High-temperature responses of heat-tolerant tomato (Lycopersicon esculentum Mill. cvs. Saladette, PI 262934, BL6807, S6916, CIAS161, and VF36) were studied at 38/27°C day/night temperature. Flower production was reduced in all cultivars except BL6807 which partitioned a greater proportion of total assimilates to the flowers. Only ‘Saladette’ and VF36 showed a total lack of stigma exsertion, which in effect is functional male sterility. Pollen production was reduced in all cultivars, and there was a lack of pollen dehiscence. Several techniques were used to evaluate gamete viability. There was poor agreement among the 3 methods used to assess pollen viability. ‘Saladette’ suffered the least reduction for in vitro germination but had the greatest loss in seed set when high-temperature pollen was used. Seed set is probably the most reliable method to measure gamete viability. CIAS 161 and S6916 had the least reduction in pollen viability according to seed set criteria. Ovule viability is much more difficult to adequately evaluate. According to seed set criteria PI 262934 ovules suffered the least damage due to high temperatures. Whether the male or female gamete was affected more severely depended on genotype. Pollen viability was greatly reduced in PI 262934 but ovule viability was less severely affected. In BL6807 ovule viability was more severely reduced than pollen viability.
The genetics of high-temperature fruit set was studied with a complete diallel cross using 5 cultivars with excellent high temperature tolerance and a California cultivar lacking stigma exsertion. The cultivars differed genetically for number of flowers per cluster, percent fruit set, number of seeds per fruit and stigma exsertion. At normal and high temperatures recessive genes are associated with greater flower number and heritability for this character was high. Percent fruit set is under the control of a largely additive system with a moderate heritability at high temperature. Nonallelic gene interaction was involved in seed set and dominance components exceeded additive at both temperatures. Heritability for seed set was low at high temperature. Stigma exsertion at high temperature is controlled by partially dominant genes with a high diallel additive component and heritability. The results suggest that a scheme of selection for specific combining ability would be useful to combine the strengths of the high temperature tolerant lines with needed characters from a successful cultivar.