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Abstract In order to investigate diversity and explain the relationships of essential oil yield with morphological traits, 84 populations T. polycephalum, were sown in the field using randomized complete blocks design (RCBD) with three replicates in Research Institute of Forest and Rangeland, Tehran, Iran in 2013-2014. Data were collected for flowering date, growth degree days (GDD), stem number, plant height, dry matter (DM) yield, essential oil percentage, and essential oil yield. The relationships among traits were determined using correlation, regression and factor analysis. The genetic distance and genotype classification were determined using cluster analysis. DM yield was positively correlated with plant height, canopy cover, stem number and oil yield (p<0.01), whereas oil% was negatively correlated with plant height and stem number (p<0.05). Result of stepwise regression in which essential oil yield were considered as dependent variable showed that essential oil%, and DM yield accounted for 39.95 and 47.75%, of the total variations in essential oil production, respectively. In factor analysis, the Eigen values obtained from 1 to 3 factors were higher than one with 88.9% of total variations. In the first factor, traits of DM yield, plant height, canopy cover, stems number, and in the second factor flowering date, GDD and oil yield and in the third factor oil% had higher Eigen vector coefficients for genotype classification. In cluster analysis, genotypes were categorized in four groups. According to the obtained results, cluster 1 for early maturity, clusters 3 for plant height canopy cover, stems number, DM yield and essential oil production and cluster4, for essential oil%, had significantly higher mean values than other groups. There was a good agreement between the results obtained from cluster and factor analyses in scatter diagram representation of 84 genotypes based on the first and second factors. It was concluded that population of cluster 3 are capable to breeding improved new cultivars and should be focused on selection higher aerial biomass yield, plant height, flower number, essential oil%, coupled with early flowering date.
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