Wednesday, 28 February 2018

Psophocarpus lancifolius - Mysterious Misobwa

One of the joys of doing a botany degree (back when it was still possible in the UK, happy days) was the opportunity for a little bit of extracurricular reading down between the shelf stacks. First port of call was always the purple-bound volumes of  Economic Botany. Many an hour of sweet happiness was spent engaged in that noblest of activities.

On one such occasion, I picked a random volume, opened it at a random page and was soon captivated; the enigmatic tale of an undervalued Afromontane legume unfolded, a crop wild relative of the winged bean (Psophocarpus tetragonolobus) known, at least sometimes, as misobwa (P. lancifolius). Oh - and it had edible roots too.

The Afromontane zones, with their high mountains, dense jungles, miombo woodlands, swamps and dambo grasslands have always appealed to my inner armchair explorer. Likewise, I've always found crop wild relatives intriguing, both in their own right and as the source of disease and drought resistance, among other useful qualities. Where would wheat, tomatoes and innumerable other crops be without them?  So, with misobwa's double whammy hitting me in all the right places, I knew it was time to roll with the blows, stiffen my sinews and start searching; this plant would surely be a worthy addition to my bucket list, filed under the category 'roots to know and grow'.

At this juncture, I really should acknowledge my sources; I complain enough about others who fail to do so. Most of what I know about misobwa comes from the following paper:

Uses, nutritional composition and ecogeography of four species of Psophocarpus (fabaceae, phaseoleae) in Zaire

By Daniel Harder, Oneyembe Pend Mbutu Lolema and Musasa Tshisand

According to Harder et al, misobwa grows at altitudes up to around 2500 metres above sea level in the Democratic Republic of the Congo; this is a bit higher than its close relative, the winged bean, which usually runs out of steam at around 2000 metres, although it sometimes struggles a  little higher in Papua New Guinea before chilling and frost finish it off.  I generally take an elevation of around 3000 metres in the tropics as my sweet spot for a sporting chance that a plant might make it here outdoors in Cornwall during the summer. Being a soft-hearted soul, I'm always prepared to exercise a little latitude in the application of my altitude rule, however - if a plant has some other merits. Misobwa - in my opinion at least - does.

What merits, you ask? Well, by all accounts its tubers are an outstanding food, containing around 10% protein and 5% lipid. But don't take my word for it. To quote that paper yet again:

The nutritious tuber is comparable to the seeds of soybean and peanut for protein and to soybean, peanut, and winged bean for lipids; it has a medium to low complement of carbohydrate (Smartt 1976). The tubers of P. lancifolius constitute an excellent food source and, as such, are promising in tropical areas experiencing seasonal rains.


You would hardly expect me to refrain from a madcap misobwa chase after that kind of write-up. Resistance is futile in the face of the Borg of tuberous beans. My peanut growing attempts have been pitiful and my soya beans' performance always so-so. Here was a root from the family Fabaceae that might just help me to regain my self-respect, I thought. 

Not only does the misobwa plant form carrot-shaped, edible roots with the properties described above, but its long, rambling stems will also root at the nodes where they touch the ground and form secondary tubers. Under cultivation, might it be possible to persuade the plants to produce more tubers by pegging the vines down?

But aren't we missing something here?  It's all very well to concentrate on a plant's apparent virtues as a crop,  but what does it actually taste like? Daniel, for all his evident enthusiasm, doesn't mention this. Luckily I have some idea, thanks to this book, which I discovered online:

Pygmées d'Afrique Centrale, by Stefan Seitz

It's an ethnobotanical treatise on, well, I think you can guess if your fluency in French is equal to or greater than mine. 

Anyway, it seems that the Bashi and Barwha peoples eat misobwa roots, which are said to resemble a potato in taste and are relished after cutting up and grilling. They gather the roots from open country rather than the rainforest proper. Although sketchy, this at least indicates that misobwa is not only edible, but palatable too. Again, this is an endearing tendency in a world populated by foul flavoured impostors like mashua (Tropaeolum tuberosum). Make that a triple whammy. 

Until 2016 this was pretty much the sum total of my knowledge about misobwa; then, after years - decades - of vainly searching, letter writing and emailing, I finally got lucky. My seeds came not from the DCR, but by a rather unexpectedly circuitous route: from Zimbabwe, via Australia. It seems that they were collected in the 1980s near Harare, by the banks of the Umwindsi River which wends its way through the area around Gaydon Road. And, perhaps disappointingly, they were collected at an altitude of only 1480 metres, although this is about 15 degrees further from the equator than the Congo sites, so average temperatures ought to be commensurately lower.

Eventually it's time to move on from all the the book learning and received wisdom and actually get up close and personal with a plant in real time.  How exactly did I get on with my first misobwa crop?

I received the aforementioned seed rather late in the year, towards the end of May. Germination was good, but that year I was caught short on the greenhouse front, so I grew them on the very
windowsill, where, a few years earlier, I had grown my asbin.
Misobwa seeds, left; winged bean/asbin right
According to Daniel Harder, the whole misobwa plant is frequently covered in dense yellow hairs. I was rather disappointed that my plants seemed completely hairless. In fact, they looked very similar to talet (Amphicarpaea bracteata), with the same twining habit of growth and trifoliate leaves. I needed some reassurance that what I had was the real McCoy and not some cheap impersonator. It's been claimed by some that I have 'trust issues'; I can't comment on that, but I do like to know the true identity of the plants I'm growing. One distinctive characteristic of Psophocarpus lancifolius seeds is that they are described as having "a hilum which transversely bisects the pale, broadly elliptical aril". Taking my faithful hand lens, I looked at the remaining seeds and could see that this was true: my plants were indeed misobwas - clean-shaven Zimbabwean versions, maybe, but misobwas just the same, no mistaking it.

Misobwa seedlings have a typical trifoliate leaves and a slightly kinked, zig -zag habit of growth which you may be able to make out in this picture:


Misobwa seedlings with zig-zag growth habit
They grew away quite well, creating a sinuous tangle of leafy, twining stems. With a little leap of imagination I could see myself squelching through a wet dambo grassland, with the lofty peaks of the Virunga National Park looming in the background. Stooping to examine a ripe misobwa pod, I could hear a silverback gorilla drumming on his chest somewhere in the undergrowth.  Like I said, I'm an experienced armchair traveller.
My very own entangled misobwa thicket
Back to the real world. I waited, patiently, for flowers to appear, but sadly, they never did. Towards the end of January 2017, the spell was broken and my beloved plants were starting to look a bit sorry for themselves, with withered leaves coated by sticky, aphidly secretions which really marred their appearance. I made the executive decision to tip them out of their pots and see what, if anything they'd made underground.

Here's the result:
Misobwa roots. Small and fleshy. 
Thickened roots were indeed present. These were, as predicted, carrot shaped, but more the size and colour of a stunted wild carrot's tap root than a chunky Autumn King. I saw no signs of nodulation, so I'm guessing that the correct cowpea miscellany rhizobia were absent. Neither a triumph nor a tragedy, these roots looked like they might be tasty roasted, but I rather doubted it would be worth the effort. Alas and alack - my attempts to overwinter them were a failure and I now wish I'd followed my baser instincts and eaten them straight away. 

As is so often the case, my initial misobwa cultivation attempt raised more questions than it answered. Here are a few that spring to mind: what controls storage root  development and size? How can I persuade plants to flower? And does anyone know anyone who can collect me some seeds from the highest parts of misobwa's range in Congo, Rwanda or Burundi? Maybe together we can explore the mysteries of this enigmatic root, a gift from the lovely, troubled highlands of equatorial Africa.  






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