FLORAL BIOLOGY AND POLLINATION ECOLOGY OF RHIZOPHORA MUCRONATA LAMK. IN GULF OF KACHCHH, GUJARAT,...

International Journal of Botany and

Research (IJBR)

ISSN 2277-4815

Vol. 3, Issue 4, Oct 2013, 39-50

© TJPRC Pvt. Ltd.

FLORAL BIOLOGY AND POLLINATION ECOLOGY OF RHIZOPHORA MUCRONATA

LAMK. IN GULF OF KACHCHH, GUJARAT, INDIA

R. PANDEY1 & PANDEY

C. N

2

1Project Coordinator-CCS, Gujarat, India

2Principal Chief Conservator of Forests (Wildlife), Gujarat, India

ABSTRACT

Rizophora mucronata is a viviparous mangrove species of the Rhizophoraceae family. The species is reported to

be wind pollinated. However, the floral biology and pollination ecology of the species have not been studied in detail in

Gulf of Kachchh. To fill this information gap, the present work has involved detailed field and laboratory investigations

regarding the flower biology and pollination ecology of R. mucronata. The major objective of the research was to

document the floral biology and pollination ecology of R. mucronata by examining the temporal relation between the

anthesis, anther dehiscence, stigma receptivity and pollinator visitation pattern. Further, the study also investigated the

chemical nature of the floral exudate. The visitation pattern of floral visitors was observed at different levels to classify

them into floral visitors and pollinators. The flowers of R. mucronata have been found to exhibit protandry. Further, the

flower is inverted and exhibits herkogamy. These features enhance the possibilities of cross breeding over self breeding.

A crucial role played by petals has been found in regulating, with help of corona, the pollen dispersal for a longer period

after anther dehiscence. Further, the observed asynchrony between the anther dehiscence and stigma receptivity also appear

to be promoting cross breeding. Wind appears to be a major pollinator. However, some insects have also been found to

play a definite role as pollinators. The sugar content of the floral exudate indicates that these exudates are nectar and

further accounts for the bees and ants in later floral stages when the exudate is secreted.

KEYWORDS: Anther Dehiscence, Floral Phenology, Only Four Mangrove Species

INTRODUCTION

Mangrove species, in general, show structural and functional features which favor outbreeding

(Primack et al 1980). Mangroves are often pollinated by animals and the classes of flower visitors are remarkably diverse.

The floral biology, to some extent, determines the breeding type (viz. self or cross breeding) in mangroves. Recently the

reproductive biology of some mangrove species such as Avicennia marina (Clarke & Myerscough 1991, Coupland et al

2005), Aegiceras corniculatum (Pandit & Chaudhary 2001), Rhizophora stylosa, Ceriops australis Sonneratia alba

(Coupland et al 2005 ), , and C. decandra (Raju et al 2006) has been studied in different mangrove forests of the world.

The floral biology and pollination ecology of R. mucronata, however, has not been studied in details for Gulf of Kachchh.

The reproductive biology, in general, depends on the local biotic and abiotic factors such as climatic condition,

pollinator resources etc. It is in this context that the present study focuses on the floral biology and pollination ecology of R.

mucronata in Gulf of Kachchh (GOK), Gujarat, India. The genus is represented by eight species (including 3 hybrids).

Although the genus is quite common, none of the species shows a worldwide distribution. The genus Rhizophora

belonging to the family Rhizophoraceae is generally considered wind pollinated. The floral features which indicate the

occurrence of wind pollination are a high pollen/ovule ratio and light powdery pollens. Chai (1982) reported that

Rhizophora sets fruits when animal visitors are denied access to flowers enclosed in fine mesh bags that does not exclude

40 R. Pandey & Pandey C. N

wind born pollens and Tomlinson (1980) recognized the probability of self- compatibility in the genus due to the above

mentioned features. However, the exact pollination strategy has not been experimentally tested. Notwithstanding the strong

possibility of wind pollination, the absence of an elaborate stigma to catch wind born pollens, the observed frequent visits

by bees and wasp to the flowers and the significant presence of floral exudates definitely suggest the possibility of biotic

vectors having some role in the pollination process. Hence, there is a need to have a closer examination of the pollination

biology of the genus of which a particular member - R. mucronata - was selected for this study. R. mucronata

inflorescences are in axillary cymes bearing 2-4 pedicellate, tetramerous flowers. The flower has white corolla, eight

epipetalous anthers and inferior ovary with bifid stigma The objectives of the present work on R. mucronata was to

investigate: (i) floral phenology, (ii) floral biology, (iii)pollination ecology, (iv)synchrony among anthesis, anther

dehiscence, stigma receptivity, exudate secretion and visitation pattern of floral visitors, and (v) the chemical composition

of floral exudates.

MATERIALS AND METHODS

Study Area

The present work has been carried out in the Marine National Park and Sanctuary, located in the Gulf of Kachchh,

Gujarat, India. Gujarat has the longest coastline (1650 Km.) and hosts two of the three gulfs of India viz. Gulf of Kachchh

and the Gulf of Khambhat. Gulf of Kachchh is located between the latitudes of 22015’N and 23

040’N and the longitudes of

68020’ E and 70

040’ E. The coastline is marked by very high tidal amplitudes of up to 12 ms. As a semiarid to arid zone,

the rate of evaporation in most parts of coastal Gujarat (particularly around Gulf of Kachchh) exceeds the rate of

precipitation. The mangrove forests in the state have relatively less species diversity as compared to many other mangrove

regions of the country. So far, 13 mangrove species have been reported from Gujarat. Further, the mangrove species are not

uniformly distributed in the state and the community distribution is highly skewed in favour of Avicennia which is

represented by three species A. marina, A. alba and A. Officinalis. Out of these, only A. marina is abundant and other two

species are conspicuously rare (Pandey and Pandey 2008). Therefore, even at the species level, the mangrove community

of Gujarat shows a highly skewed distribution in favour of only one species, A. marina.The two islands Pirotan and Bhens

Bid and the coastal site Sikka were selected as study sites. The islands are located in the eastern part of Gulf of Kachchh

(Figure 1). During the field observations, only four mangrove species A marina, R. mucronata, C.. tagal and A.

corniculatum could be found on the two islands of which, A marina was the most dominant and A. corniculatum was the

rarest species. Since the selected islands are quite remotely located and situated in a protected area, the anthropogenic

pressure is limited.

Source: Status of Magrove in Gulf of Kachchh(2005)

Figure 1: Location of Study Area in MNP&S, in GOK

Floral Biology and Pollination Ecology of Rhizophora mucronata Lamk. in Gulf of Kachchh, Gujarat, India 41

METHODOLOGY

Reproductively matured trees of different heights were selected for making field observations. The field

observations were made between March 2007 and December 2008 covering two flowering seasons. The phenology and

floral biology of R. mucronata was studied by systematically observing all the present inflorescence on 30 selected trees

(10 trees on each site) and studying temporal relation between various flowering processes such as anthesis, anther

dehiscence, stigma receptivity and nectar secretion. For this, the flowers and buds were categorized into different floral

stages based upon morphological observations and the current phnological process such as anthesis, anther dehiscence,

pollination etc.

To study the process of anthesis, 100 buds were selected and tagged at each of the three sites (total 300 buds).

The 100 buds were selected in a way that the initiation of anthesis occurred equally in different periods of the day

including mornings, evenings and nights. The pollen presentation duration was calculated by noting the average time gap

between the anthesis and the petal fall. It was noted that anther dehiscence commenced and almost completed before the

initiation of anthesis. However, pollen dispersal was observed to start after anthesis. Generally, the duration of anther

dehiscence corresponds to the pollen dispersal period (also known as the pollen presentation time or pollen availability

period). However, in case of R. mucronata anther dehiscence takes place before anthesis. Therefore, although the pollen

have been released from anther they do not disperse as the anthesis has not initiated. The pollen dispersal takes place only

after initiation of anthesis. Hence, the duration of pollen dispersal was taken to represent the pollen presentation time.

The role of petals in pollen dispersal was examined by enumerating the pollen load on petal hair (corona).

The petals of t 102 flowers of different floral stages were examined for the pollen load by using light microscopy method.

To study the stigma receptivity, 2000 buds/flowers of different floral stages were examined by the H2O2 method

(Dafni et al 1998) where the formation of bubbles indicated receptivity of stigma and vice versa. The observed intensity of

bubbling- measured by number of bubbles produced- was then directly proportional to stigma receptivity. To estimate

pollen production per flower, all the anthers of each matured bud (N=120 buds) were put in one ml of distilled water before

anther dehiscence and a homogenous solution was prepared. Pollens from 0.1 µl of this solution were then quantified in a

heamocytometer (B S. 748-I. S. 10269 ROHEM India). Ovule production per flower was separately examined by

dissecting the same 120 flowers. The P/O ratio (pollen produced per ovule) was calculated to examine the reproductive

success of the species. Further, the pollination success was examined by the ratio of pollen load on stigma and pollen

production.

The floral visitors were examined at the three levels. At the first level, the 10-15 trees per site were examined for

floral visitors (the visitor on other parts of the tree were not included) for 15 minutes. This observation was taken at the

interval of 2 hrs and it was carried out for two continuous days. Further, the whole set of observation was carried out twice

in each flowering season for two consecutive flowering season. Among all observed floral visitors, those which were

observed more than five times were treated as major floral visitors. At the second level observation, the visitation pattern of

major floral visitors, identified at the first level, was observed to assess the duration and frequency of visits, movement

pattern among the inflorescences (on a single tree or between different trees) and the foraging location etc. Lastly, at the

third level, the major floral visitors were collected and examined for the pollen load on their body.


RESULTS

The reproductive phenology of R. mucronata in Gulf of Kachchh was found to continue throughout the year.

The reproductive cycle started by the initiated production of inflorescence during October-November followed by the

maturation of buds which continued till April-May, i.e. for about six months (Figure 2). Peak flowering was observed in

June; however, smaller amounts of flowers occurred throughout the year. Different floral stages along with the

morphological characters are shown in Figure 2. The propagule maturation prolonged for about one year after termination

of flowering.

Floral Stage Picture Morphological Characters

St-1

Matured yellowish bud

St-2

Mature bud with initiation of

anthesis by 1 or 2 slits

St-3

Anthesis continue, but not

completed, pollen dispersal

initiated

St-4

Anthesis completed and

flowers fully opened, pollen

dispersal completed

St-5

Fully open flower with white

petals


St-6

Fully open flower with

petals becoming slightly

brown

St-7

Initiation of senescence of

anther and petals

St-8

Completion of senescence of

anther and petals, the sepal

base spotless and

St-9

Sepals are yellow and the

sepal base are light brown

St-10

Sepals are yellow and the

sepal base are dark brown

St-11

Sepals are green and

perpendicular to the

orientation of ovary

Figure 2: The 11 Distinct Floral Stages of R. mucronata


FLORAL BIOLOGY

The floral biology was studied by examining process of anthesis, anther dehiscence, pollen presentation, stigma

receptivity and nectar secretion and exploring the temporal relation among them. Anthesis was initiated in St-2 and

observed to be completed in St-4. As mentioned above, to study the different flowering processes, the 24 hours was

divided into four time zones 6 am-12 pm, 12 pm-6 pm, 6 pm-12 am and 12 am-6 am, however, field observations between

12 am and 6 am was not possible.

Anthesis was seen to occur in all intervals and the average duration for anthesis was found to be 21+6 SD hrs

(n=39). However, more than 50% of the samples (Figure-3 A) initiated anthesis between 6 pm and 12 am and 62% of the

samples (Figure 3 B) had completed anthesis between 6 am and 12 pm. It appears that the flowers tend to open during day

time (Figure 3)

Initiation of anthesis

6AM-12 PM

26%

12PM-6PM

21%

6PM-12AM

53%

Completion of anthesis

6AM-12 PM

62%

12PM-6PM

23%

6PM-12AM

15%

Figure 3A & B: Initiation and Completion of Anthesis by Three Different Time Intervals

Anther dehiscence was found to initiate before the initiation of anthesis. However, the pollen dispersal initiated

only after commencement of anthesis. Therefore, the anther dehiscence was ignored. In stead, the duration of pollen

dispersal was treated as pollen presentation time.

Pollen dispersal by anther continues from St-3 to St-4 i.e. about 12 hours. After St-4, the pollen dispersal was

found to be regulated, to some extent, by the corona or hair like structures present at the lateral margins of petals. When the

petal hairs of St 3, St 4-6 and St 7 were examined for their pollen load this load was found to decrease with flowering stage

(Figure 4 and 5) .The transformation from St 4 to St 6 went very fast, therefore the petals of these stages were treated in the

same category.

0.0

100.0

200.0

300.0

400.0

500.0

600.0

700.0

800.0

900.0

1000.0

St-3 St-4-5-6 St-7

Floral stage

Nu

mb

er

of

po

lle

ns

Figure 4: Pollen Load per Flower in Different Floral Stages


0.0

100.0

200.0

300.0

400.0

500.0

600.0

St- 3-4 St-4-7

Floral stage

Nu

mb

er

of

po

llen

s

Figure 5: Relative % of Pollen Dispersal per Flower in Different Floral Stages

The stigma receptivity initiated in St-4, and continued till St-10, totaling 109 hours. The intensity of receptivity

was low, however up to St 6 (Figure 6) but showed maximum values from St 7 to St 10 (i.e. for about 80 hours). It may be

noted that pollen dispersal from anthers terminated in St 5. However, some pollen mingled in the petal hair and, therefore,

the pollen presentation continues till St 7, but the quantum of pollens available (loaded on petal hair) was very low,

compared to pollen production.

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

St 1

St 2

St 3

St 4

St 5

St 6

St 7

St 8

St 9

St 10

St 11

Floral stages

Re

lati

ve

% o

f in

ten

cit

y o

f s

tig

ma

re

ce

pti

vit

y

Figure 6: Relative % of Stigma Receptivity and its Intensity in Different Floral Stages (the Full Line Indicate the

Value While the Broken Line Indicate the Polynomial Trend Line of 5th

Order)

All the eight anthers were found to produce pollens. The pollen production ranged between 5 and 30 x 105 per

flower with an average production of 12+4 x 105 SD (n=60) (Figure 7). Ovule production, on the other hand, showed no

variation; it was in all cases four ovules per flower (n=60).

0

2

4

6

8

10

12

14

16

18

20

6-8 8-10 10-12 12-14 14-16 16-18 18-20 20-22 22-24

Number of pollen (in lakhs)

Fre

qu

en

cy o

f O

ccu

rren

ce

Figure 7: The Distribution of Pollen Production per Flower


All the eight anthers were found to produce pollens. The pollen production ranged between 5 and 30 x 105 per

flower with an average production of 12+4 x 105 SD (n=60) (Figure 7). Ovule production, on the other hand, showed no

variation; it was in all cases four ovules per flower (n=60).

0.0

1.0

2.0

3.0

4.0

St 4/6 St 7 St 8 St 9 St 10

Floral stage

Nu

mb

er

of

po

llen

s

Figure 8: Pollen Grains Observed on Stigma in Different Floral Stages

As mentioned above, reproductive success was calculated as the number of pollen produced per ovule and

showed a high value of 3 X 105. Further, the pollen load on stigma was investigated to examine pollen availability which

was found to be very low compared to pollen production,(Figure 9). Further, the pollen loads on stigma in different floral

stages were also examined (Figure 8). The pollen load on stigma did not show significant variation among the floral stages.

0

5

10

15

20

25

30

35

40

0 1 2 3 4 5 6 7 8 9 10

No of pollens

Fre

qu

en

cy

Figure 9: The Distribution of the Number of Pollen Grains on Stigma

after St- 9

9%St- 9

5%St- 8

9%St- 7

0%

St 4-6

9% St- 3

68%

Figure 10: The Visitation Frequencies of Bees on Flowers in Different Floral Stages


Floral Visitors

Though, the spectrum of floral visitors of R. mucronata was found to be diverse, the visitation frequencies of most

species were low. Among various floral visitors, honey bees and ants were found to be the most frequent visitors. Although

beetles were not very frequent visitors they showed the longest visitation durations among all floral visitors. The range of

floral visitation duration was between 2 s and 3 min, 2 s and 75 min, and 2 sec and about 6 hours for honey bees, ants and

beetles, respectively. Further, the floral visitors were most frequently recorded on St 3, St 8, St 9, and St 10. Bees were

found to visit St 3 more frequently (68%) than other floral stages (Figure 10). However, it was also reported to visit flowers

in St 8, St 9 and St 10. The bees were found to forage upon the floral rewards in several floral stages. During visit to the

opening flowers of St 3 they foraged upon anther while in the later floral stages (St 8-10), they foraged upon nectar. Unlike

bees, ants were more frequently observed in the later floral stages (St 8-10) foraging upon the nectar (Figure 11).

St 4-6

6%St- 7

13%

St- 3

6%After St 9

25%

St- 9

31%

St- 8

19%

Figure 11: The Visitation Frequencies of Ants on Flowers in Different Floral Stages

The visitation patterns of the floral visitors were examined in terms of their physical contact with anther and/or

stigma. It was found that in more than 45% (Figure 12 ) of the visits, bees physically contacted only anthers, in more than

25% of the visits they contacted only stigma and in about 30% of the visits they touched both. It may be noted that these

bees have pollen pockets on their hind legs in which the pollens are stored. During landing on the flowers these pollen

pockets come in contact with the floral parts such as anther stigma etc. Hence, in most cases pollens would be present on

their bodies which lead to pollination of stigma when it touch stigma. Therefore in more than 50% of visits

(when they contact stigma or stigma and anther both) there would be a high probability of pollinating the flowers

(Figure 12).

Unlike bees, in about 80% of their visits, ants did not directly contact either anther or stigma. However, in 13% of the

visits they were found to climb on stigma (Figure-13). Since high number of pollens were present at the base of the ovaries,

pollen was recorded on the bodies of the ants. Hence, it is possible that ants too can pollinate stigma.

Figure 12: Relative Visitation Pattern of Bee Figure 13: Relative % Visitation Pattern of Ant


The three frequent floral visitors were examined for the pollen load on their body and ants, bees and beetles all

showed positive results.

It may be noted here that ants were mostly reported to forage upon the flowers without physically touching anther

or stigma. Therefore, it appears that ant would be poor pollinators, despite their potential to transport pollen. However,

organisms that prey upon ants may pollinate the flowers.

The chemical composition of the floral exudate was examined by the paper chromatography method

(Harborne 1964) which showed a significant presence of monosaccharides such as glucose and fructose and the

disaccharide sucrose. It showed that the floral exudate is nothing but the floral nectar. The nectar secretion initiated in St 7

and peaked in St 8. After St 8 nectar secretion was almost terminated.

DISCUSSIONS

Pollen dispersal by anthers was found to terminate at St 4 while the pollen dispersal by petals continued till St 6

(less than 10%). Thus pollen was deposited on the petal hairs, forming a bridge between the anther dehiscence and early

stigma receptivity, though, the quantum of pollen present on the petal hair was found to range between 0.06 and 0.01% of

the pollen production by anthers.

Further, stigma receptivity was found to initiate in St 7 (more than 10%). After St 7 stigma receptivity increased

and was found in more than 50% of the flowers. Therefore, pollen dispersal and stigma receptivity were out of phase

indicating a strong possibility of cross pollination (Figure 14). This finding counters the generalized theory that mangroves

are generally self pollination species (Tomlinson 1980).

0

10

20

30

40

50

60

70

80

ST

1

ST

2

ST

3

St

4

ST

5

ST

6

ST

7

ST

8

ST

9

ST

10

ST

11

Floral stages

Perc

en

t o

ccu

rren

ce

A

B

C

Figure 14: Distribution and Temporal Relation among Pollen Dispersal by Anthers (A),

Pollen Dispersal by Petal Hair (B), and Stigma Receptivity (C)

It was observed that bees visited flowers more frequently during St 3 (about 70%) compared to any other floral

stages. In St 3 they foraged upon the anthers but bees were never observed to forage on St 4 and St 5 flowers (Figure 15).

During these stages, pollen dispersal by anthers was less than 30% and pollens were transferred to the petal hair or blown

away with wind and water.

It thus appears that bees would not prefer to forage upon pollens present on petal hairs. However, they certainly

visited the flowers at the later stages.


Figure 15: Distriibution and Temporal Relation among Bee Visits (A), ant Visits (B) and Nectar Secretion (C)

Unlike bees, ants were mostly seen to forage on later floral stages such as St 6 to St 11 (Figure 15). During our

observations we found that ants would not allow bees to forage on the same flower. Even if the ant arrived on the flower

after the bee, it would chase away the bee within less than a second. Therefore, as the frequency of visits by ant increased

the frequency of visits by bees decreased.

CONCLUSIONS

R. mucronata is known to be a wind pollinated species. However, the role of floral exudates and frequent floral

visits by insects has not been examined hitherto. The present work suggest a strong possibility of insect pollination and yet

it does not exclude the possibility of wind pollination. In Gulf of Kachchh, the flowering of R. mucronata was taking place

just before the monsoon, when the wind velocity would was very high (15-30 km/hr) Such strong winds may play a

positive as well as a negative role in pollen transfer. Therefore, it appears that the plant has developed the possibility of

insect pollinations and yet retained wind as an alternative pollen vector.

ACKNOWLEDGEMENTS

PCCF Wildlife Gujarat, Staff of marine National Park and Sanctuary. Dr. Salvi Harshad, Mr. Dodiya, Ms. Anjali,

Ms Urvi, Mr. Nilesh, Mr. Amit, Mr Jayesh, Mr. Harshad solanki, Mr Irshad, Mr Arif, Mr. Neeraj. Mr. Chetan, Mr. Prateek,

Ms Puja. Ms Reshama, Mr Jusus Bhai and his team.

REFERENCES

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eastern Australia” Aust. J. Bot. 39:283-93

3. Coupland G. T. , Paling E. I. and McGuinness K. A. (2005) “Floral abortion and pollination in four species of

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4. Dafni A et. Al. (1998) “A Rapid and Simple Procedure to Determine Stigma receptivity” Plant Sexual Production

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5. Harborne, J. B. (1964) “Phytochemical Methods- A guide to modern techniques of plant analysis” Pub. Springer

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