Pertanika 5(2), 234-239 (1982)

A Scanning Electron Microscope Study of Flowers of Carambola,Durian and Rambutan

H.F. CHIN and A.C.G. PHOON1

Agronomy and Horticulture Department, Universiti Pertanian Malaysia,Serdang, Selangor, Malaysia.

Key words: SEM; Flowers; carambola, durian, rambutan.

RINGKASAN

Spesimen-spesimen segar bunga belimbing manis (Averrhoa carambola L.), durian (Durio zibethinusMurr.) dan rambutan (Nephelium lappaceum L.) telah diperiksa dengan mikroskop elektron pengimbasan(SEM). Bunga menyeluroh dan bahagian-bahagian bunga telah dihuraikan dan digambarkan dalam mikro-graf Struktur-struktur bunga dan perbezaan-p, *bezaannya dalam spesis telah diberi penekanan, pertaliandan fungsi mereka dibincangkan dengan rujukan khusus kepada mekanisma pendebungaan dan morfologide bunga.

SUMMARY

Fresh specimens of flowers of carambola (Averrhoa carambola L.), durian (Durio zibethinus Murr.)and rambutan (Nephelium lappaceum L.) were examined using the scanning electron microscope (SEM).Whole flowers and floral parts were described and illustrated in the micrographs. Floral structures andtheir differences within species were highlighted, their relationship and functions are discussed with specialreference to pollination mechanisms and pollen morphology.

INTRODUCTION

The carambola or star fruit (Averrhoa caram-bola LJ, Durian (Durio zibethinus Murr.,/ andrambutan (Nephelium lappaceum L.) are someof the more important and popular fruits of thetropics particularly in South East Asian countries.These economically important crops have beencultivated and described over half a century agoby Popenoe (1920), Ochse and Barkhuizen(1931) and Burkill (1935). Clonal or varietalselection, identification and propagation of therambutan were carried out later as reported byWhitehead (1959) and Milsum (1960). Despitethe economic value and importance of thesefruits, for example the durian in Malaysia, there isno large scale plantation or estate in this region(Lai, 1974) nor is there a well documented pro-gramme on the selection and breeding of thesefruits. Studies are lacking on the autecology,flowering biology, cytology and breeding systems,but these are the pre-requisites in any breedingproject. Lately selection of individuals followedby vegetative propagation is commonly used,

instead of breeding by various crosses. The re-productive biology of durian has been studied byValmayor et al, (1965); Soepadmo and Eow(1976) and the floral biology of carambola byKnight (1965); and Nand (1967, 1971).

The study of floral structures using a scanningelectron microscope (SEM) gave greater detailsand depth of focus than the ordinary light micro-scope (Troughton and Donaldson, 1972).Scholefield (1982) used this technique to illustrateand described four tropical and sub-tropicalspecies, the avocado (Persea americana Mill J, litchi(Litchi chinensis SonnJ, macadamia (Macadamiaintegrifolia Maiden and Betchej and mango (Man-gifera indica LJ.

There is still a lack of interest and informationon the floral biology of the tropical fruit species.This paper contains scanning electron micrographsof the structure of our most important fruits,the durian, rambutan and carambola. The aim ofthis paper is to provide further information andappreciation of the floral biology of these three

1 Biology Department, Faculty of Science and Environmental Studies, Universiti Pertanian Malaysia.

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popular fruit species to breeders and horticulturists.Emphasis is also given to the relationship of floralparts and the pollen grains in the process of polli-nation, fertilization and subsequently fruit andseed set.

MATERIALS AND METHODS

Plant MaterialsFlowers were picked and collected both in the

morning and evening from grafted trees of caram-bola (5 years old), durian (10 years old) and ram-butan (8 years old) in the university farm. Flowersof carambola are available all year round but thoseof durian and rambutan are seasonal and varietiescan overlap in their flowering dates/In Malaysia,the durian flowers in March-April and September-October; the rambutan between March-May andAugust-September. Flowers for this study werepicked in mid August, and were placed in clearplastic bags with moist cotton wooL

Method of ScanningThe durian flowers are very large compared

with those of the rambutan and carambola. Hencethe floral parts of durian flowers were scannedseparately. Fresh flowers were dissected with theremoval of the epicalyx, calyx and corolla. The

stigma and stamens (uncoated with gold) weremounted on a SEM brass stub and examined.

With carambola flowers three sepals andpetals were first removed before mounting on thestub; in case of the rambutan flowers the wholeflower was mounted on the brass stub. Thesemounted specimens of floral parts or wholeflowers were examined in a JEOL JSM 35Cscanning electron microscope using an acceleratingvoltage of 6kV. During the examination of speci-ments over a period of 15 to 30 minutes desicca-tion effects were noted when specimens wereunder high power of x 3000. Pollen morphologywas studied by dusting pollen grains on to adouble sided cello-tape on a stub and examinedin the same manner.

RESULTS AND DISCUSSION

The Carambola or Star Fruit (Fig. la and IbjThe carambola flower is heterodistylous

borne in small cymose inflorescences at theaxils of leaves. Individual trees may have the'long-style' or 'pin-eye' flowers or 'short-style','thrum-eye' flowers (Fig. la and lb) but neverboth. This condition was noted by Ochse et al(1961) and Knight (1965). Nand (1966, 1967)

.

fig. l(a) Fig. l(b)

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A SCANNING ELECTRON MICROSCOPE STUDY OF FLOWERS

studied the morphology and abnormalities ofthe 'pin-eye' flower and its fruit respectively.

Each flower is about 6-7 mm tall by 9-10 mmwide at the mouth of the corolla. The calyx has5 imbricate rose-pink sepals with whitish bordersand almost half as tall as the corolla tube. Thecorolla is gamopetalous with five recurved lobes.Each petal is rose-pink with the basal portion andthe borders whitish. The inner surface is coveredby tiny hairs which give it a velvety appearance(Fig. Ig).

Fig. l(g)Fig. l(c)

Fig. Iff) Fig. l(d)

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H.F. CHIN AND A.C.G. PHOON

Inner to the corolla is a whorl of 10 stamensof which five are staminodes and are modified asnectaries. (Fig. lcj. In the 'thrum-eye' flower thefive functional stamens are tall and reach themouth of the flower. The anther is bilobed anddehisces along a longitudinal line to release largeamounts of creamy white pollen grains (Fig. If).The yellowish green ovary is five-ridged, eachbearing a short stigma positioned between alternatefilaments (Fig. Id). The stigmatic surface facesthe corolla wall so that pollen deposition isenhanced from the side.

In the 'pin-eye' flower the five functionalstamens are short with the anthers facing towardsthe corolla. The five-ridged ovary has the styleselongated and joined as a stylar column and endsnear the mouth of the flower with the stigmasclosely adhered together (Fig. le). The stigmaticsurface is highly papillous.

The pollen grain is sticky and barrel-shaped atanther dehiscence (Fig. If). When wetted it swellsup to about 20-24 microns and reveals three largecolpi. The exine is pockmarked with tiny sphericaldepressions (Fig. Ih).

Fig. l(e)

The flower opens at about 07:45-08: 15 hourswith simultaneous secretion of nectar. The stig-matic surface is creamy white and is immediatelyreceptive. The pollen grains are released about 15minutes later. By 15:30 hours some flowers beginto close whereas the majority are closed by 17:00hours. On the next morning the corolla tube isdetached exposing the ovary which will developinto the fruit.

Apart from the bright colour of the flowers,its nectar and pollen are attractive to insectsespecially bees of Apis sp. and Trigona sp. Thedistylous condition of the flowers favours crosspollination between these two morphologicalfloral types. Apart from cross pollination, selfingis highly probable in the 'thrum-eye' flower byvirtue of the higher position of the anthers to thatof the stigma. The many tiny hairs on the petalspossibly prevent selfing by trapping the pollenbefore it could come into contact with the stigma.

Pollination studies by Knight (1965) showthat self and cross-incompatibility in the carambolaare associated with the distylous condition andattributed fruit-set by crossing similar floral typesto occasional break down of this system. This isexemplified in the 'Golden Star' cultivar whichfruits freely even in isolation. Nand (1971), on theother hand, obtained successful fruit-set by crossingsimilar floral forms within the same plant. SomeMalaysian carambola behave similarly to the'Golden Star' cultivar which suggests that self orcross-incompatibility is absent or it could havebeen bred out inadvertently along a similar line asthe 'Golden Star' cultivar.

The DurianThe tree starts to bear flowers and fruits at

the age of five to twelve years. There seems to betwo flowering seasons; flowers are cauliflorous ingroups of 3-20 arising from older branches. The

Fig. l(h)Fig. 2.

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A SCANNING ELECTRON MICROSCOPE STUDY OF FLOWERS

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H.F. CHIN AND A.C.G. PHOON

floral structure has been described by Ochse et al(1931, 1961). The development of flowers andfruits has been described by Valmayor etal (1965)while Soepadmo and Eow (1976) have describedthe floral differentiation and floral anatomy. Thedurian flower is perfect and regular. It is enclosedby an epicalyx, inside which are five to six bundles,each bundle consisting of six to ten stamens(Fig. 2). The durian exhibits the heterostylouscondition among the clones. The ovary is superior,ovoid with a long slender style terminated by ayellow orangy stigma

The flowers open late in the afternoon withthe splitting of epicalyx into two or three lobesfollowed by the calyx. Soon after dark petalsbecome recurved outwards exposing both stamensand stigma (Fig. 3a). Around 18:00-20:00 hoursthe anthers dehisce (Fig. 3b). The stigmaticsurface is receptive at anthesis, a sticky mucila-ginous secretion is produced on the surface andpollen grains can become attached to it (Fig. 3c).The stigma remains receptive till 01:00 hours(Soepadmo and Eow, 1976) while Valmayor etal(1965) reported receptivity lasted until 06:00 hoursthe following morning.

The flowers of the durian can be consideredto be very large; the pedicels are violet to 7 cmlong, the calyx measures 2 cm and 1.5 cm wideand petals are 3 cm long by 1.5 cm wide; thestigma and the stamens are 5-7 cm and 2-3 cmlong respectively. Pollination is attributed to batswhich feed on the nectar or the large pollen grainsor both. The nectary, made up of large cells,is located at the base of calyx tube (Fig. 3g).Mature pollen grains are more or less spherical,three to four porate measuring 80-150 microns indiameter (Soepadmo and Eow, 1976). At anthesisthey are released singly or in clumps (Fig. 3d). Thepollen grain is large and covered with mucilage(Fig, d) which can be removed with ether revealingthe three porate pollen grain (Fig. 3f).

The receptive stigma is papillate and sticky(Fig. 3c). The stigma located at the tip of the longstyle is placed in such a position that pollinatorswill come into contact with it while feeding on thenectar or pollen. Self-incompatibility has beenreported by Arasu, (1976); Valmayor et al. (1965),but compatibility by Soepadmo and Eow (1976).The pollen grains germinate within three or fourhours through the pore (Fig. 3e); the pollen tubesare sent through the stigmatic papillae into thestyle; and the successful one finally enters theembryo sac in the process of fertilization. Subse-quently the calyx, petals and stamens begin todrop off leaving only the ovary and stigma attachedto the branch. The ovary is initially covered with

scales, a cross section of which shows the thornyovary surface (Fig. 3h) and scales over it. Within aweek all unsuccessfully pollinated ovaries drop offleaving one or two per inflorescence. The fertilizedspiny ovary will develop and enlarge changingfrom light brown to the light green fruit. Thespiny fruit is made up usually of five carpelscontaining 10 to 20 very large seeds which maturein about three months.

The Rambutan (Fig. 4a)The rambutan is androdioecious with separate

male and hermaphrodite trees. The male tree isseldom found since nowadays the rambutan isusually planted from budded material. The herma-phrodite tree is often referred to as the female be-cause it bears abundant fruits. Brief descriptions offlowers have been given by Popenoe (1920), Corner(1952), Ochse et al (1961) and Pursglove (1974).

The flowers are borne on terminal or axillarycymose inflorescences. The flowers are apetalous,greenish white in colour about 2 mm in diameter.The calyx has four to six pubescent lobes.

In the male flower there are five to sevenstamens which arise from the disc between thelobes of the nectaries (Fig. 4a). Each stamen has awhitish tomentose filament with a yellowishbilobed anther. The anther lobes split along alongitudinal line to release large amounts of pollen(Fig. 4e). At the centre of the flower is the abortiveovary which is highly pubescent (Fig. 4c).

The hermaphrodite flower has six to sevenstamens but the anthers do not dehisce to releasethe well developed pollen grains inside. Func-tionally it serves as a female flower. The ovary istwo or three lobed and bears a bifid or trifidstigma. The outer surfaces of the ovary and stigmaare pubescent while the stigmatic surface is highlypapillose (Fig. 4d). Only the nectaries are notpubescent.

The male pollen grain is sticky and is barrel-shaped (Fig. 4g). When wetted it swells up to a sizeof about 20 microns and reveals three narrowcolpi. The exine is finely patterned with minutefusiform depressions (Fig. 4h),

The rambutan flowers open at all times of theday but the majority do so at about 06:30 hours.The first sign of anthesis is the parting of the calyxin the male flower whereas in the hermaphroditeflower this is indicated by the recurving of thebifid stigma. The greenish white stigma is receptiveat anthesis and remains so for a day after which itturns brown. Nectar is secreted at anthesis whereasanther dehiscence in the male flower begins atabout 08:30 hours onwards.

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Fig. 4.

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H.F. CHIN AND A.C.G. PHOON

The male flower aborts soon after the release ofits pollen. Insects especially bees and flies are muchattracted to the pollen and nectar. The 'female'flower attracts insects with its nectar and pollina-tion is achieved when the male pollen is transferredto the stigma by visiting insects. Usually one ovarydevelops while the others remain attached to thefruit. Ovary development can be apomictic (Arasu,1976 and unpublished data) and is usually so inrambutan orchards where the male tree is notgrown. Each flower is able to develop partheno-carpically so that the 'female' flower persistslong after its functional life. Most of these flowers,however, eventually abort because of the excessivenumbers of developing fruit-lets on each panicle.Many panicles may not bear any mature fruits butmany others will bear between about 2—20 fruitseach so that the tree appears red at a distance fromthe abundance of fruits. Although the rambutanis apomictic, the number of fruits a tree can bear isobserved to depend on its nutrient supply and theavailability of moisture during fruit development.

CONCLUSIONS

The flowers of the three important species offruits described revealed distinct morphologicalcharacteristics and pollination mechanisms. Thecarambola has relatively small, brightly colouredflowers which are visited by bees. The flowers ofthe durian are large, white and visited by batsduring the night. Both have distylous flowers, acondition which is often associated with self-incompatibility to avoid self-fertilisation. Self-incompatibility in fact has been reported incarambola (Knight, 1965) and in the durian(Arasu, 1976; Valrnayor et al. 1965) but on theother hand self-compatible plants of carambola(Knight, 1965) and the durian (Soepadmo andEow, 1976), have been found indicating thatself-incompatibility occasionally breaks downamong individuals in a population.

The sex of the rambutan is functionally dividedinto male and female individuals. Therefore cross-pollination between trees is a necessity for fruit-set.It is visited by bees and flies which are thereforerequired to visit the male tree prior to the femalein order to effect the transport of pollen. However,under orchard conditions the male tree is neverplanted and it is found that the rambutan is in factalso apomictic (Arasu, 1976, unpublished data).

It is obvious that all three species of fruitsstudied have highly evolved pollination mechanismswhich avoid self-fertilisation. The cultivation ofthese species has inadvertently selected memberswhere such mechanisms have broken down and areself-fruitful.

ACKNOWLEDGEMENTS

We wish to thank Miss Lai Chooi May andAminah Jusoh of the Universiti Pertanian Malaysiaelectron microscope unit for their help in thescanning of specimens and also all the photographs.

REFERENCES

ARASU, N.T. (1976); Tree fruit improvement. Workingpaper No. 14, MARDI Plant Breeding Workshop.Kuala Lumpur.

BURKILL, I.H. (1935): A dictionary of economic pro-ducts of the Malay Peninsula. Vol. 1: 871-875.Crown Agents for the Colonies. London.

CORNER, E.J.H. (1952): Wayside Trees of Malaya.Singapore Government Printing Office.

KNIGHT, R.J. JR. (1965): Heterostyly and pollination incarambola. Proc. Fla. St. Hort. Soc. 76: 375-378.

LAI, A.K.K. (1974): The economics of establishing 500acre fruit orchard in Peninsular Malaysia. Mai. Agric.I 49:421-432.

MlLSUM, J.N. (1960): The Rambutans. World Crops.12(7): 254-255 & 264.

NAND, D, (1967): Some interesting abnormalities in thecarambola fruit (Averrhoa carambola). Sci. and Cut.33(5): 240.

NAND, D. (1971): Pollination, fruit set and fruit develop-ment in carambola {Averrhoa carambola L.). Ind. J.Hort. 28(4): 278-284.

OCHSE, J.J. and BAKHUIZEN VAN DEN BRINK, R.C-(1931): Fruits and fruit cultures in the Dutch EastIndies, p. 180 Batavia G. Kolff & Co.

OCHSE, J.J., SOULE, M.J. JR., DIJKMAN, MJ. andWEHLBURG, C. (1961): Tropical and subtropicalagriculture. New York. The Macmillan Company.

POPENOE, W.P. (1920): Manual of tropical and sub-tropical fruits. New York. Macmillan.

PURSEGLOVE, J.W. (1974): Tropical crops. London.The English Language Book Society and Longman.

SCHOLEFIELD, P.B. (1982): A scanning electron micro-scope study of flowers of Avocado, Litchi, Macada-mia and Mango.

SOEPADMO, E. and Eow, B.K. (1976): The reproductivebiology of Durio zibethinus Murr. Gardens Bulletin29: 25-33,

T R O U G H T O N , J. and D O N A L D S O N , L.A. (1972):Probing plant structure. A.H. and A.W. Reed.Wellington. 116 p.

VALMAYOR, R.V., CORONEL, R.E. and RAMIREZ,D.A. (1965): Studies on floral biology, fruit set andfruit development in Durian. Phillip. Agric. 48:355-366.

W H I T E H E A D , C. (1959): The rambutan, a description ofthe characteristics and potential of the more impor-tant varieties. Malayan Agric. J. 42: 53-75.

(Received 21 August 1982)

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