Abstract
 
Background and Objectives
As the world's population grows, increasing demand for edible oils is inevitable. In addition to plant genetics, environmental variability plays a key role in plant yield. Drought is one of the most important environmental stresses that reduce the productivity of sunflower and other crops. Understanding the nature of phenotypic traits that improve performance under stress conditions and identifying the complex physiological and genetically mechanisms is fundamental to plant stress. Improving drought tolerance and increasing water productivity in plants is one of the most important goals of the breeders. Gaining knowledge of the amount of phenotypic and genetic parameters in the plant species for genetic and plant breeding studies is very important for the exact selection of the genotypes according to the objectives of the breeding programs. Understanding the nature of morphological, physiological, genetic and molecular mechanisms under the influence of drought tolerance can play an important role in the development of drought tolerant genotypes for cultivation in arid and semi-arid areas. The main aims of this study included investigation of genetic diversity of sunflower germplasm (to understand the relationships between morphological traits and their heritability) and accurate identification of the most important traits that affect the sunflower yield in each of the normal conditions and drought stress.
 
Materials and Methods
An experiment was conducted in a randomized complete block design with three replications in normal and drought stress conditions in 2014 growing season on 12 sunflower genotypes in a research field of Islamic Azad University, Karaj Branch. Each block consisted of 12 plots and each plot contained five rows. Each row was 5 meters length with 60 cm spacing, and plant spacing on rows was 20 cm. The studied traits included grain yield, seed length, seed width, seed diameter, leaf length, leaf width, plant height and stem diameter.
 
Results
The results of combined analysis of variance confirmed the significant diversity among genotypes for most traits. Grain yield showed higher values for parameters of genetic efficiency, genotypic and phenotypic variation coefficients under drought stress and non-stress conditions compared to other traits. While the highest percentage of heritability in the normal condition belonged to grain yield, it belonged to plant height in the stress conditions. Plant height, leaf width, stem diameter and leaf length showed the highest change in terms of traits in stress conditions compared to normal conditions. Dendrograms derived from cluster analysis classified genotypes into three groups based on studied traits under both drought stress and non-stress conditions. According to the cluster analysis, under drought stress condition, the Sor and Progress genotypes were identified as tolerant genotypes and, in contrast, genotypes of Favorite, Record, SHF81-90, Master and Lakomka were recognized as sensitive genotypes.
 
Discussion
In the combined analysis of variance the significance of the environmental effect shows that drought stress has significantly affected all traits. Therefore, the applied stress reveals the differences between the cultivars and the evaluation of the cultivars in the stress environment can lead to the selection of suitable cultivars. The effect of genotype was not significant for some traits, which suggests that the selection of cultivars for these traits is complex because the differences between the cultivars are not clearly defined. Selection is more reliable for traits that are significant, such as grain yield. The interaction between genotype and environment also increases the complexity of selection, which was significant only for plant height in this study. The high coefficient of phenotypic variation for traits indicates that the expression of these traits is greatly affected by the environment. Also, the high coefficient of genotypic and phenotypic variation for some traits indicates the wide range of changes for these traits. In general, the correspondence of the phenotypic coefficient and the genotypic coefficient in some traits indicates that the environmental effects on trait expression are negligible. However, when the phenotypic variation coefficient is higher than the genotypic variation coefficient, it indicates a high environmental effect. The low difference between the phenotypic and genotypic coefficient of variation for some traits suggests that these traits are more controlled by genetic factors. Therefore, the selection of parents for these traits is appropriate for hybridization programs.