Tropical almond (Terminalia catappa): A holistic review

Abstract

Terminalia catappa L. known as tropical almond, has a global distribution. Further, it is a popular choice for avenue planting in the tropics including India. In India, more than 20 species of the genus Terminalia are reported and grown for its multipurpose uses. However, T. catappa is an untapped as well as underutilized nut-yielding tree, despite having comparable nutritive value to Almonds but with lower yield. The propagation of the tree is typically done with seeds and there is lack of documented tree breeding and improvement initiative with regard to this species. In this review, we summarize the available information on the ecological and economic utility of this tree reported across the world. There are numerous studies on this species detailing its uses as an avenue tree, medicinally important plant, small timber tree, biosorbent and other uses. The tree is also reported as a good choice for agroforestry owing to its unique features including its salinity tolerance and canopy architecture. Overall, this review concludes with details on various prospects of this tree as well as alternative for agroforestry and trees outside forests.

1. Introduction

Terminalia catappa L., commonly known as Indian Almond, belongs to the Combretaceae family [1]. The generic name comes from the Latin ‘terminalis’ (ending) which refers to the habit of the leaves being crowded at the ends of the branches [2]. This is a large, spreading type of tree and usually grows in tropical environments and coastal communities [1]. It provides edible nuts and wide range of services including avenue, shade, and ornamental trees along sandy seashores [3,4].

Tropical almond is an underutilized fruit which is rich in antioxidants, vitamins, and pigments [5]. Its fruits are eaten especially by humans, birds and animals [6]. Ezeokonkwo and Dodson (2004) reported that the global production of its fruit/seed is estimated at 700,000 tons/year. Kernel yield is estimated to be about 5kg/tree/year [8]. The timber of the tree is medium density with normal strength and elasticity and rupture but strong in shear strength [9]. It is used as herbal medicine for treating various ailments like liver and intestine infections, and other related diseases [4,10].Despite numerous potentials and uses, the tree is not fully appreciated nor utilized as a multipurpose tree (MPT). Similar to other Terminalia species, T. catappa seeds and other economic products are collected from wild or naturally occurring trees. There are few reports of commercial-level plantations on this species or other Terminalia species such as T. arjuna or T. bellerica. This can be attributed to various reasons including the lack of awareness, lack of post-harvesting techniques, market linkage and value chain, etc. It can be called an untapped or underutilized tree [11]. There is a potential to promote this species for various purposes including the enhancement of the green cover in urban areas where it is not found prominently. This review aims to explore the potential of T. catappa – a multipurpose tree species.

2. Ecology of T. catappa

The genus Terminalia comprises about 200 species of trees and shrubs distributed in tropical and subtropical environments, mainly of humid coastal areas [[13], [14], [15]]. Tree species closely related to T. catappa include T. glabrata and T. littoralis [8]. In India, 20 species of Terminalia have been reported including T. alata, T. arjuna, T. bellerica, T. berryi, T. bialata, T. chebula, T. citrina, T. coriacea, T. crenulata, T. gella, T. manii, T. moluccana, T. myriocarpa, T. pallida, T. paniculata, T. parviflora, T. procera, T. tomentosa and T. travancorensis [12].

T. catappa is believed to have originated in Malaysia and to have reached the Andaman Islands through fruits carried by ocean currents and now it is found throughout India [4]. The native range of the tree is largely restricted to tropical areas of the Asia-Pacific region including Comoros, Madagascar, North Australia, Cambodia, India, Japan, Laos, Malaysia, Thailand, and Vietnam [15] (Fig. 2). It was introduced in several regions including Central and South America, West and Central Africa, and Caribbean islands.

Fig. 2.

Map showing the global distribution of T. catappa.

The tree is well adapted to well-drained, aerated, sandy and rocky coastal habitats and grows in an elevational range of 0–800m [16]. Soil pH suitable for the growth of trees is usually neutral to moderately alkaline and rich in bases. However, it can be grown in strongly acidic soils. Good drainage is required on clay soils. T. catappa has a strong tolerance to moderate to high salinity in the root zone. The best tree growth is observed in regions with a mean annual temperature of 15–35 °C and mean annual rainfall between 750 and 3000 mm [2]. Trees growing solitary in an open area produce more seeds than trees grown in shaded areas [17]. Da Silva et al. (2017) studied the effect of seawater salinity on T. catappa and reported that exposure did not alter the macronutrient contents in the tree. The tree can protect itself from seawater salinity through the compartmentalization of sodium salts in the pericarp of the fruit. The DBH (diameter at breast height), increment rate reported for this species in Vanuatu is 2–3 cm/year. It is also a drought-tolerant tree species and their seedlings are adapted to heat stress [19].

3. Botanical description

T. catappa is a large deciduous and erect tree that can reach a height of 15–30m with a trunk diameter of 1–1.5 m [[20], [21], [22]] (Fig. 1 shows different parts of this tree). It has a pyramidal shape at the top with a buttressed base, with a symmetrical crown and horizontal branches which are characteristically arranged in tiers. T. catappa has a grey-brown smoothish and thin bark that becomes rough with age [23,24,29] and has a spreading and fibrous root system [3].

Fig. 1.

Morphology of T. catappa in India.

The phenology of a tree comprises five main events including leaf-flushing, flowering, fruiting, fruit-fall and leaf-fall. There are two leaf-flushing periods immediately after leaf shedding at the equinoxes (March and September). [25] reported that the lowest canopy cover age is recorded in January (83.7 %) while the highest canopy coverage in September (97.0 %). Flowers of T. catappa bloom in summer [26] but it is also reported that flowering occurs in three main periods: March to April, July to August, and September to October [27]. Fruiting starts at around 3–5 years after planting [1,28] generally from October to April.

Leaves are alternate obovate with short petioles clustering towards the ends of the branches. Each leaf is15-36cm long, 8–24 cm wide, dark green above, paler beneath, leathery, glossy dark green and turns bright scarlet, dark red, dark purplish-red, yellow or gold and copper brown during the dry season [2,13,17]. Flowers are slightly fetid, very small, and greenish-white, with no petals. The tree is monoecious, with distinct male and female flowers on the same tree. Most of the flowers are male and borne towards the apex, while a few hermaphroditic ones appear below. Notably, some spikes in T. catappa have only male flowers [2].

Fruits of T. catappa are hard, green-red, rounded and flattened, egg-shaped, with 2 ridges but no wings. The fruits resemble closely the other drupe fruits of the Roseaceae and Anacardiaceae families such as plum, apricot, peach, cherry, Spanish plum (Spondias purpurea) and mango [30]. It has a membranous epicarp, succulent mesocarp, and fibrous endocarp that enclose the seed and attract fruit flies [31,32,35]. This fruit is green in colour when unripe and turns yellow or red when ripe. The fruit contains a large edible nut formed after embryonic development that looks somewhat like a true almond [33,34]. The kernel can be carried considerable distances on the ocean currents and remain viable [34]. There are ∼24–25 fresh fruits/kg and 160–180 nuts/kg [8]. Fruit bats and birds eat the fibrous fruits and play a key role in their distribution. The fruits are a source of rich protein (1.95 g) and source of beta-carotene and vitamin C.

T. catappa is popularly known as the Indian almond, wild almond, sea almond, tropical almond and Malabar almond [22,33,36]. The tropical almond (T. catappa) is different both taxonomically and nutritionally from the Almond (Prunus dulcis) which belongs to the Rosaceae (Table 1). Despite the taste variation [37], the nutritive value is similar to true almond, however, the commercial productivity of T. catappa is not comparable.

Table 1.

Nutritional composition of T. catappa and Prunus dulcis.

Nutritional composition	Tropical Almond	Almond
	(T. catappa)	(Prunus dulcis)
Moisture	6.23 ± 0.09 %	3.05–4.33 %
Lipids	52.02 %	43.37–47.50 %
Proteins	25.42 %	20.68–23.30 %
Sugars	5.98 %	5.35–7.45 %
Carbohydrates	6.67 %	25.0 %
Dietary Fiber	9.97 ± 0.08 %	11–14 %
Sources:	[7,33,38]	[[39], [40], [41]]

4. Phytochemical characterization of T. catappa

One of the most researched aspect of T. catappa is its phytochemical and ethnobotanical utility. It contains numerous phytochemicals like carbohydrates, flavonoids, polyphenols, phytosterols, tannins, and saponins [13]. Air-dried T. catappa seeds contain 52.02 % fat, 25.42 % protein, 14.6 % fiber and 5.98 % sugar. One more important aspect of this tree is the fatty acid composition of its kernel oil, commonly referred to as T. catappa Kernel Oil (TCKO) (Table 2).

Table 2.

Fatty acid composition of the T. catappa Kernel Oil.

Fatty acid of TCKO	Menkiti et al. (2017) [42]	Iha et al. (2014) [43]
Lauric acid	0.94	–
Myristic acid	0.54	0.10
Palmitic acid	36.01	28.30
Palmitoleic acid	–	0.90
Stearic acid	6.4	4.90
Oleic acid	33.25	30.00
Linoleic acid	22.26	32.80
Linolenic acid	0.59	1.70
Saturated fatty acids (%)	43.89	34.20
Mono-unsaturated fatty acid (%)	33.25	30.00
Polyunsaturated fatty acid (%)	22.85	34.50
Unsaturated fatty acids (%)	56.10	64.50

T. catappa leaves contain a significant amount of bioactive components such as antioxidants, and flavonoids like tannins, phenols, kaempferol and polyphenols [44]. Similarly, its gum is reported to contain galactose (20 %), arabinose (59 %), mannose (1 %), xylose (1 %) and uronic acids (19 %), with an average molar mass of 9.8 × 10⁵ g/mol [45].

Mineral analysis of defatted kernels of T. catappa indicates that phosphorus (P) is the most abundant (1804 mg/100g) macro-elements, followed by potassium (K) (1718 mg/100g), magnesium (Mg) (729.09 mg/100g), calcium (Ca) (415.01 mg/100g), and sodium (Na) (37.60 mg/100g). Among the microelements, iron (Fe) (16.15mg/100 g) is dominant, followed by zinc (Zn) (9.67 mg/100g), copper (Cu) (4.63 mg/100g), and manganese (Mn) (4.25 mg/100g) [1]. Several essential amino acids are found in its nut/seed including leucine, isoleucine, valine, phenylalanine, tryptophan, methionine, lysine, threonine, histidine, tyrosine, arginine, aspartic acid, glutamic acid, glycine, alanine, serine, cysteine, and proline [7]. The flower's nectar is also a source of five essential amino acids-threonine, arginine, histidine, lysine, methionine and some non-essential amino acids. Ascorbic acid (14.58 ± 0.09 mg/100g) is a major vitamin followed by retinol (6.45 ± 0.00 mg/100g), niacin(2.52 ± 0.09 mg/100g), thiamine (0.345 ± 0.10 mg/100g), and riboflavin (0.09 ± 0.01 mg/100g) [46]. [47] observed the removal of extra floral nectar by the Japanese White-eye bird in the Bonin Islands.

5. Uses and environmental services of T. catappa

Apart from the widely known uses like edible nuts, shade, and ornamental purposes, the tree is also regarded as the source of biodiesel [48]. Apart from this, this tree is reported to provide numerous environmental services like erosion control, ornamental service, shade provision, soil reclamation and restoration.

5.1. Ornamental and shade tree

The umbrella-shaped crown of T. catappa makes it an optimal choice for ornamental and shades provision for landscaping, particularly in beaches, town squares and other urban landscapes. Across the globe, the tree is widely used in the landscaping of cities especially along with other Terminalia species like T. manataly ([49]; [50]; Marjenah, 2021). The distinctive 'pagoda-like' shape [51], the handsome red colour of leaves before shedding, and fast growth make it a popular choice for avenue trees [27,51]. Further, T. catappa is tolerant of air pollution and can be grown as a roadside tree in urban settings [52,53].

5.2. Food and nutritional value

Tropical almond nuts are considered a human food source and are either eaten fresh shortly after extraction from the shell or else preserved by smoking or roasting [34,54]. Besides fresh consumption, the fruit can also be consumed in the form of juices, mousses, and jams [21]. Sometimes, the flour of nuts/kernels is used as a thickening agent and it is a good protein supplement which is used in soups, baby foods, and high-protein foods [55,56]. In Côte d'Ivoire, West Africa, wheat flour cakes can be enriched with its kernel flour [57].

The rich dietary protein and essential amino acids, and high digestibility of the kernel/seeds have the potential to support growth and alleviate the problem of malnourishment in poor and developing countries [50,58]. The total soluble fibre in T. catappa gum (85 %) is relatively high compared to A. senegal, A. seyal and A. siebarana exudates which confers it a nutritional value and much work is needed in this context [45]. A high oil yield, iodine value (IV) and total phenolic content (TPC) are reported and can be utilized in the food industry (Jahurul et al., 2022). The fibrous fruit also contains a higher content of minerals, vitamins, and antioxidants than other fruits such as cherry, peach, plum, apricot, and mango. The taste may not be liked by all humans but they can used as animal feed. The grey-bellied tree squirrel and long-tailed macaques feed on the fruit [37,59] and it is a habitat for insects such as Aleurodicus rugioperculatus [60].

Terminalia catappa kernel (TCK) has a relatively high oil yield and its oil yield ranges from 49 to 65 % [[61], [62], [63], [64]].Total saturated fatty acid and total unsaturated fatty acid in TCK are quantified to be 34.2 % and 64.5 %, respectively (Iha et al., 2014).It can be regarded as one potential underutilized oil crop For instance, oil yield from TCK is about 60.45 % (dry mass) more than the cottonseed (Orhevba and Efomah, 2012), soybean (Lawson et al., 2010) and other commercial oil sources (Cerutti et al., 2012). It can be used as an alternative to some conventional oil seeds in countries like India where T. catappa is underutilized. There are reports that T. catappa seed oil follows the dietary standard of edible oil [65]and is being used for cooking in some parts of South America [11]. Tropical Almond fruit pulp can be used to produce Ready-To-Serve (RTS) beverages [5].

5.3. Medicinal use

T. catappa constitutes numerous medicinal properties [66]. All parts of the tree are useful ethnomedicinally [22]. For instance, the leaves are used for treating diabetes, high blood pressure, haemorrhoids, leprosy wounds and other skin diseases [67]. The bark is a remedy for dysentery in the Southeast Asian countries and its root bark is for bilious fever, diarrhoea, and thrush. The nut is also used for antibacterial and aphrodisiac properties. The kernel of T. catappa fruit mixed with beeswax is used to treat putrid exudation and bloody faeces [2].

A wide array of medical and pharmacological properties have been documented for T. catappa like analgesic, modulatory, antiaging, wound healing [20,23], antimicrobial, antibacterial, and erythropoiesis enhancer [23], anti-inflammatory [68], antidiabetic (Teotia and Singh, 1997; [69]), antimetastasis [70], anthelmintic [71], anti-quorum sensing (QS) activity [72] andantioxidant activities [73], hepatoprotective [74], antifungal [75], and aphrodisiac [76].

The anti-tumorogenic activity of T. catappa is due to the free radical quenching property of its phyto constituents [73]. It is also rich in tannins that exhibit anti-diabetic properties.A tannin-rich fraction from T. catappa plays a vital role in inhibiting quorum sensing in bacterial populations, which consequently prevents bio-film formation [72]. This potential provides a scientific basis for the traditional use of leaf extracts of this plant as an antiseptic [72,77]. Madhavan et al. (2023) evaluated the anti-Bacillus potential of T. catappa leaf extract and concluded that ethanolic T. catappa leaf extract exhibited antibacterial and sporicidal activities against Bacillus spp.

5.4. Timber value

The timber of T. catappa is used in buildings, floors, boxes, crates, boats, bridges, planks, carts, water troughs, wheelbarrows, and barrels. Quartey (2022) determined the mechanical properties of the tree from Ghana, Africa and reported that its wood is of medium density (520 kg/m³) with medium strength in elasticity (10,500Mpa) and rupture (86.04Mpa) but strong in shear strength (16.42N/mm²). Its heartwood is reddish brown while the sapwood is pale yellow with changing colouration through age [9]. The monthly growth rate of cambium closure of this tree is 8.36 ± 0.15 cm²/month [78]. It has strength properties comparable to Canarium schweinfurthii, Scottellia klaineana, Gmelina arborea, Pterygota macrocarpa and Sterculia oblonga. Its wood can be used for heavy construction, sleepers, flooring, and turnery as an alternative to the commonly used wood. It is also reported that the wood density, heartwood colour as well as proportion varies with location and much work is needed on this aspect.

5.5. Biosorbents

Biosorbents made up of T. catappa fruit shell effectively adsorb synthetic dyes [79]; Cd(II), Pb(II), and Cu(II) metal ions [80], methylene blue (cationic) dye [81], Indigo Carmine dye [82]; micropollutants such as diclofenac [83] as well astrace metals from environmental aqueous solutions [84]. Its leaves can also be used as a low-cost biosorbent for dicloxacillin from pharmaceutical waste water and natural coagulant to remove turbidity [85].

5.6. Synthesis of nanoparticles

Devadiga et al. (2017) synthesized silver nanoparticles (AgNps) using the aqueous extract of an agro waste: T. catappa leaves and these nanoparticles exhibited antimicrobial properties [86]. These AgNPs exhibit significant antibacterial activity against water-borne pathogens. Muthulakshmi et al. (2022) studied the antibiofilm efficacy of novel biogenic silver nanoparticles (AgNPs) from leaf aqueous extract of T. catappa against food-borne Listeria monocytogenes.The extract of T. catappa leaves is also reported to be used for the synthesis of gold nanoparticles [87]and Nickel ferrite (NiFe₂O₄) nanoparticles [88].

5.7. Other uses

[89] evaluated the corrosion inhibitory effect of tropical almond leaf and reported that leaf extract added to soybean oil biodiesel can be used as a good corrosion inhibitor for zinc and carbon steel 1020. The fruits/seeds are a potential candidate as a natural food preservative due to their antibacterial and sporicidal activities against Bacillus spp [14]. It can be used as a substrate for growing orchids-Oncidium flexuosum, Dendrobium nobile and Brassavola tuberculata [90].

The gum from the tree bark has excellent swelling properties and the ability to sustain the release of dextromethorphan hydrobromide from matrix tablets and therefore tablet formulations with its gum can be used in controlled drug delivery [91].

Yellow and black dye obtained from T.catappa is used in leather preparation and as a base for inks. The bark, leaves, fruit shells and roots are used for staining/colouring fabrics and ink-making [3].Furthermore, its leaf extracts exhibit good corrosion inhibitory effects on stainless steel in 1M HCl [92]. The natural dye extracted from T. catappa can be used for fabricating dye-sensitized solar cells [93].

T.catappa leaves make good-quality fodder [3] and feed for silkworms and other animals [2]. [94] evaluated the leaves of T. catappa as non-conventional green roughages to highlight their nutritional characteristics.

Bioactive compounds from the nano emulsions of this fruit have potential for cosmetic use [21].TCKO and epoxidized-esterified methyl ester of TCKO (MTCKO^e) have the potential to be used in transformers as dielectric fluids [95,96] and its crude oil transesterified using conventional catalysts and methanol to form biodiesel [97]. Biochar can be formed from dry T. catappa leaves and Low-Density Polyethylene (LDPE) waste (biomass-plastic mass ratio of 35:1) [98].

Tropical Almond leaf extract is also used to promote the breeding activity of ornamental and food fishes [99].TCK meal and cake-based diets as aqua feed for Clarias gariepinus juveniles and leaves used to promote the growth and survival of juvenile Nile tilapia and larvae of silver therapon [100,101].

6. Natural and artificial regeneration of T. catappa

The seeds remain viable for more than 4–5 months and germinate readily [1] but mass vegetative propagation by rooted cuttings has also been reported but not widely adopted. In India, entire fruits of T. catappa are planted fresh in July in nurseries and the germination rate is about 25–50 % [4]. The seedlings should be planted in the field during the monsoon. In poor soils, 15–20 L of horse manure/pit is recommended before planting together with a mixture of 200g superphosphate, 150g chloride of potassium, 300g bone meal, 100g ammonium sulfate or potash, and 500–800g of lime [4].

Species selection and quality planting material play a vital role in successful plantations [102]. Despite highlighting the potential of this tree by Morton (1985), the species has not garnered much attention. It can be attributed the variation in the growth and yield of tree which in turn can be attributed to wide genetic diversity among this species across the globe. Fig. 3 depicts the research output on this species since 1985 in the Web of Science (WoS) and Scopus database. On examining the frequency of publication country-wise, countries like Brazil (n = 251) and India (n = 238) dominated in the number of publications (n) where the tree are found predominately.

Fig. 3.

Research output of T. catappa tree species since 1980.

However, researchers from other countries like Japan, Iran, Spain, etc. are also working on T. catappa where its presence is not recorded abundantly.

Promoting vegetative propagation of T. catappa is critical for upscaling this species in agroforestry and avenue plantations whilst carrying out tree breeding and improvement works including genomics. The supply of QPM to different stakeholders would encourage this species at various landscapes such as farmlands, streets, roads, institutional areas, parks, cultural sites and historical monuments.

7. Agroforestry potentials of T. catappa

The architecture of the tree is an advantage for agroforestry practice. Also, there are excellent case studies on land reclamation using NTFP species like T. catappa in Brazil, Cambodia, Indonesia, and Peru [103]. However, the scientific literature or case studies focusing on the development and promotion of this species in agroforestry systems are scarce. Tropical almond has much to offer growers of agroforestry systems as it can be integrated with crops in the farmlands for obtaining edible nuts and timber. Timber from mature T. catappa trees is highly regarded in Vanuatu. It is a potential crop for agroforestry initiatives in the Sigatoka Valley of Fiji [104] and planted along with agarwood plantations in East Flores [105].

In Central Java, along with other species Syzygium polyanthum, Parkia roxburghii and Dysoxylum gaudichaudianum and Aleurites moluccana, tropical almond is also recommended for multi-purpose trees combination for income augmentation and land restoration [106]. Other possible scenarios, where this multipurpose tree can be grown in agroforestry include shelterbelts/wind breaks along the sea beaches and degraded low-lying areas. These shelterbelts protect the shores/beaches, provide valuable forest products and conserve wildlife through giving shelter. The domestication of seed-bearing halophytes (e.g. T. catappa) through agroforestry is pivotal to protecting coastal areas and rehabilitating degraded salt-affected areas [107,108].

This tree is also amenable for silvomediculture i.e. agroforestry practice specifically for medicinal and aromatic crops like such as Aloe vera, Catharanthus roseus, Cymbopogon flexuosus, Adhatoda vasica, Euphorbia antisyphilitica, Lepidium sativum, Withania somnifera, Cassia senna, Plantago ovata, Matricaria chamomilla, and Vetiveria zizanoides can be successfully grown as sole crops or in combination with trees in agroforestry systems using a few irrigations with saline water up to EC_iw 10dSm⁻¹ [108]. In South Asia, it is grown as a woody species in the homegardens of the dry zone area of Moneragala district, Sri Lanka [109].

8. Toxicological and safety profile

Possibilities of organ toxicity such as hepatotoxic and nephrotoxicity cannot be ignored on the unpredictable intake of herbal products [112]. Toxicological profile of plant extracts is important for conducting in vivo tests of possible damage to living organisms and exploration of its strategies of utilization with maximum effectiveness [115]. Overall, these toxicity tests are crucial for detecting the degree of danger in biological systems [116]. [114] assessed the acute and sub-acute oral toxicity of Terminalia catappa aqueous leaf extract (TCA) in male Wistar rats. The LD₅₀ value of TCA was assessed to be greater than 5000 mg/kg bwt, indicating that it is practically non-toxic as classified by Organisation for Economic Co-operation and Development (OECD) [113]. [117] also evaluated the acute oral toxicity of T. catappa. In this study, acute oral toxicity of the n-hexane extract of T. catappa was evaluated in female albino Wistar rats following the OECD Test Guideline 425. The study shows that the LD₅₀ value of n-hexane extract is higher than 5000 mg/kg indicating impressive safety profile and low toxicity risk of acute doses in animals.

9. Future prospects of T. catappa

T. catappa is still an underutilized multipurpose tree in India and other countries, which have tropical monsoon climate. Therefore, this tree has enormous potential for cultivation owing to strong salt, drought and wind tolerance [32]. Thus, areas affected by salinity and strong winds can be reclaimed and restored by this tree. As stated by Dagar et al. (2023), who reported the possibility of reclamation of sodicity in the Indo-Gangetic plain, India by promoting tree-based farming systems whilst integrating salt tolerant tree species like T. catappa. There is a need to understand the physiological process of salt tolerance in this tree [18]. There are successful case studies of tropical almonds being used for kernel oil, nuts, and wood/timber in certain regions of the world. This warrants a complete reorienting of the research on this species in the tropics.

Similar to other nut-yielding tree species like almond (Prunus dulcis), and walnuts (Juglans regia), a tree improvement programme needs to be carried out in T. catappa too. Genomics is a breakthrough in tree improvement, specifically in walnuts by sequencing both nuclear genomes and chloroplast [110]. Therefore, a coordinated tree breeding and improvement programme is needed in the future. With micropropagation of T. catappa possible, the mass multiplication of superior germplasm can pave the way for the establishment of plantations. However, there is a need to focus on agronomic practices as well as silviculture practices like pruning and the effect of thinning on nut yield. Furthermore, future researchers must also focus on the post-harvesting techniques and processing of seeds/kernels for effective utilization. Regarding phytochemical activities, a vast array of studies are being carried out and it would be appropriate to focus on clinical trail-based results in the future [111].

Overall, this tree species can be a good choice for planting in different landscapes outside the recorded forest areas such as roadside, educational institutes, parks, historical sites, homegardens, and community lands. Despite, being an ornamental tree by choice, it can be tapped for its multipurpose utilities like seed oil, timber, and medicines. Also, this tree species is not regulated under felling and transit rules which makes it an ideal choice for promoting in Trees Outside Forests. Notably Trees Outside Forests are the only option for achieving 33 % forest and tree cover in countries. Therefore, T. catappa can provide economic and ecological utility to a greater extent.

CRediT authorship contribution statement

Suresh Ramanan S: Writing – original draft, Formal analysis, Conceptualization. A. Arunachalam: Writing – review & editing, Supervision, Project administration. Rinku Singh: Writing – original draft. Ankit Verdiya: Project administration.

Data and code availability statement

Data will be made available on request.

Declaration of generative AI and AI-assisted technologies in the writing process

During the preparation of this work, the author(s) have not used to review and edit the content as needed and take(s) full responsibility for the content of the publication.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.