Tree bean, Parkia roxburghii G. Don (Syn. P. timoriana (DC.) Merr.), a lesser known nutritious legume of leguminosae family is an important tree vegetable of Manipur and north-eastern states of India. In Manipur, the tree is called ‘Yongchak’. Commonly grown in the backyard, jhums and forest, it is considered one of the most important tree having many health and commercial benefits.
The tree is native to Southeast Asian countries such as Burma, Bangladesh, Indonesia, Java, Philippines, Thailand and the Malaysian region, and Northeastern India. It is also distributed in the tropical and subtropical regions. In the north-eastern region it prolifically grows in Arunachal Pradesh, Assam, Manipur, Meghalaya, Mizoram, Nagaland, and Tripura.
The ethnic community of Manipur uses the flower and pod as vegetables and in preparation of Singju, a typical salad. It may also be mixed with fermented fish to prepare the local delicacy, Iromba. The wood is used as firewood and for making furniture. The young tender pods and bark are used for treating intestinal disorders, piles, diarrhoea and dysentery. The bark decoction is consumed to manage diabetes. Pods pounded in water are used in cleansing the face and head. The oil extracted from the seeds has insecticidal properties against aphids.
In Malaysia, pods are consumed to cure kidney disorders, diabetes, urinary tract infection and hypertension. The local people of Ghana apply fruits of tree bean to manage leprosy and hypertension. Being a fast-growing leguminous plant, tree bean can be used for the reclamation of abandoned jhum land and enrich the soil fertility through nitrogen fixation. It is an ideal tree legume for agro-forestry systems in hill regions and can also be planted as a wind-break and avenue tree.
The tree grows well in a wide range of soil and climatic conditions from colder hilly to hotter plains regions with less care. It grows in humid tropical to subtropical climatic conditions between 400 and 1,200 mean sea level. The optimum temperature for luxuriant growth is 15 to 27°C and can tolerate up to 35°C. It grows well in regions with an annual rainfall of about 3500 mm per annum but will survive down to 1750 mm. It grows well in different ranges of altitudes up to 3,000 m above sea level. The ideal soil pH for growth is in the range of 5.5 to 7.5. In north-eastern hilly regions of India, the tree is well adapted to a wide range of soil from clay loam, and deep clay loam to lateritic/acidic soils.
The eatable parts of tree bean such as flowers, pods and seeds are a good source of protein (32.82 per cent), ascorbic acid (26.0 mg/100g), fats (20.28 per cent), carbohydrates, and minerals (4.45 per cent). Although the protein content of tree bean seeds is lower than soybean (43.0 per cent), it is higher than other eatable legumes such as Bengal gram, cowpea, green gram and red gram which range from 22 to 24 per cent.
The seeds are a good source of essential amino acids and fatty acids such as isoleucine, leucine, phenylalanine, tyrosine, oleic acid, and linoleic acid. The content of minerals such as Ca, Cu, Zn, P, Fe, Mn, and Mg in pods and seeds is also at par with other legumes. The content of protein (15-36 per cent) and fats (11-69 per cent) are increased in the various processing and cooking methods.
Processing methods also reduce the concentration of anti-nutritional compounds such as tannins, phytate, saponins, trypsin, and chymotrypsin inhibitors thereby enhancing the digestibility of nutrients. The processing further increases the content of total phenolic compounds. The mature parts contain a higher percentage of protein and fats whereas, fiber and carbohydrate content decreases when compared with the immature pods and seeds.
The essential amino acids such as histidine, isoleucine, leucine, lysine, methionine, phenylalanine, and valine content also increase as the pods and seeds reach maturity. Similarly, essential fatty acids oleic acid, linoleic acid and unsaturated fatty acids content increase as the pods and seeds age. However, fermentation methods lead to the loss of essential amino acids such as trypsin, cystiene and methionine in seeds.
Tree bean has several medicinal and health-beneficial properties that can be used as important human dietary supplements. The various parts of the plant have anticancer, antioxidant, antidiabetic and antibacterial activities. Thiazolidine 4-carboxilic acid responsible for the pungent smell of tree bean is an efficient nitrite scavenger thereby inhibiting the formation of carcinogenic N-nitroso compounds in the human body. The compound inhibits stomach cancer in rats. The compound extracted from seeds demonstrates cell death of human liver and other cancer cell lines with less toxicity to normal cells. The pungent smell also contains phytochemical compounds like phenols and flavonoids.
The other edible parts of the tree also contain phenolic and flavonoid compounds. These compounds are shown to possess antioxidant, antibacterial, anti-inflammatory, and antiallergic activity. It also shows antidiabetic, antihypertensive, antiulcer, antimicrobial, and antitumor. The phenolic content is highest in pods followed by leaves, seeds and flowers. The pods exhibit the highest antioxidant activity followed by leaves, seeds and flowers. Processing methods increase the health benefits potential. Processing and heating methods of seeds further increase the total phenolics content and considerably enhance reducing power and free radical scavenging activities over raw seeds. Among the various processing methods employed, dry heating and fermentation were found to retain potent antioxidant activities.
The pods of yongchak display antidiabetic activity in a scientific study using rats as a model. It also has a capacity to protect the liver (hepaprotective). The antidiabetic and hepatoprotective effects may be due to health benefits compounds such as epigallocatechin gallate and hyperin. Among the various edible parts, seeds show the highest antimicrobial activity followed by bark, pods and leaves against human bacterial pathogens including E. coli, Vibrio cholera, Staphylococcus aureus, Salmonella typhimurium Pseudomonas aeruginosa and Bacillus cereus. The health benefits activity of various parts of yongchak are illustrated in the figure.
Figure: Health benefit activity of various parts of tree bean
Declining of population
Yongchak is prone to insect-pests and disease infestation. Many diseases caused by insect-pests have been found infecting the tree. Manipur state has witnessed a rapid decline threatening the cultivation in the last decade. The state reported the first decline in 2002 and the death of a plantation linked to a stem borer, Bactocera sp. Lack of proper management and negligence of post plantation management also contribute to the major reason for the decline.
The common symptoms of tree bean decline are wilting, excessive gummosis, drying up of branches along with shedding of leaflets, presence of bore holes with the emission of trashes. Abscission of the infected tree starts from the tip progressing downwards with dieback being seen as the most prominent symptoms. Once these symptoms start appearing, the entire tree dries up in the next few days. The plant shows the presence of vascular black streaks running longitudinally with excessive gummonis in the trunk of declining trees. As a result, the supply of nutrients and water is blocked causing leaf shedding, loss in vigour, wilting and ultimately death of the tree. The young plants (up to 10 years) are more prone to decline as compared to older plants (more than 30 years).
Various factors are found to be associated with the decline of tree bean which includes insect-pests, fungal pathogens, abiotic stresses and nutrient imbalances. The incidence of insects-pests and diseases is the major cause of decline. An array of insect-pests viz., aphid (Aphis craccivora), thrips (Scirtothrips dorsalis), green stink bug (Nezara virudula), Asian long horned beetle (Anoplophora glabripennis), jassid (Empoasca kerri), bark eating caterpillar (Indarbela spp.), spotted pod borer (Cadra cautella), Coreid bug/tur pod bug (Clavigralla gibbosa) etc. infect the tree bean and thereby associated with the declining.
Interestingly, a frequent association of Asian long horned beetle, bark eating caterpillar and spotted pod borer has been observed with the decline in yongchak. Fungal pathogens are also associated with decline of tree bean including Botrydiplodia theobromae, Pestalotiopsis disseminate and Pestalotiopsis guepeni. The dieback and other pest infections in the tree are reportedly due to the association of Botrydiplodia theobromae, a fungal pathogen spread by bark boring insects. Other diseases infecting are Verticillium wilt and collar rot (Phytophthora sp.) Recently a fungal pathogen, Fusarium oxysporum has been discovered to cause root rot followed by wilting, dried stem and complete mortality of the tree bean seedlings.
Tree bean is a fast-growing leguminous tree that increases the soil’s fertility through nitrogen fixation. It can help maintain ecological balance by enriching and improving soil health. It is associated with various beneficial bacterial populations that help in releasing nutrients to the soil, thus improving the health and properties of the soil. The leaves and other parts of the tree are easily decomposed enhancing the organic matter content of soils. Due to a large aboveground biomass, it can also play a vital role in the sequestering of atmospheric greenhouse gases especially carbon dioxide which help in the mitigation of regional climate change.
In Kamjong district where jhum cultivation is commonly practiced, tree bean can be used as a tool for the reclamation of degraded jhum land and checking of soil erosion. The tree can be intercropped with horticultural and agricultural crops therefore, yongchak based agroforestry systems have a dynamic role to play in supporting rural livelihoods through products and services. However, due to the rapid decline in the last decade, the production of yongchak has significantly decreased which negatively affects the food security and income of the rural communities.
Even though the tree has potent health and environmental benefits, it has not received sufficient research attention in terms of multiplication, processing, disease control and maintenance. The need of the hour is to promote more scientific research on the control of diseases and maintenance of post plantation. The insect-pests can be controlled by the application of synthetic pesticides or fungicides however; these chemicals cause adverse effects on health and environment. The quest for controlling the insect-pests by the use of biocontrol (living) agents should be encouraged and promoted for sustainable production of a lesser known nutritious and important native tree vegetable yongchak.