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Assessment of genetic control, mode of inheritance, general and specific combining abilities and effect of drought stress on genetic parameters of harvest index and biological yield traits in bread wheat were achieved by using Diallel mating design. Parents (eight cultivars) along with F1 progenies (28 crosses) were sown in a randomized complete blocks design with three replications under stress condition in Karadj Agricultural Research Center. The data were analyzed according to methed of Hallauer and Miranda as well as fixed model of Griffing’s method II. Jinks-Hayman model was used to estimate broad and narrow-sense heritabilities and mean degree of dominance. There were significant differences between genotypes for mentioned traits in both environments. Studying mean of squares of general combining ability (GCA), specific combining ability (SCA), the ratio of GCA to SCA mean of squares and portion of additive and dominance variances showed importance of both additive and non additive gene effects for harvest index, but in biological yield heredity, additive effect was more important. Estimating broad-sense and narrow-sense heritabilities showed low efficiency of harvest index and high efficiency of biological yield for selection programs in stress environments.

In order to compare mode of inheritance, combining ability, heterosis and gene action in genetic control of traits flag leaf area, relative water content and grain filling rate of bread wheat under drought stress, a study was conducted on 8 cultivars using of Griffing?s method2 in fixed model. Mean square of general combining ability was significant also for all traits and mean square of specific combining ability was significant also for all traits except relative water content of leaf which show importance of both additive and dominant effects of genes in heredity of these traits under stress. GCA to SCA mean square ratio was significant for none of traits. Results of this study showed that non additive effects of genes were more important than additive effect for all traits. According to results we can understand that genetic improvement of mentioned traits will have low genetic efficiency by selection from the best crosses of early generations. Then it is better to delay selection until advanced generations and increase in heritability of these traits.

Mode of gene action, heritability and determination of the effective breeding strategy for improvement of physiological and traits specifically in drought stress conditions is very important. Therefore, this study was conducted by using two drought susceptible and tolerant wheat cultivars. Cultivars Sakha8 (tolerant) and Pishtaz (susceptible) as parents along with F1, F2, BC1 and BC2 generations were sown in a randomized complete block design with three replications in drought stress conditions. Results of analysis of variance indicated significant difference between generations as well as degree of dominance revealed over-dominance for the both traits. Fitting simple additive-dominance model designated that this model was not able to account for changes of traits relative water content and mean of grain filling rate. It was revealed that m-d-h-i-j model for relative water content and m-d-h-i model for mean of grain filling rate are the best models. Estimation of heritability and mode of gene action indicated that selection for improvement of traits studied in stress condition and specifically in early generations have medium genetic gain. In conclusion, grain filling rate is better than relative water content as indirect selection criteria to improve plant grain yield in drought stress condition.