High-temperature fruit set (heat tolerance) is a critical trait of tomato (Lycopersicon esculentum Mill.) cultivars targeted for lowland wet season production in the tropics and subtropics. Heat-tolerant Asian Vegetable Research and Development Center (AVRDC) tomato line CL5915-93D4-1-0-3 (CL5915) is a valuable source of heat-tolerance genes for tomato genetic improvement. The gene action of heat tolerance in CL5915 was determined by evaluating the F1, F2, BCP1, and BCP2 of a cross between CL5915 and heat-sensitive line UC204A for fruit set traits in two wet-season trials at AVRDC. Parent-offspring regression of F2-derived F3 (F2:3) family means on the F2 plants from CL5915 × UC204A was used to estimate the heritability of F2 single plant selection for heat tolerance. Mean percentage of fruit set and fruit number per cluster of the F1 and BCP1 exceeded midparent values and were not significantly different from those of CL5915, indicating complete dominance for heat tolerance. Generation means analyses indicated that a model including simple additive and dominance effects adequately explained the inheritance of mean fruit number per cluster both years. For mean percentage of fruit set, a model including simple additive-dominance effects produced an adequately fitting model in the 1996 season but the best-fitting model included an epistatic component in the 1997 season. Heritabilities estimated for fruit set traits in 1996 and 1997, respectively, were: 0.31 and 0.21 for percentage of fruit set; 0.28 and 0.14 for mean fruit number per cluster; and 0.53 and 0.15 for flower number per cluster. The low heritabilities for percentage of fruit set and mean fruit number per cluster under high temperatures imply that single plant selection in the F2 for heat tolerance from crosses involving CL5915 is not effective and that selection should be based on replicated family testing in the F3 and later generations.
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