Entomophily,
ornithophily and anemochory in the self-incompatible Boswellia
ovalifoliolata Bal. & Henry
(Burseraceae), an endemic and endangered medicinally important tree species
A.J. Solomon
Raju 1, P. Vara Lakshmi 2, K. Venkata Ramana 3& P. Hareesh Chandra 4
1,2,3,4 Department
of Environmental Sciences, Andhra University, Visakhapatnam, Andhra Pradesh
530003, India
Email: 1 ajsraju@yahoo.com
(corresponding author), 2 varalakshmi83@gmail.com, 3 vrkes.btny@gmail.com, 4 hareeshchandu@gmail.com
Date of publication
(online): 26 July 2012
Date of publication (print):
26 July 2012
ISSN 0974-7907 (online) |
0974-7893 (print)
Editor: Cleofas
R. Cervancia
Manuscript details:
Ms # o2964
Received 09 October 2011
Final received 12 May 2012
Finally accepted 16 June
2012
Citation: Raju, A.J.S., P.V. Lakshmi, K.V. Ramana & P.H.
Chandra (2012). Entomophily, ornithophily and anemochory in
the self-incompatible Boswellia ovalifoliolata Bal. & Henry (Burseraceae), an endemic and endangered medicinally
important tree species. Journal of Threatened Taxa4(7): 2673–2684.
Copyright: © A.J. Solomon Raju, P. Vara Lakshmi, K.
Venkata Ramana & P. Hareesh Chandra 2012. Creative Commons
Attribution 3.0 Unported License. JoTT allows unrestricted use of this
article in any medium for non-profit purposes, reproduction and distribution by
providing adequate credit to the authors and the source of publication.
Author
Details: Prof. A.J. Solomon Raju is the
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 UGC and MoEF.
P. Vara Lakshmi is project fellow working
in the major research project on Reproductive Biology, Conservation and
Management of Endemic and Globally Endangered tree species, Boswellia ovalifoliolata(Burseraceae) and Terminalia
pallida (Combretaeae) at Seshachalam Hills, Andhra Pradesh, funded by the
University Grants Commission, New Delhi, under the supervision of Prof. A.J.
Solomon Raju.
K. Venkata Ramana and P. Hareesh Chandra are junior research
fellows working in another research project under the supervision of Prof. A.J.
Solomon Raju.
Author
Contribution: AJSR has done part of the field work and
write-up of the ms while PVL, KVP and PHC were involved in field work and
provided assistance in the preparation of the ms.
Acknowledgments: This study is a part of the research work
carried out under a major research project on reproductive biology, conservation
and management of endemic and globally endangered tree species, Boswellia
ovalifoliolata (Burseraceae) and Terminalia pallida (Combretaeae) at
Seshachalam Hills, Andhra Pradesh, sanctioned to the first author by the
University Grants Commission, New Delhi (F. 34-69/2006(SR). All authors thank
Dr. V.V. Ramamurthy, Division of Entomology, Indian Agricultural Research
Institute, New Delhi, for identification of some insects reported in the
present study.
Abstract: Boswellia ovalifoliolata(Burseraceae) is a narrow endemic and endangered deciduous tree species. Its flowering, fruiting and seed
dispersal events occur in a leafless state during the dry season. The flowers are small, bisexual, mildly
odoriferous and actinomorphic; weakly protandrous but
strictly self-incompatible. While
insects and sunbirds pollinate the flowers, floral characteristics suggest that
entomophily is the principal mode. Both bud and flower feeding by a weevil and flower and fruit feeding by
the Palm Squirrel have been found to affect the success of sexual
reproduction. The Garden Lizard
serves as a predator of pollinating insects, especially bees and wasps, thus
influencing pollination of this tree species. Fruit set in open pollination is below
10%, rising to 34% in manual cross-pollination. Limitation of cross-pollination, space
constraints for seed production from all flower ovules and availability of
limited resources in rocky, dry litter of the forest floor appear to constrain
higher fruit set. Mature fruits
dehisce and disseminate their lightweight, papery winged seeds with the aid of
wind. The study site being windy provides
the necessary driving force for effective dispersal of seeds away from parent
trees. Seed germination occurs
following rainfall but further growth depends on soil water and nutritional
status. The success rate of
seedling recruitment is highly limited, and it could be due to nutrient-poor
soil and water stress resulting from dry spells during the rainy season.
Keywords: Anemochory, Boswellia ovalifoliolata, entomophily, ornithophily,
self-incompatibility.
For
figures, images, tables -- click here
INTRODUCTION
The genus Boswellia belongs to the
Burseraceae family and is widely distributed in the dry regions of tropical
Africa, Arabia and India. In
Africa, it is distributed in Somalia, Ethiopia, Eritrea, Kenya, Sudan,
Tanzania, Madagascar and some other countries. In Arabia, it is mainly restricted to
Yemen, Oman and Socotra. In India,
it is distributed in a few regions such as Rajasthan, southeast Punjab,
Danwara, Madras, etc. There are
about 18 species of Boswellia which are shrubs
or trees with outer bark often flaking. They include B. sacra, B. frereana, B. neglecta, B. microphylla, B.
papyrifera, B. ogadensis, B. pirottae, B. rivae, B. madagascariensis, B.
socotrana, B. popoviana, B. nana, B. ameero, B. bullata, B. dioscoridis, B.
elongata, B. serrata and B. ovalifoliolata. Only the last two
species have been reported to be distributed in India
(Arabia 2005; Latheef et al. 2008). Sunnichan et al. (2005) mentioned that B. serrata is the only
species found in India. But, other
workers reported that B. ovalifoliolata occurs on the foothills of the
Seshachalam hill ranges of Eastern Ghats in Chittoor, Cuddapah and Kurnool
districts of Andhra Pradesh up to an altitude of about 600–900 m. It is a globally endangered, strict
endemic medium-sized deciduous medicinally important tree species and listed in
CITES Red Data book under medicinal plants (Rani & Pullaiah 2002; Reddy et
al. 2002). Chetty et al. (2002)
reported that both B. serrata and B. ovalifoliolata occur at the
foothills of Seshachalam hill ranges of Eastern Ghats.
Reproductive biology information is
available for only a few species of Burseraceae such as Commiphora weightii,
Bursera medranoana, Sentiria laevigata and Boswellia serrata(Sunnichan et al. 2005). Boswellia
ovalifoliolata has not been investigated for its reproductive biology
despite its medicinal importance in India. Our field surveys in the areas of Tirumala Hills have shown that the
local tribes and others make deep incisions on the main trunk to extract the
gum and resin, causing damage to trees which leads to
population depletion. The gum is
used to treat a number of conditions including ulcers, fever, stomach pain,
scorpion sting, amoebic dysentery and hydrocele, while bark decoction is used
for joint and rheumatic pains (Henry 2006; Latheef et al. 2008). We have investigated the floral biology,
breeding behaviour, pollination and foraging behavior
of pollinators of B. ovalifoliolata in its natural area, and the
observations and results obtained are discussed in the light of the existing
relevant information.
MATERIALS AND METHODS
Study area
The study area included Kapilatheertham
and Deer Park areas of Tirumala Hills (13042’N & 79020’E,
elevation 751m) of the southern Eastern Ghats in Andhra Pradesh. The approximate number of trees of Boswellia
ovalifoliolata was 150 at Kapilatheertham and 60 at Deer Park. The trees occur mostly as small clusters
at the former area while they are mostly scattered at the latter area. In both
areas, the associated tree species are almost same and they include Zizyphus
rugosa (Rhamnaceae), Erythroxylum monogynum (Erythroxylaceae), Spondias
pinnata, Buchanania axillaris (Anacardiaceae), Gyrocarpus asiaticus(Hernandiaceae), Dalbergia paniculata (Fabaceae), Schleichera oleosa(Sapindaceae), Ochna obtusata (Ochnaceae), Hugonia mystax (Linaceae),Ficus mollis (Moraceae) and Azadirachta indica (Meliaceae). Of these, only the last one blooms
during the flowering season of B. ovalifoliolata. The floor of the area is completely dry
with exposed rocks during summer but it is covered with luxuriant growth of
herbaceous flora and grasses during rainy season. The field studies were carried out on
the flowering season, floral biology, foraging activity, behavior and
pollination by pollinators, and fruit, seed and seedling aspects of B.
ovalifoliolata during 2007–2010.
Floral biology
The overall timing of leaf fall, leaf
flushing, flowering and fruiting events was recorded. The number of flowers per
inflorescence (N = 20) was recorded for 10 selected inflorescences, two each
from five trees. These
inflorescences were simultaneously followed for their flowering duration. The floral characteristics were recorded
from 25 flowers collected from five each from five trees. Mature flower buds on ten inflorescences
were tagged and followed for recording the time of flower opening. The same flowers were followed for
recording the time of anther dehiscence. The pollen grain characteristics were recorded by consulting the book of
Bhattacharya et al. (2006). Pollen
production per flower was calculated following the method described by Cruden
(1977). Pollen
fertility was assessed by staining them in 1% acetocarmine. Stigma receptivity and nectar volume,
sugar concentration and sugar types were assessed by following the methods
prescribed by Dafni et al. (2005).
Breeding behavior
Fifty mature buds, five each from 10
inflorescences on five trees were bagged a day before anthesis without manual self pollination to know whether fruit set occurs through
autogamy. Another set of 50 mature
buds was selected in the same way, then emasculated and bagged a day prior to
anthesis. The next day, the bags
were removed and the stigmas were brushed with the freshly dehisced anthers
from the flowers of the same tree and re-bagged to know whether fruit set
occurs through geitonogamy. Ten
trees were selected for manual cross-pollination and open-pollination. One
hundred and twenty five flowers were used per each tree for manual
cross-pollination. For this, mature
buds were emasculated and bagged a day prior to anthesis. The next day, the bags were removed;
freshly dehisced anthers from flowers of another tree were brushed on the
stigma and re-bagged. Ten
inflorescences on each tree were tagged and followed for fruit set in
open-pollination (Sunnichan et al. 2005). The length of time followed for each of these breeding systems was six
weeks. Twenty stigmas, four each
from five trees were removed at 1500h and observed under the microscope for the
number of pollen grains deposited by pollen vectors. The per cent of
flower predation by an unidentified weevil was calculated by counting the
number of damaged flowers on 50 selected inflorescences on 10 trees.
Foraging behavior of pollinators and
pollination
Preliminary investigations on foraging
activity were made at different times of the day including dawn and dusk. Based on this information, the number of
foraging visits made by each species was made for 15 minutes in each hour
during the entire period of the day. This data was used to calculate the total number of foraging visits made
by each species for the entire day and also to calculate the total number of
foraging visits made by each category of foragers in order to evaluate their
relative importance and role in effecting pollination. The forage collected and the area of
contact of the species with the floral sex organs were also observed to
understand their role in pollination. Binoculars were specially used for this
purpose.
Fruit, seed and seedling ecology
Field observations on fruit, seed and
seedling ecology were also made to the extent possible due to certain
restrictions in the study areas.
RESULTS
Floral biology
In B. ovalifoliolata (Image 1a),
leaf shedding occurs during December–February, and flowering from first
week of March to second week of April at population level. An individual tree flowers for about
three weeks only. Leaf flushing
occurs from the three week of April and continues through rainy season. In a few trees, flowering occurs before
the fall of old leaves but complete leaf shedding occurs when flowering is at
its peak. The leaves are
imparipinnate and crowded at the ends of branches. The flowers are borne in branched
panicles at the ends of the branches (Image 1b). Each branch produces 8–10
inflorescences and each inflorescence produces 35.2±13.77 (Range 16–72)
flowers over a period of 5–14 days. The flowers are pedicellate, greenish-white, 6mm long, 5mm across, mildly fragrant, cup-shaped, bisexual and
actinomorphic. The sepals are five,
minute, basally connate, imbricate, light green, lightly pubescent outside and
persistent without any further growth during post-fertilization stage in
fruited flowers. The petals are
five, white, free, imbricate, 5mm long and erect. Stamens are inserted outside
a fleshy annular pinkish-red nectary disc which is
present outside the ovary at the flower base. They are 10 arranged in two whorls, each
with 2mm long white filament and 1mm long dorsifixed yellow dithecous anther
(Image 1c). The pistil is clearly
distinguished into ovary, style and stigma. The ovary is superior, trilocular, each
locule with two pendulous ovules borne on axile placentation. The style is light pink at base and dark
green above, 4mm long and trilobed. The stigma is short, capitate, shiny and wet papillate type (Image
1e,f).
The flowers open for a brief period daily
during 1100–1300 hr. A fully
open flower shows petals in erect position exposing the stamens and
stigma. The stigma extends 1mm
beyond the anthers and remains in that state throughout the flower’s life. Anther dehiscence is nearly synchronous
with flower opening. The anthers
dehisce by longitudinal slits along the theca and release pollen grains. The pollen grains are light yellow,
sticky, quadrangular, tricolporate with smooth exine and 66.4 µm in size (Image
1d). An anther produces 683.4±40.97
(Range 602–748) pollen grains while the total pollen output per flower is
6834 of which 72% is fertile and the remaining is sterile. The fertile pollen to ovule ratio is
820.1:1. The stigma attains
receptivity two hours after anthesis and remains receptive until the noon of
the next day. A flower produces 0.4±0.15 µl of nectar with 53.8±1.75%
(51–56 %) sugar concentration. The nectar sugars include glucose, fructose and sucrose with the last as
more dominant. The nectar also
contains both essential and non-essential amino acids. The essential amino acids are arginine,
histidine, lysine and threonine while the non-essential amino acids are
alanine, aspartic acid, cysteine, glysine, hydroxyproline, serine, glutamic
acid and tyrosine. The flowers drop
off by the evening of the second day if not pollinated while only pistil and
sepals remain intact in pollinated flowers.
Foraging activity and pollination
The flowers offer both pollen and
nectar. They were foraged by
insects and sunbirds during daytime throughout the flowering season. The insect foragers included bees,
wasps, flies and butterflies. The bees included Apis dorsata (Image 1g),
A. cerana, A. florea, Trigona iridipennis (Image 1h), Ceratina sp.
(Image 1i), Xylocopa latipes (Image 1j) and X. pubescens (Image
1k). Juvenile Xylocopa bees
were nectar foragers while all other bees were nectar and pollen
foragers (Table 1). Apis andTrigona bees foraged throughout the day from 0700–1800 hr while
the other bees during 0800–1300 hr (Fig. 1). The wasps included Scoliasp., Rhynchium sp. (Image 1o), Eumenes sp. (Image 1n),Eumenes petiolata (Image 1m) and E. conica (Image 1l). They were exclusively nectar foragers
and their foraging visits were almost confined to 0800–1400 h (Fig.
2). The flies were represented by Hyperaloniasp. only (Image 1p); it collected only nectar during 0800–1200 hr
(Fig. 3). Butterflies included four
species - Catopsilia pomona (Image 1q), Junonia
lemonias (Image 2a), Acraea violae (Image 2b,c) and Danaus
chrysippus (Image 2d). They are
nectar foragers and visited the flowers during 0800–1700 hr (Fig.
4). The sunbirds,Nectarinia asiatica (Image 2f,g) and N. zeylonica visited the
flowers day long from 0700 to 1800 hr with more foraging activity during
1000-1300 hr (Fig. 5). Of
the total insect and sunbird visits, bee visits constituted 62%, wasps 17%,
sunbirds 12%, butterflies 7% and flies 2% (Fig. 6). Other passerine birds such as Pycnonotus
jocosus, P. cafer, Pericrocotus cinnamomeus, Dicrurus adsimilis,
D. caerulescens, Parus xanthogenys, Turdoides striatus, Motacilla
flava, and a non-passerine bird, Megalaima haemacephala also visited
the flowering trees in quest of nectar but discontinued flower-probing
immediately (Table 1).
All insect categories after landing
probed the flowers for nectar and/or pollen. The forehead and ventral surface of the
body of the insects except butterflies were found to be contacting the anthers
and stigma invariably while probing the flower for nectar. The bees while collecting pollen from
the anthers normally contacted the stigma on their underside and hence were
considered to be transferring pollen and effecting pollination. Trigona bees mostly forage on one
tree largely effecting self-pollinations. Apis, Ceratina and juvenile Xylocopa bees made frequent
inter-tree flights in search of more forage. Wasps also exhibited similar foraging
behaviour. The fly tended to forage
mostly on the same tree collecting nectar very slowly from each flower. The butterflies made frequent inter-tree
flights in quest of more nectar; they inserted proboscis through the stamens as
well as from the sides of the petals for nectar collection. The Oriental Garden Lizard, Calotes
versicolor (Squamata: Agamidae) was found to lie in wait closely to the
flowers to capture the foraging insects (Image 2e). The prey species for this lizard were
mainly the foraging bees and wasps. Sunbirds landed on the inflorescence
branches, walked to the flowers and inserting their curved beak to collect
nectar; while doing so, the beak invariably contacted both the stigma and
stamens and such a contact was considered to be transferring pollen and
effecting pollination.
Breeding behavior
The inflorescences with mature buds when
bagged without emasculation did not set any fruit. Further, the manual flower-to-flower
selfing on certain inflorescences of the same tree also did not produce any
fruit. In different trees, the
fruit set varied from 10.8 to 33.7 % in manual cross-pollinations while it
ranged from 1.8 to 9.8 % in open pollinations (Table 2). The results indicated that the site has
no effect on fruit set rate from open or hand-cross pollinations. Further, the
difference in fruit set rate in these two pollination modes is quite
significant (Pearson’s Correlation Coefficient 0.877). A weevil species was found feeding on
buds and flowers (Image 2j,k); the percent of bud predation is 18% and that of
flower predation is 27%. Further, Three-Striped
Palm Squirrel Funambulus palmarum (Family: Sciuridae) was found to be
feeding on flowers and fruits (Image 2h,i). The exact percentage of flowers and
fruits fed could not be estimated due to difficulty in accessing the flowering
branches and in following the feeding activity of the squirrel in the
forest. But, visual observations
indicated that the squirrel fed voraciously on flowers and growing fruits
showing a significant effect on the reproductive success of the plant.
Fruit, seed and seedling ecology
Natural fruit set rate is 9.3±4.63 (Range
2–24) at inflorescence level (Image 2l). The average flower to fruit ratio is
3.7: 1. The fruit is initially
light green (Image 2m), then creamy white and light brown when mature. It grows to a maximum length of
13–14 mm and of 6mm width in four weeks. It is a simple septicidal trigonous
capsule with a weight of 179±26.6 mg and invariably produces three seeds
against the actual six ovules in a flower. The ovule to seed ratio is 2:1. The seeds are winged, papery,
compressed, 7mm long, 4mm wide and 19.9±3.1 mg weight. The fruits dehisce along the septa to
disseminate seeds into the air by the end of May (Image 2n,o). The seeds being light in weight
disseminate easily by the wind. The study site is windy and the seeds travel
distances up to 400m downhill. The
seeds germinate following monsoon showers in June-July (Image 2p,q) but the
success rate seemed to be dependant on the continuity of rain and the nutritional
status of soil. Some seedlings were
found to show symptoms of chlorosis which may be due
to water and nutrient deficient soils in rocky habitats.
DISCUSSION
Boswellia ovalifoliolata is a deciduous tree species because it
is leafless when in bloom. Leaf
flushing occurs almost at the end of fruiting. In a few trees, leaf flushing is
little bit early when fruits are still green and young. The short flowering period evidenced at
individual as well as population level, massive blooming and the position of
panicle inflorescences at the end of branches serve as a cue for foragers to
collect floral rewards from the flowers. The floral characteristics of B. ovalifoliolata, such as fresh
mild odour, hidden nectar in moderate quantity and pinkish-red nectary disc
serving as nectar guide, conform to bee-flowers (Faegri & van der Pijl
1979). However, the small flower
size, delicate petals and actinomorphic symmetry are not suitable for foraging
visits by adult Xylocopa bees (Faegri & van der Pijl 1979), although
the flowers can withstand juveniles. The observed Xylocopa bees are juveniles because they emerge from
brood during March–April (Raju & Rao 2006) and hence they are
suitable for probing the flowers to collect nectar. These juvenile bees in quest of nectar
for instant energy and for overcoming dehydration make multiple visits to
closely and distantly spaced flowering trees of B. ovalifoliolata. Such
consistent flower visits between trees effect and enhance cross-pollination
rate. Apis, Trigona and Ceratinabees collect pollen and nectar with ease due to cup-like flower shape with
exposed floral rewards. Baker & Baker (1982; 1983) stated that
short-tongued bees such as the bees observed in this study tend to be
rewarded with sucrose-rich nectar. Further, Cruden et al. (1983) reported that in dry seasonal forest
plants, the nectar concentration is usually high and bee-flowers produce a
small volume of nectar with high sugar concentration. In B. ovalifoliolata, the flowers
produce a small volume of sucrose-rich nectar with high sugar concentration and
hence conform to the generalizations stated by Baker & Baker (1982; 1983)
and Cruden et al. (1983). In line
with this, bees recorded consistently visit the flowers of different trees to
collect forage and in doing so effect pollination. Apis dorsata being a
large-bodied bee requires more energy and hence efficiently probes the flowers
in quick succession on the same and different trees; its foraging visits to
different conspecific trees not only effect but also enhance cross-pollination
rate. Other Apis species, Ceratinaand Trigona bees with slow mobility between conspecific trees for forage
collection mostly effect self-pollination which is not
the mode of breeding system in B. ovalifoliolata. Hence, these bees have a minor role in
cross-pollination. Wasps usually take nectar as a supplement food, especially
when brood nursing is over. They
are active in blossoms towards the end of flowering season in seasonal
climates. Wasp-flowers are also
sucrose-rich but are usually unreliable and unsteady pollinators (Faegri &
van der Pijl 1979). The floral nectar of B. ovalifoliolata being sucrose-rich is
favoured by the wasps, Scolia, Eumenes and Rhynchium. Their visits to the flowers throughout
the flowering season suggests that their brood nursing
period is over and hence, they are active in flowers to take nectar as a
supplement diet. However, they are
not consistent foragers like bees but they use this floral source until
exhausted and their frequent inter-tree movement during their foraging period
contributes to cross-pollination. The garden lizard is an ambush predator capturing the foraging insects
at the flowers of B. ovalifoliolata. The foraging insects cannot perceive the
lizard and do not respond by predator-avoidance behaviour. The lizard does not attack the prey
until it forages on a flower for a considerable period. It is for this reason that the
pollinator insects have greater opportunity of being approached and attacked by
the lizard. The predation of the
lizard on pollinating insects has its share in reducing the cross-pollination
rate in B. ovalifoliolata. The role of dipteran fly in cross-pollination appears to be negligible
due to its restricted inter-tree mobility. Butterfly-flowers also produce a small volume of sucrose-rich nectar
with high sugar concentration (Opler 1983; Cruden et al. 1983; Baker &
Baker 1982; 1983). As the floral
nectar of B. ovalifoliolata is characterized in this way, the foraging
visits of the observed species of butterflies on this tree are not
surprising. As they make frequent
flights between trees, their foraging visits also contribute to cross-pollination. All these insect species carry
considerable number of pollen grains on their body/proboscis, the character of
which qualifies them as effective and efficient pollinators. The foraging activity of these insects
coincides well with the timing of anthesis; it gradually increases from
anthesis onwards, reaches to peak around noon and gradually decreases towards
the evening. In B. serrata, honey bees have been reported to be the exclusive
pollinators (Sunnichan et al. 2005).
Ornithophilous flowers tend to be large,
red and deep seated with concealed nectar. They secrete high volumes of hexose-rich nectar with low sugar
concentration (Cruden et al. 1983; Opler 1983; Baker & Baker 1990). On the contrary, in the present study,
the sunbirds visit B. ovalifoliolata flowers whichare small, cup-shaped and white with a small volume of sucrose-rich
nectar with high sugar concentration. Since the nectar volume is very small and sunbirds require a greater
amount of energy per flower, they visit different conspecific trees in quest of
more nectar. Such a foraging
behaviour results in cross-pollination. These sunbirds exhibit fidelity to this
floral source until exhausted. Several other birds also attempt to collect
nectar from B. ovalifoliolata but soon discontinue probing the
flowers. The study shows that B.
ovalifoliolata is not ornithophilous but sunbirds use it as nectar source
for survival during dry season while other birds are unable to use it even in
the absence of dry season blooming ornithophilous tree species in the study
area. Therefore, the birds recorded
in the study area appear to be searching for the floral nectar to meet their
energy requirement during dry season.
Insects require ten essential amino acids
but all of them are not normally found in all nectars. Usually, three to four essential amino
acids and several non-essential amino acids are found in floral nectars (Baker
& Baker 1982; 1983). Baker
& Baker (1986) reported that the amino acids add taste to the floral nectar
and it depends on their concentration. Their presence serves as an important cue for insects to visit flower
and in the process effect pollination. In B. ovalifoliolata, the nectar contains some essential and
non-essential amino acids. Its
nectar is an important source for four of the ten essential amino acids
required by insects for their growth and development (DeGroot 1953). They include arginine, histidine, lysine
and threonine. Non-essential amino acids are metabolized by
insects from the food they take; however, floral nectar provides some of
these amino acids instantaneously. The nectar of B. ovalifoliolata provides alanine, aspartic acid,
cysteine, glysine, hydroxyproline, serine, glutamic acid and tyrosine. Therefore, the insects and also sunbirds
by visiting and pollinating the flowers derive the dual benefit of sugars and
amino acids from the nectar of B. ovalifoliolata.
In B. ovalifoliolata, the flowers
are weakly protandrous, produce considerable per centof sterile pollen and present the capitate, wet papillate tri-lobed stigma
above the stamens as in case of its allied species B. serrata (Sunnichan
et al. 2005). The stigma
receptivity ceases around noon on the next day. These characteristics suggest that the
tree species is adapted for cross-pollination which is
further substantiated by the lack of fruit set in manual self-pollination
treatments. The reason for the
failure of fruit set in self-pollination appears to be related to inhibition of
self-pollen tubes after their entry into the style. Therefore, the study suggests that B.
ovalifoliolata is strictly self-incompatible and obligate outcrosser like B.
serrata (Sunnichan et al. 2005). According to Cruden (1977), the pollen production rate at flower level
is not commensurate with out-crossing breeding system but it seems to be appropriate
if the fruit set rate in manual cross-pollination is considered. Fruit set in open-pollination among
individual trees is less than 10% but it is most likely to increase in the
absence of bud/flower predation by weevil and squirrel. The extent of increase in fruit set in
manual cross-pollination also has not exceeded 34% and this suggests that there
are inherent constraints such as dry conditions, nutrient-deficient environment
to fruit set in addition to limitation of cross-pollination. The distribution of fruits on the
inflorescence is sparse and hence, space is not a constraint for increased
fruit set. As all fruits invariably
produced three seeds, there seems to be a space constraint in the ovary for
seed set from all six ovules of the flower. The uniform number of seeds in each fruit
seems to be an evolved and adaptive trait to compensate the lower fruit set in
open-pollinations. It also suggests
that cross-pollen availability is not a constraint in fruited flowers. In self-pollinated flowers, the
deposited self pollen and the pollen tubes formed may
prevent or block if the cross-pollen is subsequently deposited by
insects/sunbirds. Further, the
trees being leafless during the entire flowering and fruiting period have to
utilize the available limited resources for fruit and seed loading. In consequence, the trees may even
selectively disallow genetically inferior cross-pollinations to proceed further
to set fruit in order to save available resources for pumping into the
genetically superior fruits and seeds. The floor of the forest being rocky, dry and litter free during
flowering and fruiting season deprives this tree species of nutrient
resources. Therefore, B.
ovalifoliolata with poor-nutrient environment is capable of performing
reproductive events and produce some per cent of fruit
set as a self-incompatible and obligate outcrosser. Similar reproductive events and
constraints have been reported in B. serrata (Sunnichan et al.
2005). A recent experimental study
with Boswellia papyrifera by Toon et al. (2006) shows that intensive
tapping for gum causes the trees to divert too much carbohydrate into resin at
the expense of reproductive organs, such as flowers, fruit and seeds. In consequence, the trees produce fewer
smaller fruits with seeds of lower weight and reduced vitality than non-tapped
trees. Such a situation in B.
ovalifoliolata cannot be ruled out since it is an important source of gum
resin for local tribes and hence there is a great threat to this globally
endangered and endemic species.
Fruits mature in a short period and
dehisce along the septa to disperse seeds for which dry conditions are
essential. Their dispersal takes
place in the month of May when temperature is at its maximum and which provides
ideal conditions for seed dispersal by wind. The seed characteristics such as small
size, light weight, papery and winged nature are
adapted for anemochory. As the
study site is windy, anemochory is very effective, dispersing seeds up to 400m
away from the parental site. Therefore,
in B. ovalifoliolata, dry season seems to be the prerequisite for
flowering, fruiting and seed dispersal. Leaf flushing occurs immediately after seed dispersal and the water
stress is released by rainfall in June and thereafter. During this period, with foliage, the
tree has to produce and store the required energy for the recurrence of sexual
reproduction in the next dry season. The dispersed seeds germinate readily following rainfall but their
continued growth and development is related to soil water and nutritional
status. Since the natural area of B.
ovalifoliolata is rocky with little litter and soil moisture, the success
rate of seedlings each year is very much limited and hence, this could be one
of the factors that give it the “endemic and endangered” status.
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