Combining Ability Estimates for Yield Some Traits in Tomato (Lycopersicon esculentum L.) by Line×Tester

12 single crosses (F1) achieved by combining four testers and three commercial lines of tomato were evaluated for their response to water deficit stress and the combining ability of yield and some yield components. The layout was split plots according to randomized complete block design with three replications and three irrigation levels (FC, 60%FC and 40%FC). Crossing among lines and testers, seedling production and field evaluation, were conducted in greenhouses and field of Ilam University during 2014-2016. The studied genotypes were L1: Bitstoik, L2: Kingstone, L3: Petoearly, as lines, and T1: LA1607, T2: LA2656, T3: LA2080 and T4: LA1579 as Testers. The studied traits consisted of total yield, potential yield, yield per plant, number of fruits per plant, plant height and days to 50% flowering. The analysis of variance indicated that significant differences existed among, genotypes, parents, parents vs. crosses, crosses, lines, testers and line × tester for all studied traits on both non- stress and water deficit stress, which showed a significant difference between general combining ability (GCA) of parents and specific combining ability (SCA) of hybrids. In general, the parental testers, LA2656 (T2), LA1607 (T1) and Line L1 (Bitstoik) were found to have the highest GCA for total yield and potential yield, and the number of fruits per plant in both conditions. The Petoearly×LA1579 in non- stress, Bitstoik× LA1607 and Kingstone×LA1579 in mild and severe-stresses had the highest SCA for total yield. Analysis of variance for combining ability manifested the predominance of dominance gene action for total yield, yield per plant and potential yield. The general combining ability (GCA) effects were generally found higher than SCA effects in terms of the agronomic traits. As a result, the low ratio of δ2A/ δ2D showed that non-additive effects controlled the studied traits.