Cajanus scarabaeoides

''Cajanus scarabaeoides'' is a flowering plant in the genus ''Cajanus''. Of the 32 different species within the genus ''Cajanus'', only one, '' C. cajan'' (pigeonpea), is cultivated. ''Cajanus scarabaeoides'' is the closest wild relative to ''C. cajan'', and is one of the easiest wild species to cross with pigeonpea cultivars. ''C. scarabaeoides'' is found naturally in both temperate and tropical zones around the globe. This species has higher levels of drought tolerance, is found to have greater protein content, and has higher levels of resistance to insect pests compared to cultivated types. These genetic traits can be crossed with ''C. cajan'' to improve the crop's productivity. For subsistence farmers, this can reduce economic losses and drastically improve overall crop yield.

Description

''Cajanus scarabaeoides'' is a very close wild relative species of ''Cajanus cajan'' (common name, pigeonpea). It is a dicot angiosperm of the family Fabaceae. ''C. scarabaeoidis'' may be an annual or a perennial, making it a flexible crop for subsistence farmers.

The branches of ''C. scarabaeoides'' can be straight or winding and up to 135 cm in length. ''C. scarabaeoides'' has pinnate leaves, typically arranged in a trifoliate manner with flowers that are yellow with red veins. The pods of ''C. scarabaeoides'' are oblong in shape, typically 11–34 mm in length and 6–10 mm in width. The seedpods are densely covered in a combination of short and long hairs and are typically a dark purple colour, containing anywhere from 1-7 seeds. The seeds of ''C. scarabaeoides'' range from 2.4–4 mm long, 1.8–3 mm wide, and 1–2 mm thick and are either black in colour or speckled. Compared to the pigeonpea cultivars, ''C. scarabaeoides'' has a higher pod seed percentage, 74% compared to 20%, and has more multiseed pods, on average 6.04 seeds compared to 3.0 seeds

History and geography

''C. scarabaeoides'' is the most widely distributed wild species of ''C. cajan'' and is native to many countries in both temperate and tropical zones. It is native to Madagascar in Africa. In temperate Asia it is native to China, Japan and Taiwan. In tropical Asia it is native to Bangladesh, Bhutan, India, Nepal, Pakistan, Sri Lanka, Myanmar, Thailand, Vietnam, Indonesia, Malaysia, Papua New Guinea and the Philippines. In Oceania it is native to Australia and Fiji.

In Asia, ''C. scarabaeoides'' is the most commonly disbursed wild species of ''Cajanus'' and can be found in abundance Taiwan and in the Chinese provinces of Yunnan, Guizhou, Guangxim Guangdong, Hainan and Fuijan. In China there are several names for this species. In Mandarin Chinese it is called “Man Cao Chong Duo”. In Guangdong dialect it is called “Shui Kom Ts’o”. In Yunnan dialect it is called “Jia Yan Pi Guo”.

Growing conditions

''C. scarabaeoides'' occurs naturally in the wild, and can be found in open grassland and dry vegetation areas and in deciduous forests. It is often found along the ridges of cultivated fields, along roads or footpaths, or on hill slopes. It is typically found where there is a decent amount of sunlight, and populations tend to dwindle in dark bush areas or dense forests. This crop is known as a “creeper-climber” that supports itself on surrounding grass and small shrubs. In the Tiandong county in the Guangxi province of China, it can be found growing in wastelands at elevations of 180 m. In the Yi Oun Yang mountains, it can be found growing wildly in the dry hills and beside rivers. The vast areas in which this crop has the ability to grow provides advantages for farmers in both rural and peri-urban areas, as it can be supported by a variety of environments.

While pigeonpea is already a particularly good crop in terms of resistance to drought, ''C. scarabaeoides'' has even greater drought tolerance properties and is therefore capable of thriving with very little annual rainfall.

In many accessions studied of ''C. scarabaeoides'', many have been found to flower early compared to pigeonpea cultivars. On
ICRISAT
study reports flowering in some ''C. scarabaeoides'' accessions as early as 34 days compared to 60 days. Another ICRISAT study reports ''C. scarabaeoides'' accessions flowering within 70 days compared to 126 days. If ''C. scarabaeoides'' can be crossed with pigeonpea cultivars for this desirable trait, farmers can reduce harvest time and increase overall yield. With the world's changing climate, this trait is useful to improve the long-term sustainability of the pigeonpea crop

Additional uses

In China, ''C. scarabaeoides'' is sometimes used as fodder, and has shown to be effective in reducing diarrhea in cattle. In addition, the leaves of the plant species have been used to improve indigestion in traditional medicines as well as limit the excessive production of urine.

Major pests

Within the ''Cajanus'' species, the pod borer, ''Helicoverpa armigera'', is a major constraint that limits crop productivity. This insectivorous pest attacks the pods during the developmental stage, which reduces the total grain yield of the plant. This pest is very difficult to manage, largely due to its extensive host range and migratory capabilities. Additionally, ''H. armigera'' has become more resistant to certain insecticides in recent years, increasing the degree of difficulty to which management of this pest is possible.

Wild relatives of pigeonpea, specifically ''C. scarabaeoides'', have high levels of resistance to this destructive insect pest. The larval survival rate of ''H. armigera'' on ''C. scarabaeoides'' is only 21%, where it is 78% on pigeonpea. There is significant evidence showing that these species have different mechanisms than that of the cultivated types, all of which limit the ability for ''H. armigera'' to thrive on the plant. For scientists, breeders, and subsistence farmers, identifying these mechanisms can improve host plant resistance in cultivated types and reduce economic losses.

Structural mechanisms

Research has been done on the types of plant trichomes that different ''Cajanus'' species possess. Typically, there are 5 types of trichomes found, where types A, B and E are glandular and types C and D are non glandular. ''C. scarabaeoides'' was found to have a greater proportion of type C, short non-glandular, and type B, short glandular, trichomes. It lacks the type A, long glandular, trichomes that cultivated pigeonpea possess. The high density of short, nonglandular and glandular trichomes on ''C. scarabaeoides'' act as a barrier against the young larvae of ''H. armigera''. This barrier prevents larvae from feeding on the pods, causing mortality due to starvation before they are able to reach maturity. ''H. armigera'' lays 80% of its eggs on the pod surface of ''Cajanus'' species, so possessing type C and B trichomes is extremely beneficial in contributing to larval mortality.

Chemical mechanisms

Other research has been done on the chemical components extracted from the pod surface of ''C. scarabaeoides'' and compared with that of cultivated pigeonpea. The β-carophyllene and guaiene that is emitted from cultivated pigeonpea, which attracts ''H. armigera'', was found to be absent in ''C. scarabaeoides''. Acetone that was extracted from the pod surface of pigeonpea was found to stimulate larval feeding, where in ''C. scarabaeoides'' the extracts did not possess this characteristic. The water extracted from the pods of both pigeonpea and ''C. scarabaeoides'' showed greater antifeedant activity in that of ''C. scarabaeoides''. For scientists and breeders, being able to identify feeding stimulants associated with host plants and related insect pests allows for the selection of less susceptible genotypes during hybridization

Economic implications

''H. armigera'' is the most damaging insect pest to pigeonpea cultivars, causing annual yield losses of more than $300 million globally. A study in India in 1992-93 and 1997-98 showed an average yield loss from ''H. armigera'' as high as 90-100%. For what is typically a low value and easy to manage crop, identifying resistant cultivars and their genotypic traits can bring significant economic benefits for poor, subsistence farmers 0

Nutritional information

Similar to the cultivated types, ''C. scarabaeoides'' is rich in protein and essential amino acids. The seed protein content can range from 17.8-27%, typically being in the upper portion of the range, where cultivated types typically only have around 20% protein content. ''C. scarabaeoides'' leaves are also rich in protein, around 13%. This allows farmers to make greater use of the whole crop, as both the seeds and leaves can be eaten for their protein. Additionally, ''C. scarabaeoides'' is rich in the amino acids methionine and cysteine, around 3% of protein compared to only 2% in cultivated pigeonpea. These sulfur-based amino acids play an essential role in building the protein structures within this crop.

The sugar content in the pods of ''C. scarabaeoides'' was found to be much lower than that of cultivated species. Furthermore, the pods of ''C. scarabaeoides'' were also shown to have higher levels of condensed tannins. Studies suggest that these two mechanisms could be possible factors that limit the larval feeding and the growing ability of ''H.armigera'', respectively. These beneficial traits can be used by breeders to cross into cultivated types to improve insect pest resilience on pigeonpea.

Genetic stocks

ICRISAT
currently maintains 213 accessions of 19 ''Cajanus'' species that represent a total of 9 countries. ''C. scarabaeoides'' comprises one of the largest collections at the gene bank, with a total of 102 accessions. By selecting for specific genetic traits in the wild species and incorporating these genes into the cultivated relatives, improved characteristics such as pest resistance and drought tolerance can improve the overall productivity and production of the crop. Improving the diversity of traits in ''Cajanus'' species can be favourable to farmers who face challenges of drought and have crops susceptible to pests

References

{{Taxonbar, from=Q15237994

Phaseoleae
Edible legumes
Flora of Madagascar
Crops originating from Africa
Nitrogen-fixing crops
Tropical agriculture
Taxa named by Carl Linnaeus