Evaluation of cause and effect relationships between grain yield and its components in Trachyspermum copticum L. under normal irrigation and drought stress conditions

Ajowan (Trachyspermum copticum L.), belonging to the Apiaceae family, is remarked as an important pharmaceutical and economical product. In order to determine the most important traits affecting grain yield, a split plot experiment was conducted in two conditions of irrigation (normal and drought stress) as the main factor and 36 ecotypes of ajowan, belonging to nine different provinces of Iran, as a sub plot based on a randomized complete block design with two replications in 2015 and 2016 crop year in Kerman. The results of linear multiple regression analysis with stepwise method showed that in drought stress condition, number of umbrellas per plant, number of seeds per umbrella and essential oil yield, and in normal irrigation conditions, biological yield, 1000-seed weight and seed number per umbrella had the highest effect on grain yield, explaining 87% and 99% of the changes, respectively. Based on the results of path analysis, traits including number of umbels per plant under stress conditions and biological yield under normal conditions had the most direct effect on the grain yield and were recognized as the most important components of the grain yield. Therefore, these traits can be considered as main appropriate criteria for facilitating the indirect selection process and evaluating the seed yield potential in ajowan breeding programs.