Floral ecology and
pollination in Eriolaena lushingtonii (Sterculiaceae), an endemic and threatened deciduous tree
species of southern peninsular India
A.J. Solomon Raju 1, K. Venkata Ramana 2 & P. HareeshChandra 3
1,2,3 Department of Environmental
Sciences, Andhra University, Visakhapatnam, Andhra Pradesh 530003, India
1 solomonraju@gmail.com (corresponding author), 2 vrkes.btny@gmail.com,3 hareeshchandu@gmail.com
doi: http://dx.doi.org/10.11609/JoTT.o3168.4359-67
Editor: Cleofas Cervancia, University of Philippines Los Baños College Laguna, Philippines. Date of publication: 26
May 2013 (online & print)
Manuscript details: Ms #
o3168 | Received 18 April 2012 | Final received 19 April 2013 | Finally
accepted 25 April 2013
Citation: Raju,
A.J.S., K.V. Ramana & P.H. Chandra (2013). Floral ecology and pollination in Eriolaena lushingtonii (Sterculiaceae),
an endemic and threatened deciduous tree species of southern peninsular India. Journal of Threatened Taxa 5(9): 4359–4367; http://dx.doi.org/10.11609/JoTT.o3168.4359-67
Copyright: © Rajuet al. 2013. Creative Commons Attribution 3.0 Unported License. JoTTallows unrestricted use of this article in any medium, reproduction and
distribution by providing adequate credit to the authors and the source of
publication.
Funding: Ministry of Environment and
Forests, Government of India, New Delhi
Competing Interest: None.
Acknowledgements: This
study is a part of the research work carried out under an All India Coordinated
Research Project on Reproductive Biology of RET Tree species (MoEF No. 22/6/2010-RE) funded by the Ministry of
Environment & Forests, New Delhi sanctioned to AJSR. The second and third
authors are Junior Research Fellows working in this project. We thank Mr. K. Venkanna, Technical Officer, Central Research Institute for
Dry land Agriculture, Hyderabad, for carrying out soil chemical analysis. We
also thank Prof. B. Ravi Prasada Rao,
Department of Botany, Sri Krishna DevarayaUniversity, Ananthapur, for the help to confirm the
location of the natural population of Eriolaena lushingtonii.
Author
Contribution: Prof. A.J. Solomon Raju is Head of the Department of Environmental
Sciences, Andhra University, Visakhapatnam. He is the
recipient of several national and international awards. He has more than 250
research papers in international and national journals. He is on the editorial
board of several international journals. He is presently working on endemic and
endangered plant species in southern Eastern Ghats forests with financial
support from MoEF and CSIR, and on mangroves with
financial support from MoEF. Dr.
K. Venkata Ramana is
a Research Associate in CSIR Project on Pollination ecology of Wendlandia glabrataDC. and Wendlandia tinctoria (Roxb.) DC. (Rubiaceae), the rare tree species of Eastern Ghats of
Andhra Pradesh. Mr. P. HareeshChandra is a Junior Research Fellow working in All India Coordinated
Research Project on Reproductive Biology of Rare Endangered and Threatened
(RET) Tree species funded by the Ministry of Environment and Forests,
Government of India, under the supervision of Prof. A.J. Solomon Raju, Andhra University,
Author Details: AJSR has done part of the field
work and write-up of the ms while VR and HCwere involved in field work and provided assistance in the preparation
of the manuscript.
Abstract: Eriolaena lushingtonii is an endemic and
threatened medium-sized deciduous tree species. The flowering is very brief and
occurs during the early wet season. The flowers are solitary, remain within the foliage and attract a few
bee foragers only in the presence of nectariferousand polleniferous plants such as Holarrhena pubescens, Grewia tiliaefolia and Orthosiphon rubicundus which are common, exhibit gregarious
flowering and attract a wide array of insects. In E. lushingtonii,
the floral characteristics suggest entomophily but it is exclusively melittophilous involving Apis,Trigona and Xylocopabees in the study area. The hermaphroditic flowers with the stigmatosestyle beyond the height of stamens and the sticky pollen grains do not
facilitate autogamy but promote out-crossing. The study showed that pollinator
limitation is responsible for the low fruit set but it is, however, compensated
by multi-seeded fruits. Anther predation by a beetle also affects the
reproductive success. Explosive
fruit dehiscence and anemochory are special
characteristics but these events are not effective during the wet season. The
locals exploit the plant for treating snake bites,
scorpion sting, making ropes and fuel wood. Therefore, the pollinator
limitation, ineffective anemochory, seedling establishment
problems and local uses largely contribute to the endemic and endangered status
of E. lushingtonii.
Keywords: Eriolaena lushingtonii, vulnerable, melittophily,
explosive fruit dehiscence, anemochory.
For images, table -- click here
INTRODUCTION
Eriolaena meaning the “woolly calyx
lobes” is a genus of the family Sterculiaceae with
8–10 arborescent deciduous tree and shrub
species distributed in India, South East Asia and southern China (Kubitzki & Bayer 2003). Tang et al. (2007) mentioned
that this genus has about 17 species but the authors have not given the list;
the state of information indicates that the taxonomic aspects have been poorly
studied. The
genus is characterized by actinomorphic, bisexual flowers lacking staminodes and ovary tipped with a long style and
spreading stigmas developing into a woody dehiscent capsule with numerous
winged seeds (Hutchinson 1967). The
identified species include E. kwangsiensis, E. glabrescens, E. quinquelocularis,
E. spectabilis, E. stocksii,
E. candollei, E. wallichii,
E. hookeriana and E. lushingtonii.E. kwangsiensis is
endemic to China, E. glabrescens to south
Yunnan, Thailand and southern Vietnam while all other species including E. hookeriana and E. lushingtoniioccur in India and elsewhere in tropical and subtropical Asia. E. hookeriana occurs in the
peninsular India, Maharashtra and Sri Lanka while E. lushingtoniiis an endemic and endangered deciduous tree species of southern peninsular
India where it is restricted to open slopes of moist deciduous forests at
350–900 m (Hutchinson 1967; Kubitzki &
Bayer 2003; Tang et al. 2007; Rao & Pullaiah 2007). The chronology of field collection of E. lushingtoniiin peninsular India has been noted by Rao & Pullaiah (2007). In Andhra Pradesh, the
type collection was made by Lushington during
pre-independence time from the Nallamalai Hills in
Kurnool District. After a lapse of
about 70 years, Ellis collected it from ChelamaReserve Forest of Kurnool District on 05 July 1963 and from the adjacent
locality Rollapenta on 16 August 1972 in the Nallamalais. In
Tamil Nadu, it was collected from the south Srivalliputhur Reserve Forest on 24 July 1965 by E. Vajravelu. Malick (1993) noted the distribution of this
species in Karnataka and Kerala. None of these species have been investigated for their reproductive
ecology. But, there is one
reference to the pollination aspect of E. quinquelocularisin a study on the reproductive phenology of a tropical dry forest in Mudumalai in southern India by Murali& Sukumar (1994). In this study, these authors noted that E.quinquelocularis is entomophilousand anemochorous. Keeping this state of information in
view, E. lushingtonii has been studied for its
floral ecology and pollination in recognition of its endemic and Vulnerable (WCMC 1998) status and the same is presented and
discussed in this paper.
MATERIALS AND METHODS
In Andhra Pradesh, Eriolaena lushingtoniioccurs in two localities only. A
small population consisting of 25 individuals occurs in the dry deciduous
forest near Chinarutla Gudem(16002’N & 78057’E; elevation 727.5m) which is a part of Nallamalaiforest in Markapuram Division, PrakasamDistrict (Images 1a-c). Another
population occurs in Nagarjuna Sagar-SrisailamTiger Reserve (15053’–16043’N & 78030’–79028’E;
837.4m) which covers an area of 3,568sq.km in the Nallamalai Hill Range. The first population was used for study during May 2010–September
2011. The second population could
not be accessed due to entry restrictions.
Twenty five tagged mature buds were
followed for recording the time of anthesis and
anther dehiscence; the mode of anther dehiscence was also noted by using a 10x
hand lens. Five flowers each from
ten trees selected at random were used to describe the flower morphology such
as flower sex, shape, size, colour, odour, sepals, petals, stamens and ovary. Ten mature but undehiscedanthers were collected from five different plants and placed in a petri
dish. Later, each time a single
anther was taken out and placed on a clean microscope slide (75x25 mm) it was
dabbed with a needle in a drop of lactophenol-aniline-blue. The anther tissue was then observed
under the microscope for pollen, if any, and if pollen grains were not there,
the tissue was removed from the slide. The pollen mass was drawn into a band, and the total number of pollen
grains was counted under a compound microscope (40x objective, 10x eye
piece). This procedure was followed
for counting the number of pollen grains in each anther that was
collected. Based on these counts,
the mean number of pollen produced per anther was determined. The characteristics of pollen grains
were also recorded. Five flowers
each from ten trees were used for testing stigma receptivity. It was tested
with hydrogen peroxide from mature bud stage to flower drop as per Dafni et al. (2005). Hydrogen peroxide when applied to stigma does not stain but produces
bubbles as a result of catalase (peroxidase) presence. The period of bubble production was
taken as the duration of stigma receptivity. A sample of 50 just
fallen flowers were taken to record the percentage of anther predation
by an unidentified beetle. The fruit set rate in open-pollinations was recorded
by counting the number of fruits per tree. Fifteen trees were used for this purpose. Regular field visits were
conducted to observe the foraging activity of insects but the insects were not
regular and consistent in forage collection from E. lushingtonii. However, we could record and identify
the foragers from their occasional foraging activity. The insects were observed with reference
to the mode of approach, landing, probing behaviour,
the type of forage they collect, contact with essential organs to result in
pollination, inter-plant foraging activity in terms of cross-pollination,
etc. Fruits and seed
characteristics were also recorded. Field observations were made on seedling
establishment rates. In the entire
study area, only 21 out of 214 seedlings were established and growing
continually while all others perished subsequently within three weeks. The soil samples were collected from
this area for pH, electrical conductivity, organic carbon, Pb,
Na, Cd, Cu, Zn, Mn, Fe and soil microbial carbon. Thesoil analysis was done by the Central Research Institutefor Dry land Agriculture, Hyderabad. The plant habit, flowers and fruits were photographed with Nikon D40X
Digital SLR (10.1 pixel). Magellan Explorist 210 Model Digital Global Positioning System was
used to record the coordinates—latitude, longitude and altitude.
RESULTS
The habitat of E. lushingtonii is rocky with black clay soil. The soil is
principally transported from other areas during rainy season. Its pH is slightly acidic; salinity, Pb, Cu, Fe, Na, SMBC levels are very low; Mn and organic carbon are very high; Cd and Zn levels are
at normal range (Table 1). The soil
chemical profile for the analyzed parameters indicated that the slightly acidic
pH appeared to be playing an important role in the availability of
micronutrients to the plant growth (Rengasamy &
Olsson 1991; Doran & Parkin 1996; Moody &
Aitken 1997). In such soil
environments, the medium-sized deciduous tree species, E. lushingtonii occurs naturally. The plant displays leaf flushing during
May–June. The flowering event
occurs from the 3rd week of June to the 3rd week of
July. An individual tree flowers
for about two weeks while the flowering duration at population level is about
four weeks. The flowers are solitary,pedicellate and borne in the leaf axils hanging
downwards or at right angles to the axis (Image 1d). They are large, 25mm long, 42mm wide,
yellow, mildly fragrant, hypogynous, actinomorphic
and bisexual. The calyx is composed
of five pale green 22mm long free linear sepals with stellate-hairs on the
outside and villous on the inside. The corolla has five free petals and arranged alternate to sepals; they
are golden yellow, reflexed and each petal is 20mm long and 5mm wide. The stamens are 18–20, yellow and
fused at their base to form a stamina column. Free filaments arise from the length of
the tube and each is 6mm long and surmounted by a 2mm long bilocular, tetrasporangiateanthers with parallel locules. The staminodesare completely absent and the fertile anthers are of varying heights due to
varying filament length. The ovary
is 5mm long, 4–5 locular and each locule is 6–8 ovuled; the
total number of ovules per ovary is 35–40 (Image 2g). The style is 18mm long, brownish, stigmatose and stellately tetra
or pentafid at its apex. The stigmatic lobes are equal, pubescent
and each of them is 2mm long. The stigmatose style extends far beyond the height of stamens.
The mature buds are open from
0700–0800 hr (Image 1e). The sepals and petals unfold and reflex
backwards exposing the entire length of staminalcolumn and style and stigmatic lobes (Image 2a,b). Anther dehiscence and stigma receptivity
occur after anthesis; anthers dehisce first by
longitudinal slits (Image 2c) while the stigmatic lobes unfold completely about
3 hours later (Image 1f,g). In this
state, the stigmatic lobes are semi-wet, pubescent and remain wet for two days
(Image 2f). During this
period, they are receptive to pollen and it is further confirmed by hydrogen
peroxide test. The pollen output per anther is 763±30 and it varies from
13,734–15,260 depending on the number of stamens. The pollen grains are spherical, echinate, panporate, sticky and 58.1µm in size (Image 2d). The pollen-ovule ratio is also not
constant due to the varying number of ovules per flower. A flower produces 0.8±0.1 µl of nectar
at the base of the ovary, which is firmly held by the pubescent hairs, and the
nectar glistens against sunlight. The flowers fall off on the 4th day if not pollinated while in
the pollinated ones, the ovary remains and all other flower parts fall off over
a period of 4–10 days.
During the entire flowering
season, the flowers were occasionally foraged by the honey bees Apis dorsata, A. cerana and A. florea,
the stingless bee, Trigona iridipennis, and the carpenter bees Xylocopa pubescens and X. latipesduring day-time especially during the forenoon period due to availability of
fresh nectar and pollen. The
carpenter bees were found to be feeding only on nectar while the other bees
also on pollen. These bees
exhibited two different approaches, the first included
an approach of the bee in upright position from the side bypassing the stamens
and stigmas for probing the ovary base directly for nectar collection while the
second included an approach of the bee in upright position via the stamens and
stigmatic lobes for collecting nectar from the ovary base. In both the approaches, the pollen
collecting bees after nectar collection ascended to the place of stamens for
pollen collection; sometimes these bees first
collected pollen and then descended into the ovary base for nectar
collection. All the bees except the
stingless bees had contact with the stamens and stigmatic lobes while collecting
nectar and/or pollen in the same or different visits; such a contact was
considered to result in pollination. The production of a small number of flowers at tree level was found to
be driving the bee foragers from one plant to the other to seek more forage and
such a foraging behavior was considered to result in cross-pollination. The stingless bee had contact with the
stigmatic lobes occasionally while collecting pollen from the anthers and this
contact was considered to be effecting pollination. A beetle (unidentified) feeds on the
anthers leaving the filaments and other floral parts intact; the percentage of
such flowers accounts for 5% of the sampled flowers (Image 1h).
In the habitat of E. lushingtonii, the trees Holarrhena pubescens (Apocynaceae),Grewia tiliaefolia(Tiliaceae), Pterocarpus marsupium and the herb Orthosiphon rubicundus (Lamiaceae)
were found to be flowering simultaneously. P. marsupium is a rare species while the other
species are common. The flowers are
white in H. pubescens, white lilac in O. rubicundus and yellow in G. tiliaefoliaand P. marsupium. The flowers are borne in small to large
clusters or aggregates, anthese during early morning
and appear prominently against the foliage in these species. Further, the flowering is profuse in G.tiliaefolia and O. rubicundus. The flowers of all four species are nectariferous and offer nectar and pollen as rewards to
pollinators. With the production of ample floral forage, these plant species
attracted a wide array of insects representing bees, wasps and
butterflies. With simultaneous
flowering of these plant species in the habitat, E. lushingtoniiwith a small number of solitary flowers covered in the foliage was found to be
unattractive and hence were visited only occasionally by some bees.
In fertilized flowers of E.lushingtonii, the ovary gradually bulges,
transforms into fruit and produces seeds (Image2h,i). The total duration from fruit initiation
to maturation and dispersal is about four weeks. A tree produces 25–39 capsular
fruits. The fruit is a
3.5–3.7 cm long woody 4–5 lobed capsule with 13–22 ascending
seeds and wool filling the spaces between them. The seeds possess apical membranous wings which are about as long as the body of the seed. The mature, dry capsules dehisce loculicidally a bit explosively liberating winged seeds
into the air (Image 2j,k). The
seeds thus released are driven away by the prevailing wind and they germinate
readily and produce seedlings (Image 2l,m). The rocky habitat with migrated soils
was found to be unfavourable for seedling establishment
even if seed germination is successful. The aged trees cut to ground level and beyond by locals were found to
show new vigorous growth continually from the root system left in the ground
(Image 3a-c).
DISCUSSION
Eriolaena is a genus distributed
exclusively in dry or moist deciduous open forest. Its species flower either in
the dry or wet season (Tang et al. 2007). In India, Murali & Sukumar (1994) reported that E. quinquelocularis,
occurring in a dry forest of Mudumalai in southern
India, exhibits leaf flushing and flowering events successively within the dry
season. Chettyet al. (2008) noted that in E. hookeriana,
flowering and fruiting events take place during January–October. E. lushingtonii is also a dry
deciduous open forest species but shows leaf flushing in late dry season while
flowering during the early wet season. Tang et al. (2007) reported that the flowers of all Eriolaena species are characteristically yellow. E. hookeriana, a dry deciduous species confined to
India and Sri Lanka also produces yellow flowers. E. quinquelocularis, a dry deciduous species
distributed in and outside India also bears yellow flowers which are more
conspicuous due to its flowering during the dry season when the herbaceous
flora disappear and tree species remain leafless (Murali & Sukumar 1994). In E. lushingtonii,
the flowering occurs for a brief period during which a few yellow flowers are
produced daily at tree level. Further, the flowers are solitary, remain covered in the foliage and
hence, go unnoticed by the flower visitors due to emergence of herbaceous flora
and leaf flushing in deciduous tree species following rainfall in early
June. Such flowering pattern and
flower position appear to be disadvantageous for the plant to attract flower visitors,
especially in the presence of other simultaneously flowering plant species in
the area. The nectariferousand polleniferous plant species, Holarrhena pubescens, Grewia tiliaefolia and Orthosiphon rubicundus with common occurrence, gregarious
flowering and very conspicuously present grouped flowers against the foliage
attract a variety of insects comprising of bees, wasps and butterflies while E.lushingtonii receives only occasional foraging
visits of bees. All the
co-flowering plant species and E. lushingtonii produce
a minute amount of nectar. G. tiliaefoliawith numerous stamens is a copious pollen producer. E. lushingtonii produces a moderate amount of pollen
while the other two species a small amount of pollen. The aggregate arrangement of flowers and
a profuse flowering pattern appear to play an important role in the
co-flowering plant species to compete well with E. lushingtoniifor flower visitors.
Murali & Sukumar (1994) reported that E. quinquelocularisis insect-pollinated in the Mudumalai deciduous
forest. In the Nallamalaideciduous forest, E. lushingtonii with morninganthesis,exposed dehisced anthers presenting pollen and exposed flower base presenting
nectar after anthesis indicates that it is adapted
for day-active flower foragers for pollination. In line with this, bees visit the
flowers for forage, the honey bees and stingless bees
for both the pollen and nectar while carpenter bees only the nectar. The low energy-requiring small bodied stingless bees habitually confine to an individual
plant for forage collection while high energy-requiring moderate-sized honey
bees and large-bodied carpenter bees fly between plants for more forage
collection (Raju 2007). In
the present study, E. lushingtonii with a
small number of solitary flowers at tree level drive even the stingless bees to
fly between plants to collect as much forage as possible for meeting their own
needs and for their colony members as well. Such a foraging activity by these bees
contributes to the maximization of cross-pollination rate. The flowers with 2-day long stigma
receptivity have the option to receive pollen and subsequently to achieve
pollination by the visiting bees on the second day of anthesis. The floral characteristics of E. lushingtonii as detailed previously conform to the entomophilous pollination syndrome (Faegri& van der Pijl 1979). Since only bees are involved in
pollination, it can be stated that E. lushingtoniiis melittophilous in the study area. Sharma (Sharma
1967) stated that Eriolaena pollen
grains are stenopalynolous and also described the
pollen grain characteristics of Eriolaenaspecies, E. wallichii, E. hookerianaand E. spectabilis. These three species exhibit similar
pollen morphology and hence have been referred to as E. wallichiitype by him. The panporate and spinulatecharacters typify this type of pollen grains. He considered the panporate condition as a climax in the line of apertual evolution. The spherical, echinate and panporate pollen grains of E. lushingtoniirepresent the E. wallichii type. Further, the pollen grains are sticky
and there is no possibility of their dissemination into the air during the wet
season. The sticky and spinulate nature of the grains in this species is an
adaptive feature for melittophily and is also
advantageous for the bees to load and pack them into corbiculaefor transportation to their nest (Gottsberger 1989).
E. lushingtonii with hermaphroditic sexual system and weak protandryfacilitates both self- and cross-pollination. The ability to fruit through both modes
of pollination is adaptive but it is essentially insect-dependent since the
extension of the stigmatose style beyond the height
of stamens and the sticky nature of pollen grains which is further dampened by
the high humidity during wet season preclude the occurrence of autogamy. The study indicates that the pollinator
limitation appears to be playing a considerable role for the low fruit set, and
the fruit number per tree in the entire population stands below forty. However, the low fruit set is
compensated to a small extent by the production of several seeds per
fruit. The seed set rate could be
attributed to the number of fertilized ovules and to the nutrient status of the
soil. Further, the flowers attract
a beetle which consumes anthers and such flowers do
not participate in sexual reproduction. Murali & Sukumar(1994) reported that in E. quinquelocularis,
the fruits mature within a short duration and this reason could be the
investment of more resources in flowers. Further, the fruit dehisce explosively to disseminate seeds into the air
and hence the seeds are characteristically wind-dispersed for which the dry
season is very effective because of low humidity at that time. All the reproductive events, flowering,
fruiting and seed dispersal of this plant occur within the dry season only. In E.lushingtonii, fruits mature within a month,
dehisce explosively liberating winged seeds into the air and the seeds are
wind-dispersed; and hence the plant is anemochorous (Maury-Lechon & Curtet1998). But, the wet season is not
ideal for anemochory due to high humidity and
precipitation. However, seed
dispersal during the wet season enables seeds to avoid exposure to desiccation
during the dry season. The seeds are not dormant due to their immediate
germination following their dispersal. But, the seedlings do not grow properly and subsequently wither
away. There are very few seedlings
that finally establish and produce new plants. The non-establishment of the seedlings
could be attributable to several factors. The rocky habitat with migrated soil due to runoff from the uphill areas
does not favour the establishment of the
seedlings. The soil reaching the
habitat does not remain there and is subject to erosion by rainwater. Further, the soil chemical analysis
indicates that it contains micronutrients at low or normal range with high
organic carbon. But, its slight
acidic pH appears to be limiting the availability of micronutrients and the
high organic carbon in soils may also tie up the micronutrients in unavailable
forms (Sarwar et al. 2010). The quick growth of grass cover is also
another factor that is affecting the growth of seedlings of E. lushingtonii. The successful establishment and the continuous growth of the seedlings
of E. lushingtonii into new plants could be
related to the existence of some pockets of deep soil within the rocky
habitat. The areas where the plants
are established have deep soil which enables them to
penetrate gradually through crevices and cracks of rocks. The study suggests that the soil layer
and its composition, and also robust grass growth have a great bearing on the
build-up of population of E. lushingtonii.
Further, the locals use leaf extract as antidote for treating snake bites and scorpion
sting and bark fiber for making ropes. They also cut down the trees to use as fuel wood; new shoots arise from
the underground root stock if the latter is
intact. Therefore, the pollinator
limitation, anther predation, seed dispersal during the wet season,
establishment problems and local uses collectively contribute to the endemic
and endangered status of E. lushingtonii. These aspects are to be taken into
account while framing effective conservation and management measures as well as
the recovery of the population of E. lushingtonii.
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In: IUCN 2012. IUCN Red List of Threatened Species.Version 2012.2. <www.iucnredlist.org>.
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