Drought and Heat Tolerance Mechanisms in Underutilised Legume Species: A Systematic Review
Department of Crop and Horticultural Sciences, Faculty of Agriculture, University of Ibadan, Nigeria
tb.akinrinola@gmail.com
Department of Crop and Horticultural Sciences, Faculty of Agriculture, University of Ibadan, Nigeria
Department of Soil Resources Management, Faculty of Agriculture, University of Ibadan, Nigeria
Department of Crop and Horticultural Sciences, Faculty of Agriculture, University of Ibadan, Nigeria
Department of Crop and Horticultural Sciences, Faculty of Agriculture, University of Ibadan, Nigeria
Abstract
Underutilised legume species such as bambara groundnut, horse gram, pigeonpea, and grass pea possess remarkable adaptations to drought and heat, two stresses projected to increasingly limit crop production in sub-Saharan Africa. This systematic review draws on experimental studies published between 2010 and early 2026, focusing on morphological, physiological, biochemical, and molecular mechanisms that confer tolerance. After screening 1,146 records, 28 peer-reviewed studies meeting strict eligibility criteria were included. Results indicated that deep rooting, paraheliotropic leaf movement, early flowering, osmotic adjustment via proline and glycine betaine, robust antioxidant systems, and upregulation of stress-responsive transcription factors like DREB are common adaptive strategies. Bambara groundnut and horse gram are identified as the most resilient under combined stress. The review recommends advancing promising genotypes into participatory breeding programmes in sub-Saharan Africa and prioritising multi-location field trials and integration of socioeconomic factors to enable successful adoption and climate-resilient food systems.
Keywords
Introduction
Legumes belonging to the Fabaceae family are among the most economically and nutritionally important plant groups. Beyond major staples, many underutilised legumes such as bambara groundnut, moth bean, horse gram, chickpea, lentil, hyacinth bean, and marama bean remain largely neglected despite possessing adaptive traits that allow them to withstand abiotic stresses like drought and heat. Climate change poses profound threats to agricultural productivity, with projections indicating rising global temperatures, erratic rainfall patterns, and prolonged drought episodes. In tropical and subtropical zones, these changes are already manifesting as reduced crop yields, with legumes being particularly vulnerable during reproductive stages like flowering and pod filling. The combined effects of drought and heat stresses are synergistic, often leading to oxidative damage, membrane instability, and yield losses of up to 50-70% in susceptible varieties. A systematic review of drought and heat tolerance mechanisms in underutilised legumes is timely to synthesise existing knowledge, identify adaptive strategies, and guide future research toward climate-resilient agriculture.
Methodology
This systematic review adhered to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 guidelines. Searches were conducted across Scopus, Web of Science, PubMed, Google Scholar, AGRICOLA, CAB Abstracts, and OpenGrey. The review focused on underutilised legume species including pigeonpea, grasspea, lupins, faba bean, bambara groundnut, and horse gram. Peer-reviewed studies published between January 2010 and February 2026 were included. Eligible studies investigated drought or heat tolerance mechanisms including omics approaches, physiological experiments, biochemical assays, and molecular research. After screening 1,146 records, 187 were assessed for full-text eligibility, and 28 peer-reviewed studies were ultimately included: 18 focused on drought tolerance, 7 on heat tolerance, and 3 on combined drought-heat stresses. An additional 12 grey-literature sources were incorporated narratively. Most included studies were greenhouse-based (62%), 28% were field trials, and 10% focused on molecular or omics approaches.
Tolerance Mechanisms
Morphological mechanisms include deep rooting in bambara groundnut that accesses subsoil moisture, reducing yield losses by 20-30% under water deficit. Reduced leaf area and increased root-to-shoot ratios facilitate water conservation, while narrow leaves and winged stems in horse gram minimise transpiration. Physiological mechanisms include stomatal regulation mediated by abscisic acid (ABA) in pigeonpea and bambara groundnut, preserving Relative Water Content above 70% under moderate drought. Biochemical mechanisms include accumulation of osmoprotectants like proline and glycine betaine, with proline levels rising 2-5 fold under drought in horse gram and lupin. Antioxidant enzymes including superoxide dismutase and catalase scavenge Reactive Oxygen Species, preventing oxidative damage. At the molecular level, gene expression studies revealed upregulation of DREB proteins, heat shock factors, and aquaporins. In bambara groundnut, transcriptomics showed differential gene sets for dehydration escape, while pigeonpea wild relatives exhibited QTLs for heat tolerance involving calcium signalling and ROS pathways.
Species-Specific Findings
Bambara groundnut demonstrated superior drought tolerance through hydrotropism and osmotic adjustment, with accessions maintaining high photosynthetic rates under stress and yield reductions of less than 20%. Pigeonpea heat tolerance involved HSP expression and DREB2A, with cultivars enduring temperatures up to 45°C. Horse gram showed extreme drought resistance via antioxidant mechanisms and osmotic regulation, with QTLs on linkage groups 2-5 controlling root traits. Grasspea demonstrated dual tolerance to drought and waterlogging via compartmentation of ions and phenolic accumulation, with Mediterranean ecotypes escaping stress through early flowering. White lupin showed genetic variation in drought tolerance, with GWAS identifying QTLs for yield under stress. Subgroup analysis revealed tropical species such as bambara and pigeonpea favoured avoidance via root modifications, while temperate-adapted species relied on escape through early maturity. Combined stresses amplified molecular responses, with 1.5-2 times higher gene upregulation than single stresses.
Implications for Crop Improvement
Synthesised mechanisms have significant implications for breeding climate-resilient legumes, particularly in tropical regions such as Nigeria. Marker-assisted selection targeting QTLs for root architecture and osmoprotectants can accelerate variety development. Introgression of DREB genes from pigeonpea wild relatives into major crops like chickpea can enhance heat tolerance. CRISPR-Cas9 editing provides precision for introducing HSPs or aquaporins to strengthen combined stress resistance. In sub-Saharan African farming systems, promotion of underutilised legumes aligns with agroecological principles, reduces reliance on inputs, and improves nutrition. Bambara groundnut's low water requirements suit Nigeria's northern drylands, diversifying diets with proteins and micronutrients. Policy integration through initiatives such as the African Orphan Crops Consortium can fund gene banks and farmer participatory breeding, ensuring landrace conservation during biodiversity loss.
Conclusions and Recommendations
Research shows that many lesser-known legumes have developed unique strategies to tolerate environmental stress, making them valuable for sustainable agriculture in the face of climate change. These plants use a combination of physical traits, efficient water use, and biochemical adaptations to survive drought and high temperatures. Applying this knowledge to breeding programmes in countries like Nigeria can help develop crop varieties better equipped for future climate uncertainties. Future research should integrate genomics, proteomics, and metabolomics to better understand gene-environment interactions in field conditions. Long-term trials in tropical locations including climate projections will help test mechanisms in practice. Comparing the genomes of less-studied and widely cultivated legumes may reveal useful traits to speed up hybrid crop development. Research into epigenetics and the microbiome, including how rhizobia adapt to stress, presents promising directions. Focusing on a wider range of legume species, rather than relying only on major commercial crops, offers a pathway to more resilient and diverse agricultural systems.