Common beans, biodiversity, and multiple stresses: challenges of drought resistance in tropical soils

Abstract

Common bean (Phaseolus vulgaris L.) is the grain legume with the highest volume of direct human consumption in the world, and is the most important legume throughout Eastern and Southern Africa, cultivated over an area of ~4 million ha. In Sub-Saharan Africa (SSA) drought is the most important production risk, potentially affecting as much as one-third of the production area. Both terminal and intermittent drought prevail in different production regions. The Pan-African Bean Research Alliance (PABRA), coordinated by the International Center for Tropical Agriculture (CIAT by its Spanish acronym), has participated in projects for both strategic and applied research to address drought limitations, with research sites in six SSA countries. Bean originated in the mid-altitude neo-tropics, and by its nature is not well adapted to warm, dry climates. Efforts at genetic improvement of drought resistance have a long history, exploiting variability among races of common bean, as well as through interspecific crosses. Useful traits are found both in roots and in shoots. Many authors have stressed the importance of harvest index and related parameters to sustain yield of common bean under drought stress, and our field studies substantiate this. Additionally, in tropical environments, soil-related constraints can seriously limit the potential expression of drought resistance, and it is especially important to address multiple stress factors to confront drought effectively in farmers’ fields. Poor soil fertility is widespread in the tropics and constrains root and shoot growth, thus limiting access to soil moisture. Phosphorus and nitrogen deficiencies are especially common, but are not the only limiting soil factors. Soil acidity and accompanying aluminium toxicity limit root development and inhibit access to moisture in lower soil strata. Soil physical structure can also limit root development in some soils, as can poor soil management that leads to compaction. We review efforts to address each of these constraints through genetic means in combination with drought resistance per se.

Root traits and their potential links to plant ideotypes to improve drought resistance in common bean

“…Thus, a vigorous and deeper root system, with rapid growth rate is useful but not enough to have resistance to drought in common bean. Our results indicate that for water spender type of genotypes, a strategic combination of root and shoot traits such as deep root system combined with the ability to remobilize photosynthates from vegetative structures to the pods and subsequently to grain production could contribute to superior performance under intermittent drought stress (Beebe et al 2014;Rao 2014). It also appears that for water saving genotypes, with a combination of development of fine root system with high water use efficiency mechanisms at leaf level will contribute to improved adaptation to prolonged or terminal drought stress (Polania et al 2016a).…”

“…Genotypes with superior grain yield under drought stress under field conditions and classified as water spenders (Polania et al 2016a) showed higher root vigor with deeper rooting ability under drought stress in the greenhouse. Deep roots may develop from the basal roots that change their root angle to turn downward, or from lateral roots that develop from a tap root, or both (Bonser et al 1996;Ho et al 2005;Basu et al 2007;Lynch 2011;Miguel et al 2013;Beebe et al 2014;Burridge et al 2016). High root vigor and deeper rooting ability in water spender type genotypes allows the plant to access greater amounts of available water, permitting the processes of gas exchange to continue, with the accumulation of water soluble carbohydrates in the stem and their subsequent remobilization to grain filling as was observed in some wheat genotypes (Lopes and Reynolds 2010).…”

“…Better remobilization of photosynthates to the grains is essential for the success of high yielding genotypes under drought stress (Polania et al 2016c;Rao et al 2017). Several studies using common bean have demonstrated the importance of better plant growth, accompanied by a superior photosynthate remobilization from plant structures to pod formation (pod partitioning index), and subsequently to grain filling and yield (pod harvest index) in improving adaptation to drought and also adaptation to low fertility soils Rao et al 2013;Assefa et al 2013;Yang et al 2013;Beebe et al 2014;Araújo et al 2015;Polania et al 2016c). Moreover it is possible to improve symbiotic nitrogen fixation (SNF) ability of common bean during drought stress, through the identification of genotypes that present greater ability to fix nitrogen under stress conditions (Devi et al 2013;Polania et al 2016b).…”

“…Keeping this in mind, some rapid and cost effective methodology has been used to carry out root phenotypic evaluations, such as small soil cylinders under greenhouse conditions, which allow to evaluate several root traits under different types of abiotic stress (Polania et al 2009;Butare et al 2011Butare et al ,2012. Phenotypic evaluations of root traits in common bean under drought stress have shown the importance of different rooting patterns, including deep rooting which allows access to water from deeper soil layers (Sponchiado et al 1989;White and Castillo 1992;Lynch and Ho 2005;Polania et al 2009Polania et al , 2012Beebe et al 2013Beebe et al , 2014Rao 2014;Burridge et al 2016). Different ideotypes of root system have been proposed for better crop adaptation to individual and combined abiotic stress conditions (Yang et al 2013;Rao et al 2016).…”

Improved production systems for common bean on Phaeozem soil in South-Central Uganda

“…Common bean (Phaseolus vulgaris L.) is the most important grain legume in Uganda (Beebe et al, 2014) and is produced primarily by smallholder farmers (Ugen et al, 2002). About two decades ago, per capita bean consumption in Uganda exceeded 50 kg year -1 in some regions (Wortmann et al, 1998a), but more recent countrywide consumption averages about 11 to 16 kg person -1 year -1 (Broughton et al, 2003).…”