Research Progress of Adansonia digitata

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1. Sanya Sci-Tech Academy of Hainan National Breeding and Multiplication, Sanya 572000, China; 2. Hainan University, Haikou 570228, China

Abstract The origin, introduction, cultivation, active ingredients, use and advances in propagation methods of Adansonia digitata are reviewed in this paper, with focus on the rapid propagation techniques of plant tissues, to provide theoretical basis for seedling breeding and promotion of A. digitata.

Key words Adansonia digitata, Research progress, Tissue culture, Rapid propagation technology

1 Introduction

Adansoniadigitata(Bombacaceae: Adansonia), named baobab in English, is a large deciduous tree with short trunks and numerous branches. It is native to tropical arid regions of Africa.A.digitatahas huge canopy. Its branches are so strange and spectacular, resembling tree roots. The fruit ofA.digitatais shaped like bread, and the flesh juice is sweet, so it is the favorite food of monkeys, elephants, and cockroaches[1].A.digitatais thermophilic. It is usually located in the savannah of Africa, where the weather is drought from April to October.A.digitatacan withstand high temperature above 40℃, and can also endure the extreme low temperature below 0℃. Through introduction and cultivation at the initial stage, the research team of Sanya Sci-Tech Academy of Hainan National Breeding and Multiplication found thatA.digitatais adapted to the Sanya’s climate, with fast growth, numerous lateral branches, tender leaves and good taste.

The development and utilization ofA.digitatain most sub-Saharan African countries has increased its commercial value and increased the demand forA.digitataseedlings. However, the cutting propagation ofA.digitatais extremely low, with low germination, which hinders the industrialization of its development. Therefore, the rapid propagation technique ofA.digitatahas been on rise.

2 Domestic research progress

2.1IntroductionandcultivationofA.digitatainChinaA.digitatais introduced and cultivated in only a few places in China. There are two relatively old baobab trees in two places of Hainan, one at the Baoting Tropical Crops Research Institute and one in the Fucheng Forest Seed Testing Farm in Qiongshan District, Haikou City. The baobab tree in the Fucehng Forest Seed Testing Ground has a height of 30 m, a circumference of 5.8 m and a diameter of 1.6 m. According to records, this tree was introduced to the farm in the early 1960s, and flowered after eight years[2]. MatureA.digitatafruit can be harvested every year. In the past 50 years, the tree has withstood many typhoons and still stands still. It is reported that there is one baobab tree in the Fujian Subtropical Plant Research Institute. It was introduced from Africa in the 1960s. It is currently the tallest and largest baobab tree in Fujian Province[3]. The Yunnan Tropical Crop Research Institute planted two baobab trees in Jinghong City, Xishuangbanna in the 1960s. They grow slowly. At present, the diameter of the baobab trees is 120 and 130 cm, respectively. They began to bloom and fruit around 2011[4]. The baobab trees introduced by Sanya Sci-Tech Academy of Hainan National Breeding and Multiplication in 2015 have grown rapidly after transplanting. They began to bloom after two years of transplanting and their branches and leaves flourish.

2.2ApplicationofA.digitatainChinaA.digitatais only used as a landscaping tree species in Yunnan, Guangdong, Fujian, Hainan, Taiwan, Guangxi and other regions in China. It is one of the landscaping trees[5]. In Yunnan, the local farmers useA.digitataas a kind of woody vegetable. The flesh ofA.digitatais rich in sugar, so it is used for roasting, porridge cooking, wine making and beverage making. The seeds can be used to refine edible oil, make flavor agents, or for direct consumption. The young leaves can be used as vegetable for salads and flavor agents. The current application ofA.digitatain other fields has not been reported.

3 Foreign research progress

3.1TaxonomyandbiologicalcharacteristicsofA.digitataA.digitatais mainly distributed in Africa to Oceania. In China, it is mainly distributed in Yunnan, Guangdong, Fujian, Hainan, Taiwan and Guangxi.

A.digitatais a deciduous tree with a plant height of about 20 m. In the place of origin, the height ofA.digitatacan reach 35 m. The base of the trunk is swollen, and the diameter can reach 6-12 m. The girth can reach 50 m. The shape of trunk ofA.digitatais diverse, bottle-shaped, conical, cylindrical or irregular. The bark is purple, gray, or silver, smooth and shiny[6]. The blade is a palmate compound leaf composed of 37 leaflets[7]. The flower is solitary at the tip of a 25 cm long flower stalk. The corolla has five thin white fleshy petals with a diameter of about 20 cm. The stamens are mostly gamogenic with style and stigma in the middle.A.digitatacan bloom at any time except the peak of the drought. The pollination ofA.digitatamainly depends on wind, bats, lemurs,etc[8-12]. The fruit is long oval or sausage-shaped, in length of 30-35 cm and longitudinal section of 15-17 cm. The sour flesh wraps numerous seeds. According to observation,A.digitatacan grow all the year round in Sanya, without obvious defoliation. After planted for two years, it blooms around mid-October, but does not fruit.

3.2ActiveingredientsandusesofA.digitata

3.2.1 Active ingredients.A.digitatahas many uses. It is a raw material for food, clothing materials, medicine and many useful items. Its flesh, seeds, leaves, flowers, roots and bark are edible[13]. Scientists have studied many useful properties. The flesh is rich in vitamin C, calcium, phosphorus, carbohydrate, cellulose, potassium, protein and lipid[14], and it can be used for seasoning as an appetizer or made into fruit juice. The seeds contain appreciable amounts of phosphorus, magnesium, zinc, sodium, iron, and manganese, as well as lysine, thiamine, calcium and iron. The bark contains ingredients such as tannins, gums, mites and albumin. The bark and seed oils contain β-sitosterol, which has a fever-reducing effect. The leaves and bark contain lupenyl acetate, scopoletin, friedelin and betulinic acid[15], so the bark ofA.digitatawas once exported to Europe as an antipyretic drug. The leaves are dried for home dining and are marketed during the dry season. The leaves are usually dried and stored in the form of entire leaf or chopped and sieved into fine powder[16]. Young leaves are widely used, cooked as vegetables, dried and powdered, for sauces, porridge, whole grains or boiled rice[17]. It is understood that the wild fruit pulp has a very high vitamin C content (280-300 mg/hg), which is 7-10 times higher than that in oranges (51 mg/hg)[18].A.digitatahas many biological properties including antibacterial, antimalarial, anti-diarrhea, anti-anemia, anti-asthma, antiviral, antioxidant and anti-inflammatory activities. A phytochemical investigation revealed that it contains flavonoids, plant sterols, amino acids, fatty acids, vitamins and minerals.

3.2.2 Use.A.digitatais a treasure in African life[19]. Its main uses are as follows. The first is the value of food. The fruit ofA.digitatais shaped like baked bread. The fruit is edible, delicious, and rich in vitamin C and calcium. The fruit can be baked, chopped, and boiled as a drink. The soup made of leaves tastes delicious like asparagus. During several famine periods in African history, this "natural bread" has saved the lives of thousands of hungry people. The tubers, twigs, fruit, seeds, leaves and flowers ofA.digitatacan be used as a common ingredient to make traditional dishes[20]. The seeds and pulp contain organic acids and gums. After boiled, it is a delicious refreshing drink. The oil content of seeds ofA.digitatais 15%. It is a non-drying oil, light yellow, and is a good edible oil[21]. The seeds can also be mixed with whole grains. The second is medicinal value.A.digitatacan be used as medicine entirely, and the fruit, leaves and bark can be used as medicine, with effects of nourishing the stomach and gallbladder, clearing away heat and swelling, stopping bleeding, stopping diarrhea, and calming the nerves[22]. The infusion of leaves can be used as a medicine to treat malaria, kidney disease, asthma, diarrhea and poisonous bites. It can be used to treat toothaches by adjusting it into a paste[23]. The leaves are rich in anti-free radical flavonoids, which also have the function of preventing sun damage. Scientists have isolated a substance fromA.digitatathat inhibits the formation and expansion of gastric cancer cells, thus uncovering the mystery that stomach cancer almost does not occur in Africans who eatA.digitata. It has also been used to treat malaria, with fever-reducing effect. The slurry of leaves and fruit ofA.digitatais still a commonly used anti-inflammatory drug in the local area[24]. The global demand forA.digitatain the industries such as the medical industry, the food industry, and the cosmetics industry has grown dramatically. In fact, due to its interesting pharmacological properties,A.digitatahas been used for many drug purposes in many parts of Africa and in other continents. The third is economic value. Bark is a good material for papermaking and weaving, and it can be woven and roped[25]. The fruit shell can be used as a scoop, the pulp can be eaten raw, and the juice can be used as tea. The wooden husk is oval and very hard and can be used as a container. The fruit is rich in potassium carbonate after burnt. It is a good soap. Fruit juice is a shampoo for women. The red juice of the roots can be used as a lotion, and it is said to be able to make the skin white and tender. The pollen can be used to make glue. The seeds contain high-quality edible oils. In recent years, as the industry has sought natural alternatives, the demand for seed oils ofA.digitataas a food, cosmetic and biofuel has increased significantly. The fourth is ecological value and landscape value.A.digitatahas the advantages of resistance to typhoon, rain, infertility, high temperature and waterlogging. There is no need for frequent tillage and weeding, which is conducive to soil and water conservation. It can also be interplanted. The shape ofA.digitatais fantastic and spectacular, and it is an excellent ornamental tree species with a strong tropical flavor. It can be planted in flower beds, tall and straight. It can also be planted in gardens or lawns, and it can also be used as road green belts.

4 Research progress on the propagation methods of A. digitata

4.1SeedgerminationA.digitatauses seeds to reproduce. Seedlings can also be reared by seeding, cutting and grafting. The seed coat ofA.digitatais very thick. The seed germination rate is less than 15% without any treatment in Sanya, Hainan. After pretreatment, the seed germination rate can be effectively improved. The germination rate can be effectively increased by physical and chemical treatment or the like. In Sanya, Hainan, the germination rate of pretreatedA.digitatacan reach about 50%.

4.2CuttingandgraftingCutting and grafting are important technical measures to keep the excellent agronomic traits of the mother tree, bring blooming and fruiting forward, and achieve dwarfing operations. It is easy to operate but has a low survival rate and strict seasonal requirements and cannot be reproduced at any time.

4.3RapidpropagationtechnologyofA.digitataPlant tissue rapid propagation, as a basic method of plant biology research, is widely used in the breeding of high-quality seedlings, crop breeding, production of pathogen-free plants and conservation of plant germplasm resources. The application of woody plant tissue culture technology in forestry scientific research and production is the same as other plants. It is mainly reflected in the rapid propagation of plants, detoxification of plants, cultivation of new varieties and preservation of germplasm resources. The problem of browning and vitrification of cultures in subculture of woody plants is difficult to overcome. So far, woody plant tissue culture technology is still difficult to find a rule that can be generally followed[26]. At present, the research on rapid propagation ofA.digitatahas not been reported in China.

In most sub-Saharan African countries, the use ofA.digitatain the food industry has increased due to the high nutritional value and medicinal value ofA.digitata, and the demand forA.digitataseedlings has greatly increased due to its increased business value. However, due to the limitations of the natural reproductive capacity and the regeneration cycle ofA.digitata, there is an urgent need to find new ways to quickly acclimate and cultivate to preserve genetic diversity, save multiple specimens, and promote the cultivation of varieties suitable for commercial cultivation[27]. Due to environmental and genetic factors and low seed germination rate, the proliferation and storage ofA.digitatagermplasm can be achieved by vegetative propagation methods[28].Invitropropagation techniques have been widely used in a variety of species, with certain advantages[29]. In foreign countries, this biotechnological approach has been widely used inA.digitata. In 2007, Ishiietal.[30]carried outinvitroculture ofA.digitataand determined the effects of different media and 6-benzylaminopurine (BAP) concentrations on one-month-oldinvitroseedling explants. Vitrified cultures were obtained by inoculating sections ofinvitrogerminated seedlings at different concentrations of BAP. Singetal.[31]focused oninvitroseed germination rather thaninvitroproliferation. In 2010, Cunietal.[32]examined embryonic development of mature and immature seeds. It was found that seed germination was significantly affected by age, mature seeds could not germinate, and theinvitrogermination ofA.digitatapretreated with sulfuric acid was not synchronized, taking 3-30 d to extend the radicles. In 2012, N’Doyeetal.[33]described the proliferation ofA.digitataexplants added with various types of cytokinins, indicating that BAP is more effective in forming new shoots. In the same year, Rollietal.[34]proliferatedA.digitataby cultivating cotyledons and scorpions. It was observed that abnormal seedling growth or shoot tip necrosis might occur in germination seedsinvitro, making it difficult to obtain a reliable culture to initiate. In 2014, Rollietal.[35]achievedinvitroshoot proliferation by enhancing axillary bud proliferation in sterile two-node segments.

4.3.1 Explant selection and treatment. N’Doyeetal.[33]used mature seeds of the same source with no obvious disease as explants. After soaked in concentrated sulfuric acid (95%) for 12 h, they were subjected to chemical scratching. Then, they were rinsed thoroughly with sterile distilled water, soaked in sterile distilled water added with a few drops of Tween for 10 min, disinfected with bleach (NaOCl, chlorination 8°) for 5 min, and rinsed four times with sterile distilled water. The pretreated seeds were sown under sterile conditions in MS mineral medium without hormones. The flaks were placed in an incubator (dark culture, 27±1℃). After the seed radicles grew out, they were transferred to lighting culture (16 h/d).

Rollietal.[35]disinfected the two nodes obtained after seed germination as explants. The seeds were soaked in sulfuric acid (98%) for 24 h, washed with running water, and placed in 100 mmol/L NaHCO3solution for 30 min to neutralize the remaining acid. After the seeds were rinsed clean, they were incubated in deionized water at 37℃ for 24 h and placed in a plastic box containing sterilized vermiculite wetted with distilled water. They were covered with thee same substrate of 2 cm and then incubated in a climate chamber (22-28℃, lighting 16 h/d) for four weeks. The four-week-old seedlings were collected and cut to obtain double-node sections (about 3 cm long, without base leaf). The sections were disinfected in 70% (V/V) ethanol solution for 30 s, infiltrated with 1% or 2% (V/V) sodium hypochlorite for 5 or 10 min. The disinfected double-node explants were vertically inserted into glass culture tubes containing MS medium separately.

4.3.2 Propagation. The division ofA.digitatabuds is dependent on the supply of cytokinins. The combination of 10.0 μmol/L zeatin nucleoside (ZR) and 10.0 μmol/L indole-3-butyric acid (IBA) had the best proliferation effect. N’Doyeetal.[33]tested three types of explants: cotyledonary nodes, axillary nodes and tip ends. In the medium of MS plus BAP (0.5 mg/L), the proliferation coefficient of the tip ends was 2.31, with an average of 1.25 buds. In the medium of MS plus BAP (0.5 mg/L), the proliferation coefficient of axillary nodes was 1.88, with an average of 1.33 buds. In the medium of MS plus BAP (1.0 mg/L), the average bud number of cotyledonary nodes was 1.47.

Rollietal.[35]studied the effects of three different cytokinins on the proliferation ofA.digitata. In the MS medium, 0.1, 1.0 and 10.0 mmol/L BAP, 2-i P and ZR was added separately, or accompanied by 1.0 and 10.0 mmol/L IBA. MS (no hormone) was used as the control. The results showed that 10 μmol/L of ZR was more effective for inducing shoot regeneration. When both cytokinin and auxin were present, the efficiency of bud regeneration of double-node sections was enhanced, and the shoots were effectively stretched. In the medium added with 10.0 mmol/L ZR and 10.0 mmol/L IBA, the maximum proliferation rate was obtained, and the shoots developed fully.

4.3.3 Rooting and transplanting. NAA is more effective than IBA in inducing rooting ofA.digitata. The rooting effect of appropriate amount of NAA is obvious. Before transplanting, if rooted seedlings are domesticated for 30 d in an adapted environment, the survival rate of transplanting can reach more than 70%.

5 Conclusions

A.digitatais common vegetable in Africa, and its tubers, twigs, fruit, seeds, leaves and flowers all can be eaten, with good and unique taste. Its fruit, bark and leaves can be used as medicines and their shape is unique with certain ornamental value. It is a tropical woody plant that is well worth developing.A.digitatahas the advantages of resisting typhoon, rain, infertility, high temperature, and waterlogging. The incidence of diseases and pests is light inA.digitata. The vegetable production period is long during the year. It is adapted to Hainan’s climate, with fast growth, numerous branches and young leaves, and the nutritional value is high, conducive to the development of special dishes and suitable for Hainan’s characteristic tourism market. However, the germination rate ofA.digitataseeds is low, and the breeding of seedlings is difficult. The rapid propagation of plant tissues can solve the breeding problem ofA.digitata. Establishing a set of rapid propagation technology system forA.digitatawill lay a foundation for the development and use of this plant with edible, medicinal and ornamental value, and provide a scientific and technical basis for the rapid propagation of tropical woody plants. This is of great significance for the innovative use of tropical woody resources and the maintenance of tropical diversity.