1. Introduction

Tectona grandis L.f.), belonging to the family of Lamiaceae, is a large deciduous tree up to 40 m in height. The leaves are ovate (40 cm long, 20 cm wide) and hairy on the lower surface. It has small, fragrant white flowers attached in panicles at the end of its branches. Teak is one of the most valuable timber species because of its beautiful appearance and durable property ^[1][2]. Although native to South and Southeast Asia, the significant economic potential of teak wood led the species to be introduced into the agroforestry systems of many countries across tropical Asia, Africa and Central and South America ^{[3][4][5][6][7]}.

Home gardens in tropical and subtropical countries surround residences of local inhabitants, and create small forest-like structures. They are considered to be the traditional agroforestry systems characterized by complexity of the structure and multiple functions. Home gardens consist of various kinds of tree species, with crops, livestock, poultry and fishes—those species have been selected by local inhabitants for their preference, productivity and sociocultural aspects. Home gardens provide various food and goods, including commodities such as animal products, fruits, vegetables, folk medicine, ornamentals, fodder, timber and fuel ^{[8][9][10][11][12][13]}. Teak is one of the most essential trees in home gardens in South Asia, because it is a very precious wood species and important in folk remedies ^[8][14][15]. Hot water extracts of teak barks are applied for the treatment of bronchitis, biliousness, hyperacidity, diabetes, dysentery, and leprosy. Water extracts of teak leaves are used in pruritus, stomatitis, ulcers and wounds. Hot water extracts of teak roots are applied for anuria treatments. Oil extracts of the flowers are useful for scabies and hair growth. It has also been used as an important plant in Ayurvedic treatments ^[2][16].

Evidence of the pharmacological properties of teak plants has been accumulated over the past decades. Ethanol extracts of teak leaves have shown significant wound healing activity ^[17]. Ethanol extracts of teak roots have hyperglycemic activity ^[18]. Ethanol extracts of teak barks show anti-inflammatory and analgesic potentials ^[19]. Petroleum ether extracts of teak seeds have hair growth activity ^[20]. Many compounds with pharmacological activities were also isolated from various parts of teak plants ^[2][16][21].

Some plants have shown excellent weed control abilities as soil additives and/or in intercropping, due to their characteristics of allelopathy ^[22][23]. Plants produce hundreds of secondary metabolites. Some of those compounds are released into the surrounding environments through root exudation, volatilization, leaching and decomposition of the plants. Those compounds with allelopathic activity are able to inhibit the growth and germination of neighboring plant species ^[24][25][26]. Therefore, allelopathy of plants is potentially useful for weed management options in several agriculture settings, including agroforestry systems, for the reduction of commercial herbicide dependency ^[27][28]. Many phytotoxic substances with allelopathic activity in teak have already been isolated and characterized. However, there has been no review article about the phytotoxic substances involved in teak allelopathy.

2. Ecological Impact of Teak

The effects of teak on biodiversity, and its allelopathic effects, are discussed in this section. Teak plantations have replaced a significant proportion of natural forests in the tropical and subtropical regions in the world, and have, subsequently, affected the biodiversity of those forests ^{[29][30][31][32]}[29,30,31,32]. The population and diversity of the understory vegetation of the 10-year-old teak plantation were reported to be significantly less than those of native rehabilitated forests nearby, although sunlight intensity on the forest floor was not different between them [4]. Weed diversity under teak trees was also lower than that under the trees of Albizzia procera (Roxb.) Benth., Aleurites fordii (Hemsl.) Airy Shaw, Arceca cantechu L., Azadiratcha indica A.Juss., Gmelina arborea Roxb., and Toona ciliata M.Roem. [33]. There were also more understory plant species outside of the teak canopy than under its own canopy [34].

The shade produced by large teak leaves may partially explain this phenomenon. The allelopathic characteristics of teak may also be one of the reasons to reduce the population and diversity of undergrowth plant species of teak forests. Several phytotoxic substances were identified in teak leaves ^{[2][16][21][35]}[2,16,21,35]. Teak leaves drop to the forest floor through annual defoliation, and the phytotoxic substances in the leaves may be common in the forest soil due to the decomposition process of the leaves. Those substances may affect the growth of undergrowth plant species. Therefore, many researchers have focused on evaluating the allelopathic potential of the leaves, as teak is a deciduous tree species ^{[2][16][21][35]}[2,16,21,35]. In addition, the amount of fallen leaf litter is significant [36].

3. Allelopathic Property of Teak

The allelopathic activity of leachate, leaves, and extracts of teak are discussed in this section.

3.1. Leachate

Teak leaves were soaked in water for 24 h and the soaking water was applied as leaf leachate. The soaking water suppressed the germination and growth of cowpea (Vigna unguiculata (L.) Walp.), Momordica charantia L. and eggplant (Solanum melongena L.) in Petri dish and pot culture conditions [37]. Leaf litter under teak trees was mixed with washed sand and the mixture was percolated with water. The water from the mixture also suppressed the germination and seedling growth of Cicer arietinum L. [38]. Those results suggest that some phytotoxic substances may be released from teak leaves by water as leaf leachate.

3.2. Effects of Teak Leaves

Teak leaves were used as mulch for the cultivation of turmeric (Curcuma longa L.) for six months, and the treatments resulted in significantly lower yield of turmeric rhizome [15]. Powder (100 g/7.2 m²) of fallen teak leaves was applied on a field of wheat (Triticum aestivum L.), and weed emergence on the field was monitored. The dominant weed species in the field were Cyndon dactylon (L.) Pers., Echinochloa colona (L.) Link, Cyperus rotundus L., Cyperus difformis L., Amaranthus viridis L., Chenopodium album L. and Melilotus alba L. At 21 days after the powder application, a 45% reduction in weed population was observed. However, the treatment did not affect the growth of wheat [39]. Those observations indicate that some phytotoxic substances were released from teak leaves, and suppressed weed eminence and growth.

3.3. Extract of Teak Soil

The chemicals in soil under teak trees were extracted with water, and allelopathic activity of the extracts was determined by tomato (Solanum lycopersicum L.). The extracts inhibited the germination and growth of tomato with extract concentration dependently ^[40][41][40,41], which indicated that the soil contained some phytotoxic substances.

3.4. Extracts of Teak Leaves

Aqueous extracts of fallen teak leaves inhibited the seedling growth and protein contents of Vigna mungo (L.) Happer and Vigna radiata (L.) R.Wilczek [42]. Methanol extracts of fallen leaves of teak reduced the germination of Echinochloa colona L. and Cyperus difformis L. [43]. Methanol extracts of fallen teak leaf powder were applied in a wheat field (10.8 g methanol extract residue/7.2 m²) and weed emergence in the field was monitored. The highest reduction (56%) in weed population was recorded 14 days after treatments. However, the treatment did not affect the growth of wheat [39].

Aqueous extracts of fresh teak leaves also suppressed the germination and seeding growth of Vigna mungo (L.) Happer [44] and Plumbago zeylanica L. under laboratory conditions [45], and rice (Oryza sativa L.), maize (Zea mays L.), Vigna radiate (L.) R. Wilczek, Vigna umbellate (Thumb) Ohwi & H. Ohashi, and Arachis hypogeae L. under Petri dishes and pot culture conditions [46]. Aqueous extracts of fresh teak leaves recorded more than 30% germination inhibition of Luffa cylindrica Mill., okura (Abelmoschus esculentus (L.) Moench), and brown mustard (Brassica juncea Jorb. et Hem.) at 10 days after sowing of these seeds [47]. In addition, aqueous extracts of teak leaf powder inhibited the seedling growth and the contents of photosynthetic pigments, such as chlorophyll and carotenoid in chilli (Capsicum frutescent L.), Vigna radiata (L.) R.Wilczek [48], Pennisetum glaucum (L.)R.Br., and Eleusine coracana Gaertn [49]. Methanol extracts of fresh teak leaves also inhibited the growth of Amaranthus spinosus L. [50].

Aqueous extracts of teak roots suppressed the germination and seedling growth of Hibiscus esculentus L. [51]. Inhibitory activity of the extracts of teak leaves, barks and seeds was compared against the growth of maize (Zea mays L.), and leaf extracts showed the greatest inhibitory activity [52]. Those findings described in this section indicated that the extracts of the fallen and fresh teak leaves, roots and barks suppressed the germination and growth of many plant species, both weeds and crops. Those findings also suggested that the leaves, roots and barks may contain some phytotoxic substances, which are extractable with water and/or methanol. Allelopathic activity of the leachate, leaves, and extracts of teak and target plant species, are summarized in Table 1.

Table 1.

Allelopathic activity of the leachate, leaves and extracts of teak and target plant species.

Source	Inhibition	Target Plant Species	Reference
Inhibition	Reference
Leachate from leaves	Germination, plant growth
Phenolic	Vigna unguiculata, Momordica charantia, Solanum melongena	1: Acetovanillone	[37]
Wheat	Plant growth	[	56	]			Cicer arietinum	[38]
Benzofuran	2: Dehydrololiolide	Wheat	Plant growth	[55]	Leaf mulch	Rhizome growth	Turmeric
Anthra quinone	3: Naphthotectone	[	Wheat, onion, tomato, lettuce15]
Plant growth, germination	[	53	]	Leaf powder	Weed emergence
Monoterpene	Cyndon dactylon, Echinochloa colona, Cyperus rotundus, Cyperus difformis, Amaranthus viridis, Chenopodium album, Melilotus alba	4: (6RS)-(E)-2,6-Dimethyl-2,7-octadiene-1,6-diol	[39]
Wheat	Plant growth		Extracts
Sesquterprne	5: lβ-6α-Dihydroxy-4(15)-eudesmene	Wheat	Plant growth	[	Soil under teak trees	Germination, plant growth	Tomato	[40][41
Rice, maize,
Vigna radiate, Vigna umbellate, Arachis hypogeae
[	46	]
54	]
	6: (1S,3aR,4R,7aS)-1-(2-hydroxypropan-2-yl)-3a-methyl-7-methyleneoctahydro-1H-inden-4-ol	Wheat	]	[40,41]
Plant growth	[	54	]	Fallen leaf	Plant growth, protein content	Vigna mungo, Vigna radiata	[42]
Diterpene	7: Phytol	Wheat	Plant growth	[54]		Germination,	Echinochloa colona, Cyperus difformis	[43
	]
8: Rhinocerotinoic acid	Wheat, lettuce	Plant growth, germination	[	54]		Weed emergence	Cyndon dactylon, Echinochloa colona, Cyperus rotundus, Cyperus difformis, Amaranthus viridis, Chenopodium album, Melilotus alba	[39]
	9: 2-Oxokovalenic acid	Wheat, onion, lettuce	Plant growth, germination	[54]	Fresh leaf	Germination, plant growth	Vigna mungo	[44]
	10: Lab-13-en-8β-ol-15-oic acid	Wheat, onion, lettuce	Plant growth	Germination	Luffa cylindrical, Abelmoschus esculentus, Brassica juncea	[47]
	Plant growth, contents of chlorophyll and carotenoid	Chilli,	Vigna radiata	[48]
	Plant growth, contents of chlorophyll and carotenoid	Pennisetum glaucum, Eleusine coracana	[49]
	Plant growth	Amaranthus spinosus	[50]
Root	Germination, seedling growth	Hibiscus esculentus	[51]
Leaf, root, bark	Plant growth	Maize	[52]

4. Phytotoxic Substances with Allelopathic Activity in Teak

Phytotoxic substances with allelopathic activity identified in teak are discussed in this section. All phytotoxic substances listed in

Table 2

and

Figure 1

were isolated from fresh teak leaves with water. Naphthotectone (3) inhibited the germination and seedling growth of wheat (

Triticum aestivum

L.), onion (

Allium cepa

L.), tomato (

Lycopersicon esculentum

L.), and lettuce (

Lactuca sativa

L.) [53]. Rhinocerotinoic acid (8) suppressed the germination and seedling growth of wheat and lettuce [54]. 2-Oxokovalenic acid (9) and 19-hydroxyferruginol (11) inhibited the germination and seedling growth of wheat, onion, lettuce; 3β-hydroxy-7,8-dihydro-β-ionol (15) inhibited the seedling growth of wheat, onion and tomato; and 3β-hydroxy-7,8-dihydro-β-ionone (16) inhibited the seedling growth of wheat, onion, tomato and lettuce [55]. Other compounds listed in

Table 2 inhibited the seedling growth of wheat ^[54][55][56]. Although those compounds were isolated and identified from teak leaves for potential use as a source of natural herbicide model and/or bioactive compounds, the allelopathic effects of those compounds were determined only by crop plants. It may be necessary to determine the activity of those compounds on weed species.

inhibited the seedling growth of wheat [54,55,56]. Although those compounds were isolated and identified from teak leaves for potential use as a source of natural herbicide model and/or bioactive compounds, the allelopathic effects of those compounds were determined only by crop plants. It may be necessary to determine the activity of those compounds on weed species.

Figure 1. Phytotoxic substances in teak.

Phytotoxic substances in teak.

Table 2.

Phytotoxic substances in teak with allelopathic effects on crop plant species.

Phytochemical Class	Compound	Terget Plant Species
[
54
]
Germination, plant growth
Plumbago zeylanica
[	45	]
	11: 19-Hydroxyferruginol	Wheat, onion, lettuce	Plant growth, germination	[54]
	12: Solidagonal acid	Wheat	Plant growth	[54]
Apocarotenoid	13: Tectoionol A	Wheat	Plant growth	[55]
	14: Tectoionol B	Wheat	Plant growth	[55]
	15: 3β-Hydroxy-7,8-dihydro-β-ionol	Wheat, onion, tomato	Plant growth	[55]
	16: 3β-Hydroxy-7,8-dihydro-β-ionone	Wheat, onion, tomato, lettuce	Plant growth	[55]
Phenylpropanoid	17: Syringaresinol	Wheat	Plant growth	[56]
	18: Medioresinol	Wheat	Plant growth	[56]
	19: Lariciresinol	Wheat	Plant growth	[56]
	20: Balaphonin	Wheat	Plant growth	[56]
	21: Tectonoelin A	Wheat	Plant growth	[56]
	22: Tectonoelin B	Wheat	Plant growth	[56]

Several phenolics were also identified in teak barks and leaves [39,50,57]. Phenolic compounds have been found in a wide range of plants and soils, and often mentioned as putative allelopathic substances [58,59]. The importance and contribution of those phenolics found in teak are not clear because no information regarding the phytotoxic activity of those compounds for teak allelopathy is available in the literature. However, gallic and ellagic acids were identified in teak leaf extracts [60], and the allelopathic activity of those compounds isolated from other plant sources were reported [61,62]. Therefore, some phenolics in teak plants may contribute to the allelopathy of teak. Phenolic compounds inhibit some enzyme activities and physiological processes, such as plant hormone functions, water balance and mineral uptake, as well as stomatal functions, respiration, and photosynthesis [58,63].

Several phenolics were also identified in teak barks and leaves ^[39][50][57]. Phenolic compounds have been found in a wide range of plants and soils, and often mentioned as putative allelopathic substances ^[58][59]. The importance and contribution of those phenolics found in teak are not clear because no information regarding the phytotoxic activity of those compounds for teak allelopathy is available in the literature. However, gallic and ellagic acids were identified in teak leaf extracts ^[60], and the allelopathic activity of those compounds isolated from other plant sources were reported ^[61][62]. Therefore, some phenolics in teak plants may contribute to the allelopathy of teak. Phenolic compounds inhibit some enzyme activities and physiological processes, such as plant hormone functions, water balance and mineral uptake, as well as stomatal functions, respiration, and photosynthesis ^[58][63].

A number of secondary metabolites in many classes have been isolated and identified from various parts of teak plants, such as barks, flowers, fruits, leaves and roots. Those compounds were quinones, terpenes, apocarotenoids, phenolics, flavonoids, saponins, lignans and norlignans ^[16][64]. Teak wood shows resistance to termite and fungal damages, and napthoquinones and anthraquinones contribute a resistance property ^{[65][66][67][68]}. Some other compounds were also related to the pharmacological activities of teak ^{[2][16][21][35][57]}. Although those compounds have been associated with the pharmacological effects and property of its wood characteristics, some of those compounds may possess phytotoxic activity.

A number of secondary metabolites in many classes have been isolated and identified from various parts of teak plants, such as barks, flowers, fruits, leaves and roots. Those compounds were quinones, terpenes, apocarotenoids, phenolics, flavonoids, saponins, lignans and norlignans [16,64]. Teak wood shows resistance to termite and fungal damages, and napthoquinones and anthraquinones contribute a resistance property [65,66,67,68]. Some other compounds were also related to the pharmacological activities of teak [2,16,21,35,57]. Although those compounds have been associated with the pharmacological effects and property of its wood characteristics, some of those compounds may possess phytotoxic activity.