T REPRO N DU LA C The International Journal of Plant Reproductive Biology 7(2) pp.189-194, 2015 P T I F V O E

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DOI 10.14787/ijprb.2015 7.2.189-194 T

Observations on the presence of an aril in the seeds of Aquilaria malaccensis Lam. (syn. A. Agallocha Roxb.) (Thymelaeaceae) growing in Meghalaya, North-east India

N. Venugopal*& Ester Jones Marbaniang**

Department of Botany, Centre of Advanced Studies in Botany, North Eastern hill University, Shillong-793 022,India

*e-mail: [email protected]; **[email protected].

Received : 31.03.2015; Accepted & Published on line: 30.04.2015

ABSTRACT

Aquilaria malaccensis Lam. (syn. A. agallocha Roxb.) is an economically important tree species for the production of agar wood. The species has been placed on Appendix II of CITES and belongs to the “Threatened” category of the IUCN Red List. The flowers are small, yellowish green and clustered on the leaf axis. The fruit capsules are bi-valved and the pericarp of the matured fruit split opened thereby the seeds are exposed. The seeds remain hanging upside down by a vasculature which usually is intact with the persistent funicle. During the sectioning of the plant materials, the authors came across the presence of an aril in the seeds of A. malaccensis and enwrap the seeds completely. The aril in this plant is true aril because it develops from the funicular part. The aril is soft and fleshly and contains several food reserves which supply food and nutrition to the developing seeds and is eaten by ants. For ants, arils are edible enticement and serve as bait for myrmecochorous dispersal.

Keywords : Agar wood, Aquilaria malaccensis Lam., aril, Myrmecochorous, Threatened.

INTRODUCTION Arils are best defined as any soft and persisting tissue emerging from the seed surfaces without Aril was defined for the first time as an considering the exact position on the seeds (Endress accessory integument which is attached to the hilum 1973). It is an extra envelop outside the and enveloping the seed completely or integuments, which grows up around the ovule from incompletely (Gaertner 1788). Arils do not have a the point of attachment of the funicle to the ovule fixed morphology and may developed from any part (Sporne 1974). It enwraps the seed either partially of the integument, funiculus or even the inner or completely and presents in diverse epidermis of the ovary wall (Puri 1970) and as such morphological forms. Most arils are vividly the different parts of the ovule are not exactly coloured and occur in at least 15% of dicotyledons definable from each other in practice, so separate as well as in few monocotyledons mostly to terms are given to arils. Arils or secondary attract animals. The aril at seeds are often structures and they are not necessarily to be strictly protected by sclerotic layers (Boesewinkle & correlated with any particular parts of the ovule. Bouman 1984). 190 The International Journal of Plant Reproductive Biology 7(2) pp.189-194, 2015 July, 7 (2)

Aquilaria malaccensis Lam. (syn A. (O'Brien & Mc Cully 1981). The seeds were agallocha Roxb.) a member of the family sectioned at a thickness of 7-10µm by using Leica Thymelaeaceae, commonly known as agar treeis an RM 2125RT rotary microtome and stained with economically important tree for the production of saffranin and fast green (Johanson 1940). The good-quality agar wood (Chang & Kadir 1997). It sections were also stained with Periodic acid and is found in the tropical and sub-tropical forest of Schiff's Reagent (PAS) for total insoluble Bangladesh, Bhutan, India, Indonesia, Loas, polysaccharides (Jensen 1962, Fedder & Malaysia etc. In North-east India, it is confined in O'Brien1968), Sudan III and IV dyes for lipids the dense tropical evergreen forests of Meghalaya (Baker 1947, Gomori 1952), Mercuric below 800m (Haridasan & Roa 1987). The genus is bromophenol blue (Mazia et al. 1953, Chapman best known as the principal producer of the resin- 1975) for total proteins, Iodine – Potassium Iodide suffused agar wood and this resin is valuable for its (Johansen 1940) for starch and photomicrographs uses in medicine, perfumery and other aromatic were taken under Olympus -BX 43 microscope. products (Persoon 2007). Because of its immense value and rarity, indiscriminate cutting of trees and RESULTS over harvesting has lead to the depletion of A. malaccensis in nature. Aquilaria malaccensisis a relatively slow- Population of A. malaccensis has decreased growing, medium-sized tree, on average 15–25 m in the natural forests of north-east India due to over tall with a moderately straight stem. The flowers harvesting of mature trees and as a result of which are small, yellowish green in colour and flower in its natural populations have diminished into critical clusters on the leaf axis (Fig. 1A). Flowers are minimum level and now the trees are found only in bisexual, ebracteate and about 8-10 mm in diameter. few protected pockets in the NE India (Broad 2007, Sepals are 5 to 6, gamosepalous, larger than the Barden 2000). Hence the species was placed on petals, persistent, petaloid and pubescent. Petals Appendix II of the Convention on the International are 10 to 11, free, highly pubescent and reduced to Trade in Endangered Species (CITES) and belongs scale like appendages. The androcium consists of to the “Threatened” category according to the usually 10 and occasionally 11 stamens, IUCN Red List (Oldfield et al.1998, CITES 2005, epipetalous, dorsifixed and hypogynous; pistil 1, Saikhia & Khan 2012). ovary superior, obovoid, massive and densely pubescent, biloculate with a single ovule in each MATERIALS & METHODS locule and ovules are bitegmic; stigma capitate, dry and style inconspicuous. Fruits are capsules, bi- Aquilaria malaccensis was collected from valved consisting of one seed per locule, green and Mawlyngroh, South West Khasi Hills District of oval in shape, exocarp leathery with fine white hairs Meghalaya, India. This area lies between (Fig. 1B). These fruits normally have two seeds but 25o14`48.6`` North latitudes and 91o24`57.90`` East sometimes only one seed. The matured fruits are longitudes at an altitude of about 487m AMSL. The persistent in the tree until the seeds got dispersed. fruits of A. malaccensis at various development As soon as the fruits dehisced, the pericarp of the stages were collected from the collection site during fruit splits opened and the seed are exposed and the fruiting season and fixed in FAA (formalin: remains hanging upside down by a thread like acetic acid: 70% ethyl alcohol = 5: 5: 90 v/v). The structure or the vasculature which usually are intact seeds were taken out from the different stages of with the persistent aril (Figure 1C). Seeds of A. fruit development and the seeds were processed malaccensis are brown, connoid in shape and through the customary methods of dehydration, covered with short and stiff brown hairs. There is a infiltration and embedding in paraffin were used minute invisible opening at the micropylar end of 2015 Aril in the seeds of Aquilaria malaccensis Lam. (syn. A. agallocha Roxb.)- Venugopal & Marbaniang 191

A B C

D E F G

H I J K

Figs. 1— A-C. Flowers and fruits of Aquilaria malaccensis. A. Small and yellowish green flowers in leaf axis. Bar= 0.8 cm; B. Green and young fruits. Leathery exocarp with fine white hairs. Bar= 0.3 cm; C. Splitted fruit, seeds exposed, which remains hanging by a thread like structure intact with the persistent aril. Bar=1.5 cm; D & E. Brown mature seeds covered with short stiff brown hairs. Bar= 3.3 cm; F. Separated aril from the seed due to the presence of palisade layer at hilar region. Bar = 200µm; G. Enlarged view of palisade at the hilar region. Bar= 500µm. H-K. Histochemical tests showing carbohydrates (H), lipids (I), proteins (J) and starch (K). Bars=500µm. 192 The International Journal of Plant Reproductive Biology 7(2) pp.189-194, 2015 July, 7 (2)

the seed for the absorption of water and other Sernander (1906) introduced the term elaiosome, an nutrients as to ensure the proper germination of the ecological term given to all fleshy and edible parts seed. of seeds dispersed by ants. This structure is soft, The aril in A. malaccensis is true because it fleshy and attracts ants and get easily detached from develops from the funicular outgrowth. True aril the seed. Therefore, the seeds are only collected by develops near the top of the funicle and surrounds the ants for the elaiosome. Usually, for ants, the seed and indicated as funicular aril (as stated by elaiosomes are a high-energy food, and for plants Vander Pijl 1982). Initially they are fleshly and they are seed appendages that serve as bait for white in colour but as the seed matured they become myrmecochorous dispersal (Lisci et al. 1996). papery and the colour changes to orange or rusty In A. malaccensis if fertilization does not take and covered by brown hairs (Fig. 1D) except the place the area remains unchanged, and after some ventral portion which is white (Fig. 1E). As the aril days its cells begin to degrade and die together with develops, the aril gets separated from the seed by the ovule, a phenomenon that has been reported the presence of some palisade layer at the hilar previously by other workers (Corner 1949, Kloos & region (Figs. 2F & G). Bouman 1980, Johri et al. 1992). Generally in The development of the aril is stimulated by herbaceous angiosperms the ovule attains nearly fertilization and completed with the development of full development prior to pollination but in some the endosperm. In A. malaccensis, if fertilization epiphytic orchids (Swamy & Krishnamurthy 1990) does not take place the area remains unchanged, and and in A. malaccensis pollination is prerequisite for after some days its cells begin to degrade and die the initiation of the ovule primordium. together with the ovule. Fertilization reactivates the The aril in the mature seed has abundant starch, aril primordia to undergo development, but this carbohydrates, lipids and total proteins as reserve reactivation is complete by the normal development foods. Hence arils are edible enticement, of the endosperm. By increasing the number of encouraging transport by animals and thereby endosperm nuclei the size of the ovule also gets assisting in seed dispersal. Like in the case of A. increases which leads to an increase in the cells of malaccensis, in Cytisus multi florus and C.striatus the aril. The size particularly the elongation of the (Rodri-Guez et al. 2006) the arils are also eaten by aril also increases with the increase in the size of the ants, in Myristicaceae arils are eaten by birds ovule. The increase in size involves a functional (Endress 1973) and human (Uphof 1959). Apart change in the aril cells, from meristematic cells to from wind and ants, wasps (Manohara 2013) are storage cells. Histochemical tests indicate the also the major agents for the dispersal of seeds of presence of food reserves such as carbohydrates, A. malaccensis. He also stated that the wasp carries lipids, proteins and starch (Figs. 1 H, I, J & K) hence the seed over varying distances before it lands on supplying food and nutrition to the developing the leaves or branches of the same or any other tree. seeds. The entire aril is consumed by ants and It feeds on the aril after it lands, leaving behind an caterpillars leaving the seed portion. Ants are often undamaged seed that falls to the ground. seen to carry the seeds to some distances and as a The present observation supports the views that result of which helps in the dispersal mechanism. arillate seed are more abundant in the tropical rain forest than in the others part of the world. It is also DISCUSSION & CONCLUSION true that the arillate seed are usually borne in rather dry fruits which splits open when ripe, to expose the Different terminology has been used by the sweet and juicy arils within (Sporne 1974). various workers for the seminal appendages have led to great confusion in correct naming of each of Acknowledgements—We would like to thank the the different seminal appendages (Corner 1949 Head of the Department of Botany for providing 1976, Van der Pijl 1955, Kapil et al. 1980). facilities. The authors are also grateful to the DFO 2015 Aril in the seeds of Aquilaria malaccensis Lam. (syn. A. agallocha Roxb.)- Venugopal & Marbaniang 193

and staff of Mawsynram forest division, Meghalaya Johanson DA 1940. Plant Microtechnique. Mc and to Mr. B. Thongni for collection of the plant. We Graw-Hill Book Company, New York & London. also expressed gratitude to the Ministry of Environment and Forest (MoEF), for funding the Johri BM, Ambegaokar KB & Srivastava PS 1992. project No. 22/3/2010-RE, New Delhi, Govt. of Comparative embryology of Angiosperm. Vol.1. India. Springer-Verlag, Berlin.

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