Introduction
Wheat plays a very important role in meeting the food security in the world. The wild relatives of wheat have high genetic diversity for various traits. These traits can be exploited in order to improve the yield of bread wheat, and as a result, answer the question of food security for the growing population. Aegilops tauschii or Aegilops squarrosa L. is a diploid and self-pollinating species that has given the D genome to bread wheat as a donor parent. Genome D is an unexploited gene reservoir for genetic diversity that can be used to increase the quantity and quality of grain yield in common bread wheat. Aegilops tauschii as a valuable gene pool, has always been of interest to researchers to improve yield and related traits in wheat. Based on this, identifying and using allelic diversity for various root, phenological and morpho-physiological traits in this species is very important in order to expand the genetic base of wheat to improve wheat yield. Therefore, the aim of this research was to study the relationships between different traits and grain yield in order to identify the most important traits affecting the changes of grain yield in ten different ecotypes of Aegilops tauschii.

Matherials and Methods
This research was conducted in a repeated manner during the years 2016 and 2017 in the Research Greenhouse of Bu-Ali Sina University, Hamedan, west of Iran, in the form of a randomized complete block design with three replications under normal moisture conditions (supplying 95% of field capacity). The plant materials of this research included 10 ecotypes of Aegilops tauschii. The seeds were germinated and grown in germination trays containing equal proportions of perlite and cocopeat. After the seeds germinated, all the seedlings in the trays were transferred to the cold room at the temperature of 4 degrees Celsius for five weeks in the two-leaf stage for vernalization. Then, the seedlings were transplanted into 10 kg plastic pots (pots had a diameter of 40 cm and a depth of 50 cm), which contained an equal ratio of agricultural soil, sand, and rotted animal manure. In this research, 34 attributes were measured and evaluated.

Results and Discution 
High genetic diversity among ecotypes was observed for most of the important traits related to grain yield. The desired ecotype A19 had the highest, and ecotypes A16 and A17 had the lowest yield and grain yield components. The results of correlation analysis revealed that an array of traits including grain water use efficiency, plant height, harvest index, grain weight per main spike, main spike weight, peduncle weight, spikelets/spike, peduncle length, thousand grain weight, grain filling period length, and main stem weight showed the highest positive and significant correlation (p≤0.01) with grain yield. Additionally, grain yield had the most significant (p≤0.01) and negative correlation with days to heading, water consumption, days to anthesis, root dry weight, and days to maturity. According to the stepwise regression results, water use efficiency, grain weight per main spike (both with a positive regression coefficient), and root dry weight (with a negative regression coefficient) were identified as the most important traits influencing grain yield variations. Water use efficiency had the highest direct positive effects on grain yield, while grain weight per main spike and root dry weight had the highest indirect positive and negative effects on grain yield, respectively, through increasing and decreasing water use efficiency. Therefore, it may be infered that due to the relative similarity of the genetic background of common bread wheat with Aegilops tauschii species, as one of its wild ancestors, the selection for higher grain water use efficiency and grain weight in the main spike, as well as root and phenological traits (except grain filling period) leads to the improvement of common wheat yield in normal moisture conditions. Ecotypes A16 and A17 were identified as the most unfavorable ecotypes, mainly because they needed greater number of days to heading, days to pollination, and days to physiological maturity, and invested more photosynthetic products to rooting attributes, including root dry weight. According to the results of this research, Aegilops tauschii, is an important gene reservoir for improving wheat yield. In this research, ecotype A19, where its higher amounts of grain yield and related traits, including water use efficiency and grain weight in the main spike are accompanied by lower amounts of phenological traits as well as lower dry weight of roots, was the most desirable ecotype.

Conclusion
Water use efficiency, grain weight per main spike and root dry weight were identified as the most important traits affecting grain yield. Therefore, it may be possible to select for higher values of water use efficiency and grain weight per main spike and lower values of phenological traits and root traits. These traits, which have high heritability and respond well to selection, can be used to identify ecotypes of this wild wheat. Ultimately, they can be used in future wheat breeding programs.