November 8, 2021, by Lexi Earl

Surviving drought: The Root of the problem in Bambara groundnut

We must pay attention to those parts of plants we cannot see, to fully understand why some plants withstand environmental stress better than others, says Kumbirai Ivyne Mateva.

Imagine walking out into a crop field. What do you notice? Perhaps you see lush green vegetation swaying ever so majestically in the breeze (Figure 1). Maybe you see the fruit high up in the tree tops. Or the colourful flowers around about. But, would you ever consider the investment beneath your feet, what lurks in that hidden half of plants and why our lives depend on it?

Let me explain why!

Figure 1: Bambara groundnut grown in a Polyvinyl chloride (PVC) column phenotyping system at Crops for the Future – Field Research Centre (CFF-FRC), Malaysia.

Climate change is real, and of all the ways climate change inflicts harm, drought is most concerning to many people. Droughts have severe ramifications for people’s livelihoods, impacting everything from water supplies to agriculture. Given that agriculture provides a living for over 2.5 billion people, if severe droughts cause significant water shortages in agricultural areas, we are all in trouble. Are you worried? We all should be!

The world currently relies on only three plant species for most of global food production (rice, wheat and maize). These plants account for more than 60% of our caloric intake. When droughts strike, most of those crops suffer. But chances are Bambara groundnut (Vigna subterranea (L) Verdc) (Figure 2), a neglected, naturally drought tolerant grain legume just might.

Figure 2: Dodr a Bambara groundnut parental genotype sourced from Tanzania. 1) above ground shoots; 2) single trifoliate leaflet; 3) enlarged and untangled branching zone; 4) enlarged branching zone with two elongated peduncles; 5) below ground tap root system; 6-8) pods with cream, brown, and red seeds respectively.

Bambara groundnut thrives under strikingly contrasted environments relative to other grain legumes. Originating in West Africa, its distribution spans across aridity gradients from tropical dry climates in Senegal and Kenya, down to arid and semi-arid regions in southern Africa. The crop is considered a complete food in that it has a good and comparable balance of carbohydrates, protein and fats, and is able to do very well in marginal and hostile environments,  those areas with limited water where farmers have little to no chemical inputs.

However, there is a down side. We do not know the actual mechanisms deployed by the crop to deal with drought. Edgar Allan Poe, father of the modern detective story once said “sometimes the most elusive solutions are the ones that hide in plain sight,” (Poe, 1910). In our case, we need to study the plant’s hidden half – the roots (Figure 3) – which have been largely ignored.

Figure 3: Root system of a 15-day old S19-3, a parental Bambara genotype sourced from Namibia. Bambara groundnut forms only one primary/tap root during its development, which branches out through lateral roots. White scale bar = 5cm.

My research provides an in-depth examination of Bambara groundnut below-ground (root trait variation) and its role in drought tolerance. Accordingly, my research has shown that Bambara groundnut possesses differential deep root foraging and density patterns with two extremes, i.e., deep-cheap rooting in the genotypes mainly sourced from dry regions and a shallow-costly rooting system in genotypes suited to higher rainfall areas (Figure 4).

Figure 4: Images of the entire root system for Bambara groundnut genotypes and mean deep rooting (taproot length in the 0-110cm soil depth; TRL) and root distribution (root length density in the 0 to 30cm soil depth) at 35 days after emergence (DAE) grown in a soil-filled PVC column of 20x110cm (diameter and length). White scale bar = 15cm. (Source: Mateva et al. (2020)).

Farmers have inadvertently selected for these variations over time due to their effect on yield in both dry and wet conditions. Drought resistance breeding for Bambara groundnut will more likely accelerate as a consequence of a better understanding of the hidden half – root systems and foraging patterns.

The Future Food Beacon Bambara groundnut breeding research project (BamBREED) at the University of Nottingham has been at the forefront of increasing the crop’s productivity under drought stress conditions. Using the decoded genome of the Bambara groundnut, multidisciplinary techniques, and strategic collaboration with universities, institutes, and farmers in South Africa and Ghana, deep-rooting, high-yielding Bambara groundnut varieties with increased adaptability are now being developed for release to the general public. This will not only provide food diversity but will also enhance the nutritional value in people’s diets. So, the next time you’re outside admiring the beauty above-ground, don’t forget that there’s an equally interesting and important world that exists right beneath your feet!

Further reading:

Poe, Edgar Allen. “The Purloined Letter” in The Works of Edgar Allan Poe. New York: Harper and Bros. 1910. https://bit.ly/2ksYlY4

Publications:

Mateva, K.I.; Chai, H.H.; Mayes, S.; Massawe, F. (2020). Root Foraging Capacity in Bambara Groundnut (Vigna Subterranea (L.) Verdc.) Core Parental Lines Depends on the Root System Architecture during the Pre-Flowering Stage. Plants 9, 645. https://doi.org/10.3390/plants9050645

Gao, X.; Bamba, A.S.A.; Kundy, A.C.; Mateva, K.I.; Chai, H.H.; Ho, W.K.; Musa, M.; Mayes, S.; Massawe, F. (2020). Variation of Phenotypic Traits in Twelve Bambara Groundnut (Vigna subterranea (L.) Verdc.) Genotypes and Two F2 Bi-Parental Segregating Populations. Agronomy 10, 1451. https://doi.org/10.3390/agronomy10101451

Mayes, S., Ho, W.K., Chai, H.H. et al. (2019). Bambara groundnut: an exemplar underutilised legume for resilience under climate change. Planta 250, 803–820. https://doi.org/10.1007/s00425-019-03191-6

Kumbirai Ivyne Mateva is a final year PhD student in the School of Biosciences and the Future Food Beacon, based at the University of Nottingham Malaysia. Kumbirai’s current research focuses on improving productivity, quality and value of food crops, especially underutilized species. His PhD is titled: Root trait variation and its contribution to drought tolerance in Bambara ground nut (Vigna subterranean (L.) Verdc.). Kumbirai received a joint Crops for the Future, University of Nottingham Malaysia Doctoral Training Partnership scholarship in 2017. He is supervised by Prof Festo Massawe (UNMC), Dr Sean Mayes (UoN), and Dr Hui Hui Chai (UNMC).

Follow Kumbirai on Twitter: @KumbiraiMateva

Photographs by Kumbirai Mateva, unless otherwise stated.

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