Drought stress impact on agronomic, shoot, physiological, canning and nutritional quality traits of navy beans (Phaseolus vulgaris L.) under field conditions in Zimbabwe

Climate change has increased the frequency of terminal drought stress substantially in navy bean (Phaseolus vulgaris L.) growing regions. This has consequently contributed to micronutrient malnutrition, in particular deficiency in zinc (Zn) and iron (Fe), which affects over 2 billion people in the world. The objectives of this study were to investigate the impact of drought stress on agronomic and shoot traits, canning and nutritional quality of navy beans, and to identify drought tolerant lines with good canning and nutritional quality. For this purpose, 110 lines were evaluated in 2019 and 2020 at Save Valley Experiment Station, Zimbabwe under drought stressed (DS) and well-watered (WW) field conditions, resulting in four environments. Combined analysis showed sig-nificant genotype and genotype x environment interaction (GEI) effects (p < 0.001; p < 0.05) for most of the evaluated traits. Under DS conditions, the predicted genotype values (G) for seed micronutrient concentrations ranged from 72.3 (NAVY LINE-48) to 120 ppm (ZABRA16575-51F22) for Fe; 31.3 (Navy 46) to 60.8 ppm (NAE70) for Zn, while washed drained weight (WDW) varied from 224.8 (Protea) to 310 g (G24), and seed yield (SYD) ranged from 494 (SIRAJ) to 2619 kg/ha (ZABRA16573-78F22). Terminal drought stress reduced mean stomatal conductance (SC), leaf chlorophyll content (LCC), SYD, number of mature pods per plant, number of seeds per pod, number of seeds per plant and 100-seed weight by 80%, 42%, 28%, 26%, 3%, 30% and 3%, respectively. Seed Fe concentration and leaf temperature increased by 1.4% and 34.2% in the DS environments, respectively, whereas seed Zn decreased by 0.9%. Terminal drought stress adversely impacted the canopy biomass, pod harvest index, hydration coefficient, WDW, uniformity, and degree of splitting. The effect of DS was less severe on the degree of clumping and percent washed drained weight. The line ZABRA16575-86F22 had better mean ranks across four SYD based drought tolerance indices, canning and nutritional quality traits under DS compared to the standard checks. This line is a potential candidate for release, genetic analysis and donor for tolerance to DS, superior canning and nutritional quality in navy bean breeding programs.