Journal of Threatened Taxa | www.threatenedtaxa.org | 26
November 2019 | 11(14): 14923–14926
On the
floral biology and pollination of a rare Twining Liana Sarcolobus
carinatus Wall. (Asclepiadoideae:
Apocynaceae) in Coringa
Mangrove Forest, Andhra Pradesh, India
A.J. Solomon Raju
Department of Environmental Sciences, Andhra
University, Waltair Junction, Visakhapatnam, Andhra
Pradesh 530003, India.
doi: https://doi.org/10.11609/jott.4870.11.14.14923-14926
Editor: Analinda C. Manila-Fajardo, University of the Philippines,
Laguna, Philippines. Date of publication: 26 November
2019 (online & print)
Manuscript
details: #4870 | Received 07
February 2019 | Final received 11 October 2019 | Finally accepted 03 November
2019
Citation: Raju, A.J.S. (2019).
On the floral biology and
pollination of a rare Twining Liana Sarcolobus
carinatus Wall. (Asclepiadoideae:
Apocynaceae) in Coringa
Mangrove Forest, Andhra Pradesh, India. Journal of Threatened Taxa 11(14): 14923–14926.
https://doi.org/10.11609/jott.4870.11.14.14923-14926
Copyright: © Raju 2019.
Creative Commons Attribution 4.0 International
License. JoTT
allows unrestricted use, reproduction, and distribution of this article in any
medium by adequate credit to the author(s) and the source of publication.
Funding: University Grants Commission, Government of India, New Delhi.
Competing interests: The author declares no competing interests.
Acknowledgements: This work has been supported by the University Grants Commission,
Government of India, New Delhi, in the form of BSR Mid-Career Award to Faculty
Members.
Sarcolobus is a genus of sub-family Asclepiadoideae,
family Apocynaceae according to APG IV System
Classification (2016). Rintz (1980) documented that this genus formerly consisted
of 17 described and eight undescribed species distributed in India, Malesia,
Melanesia, and Australia. He revised the
genus and listed only four species, S. carinatus,
S. retusus, S. globosus,
and S. oblongus as occurring in Australia. All four species are restricted to coasts and
areas of brackish water, on mud in mangrove and swamp forests, and on sand and
coral beaches scrambling over strand vegetation. Later, Forster (1991) documented that Sarcolobus is a genus of 13 species in India,
Malesia including Papuasia, Melanesia, and
Australia. Three species occur in
Australia and eight species in Papuasia. ENVIS Centre on Floral Diversity hosted by
Botanical Survey of India published a status document on Indian mangrove plant
species in 2016. In this document, S.
globosus and S. carinatus
are the only species listed under the genus Sarcolobus;
the former occurs in the Sunderbans, Mahanadi, and
the Nicobar Islands while the latter occurs along the east coast in the
mangrove areas of the Sunderbans, Mahanadi, Coringa tidal forests, and the Andaman Islands. Rintz (1980)
reported that S. carinatus is distributed from
India, Burma and the Andaman Islands with flowering during February to August
and fruiting in June. He mentioned that
the characters such as the shape of gynostegium, the
presence or absence of double corona, the shape of twin-pollinia, the shape of
fruit and the presence or absence of a coma on the seeds are important to
distinguish Sarcolobus from other genera of
the sub-family Asclepiadoideae. Forster (1991) also provided the characters
important to distinguish Sarcolobus from other
genera within the sub-family. These
include the presence of slight to strongly papillate style-head, the narrowly
oblong corpuscles that stand upright away from the anthers the length of the
pollinia, and the geniculate caudicles of the pollinaria. The
available information on Sarcolobus indicates
that the taxonomic details of its species need revision to define total species
accurately and provide distinguishing characters for each identified
species. Further, none of the species of
this genus have been studied thus far for their floral biology and pollination
in any part of the world. Therefore, the
present study is the first to document floral biology and pollination notes on S.
carinatus based on field study conducted during
January–December 2014 and January–December 2018 at Coringa
Mangrove Forest (16043’47.413”N & 82012’54.864”E),
Andhra Pradesh, India.
In the genus Sarcolobus, S. carinatus is the only species that occurs at Coringa Mangrove Forest.
Here, it grows as a semi-evergreen twining liana along the brackish
water creeks that end up after a short distance with the land (Image
1a-c). It uses Clerodendrum
inerme, a common mangrove associate distributed
towards landward side of mangrove forest, for vertical support. In areas where there is no supporting tree
species, it grows erect for some time, then the stems twist and hang downwards
almost touching the ground. It is rare
in occurrence. The stem is light yellow
initially and brown eventually. Leaf is
petiolate, blade thick, coriaceous, elliptical and bright to dull green (Image
1d). Pheno-events,
leaf fall, leaf flushing, flowering and fruiting occur sequentially. Leaf fall occurs during May and leaf flushing
commences immediately from the old stems, but it is complete by July. Floral bud initiation occurs simultaneously
(Image 1e–g) with leaf flushing and flowering commences in the last week of
July, reaches to peak phase during second week of August and ceases by the end
of August. Inflorescence is a
pedunculate spirally elongating raceme with 6–7 hermaphroditic flowers borne at
leaf axils (Image 2a). Calyx is
represented by five inconspicuous green sepals with ciliate margins. Corolla is represented by five glabrous 6–10 cm long petals each with rows of brown to
purple dots on the upper surface. Corolline corona composed of five distinct yellow pad-like
segments, positioned on the petals in one whorl, aligned with the petals and
located between the corolla and androecium; the coronal segments are separated
from the staminal corona by a narrow cleft (Image 2d). Anthers are attached with the pistil forming
papillate and capitate gynostegium (Image 2b, c); the
style-head with five grooves support the anthers. The anther wings are narrow but extend back
beneath the stigma to end up abruptly at the stamen tube without curving
downward. Anthers are represented by
twin sub-globose pollinia (Image 2e), each is attached to the caudicle by its basal end and hence held semi-erect and is
away from the style-head. Both pollinia
are attached by caudicles to the corpusculum which is
a black horny structure and quite prominent against yellow corolline
and staminal coronas. The ovary is
bicarpellary syncarpous, whitish-green and many-ovuled on axile placentation
(Image 2f); the style is one with stigma deeply 5-angled, the center with a papillose spherical knob, and five narrow
ridges radiating from it to the apex of angles.
Rintz (1980) reported that Sarcolobus genus
includes both coronal and non-coronal species.
He noted that S. retusus and S.
oblongus are non-coronal species lacking both corolline
and staminal coronas while S. carinatus is a
coronal species consisting of both corolline and
staminal coronas. He noted that corolline and staminal coronas are either present or absent
in S. globosus. Forster (1991) reported that S. globosus and S. kaniensis
are coronal species consisting of both corolline and
staminal coronas. In S. vittatus, corolline corna is present but staminal corona is absent. In S. brachystephanus,
S. hullsii, S. porcatus, S.
ritae, S. secamonoides
and S. spathulatus, corolline
corona is absent but staminal corona is present. The present study substantiates the report by
Rintz (1980) that S. carinatus
is a coronal species with well-developed corolline
and staminal coronas. In S. carinatus, the stigmatic chamber in gynostegium
has nectariferous tissues which secrete nectar in
minute volume and the nectar can accumulate in the staminal corona or at the
area where the corolline corona is connected to the gynostegium. Such a
secretary pattern is related to the specialized pollination mechanism and it is
characteristic of Asclepiadoideae (Kunze 1991, 1997;
Demarco 2008). The pollinia in the
flowers of S. carinatus represent coherent
masses of pollen grains and are transferred as single units by flower-visiting
insects when the latter collect nectar.
Field observations indicated that only juvenile carpenter bees (Xylocopa latipes
and X. pubescens) visit the flowers soon after
anthesis and until evening for nectar collection during which their legs or
tongue are trapped between the pollinia and in effect, their struggle to
withdraw these parts from the flowers result in them capturing the
pollinia. Then, they leave carrying the
pollinia and in their subsequent visit to another flower get rid of the
pollinia and deposit them on that flower effecting pollination. It is not clear if there is any chance of the
carpenter bees losing the pollinia during flight. Field observations also indicated that
carpenter bees make few visits and very often occasionally to the flowers of S.
carinatus; such a situation could be attributed
to the landward side location of the plants from tidal creeks, scattered
occurrence of plants, production of a few flowers per day, which are mixed with
the foliage and flowers of host plants, occurrence of high winds during
flowering season, which influence the foraging activity of carpenter bees.
In S. carinatus, the fertilized flowers
take 3–4 months to produce mature fruits (Image 2g-o). The fruits are of follicle type, 5–6 cm long,
ellipsoid in shape with 3 flattened ribs, keeled along the dorsal side and
terminated into a short beak. They are
initially green and brown when mature.
Each fruit produces many seeds attached to the vertical receptacle
(Image 2p). Mature seeds are dark brown,
lacking coma, papery, 10–12 mm long, broadly ovate and flattened. Fields observations indicated that individual
plants produce 2–4 fruits in a flowering season suggesting that S. carinatus is an obligate out-crosser and essentially
dependent on insect pollinators, particularly carpenter bees which act as
efficient pollinators. Further, the
fruit set rate is probably pollinator-limited but individual flowers upon
receipt of a pair of pollinia each consisting of several pollen grains are able
to produce several seeds per fruit as compensation against low fruit set. Therefore, the rare occurrence of S. carinatus is attributable mainly to specialized
pollination system that prevents insect pollinators other than carpenter bees
and pollinator limitation, resources available to the plant in areas where it
occurs and edible nature of tender fruits which are often eaten by locals.
Forster (1991) reported that Sarcolobus
seeds with a layer of seed coat and well-developed lateral margins appear to be
adapted to water dispersal but suggested field observations to confirm the
same. In this study, S. carinatus disperses seeds from mature and dry fruits
upon dehiscence into the air during November–December. The flat, papery nature of seeds without coma
appear to be a perfect adaptation for wind dispersal. The wind-driven seeds that fall into the
tidal water subsequently disperse by floating and settle in muddy areas where
they germinate and produce new plants from seedlings if the soil environment is
favourable (Image 2q). Therefore, S. carinatus is both anemochorous
and hydrochorous.
Despite efficient seed dispersal modes this plant species is not able to
recruit new plants due to its restricted habitat requirement; the sites along
the tidal creeks which interface with landward zone.
In Coringa Mangrove Forest, ecotourism,
fishing activity and fuel wood collection are regular activities and if these
activities are not regulated, the mangrove plant species that grow along
brackish water creeks interfacing with landward side are at risk. S. carinatus
is one such species that is most likely to disappear in the near future because
of its rare occurrence if proper conservation and management measures are not
implemented by the forest department.
References
APG IV System Classification (2016). An update of the Angiosperm Phylogeny Group Classification for the
Orders and Families of Flowering Plants: APG IV. Botanical Journal of Linnean Society 181: 1–20. https://doi.org/10.1111/boj.12385
Demarco, D. (2008). Glandulas
de orgaos vegetativos aereos e florais de especies de Asclepiadoideae
(R.Br.) Duby (Asclepiadoideae,
Apocynaceae) de mata atlantica de estado de Sao Paulo.
Ph.D Thesis, Universidade Estadual de Campinas, Campinas, Brazil, 392pp.
Forster, P.I. (1991). A taxonomic revision of Sarcolobus R.Br. (Asclepiadaceae:
Marsdenieae) in Australia and Papuasia.
Austrobaileya 3: 335–360.
Kunze, H. (1991). Structure and function
in asclepiad pollination. Plant Systematics and Evolution 176: 227–253.
Kunze, H. (1997). Corona and nectar system
in Asclepiadinae (Asclepiadaceae).
Flora 192: 175–183. https://doi.org/10.1016/S0367-2530(17)30774-0
Rintz, R.E. (1980). A revision of the genus Sarcolobus
(Asclepiadaceae). Blumea
26: 65–79.