Journal of Threatened Taxa
| www.threatenedtaxa.org | 26 February 2018 | 10(2): 11339–11347
Pollination ecology of Merremia
tridentata (L.) Hallier f. (Convolvulaceae)
G. Lakshminarayana 1 & A.J. Solomon Raju
2
1 Department of Environmental Sciences, Gayathri Vidya Parishad College for Degree & P.G. Courses
(Autonomous), M.V.P. Colony, Visakhapatnam, Andhra Pradesh 530017, India
2 Department of Environmental Sciences,
Andhra University, Visakhapatnam, Andhra Pradesh 530003, India
1 lakshmi.gnarayana@gmail.com, 2 solomonraju@gmail.com (corresponding author)
Abstract: Merremia tridentata is
a twining and prostrate herb. The
flowers are campanulate, bisexual, weakly protandrous, self-compatible and facultative autogamous. The
forager guilds indicate that thripsophily, melittophily and psychophily are
functional pollination syndromes. Ballistichory, anemochory and hydrochory are the seed dispersal modes. Seeds germinate as soon as they reach
the ground if the soil has sufficient moisture or else they remain dormant and
germinate during the rainy season.
Such seed dispersal modes and flexible germination responses enable the
plant to invade and colonize new areas.
Further, the plant with perennial woody root stock stays alive during
the dry season, sprouts back to life during the rainy
season to re-start its life cycle.
The dual modes of regeneration enable the plant to form extensive
herbaceous cover and bind the soil effectively. Therefore, the plant is an important
soil binder and useful at controlling soil erosion.
Keywords: Anemochory, ballistochory, facultative autogamy, hydrochory,
melittophily, Merremia
tridentata, psychophily,
thripsophily.
doi: http://doi.org/10.11609/jott.3252.10.2.11339-11347
Editor: Anonymity requested. Date
of publication: 26 February 2018 (online & print)
Manuscript details: Ms # 3252 |
Received 31 December 2016 | Final received 01 January 2018 | Finally accepted
02 February 2018
Citation: Lakshminarayana, G. & A.J. S. Raju (2018). Pollination ecology of Merremia tridentata
(L.) Hallier f. (Convolvulaceae). Journal of Threatened
Taxa 10(2): 11339–11347; http://doi.org/10.11609/jott.3252.10.2.11339-11347
Copyright: © Lakshminarayana & Raju
2018. Creative Commons Attribution 4.0
International License. JoTT allows
unrestricted use of this article in any medium, reproduction and distribution
by providing adequate credit to the authors and the source of publication.
Funding: Self-funded.
Competing interests: The authors declare no competing
interests.
Acknowledgements: We thank the Andhra
University, Visakhapatnam, for providing physical facilities to carry out this
research work..
The genus Merremia
comprises about 70–80 species with a pantropical
distribution (Deroin 2001; Demissew
2001) and most of them are sources of pyrrolizidine
alkaloids (Jenett-Siems et al. 2005). The genus is characterized by the
yellow, funnel-shaped or campanulate corolla,
spirally twisted anther thecae (after dehiscence) and non-spiny (non-echinate) pollen grains. Very few species of this genus have been
studied for their reproductive ecology.
Raimundez-Urrutia et al. (2008) reported that M.
macrocalyx is facultatively
xenogamous and melittophilous. Kill & Ranga
(2000) reported that M. aegyptia displays a
cornucopia pattern of flowering. It is self-compatible, facultatively
autogamous and melittophilous. Maimonia-Rodella
& Rodella (1987) reported that M. cissoides is self-compatible and mostly pollinated by
bees. Willmott
& Burquez (1996) reported that M. palmeri is hermaphroditic, self-incompatible and sphingophilous.
Santapau & Henry (1973) reported that the genus Merremia
is represented by 15 species in India.
Some of the species recorded include M. vitifolia,
M. quinata, M. aegyptia, M.
emarginata, M. umbellata,
M. hederacea, M. dissecta,
M. gangetica, M. hastata,
M. tuberosa, M. rajasthanensis
and M. tridentata. None of these species have been studied
for their pollination ecology. Kaladhar (2010) and Oyen (2013)
mentioned that M. tridentata is widely
distributed in tropical Africa, Asia and Australia. It is widely used in traditional
medicine in Africa and Asia. In India, M. tridentata
as a perennial soil binding creeper shows its dominance among herbaceous flora
during wet season. Recently, this creeper has been used as an effective soil
binder in rocky and erosion-prone soils.
Further, it is also planted as an ornamental in parks and gardens. Keeping in view its importance as a soil
binder and ornamental creeper, the present study is contemplated to provide
details of pollination ecology of M. tridentata.
The prime objective of the study is to understand its sexual reproduction,
pollinators, fruiting ecology and seed dispersal in this species in order to
use this information for its propagation in ecologically fragile areas. Further,
the work reported in this paper would provide a reference base for further
studies on other species of Merremia either
in India or elsewhere.
Materials and Methods
The wild patches of Merremia
tridentata growing in Visakhapatnam and its
surroundings (17.700000000N & 82.300000000E) were
used for the study. Ten inflorescences which had not initiated flowering were tagged
and followed daily to record the duration of flowering, anthesis
schedule and the timing of anther dehiscence. Twenty-five fresh flowers were used to
record the floral morphological details. Nectar could not be measured and analyzed due to its secretion in minute quantities
which was further depleted by thrips during
mature bud and flower life. Twenty
mature, but un-dehisced anthers were collected from different plants and
examined for pollen output as per the method described in Dafni
et al. (2005). The calculation of
pollen output per flower and pollen-ovule ratio was done as per the formulas
described in Cruden (1977). The method described in Mondal et al. (2009) was followed for the analysis of amino
acid types in the pollen. The
protocol described in Sadasivam & Manickam (1997) was followed for the extraction of protein
and the Lowry et al. (1951) method was followed for estimating the protein
content in the pollen. Ten flowers
each from five individuals were used to test stigma receptivity. It was tested with hydrogen peroxide
from mature bud stage to flower closure/drop as per the procedure described in Dafni et al. (2005).
Further, the receptivity was also observed visually whether the stigma
was shiny, wet or changing colours or withering. Twenty patches with 125 mature buds were
tagged and followed for four weeks to record fruit and seed set rate in
open-pollinations. The morphological
characteristics of fruit and seed were observed in detail to evaluate their
adaptations for dispersal by different means. Fields visits were made during dry and
rainy seasons to note the aspects of seed germination and production of new
plants. Further, field observations
were made on the production of fresh growth and sexual reproduction from the prennial woody root stock of the
plant.
Insects foraging at the flowers were
observed from morning to evening on four different days for their mode of
approach, landing, probing behavior and contact with
the floral sexual organs. Bees were
identified with the representative specimens available with the Department of
Environmental Sciences, Andhra University, Visakhapatnam. Butterflies were
identified by consulting the book written by Kunte
(2007). The foraging visits of
insects were recorded using 1x1 m area of flowering patch for 10mins every hour
for the entire day on four different days and the data was tabulated to record
the foraging pattern and the percentage of visits made by bees and
butterflies. The pollen/nectar
collection behaviour of insects was carefully observed to assess their role in
effecting pollination. Ten
specimens of each insect species were captured during the peak foraging period
and brought to the laboratory. Each
specimen was washed in ethyl alcohol, stained with aniline-blue on a glass
slide and observed under the microscope to count the number of pollen grains
present. From this, the average
number of pollen grains carried by each insect species was calculated to know
the pollen carryover efficiency. Raju & Ramana (2017) followed
these methods in previous studies on insect foraging activity and their pollen
carryover efficiencies.
Results
Phenology
It is an annual or perennial twining and
prostrate herb depending on the soil environment (Image 1a). It commonly occurs in open, dry and
sandy soils during rainy and winter season. It roots at the nodes and produces
several wiry radiating branches from a thick, woody root
stock. It measures up to 2m
long. It is multi-stemmed and
propagates through seeds as well as by vegetative mode. Leaves are sessile, simple, linear to lanceolate and dentate. The plant re-grows from the perennial
ground root stock during the rainy season. Individual
plants occurring close to each other form extensive mats due to their profuse
growth. It appears conspicuous
during the flowering period. The
flowering occurs early in individuals emerging from the root
stock while it is late in those emerging from the seed. The flowering begins in July in the
former and August in the latter.
The duration of flowering and the life of the plant depend on the soil
moisture condition. But, the
flowering is profuse during August-September when the soil is sufficiently
wet. If the soil is sufficiently
wet even during summer season, the plant survives and produces flowers
depending on the soil nutrient status. In such plants, the flowering occurs
throughout the year. A closer
examination of the flowers of different populations indicated that the plants
produce two floral colour forms, one exclusively white flowers (Image 1b) and
the other lemon yellow with a dark reddish throat (Image 1c). But, individual plants produce only one
flower form. The plants of lemon yellow with dark reddish throat flower form
are most common. The plants of the
two floral colour forms grow either intermingled with each other or grow
displaying distinct populations.
The number of twining branches per plant is 18.2 ± 6.46 in white flower
form and 24.4 ± 10.90 in lemon yellow flower form during the rainy season. The plants of the yellow flower form are
more profuse and spreading than the plants of the white flower form. The flowers are pedicellate
(17mm long), solitary, densely villous and borne in leaf axils. They appear quite distinct against the
foliage. The plants of both the
flower forms with profuse spreading branches with many solitary flowers in a
scattered appearance is quite attractive during the flowering season.
Flower morphology
The flowers are dichromatic only by
corolla colour. Morphometrics
for both flower colours are the same and the description provided here relate
to both flower morphs unless otherwise stated. The flowers are small (20.8 ± 0.04 mm
long, 17.2 ± 0.04 mm wide), white, campanulate,
odourless, actinomorphic and bisexual.
The calyx has five free sepals but slightly fused at the base; the
sepals are green, densely hairy, lanceolate,
acuminate, 4.8 ± 0.04 mm long and 2.2 ± 0.04 mm wide. The corolla is campanulate,
glabrous, 18.21 ± 0.13 mm long, tubular at base and
shallowly five-lobed with more or less broadly triangular lobes. It is completely white or lemon yellow
with a reddish throat. The stamens
are white, inserted on the corolla, exserted, glabrous, filaments filiform and
5.8 ± 0.04 mm long and monomorphic (Image 2a,b). The anthers are white, glabrous, 1mm long and dithecous.
The ovary (2.6 ± 0.24 mm long) is light yellow and bicarpellary,
bilocular with two ovules arranged on axile placentation in each locule.
The style is filiform, light yellow, 6.1mm long and
crowned with a sticky, bi-lobed stigma (Image 2d–f).
Table 1. List of insect foragers on Merremia
tridentata
Order |
Family |
Genus |
Species |
Common
name |
Forage
sought |
Hymenoptera |
Apidae |
Apis |
cerana F. |
Indian
Honey Bee |
Pollen
+ Nectar |
|
|
Apis |
florea F. |
Dwarf
Honey Bee |
Pollen
+ Nectar |
|
|
Trigona |
iridipennis Smith |
Stingless
Bee |
Pollen
+ Nectar |
Lepidoptera |
Pieridae |
Catopsilia |
pomona F. |
Common
Emigrant |
Nectar |
|
|
Catopsilia |
pyranthe L. |
Mottled
Emigrant |
Nectar |
|
Lycaenidae |
Leptotes |
plinius F. |
Zebra
Blue |
Nectar |
|
|
Zizeeria |
karsandra Moore |
Dark
Grass Blue |
Nectar |
|
|
Chilades |
lauis Stoll |
Lime
Blue |
Nectar |
|
|
Chilades |
pandava Horsfield |
Plains
Cupid |
Nectar |
|
|
Euchrysops |
cnejus F. |
Gram
Blue |
Nectar |
Floral biology
The dichromatic flowers on
different individuals display the same functional characters and the
description provided here relate to both.
Mature buds open from 07:00–09:00 hr on
clear sunny days and from 07:30–09:30 hr on
rainy days. The flowers open
completely on sunny days while they are partially open on rainy days. The anthesis
process from mature bud to fully open flower occurs in a time span of thirty
minutes. In mature buds, the corolla is slightly twisted and attains sub-rotate
shape after complete opening. The
anthers dehisce by longitudinal slits during anthesis
and the pollen is presented latrosely (the split on
the side of the anther positioned towards the other anthers rather than towards
the inside or outside of the flower).
The number of pollen grains per anther is 265 ± 10.25 and per flower is
1,325 ± 51.28 (Image 2c). The
pollen-ovule ratio is 331.25:1. The pollen grains are monads, light yellow,
sticky initially, powdery later, spheroidal, polyporate,
74.7 ± 1.23 µm and exine smooth with reticulate
sculpture. The pollen contains four
essential and six non-essential amino acids. The essential amino acids are
threonine, valine, isoleucine and lysine. The non-essential amino acids include
cysteine, cystine, glutamic acid, hydroxyproline,
proline and serine. The total protein content per 1mg of
pollen is 285.7µg. In vitro pollen
germination tests were unsuccessful to record pollen viability duration. The stigma becomes receptive at the end
of anthesis and remains so until 16:00hr of the same
day. The stigma is shiny, sticky, papillate during the receptive phase. The ring-shaped nectary present in the center of the corolla tube, around the base of the ovary
secretes nectar in minute quantities during the mature bud stage and it is
exposed upon anthesis. It is present in traces in
open flowers due to feeding by thrips in buds. Thrips use the growing buds for breeding and emerge by the
time the buds bloom. During mature
bud, anthesis and post-anthesis,
the thrips continually feed on nectar and pollen. The corolla together with the stamens
and stigma closes back spirally by 12:30hr on the same day. The pollinated and fertilized flower
remains in place while the entire flower falls off in un-pollinated flowers on
the morning of the second day.
Pollination mechanism
The flowers display different positions of
the stamens and the stigmas during and after anthesis. Of the five stamens, one stands close to
and/or contacts with the stigma due to the attachment of the filament at a
different position on the corolla tube throughout flower life. Such a placement of this stamen and the
stigma may facilitate autogamy but it is not definite. All other four stamens stand slightly
below the stigma preventing autogamy throughout flower life. Such differential positions of stamens
in relation to the stigma appear to be facilitating partial selfing
during flower life. The closing of
the corolla in a spiral manner at noon time
facilitates brushing of the stigma against all anthers effecting autogamy. Such a floral mechanism is considered to
be a fail-safe strategy by the plant to resort to autogamy in the event of
failure of either geitonogamy or xenogamy.
Insect visitors and pollination
The insect foragers were the same to both
the flower forms; they visited both the forms without any discrimination but
comparably, the yellow flower form was found to be more attractive to
them. Thrips
were the first feeders of both nectar and pollen. They were found to be contributing to
primarily self-pollination by feeding on both pollen and nectar during mature
bud and during and after anthesis. The flowers were
foraged by honey bees and stingless bees from
08:00–11:00 hr with concentrated foraging activity
at 09:00hr, and by pierid and lycaenid
butterflies from 08:00–10:00 hr with
concentrated foraging activity at 09:00hr (Fig. 1). The bees included Apis
cerana (Image 2i), A. florea
(Image 2g,h) (honey bees) and Trigona iridipennis (stingless bees). The pierid
butterflies were Catopsilia pomona (Image 2j) and Catopsilia
pyranthe while lycaenid
butterflies included Leptotes plinius, Zizeeria karsandra, Chilades laius, Chilades
pandava (Image 2k) and Euchrysops
cnejus (Table 1). Of the total foraging visits recorded
during the observation period, honey bees and
stingless bees made 65% while pierid and lycaenid butterflies contributed to 35% (Fig. 2). The bees approached the flowers in
upright position, landed on the corolla and probed for pollen and/or
nectar. To collect nectar, they
inserted their hairy tongue (proboscis) into the corolla throat to access the
available nectar; in so doing, the ventral side brushed against the dehisced
anthers and the stigma effecting sternotribic
pollination. To collect pollen, the
bees approached individual anthers during which only the ventral side and the
stigma brushed against the anthers effecting sternotribic
pollination. In most of the
foraging visits, the bees probed for nectar and pollen while in other visits,
they collected either nectar or pollen.
The butterflies approached the flowers with head facing the corolla
throat; then they inserted the proboscis to collect nectar during which their
ventral side and/or proboscis brushed against the anthers and the stigma
effecting sternotribic pollination. The bees and butterflies visited
9–14 flowers consecutively and in some cases up to 25 flowers depending
on the flower density before leaving the flowering patch. As the flowers were depleted of nectar
by thrips, the insects made multiple visits to the
same flowers in quest of nectar and/or pollen. The pollen carrying efficiency
evaluated by body washings of captured insects indicated that honey bees were
more efficient in carrying pollen than stingless bees and butterflies; the
average number of pollen grains recorded varied from 68.7–50.8 in case of
honey bees and 26.7 in case of stingless bees and from 19.9–12.5 in case
of butterflies (Table 2). The insects
foraged the flowers in quick succession from one flower to the other on the
same and/or different flowering patches in order to collect as much pollen
and/or nectar as possible; this inter-plant foraging activity was considered to
promote cross-pollination.
Table 2. Pollen recorded in the body washings of bees and butterflies on
Merremia tridentata
Insect
species |
Sample
size (N) |
Number
of pollen grains |
||
Range |
Mean |
S.D |
||
Apis cerana |
10 |
26–103 |
68.7 |
23.22 |
Apis florea |
10 |
32–78 |
50.8 |
12.27 |
Trigona iridipennis |
10 |
15–46 |
26.7 |
8.82 |
Catopsilia pomona |
10 |
7–24 |
14.5 |
5.31 |
Catopsilia pyranthe |
10 |
8–29 |
14.1 |
6.05 |
Leptotes plinius |
10 |
5–23 |
12.5 |
4.69 |
Zizeeria karsandra |
10 |
9–21 |
13.8 |
3.48 |
Chilades laius |
10 |
10–23 |
14.6 |
4.17 |
Chilades pandava |
10 |
12–28 |
19.9 |
4.63 |
Euchrysops cnejus |
10 |
6–26 |
15.2 |
6.8 |
Fruiting ecology
In the two flower forms, the pollinated
and fertilized flowers grow continually and produce fruits in three weeks
(Image 2l,m). Natural fruit set
rate is 89.6% and seed set rate is 74.6%.
The calyx is persistent and grows further during fruiting phase; it
envelops the growing fruit. It is a globose capsule,
green initially and brown when mature; it is stalked, non-fleshy, non-hairy,
4–5 mm long and 4–9 mm diameter. A single fruit produces one to four
seeds. One-seeded fruit set rate is
5%, two-seeded 8%, three-seeded 37% and four-seeded 50%. The seeds are dull black, smooth, ovoid-trigonous, and 3mm in diameter (Image 2n). The fruit capsules dehisce loculicidally to disperse seeds. The dispersed seeds fall to the
ground. They further disperse by
wind and rainwater. The seeds germinate and form new plants if the soil is
sufficiently wet. But, seed
germination occurs mainly during the rainy season when the soil is charged with
moisture or rain water. Erratic rainfall and long dry spells
during the rainy season terminate the growth and development of seedlings. The old plants with their robust
underground woody root system withstand water stress and continue their phenological events sequentially.
Discussion
Merremia tridentata
produces several wiry radiating branches from the thick, woody root stock. It
grows throughout the year and displays profuse to sporadic flowering depending
on the soil moisture and nutrient environment. The plant disappears if the soil is dry
but the woody root stock stays alive under-ground to
re-start its life cycle when favourable conditions return. Seed production is continuous in plants
growing in areas of moist soils; seeds germinate almost immediately if soil
environment is favourable but new plants from seeds appear mostly during the
rainy season. With these dual modes
of regeneration, it shows prolific growth during the rainy season. The plant grows well in open, sandy and
dry localities. It is easy to spot
in the field with its twining wiry branching habit, large patches and large
brightly coloured and campanulate flowers. Two flower forms are distinguished based
on corolla colour; one is completely white while the other is lemon yellow with
dark reddish throat. Individual plants produce only one flower form. The lemon yellow flower form is most
common with profuse growth due to more number of wiry branches than the other
one. This study is the first to document the existence of two flower forms in
this species. The plants of the two
flower forms grow either intermingled with each other or produce distinct
populations. The solitary flowers
borne in leaf axils project out against the foliage, appear attractive and
hence is easy to spot the plant in the field.
In Merremia,
self-compatibility and self-incompatibility have been reported in the studied
species. M. cissoides
and M. aegyptia are self-compatible (Maimonia-Rodella & Rodella
1987) and M. palmeri is self-incompatible (Willmott & Burquez
1996). The present study shows that
M. tridentata is highly self-compatible due to
which it shows the highest fruit set and seed set rates in
open-pollinations. The low
pollen-ovule ratio recorded in this plant also substantiates this claim. The low pollen output is relatable to
the pollen size also. Cruden (1977) stated pollen-ovule ratios can
serve as a reliable indicator of breeding system. High pollen-ovule ratios are
normally associated with obligate out-crossing, moderate pollen-ovule ratios
with facultative xenogamy and low pollen-ovule ratios
with obligate autogamy. The low
pollen-ovule ratio in M. tridentata can be
taken as an indicator of obligate autogamy. But, the flower behavior
differs partially with this indication.
In open flowers, one of the five stamens is positioned close to the
stigma due to which autogamy may occur but it is not absolute while all other
four stamens stand away from the stigma precluding the occurrence of
autogamy. However, all five stamens
would brush against the bi-lobed stigma during the closing of the corolla in a
spiral manner at noon and in effect, autogamy occurs since the stigma is still
receptive and the receptivity ceases long after the closure of the
corolla. The delayed autogamy keeps
the option open for cross-pollination during flower life and it has obvious fitness
benefits in habitats where there is much spatial and/or temporal variation in
the availability of pollen vectors (Morgan 2006). This means that when the pollinator
availability is constant and adequate, plants can maximize their out-crossed
seed set and then use delayed autogamy to fertilize any remaining ovules, while
if pollinators are absent the ovules may still be fertilized by autogamy and
still retain a high relative fitness if inbreeding depression is low (Pannell
2006). Therefore, the positional
aspects of the stamens and stigma during flower life and at the closing of the
corolla and the low pollen-ovule ratio collectively suggest that M. tridentata is facultative autogamous.
Merremia tridentata
flowers are nectariferous but the nectar is secreted
in minute quantities, which is not measurable. It is available during mature
bud stage and is utilized by thrips, which use the
buds for their breeding. Since the
stigma is receptive after anthesis, the nectar and
pollen feeding activity of thrips does not contribute
to self-pollination before anthesis but contributes
mostly to self-pollination after anthesis. Further, the differential positions of
stamens in relation to the position of stigma may also reduce the chances of
occurrence of selfing within the flowers. The left over nectar present in traces
in the flowers compels the actual nectar-feeding pollinator insects to make
multiple visits in search of more nectar and in effect, cross-pollination rate
is enhanced. Further, the pollen feeding activity by thrips
also indirectly increases the flower visitation rate by pollen-feeding
pollinator insects. Therefore, such
a state of standing crop of nectar before and after anthesis
due to foraging activity by thrips is advantageous
for the plant to increase cross-pollination rate. This finding is in agreement with the
note by Hodges (1995) that an overabundance of nectar may have a detrimental
effect on seed set by increasing intra-plant pollinator movement.
In the genus Merremia,
a few species have been studied for their pollination biology. Raimundez-Urrutia
et al. (2008) reported that M. macrocalyx displays
melittophilous pollination syndrome and is pollinated
by bees of Apidae and Halictidae. Kill & Ranga
(2000) reported that M. aegyptia
is pollinated by bees of Apidae and Halictidae.
Maimonia-Rodella & Rodella
(1987) reported that M. cissoides
is mostly pollinated by bees.
Willmott & Burquez
(1996) reported that M. palmeri is sphingophilous.
In the present study, M. tridentata has
patchy distribution and its showy bloom may serve as a long distance
attractant. The large patches
increase the availability of cross-pollen and encourage flower constancy by
potential pollinators. The early
morning pattern of floral nectar secretion in synchrony with the process of anthesis in this species appears to be an adaptation for
foraging activity and subsequent pollination by day-active insects. Despite such an abundance of flowers,
even during rainy season, the flowers are pollinated by a few
insects only. The flowers
are pollinated by honey bees, stingless bees and lycaenid butterflies. These insects foraged the flowers
only during forenoon period since the flowers are not available in the
afternoon due to closure of the flowers by noon. The bees contribute to sternotribic pollination while collecting nectar and
pollen. The pollen is protein-rich
and also a source of four essential and six non-essential amino acids for bees
(DeGroot 1953).
Similarly, butterflies also contribute to sternotribic
pollination while collecting nectar.
The body washings of these insects collected after foraging indicated
that bees are efficient in carrying more pollen than butterflies and hence bees
are the primary pollinators while butterflies are supplementary
pollinators. Therefore, the
pollinator guilds recorded in this study indicate that thripsophily,
melittophily and psychophily
exist in M. tridentata.
In Convolvulaceae,
dormant and non-dormant seeds have been reported. In case of non-dormant ones,
recalcitrant seeds have also been documented in this family (Daws et al.
2005). The reports by Sharma & Sen (1975) and Azania et al. (2003) indicate that Merremia species have physical dormancy and
it might have evolved from species with non-dormant recalcitrant seeds. In this study, M. tridentata
has been found to produce non-dormant recalcitrant seeds since the seeds
germinate as soon as they reach the ground but it occurs only in soils which are sufficiently moist. But, most of the seeds germinate and
produce new patches during rainy season and such a situation explains that the
seeds are partially recalcitrant and the dormancy factor is related to the
state of soil moisture and nutrient environment. In recent years, rainfall is
insufficient and also long dry spells exist within rainy season. In consequence, the seedlings are
struggling to survive and if there is not enough soil moisture, they do not
show any further growth and subsequently perish. However, the re-growth from the well
established old root stock withstands rain deficit and
produces new plants alleviating the loss of seedlings from the seeds to some
extent. Therefore, the observed
level of seed dormancy and the production of new plants from the old root stock enable the plant to occupy various habitats to
extend and expand its distribution range.
In this context, it is appropriate to mention that the capsule form of
fruit dehisces loculicidally facilitating seed
dispersal to different distances on the ground and the smooth and ovoid-trigonous seeds subsequently disperse through wind and rain
water characterizing anemochory and hydrochory. These three modes of seed dispersal benefit the
plant to invade and colonize new areas.
In the literature, there is only one
report on the natural bio-controlling agent to control the weeds of Merremia genus. Raimundez-Urrutia
et al. (2008) reported that the bruchid beetle, Megacerus flabelliger
is the natural controlling agent for M. macrocalyx. This beetle uses the seeds for its breeding and hence is considered to be a natural control
for this weed. In the present study, the seeds of M. tridentata
are not used by any beetle or other insect for its breeding. But, further studies across different
regions of its distribution range are recommended to find out whether there is
any such natural controlling agent for this weed.
The patchy distribution of M. tridentata provides the needed forage to all flower
visiting insects almost throughout the year depending on the soil moisture and
nutrient status. But, the forage is
available mainly during forenoon period since the flowers close back by
noon. The plant builds up
populations seasonally and then cover the soil, reduce soil erosion during
rainfall, and add organic matter to the soil upon withering and decomposition,
the aspect of which assumes importance with reference to infiltration and
percolation of rain water during rainy season. The study clearly indicates that M. tridentata is a successful colonizer, especially of open,
sandy dry areas and play many ecological roles in supporting local insects and
in the protection of soil cover.
Progressive anthropogenic disturbance of
natural habitats is one of the greatest problems for the last few decades. Many
environmental agencies direct to stop non-reversible environmental damage and
to promote spontaneous restoration.
In many cases, programs of environmental management are designed for
multiple purposes for which woody species are preferred while shrubs and
herbaceous species are less commonly considered for habitat restoration. Bradshaw (1987) stated that restoration
of degraded areas would more likely be successful if native species were
used. Knapp & Rice (1996) noted
that there is a widespread interest in native herbs due, in part, to the recent
availability of plant material as well as recognition of the role of native
herbal species in the restoration of biological diversity and the conservation
of endangered species and habitat.
In this context, M. tridentata is a promising
candidate for the restoration of the ecological niches where they successfully
grow and colonize. Needless to say,
this plant is an effective soil binder with its clustered root system and
spreading form of multi-stemmed branching pattern. Therefore, it is an important herb in
the natural and artificial restoration of habitats which
are either destroyed or degraded or damaged.
Merremia tridentata is
widely used in traditional medicine in Africa and Asia, especially in India. It
has ornamental value due to its large and bright showy flowers. It is a good
feed material for cattle, sheep and other livestock (Kaladhar
2010; Oyen 2013). This plant has been documented to be
useful as a supplementary feed to the grass Panicum
maximum in housed sheep during the rainy season in West Africa. It has high protein, low fiber and tannin content and hence has been considered to
be worth using as a supplement to P. maximum for young West African
Dwarf sheep. Further, it is easy to
harvest this weed and minimizes the cost of supplying the feed when used (Aschfalk et al. 2002).
Since M. tridentata is used in
traditional medicine and also as fodder, it can be scientifically used in the
modern forms of medicine and also as a potential fodder supplement while
allowing it to grow in areas where it is not a menace from the human point of
view.
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