Selection for drought and bruchid resistance of common bean populations

Common bean (Phaseolus vulgaris L.) is world’s most important grain legume for human consumption and the crop is grown annually on more than 14 million hectares. Drought stress limits common bean production worldwide. Understanding drought resistance mechanisms and identifying key plant traits may help to select the superior performers of crop under drought stress. Storage insect attacks on stored beans are also known to be substantial all over the world. Understanding the resistance mechanisms to bruchid weevils and identifying resistant genes can help to develop resistant varieties. Participatory variety selection also helps to select genotypes that possess farmers preferred plant and grain traits. The main objectives of the study were (i) to conduct phenotypic evaluation of a set of 81 genotypes along with two parents for drought resistance and identifying key plant traits related to superior performance under drought stress; (ii) to select the most promising genotypes that combine drought resistance with seed yield and market potential;( iii) to select bruchid-resistant advanced lines and apply marker-assisted selection useful for the identification of arcelin gene; (iv) to evaluate bean genotypes using participatory variety selection. In the first study, a total of 78 lines, two parents and one standard check (Awash melka) were evaluated under drought stress and irrigated (control) conditions at Melkassa research center (390 12'N and 80 24'E and 1550 meters above sea level) over two season ( 2008 and 2009) in Ethiopia. A 9x9 lattice experimental design with three replications (two rows of 3m long with 0.4m wide) was used. The seeds were planted at plant to plant distance of 10 cm. Data were taken on seed yield, seed number and pod number per plant, 100 seed weight, Shoot biomass, leaf area index (LAI) and pod harvest index (PHI). Data were analyzed using SAS 2002. Pearson correlation test and principal component analysis were used to determine the relation between and among measured variables. Significant (P< 0.05) genotypic differences were recorded in drought and irrigated conditions for grain yield, seeds per plant, pods per plant and 100 seed weight. The mean values of yield for the 81 lines ranged from 404 to 1580 kg/ha grown under moisture stress, while in the irrigated conditions, yield ranged from 1560 to 3985 kg/ha. Genotypes G80,G13, G19, G40, G87, G6, G28, G21,G24, G70, G22, G78, G60, G100 and G14 performed better under drought stress, and they also showed higher values for seeds per plant and pods per plant. Genotypes G78, G80, G6 and G19 were found to be responsive to irrigated conditions. Significant differences among genotypes for their LAI and PHI values were found under drought condition but a significance difference for canopy biomass was only found under irrigated conditions. Canopy biomass under drought conditions was higher with genotypes such as G80, G6, G87, G76 and G58 compared with the poor lines G16, G35 and G101. Genotype G103, G70, G2, G105, G74, G69, and G49 had significantly better LAI value than the standard check (Awash melka) and SxB 405 under drought conditions. There were also higher PHI recorded for G24, G78, G19, G14, G72, G60, G13, G100 and G87. Grain yield under drought conditions was positively correlated with seed number per m2, pod number per m2, 100 seed weight, canopy biomass and PHI. Genotypes such as G14, G21, G28, G60, G22, G24, G19, G78, G40 and G6 had positive association with grain yield, seed number, pod number, 100 seed weight and PHI. In the second study, a set of 40 advanced lines of RAZ (resistance against zabrotes) and susceptible commercial varieties were tested for bruchid resistance using four replicates of 30 seeds. Each replicate of advanced lines and commercial varieties at 10% seed moisture was infested with 6 pairs of newly emerged Mexican bean weevil (Zabrotes subfasciatus) from the stock rearing of CIAT Colombia. Two microsatellite markers analysis were used for the marker assisted selection scheme and protein analysis was done for presence or absence of arcelin. A field trial was also conducted in Ethiopia. Data were collected on number of eggs at 15 days, number of emerged adults, percentage emergence, adult dry weight and yield. RAZ 4, RAZ 101, RAZ 173, RAZ 44 and RAZ 174 showed consistently high resistance for all the parameters measured. The average yield of susceptible varieties (2.11 t/ha, SE = 0.05) was moderately higher than that of the resistant lines (1.8 t/ha, SE = 0.02). Arcelin protein analysis of 21 highly resistant advanced lines and 5 susceptible varieties together with the controls also showed a high level of accuracy. Resistance was associated entirely with the presence of the heavy 35KDa band representing Arcelin 1. The molecular markers BMy 11 and Pvatct 001 confirmed that they are more tightly associated with the arcelin gene and they produced bands that were 208 and 192 bp long for resistance lines. In the third study, a total of 16 farmers were invited in the 2008 season and 20 farmers in the 2009 season from Boffa and Siredodota areas to Melkassa research farm in Ethiopia to evaluate the 80 genotypes of common beans at podding and maturity growth stages. Seeds of selected genotypes were exposed to exporters and traders for quality assessment. A total of 25 genotypes were selected in 2008, both individually and in a group by farmers. Four genotypes were selected by exporters and traders. In 2009, a total of 12 genotypes from a total of 25 were selected by farmers from the two sites. Farmers from Boffa as well as from Siredodota conducted a last group selection of the genotypes under field conditions and ranked the top five genotypes (G60, G53, G40, G80 and G5) in terms of seed size, contrasting color and contrasting shape. The main selection criteria used by male farmers from both Boffa and Siredodota were grain yield, drought resistance, earliness, pod load, vegetative vigor, pod filling, marketability and color (brilliance). Female farmers also used their own selection criteria, grain yield, drought, earliness, pod load, color (brilliance) and suitability for stew. Exporters and traders evaluated and selected G40, G60, and G80. Exporters’ and traders’ selection criteria were seed size, color, shape, split seed, slightly stained (anthracnose) and moisture content of the seed. The study conducted over two years implied that there is a need to combine the classical breeding with participatory variety selection for effective and efficient selection of bean genotypes under drought conditions. Insect bioassay should also be supported by marker assisted selection for identification of better resistant genotypes to bruchids.

Il fagiolo (Phaseolus vulgaris L.) è una leguminosa tra le più coltivate al mondo per il consumo umano, su una superficie di più di 14 milioni di ettari, ma fortemente limitata dalla siccità. Un altro dei fattori limitanti per il fagiolo è rappresentato dagli insetti che attaccano i semi, appartenenti ai coleotteri bruchidi. La comprensione dei meccanismi di resistenza alla siccità e agli insetti è utile per la selezione di varietà superiori. Inoltre la partecipazione dei coltivatori al processo di selezione è importante al fine di identificare le caratteristiche migliori di piante e semi. Gli obiettivi di questo studio sono: (i) condurre una valutazione fenotipica di 81 genotipi in relazione alla resistenza alla siccità; (ii) selezionare i genotipi migliori per resistenza alla siccità, produzione e caratteristiche commerciali; (iii) selezionare linee resistenti ai bruchidi anche mediante marcatori genetici associati al gene dell’arcelina; (iv) valutare i genotipi mediante la partecipazione dei coltivatori in Etiopia. Nel primo studio sono state impiegate 78 linee, due parentali e un controllo (Awash melka) in condizioni di stress idrico e di irrigazione presso il centro etiope di Melkassa nel 2008 e nel 2009, secondo un disegno di blocchi randomizzati con tre repliche. Sono state analizzate le seguenti variabili: resa in seme, semi per baccello, peso di 100 semi, biomassa della pianta, LAI (leaf area index) e PHI (pod harvest index). I valori di resa in seme sono variati da 404 a 1580 kg/ha, con differenze significative tra i genotipi. I genotipi G80, G13, G19, G40, G87, G6, G28, G21,G24, G70, G22, G78, G60, G100 e G14 hanno dato risultati migliori in condizioni di stress idrico. I genotipi G78, G80, G6 e G19 e hanno dato buoni risultati anche con l’irrigazione, mostrando differenze significative anche per la biomassa della pianta. La resa in seme in condizioni di stress è correlata positivamente con il numero di semi per m2, il numero di baccelli per m2 , il peso di 100 semi, la biomassa e il PHI. Nel secondo studio sono state utilizzate in laboratorio 40 linee avanzate per la resistenza ai bruchidi, con 4 repliche di 30 semi ciascuna. Ogni gruppo di semi è stato infestato con 6 coppie del bruchide Zabrotes subfasciatus provenienti dall’allevamento presso il CIAT di Cali, Colombia. Due marcatori genetici microsatelliti sono stati utilizzati per la caratterizzazione delle linee in merito alla presenza del gene per l’arcelina, unitamente all’analisi della proteina stessa. Le stesse linee sono state infine utilizzate in una prova di campo condotta in Etiopia. I dati raccolti hanno riguardato variabili relative alla performance degli insetti e alla resa delle linee in campo. Le linee RAZ 4, RAZ 101, RAZ 173, RAZ 44 e RAZ 174 hanno mostrato una resistenza elevata per tutte le variabili considerate. Nel complesso la resa in campo è stata moderatamente più elevata per le linee suscettibili (2.11 t/ha, SE = 0.05) rispetto alle resistenti (1.8 t/ha, SE = 0.02). La resistenza è stata sempre associata alla presenza di una proteina da 35 kDa che rappresenta l’arcelina 1. I marcatori microsatellite BMy 11 e Pvatct 001 hanno confermato l’associazione con il gene per l’arcelina. Nel terzo studio sono state condotte indagini con coltivatori etiopi nel 2008 (16 coltivatori) e nel 2009 (20 coltivatori) provenienti dalle aree di Boffa e Siredodota. Sono state utilizzate le prove del primo studio presso il centro di Melkassa, valutando sia le piante sia il prodotto. I semi sono stati inoltre mostrati a esportatori e commercianti per la valutazione di qualità. Nel 2008 tale processo ha portato alla selezione di 25 genotipi superiori, di cui 4 apprezzati commercialmente. Nel 2009 è stato individuato un sottogruppo di 12 genotipi, all’interno dei quali è stato possibile elencare i 5 migliori (G60, G53, G40, G80 e G5) in relazione a misura, colore e forma del seme. I criteri di selezione sono variati tra coltivatori maschi e femmine. La valutazione commerciale, basata su caratteri in parte simili, ha portato alla selezione di tre linee (G40, G60, G80).