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Taxonomy

Nine named species

Of the nine named species of Psophocarpus DC., it is only winged bean, P. tetragonolobus (L.) DC., that is found extensively outside of Africa. Detailed taxonomic studies suggest that the nearest African species to winged bean are the naturally occurring species P. scandens (Endl.) Verdc. and P. grandiflorus Wilczek. Though morphologically distinct from winged bean, both species share with it the chromosome number 2n=18 as well as the same preference for moist equatorial habitats.

P. scandens has a predominantly lowland distribution
(0 to 1000 m above sea level) and is widespread in central and east Africa, including Madagascar.

 

Psophocarpus scandens in Bujumbura, Burundi [120 k].

 

 

P. grandiflorus, on the other hand, is a rare species, with a range restricted to humid pockets in the mountains of central and north central Africa (Teza, Virunga, Ruwenzori, Elgon and Southern Ethiopia) at an altitude between 1600 and 2300 m above sea level.

P. grandiflorus, South of Gojeb River, Kefa, Southern Ethiopia
(altitude > 2000m a.s.l.) [120 k].

Attempts to hybridise species of Psophocarpus DC.

Given the similarities in chromosome number and type between winged bean and P. scandens, it is notable that all attempts to hybridise the two species have failed. Nor, up until now, has winged bean been hybridised with P. grandiflorus, because when the two species have been grown alongside one another, it has not been possible to synchronise flowering to the level required for cross-pollination.

In contrast to these attempts at interspecific crosses, intraspecific crosses between winged bean varieties from widely separated parts of Asia and Melanesia, are nearly always successful.

A hypothesised progenitor of winged bean

The hypothesis that P. grandiflorus is the wild progenitor of winged bean, was first proposed in 1980.

The pods and seeds of three species of Psophocarpus:
P. scandens, P. grandiflorus, and P. tetragonolobus (the winged bean), from left to right.

At first sight, P. grandiflorus resembles winged bean more closely than does P. scandens; it has a climbing rather than creeping habit and a more open inflorescence with larger flower parts, pods and seeds. However, while natural stands of P. grandiflorus are harvested for their edible leaves, pods and beans, nowhere in modern Africa, is P. grandiflorus recorded as being planted, let alone cultivated. On the other hand, P. scandens is planted and cultivated for its vegetable parts and also as a cover crop in some parts of its natural area of distribution.

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