combinability

Wheat performance is always affected by climate change and environmental stresses such as drought stress. Development of high-yielding cultivars requires genetic diversity. Awareness about heritability of traits can be helpful in determining the appropriate breeding strategy for any environmental conditions. By progeny trial after a diallel crossing, the function of genes and inheritance of quantitative traits can be estimated. This increases the ability to select parental lines, to participate in crosses, and determines the management of generations in the segregating populations.

In order to understand the genetic structure of grain yield and some morphological traits of bread wheat under normal and drought stress conditions, eight bread wheat cultivars including Star; Tirgan; Ehsan; Shirodi; Bam; Tajan, Yang and Akbari were planted in the crossing block and direct crosses were performed among the eight cultivars to produce F1 generation. This research carries out in Gorgan agricultural research station in 2020-2021 cropping season, under normal and drought stress conditions. The progenies of the crosses with their parents were evaluated based on a randomized complete block design (RCBD) with three replications. Imposing stress by terminating irrigation was initiated at the Zadoks 60 (full heading emergence) stage.

Combined analysis of variance under normal and drought stress conditions showed that the simple effects of genotype and environment were statistically significant in all traits. Also, the interaction effects of genotype×environment was statistically significant for grain yield, biological yield and 100-kernel weight. In fact, for these traits the response of genotypes was different under normal and stress conditions. Analysis of variance of Griffing's diallel method showed a statistically significant difference between genotypes in all studied traits. Therefore, among the progenies of the crosses it is possible to select superior genotypes. In biological yield and grain yield under both normal and drought stress conditions, Tirgan cultivar had the highest general combining ability, while Star cultivar showed the lowest general combining ability. Tirgan cultivar can be used to increase grain yield and biological yield under both conditions. In plant height, peduncle length and harvest index, Tajan×Yang, Ehsan×Shirodi and Bam×Yang crosses had the maximum specific combining ability, respectively. In other traits, in each condition, an individual cross had the highest specific combining ability. Under normal conditions, Tajan×Akbari cross had the highest specific combining ability in grain yield. Under drought stress conditions, Bam×Akbari cross had the highest specific combining ability in biological yield and grain yield, it simultaneously had a high specific combinability in 100-grain weight, harvest index and number of grains per spike. Therefore, Bam×Akbari cross was determined as the best cross in genetic improvement of desirable agronomic traits with high specific combining ability in yield and yield components, under drought stress conditions. The ratio of additive variance from genetic variance for plant height and peduncle length was greater than dominance variance. Subsequently, the highest narrow-sense heritability in both normal and drought stress conditions was related to plant height and peduncle length. In both normal and drought stress conditions, the highest degree of dominance was related to harvest index traits and number of grains per spike, respectively. Therefore, in these traits the lowest narrow-sense heritability, the lowest Baker's genetic ratio and the highest ratio of dominance variance from genetic variance were observed in both normal and drought stress conditions. In grain yield, a high ratio of dominance variance from genetic variance was assigned under normal (84%) and drought stress (88%) conditions, hence low narrow-sense heritability was observed under normal (8%) and drought stress (5%) conditions. In general, in grain yield, biological yield and 100-kernel weight, narrow-sense heritability was lower under drought stress conditions, compared to normal conditions, which indicates a greater ratio of non-additive effects under drought stress conditions.

The results of this study showed that under both conditions, Tirgan cultivar had the highest general combining ability in grain yield and biological yield. Tajan×Akbari and Bam×Akbari crosses were determined as the best cross in genetic improvement of desirable agronomic traits with high specific combining ability in yield and yield components, under normal and drought stress conditions, respectively. In grain yield, biological yield and 100-kernel weight, narrow-sense heritability was lower under drought stress conditions, compared to normal conditions. Based our results under both conditions in the progenies of the crosses, genetic improvement can be done in plant height and peduncle length in the initial generations, but for other traits including grain yield and its components, selection should be postponed to more advanced generations.

Wheat is one of the most important crops in the world that plays a key role in ensuring food security. In wheat breeding programs, selection of genotypes with desirable agronomic traits and high yield potential have always been the goal of plant breeders. Increasing the selection efficiency for grain yield depends on genetic knowledge from morphological traits and the effect of these traits on grain yield.

In order to estimate genetic parameters and general and specific combining abilities, six bread wheat cultivars including Falat, Tajan, Bolani, Ehsan, Morvarid and Catbird were planted in a crossing block and direct and reciprocal crosses were performed among the six cultivars to produce F1 generation. The progenies of the crosses with their parents were evaluated based on randomized complete block design (RCBD) with three replications in Gorgan agricultural research station in 2020-2021 cropping season.

Analysis of variance of Griffing's diallel method showed a statistically significant difference between genotypes in all studied traits. Mean squares of general and specific combining abilities were statistically significant in most traits. Reciprocal, maternal and non-maternal effects were significant for the number of days to heading, grain yield, and biological yield. In grain yield and biological yield, Ehsan cultivar had the highest general combining ability and was identified as a suitable parent for the genetic improvement of these traits. The Catbird×Bolani hybrid had the highest specific combining ability in plant height, peduncle length, biological yield, grain yield and harvest index, it was determined as the best hybrid in genetic improvement of desirable agronomic traits with high specific combining ability in yield and yield components. The ratio of additive variance from genetic variance for plant height, number of days to heading and peduncle length was greater than dominance variance. The highest degree of dominance was related to the number of spikelets per spike, 100-seed weight, grain yield and biological yield, respectively.

The results of this study showed that in the progenies of the crosses, genetic improvement can be done in plant height, number of days to heading and peduncle length in the initial generations, but for other traits including grain yield and its components, selection should be postponed to more advanced generations.