African Nightshades and African Eggplants: Taxonomy, Crop

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Chapter 11

African Nightshades and African Eggplants: Taxonomy, Crop Management, Utilization, and Phytonutrients Ray-Yu Yang*,1 and Chris Ojiewo2 1Nutritionist,

AVRDC-The World Vegetable Center, Tainan, Taiwan, ROC Breeder, AVRDC-The World Vegetable Center, Regional Center for Africa, Arusha, Tanzania *E-mail: [email protected].

2Vegetable

The Solanaceae plant family contains many food crops important to agriculture, food security, human nutrition and health. These include globally-consumed peppers (Capsicum sp), potato (Solanum tuberosum), tomato (S. lycopersicum), eggplant (S. melongena) and regionally consumed, such as, African nightshades (Solanum section Solanum, S. scabrum and S. villosum) and African eggplants (S. aethiopicum, S. macropcarpon and S. anguivi). The taxonomy of Solanum is complex, and the “deadly nightshade” reputation of S. nigrum and other Solanum weeds or medicinal plants has extended to the edible African nightshades and African eggplants; this has confused people unfamiliar with these plants. The attention of scientists and policymakers has been focused on potato, tomato and peppers. Information on other regionally important Solanum crops is lacking. This paper reviews the taxonomy, crop management and the uses of African Solanum food crops as well as their nutritional values and glycoalkaloid contents.

Introduction Vegetable species are diverse in tropical Africa (1). Of the 275 species vegetable crops grown in Africa, 207 are indigenous to Africa, 45 were introduced long ago but became adapted, and 23 vegetables are recently introduced (1). African indigenous vegetables (AIVs) play important roles in nutrition, food © 2013 American Chemical Society In African Natural Plant Products Volume II: Discoveries and Challenges in Chemistry, Health, and Nutrition; Juliani, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


security, food diversity, rural development and sustainable land care (2). They may serve as income sources and can be marketed or traded locally, regionally and internationally (3). Many traditional vegetables are not only consumed as food but are used for their preventive and curative medicinal properties as well (4). Popular AIVs in urban markets and rural settings include okra, amaranth, spiderplant, jute mallow, celosia, jew’s mellow, roselle, cowpea leaf, African nightshade, and African eggplant (garden egg and gboma) (1, 4). These vegetables have fed Africans for centuries. Many of them are exceptionally nutritious (5) yet most have not received sufficient scientific attention despite their significance for food and nutritional security in Africa. African nightshade (Solanum spp) and African eggplant (S. aethiopicum, S. macropcarpon and S. anguivi) are important traditional leafy and fruit crops in many parts of East, West, Central and Southern Africa (3, 4, 6). African nightshade and African eggplant are members of genus Solanum which includes economically important crops such as potato (S. tuberosum), tomato (S. lycopersicum) and eggplant (S. melongena). Some species in the nightshade group such as S. nigrum are high in glycoalkaloids and are not suitable for human consumption; however, the “poisonous” label has been blanketedly applied to many edible nightshade species that are important foods in tropical Africa and Asia, thus reinforcing their neglect and underutilization (1, 7). African nightshades and African eggplants have much potential for expanded production and use in agriculture and human nutrition. In addition to their value in human nutrition (5), the leaves and fruits can be used as a natural dye (8, 9); their resistance to late blight disease could be exploited as a genetic source for varietal improvement of other solanaceous vegetables (10, 11).

Taxonomy The Solanaceae family comprises 90 genera and about 2300 species. The genus Solanum is the largest and most diverse, containing many essential food plants such as potato (S. tuberosum L.), eggplant (S. melongena L.) and naranjilla (S. quitoense Lam.); horticulturally useful plants such as winter cherry (S. pseudocapsicum L.) and jasmine nightshade (S. jasminoides Paxt.); medicinal plants such as bittersweet (S. dulcamara L.) and S. viarum Dun., both used as sources of corticosteroids. The main vegetable Solanum species cultivated in Africa include African eggplant (S. aethiopicum L, S. macrocarpon (Gboma) and S. anguivi) mainly cultivated for fruit and African nightshade (the Solanum section Solanum, with more than 30 species), mainly cultivated for their leaves. African eggplant is also known as garden eggs, mock tomato, and ngogweor nyanyachungu. The crop thrives in the warm, dry environments found in the savannah belt of West and East Africa. Optimal temperature ranges for the crop are 23–35 °C (day) and 18–25 °C (night) and it grow on a wide range of well-drained soils with an optimal soil pH range between 5.5 and 6.8.

138 In African Natural Plant Products Volume II: Discoveries and Challenges in Chemistry, Health, and Nutrition; Juliani, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Figure 1. Major types of African eggplants.

Solanum aethiopicum L

This phenotypically diverse species is grown for its fruit and leaves. Stems and leaves can be hairy or glabrous leaves, the flowers are bisexual and mostly self-pollinate; Fruits are produced singly or in groups from trusses or short cymes, depending on subspecies and varieties (1). The fruits vary in color (green, white, striped, multicolored), shape (round to long, smooth, grooved or ribbed) and size, (small to very big). Taste ranges from bitter to sweet, depending on saponin content, and fruit may be consumed cooked or raw. At full maturity, fruit turn red to orange due to their carotene content. The S. aethiopicum is divided into 4 groups (Figure 1):

The Aculeatum Group

Leaves and fruits of this group are inedible and members of this group are mostly used as ornamentals. They have hairy and prickly leaves and stems. The fruits are of various sizes, shapes and colors. This group is thought to have resulted from natural crosses between S. anguivi Lam and the Kumba group of S. aethiopicum (14). Members of this group are possible sources of genes for disease resistance breeding (15).

139 In African Natural Plant Products Volume II: Discoveries and Challenges in Chemistry, Health, and Nutrition; Juliani, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.

The Gilo Group


This group is very common in the humid tropics. It has hairy, inedible leaves, variable fruit shape (round, elongated, egg-shaped or spindly, ribbed or smooth), color (dark and light green, white or striped) and size (from a few to over 100 grams). This group is thought to have resulted from evolution of S. anguivi (14), and is sometimes (wrongly) referred to as S. gilo.

The Kumba Group This group is most commonly grown in the arid areas of tropical Africa. Also called red African eggplant, orange African eggplant, scarlet African eggplant, or mini pumpkin tree (12). The plants are glabrous and have large leaves and medium to big ribbed fruits (5-10 cm in diameter) that are eaten cooked or raw. The hairless leaves are sometimes consumed as green vegetables. The Kumba group probably evolved from the Gilo group (14).

The Shum Group This group is most generally grown for its glabrous leaves that are eaten as a green vegetable; its very small, slightly flattened, round or elongated fruits, though edible, are rarely consumed. This group probably evolved from the Kumba group through genome reduction (14).

Solanum anguivi Lam S. anguivi plants have small fruits which mainly differ in shape, color and taste although most are bitter. The pea-size bitter fruits are either collected from the wild or grown and consumed fresh or dried, mainly for medicinal purposes.

Solanum macrocarpon L S. macrocarpon can easily be distinguished from S. aethiopicum by its large hairless leaves and stems and its long calyx that persists after formation of its smooth green or yellow fruits. The leafy varieties are common throughout West and Central Africa, while the fruit types are mainly restricted to the humid coastal areas of West Africa. Some varieties have both edible leaves and fruits. This species is a popular traditional vegetable in West and Central Africa and also very common in Asia and tropical America. Spiny wild forms are found throughout the humid parts of tropical Africa, and occasionally collected as vegetables. It is sometimes called the Gboma eggplant, named after a village in Liberia. 140

In African Natural Plant Products Volume II: Discoveries and Challenges in Chemistry, Health, and Nutrition; Juliani, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Figure 2. Major types of African nightshades (S. scabrum, S. tarderemotum, S. florulentum, S. americanum and S. villosum). (Photos by Chris Ojiewo). 141 In African Natural Plant Products Volume II: Discoveries and Challenges in Chemistry, Health, and Nutrition; Juliani, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


African Nightshades

African nightshades are variously referred to as common nightshades, garden nightshades, nightshades, ‘S. nigrum and related species’, or ‘S. nigrum complex’, members of the genus Solanum L. subgenus Solanum L. section Solanum L. The group is comprised of approximately 30 species (16). Although they perform well in a wide range of climatic conditions, nightshades grow best at medium to high altitudes in moist conditions at optimal growth temperatures between 15°C and 35°C. They tolerate shade but grow best under full sunlight. African nightshades are generally intolerant of water deficit, and thrive in tropical rainy seasons and in areas with an annual precipitation of 500-1200 mm. They grow in various soil types, but are best adapted to sandy loams to friable clay soils with a pH range of 6.0-6.5, and high in nitrogen, phosphorus and organic matter (16). Some African nightshade species are widely consumed (Figure 2).

S. americanum These are diploid species with simple umbellate cymes with round sepals, reflexes away from mature berries. Its pedicels are erect, radiating from the peduncle. The fruits are shiny dark purple, small (≤12 mm) and globose (fruit height t= fruit width). The plants produce approximately 4500 berries per plant, but many berries drop off at maturity. It produces very small seeds with 1000 seeds weighing approx. 0.3-0.4 g (9).

S. florulentum This is rarely cultivated tetraploid species produces a bitter-tasting plant that that emits an unpleasant smell. It is used as a medicinal plant and is particularly recommended for malaria patients among the Kisii tribe in Kenya. The corolla of S. florulentum often have a purple strip along the mid-veins, usually being more conspicuous on the underside. The styles are clearly exerted beyond the anthers and often geniculate. The plants have triangular-ovate shaped sepals which adhere to mature berries. The plants produce 3000 - 4000 berries, dull dark purple, slightly flattened, and small (≤10 mm diameter) which drop off with pedicels still attached when ripe, making harvest difficult. The 1000 seed weight is 0.4 - 0.6 g.

S. grossidentatum Recent cytological data shows that S. grossidentatum is a tetraploid (9) although previously it had been erroneously identified as the morphologically similar diploid S. sarrachoides. S. grossidentatum has simple extended/lax cymes, 142 In African Natural Plant Products Volume II: Discoveries and Challenges in Chemistry, Health, and Nutrition; Juliani, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.

with fruits ranging in color from greenish-yellow to yellow, and globose in shape. Plants produce 3000 - 4000 berries per plant that drop with pedicels when ripe. The 1000 seed weight is 0.4-0.6 g. Plants develop a dense, glandular pubescence that give the leaves and stems of S. grossidentatum have a grayish color.


S. hirsutum It is probable that S. grossidentatum (collected in western Kenya by Mwai) (9), and S. hirsutum (collected in Uganda by Olet) (18) are conspecific. Morphological descriptions of the latter species by Olet (18) approximates very closely the morphological description of S. grossidentatum (9), the most notable difference being the extended cymes in the description of S. grossidentatum (9), and umbellate cymes in the description of S. hirsutum by Olet (18).

S. scabrum This hexaploid species produces simple umbellate cymes, light-purple and purple-colored corollas and brown to yellowish brown anthers. The sepals are round and reflexed away from mature berries. Except for a few green-stemmed accessions, seedlings have purple hypocotyls. Ripe berries are shiny black and larger (≥14 mm), slightly flattened (fruit height

African Nightshades and African Eggplants: Taxonomy, Crop

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