Seedcoat darkening in pinto bean (Phaseolus vulgaris L.)
Post-harvest seedcoat darkening is a major problem in many pulses, including common bean (Phaseolus vulgaris L.). In some bean market classes, such as pinto, beans that have a darkened seedcoat are discounted in the market place as it is assumed that the beans are old and will be hard-to-cook (HTC). Pinto genotypes that darken more slowly than conventional pinto beans would be more desirable and have been identified in the bean breeding program at the University of Saskatchewan. To study the slow-darkening trait, a quick, reliable, and inexpensive screening method that would not affect seed germination would be beneficial. Three potential protocols to accelerate seedcoat darkening were examined. The greenhouse protocol was conducted in the greenhouse by placing the bean seeds in polybags with a 1 cm2 piece of moistened felt. For the UV light protocol, bean seeds were placed 10 cm below an UV lamp which had a wavelength of 254 nm. For the cabinet protocol, bean seeds were placed in a cabinet set at 30¢ªC, 80% relative humidity, and full fluorescent lights. Color measurements were taken routinely using a Hunter Lab colorimeter. All three methods were successful in distinguishing darkening beans from slow-darkening beans although the UV light protocol was considered to be superior to the greenhouse and cabinet protocol as the UV light protocol was quick, consistent over years, and the most economical. Unlike the greenhouse and the cabinet protocols, the UV light protocol did not affect seed germination following accelerated darkening. The stability of the slow-darkening trait was further investigated in genotype by environment (g x e) studies across different indoor and outdoor environments. In the g x e study across different field environments, it was found that prior to accelerated seedcoat darkening the g x e interaction was significant. Following accelerated seedcoat darkening, environment and genotype were both significant and g x e was not. The slow-darkening genotypes had lighter seedcoats than the darkening genotypes and those field sites that had more favorable weather had lighter seedcoats. For the g x e study across indoor and outdoor environments, when the genotypes were split into either slow-darkening or darkening, the g x e interaction was not significant and the slow-darkening genotypes had lighter seedcoats. Genetic control of the slow darkening trait was determined. For crosses between slow-darkening genotypes and CDC Pintium, the F2 populations segregated 3 darkening : 1 slow-darkening with distinct bimodal distribution. This indicated that seedcoat darkening was controlled by a single gene and darkening was dominant over slow-darkening. For both slow-darkening by slow-darkening crosses, the F2 populations¡¯ L* values were unimodal, normal distributions, indicating there may be modifying genes for the slow-darkening trait.
genetics, protocol, genotype by environment, plant science
Master of Science (M.Sc.)