GENETICS OF HEAT TOLERANCE DURING REPRODUCTIVE DEVELOPMENT IN COMMON BEAN

Common bean (Phaseolus vulgaris L.) is grown in regions where high tem peratures during reproductive development reduce yields. The purpose o f this study was to identify sensitive growth stages during reproducti ve development and to investigate the genetics of heat tolerance durin g these stages. Exposure to high temperatures during two reproductive growth stages, namely flower bud formation and pod filling, resulted i n particularly severe damage. This damage was evaluated by scoring two corresponding traits-flower bud abortion and reduced pod fill. For ea ch trait, two tolerant and two susceptible large-seeded dry bean genot ypes were chosen and crossed in all combinations, including reciprocal s. The parents and control cultivars were evaluated under heat stress conditions at two locations in the Central Valley of California; this allowed us to determine that genotype x environment interactions were present for both traits. Genetic studies included parents, F-1 and F-2 progenies, and control cultivars at one of the above locations. Both traits displayed continuous variation, indicating quantitative inherit ance. Generation means analyses demonstrated that additive genetic eff ects were significant for both heat tolerance traits, but were more im portant for tolerance to bud abortion. Significant dominance effects f or tolerance to bud abortion were attributed to either linkage or plei otropy of the single dominant gene conferring indeterminate growth hab it. Both generation means and diallel analyses demonstrated the presen ce of cytoplasmic effects including interactions of cytoplasmic with n uclear genes. The significant additive effects observed indicate that gain from selection for improved heat tolerance should be possible for both traits.