Introduction: Corn is a C4 plant that is widely cultivated in the world and is the third most important cereal crop contributing to global food security. However, corn experiences frequent and severe heat stress. Corn grain yield is severely reduced when exposed to heat stress around flowering and early grain filling. The diallel analysis has been widely used in heritability research to investigate the heritability of any important trait in a set of genotypes. One of the diallel analysis methods is the Hayman graphical method, which provides useful information about the genetic parameters of quantitative traits and the genetic status of the parents under study.

Materials and Methods: In this experiment, six lines and their one-way hybrids (15 hybrids resulting from diallele crosses) were evaluated in two independent experiments, early sowing date (July 15, heat stress conditions) and recommended sowing date (August 15, non-stress conditions) in a randomized complete block design with three replications in the summer of 2019 at the Safiabad Agricultural Research and Education Center of Dezful, located in south western Iran. ASI (Anthesis-Silking Interval), Days to maturity, yield and yield components trait were measured. Analysis of variance of parental and F1 generation data was performed in a randomized complete block design. Graphical analysis was performed using the Hayman (1954) method when the effect of genotypes was significant and the effect of epistasis was insignificant. Gene function and genetic parameters of grain yield and related traits were estimated using the Hayman's method. All diallel analyses were performed with Dial98 software.

Results and Discussion: The results of genetic parameters showed that D, H1 and H2 indices and the average degree of dominance for corn grain yield, harvest index, number of days to maturity, number of grains per row and 1000-grain weight under normal conditions and the traits harvest index and 1000-grain weight under stress conditions had both additive and non-additive effects in controlling these traits. The parental distribution and the regression line of Wr on Vr in both normal and heat stress conditions for the grain yield trait, 1000-grain weight, intersected in the negative part of the Wr axis, and these traits are influenced by over dominant genes. The distribution of parents around the regression line for the grain yield trait showed that under normal conditions, the parents C3-95-3 and C3-95-9 and under heat stress conditions, C3-95-10 and C3-95-9 were the closest parents to the intersection of the line with the Wr axis, so the most dominant genes controlled the grain yield of the studied corn lines under these conditions.

Conclusion: From the results of this study and the findings of other researchers on the inheritance of grain yield and its components in corn genotypes, it can be seen that for the genetic improvement of these traits, selection should be postponed to advanced breeding generations and the potential of non-additive and over dominance effects and the use of heterosis in crosses should be used. However, few results are available for genetic evaluation of maize traits under heat stress. Therefore, further research in this context could provide important information for the development of heat-tolerant hybrids.