Selection of carioca bean isolines for slow grain darkening, obtained by backcrossing

Common bean (Phaseolus vulgaris L.) is cultivated in various regions of the world, with the carioca commercial group being the most consumed and commercialized in Brazil. One of the bottlenecks in the commercialization of carioca beans is the regular darkening of the seed coat after harvest, which is often associated with old grain and prolonged cooking time, leading to rejection by the consumer market. For this reason, plant breeding programs have sought to develop cultivars with slow seed coat darkening through the introduction of the sd gene, promoting longer storage time and greater commercial value. Thus, the present study aimed to phenotypically evaluate promising isolines for slow grain darkening, obtained from an F₂ population of the fifth backcross (BC₅F₂). Three segregating populations, derived from the cross between the accession ‘Carioca Claro’ and the cultivars ‘BRS Pérola’, ‘IPR Juriti’ e ‘Flor Diniz UEM’ were evaluated. To obtain BC₅F₂ seeds, backcrosses were performed between isolines (BC₄F₂) and their respective recurrent parents. Seeds from the BC₅ generation were sown to obtain plants by self-pollination (BC₅F₂). The isolines from this generation were evaluated for grain color (light-colored grains), grain size (100-grain weight - M100g), and absence of yellow hilum. Based on these criteria, isolines from populations ‘Carioca Claro’ × ‘IPR Juriti’ (28, 30, 58 and 96), ‘Carioca Claro’ × ‘BRS Pérola’ (40, 72, 86 and 95) and ‘Carioca Claro’ × ‘Flor Diniz UEM’ (6) were selected. Samples of 10 seeds from the selected isolines were exposed to UV light for 72 hours, followed by visual evaluation using a rating scale from 1 to 5 (1- very light background color of the grain seed coat; 2- moderately light; 3- light; 4- moderately dark; and 5- very dark). Selections for slow darkening were carried out together with the grains of the current parents and donor parent. Considering an average score value of ≤ 2,5 as indicative of slow-darkening genotypes, isoline 96 (‘Carioca Claro’ × ‘IPR Juriti’) stood out by presenting a score of 1, followed by isolines 86 and 95 (‘Carioca Claro’ × ‘BRS Pérola’) with a score of 2. The remaining evaluated isolines presented scores between 3 and 4. However, further molecular evaluations are necessary to confirm the presence of the sd gene. Thus, it is concluded that BC₅F₂ was effective in obtaining isolines with the slow-darkening trait, associated with grain size within market standards.