a key role with regard to
the success of invasive plants since they may alter physiological
processes, and thus influence the structure of communities (Rice,
1992; Bais et al., 2002, 2003; Inderjit and Duke, 2003). Better
known in terrestrial environment, allelopathy has also been found
in prolific submerged macrophytes (Gross, 1999, 2003; Ervin
and Wetzel, 2003) as Ceratophyllum and Myriophyllum spp.
(Jasser, 1995; Nakai et al., 2000). Most bioassays, using plant
extracts, have demonstrated growth inhibition in phytoplankton
or lettuce seedlings, but only a few studies have described
allelopathic effects of emergent macrophytes on hydrophytes
living in the same ecosystem (Elakovich and Wooten, 1989,
1995; Gopal and Goel, 1993).
The aim of this initial work is to study, over a yearly cycle of
development, the allelopathic potential of L. peploides and L.
grandiflora with regard to the germination, mortality and
seedling elongation of two target species. The first, Lactuca
sativa L., is the standard cultivar for bioassays due to its
sensitivity to allelochemicals (Leather and Einhellig, 1987;
Ervin and Wetzel, 2003). The second, Nasturtium officinale R.
Brown, has been chosen because this native European
hydrophyte germinates easily, and a few stems of this resistant
species sometimes occur in Ludwigia stands in France. Since
allelopathy involves the release of compounds into the
environment (Rice, 1984), we directly sampled water from
Ludwigia tanks for our tests.
2. Materials and methods
In May 2003, about a hundred samples of Ludwigia were
randomly taken in the naturalized Southern France populations
of L. peploides (Durance River) and L. grandiflora (Siagne
River). For each species, two tanks (90 cm 30 cm 50 cm:
0.27 m2) were placed in the open air and each tank filled with
30 L of tap drinking water. Thirty stems with roots of Ludwigia
were washed, and then placed at regular intervals within each
tank, in order to obtain the mean density observed in situ.
During the year, tap water was daily added in order to maintain
a constant volume (30 L/tank). Tap drinking water was used
because: (1) it is free from allelochemicals and various
contaminants, (2) it has an osmotic potential compatible with
seed germinations, and (3) it has the nutrient content required
for the development of both Ludwigia in tanks.
Lettuce and watercress seeds were bought from local
suppliers. Five treatments per target species were tested: (1)
water from L. peploides tank 1, (2) water from L. peploides tank
2, (3) water from L. grandiflora tank 1, (4) water from L.
grandiflora tank 2, and (5) tap water as control. Our
experiments were carried out during the three main phases
of both Ludwigia growths in Europe: February, May and
August 2004. Bioassays were carried out in vitro under
conditions of natural photoperiod and at room temperature (20–
25 8C). Each treatment was tested on seven replicates of 20
seeds, arranged at regular intervals and numbered in sterile
Petri dishes (9 cm in diameter) with a Whatman filter-paper No.
4. Thus, for each season, 140 seeds for control and 280 seeds for
each invader were tested. Daily during each month of
experimentation, 1 mL of water taken within the Ludwigia
tanks, and tap water for control, was added to each Petri dish.
At each season, the germination, viability, culture yield
percentages and seedling growth (radicle and hypocotyl
elongation) have been measured. The number of germinations
and abnormal seedlings were recorded daily. The percentage of
very abnormal, i.e. non-viable, seedlings represents the early
mortality percentage, calculated in relation to the number of
germinations. The culture yield corresponds to the percentage
of the number of seedlings with a ‘‘normal aspect’’ in relation to
the total number of seeds tested. For each normal seedling, the
measurement of the radicle and hypocotyl length was
performed 15 days after its germination.
Comparison of percentages was tested using x2-test where
we approximated the distribution of statistical tests by
permutations (1000 when necessary). Differences in mean
lengths were tested using analyses of variance (one-way or twoway ANOVA) and multiple comparison tests by Tukey’s
method. All statistical analyses were performed using S-PLUS
6 (2001).
3. Results
3.1. Germination
For all treatments, germination percentages were always
very high for lettuce (95–99%) and much lower for watercress
(48–74%). For lettuce, results did not significantly differ during
the experiment between seeds growing in water from the two
Ludwigia tanks and the control (2.11 < x2 < 3.41; d.f. = 2;
0.18 < P < 0.35). The pattern was the same for watercress in
February (x2 = 0.38; d.f. = 2; P = 0.85) and May (x2 = 5.45;
d.f. = 2; P = 0.06: Fig. 1), but in August germination
percentages of seeds growing in both Ludwigia waters were
significantly reduced (x2 = 17.62; d.f. = 2; P = 0.0001), particularly with L. peploides (48.6% vs. control: 68.6% and L.
grandiflora: 61.1%).
3.2. Mortality and yield
Lethal anomalies encountered were characterized by:
abortion of seedlings, poorly or mis-formed cotyledons,
withered or necrotized seedlings, absence of radicle and
arrested growth after germination. Mortality percentages
differed widely according to the time, the treatment (tap or
Ludwigia waters) and the target species. In February,
percentages were low for all treatments (0–2% for lettuce,
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313
Fig. 1. Mean germination (Germ) and seedling mortality (Mort) percentages of watercress according to the three treatments in February, May and August. Values not
different at a = 5% are denoted with the same letter. Bars indicate standard error.
7–4% for watercress) and without significant differences
compared to control (x2 = 2.9; d.f. = 2; P = 0.22 for lettuce, and
x2 = 1.24; d.f. = 2; P = 0.54 for watercress). In May, results
showed wide divergences according to the target species.
Mortality percentages were very low for all control seedlings
(<3.5%). For lettuce, they increased up to 4.7% with L.
peploides water and 6.6% with L. grandiflora water, but without
significant difference (x2 = 3.75; d.f. = 2; P = 0.15). Watercress
seedlings exhibited greater mortality in both Ludwigia waters:
13.5% and 12%, respectively. These percentages were
significantly different compared to control (x2 = 6.74;
d.f. = 2; P = 0.03) and similar for both Ludwigia (Fig. 1). In
August, both Ludwigia waters strongly and significantly
increased the mortality percentages of lettuce (x2 = 31.83;
d.f. = 2; P < 10 6) and watercress seedlings (x2 = 8.79;
d.f. = 2; P = 0.01). In summer, L. grandiflora water had a
more severe lethal impact on both target species (lettuce:
22.3%, watercress: 27%) than L. peploides (respectively, 11 and
17.7%).
Lastly, culture yield percentages did not significantly differ
in February and May for the two target species
(0.2 < x2 < 2.78; d.f. = 2; 0.25 < P < 0.9: Table 1). On the
other hand, in August we observed a significant decrease for
lettuce (x2 = 22.84; d.f. = 2; P = 10 5) and watercress
(x2 = 15.31; d.f. = 2; P = 10 4). For lettuce the negative impact
appeared to be greater with L. grandiflora than with L.
peploides, whereas watercress seedlings showed the same
sensitivity towards both invaders.
3.3. Chlorosis and elongation of seedlings
The first obvious result of this experiment was the similarity
of behaviour of the two target species with regard to chlorosis.
Compared to healthy controls whose hypocotyls were always
green, those of lettuce and watercress seedlings growing in both
Ludwigia waters were yellowish, particularly in spring and
summer, and even etiolated (especially lettuce seedlings).
On the other hand, with regard to seedling elongation, the
two target species showed contrasting behaviour. Whatever the
season, the size of watercress seedlings did not show significant
differences (one-way ANOVA, 0.06 < F < 2.85; 0.06 <
P < 0.9 for radicles, and 0.29 < F < 2.57; 0.07 < P < 0.7
for hypocotyls). In contrast, the response of lettuce seedlings
changed according to the season and the plant part. For radicle
lengths (Fig. 2), the three treatments had a significantly
different effect in February (one-way ANOVA, F 2,588 = 50.75;
P < 10 10), because both Ludwigia waters had a clearly
stimulating effect on root elongation. In May, this effect
decreased and only the seedlings growing in L. peploides water
were significantly different from the two others (one-way
ANOVA, F 2,649 = 9.35; P < 10 4). Lastly in August, the
impact of both Ludwigia waters became negative, especially
Table 1
Mean culture yield percentages (number of ‘‘normal’’ seedlings/total number of seeds tested), and standard error, of the two target species 15 days after germination
according to the three treatments in February, May and August
February
Control
L. peploides
L. grandiflora
May
August
Lettuce
Watercress
Lettuce
Watercress
Lettuce
Watercress
97.1 1.4
93.6 1.5 ns
95.4 1.3 ns
66.4 4
67.1 2.8 ns
65.4 2.8 ns
94.3 2
93.9 1.4 ns
90.7 1.7 ns
61.4 4.1
64.3 2.9 ns
60.4 2.9 ns
94.3 2 a
86.8 2 *** b
77.1 2.5 *** c
60 4.1 a
40 2.9 *** b
44.6 2.9 *** b
Values not different at a = 5% are denoted with the same letter. P: ns non-significant; * significant at 0.05; ** significant at 0.01; *** significant at 0.001.
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S. Dandelot et al. / Aquatic Botany 88 (2008) 311–316
Fig. 2. Mean length (cm) of lettuce radicles on normal 15-day-old-seedlings
according to the three treatments in February, May and August. Values not
different at a = 5% are denoted with the same letter. Bars indicate standard
error.
with L. grandiflora (one-way ANOVA, F 2,588 = 11.17;
P = 10 5). Conversely, for hypocotyl lengths, a slight positive
effect was observed in February, but only the treatment with L.
peploides water significantly differed from the two others (oneway ANOVA, F 2,661 = 3.87; P = 0.021). This effect increased
in May, to reach a maximum in August, especially with L.
peploides water. In May, the two Ludwigia impacts were similar
(3.7 and 3.6 cm) and distinct from the control (3.3 cm) (oneway ANOVA, F 2,649 = 14.72; P < 10 6), but in August the
three treatments significantly differed (respectively, 3.2, 3.8 and
3.6 cm) (one-way ANOVA, F 2,588 = 24.92; P < 10 10).
4. Discussion
Generally seeds, protected by their teguments, seem less
sensitive to allelochemicals than seedlings (Elakovich, 1999;
Quayyum et al., 1999). Although the lettuce germination was
never affected by the two Ludwigia waters, that of watercress
showed a clear decrease in August. However, the most
significant results of this study concern the seedling mortality
of the two target species, which increased with the yearly
development of the two invaders:
(i) no effect in February during the winter Ludwigia
dormancy;
(ii) a slight (for lettuce) or clear effect (for watercress) in May
when their leaf-rosettes grow;
(iii) a strong effect in August (particularly with L. grandiflora)
when Ludwigia are flowering.
Although watercress is one of the most resistant hydrophytes, possessing allelopathic and anti-herbivory properties
(Newman et al., 1996), its seedlings exhibit as early as the
spring and during the summer higher mortality percentages
than that of lettuce, which is well known for its sensitivity to
allelochemicals (Leather and Einhellig, 1987).
In contrast to watercress, the size of lettuce seedlings was
strongly altered during tests and exhibited an unbalanced
development pattern (radicle/hypocotyl ratio). In winter, both
Ludwigia waters had a clearly stimulating effect on root
elongation. In summer, in agreement with previous studies,
growth inhibition was greater on roots than on hypocotyls. The
negative and positive impact on seedling growth observed
suggests that both alien Ludwigia could influence, throughout
the year, the water quality. This concomitant inhibition and
stimulation may result from: (1) direct alterations of roots by
the water, (2) specific growth re-orientation related to
allelochemical stress (Pasternak et al., 2005), and (3) perhaps
the release of hormones by invaders (Rice, 1986; Neori et al.,
2000; Inderjit and Duke, 2003). With hydrophyte extracts, the
stimulation has been essentially found on lettuce and Lemna
(Quayyum et al., 1999); its intensity varied widely according to
allelochemical concentrations (maximal effect with low
concentrations) and the development period of the donor
(Elakovich and Wooten, 1989; Wooten and Elakovich, 1991).
Except in winter, all seedlings of the two target species
suffered chlorosis in Ludwigia waters. Photosynthesis disturbance is an important and widespread mode of action of
many aquatic plants against competitors, via allelochemicals
(Gross, 2003). Adding chlorosis to the disturbed development
can compromise the long-term survival of most seedlings in
invaded wetland communities.
The occurrence of allelopathy and the feasibility of
transferring bioassays to the natural environment is still a
matter of debate (Inderjit and Callaway, 2003), particularly in
aquatic ecosystems. Nearly all previous studies were carried out
either with crude extracts of adult plants, or with some of its
active compounds tested at different concentrations, on the
germination and seedling growth of target species. In fact, the
rare experiments using water extracts of lake sediments,
associated with allelopathic macrophytes, showed no significant result, because exudate concentrations were insufficient
(Quayyum et al., 1999; Seal et al., 2004). Moreover, the wellknown allelopathic compounds were not detected in any water
samples, probably due to their rapid degradation (Glomski
et al., 2002). In this context, our significant results, obtained
directly with water from Ludwigia tanks, are new, relevant and
close to certain natural situations.
According to Gross (2003) ‘‘allelochemicals released by
donor organisms into the water need to be sufficiently
hydrophilic and reach their target organisms in effective
concentrations despite considerable dilution’’. In the Mediterranean area, the long summer drought produces a strong drop
in the water-level and an increase in compound concentrations,
notably in closed systems. So compared to natural stands, our
experimental conditions probably minimise the impact,
because tanks are maintained at constant volume (30 L) and
the duration was limited to 1 year. Compounds released by
Ludwigia are probably produced by roots, foliar leachage and
decomposition, but especially by several secretory organs
(trichomes and glands) that excrete liquid lipophilic substances
creating iridescence on the water surface (Dandelot, 2004).
Lipophilic complexes, which tend to remain near the donor,
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S. Dandelot et al. / Aquatic Botany 88 (2008) 311–316
have been implicated in plant defences and as a strong potential
source of allelochemicals in wetlands (Gross, 1999, 2003;
Fahn, 2000).
Studies of putative allelochemicals will be complex
because Ludwigia spp. synthesize: tannins, triterpenes,
flavonoids, polyphenols, alkaloids, linoleic acids, fatty
substances, saponins, etc. (Ghani, 1998) that can act in
synergy. Most of them exert either phytotoxic, allelopathic
(Singhvi and Sharma, 1984) or repellent effects (Neori et al.,
2000; Ervin and Wetzel, 2003; Gross, 2003; Inderjit and
Duke, 2003). But more significant is the strong antibacterial
activity found in the closely related species L. adscendens
(L.) Hara (Ahmed et al., 2005) and in both invaders (Le Petit,
personal communication). This might explain the lack of
decay or turbidity in our aquaria, even after several years.
This property suggests that Ludwigia are not necessarily
the single source of putative allelochemicals: complex
interactions may also occur with the microflora (Algae,
Bacteria).
The relief and climate of the Mediterranean area result in
major seasonal changes in the wetlands (devastating autumnal
and spring floods, long summer drought and water resources
management). This ecosystem is inhabited by endangered flora,
with about 1/3 of short-living species (Verlaque et al., 2001),
that require regular regeneration by seeds. Colonization by
Ludwigia might contribute to the impoverishment of this flora,
by reducing the seedling survival of the more vulnerable native
taxa. Moreover, from mid-spring to mid-autumn, the strong
biomass and height of Ludwigia (>1 m) limit interspecific
competition for space, light and nutrients, and also prevent
flowering of nearly all hydrophytes. In Ludwigia stands only a
few etiolated shoots of early resistant or allelopathic macrophytes survive (e.g. Nasturtium, Myriophyllum, Ceratophyllum
spp.), before the growth period of the invaders (Dandelot,
2004).
In conclusion, this study shows that both alien Ludwigia
possess real allelopathic potential that varies strongly in
function of four main factors: (1) the season, (2) the target
species, (3) the parameter studied (germination, mortality,
elongation, chlorosis), and (4) the invaders. Further studies are
required to complete this initial work, in particular with a view
to analysing the nature of the putative allelochemicals and the
complex hydrophyte–microflora interactions. The inexorable
spread of these pests in the Mediterranean area is all the more
worrying because aquatic ecosystems harbour a rich and highly
vulnerable flora.
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