Improving dry beans (phaseolus vulgaris L.) of middle American gene pool for canning and nutritional quality traits, and drought tolerance in Zimbabwe.

Abstract

Among the different dry beans market classes, navy bean is a modern and specialised niche market-oriented product for domestic and foreign markets. Given the high navy bean import bill, increased prevalence of drought, and micronutrient malnutrition in Zimbabwe, the development of drought tolerant, high yielding and biofortified bean cultivars with superior canning quality offers a sustainable solution to the aforementioned challenges. Therefore, the objectives of this study were: (1) to identify farmers’ perceived production and marketing constraints, preferred traits and cultivars of navy bean, and strategies used to mitigate drought and heat stress in the south east lowveld region of Zimbabwe, (2) to evaluate the adaptability and stability of navy bean genotypes for grain yield and nutritional quality traits (Fe and Zn) across multiple locations in Zimbabwe, (3) to investigate the impact of drought stress on agronomic, shoot, physiological, canning and nutritional quality traits of navy beans, and identify drought tolerant genotypes with superior canning and nutritional quality, (4) to determine combining ability effects and mode of gene action of grain yield and yield-attributing traits in navy bean under drought stressed and non-stressed environments and select best combiners for effective breeding, (5) to quantify genome-wide marker-trait association of agronomic and physiological traits in dry beans under non-stressed (NS) and drought stressed (DS) conditions and to identify candidate markers for marker-assisted selection (MAS). A participatory rural appraisal (PRA) study conducted in four villages of the Lowveld region of Zimbabwe showed that the most important constraints to navy bean production were drought stress (79.5%), heat stress (56.5%), load shedding (50%), susceptibility to pod shattering (37.5%) and poor soil fertility (32.5%). Farmer-preferred traits included tolerance to drought and heat, early maturing varieties and disease resistance. Marketing constraints included nonpayment for produce in hard currency, lack of diversity in terms of off-takers, high inflation, low grain producer price, and delayed payment. Suggested mitigation strategies were mulching (18%), ridges (12%), reduced acreage (11%), and cultivating to retain more soil moisture (11%) for drought stress, while irrigating at night (32%), and adjusting planting dates (29%) were used to manage heat stress. A study on the evaluation for adaptability and stability of 84 navy bean genotypes for grain yield (GYD), seed Fe and seed Zn across four locations over two seasons in Zimbabwe was done using additive main effects and multiplicative interaction (AMMI), AMMI stability value (ASV) and yield stability index (YSI). This resulted in the identification of six genotypes (G14, G49, G37, ICA BUNSIxSXB405/3C-1C-1C-8, NAE70 and CZ108-53) with high GYD, good GYD stability and desirable seed Fe and Zn

concentrations above breeding targets of 90 and 40 ppm, respectively. The vertex genotypes ZABRA16575-26F22, ICA BUNSIxSXB405/4C-1C-1C-8 and NAE13 combined specific adaptation and high GYD with desirable micronutrient density. Furthermore, the impact of drought stress on agronomic and shoot traits, canning and nutritional quality of navy beans was conducted on 110 navy bean genotypes in 2019 and 2020 at Save Valley Experiment Station, Zimbabwe under drought stressed (DS) and non-stressed (NS) field conditions. Across environments, the genotype effect on agronomic, shoot, physiological, canning and nutritional quality traits was significant (p < 0.001; p < 0.05). Broad-sense heritability estimates were high for all canning quality traits and moderate to high for most agronomic and nutritional quality traits. Under DS conditions, the predicted genotype values (Ĝ) for seed micronutrient concentrations ranged from 72.3 (NAVY LINE-48) to 120 ppm (ZABRA16575-51F22) for Fe, 31.3 (NAVY LINE-46) to 60.8 (NAE70) for Zn, while washed drained weight (WDW) varied from 224.8 (Protea) to 310 (G24), and GYD ranged from 494 (SIRAJ) to 2619 kg/ha (ZABRA16573-78F22). Terminal drought stress reduced mean stomatal conductance (SC), leaf chlorophyll content (LCC), GYD, 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 (LT) increased by 1.4% and 34% 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, shape of seed and degree of splitting. The effect of DS was less severe on the degree of clumping and percent washed drained weight. ZABRA16575-86F22 had better mean ranks across four GYD based drought tolerance indices, canning and nutritional quality traits under DS compared to the standard checks. Combining ability analysis of 28 F2 progenies generated from an 8 x 8 half-diallel mating design and evaluated under DS and NS conditions resulted in significant general and specific combining ability (GCA; SCA) effects (p < 0.05) under both DS and NS for most traits. This indicated the importance of both additive and non-additive gene effects in the expression of the traits. Parents with best GCA for most of the studied traits were CZ113-13, G97, NAVY LINE-60, and G550 under NS, and ZABRA16575-73F22, G37, G97 and G550 under DS. ZABRA16575-73F22 and NAVY LINE-60 were tolerant to DS with high values for drought tolerance index (DTI) and geometric mean productivity (GMP) and low values for percentage grain yield reduction (%GYR) and drought susceptibility index (DSI). There were significant (p < 0.001; p < 0.05) positive correlations for number of pods per plant (NPPP) and 100-seed weight (SW; g) with GYD under both DS and NS.

Using an andean and middle-american diversity panel (AMDP) comprising of 185 dry beans genotypes, genome-wide marker-trait association analysis of agronomic and physiological traits in dry beans under DS and NS conditions was conducted. Genotyping was done using 24,450 Diversity Arrays Technology (DArT) markers, while the following traits: days to 50% flowering (DFW), plant height (PH), days to physiological maturity (DPM), grain yield (GYD), 100-seed weight (SW), leaf temperature (LT), leaf chlorophyll content (LCC) and stomatal conductance (SC) were recorded. Principal component and association analysis were conducted using filtered 9370 DArTseq markers. Population structure analysis revealed two sub-populations, which correspond to the andean and middle-american gene pools. Markers explained 0.08 – 0.10, 0.22 – 0.23, 0.29 – 0.32, 0.43 – 0.44, 0.65 – 0.66 and 0.69 – 0.70 of the total phenotypic variability (R2) for SC, LT, PH, GYD, SW and DFW, respectively under DS conditions. A total of 68 significant (p < 10-03) marker-trait associations (MTAs) and 22 putative candidate genes were identified under contrasting water regimes. Most of the identified genes had known biological functions related to regulating drought stress response, growth and development under drought stress. In conclusion, the identified farmer-preferred traits, marketing, and production constraints should be considered by the breeding programme in Zimbabwe during the development of improved cultivars. Stable genotypes identified to be drought tolerant and to possess desirable micronutrient density, superior canning and nutritional quality should be used as parents for crossing with other cultivars to improve micronutrient density, GYD and GYD stability and also recommended for deployment in their respective mega-environments. Good general and specific combiners with desirable values of drought tolerance indices and high significant positive effects under DS should be used further in breeding for drought stress tolerance. The identified markers should be validated for use in marker-assisted breeding for drought tolerance.