GENETICS OF HEAT TOLERANCE DURING REPRODUCTIVE DEVELOPMENT IN COMMON BEAN

Common bean (Phaseolus vulgaris L.) is grown in regions where high temperatures during reproductive development reduce yields. The purpose of this study was to identify sensitive growth stages during reproductive development and to investigate the genetics of heat tolerance during these stages. Exposure to high temperatures during two reproductive growth stages, namely flower bud formation and pod filling, resulted in particularly severe damage. This damage was evaluated by scoring two corresponding traits-flower bud abortion and reduced pod fill. For each trait, two tolerant and two susceptible large-seeded dry bean genotypes were chosen and crossed in all combinations, including reciprocals. 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 × environment interactions were present for both traits. Genetic studies included parents, F~ and F2 progenies, and control cultivars at one of the above locations. Both traits displayed continuous variation, indicating quantitative inheritance. Generation means analyses demonstrated that additive genetic effects were significant for both heat tolerance traits, but were more important for tolerance to bud abortion. Significant dominance ffects for tolerance to bud abortion were attributed to either linkage or pleiotropy of the single dominant gene conferring indeterminate growth habit. Both generation means and diallel analyses demonstrated the presence of cytoplasmic effects including interactions of cytoplasmic with nuclear genes. The significant additive effects observed indicate that gain from selection for improved heat tolerance should be possible for both traits. H Sa~R~SS, especially during reproductive development, causes severe yield reductions in common bean. This crop evolved under cool conditions in the highlands of Central and South America where temperatures during the growing season average 12 to 24°C (Debouck and Tohme, 1988). Despite this, common bean is grown commercially in climates that pose a marked temperature stress, such as the Central Valley of California where maximum daytime temperatures during the growing season reach 45 °C. Temperatures of this magnitude inhibit processes in common bean that affect overall productivity such as photosynthesis (Chaisompongpan et al., 1990) and nitrogen fixation (Hernandez et al., 1987) and can cause damage by disrupting plasma membrane integrity (Chen et al., 1982). More importantly, exposure to less extreme temperatures during critical reproductive stages can directly affect seed yield. Most reproductive stages in common bean are sensitive to high temperatures, including flower bud formation, flowering, pollen formation and function, fertilization, and pod and seed set (Konsens et al., 1991; Monterroso and Wien, 1990; Smith and Pryor, 1962; Weaver et al., 1985). Heat-induced abscission of flower buds before anthesis could be due to decreased carbohydrate levels G.C. Shonnard, Dep. Biology, Michigan Technological Univ., 1400 Townsend Dr., Houghton, M149931-1295; and P. Gepts, Dep. of Agronomy & Range Sci., Univ. of California, Davis, CA 95616-8515. Research funded in part by Hatch funds and Jastro-Shields awards to G.C. S. Received 8 Feb. 1993. *Corresponding author (plgepts@ucdavis.edu). Published in Crop Sci. 34:1168-1175 (1994). or translocation constraints (Konsens et al., 1991). However, heat damage occurring after anthesis, such as flower abscission and low pod and seed set, is likely to include lack of pollination or fertilization as an important factor (Bouwkamp and Summers, 1982; Konsens et al., 1991). The extreme sensitivity of common bean pollen to heat, as documented by stainability (Halterlein, 1980; Weaver et al., 1985) and germination tests (Dickson and Boettger, 1984; Weaver and Timm, 1988), could account for lack of fertilization under heat stress. It appears that pollen is more sensitive to high temperatures than are female reproductive structures (Dickson and Boettger, 1984; Monterroso and Wien, 1990). In particular, high night temperatures are very detrimental to pod and seed set in legumes, including common bean (Konsens et al., 1991), lima bean (Phaseolus lunatus L.; Fisher and Weaver, 1974), and cowpea [Vigna unguiculata (L.) Walp. ; Hall, 1990]. In both common bean (Konsens et al., 1991) and lima bean (Fisher and Weaver, 1974), high temperatures and not water stress were responsible for the damage, since increased irrigation or relative humidity did not prevent the low pod set. Despite the well documented sensitivity of common bean to high temperatures, literature on the genetics of heat tolerance during reproductive development is limited. Quantitative inheritance with large environmental effects was reported for heat tolerance at pod and seed set in snap bean (P. vulgaris) by Dickson and Petzoldt (1989). Bouwkamp and Summers (1982), ever, concluded that heat and drought tolerance at pod set was due to a single dominant gene in one snap bean accession and to two genes with epistatic action in another. Heat tolerance during reproductive development in cowpea has been more thoroughly characterized (Hall, 1990; Hall, 1992). Tolerance to inhibition of flower bud development under high temperatures and long day conditions was due to one recessive gene (Hall, 1992, data not presented). Tolerance during pollen formation, which affects pod set, was controlled by a single dominant gene, but was greatly influenced by environmental factors (Marfo and Hall, 1992). The goal of this study was to investigate the genetic control of heat tolerance in common bean at two reproductive growth stages particularly sensitive to high temperatures. Improved tolerance during reproductive development could lead to improved yield under heat stress conditions. MATERIALS AND METHODS