Induction of seed coat darkening in common beans (Phaseolus vulgaris L.) and the association with cooking time after storage

Carioca is the most important edible dry bean (Phaseolus vulgaris L.) grown in Brazil. It represents the largest dry bean market class in the world. The seed coat of carioca beans will darken under adverse harvest conditions and with the increasing of storage time. In general, darkened seeds are associated with older seeds that suffer from prolonged cooking times. A relatively new ‘slow darkening’ trait is available in carioca that delays seed coat darkening under storage. However, its effect on cooking time is unknown. The objective of this work was to evaluate two induction methods of seed coat darkening and to examine the effect of slow darkening trait on cooking time after storage. Lines derived from four segregating populations resulting from crossings between cultivar BRSMG Madrepérola with slow seed coat darkening, and the parents BRS Estilo, BRS Cometa, BRS Notável and BRS Sublime with normal darkening were evaluated. An experiment inlcuding 220 lines, 55 per population and the five parents, in a 15x15 triple lattice was conducted in the winter growing season in Brasilia. Seed coat darkening and cooking time traits were evaluated. Two methods for inducing seed coat darkening were compared: an accelerated aging test using UV light for 72 hours, and an extended storage for 90 days under ambient conditions. The correlation between the induction methods ranged from 0.77 to 0.85 for the different populations indicating either method could be used to discriminate lines with normal versus slow darkening trait. The percentage of light-colored grain lines was identified by both induction methods of seed coat darkening ranged from 75 to 85.7% in the populations. The genetic correlation between seed coat darkening and cooking time varied from -0.06 to -0.48, indicating that in some populations there is no significant genetic correlation between seed coat darkening and cooking time. Thus, light colored grains are not indicative of low cooking time, when they are considered genotypes with genetic variation for the seed coat darkening.