Water deficit is one of the most important factors limiting crop yield, and the monitoring of crop water status is important for reasonable irrigation and water saving cultivation. Drought stress tolerance is seen in almost all plants but its extent varies from species to species and even within species. In this study, seven agronomical traits (grain yield, 1000 grain weight, biomass, harvest index, plant height, main spike length and awn length) of 17 spring wheat lines with variable responses to drought stress, obtained from the agriculture research centre of Zanjan province, Iran, were investigated under three different levels: (a) normal irrigation (b) after anthesis drought stress condition, and (c) no irrigation, in a split block design in a randomized complete block design with three replications in Zanjan, Iran, from March to September 2006. This study was conducted to extract the probable correlation between different traits and stress tolerance index (STI) of wheat lines. It was also conducted to estimate the direct and indirect effects of traits on STI and heritability of these traits to provide plant breeders useful information regarding drought resistance in wheat breeding. Analysis of variance indicated that there were significant differences among the genotypes for all studied traits. For biomass, 1000 grain weight, grain yield and harvest index, there was no significant interaction between genotypes and levels of stress. The Zarrin line produced the highest amount of grain yield, biomass, and harvest index. Moreover, under normal irrigation and no irrigation conditions, the harvest index (75 and 70%) had the highest broad sense heritability. Under normal conditions, a positive and significant correlation between STI with grain yield, biomass, and harvest index was observed. Additionally, in stress conditions there was a positive and significant correlation between STI and awn length, spike length and plant height. Under normal conditions, the correlation between biomass and STI was bigger than the stress condition. In path analysis, in all three different conditions, the biggest direct effect on STI was related to grain yield resulting in a positive and significant correlation between grain yield and STI.
