Extreme temperatures can hinder development and production of tomato (Solanum lycopersicum L.) plants. Seedlings and pollen grains of F1 hybrids with their reciprocals, developed via a 7×7 diallel cross system, were evaluated under sub- and super-optimal temperatures to estimate combining ability, gene action, heterosis, heritability and correlation of thermotolerance in tomato. To assess plant survival under non-optimal temperatures, six-week-old hybrid seedlings were exposed to 0 and 40°C for 96 h. For screening plant fertility, pollen grains were subjected to 10 and 40°C for 4 h. Biometrical analyses indicated that in thermotolerance attributing traits, general combining ability (GCA) and specific combining ability (SCA) were highly significant, indicating the importance of additive and non-additive gene effects in controlling the studied characters. The reciprocal effect (REC) was significant only in seedling survival. The GCA variance was greater than SCA variance indicating predominance of additive gene action in inheritance of tolerance to heat and cold. Broad sense (H2b.s%) and narrow sense heritability (h2n.s%) percentages were high for seedling performance and moderate for pollen germination. There was a positive correlation between heat tolerance and yield, while chilling tolerance had a positive association with higher fruit weight and lower fruits number. The parental inbred line ‘CT6’ was the best combiner for seedling survival under cold and heat, while ‘Kingstone’ was the best genitor for pollen germination under low temperature and ‘Perimoga’ for high temperature. Selection following heterotic breeding could be an effective strategy for tomato improvement in terms of tolerance to unfavorable temperatures.

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